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Electrophilic Phosphinidenes: Science or Fiction?

Jansen, H.

2010

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citation for published version (APA)Jansen, H. (2010). Electrophilic Phosphinidenes: Science or Fiction?.

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5 Synthesis, Structure and

Reactivity of a Stabilized

Phosphiranylium Salt

Published in: Angew. Chem. Int. Ed. 2010, 49, 5485–5488.

‐85‐

Chapter 5

5.1 Introduction

Phosphiranes with a three‐membered PC2 ring have unique electronic

properties.[1] Although their ring strain is smaller than the one in cyclopropanes,

they are highly reactive and display remarkable chemistry, like [2+1]

cycloreversions to olefins and phosphinidenes [R–P],[2] cationic ring opening

polymerization to poly(ethylenephosphine),[3] and special properties as ligands in

transition metal complexes for catalysis.[4] Positively charged species are even

more reactive. Species like A (phosphirenylium ion) or B (phosphiranylium ion),

which may be viewed as ‐complexes of P+ to alkynes or alkenes,[5] respectively

(Scheme 1), were never isolated.[6] A notable exception is the crystalline

diphosphirenium ion C.[7]

The dibenzo[a,d]cycloheptatrienyl (trop) platform was successfully used for the

synthesis of the highly strained dibenzophosphasemibullvalene D,[8] very robust

aminophosphiranes (BABAR‐Phos) E,[9] and the phosphiranium ion F (R = iPr,

R1 = tBu).[10] Isolated examples of the latter are likewise rare.[11]

Scheme 1. Cationic three‐membered phosphorus heterocycles (A – C) and

BABAR‐Phos derivatives D – G.

‐86‐

Synthesis, Structure and Reactivity of a Stabilized Phosphiranylium Salt

Because the lone pair at phosphorus has a very high s‐orbital character,[10]

phosphiranes are generally reluctant to be alkylated, oxygenated or sulfurated

and with bulky substituents R in E these reactions are suppressed. However,

transition metal ions [Pt(II), Rh(I)] bind to the phosphorus center albeit very

weakly.[4b]

We now report on the synthesis of a sterically unshielded BABAR‐Phos derivative

that reacts with methyl triflate, a “hard” alkylation reagent, at the “hard” nitrogen

center to give the cation G, which can be described as an intra‐molecular amine

complex of the elusive phosphiranylium ion B.

5.2 Synthesis

The synthesis of the MeBABAR‐Phos 2 is straightforward as is shown in Scheme 2.

As previously reported,[4a] the reduction of the amino(dichloro)trop phosphane 1

with magnesium turnings (RT, 3 h) generates an intermediate phosphinidenoid

that gives a rather clean formation of MeBABAR‐Phos 2 (73%; δ31P –149.0) due to

the spatial proximity of the P‐unit and the olefin. The characteristic low‐frequency

shifts of the 1H and 13C resonances of the HCCHtrop unit and the J(H,P) and J(C,P)

couplings in 2 (δ1H: 2.55 (2J(H,P) = 18.3 Hz); δ13C 24.1 (1J(C,P) = 41.7 Hz) clearly

indicate its formation (vs. δ 1H 6.83 and δ 13C 130.9 in 1).

Reaction of MeBABAR‐Phos 2 with methyl triflate in toluene (RT, 2 h) resulted

exclusively in N‐alkylation (91%; δ31P –83.6; Scheme 2). The structure of the novel

salt 3 was established by an X‐ray crystal structure determination (Figure 1)[12]

and displays a remarkable long P–N bond (1.8449(14) Å) compared to the neutral

analogues E (1.73–1.74 Å)[4a,13] and phosphiranium ion F (1.628(4) Å).[10]

‐87‐

Chapter 5

Scheme 2. Synthesis of 2 and 3.

Figure 1. Displacement ellipsoid plot (50% probability) of 3. The OTf anion was omitted for

clarity. Selected bond lengths [Å] and angles [°]: P1–N1 1.8449(14), P1–C4 1.8532(18), P1–

C5 1.8593(17), N1–C1 1.541(2), N1–C16 1.509(2), N1–C17 1.510(2), C4–C5 1.523(2); N1–

P1–C4 97.68(7), N1–P1–C5 97.95(7), C5–P1–C4 48.44(7); (P): 244.1°.

‐88‐


5.3 Computational Study

DFT calculations at the B3PW91/6‐311+G(d,p) level of theory were performed in

order to understand and compare the electronic structure and reactivity of 2, 3,

and the non‐observed P‐methylated product FMe.[14] The HOMO of 2 (–5.81 eV) is

mainly located on the nitrogen atom, while the HOMO–1 (–6.35 eV; Figure 2) can

be attributed to the “lone pair” located on phosphorus. This finding is in accord

with the preferred alkylation at nitrogen. Further, the N‐alkylated cation 3 is 9.5

kcal∙mol–1 more stable than the computed P‐alkylated derivative FMe.

The nature of the P–N bond was studied by analyzing the electron density using

the theory of atoms in molecules (AIM)[15] to investigate the relative weight of the

resonance structures 3A (ammonium salt with the positive charge on N) and 3B

(intramolecular amino‐stabilized phosphiranylium cation with the positive charge

localized on the PC2 ring; Scheme 2).[5e]

Figure 2. HOMO (left) and HOMO–1 (right) of MeBABAR‐Phos 2.

The atomic charges of the basins for nitrogen Q(N) and phosphorus Q(P) of 2, 3,

and FMe are given in Table 1. As expected, the positive charge at the P‐atom in

phosphonium‐type ion FMe is the highest (+2.47e). With respect to 2,

N‐methylation diminishes both the negative charge at N and the positive charge

on P. That is, 3 contains the longest, weakest and least polar P–N bond (Q(N,P) =

2.20e [vs. 2.67e (2) and 3.88e (FMe)].

‐89‐

Chapter 5

Table 1. Characterization of the P–N bond with AIM[a]

P–N Bond H(rc)[b] Q(P) Q(N)

2 1.727 Å –0.912 1.34 –1.33

3 1.885 Å –0.670 1.09 –1.11

FMe 1.660 Å –1.102 2.47 –1.41

[a] The B3PW91/6‐311+G(d,p) calculated structures where used; [b] The bond critical point

rc corresponds to a saddle point of the electron density in the bonding region.

From the obtained energy densities (H(rc); Table 1), it can be derived that the P–N

bond in 2 and FMe has a strong covalent character.[16] However, the reduced

energy density (H(rc) = ‐0.670 hartree/Å3) of the P–N bond in 3 indicates a

significant contribution of resonance structure 3B to the electronic ground state

of 3, which may be viewed as an intra‐molecular donor‐acceptor N→P complex[17]

of an amino‐stabilized phosphiranylium ion (B).

5.4 Reactivity

The reactivity of the amino phosphiranylium salt 3 was investigated as well.

Addition of methyl lithium at –78 °C resulted in P‐alkylation under cleavage of the

P–N bond yielding the P‐methyl phosphiranes endo‐4 (δ31P –173.3) and exo‐4

(δ31P –205.7; 4:1 ratio; Scheme 3) quantitatively.[18] The reaction of 3 with

thiolate, Li(SPh), gives exclusively the sterically less encumbered isomer exo‐5

(δ31P –132.8), which is stable in solution and in the solid state.

‐90‐


N

Me

PMe

OTf

3

Me2N PR

endo

Me2N P

exo

R

+RLi

THF, -78oC

R = Me, PhS

R = Me4

R = PhS5

NMe2

P

N N

OTf10A

NMe2

P

N N

OTf10B

P P

PPP

Me

Me

Me

Me

Me

6

P

R

S SPh Ph

trop-NMe2

Ph2S2trop-NMe2N

NTHF, r. t.

7 R = Me8 R = SPh

Me2N

trop-NMe2 =

9

Scheme 3. Reactivity of the amino phosphiranylium salt 3.

Solutions of 4 can be stored at –18 °C for days, but within 24 hours RT clean

formation to the cyclic phosphinidene oligomer (PMe)5 (6; Scheme 3),[19] and (5‐H‐

dibenzo[a,d]cyclohepten‐5‐yl)‐dimethylamine (trop‐NMe2) occurred. Addition of

diphenyldisulfide to a freshly prepared solution of the phosphiranes 4 or 5

resulted in the quantitative formation of the phosphonodithious acids 7

(δ31P 85.2)[20] and 8 (δ31P 130.8),[21] respectively (Scheme 3). Formally these

reactions correspond to the expulsion of a phosphinidene [R–P],[22] but at this

stage of our investigations, we assume a [R–P]‐transfer process without the

intermediacy of free phosphinidenes as our calculations predict that this process

costs ~50 kcal.mol1.

‐91‐

Chapter 5

Interestingly, 3 also reacts with neutral nucleophiles like N‐heterocyclic carbenes,

NHC (Scheme 3). Namely, addition of IiPr2Me2 9 resulted in the NHC‐stabilized

phosphiranylium cation 10 (δ31P –214.3).[23,24]

Figure 3. Displacement ellipsoid plot (50% probability) of 10. Only one of two independent

molecules is shown. The OTf anion and ordered/disordered THF solvent molecules were

omitted for clarity. Selected bond lengths [Å] and angles [°]: P1–N1 2.581(2), P1–C4

1.908(3), P1–C5 1.896(2), P1–C18 1.881(3), N1–C1 1.478(3), N1–C16 1.467(3), N1–C17

1.466(3), N2–C18 1.350(3), N2–C19 1.384(3), N3–C18 1.351(3), N3–C20 1.391(3), C4–C5

1.500(3), C19–C20 1.354(4); C4–P1–C5 46.43(11), C4–P1–C18 95.32(11), C5–P1–C18

96.22(11), C5–C4–P1 66.36(13), C4–C5–P1 67.21(13), N2–C18–N3 106.8(2); (P): 238.0°.

The molecular structure of 10, established by an X‐ray crystal structure analysis

(Figure 3),[12] may be viewed as an olefin carbene PI cation. The sum of bond

angles at the phosphorus center ((P): 238.0°) is small (cf. 244.1° for 3) despite

the sterically demanding substituents. The strength of the P–Carbene interaction in

10 (P1–C18 1.881(3) Å) was investigated with a bond decomposition scheme (ADF

on model system 10’ with H for all NHC substituents).[25] Strong ‐donation of the

‐92‐


lone pair of the C‐center of the carbene to the empty p‐orbital at the phosphorus

atom makes up for 91% (130 kcal.mol1) of the orbital interaction energy (Eoi) to

which a small contribution from ‐backdonation of the phosphorus lone pair into

the empty *‐orbital of the carbene is added (13 kcal.mol1, 9% of Eoi). In

contrast in Macdonalds bis‐carbene PI cation [P(IiPr2Me2)2]+,[26] the contribution of

the ‐backdonation to the interaction energy is significantly higher (27

kcal.mol1; 13%).[24] This is reflected in the PC bond dissociation energies (BDE’s)

which are much smaller in 10’ (54.9 kcal.mol1) then in [P(NHC)2]+ (105.6

kcal.mol1). Interestingly, the low energy density at the bond critical point of the

PCarbene bond in 10 (H(rc) = ‐0.763 hartree/Å3) is comparable to that of the PN

bond in 3, which underlines the contribution of resonance structure 10B (Scheme

3) to the electronic ground state of 10.

5.5 Conclusion

Our computational and experimental results indicate that [N,N‐dimethyl‐BABAR‐

Phos]OTf 3 and [(Me2N‐trop)P(IiPr2Me2)]OTf 10 may be regarded as the first

isolated examples of intra‐ and inter‐molecular amino‐ or carbene‐stabilized

phosphiranylium salts, respectively.

5.6 Experimental Section

The sterically unencumbered MeBABAR‐Phos 2 could be obtained in a few steps from the

commercially available dibenzosuberenone (11). The common method previously used to

synthesize the trop‐amines,[27] is not practical with small amines (i.e MeNH2); therefore

the synthetic route was slightly altered (see Scheme 4).[28]

‐93‐

Chapter 5

Scheme 4. Synthesis of [N,N‐dimethyl‐BABAR‐Phos]OTf 3.

General Procedures. All syntheses were performed with the use of a glove box and

Schlenk techniques under argon or nitrogen. All solvents were dried and purified by

standard procedures and were freshly distilled under nitrogen from

sodium/benzophenone (THF), from sodium (toluene), or from lithium aluminum

anhydride (pentane) prior to use. Reagents were used as purchased. 1,3‐Diisopropyl‐4,5‐

dimethylimidazol‐2‐ylidine[29] was synthesized according to literature procedures. NMR

spectra were recorded (at 298 K unless otherwise noted) on a Bruker Advance 250, an

Advance DPX 300 or MSL 400 and referenced internally to residual solvent signals (CDCl3

[1H: δ 7.25 ppm, 13C{1H}: 77.0 ppm], CD2Cl2 [1H: δ 5.32 ppm, 13C{1H}: 53.5 ppm], C6D6 [

1H: δ

7.16 ppm, 13C{1H}: 128.1 ppm], or d8‐THF [1H: δ 1.73 and 3.58 ppm, 13C{1H}: 25.5 and 67.7

ppm]). High‐resolution mass spectra (HR‐MS) as performed on a Finnigan Mat 900 mass

spectrometer operating at an ionization potential of 70eV and the Electrospray Ionisation

(ESI) mass spectrometry was carried out with a micrOTOF‐Q instrument in positive ion

mode. Melting points were measured in capillaries, sealed when necessary, and are

uncorrected.

N‐methyl‐5H‐dibenzo[a,d]cyclohepten‐5‐imine (12):[30] Dibenzosuberenone (11) (30.0 g,

145 mmol) was dissolved in toluene (1.0 L) and the solution was cooled to –10 °C

(NaCl/ice). Titanium(IV) chloride (24.0 mL, 218 mmol) was slowly added, which resulted in

a dark red suspension. Subsequently, methylamine (50 g, 1.6 mol) was added in one

portion and the orange suspension was allowed to warm to room temperature and was

‐94‐


stirred overnight. The reaction mixture was poured over an ice/water mixture and the

light yellow organic layer was separated. The milky white aqueous layer was extracted

with toluene (3 x 300 mL). The combined organic layers were dried over MgSO4 and

concentrated, yielding imine 12 analytically pure as a pale yellow, viscous oil in quantative

yield. Recrystallisation from hexane at –20 °C provided colorless crystals. 1H NMR (250.1

MHz, CDCl3); δ 3.33 (s, 3H, CH3), 6.95 (s, 2H, HC=CH), 7.25‐7.63 (m, 8H, ArH). 13C{1H} NMR

(75.5 MHz, CDCl3); δ41.0 (s, CH3), 127.0 (s, ArCH), 127.4 (s, ArCH), 127.6 (s, ArCH), 128.4 (s,

ArCH), 128.6 (s, ArCH), 128.6 (s, ArCH), 129.1 (s, ArCH), 129.3 (s, ArCH), 130.2 (s, HC=CH),

131.4 (s, HC=CH), 133.0 (s, ArCq), 133.8 (s, ArCq), 134.2 (s, ArCq), 141.7 (s, ArCq), 169.0 (s,

N=Cq); MS (EI, 70 eV): m/z (%): 219 (88) [M]+, 218 (100) [M – H]+, 178 (80) [M – C=NCH3]+,

HR‐MS (EI): calcd for C16H13N: 219.1048, found: 219.10434.

(5H‐Dibenzo[a,d]cyclohepten‐5‐yl)‐methylamine (13):[28,31] N‐methyl‐5H‐dibenzo[a,d]

cyclohepten‐5‐imine (12) (7.76 g, 35.4 mmol) was dissolved in methanol (360 mL) and

treated with four portions of NaBH4 (1.60 g, 42.3 mmol each) at an interval of 1.5 hours.

Subsequently the reaction was stirred for 20 h and concentrated under reduced pressure

and the obtained white solid was partitioned between 130 mL of 20% Na2CO3 (aq) and 130

mL DCM. After phase separation, the aqueous layers were washed with 2 x 80 mL DCM.

The combined organic layers where dried over MgSO4 and concentrated under reduced

pressure, resulting in 7.69 g (98%) of a light yellow viscous oil, which solidified upon

standing. Due to endo‐exo isomerization of the amine in solution the NMR‐signals are

strongly broadened at r.t, at 233 K the signals are sharp with the ratio 2.96 to 1.00 in favor

for the endo‐isomer. M.p. 55‐56 °C; Endo‐13: 1H NMR (400.1 MHz, CDCl3, 233 K) δ 2.09 (s,

3H, CH3), 2.10 (bs, 1H, NH), 4.72 (s, 1H, NCH), 7.03 (s, 2H, HC=CH), 7.22‐7.37 (m, 8H, ArH). 13C{1H} NMR (75.5 MHz, CDCl3, 213 K); δ 34.1 (s, CH3), 70.8 (s, NCH) 126.9 (s, 2C, ArCH),

128.4 (s, 2C, ArCH), 129.6 (s, 2C, ArCH), 129.7 (s, 2C, ArCH), 130.2 (s, 2C, HC=CH), 132.8 (s,

2C, ArCq), 138.9 (s, 2C, ArCq). Exo‐13: 1H NMR (400.1 MHz, CDCl3, 233 K); δ 1.95 (bs, 1H,

NH), 2.54 (s, 3H, CH3), 3.99 (s, 1H, NCH), 7.17 (s, 2H, HC=CH), 7.22‐7.37 (m, 8H, ArH). 13C{1H} NMR (75.5 MHz, CDCl3, 213 K); δ 35.2 (s, CH3), 62.3 (s, NCH) 121.8 (s, 2C, ArCH),

125.5 (s, 2C, ArCH), 127.5 (s, 2C, ArCH), 128.5 (s, 2C, ArCH), 130.9 (s, 2C, HC=CH), 133.4 (s,

2C, ArCq), 140.0 (s, 2C, ArCq). MS (EI, 70 eV): m/z (%): 221 (60) [M]+, 191 (100) [trop]+, HR‐

MS (EI): calcd for C16H15N: 221.1204, found: 221.1198.

‐95‐

Chapter 5

(5H‐Dibenzo[a,d]cyclohepten‐5‐yl)‐methyl‐amino dichlorophosphine (1). A solution of

(5H‐dibenzo[a,d]cyclohepten‐5‐yl)‐methylamine (13) (3.18 g, 14.4 mmol) in Et2O (50 mL)

and THF (20 mL) was cooled to ‐78 °C (dry ice/acetone). n‐BuLi (1.6 M in hexanes, 9.5 mL,

14.5 mmol) was slowly added, resulting in the formation of an orange suspension, which

was stirred for 30 min at the same temperature. The cold reaction mixture was slowly

added to a ‐78 °C solution of PCl3 (9.4 mL, 107.7 mmol) in diethyl ether (5 mL). The

obtained yellow suspension was allowed to warm to rt. in 2 h and the solvent was

evaporated. The yellow viscous oil was dissolved in toluene (50 mL) and filtered over pre‐

dried Celite to remove the insoluble LiCl. Concentration of the filtrate under reduced

pressure gave 3.98 g of 1 (86%) as a pale yellow solid. M.p. 71‐73 °C; 31P{1H} NMR (101.3

MHz, C6D6); 168.9 (s); 1H NMR (300.1 MHz, C6D6); δ 2.49 (d,

3J(H,P) = 4.2 Hz, 3H, CH3),

5.12 (d, 3J(H,P) = 4.8 Hz, 1H, NCH), 6.83 (s, 2H, HC=CH), 7.08‐7.11 (m, 8H, ArH); 13C{1H}

NMR (75.5 MHz, C6D6); δ 33.4 (d, 2J(C,P) = 7.3 Hz, CH3), 73.7 (d,

2J(C,P) = 33.2 Hz, NCH)

128.4 (s, 2C, ArCH), 128.9 (s, 2C, ArCH), 130.2 (s, 2C, ArCH), 130.3 (d, 4J(C,P) = 2.5 Hz, 2C,

ArCH), 130.9 (d, 5J(C,P) = 2.8 Hz, 2C, HC=CH), 135.3 (d, 4J(C,P) = 4.5 Hz, 2C, ArCq), 135.5 (d, 3J(C,P) = 7.5 Hz, 2C, ArCq). MS (EI, 70 eV): m/z (%): 321 (20) [M]+, 191 (100) [trop]+, HR‐MS

(EI): calcd for C16H14Cl2NP: 321.0241, found: 321.02463.

N‐Methyl‐BABAR‐Phos (2). Dichlorophosphane 1 (3.98 g, 12.4 mmol) was dissolved in THF

(70 mL) and magnesium turnings (0.34 g, 14.0 mmol) were added. After stirring the

reaction mixture for 3 hours at rt., 31P NMR indicated that the reaction was completed.

After filtration and concentration, a yellow solid remained, which was dissolved in 40 mL

toluene and washed with 40 mL saturated, degassed NH4Cl‐solution (aq). The organic layer

was separated, dried over MgSO4 and quickly concentrated under reduced pressure to

yield 2.29 g of 2 (73%) as a yellow solid. M.p. 98‐103 °C; 31P{1H} NMR (300.1 MHz, C6D6); δ

–149.0 (s); 1H NMR (300.1 MHz, C6D6); δ 2.36 (d, 3J(H,P) = 16.6 Hz, 3H, CH3), 2.55 (d,

2J(H,P)

= 18.3 Hz, 2H, PCH) 4.60 (d, 3J(H,P) = 5.3 Hz, 1H, NCH), 7.00‐7.21 (m, 8H, ArH); 13C{1H} NMR

(75.5 MHz, C6D6); δ 24.1 (d, 1J(C,P) = 41.7 Hz, PCH), 38.3 (d, 2J(C,P) = 22.5 Hz, CH3) 68.6 (d,

2J(C,P) = 6.6 Hz, NCH), 125.5 (s, 2C, ArCH), 125.8 (s, 2C, ArCH), 127.5 (s, 2C, ArCH), 129.7 (d,

J(C,P) = 0.7 Hz, 2C, ArCH), 133.1 (d, J(C,P) = 1.2 Hz, 2C, ArCq), 135.3 (d, J(C,P) = 1.8 Hz, 2C,

ArCq); m/z (%): 251 (16) [M]+, 191 (100) [trop]+, HR‐MS (EI): calcd for C16H14NP: 251.0864,

found: 251.08587.

‐96‐


[N,N‐Dimethyl‐BABAR‐Phos]triflate (3). To a filtered solution of N‐methyl‐BABAR‐Phos (2)

(200 mg, 0.80 mmol) in toluene (8 mL), methyl triflate (0.09 mL, 0.80 mmol) was added,

which resulted in the formation of a white precipitate. The reaction was stirred at rt. for

two hours, after which the white precipitate was washed with 5x5 mL diethyl ether. After

drying under vacuum the product was obtained as a white solid (302 mg, 91%). Crystals

suitable for single‐crystal X‐ray structure determination were obtained by slow

evaporation of a saturated THF solution at room temperature in the glove box. M.p. 173‐

177 °C; 31P{1H} NMR (101.3 MHz, CD2Cl2); δ –83.6 (s); 1H NMR (300.1 MHz, CD2Cl2; δ 2.97

(d, 3J(H,P) = 9.6 Hz, 6H, CH3), 3.60 (d, 2J(H,P) = 20.4 Hz, 2H, PCH), 5.97 (d, 3J(H,P) = 2.4 Hz,

1H, NCH), 7.43‐7.64 (m, 8H, ArH); 13C{1H} NMR (75.5 MHz, CD2Cl2); δ 30.2 (d, 1J(C,P) = 47.1

Hz, 2C, PCH), 49.3 (d, 2J(C,P) = 14.6 Hz, 2C, CH3) 75.0 (s, NCH), 120.9 (q, 1J(C,F) = 320.5 Hz,

SO3CF3), 127.3 (s, 2C, ArCq), 128.3 (s, 2C, ArCH), 128.6 (s, 2C, ArCH), 130.5 (s, 2C, ArCH),

130.8 (s, 2C, ArCH), 130.9 (s, 2C, ArCq). MS (EI, 70 eV): m/z (%): 251 (40) [M‐CH3]+, 191

(100) [trop].

N,N‐Dimethyl‐trop methylphosphirane 4. To a cooled suspension of [N,N–Dimethyl–

BABAR–Phos]triflate (3) (201 mg, 0.49 mmol) in THF (8 mL) at –78 °C (dry ice/acetone), a

solution of methyl lithium (1.6 M in Et2O, 0.29 mL, 0.49 mmol) was added. The clear, pale

yellow solution was stirred at –78 oC for 15 min (31P–NMR spectroscopy indicated

complete conversion). The resulting methyl‐phosphirane 4 was not isolated in pure form;

Characterization and structural assignment are based on 13C‐1H HMQC, HMBC, 31P‐1H COSY

and 1H‐1H NOESY NMR experiments at –20 oC. Endo‐4; 31P{1H} NMR (202.4 MHz, d8‐THF); δ

–173.3 (s); 1H NMR (500 MHz, d8–THF); δ 0.83 (d, J(H,P) = 11.5, 3H, PCH3), 1.74 (s, 6H,

NCH3), 2.86 (s, 2H, PCH), 3.81 (s, 1H, NCH), 7.00‐7.55 (m, 8H, ArH); 13C{1H} NMR (125.7

MHz, d8‐THF); δ 19.4 (d, 1J(C,P) = 38.2 Hz, PCH3), 37.9 (d,

1J(C,P) = 45.0 Hz, 2C, PCH), 41.2

(s, 2C, NCH3) 78.4 (s, NCH), 125.6 (s, 2C, ArCH), 128.0 (s, 2C, ArCH), 130.0 (s, 2C, ArCH),

133.2 (s, 2C, ArCH), 138.7 (d, 3J(C,P) = 1.7 Hz, 2C, ArCq), 139.3 (d, 2J(C,P) = 8.9 Hz, 2C, ArCq).

Exo‐4; 31P{1H} NMR (202.4 MHz, d8‐THF); δ –205.7 (s); 1H NMR (500 MHz, d8–THF); δ 1.02

(d, J(H,P) = 11.5, 3H, PCH3), 2.47 (s, 6H, NCH3), 3.19 (s, 2H, PCH), 5.21 (s, 1H, NCH), 7.00‐

7.55 (m, 8H, ArH); 13C{1H} NMR (125.7 MHz, d8–THF); δ 11.5 (d, 1J(C,P) = 33.1 Hz, PCH3),

33.0 (d, 1J(C,P) = 35.6 Hz, 2C, PCH), 45.2 (s, 2C, NCH3) 70.9 (s, NCH), 123.8 (s, 2C, ArCH),

126.6 (s, 2C, ArCH), 126.7 (s, 2C, ArCH), 130.3 (s, 2C, ArCH), 134.9 (d, 3J(C,P) = 10.5 Hz, 2C,

ArCq), 145.5 (d, 2J(C,P) = 4.3 Hz, 2C, ArCq).

‐97‐

Chapter 5

Reactivity of N,N‐Dimethyl–trop methylphosphirane 4.

Thermal behavior; A solution of 4 in THF was allowed to stand at room temperature under

inert atmosphere. Within 24 h complete decomposition of the phosphirane into trop–

dimethylamine 14 and pentamethyl–cyclopentaphosphane (δ31P 18.0 (m))[32] is observed.

Aqueous workup of the mixture allows the isolation of 14 in 80% yield. Solution of 4 can

be stored in the freezer at –18 oC for one week without decomposition.

Trapping reaction with phenyldisulfide; To a freshly prepared solutions of 4 (0.132 mmol)

in THF at –78 oC, phenyldisulfide (328 mg, 1.5 mmol) was added. The solution was allowed

to warm to room temperature and stirred for 15 min. 31P NMR of the solution shows

complete conversion of the phosphirane and the formation of the known (PhS)2PMe (7),

[δ 31P 85.2 (s)][33]

N,N‐Dimethyl‐trop thiophenyl phosphirane 5. To a cooled suspension of [N,N–Dimethyl–

BABAR–Phos]triflate (3) (32 mg, 0.078 mmol) in d8–THF (0.5 mL) at –78 °C (dry

ice/acetone), a solution of lithium thiophenolate (9 mg, 0.078 mmol) in d8–THF (0.3 mL)

was added. The reaction mixture gave immediately a clear yellow solution. 31P NMR

spectroscopy showed that the reaction was complete. The resulting exo phosphirane 5

was not isolated in pure form; Characterization and structural assignment are based on 13C‐1H HMQC, HMBC, 31P–1H COSY and 1H–1H NOESY NMR experiments. 31P{1H} NMR

(101.3 MHz, d8–THF); δ –132.8 (s); 1H NMR (400.1 MHz, d8–THF); δ 1.76 (s, 6H, NCH3), 3.44

(s, 2H, PCH), 3.98 (d, 5J(H,P) = 4.3 Hz, 1H, NCH), 7.07–7.51 (m, 8H, ArH); 13C{1H} NMR

(100.6 MHz, d8–THF); δ 40.8 (d, 1J(C,P) = 51.0 Hz, HCP), 41.4 (d, 6J(C,P) = 4.8 Hz, NCH3), 77.1

(s, NCH), 126.3 (s, SArCH), 126.9 (s, 2C, tropArCH), 129.0 (s, 2C, tropArCH), 129.3 (s,

SArCH), 129.4 (s, 2C, tropArCH), 129.7 (d, 2J(C,P) = 8.1 Hz, SArCq), 132.9 (s, SArCH), 133.6

(s, 2C, tropArCH), 137.1 (d, 3J(C,P) = 7.2 Hz, 2C, tropArCq), 138.6 (d, 2J(C,P) = 2.7 Hz, 2C,

tropArCq).

Reactivity of N,N‐Dimethyl‐trop thiophenyl phosphirane 5.

Trapping with phenyldisulfide; To a freshly prepared solution of 5 (0.078 mmol) in d8‐THF

(0.3 mL), phenyldisulfide was added (347 mg, 1.46 mmol). 31P NMR spectroscopy of the

solution shows complete conversion of the phosphirane and the formation of the known

‐98‐


(PhS)3P (8) [δ31P 132.5 (s)][34]. 1H NMR indicates the formation of dimethylamine‐trop 14

[(250.1 MHz, d8‐THF) δ 1.80 (s, 6H, CH3), 4.13 (s, 1H, NCH), 6.98 (s, 2H, HC=CH), 7.30‐7.47

(m, 8H, ArH)]; HR‐MS (EI): calcd for C18H15NPS3: 358.00735 found: 358.00645.

(5‐H‐Dibenzo[a,d]cyclohepten‐5‐yl)‐dimethylamine (14): To a solution of 5‐chloro‐5‐H‐

dibenzo[a,d]cycloheptene[35] (9.39 g, 41.5 mmol) in dry toluene (200 mL), N‐

trimethylsilyldimethylamine (8.8 mL, 53.9 mmol) was added. The reaction was stirred

overnight and concentrated. Purification by column chromatography (SiO2; using a

gradient from DCM till 10% methanol in DCM, Rf = 0.23) resulted in 9.66 g of 14 as a

colorless solid (99%). 1H NMR (250.1 MHz, CDCl3); δ 1.89 (s, 6H, CH3), 4.11 (s, 1H, NCH),

7.09 (s, 2H, HC=CH), 7.28‐7.43 (m, 8H, ArH). MS (70 eV): m/z (%): 235 (12) [M]+, 191 (100)

[trop]+, HR‐MS (EI): calcd for C17H17N: 235.1361, found: 235.1355.

N,N‐Dimethylamino‐trop carbene phosphirane 10. To a stirred suspension of [N,N‐

dimethyl‐BABAR‐Phos]triflate (3) (26.0 mg, 0.063 mmol) in d8‐

THF (0.4 mL), a solution of 1,3‐diisopropyl‐4,5‐

dimethylimidazol‐2‐ylidine (0.224 M in d8‐THF, 0.28 mL, 0.063

mmol) was added dropwise, over 5 min. The resulting clear

yellow solution was stirred at r.t. for 5 min, after which 31P and 1H NMR spectroscopy indicated full conversion to 10. In

addition to the phosphirane 13, 10 % of imidazolium salt 15 was formed as well. Suitable

crystals of 10 for X‐ray analysis were obtained from d8‐THF at –20 °C. 31P{1H} NMR (161.9

MHz, d8‐THF); δ –212.9 (s); 1H NMR (500.2 MHz, d8‐THF); δ 1.76 (bs, 12H, CH(CH3)2), 1.83

(s, 6H, NCH3), 2.37 (s, 6H, =CCH3), 4.13 (s, 1H, NCH), 4.14 (s, 2H, PCH), 5.45 (dsept, 3J(H,H)

= 7.0 Hz, 4J(H,P) = 3.0 Hz, 2H, CH(CH3)3), 7.22 (dt, 3J(H,H) = 7.3 Hz, 5J(H,P) = 1.3 Hz, 2H, H9 +

H14), 7.31 (dt, 3J(H,H) = 7.5 Hz, 4J(H,P) = 1.5 Hz, 2H, H10 + H13), 7.35 (dd, 3J(H,H) = 7.5 Hz, 4J(H,P) = 1.0 Hz, 2H, H8 + H15), 7.61 (d, 3J(H,H) = 7.5 Hz, H11 + H12). 13C{1H} NMR (125.8

MHz, d8‐THF); δ 10.6 (s, =CCH3), 41.0 (s, NCH3), 41.5 (d, 1J(C,P) = 42.6 Hz, PCH), 53.3 (d,

3J(C,P) = 5.8 Hz, CH(CH3)2), 77.5 (s, NCH), 122.3 (q, 1J(C,F) = 323.3 Hz, SO3CF3), 128.0 (s, C9 +

C14), 128.9 (s, =CCH3), 129.4 (s, C10 + C13), 130.8 (s, C8 + C15), 133.9 (s, C11 + C12), 135.8

(d, 2J(C,P) = 8.8 Hz, C3 + C6), 139.7 (s, C2 + C7), 153.8 (d, 1J(C,P) = 114.5 Hz, PC), CH(CH3)3

signals are unresolved. 19F{1H} NMR (235.4 MHz, d8‐THF); δ ‐78.9 (s, SO3CF3). HR MS (ESI):

N PN

N

15

2 345

67

8

9 10

11

12

1314

1

17

16

18

19

20

212223

24

25

2628

27

‐99‐

Chapter 5

calcd for C28H37N3P: 446.2720 found: 446.2710 [M‐H]; m/z (%): 446 (5) [M‐H]+, 266 (50)

[M‐NHC]+, 181 (100) [NHC]+.

Imidazolium salt 15: 1H NMR (500.2 MHz, d8‐THF); δ 1.54 (d, 3J(H,H) = 7.0 Hz 12H,

CH(CH3)3), 2.26 (s, 6H, =CCH3), 4.56 (sept, 3J(H,H) = 7.0 Hz, 2H, CH(CH3)3), 9.17 (s, CH).

13C{1H} NMR (125.8 MHz, d8‐THF); δ 8.3 (s, =CCH3), 22.5 (s, CH(CH3)), 51.4 (s, CH(CH3)),

122.3 (SO3CF3), 127.3 (s, =CCH3), 132.0 (s, CH), 19F{1H} NMR (235.4 MHz, d8‐THF); δ ‐78.1 (s,

SO3CF3)

X‐ray crystal structure determinations. X‐ray intensities were measured on a Nonius

KappaCCD diffractometer with rotating anode (graphite monochromator, = 0.71073 Å)

at a temperature of 150(2) K. Data were integrated with the EVAL14[36] (compound 3) or

HKL2000[37] software (compound 10) and corrected for absorption based on multiple

measured reflections. The structures were solved with Direct Methods using the program

SHELXS‐97.[38] Least‐squares refinement was performed with SHELXL‐97[38] on F2 of all

reflections. Non‐hydrogen atoms were refined with anisotropic displacement parameters.

Hydrogen atoms were located in difference‐Fourier maps (compound 3) or introduced in

calculated positions (compound 10). In compound 3 all hydrogen atoms were refined

freely with isotropic displacement parameters. In 10 the hydrogen atoms of the

phosphirane ring were refined freely with isotropic displacement parameters; all other

hydrogen atoms were refined as rigid groups. Drawings, structure calculations and

checking for higher symmetry were performed with the PLATON software.[39]

Compound 3: [C17H17NP](CF3O3S), Fw = 415.36, colourless plate, 0.48x0.36x0.04 mm3,

monoclinic, P21/c (no. 14), a = 12.0365(2), b = 12.6154(2), c = 14.1033(2) Å, =

120.919(1)°, V = 1837.20(5) Å3, Z = 4, Dcalc = 1.502 g/cm3, = 0.31 mm‐1. 37241 Reflections

were measured up to a resolution of (sin /)max = 0.65 Å‐1 of which 4236 were unique (Rint

= 0.031) and 3365 observed [I > 2(I)]. Absorption correction range: 0.84‐0.99. 312

Parameters were refined with no restraints. R1/wR2 [I > 2(I)]: 0.0343/0.0789, R1/wR2

[all refl.]: 0.0511/0.0873. S = 1.024. min/max = ‐0.32/0.40 eÅ3.

‐100‐


Compound 10: [C28H37N3P](CF3O3S) ∙ 2C4H8O + disordered THF, Fw = 739.85[*], colourless

block, 0.30x0.18x0.12 mm3, triclinic, P 1 (no. 2), a = 13.1887(1), b = 17.2207(2), c =

18.1037(2) Å, = 94.2947(5), = 95.3125(5), = 91.7172(7)°, V = 4079.71(7) Å3, Z = 4, Dcalc

= 1.205 g/cm3[*], = 0.17 mm‐1[*]. 71939 Reflections were measured up to a resolution of

(sin /)max = 0.61 Å‐1 of which 15480 were unique (Rint = 0.084) and 9971 observed [I >

2(I)]. Absorption correction range: 0.86‐0.98. Four THF molecules in the asymmetric unit

were modeled in the least‐squares refinement. Additionally, the crystal structures

contains severely disordered THF molecules, which were treated as diffuse electron

density using the SQUEEZE routine in PLATON[ ], resulting in 83 electrons / unit cell. The

diffuse electron density is located on inversion centers in two solvent accessible voids (313

Å3/ unit cell). 960 Parameters were refined with 148 restraints concerning puckering

disorder in two of the modeled THF molecules. R1/wR2 [I > 2(I)]: 0.0542/0.1445, R1/wR2

[all refl.]: 0.0872/0.1624. S = 1.115. min/max = ‐0.33/0.56 eÅ3.

39

[*] Derived values do not contain the contribution of the disordered solvent molecules.

Computations. Density functional theory calculations were carried out with Gaussian 03 at

the B3PW91/6‐311+G(d,p) level[14] and all Cartesian coordinates are given in Angstroms.

The optimization was done using the modified GDIIS algorithm.[40] The nature of each

stationary point was confirmed by a frequency calculation. The bond characterization and

atomic charges were obtained with the programs AIM2000[41] and AIMALL.[42]

The bonding interactions of model carbene P+ adducts NHC‐adduct 10’orth and NHC2P‐Cs

(without substituents on the carbene and a rotated carbene, Figure 4) were analyzed in

terms of f°ment orbitals with the ADF 2007.01 package[43] at the OPBE level[44] using a

TZ2P basis set.[43] According to the extended transition‐state model,[45] the net bond

energy (BDE) can be decomposed into four contributions: the preparation energy (EPrep)

required for deformation of the f°ments from their equilibrium structure to their

geometry in the adducts; the steric interactions between the f°ments due to Pauli

repulsion (EPauli) and electrostatic attraction (Velstat); and the orbital interaction energy

EOi (negative stabilizing):

BDE = Eadduct – (EBPcat + Ecarbene)

= EPrep + (EPauli + Velstat +EOi)

‐101‐

Chapter 5

‐102‐

The latter three contributions are usually summed to give the interaction energy. Rotation

of the carbenes was needed to achieve symmetry partitioning between ‐ and ‐

contributions in the total orbital interaction (Eoi). The ‐contribution is then located

perpendicular to the symmetry plane. The energies obtained after bonding energy analysis

of the full systems 10 (‐68 kcal.mol‐1) and [P(IiPr2Me2)2]+ (‐112 kcal.mol‐1), the structures

without carbene substituents NHC‐adduct 10’ (‐56 kcal.mol‐1) and NHC2P‐C2 (‐109 kcal.mol‐

1), and for NHC‐adduct 10’orth (‐55 kcal.mol‐1) and NHC2P‐Cs (‐106 kcal

.mol‐1) were the

carbene is rotated, are all very similar.

A B

Figure 4. The structures of A) NHC‐adduct 10’orth and B) NHC2P‐Cs.

5.7 References and Notes

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1997, Vol 1A, 277–304; d) F. Mathey, M. Regitz in Phosphorus‐Carbon

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Amsterdam, 2001, 17–55.

[2] a) K. Lammertsma, Top. Curr. Chem. 2003, 229, 95–119; b) K. Lammertsma. M. J.

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Marinetti, Helv. Chim. Acta 2001, 84, 2938–2957; d) X. Li, S. I. Weismann, T.–S.

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‐103‐

7900; e) G. Bucher, M. L. G. Borst, A. W. Ehlers, K. Lammertsma, S. Ceola, M.

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[3] S. Kobayashi, J.–I. Kadokawa, Macromol. Rapid Commun. 1994, 15, 567–571.

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Frison, H. Grützmacher, A. C. Hillier, W. Sommer, S. P. Nolan, Organometallics

2003, 22, 2202–2208; and references therein.

[5] a) R. Chen, Y.–H. Cheng, L. Liu, X.–S. Li, Q.–X. Guo, Res. Chem. Intermed. 2002, 28,

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[6] a) A in solution at low temp.: K. K. Laali, B. Geissler, O. Wagner, J. Hoffmann, R.

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stable transition metal complexes of A, see: b) J. Simon, U. Bergsträsser, M.

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Hitchcock, J. F. Nixon, D. M. Vickers, C. R. Chimie 2004, 7, 931–940; d) Gas phase

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Chapter 5

‐104‐

[9] J. Liedtke, S. Loss, G. Alcaraz, V. Gramlich, H. Grützmacher, Angew. Chem. 1999,

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[10] The phosphiranium F was obtained by a halide abstraction reaction from trop–

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[11] The only other example is reported in: D. C. R Rockless, M. A. McDonald, M.

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by alkylation at the P‐atom of PhP(CH2)2.

[12] CCDC‐765756 (3), and 765757 (10) contain the supplementary crystallographic

data for this paper. These data can be obtained free of charge from The

Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/

data_request/cif.

[13] a) F. B. Läng, H. Grützmacher, Chimia 2003, 57, 187–190; b) J. Liedtke, Diss., ETH

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[14] Gaussian 03, Revision C.02; Frisch, M. J. et al.; Gaussian, Inc., Wallingford CT,

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[15] R. F. W. Bader in Atoms in Molecules: a quantum theory, Clarendon Press: R.F.W.

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‐105‐

[18] It is unclear whether exo‐4 and endo‐4 are in a very slow equilibrium (not

detectable on the NMR time scale) via an inversion of the central ring or are

formed through an attack of MeLi form either side on the P‐center.

[19] L. R. Smith, J. L. Mills, J. Am. Chem. Soc. 1976, 98, 3852–3857.

[20] a) S. C. Peake, R. Schmutzler, J. Chem. Soc. A 1970, 1049–1054; b) A. I. Razumov,

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optimized structures[ref. 14] using ADF2007.01, SCM, Theoretical Chemistry,

Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm.com.

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‐106‐

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S. Loss, C. Widauer, H. Grützmacher, Tetrahedron 2000, 56, 143–156; c) C.

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[28] F. B. Läng, H. Grützmacher, Chimia 2003, 57, 187–190.

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[41] AIM2000, Version 2.0, designed by Friedrich Biegler‐König and Jens Schönbohm

[42] AIMAll, Version 09.04.23, designed by Todd A. Keith, 2009 (aim.tkgristmill.com)


‐107‐

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