Polyhedron 48 (2012) 9–20

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Polyhedron

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Syntheses and properties of cycloborazines and cyclocarborazines

Miroslav Kavala, Peter Zálupsky, Peter Szolcsányi ⇑Dept. of Organic Chemistry, Slovak University of Technology, Radlinského 9, SK-812 37 Bratislava, Slovakia

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 May 2012Accepted 4 September 2012Available online 15 September 2012

Keywords:BorazinesSynthesisPropertiesHydrolysis

0277-5387/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.poly.2012.09.003

⇑ Corresponding author. Tel.: +421 2 59325162; faxE-mail address: peter.szolcsanyi@stuba.sk (P. Szolc

We present a review of preparative methods as well as physico-chemical properties of selected types ofcyclic boron–(carbon)–nitrogen containing compounds. The review is compiled as a synthetic manual forpreparation of compounds with the desired structure, which are to be further modified to novel, as yetunknown derivatives.

The compilation is subdivided according to various types of ring sizes. Each section presents knownmethods of synthesis of respective cyclo(car)borazines highlighting their possible functionalisation.The important data on hydrolytic stability of compounds are also included.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Cycloborazines are cyclic compounds formed of alternatingnitrogen and boron atoms [1–9]. Such systems are isoelectronicwith their all-carbon analogues, however, due to the dipole mo-ments of adjacent heteroatoms, the bonding contains a significantionic component [10–13]. In contrast to cyclosilazanes, a familyencompassing numerous derivatives with various ring size, 4- and8-membered cycloborazines substituted at boron are as yetunknown. Therefore, we shall concentrate on properties and prep-aration of 6-membered cyclotriborazines.

2. Cyclotriborazines (borazines)

Cyclotriborazines form 6-membered rings, the basic structureof which is shown on Fig. 1. Positions 1, 3 and 5 are occupied bynitrogen atoms, while positions 2, 4 and 6 are taken up by three-valent boron atoms. Although boron is normally three-valent, withits empty p-orbital it can form a four-valent anion. The name cyclo-triborazines has not become a common name, they are routinelyknown under the name B-trisubstituted borazine. In this sectionwe shall deal with cyclic borazines with unsubstituted nitrogenatoms, carrying at boron either bulky substituents, or such thatallow further functionalisation.

2.1. Preparation of borazines

Presently several methods for preparation of borazine deriva-tives are known. They encompass aminolysis of trichloroborane,

ll rights reserved.

: +421 2 524 953 81.sányi).

functionalisation of hexahydroborazine or derivatisation of boricacid esters.

2.1.1. Cyclocondensation of iminoboranesConceptually this is the most effective method of borazine prep-

aration, because it is experimentally rather simple. In its first step,it involves reduction of alkylboroxines (anhydrides of boric acid)with LAH in the presence of trialkylamine, to give trial-kylaminoalkylboranes 1–4 (Scheme 1) [14]. Next, the latter aretreated with an excess of ammonia in diglyme, in the presence ofammonium chloride as catalyst. The reaction is accompanied byvigorous release of hydrogen and dimethylamine, forming the cor-responding cyclic borazines. Although the reaction has not beendescribed with alkylamines, authors claimed reactions proceededalso with various alkylsubstituted tertiary amines. Cyclic borazineis considered to be an isoelectronic analogue of benzene, however,its thus supposed aromaticity is still a matter of debate [15–21].According to the postulated mechanism (Scheme 2), the first stepinvolves transamination of trimethylamine by ammonia. The sub-sequent action of ammonia and catalytic amount of ammoniumchloride elicits a sequential elimination of hydride anion from bor-on and proton from nitrogen, the evolving gaseous hydrogen shift-ing the reaction equilibrium towards products 5–13. The key stepof the synthesis is the cyclocondensation reaction of three mole-cules of iminoborane.

It turned out that although the above mentioned borazines didnot succumb to air moisture, nevertheless after stirring with waterfor several hours they completely hydrolysed to boric acid withconcomitant release of ammonia. In addition, storing samples indry conditions under nitrogen atmosphere caused yellowing ofthe originally colourless compound and its transformation to afurther unspecified solid.

R1 - R3 = chlorine, alkyl, aryl, N-alkyl,N-silyl, O(S)-alkyl

HNB

NH

BNH

B

12

34

56

R1 R3

R2

Fig. 1. General formula of cyclotriborazines.

10 M. Kavala et al. / Polyhedron 48 (2012) 9–20

2.1.2. Ammonolysis of alkylthioboronic acidThis sequential method was described for the synthesis of alkyl-

borazine derivatives 5–7. Authors claim it to be a facile method ofpreparation with easy isolation of products and good yields [22].In the first step, anhydride of alkylboric acid reacts with tribrom-oborane, giving rise to alkyldibromoboranes 8–10 (Scheme 3). Itmay be assumed that low yields can be accounted for by polysub-stitution of the created monosubstituted borane. Next, dibromo-boranes are transformed to n-butylesters of alkylthioboric acid11–13; the experiment has however been described in detail in caseof isopropyl derivative. These esters underwent ammonolysisalready at laboratory temperature, giving the required borazines5–7 in good yields (�80%). The tentative mechanism of ammonoly-sis assumes in its first step a substitution of thiolate group withammonia to give A and self-condensation in the next step to giveB (Scheme 4). The thus formed intermediate next condenses totriborazane structure 14, which in the last releases a molecule ofthiol to give the cyclic borazine.

O

BO

B

OB

R

R R

3 R BH

HN

1: R = nPr (65%)2: R = iPr (65%)3: R = nBu (64%)4: R = sBu (66%)

a

a) Me3N (3 equiv), LAHb) NH3(g), NH4Cl, digly

b

Scheme

R BH

HN

+NH3

R BH

HN H

H

H

NH4- Me3N +

-H2, -NH

NH4+ -H2, -NH3

B NH

H

+NH3

NH4-

BN

H

B

NHB

NH

R BH

HN

R

R R

R

Scheme

2.1.3. Ammonolysis of diethylaminodichloroboraneThe methodology has been demonstrated on a single substrate

with free amino group at the borazine ring [23]. First, the diethyl-aminodichloroborane 15 was prepared by treating trichloroboranewith diethylamine (Scheme 5). Compound 15 was then subjectedto ammonolysis to give the cyclic product 16 in only average yield.

This section also presents a method based on ammonolysis ofaryldichloroboranes 17–19, accessible by reaction of aromaticchloromercury derivative with trichloroborane (Scheme 6) [24].Although the ammonolysis itself furnishes very good yields of aryl-borazines 20–22 (80–97%), its major drawback is the use of toxicmercury(II) salts.

2.1.4. Reaction of HMDS with dichloroborane derivativesThe method was used to prepare an ethyl and phenyl derivative

of borazine [25,26]. It is a ‘‘one-pot’’ reaction of ethyl- or phenyldi-chloroborane (23 and 24) with HMDS, releasing TMSCl and formingcyclic borazine derivatives 25 and 26 (Scheme 7). B-triphenyl bora-zine could be isolated in almost quantitative yield which makesthis preparation synthetically very attractive.

2.1.5. Reaction of bis(diisopropylamino)ethynylborane withammonium chloride

Further functionalisation of borazine derivatives has been the to-pic of very few reports. This method was used to prepare triethynyl-borazine (30 or 31) [27], the triple bonds of which lend itself tofurther functionalisation. In its first step, the synthesis uses the reac-tion of BCl3 with diisopropylamine (DIPA) in toluene, leading to dia-minochloroborane 27 (Scheme 8). Chloroborane 27 then reacts with

, Et2O, reflux, 3 h;m, 100-150 °C, 2 h.

HN

BNH

B

NHB

R

R R

5: R = nPr (88%)6: R = iPr (79%)7: R = nBu (91%)8: R = sBu (85%)9: R = iBu (70%)

10: R = tBu (65%)11: R = nPen (88%)12: R = nHex (86%)13: R = Bn (70%)

1.

HN

BNH

B

NHB

R

R R

RB

HN H

H

H3

RB

HN

H

H

+NH3

NH4-

HN

BNH

B

NHB

R

R R

2.

O

BO

B

OB

R

R R

R B

8: R = nPr9: R = iPr (59%)10: R = nBu

a

a) BBr3, r.t.;b) nBuSH, reflux, 12 h;c) NH3(g), r.t., 1 h.

c HN

BNH

B

NHB

R

R R

5: R = nPr (86%)6: R = iPr (82%)7: R = nBu (80%)

S

S nBu

nBuR B

Br

Brb

11: R = nPr12: R = iPr (78%)13: R = nBu

Scheme 3.

R B

HN

BNH

B

NHB

R

R R

S

S nBu

nBu +NH3R B

S

NH2

nBu

A

+A

-nBuSHR B

HN

NH2

BS

R

nBu

R BHN

NH2

BHN

R

BR

SnBu

+A

-nBuSH

14

-nBuSH

5-7

-nBuSH

B

Scheme 4.

M. Kavala et al. / Polyhedron 48 (2012) 9–20 11

lithium acetylide (or TMS/lithium acetylide) to furnish the boranederivatives 28 and 29, which, in the final step afford the target bora-zine 30 and 31. It is evidently one very effective method for prepara-tion of borazines with the propensity for further functionalisation.

2.1.6. Substitution reactions of B-trichloroborazineB-Trichloroborazine 32 is a labile compound, completely hydro-

lysed by water to boric acid and ammonium chloride [28]. In spiteof the wealth of data in the literature, it has become commerciallyavailable, we shall concentrate here on derivatives 32. Derivatives32 can be approached by four methods we shall describe in chro-nological order.

The first described method involves treatment of trichlorobor-azine 32 with Grignard reagents MeMgI, EtMgI and PhMgBr(Scheme 9) [29]. Triethylborazine 25 and triphenylborazine 26are isolated from the reaction mixture after the solvent has beendistilled off and the crude reaction mixture pyrolysed at 150 �C.Compounds 25 and 26 are better prepared by the method givenin Section 2.1.4.

ClB

Cl

Cl

15

a

a) Et2NH, benzene, -78 °C - r.t., 3 h, 81%;b) NH3(g), benzene, -78 °C - r.t., 20 h, 52%.

HN

BNH

B

NHB

NEt2

Et2N NEt2

16

Et2N BCl

Cl b

Scheme 5.

In 1961 a synthesis was published [30] the first step of whichinvolved substitution of chlorine atoms in 32 for butanethiolatecoming from n-butyl lead mercaptide (Scheme 10). The thus ob-tained B-tri-n-butylmercaptoborazine 34, when treated withammonia, dimethylamine, aniline and methanol respectively, givesthe corresponding derivatives 35–38. However, certain limitationsof this methodology lies in the use of toxic lead compounds andfoul smelling sulfides and/or thiols.

Gerrard and coworkers carried out reactions of trichlorobor-azine 32 with a series of secondary amines [31], leading in turnto the corresponding amino borazines (16, 36 and 39–44) in lowyields (Scheme 11). Only substitution with diphenylamine gaveproduct 44 in good yield (76%).

Tris(diethylamino)borazine 16 arises from the reaction of 40with trimethylsilyldiethylamine (Scheme 12) [32]. As far as theyields are concerned, there was hardly any improvement (54%).

2.1.7. Rhodium-catalysed hydroboration of alkenesAll methods described so far produced derivatives with sym-

metrical substitution pattern at boron atoms. Sneddon carriedout a series of experiments [33], in which he succeeded in prepar-ing (apart from symmetrically trisubstituted borazines) also mono-and disubstituted borazines by Rh-catalysed reaction of olefinswith the borazine 45 (Scheme 13 and Table 1). The Sneddon meth-odology not only allowed preparation of monosubstituted bora-zines, it also allowed a step-by-step procedure introducing threedifferent substituents into the molecule of borazine.

Authors have also postulated a mechanism of such Rh-catalysedhydroboration of olefin (Scheme 14). The first step involves disso-ciation of fosfine ligand from the central rhodium metal (A), fol-lowed by coordination of olefin (B), addition of hydride to olefin(C), oxidative addition of the B–H group of olefin (D) and finallyby reductive elimination of alkylborazine (E).

This method has a considerable synthetic potential, since themetal catalysts is used only in 0.015–4.4 mol%, in addition to thepossibility to introduce different substituents – a feature uniqueto this methodology but one. It gives fairly good yields of desiredproducts, thus making it unequivocally the method for preparationof borazines.

Table 2 compiles data on melting and/or boiling points of 39borazine derivatives.

Rhodium(I) complexes were also used in the catalytic dehydro-coupling methodology [34] that transforms ammonia–borane ad-ducts to cyclic borazines under mild conditions (max. 45 �C,glymes). However, isolation of products from the reaction mixtures(by vacuum fractionation) proved difficult; pure borazines wereisolated low yields (10–30%) with the major products being non-volatile, oligomeric species.

Ar B

Ar = 2-Me-C6H4,3-Me-C6H44-Me-C6H4

a) BCl3, benzene, reflux, 4 h;b) NH3(g), benzene, -78 °C - r.t., 20 h.

b HN

BNH

B

NHB

Ar

Ar Ar

20: Ar = 2-Me-C6H4 (85%)21: Ar = 3-Me-C6H4 (97%)22: Ar = 4-Me-C6H4 (80%)

Cl

ClAr

HgCl

a

17: Ar = 2-Me-C6H4 (67%)18: Ar = 3-Me-C6H4 (57%)19: Ar = 4-Me-C6H4 (74%)

Scheme 6.

a

a) HMDS 25: benzene, -78 °C - r.t., 20 h, 61%;26: CH2Cl2, -78 °C - 55 °C, 10 d, 98%.

HN

BNH

B

NHB

R

R R

25: R = Et26: R = Ph

R BCl

Cl

23: R = Et24: R = Ph

Scheme 7.

a

a) RMgX (X=I, Br), Et2O, r.t. - 150 °C.

HN

BNH

B

NHB

R

R R

33: R = Me25: R = Et (70%)26: R = Ph (60%)

HNB

NH

BNH

B

Cl

Cl

Cl

32

Scheme 9.

12 M. Kavala et al. / Polyhedron 48 (2012) 9–20

2.2. Reactions of borazines

Borazine derivatives being relatively unstable compounds canalso be expected to be fairly reactive. This has been amply demon-strated in their preparation starting from trichloroborazine. Allreactions are carried out under the blanket of nitrogen or argonatmosphere, without access of oxygen and air moisture. We shallnevertheless try to map their hydrolytic stability.

2.2.1. Hydrolysis of aminoborazine compoundsAll borazines undergo hydrolysis, the outcome of which de-

pends a great deal on volume of substituent and electronic effects.Table 3 summarises the results of the study by Gerrard, who stud-ied the hydrolytic stability of aminoborazines [31] in refluxingwater, or aqueous NaOH.

The reaction products were naturally amines (neutralised by0.1 N HCl), and boric acid, determined by titration (phenolphtha-

a

a) BCl3, toluene, 0 °C - r.t., 14 h,b) Li-C C-R, 12-crown-4 (cat.)c) NH4Cl, toluene, reflux, 12-24 h

NHN

BCl

N

b N

28: R = H29: R = T

27

Scheme

lein-mannitol). The highest hydrolytic stability demonstrated theelectron-rich tris(diphenylamino)borazine 52, which, after 30 min-ute reflux hydrolysed merely to 7% – a fact testifying to its rela-tively high stability towards hydrolysis.

It also means that in manipulating other derivatives contactwith water should be avoided which is a fairly serious practicallimitation. In spite of this, various derivatives can be preparedand used in further functionalisation.

3. Cyclocarborazines

Cyclocarborazines are cyclic compounds, analogues of saturatedcarbon azaheterocycles – azetidine, pyrrolidine and piperidine.Their parent structure contains in positions 2 and (3 + n) boronatoms instead of carbon atoms (Fig. 2). The stability and reactivityis what makes these compounds truly interesting. The reports oncyclocarborazines in the literature are scarce, derivatives with free

90%;, THF or Et2O, 0 °C - 80 °C, 4 h;.

HNB

NH

BNH

B

30: R = H (92%);31: R = TMS (83%)

B

N

Rc

R

RR

(98%);MS (89%)

8.

a

a) (nBuS)2Pb, benzene, reflux, 2 h, 91%b) RH, benzene, r.t., 1 h.

HN

BNH

B

NHB

R

R R

35: R = OMe (50%)36: R = NMe2 (97%)37: R = NHPh (70%)38: R = NH2 (80%)

HN

BNH

B

NHB

Cl

Cl

Cl

32

HN

BNH

B

NHB

S

S

S

34

nBu

nBu nBu

b

Scheme 10.

a

a) benzene, r.t.

HN

BNH

B

NHB

N

N N

36: R1 = R2 = Me (51%)16: R1 = R2 = Et (58%)39: R1 = R2 = nPr (47%)40: R1 = R2 = iPr (52%)

HN

BNH

B

NHB

Cl

Cl

Cl

32

+R2

NH

R1

R1

R2

R2

R1

R2 R1

41: R1 = R2 = nBu (42%)42: R1 = R2 = iBu (28%)43: R1 = Me, R2 = Ph (64%)44: R1 = R2 = Ph (76%)

Scheme 11.

a

a) toluene, r.t., 54%.

HN

BNH

B

NHB

N

N N

HN

BNH

B

NHB

Cl

Cl

Cl

40

+ NTMS

16

Scheme 12.

a

a) RhH(CO)(PPh3)3 (0.015-4.4% mol), neat / CH2Cl2, -196 °C - r.t.

HN

BNH

B

NHB

R2

R1 R3

HN

HBNH

BH

NHHB

45

+

Products in Table 1

alkene

Scheme 13.

M. Kavala et al. / Polyhedron 48 (2012) 9–20 13

amino group being practically unknown. We shall therefore con-centrate here solely on preparation and properties of N-substitutedcyclic derivatives.

3.1. Cyclomonocarbodiborazines

Cyclomonocarbodiborazines form 4-membered rings with thebasic structure shown in Fig. 3. The name of this group of com-pounds has been derived from the saturated azaheterocycle –azetidine. Instead of carbons atoms in positions 2 and 4 it containsboron atoms. The valence of boron allows it carry a substituent,thus a typical representative of this group of compound is a 2,4-disubstituted 2,4-diboraazetidine, also called 2,4-disubstituted2,4-dibora-1-azacyclobutane. All so far reported compounds of thistype carry a substituent at nitrogen.

3.1.1. Preparations of 2,4-diboraazetidinesCyclomonocarbodiborazines are structurally and chemically

intriguing compounds, so far accessible by only a handful of meth-ods. There is no procedure so far capable of producing such hetero-cycle without a substituent at nitrogen.

3.1.1.1. Insertion of isonitriles and carbenes. The key substrate, onwhich this method of preparation of 4-membered rings is based[35], is the 1,2,3-tri-tert-butylazadiboridine 65. It is a highly reac-tive compound (owing to the extremely weak B–B bond), preparedin two steps (Scheme 15) [36,37]. Authors reported an aromaticcharacter for this azadiboracyclopropane, the free electron pair atnitrogen being delocalised over the ring.

The first of its synthetic transformations is the reaction of 65with aromatic isonitriles, of which only one has been describedin detail (Scheme 16). According to the postulated mechanism(Scheme 17) it involves an insertion of isonitrile 66 into the B–Bbond, bringing about an expansion to a 4-membered ring of

1,2,4-azadiboretidine 67 as red oily liquid. The method has onesubstantial limitation though, being demonstrated on a single aro-matic isonitrile. In case of compound 67 NMR spectra revealed aninteresting feature, namely the non-equivalence of tert-butylgroups. This can be accounted for by assuming presence of aC@N bond without free rotation. Such structure would also beresponsible for a single 1H NMR signal for ortho-methyl groups atthe benzene ring, thus their magnetic equivalence.

Table 1Compounds prepared by Rh-catalysed reaction of olefins with borazine 45.

Compounds Alkene 45/alkene Time (h) R1 R2 R3 Yield (%)

25 ethylene 1/3 2 Et Et Et 9246 propene 8/1 2 H H nPr 9847 propenea 1/3.3 1 H nPr nPr 435 nPr nPr nPr 5548 1-butene 5.8/1 3 H H nBu 9149 Z-2-butene 5/1 47 H H nBu 9050 H nBu nBu 351 E-2-butene 6/1 185 H H sBu 7252 styrene 7/1 2.5 H H (CH2)2C6H5 8453 4-allylanisol 10.5/1 96.5 H H (CH2)3C6H4-4-OMe 5754 ethyleneb 1/2.2 2 Et Et nPr 9755 ethylenec 1/3.4 2 Et nPr nPr 9756 acetylenec 1/3.9 120 vinyl nPr nPr 54

a Di- and trisubstituted products are formed in the ratio 44/56.b Starting substrate is 2-propylborazine.c Starting substrate is 2,4-dipropylborazine.

P RhPP

H

CO

-P+P

RhOC

P HP

R

R

RhPP

H

CO

(A)

(B)

(C)RhP CO

PR

HRh

P HP

B

CO

B3N3H6

(D)

NH

BH

HN

HB

HNH

R

(E)

BNH

BHHN

HB

HN

R

H

(A) release of phosphine ligand(B) olefin coordination(C) hydride addition to olefin(D) oxidative addition of borazine(E) reduction elimination of alkylborazine

P = PPh3

Scheme 14.

Table 2Compiles data on melting and/or boiling points of 39 borazine derivatives.

Compounds mp (�C) bp (�C)

5 – 70 (0.6 Torr)6 – 70 (0.5 Torr)7 – 102–105 (0.2 Torr)8 – 94 (0.7 Torr)9 – 72 (0.03 Torr)10 – 60 (10 Torr)11 – 125 (0.07 Torr)12 – 140 (0.05 Torr)13 – not given16 120 (0.1 Torr)20 89–92 –21 140 –22 189–190 –25 �46 56–57 (2 Torr)26 180–185 –30 132 90 (75 Torr) sublimation33 36 35 (20 Torr)35 114 –36 112–115 –37 204–206 –38 >250 –39 32–35 170–172 (0.4 Torr)40 138–143 –41 – 200 (0.05 Torr)42 47–52 167 (0.3 Torr)43 128–132 –44 152–155 –47 – 119–124 (760 Torr)52 123–125 –

Table 3Compiles date on the hydrolytic stability of aminoborazines.

Compounds R2N Reagent Hydrolysis (%)

44 Me2N NaOH solution (reflux) 10048 iPr2N water (reflux) 10048 iPr2N water (cold) 5551 PhMeN water (reflux) 5852 Ph2N water (reflux) 752 Ph2N NaOH solution (reflux) 29

14 M. Kavala et al. / Polyhedron 48 (2012) 9–20

Compound 67 is rather unstable and exposed to air quicklydecomposes. Authors do not report about its thermal stability, ormoisture sensitivity. It can be assumed though, that the iminobond quickly hydrolyses, making the derivatives hydrolyticallyunstable.

The second type of transformation of 67 to 4-membered ring isthe carbene insertion of lithiated a-bromoalkanes. Using thismethod, authors prepared the 4-membered spiro-compound 69,starting from the cyclopropane derivative 68 (Scheme 18). Thistransformation starts with lithiation of the dibromocyclopropane68, followed by insertion of organometallic cyclopropylidene com-pound 68A into the B–B bond of 65 to give the 1,2,4-azadiboreti-dine 69 (80%) and free LiBr (Scheme 19).

R1, R2, R3 = alkyl, aryl

12

34

NB BR1

R2

R3

n

Fig. 2. General formula of cyclocarborazines.

3.1.1.2. Cycloaddition reaction of iminoboranes. An interesting meth-od of preparation of 1,2,4-azadiboracyclobutanes is the cycloaddi-tion of iminoborane with unsaturated alkylidene tantalumcomplex [38]. Alkyl(tert-butylimino)boranes 70–72 add to the tan-talum complex 73 (in a molar ratio 2:1) to give the intermediacydiazadiboratantalocyclohexanes 74–76 (Scheme 20) [39]. Whenheated, these 6-membered derivatives eliminate the tantalum

R1, R2, R3 = alkyl

12

34

NB BR1

R2

R3

Fig. 3. General formula of cyclomonocarbodiborazines.

NB B

+

65

NC

66

a

a) CH2Cl2, -78 °C, 2 h, 61%.

NB B

N

67

Scheme 16.

M. Kavala et al. / Polyhedron 48 (2012) 9–20 15

complex 77 causing a ring contraction to the target 1,2,4-azadib-oracyclobutanes 78–80. Although they could not be analysed inpure form, their presence was deduced from NMR spectra of reac-tion mixture.

The reaction of analogous tantalum complex 82 with iminobo-rane 81 (in 1:1 molar ratio) gives rise to the 1-aza-2,4-diboracyc-lobutane derivative 83 (Scheme 21), which could be isolated bymultiple crystallisations in 26% yield. Authors give the tentativestructure of leaving tantalum complex 84 without being able toconfirm its structure.

1,2,4-Azadiboracyclobutanes have been fully characterised byNMR spectroscopy, but were not further chemically transformeddue to their high instability (Table 4).

4. Cyclodicarbodiborazines

Cyclodicarbodiborazines form 5-membered rings with the corestructure shown in Fig. 4. The name of this group of compoundshas been derived from the saturated azaheterocycle – pyrrolidine.Instead of carbons atoms in positions 2 and 5 it contains boronatoms. The valence of boron allows it carry a substituent, thus atypical representative of this group of compound is a 2,5-disubsti-tuted-1,2,5-azadiborolane, also called 1,2,5-azadiborolidines or 1-aza-2,5-diboracyclopentane. All so far reported compounds of thistype carry a substituent at nitrogen.

4.1. Preparation of 2,5-diborapyrrolidines

So far, only two methods of preparation of diborolanes areknown. One of them involves cyclisation of dichlorodiboralkaneswith a tertiary silazane, the other relies on thermal cyclisation ofaminoboranes. Similarly as in the earlier section on 4-memberedderivatives, these methods were so far able to furnish only N-substituted cyclic compounds.

4.1.1. Cyclisation of chloroboranes with silazanesSyntheses of 5-membered nitrogen heterocycles containing a

B–N–B moiety described in the literature are few and far between.Only several derivatives have been known. A method reported forthe preparation of such heterocycles starts from dichlo- or tetra-chlorodiboralkane derivative [40]. The action of diborane on ethyl-ene at �80 �C gives rise to 1,2-bis(dichloroboryl)ethane 85 [41],

NSn

Sn

Cl

Cl

+ BBr

Br

a

a) CH2Cl2, -30 °C,b) Na-K, hexane, re

Scheme

which next undergoes a reaction with hexamethyldisilazane 86to give cyclic 2,5-dichloro-1,2,5-azadiboracyclopentane 88 as col-ourless liquid in 95% yield (Scheme 22). However, the compounddecomposes on standing at laboratory temperature. However, areaction of 85 with hexamethylsiladimethylborazines 87 failed toproduce isolable target compound 89. Its formation was onlyproved by experiment carried out in a NMR tube. Substrates 86and 87 were prepared from LiHMDS and the corresponding elec-trophiles (methyl iodide, or dimethylchloroborane) [42].

Compound 88 decomposes on standing too, but boron atoms,being shielded by a nitrogen bridge, are no longer attacked by ac-tive reagents such as SbF3, or Me4Sn. Dimethyldiborazine 90 is ob-tained from tetrachloroalkyldiborane 85 by double methylationwith tetramethyltin 91. Dimethyl derivative cyclodicarbodibora-zines 92 arises from cyclisation reaction of 90 and 86, taking placewithout solvent, and in good yield (Scheme 23).

4.1.2. Reductive cyclisationAnother known methodology leading to azadiboracyclopentane

is based on a reaction of tetraalkyldiboranes with dialkylaminobor-anes [43]. The diethyl-N-boroaniline 95 arises in the dehydrogena-tion reaction of aniline 93 in 88% yield (Scheme 24). After isolationand purification, the secondary amine 95 reacted with 1.5 equiv. oftriethylborane 96 in the presence of 0.3 equiv. of tetraethyldibora-ne 94 in an autoclave for 16 h at 180 �C and 9 h at 200 �C. Subse-quently, the reaction mixture was reheated to 225 �C for another2 h. This procedure, followed by a fraction distillation furnishedthe desired cyclic product 97 in 8% yield. However, authors didnot propose any plausible mechanistic course of reaction.

Table 5 displays boiling points of isolated cyclic products.

5. Cyclotricarbodiborazines

Cyclotricarbodiborazines are 6-membered rings with the parentstructure shown in Fig. 5. Once again, positions next to nitrogen(2,6) are occupied by boron atoms. The difference in valence be-

30 min., 97%;flux, 4 h, 33%.

NB

B Cl

Cl b NB B

65

15.

NB B

+

65

b

a) LiBr, BuLi, THF/Et2O/pentane (3:1:1), -105 °C, 30 min.;b) -110 °C to 20 °C, 8 h, 80%.

NB B

69

BrBr

68

BrLi

68A

a

Scheme 18.

NB

BR

R +

65 68A

NB B

69

R

R = tBu

B BN

R

R

RLi

Br

LiBr

Scheme 19.

C TaH

Cl

THF

THF

Cl

Cl

73

B NR +

70: R = Et71: R = Pr72: R = Bu

CB N

TaNBH

R

R

ClCl

Cl

74: R = Et75: R = Pr76: R = Bu

NB BR R

H

78: R = Et79: R = Pr80: R = Bu

+N

TaCl Cl

Cl

77

-78 °C 60 °C

Scheme 20.

C TaH

Cl

PMe3

PMe3

Cl

Cl

82

B NN +

81

NB BN N

H

83

+ N

TaCl ClCl

84

a

TMS TMS

a) toluene,-78 °C - r.t., 16 h, 26%.

PMe3Me3PTMS

Scheme 21.

NB

BR

R +

65 66

NB B

N

67

R

R = tBu, R' = 2,6-(CH3)2-C6H3

CN

R'B B

NR

CN

R'

R

R

Scheme 17.

16 M. Kavala et al. / Polyhedron 48 (2012) 9–20

8586: R = Me87: R = B(Me)2

a) isopentane, -78 °C - r.t.

BB

Cl

Cl

Cl

Cl

+NTMS

RTMS B

NB

R

Cl Cl

88: R = Me (95%)89: R = B(Me)2 (0%)

a

Scheme 22.

Table 5Boiling points.

Compounds bp (�C)

88 20 (0.5 Torr)92 20 (22 Torr)97 65–73 (0.2 Torr)

R2 = H, alkylR1, R3 = alkyl, N, S

12

34

5

B BNR1R2

R3

6

Fig. 5. General formula of cyclotricarbodiborazines.

Table 4Boiling points.

Compounds bp (�C)

67 –69 60 (0.002 Torr)83 –

R1, R2, R3 = alkyl

12

3 4B

NBR1 R3

R2

5

Fig. 4. General formula of cyclodicarbodiborazines.

M. Kavala et al. / Polyhedron 48 (2012) 9–20 17

tween carbon and boron predetermines the substitution pattern.Substituents are carried in positions 2 and 6, thus giving rise to2,6-disubstituted 1-aza-2,6-diboracyclohexanes, or else 2-aza-1,3-diborinanes. In this section, we shall deal with compoundshaving (un)substituted nitrogen.

85

a) pentane, -78 °C - r.t. - -78b) -78 °C - r.t., low-temp disti

BB

Cl

Cl

Cl

Cl

a+ Sn

91 9

BCl

Scheme

NH2

+ B BH

H

93 94

a

HNB

95

+

9

a) -78 °C to 80 °C, 90 min.b) 200 °C - r.t. - 220 °C, 25

Scheme

5.1. Preparation of 1-aza-2,6-diboracyclohexanes

Preparations of cyclotricarbodiborazines parallel those de-scribed in Sections 3.1.1 and 4.1.1.

5.1.1. Cyclisation of chloroboranes with silylaminesSyntheses of 6-membered nitrogen heterocycles containing a

B–N–B moiety described in the literature are scarce. The reportedsynthesis of trialylborane 99 starts with the reaction of alylmagne-sium bromide 98 with BF3.OEt2 (Scheme 25) [44]. Trialylboranewas next subjected to a transformation with diborane to a polymerof unknown composition, which was nevertheless subjected toreaction at high temperature with trichloroborane, to give the1,3-bis(dichloroboryl)propane 101 [45]. It was expected that theaction of hexamethyldisilazane 86 on organoborane 100 in pen-tane would furnish the cyclic 2,6-dichloro-1-aza-2,6-diboracyclo-

°C, fraction distillation, 82%llation, 70%.

+ NTMS

TMSB

NB

0

B

Cl

86

b

92

23.

+ B BH

H

94

Bb

BN

B

976

, fraction distillation, 88%+ 2 h., fraction distillation, 98%.

24.

Br aB

b, cBB

Cl

Cl

Cl

Cl

d B BNCl Cl

BBCl

Cl

Cl

ClN

TMS

TMS+

98

99

100

100 86 101

a) BF3.OEt2, I2, Mg, Et2O, reflux, 3 h, Kugel-Rohr, 84%;b) B2H6, THF, 1 h, concentrated 3 h, 100 °C;c) BCl3, 200 °C, 20 h, dist., 92%;d) pentane, r.t., 11B-NMR analysis if reaction mixture.

Scheme 25.

18 M. Kavala et al. / Polyhedron 48 (2012) 9–20

hexane 101 [46]. Nevertheless, authors failed to isolate the targetcompounds and thus tried to prove the presence of compound101 in reaction mixture by 11B NMR monitoring. The reported sig-nals that may well have been assigned to the derivative 101, but itis still an assumption. A similar attempt was undertaken with the1,3-bsi(chloromethylboryl)propane 102, obtained by treating 100with tetramethyltin (Scheme 26). The situation repeated itself inthat no desired cyclic product 103 could be isolated and its pres-ence was assumed from the 11B NMR analysis of the reaction mix-ture. It appears that the described methodology cannot be used forpreparation of 1-aza-2,6-diboracyclohexanes after all.

The only success of this method was the preparation of 1-aza-2,6-diboracyclohexane derivative 105 [47]. In its first step 101was functionalised by diisopropylamine to compound 104 in 98%

aBBCl

Cl

Cl

ClB

Cl

100 102

a) Me4Sn, pentane, -78 °C - rd) pentane, r.t., 11B-NMR ana

Scheme

aBB

Cl

Cl

Cl

Cl

100

a) (iPr)2NH, pentane, reflub) tBuNH2, pentane, 0 °C

BCl

N

104

BCl

Scheme

yield (Scheme 27). Action of tert-butylamine on 104 elicits cyclisa-tion to cyclotricarbodiborazane derivative 105 (73%).

5.1.2. Cyclisation of thioboranes with amines1,1,5,5-Tetramethoxy-1,5-diboropentane 106 serves as the

starting compound in the methodology. It is prepared by the reac-tion of 100 with methanol (Scheme 28) [45]. The subsequent reac-tion with tris(ethylsulfanyl)boranes 106 and 107 affords theborasulfane 108. 1,1,5,5-Tetraethylmercapto-1,5-diborapentane108 represents a sulfur analogue of boric acid esters and allowsthe exchange of mercaptoethyl groups by substitution reactionwith ammonia, or amines [48]. The action of gaseous ammoniaon 108 affords the cyclic product – 2,6-diamino-1-aza-2,6-dibora-cyclohexane 109 in 44% yield, with concomitant loss of ethanethi-ol. If methylamine or ethylamine are used, analogous N-substituted cyclic compounds 110 and 111 are obtained in 70%and 93% yield, respectively.

The cyclic product 112 could be obtained in 38% yield(Scheme 29) by treating the tetraethyl ester of the propane-1,3-dithioboronic acid 108 with 1 equiv. of methylamine.

Table 6 displays an overview of physical properties of isolatedcyclic products.

As in the case of earlier mentioned 4- and 5-membered cyclo-carborazines, the products given here were not further trans-formed and their preparation only served to demonstrate thesynthetic viability of the suggested methodology.

6. Summary

As can be seen from facts given so far, cyclomono- and cyclod-icarbodiborazines are hydrolytically unstable (with the exceptionof 69, 88, 92 and 97) and thus, none of the compounds could be iso-lated in pure state. On the other hand, cyclotricarbodiborazines arethe most stable in this group, their synthesis by cyclisation of chlo-roboranes or thioalkylboranes is straightforward and effective.These compounds can be isolated from reaction mixtures by stan-

b B BN

B

ClN

TMS

TMS+

86 103

.t. - -78 °C, fraction dist., 70%;lysis of reaction mixture.

26.

B BNN N

105

x, 2 h, 98%;- reflux, 5 h, 73%.

Nb

27.

100

a) MeOH, hexane, 0 °C, distillation, 95%;b) reduced pressure, 120-140 °C, distillation, 88%;c) 109 and 111 : CHCl3; 110: benzene, 60-70 °C.

BB

Cl

Cl

Cl

Cl

a

BB

O

O

O

O106

b+

BSS

S

107

BB

S

S

S

S

108

BB

S

S

S

S

108

+ R NH2c

B BN

R

R R

109: R = H (44%)110: R = Me (70%)111: R = Et (93%)

Scheme 28.

a) Et2O, 0 °C, 70 min., fraction distillation, 38%.

BB

S

SS

S

108

+ Me NH2a

B BNS S

112

(1 equiv)

Scheme 29.

Table 6Displays an overview of physical properties of isolated cyclic products.

Compounds mp (�C) bp (�C)

105 – 115 (0.01 Torr)109 63–66 –110 – 90–91 (7 Torr)111 – 120–121 (15 Torr)112 – 124–126 (3 Torr)

M. Kavala et al. / Polyhedron 48 (2012) 9–20 19

dard procedures, they even withstand higher temperatures (e.g.during distillation), but similarly as 4- and 5-membered cycles re-main sensitive to water.

Acknowledgments

We thank Prof. Roman Boca for helpful discussions. This workwas supported by the Science and Technology Assistance Agencyunder contract No. APVV-0014-11.

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