Tetrahedron Letters, Vol32, No 31, pp 3787.3790,199l Punted III Great Britain

OMO-4039/91 $3 00 + ( Yergamon Press plc

BIPHENYLDIALKYLSILYL CHLORIDES: REAGENTS FOR THE FORMATION OF CRYSTALLINE DERIVATIVES OF SMALL

TERMINAL ALKYNES

John Anthony and FranGois Dledench*

Department of Chemistry and Biochemistry, University of California

Los Angeles, California 90024-1569, U.S.A.

ABSTRACT BiphenyldimethylsiIyl chloride (BDMS-Cl) and blphenyldiisopropylsilyl chloride (BDIPS-Cl) are readily prepared reagents for the introduction of the BDMS and BDIPS protecting groups onto small termmal alkynes, leadmg to stable, sokd, and easily recrystallizable derwatives of these thermally unstable molecules.

Our research into acetylenic all-carbon rings’ and polymer networks2 reqmres us to work with large

quantities of small, often thermally unstable acetylenic precursors. For the storage, protectron, and selectrve

functionahzation of terminal alkynes, the trirnethylsilyl (TMS), trnsopropylsilyl (TIPS) and t butyldrmethylsllyl

(TBDMS) groups are the most commonly used protecting groups. 3 However, these denvativcs of small alkynes

tend to be oils or liqutds, and the only practical way for punfication on a large scale IS distillation, a method that

can lead to explosions. This has recently occurred twice rn our laboratory, fortunately wnh minor damage, dunng

the distillation of 2-TBDMS-l-chloroacetylene and 3-TMS-2-propyn-l-al. To eliminate the need for drstillatron as

the purification method, we now have developed new srlyl-protecting groups which yield solid, stable, and easrly

recrystallizable alkynyl derivatives. Here, we report on the synthesis of the two biphenyldialkylsilyl chlortdes 1

and 2 and their use in the protection of small termmal alkynes.

I

1. BDMS-Cl

3\

(&L

,CH3

7” Si-Cl

LH CH/ ‘CH,

2, BDIPS-Cl

Blphenyldimethylsilyl chloride (BDMS-Cl, 1) IS a white crystalbne solid, mp 60-61 ‘(1, which is readrly

prepared in 85% yield from 4-lithiobiphenyl and (CH3)2S1C12 following a method reported by Smith et a1.4 The

reaction of 1 with lithium acetylides in THF affords the BDMS-protected derivatives as relatively high-melting

sohds in very good yields (Table 1)s The corresponding TMS, TIPS, or TBDMS denvatlves are liqmds or 011s

Scheme 1 shows the preparation of additional crystalline BDMS-denvatives of commonly used small alkynes.

The new protecting group is conveniently removed with catalytic amounts of K2CO3 in methanol, yieldmg the

deprotected terminal alkynes 1n high yields. Examples for deprotectlons are shown in Table 2.

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Table 1: Yields and Melting Points of Crystalline BDMS-Protected Terminal Alkynes

R-C-_-C-H

3a-g

n-BuLi, THF

1 R’-C-_-C-BDMS

4a-g

Alkyne3 R Product 4 R’

a H a H

b H b BDMS a

C Ph C Ph

d CHjCHzCH2 d CHjCH2CH2

e BDIPS e BDIPS

f THPOCH2 b f HOCH2 c

g cis-CH30CH=CH d g cis-CHgOCH=CH

Yield MP -.

80% 75 “C

98% 172-173 “C

92% 84-85 ‘C

85% 24-25 ‘C

96% 85 “C

88% x4 “C

75% 72 “C

[a] 2 Equivalents of n-BuLi and 1 used. [b] THP = Tetrahydropyranyl. [c] The THP-ether was cleaved with

MeOH/HCl during workup. [d] Ref. 3d.

BDMS-C=C-H

Yield Mp

1) MeLi, - 78 ‘C, Et,0 2) 1, -78 “C 4 20 ‘C BDMS-C=C--Cl (4h) 92% 38 “C

(da) 1) n-BuLi, 0 “C, Eta0

- BDMS-CEC-Br (4i) 60% 43 “C 2) Brz, pyridine

BDMS-C=C-CHaOH (4f) PCC on alumina

hexane BDMS-CEC-CHO (4j) 92%, 42 “C

H

BDMS-C=C-A LDA. THF

\c,H (4g) BDMS-CrC-CCC-H (4k) 65%, 77 “C

-78 ‘C -+ 20 ‘C

I

For the intrcduct1on of the biphenyldi1sopropylsilyl (BDIPS) group, an analogue of the TIPS group, reagent

2 (BDIPS-Cl) was prepared. The reaction of C13S1H with two equivalents of (CH3)#IMgCI, followed by one

equivalent of 4-lithiobiphenyl, afforded biphenyldlisopropylsllane in 75% yield. The subsequent rcactmn ~1111

anhydrous CuCl2 in CH3CN gave 2 in 91% yield. BDIPS-protected alkynes tend to be oils, e.g. BDIPS-

phenylacetylene, which was obtained 1n 81% yield from 2 and lithiated phenylacetylene 1n Et;?O. However, the

BDIPS-group was found to be quite useful in differential double protections. Whereas i-BDIPS-2-BDMS-echynf-

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After 16 h at 20 ‘C, a 0.575 M solution (174 mL, 0.1 mol) of 4-lithiobiphenyl in Et;?O/hexane was added as

described for 1. The mixture was allowed to react for 16 h. The solution was filtered and the solvent cvaporatzd.

Recrystallization of the resulting solid from hgroin (40-50 ‘C) gave 20 g (75%) of biphenyldiisopropylsrlane as a

whtte solid, mp < 22 ‘C. A total of 10 g (37 mmol) of this material was then added to anhydrous CuClz (14 g,

0.104 mol) in refluxing CH$N and stirred for 18 h. After the resulting black solution was cooled to 0 ‘C, the

precipitated copper salts were removed by filtration. The acetonitnle mixture was exhaustively extracted with

pentane. Evaporation of the combined pentane extracts followed by recrystallization from pentane gave 10.3 g

(91%) of 2 as a white solid, mp 2X-30 “C; 13C NMR (CDCl3) 6 13.8, 16.8, 17.6, 126.5, 127.2, 127 6, 118.8,

130.9, 134.8, 140.7, 142.6.

General Method for the Prendrdtlon . , . a f BDMS-alkvneS; To a 0 ‘C-cold solution of terminal alkyne (100

mmol) in dry THF (75 mL) was added n-BuLi in hexane (100 mmol). After 20 min, reagent 1 (100 mmol) was

added and the solution warmed to 20 ‘C After stirring for 2 to 3 h, the reactton was quenched with %aturated

NH4Cl. The orgamc phase was separated, washed with saturated NaCI, and dried over MgSG4. Evaporatmn of

the solvent followed by recrystalhzation from hexanes or methanol yielded the pure BDMS-alkynes (Table 1)

ACKNOWLEDGMENT We are grateful to the National Science Foundation for supporting this work.

REFERENCES

PI

121

131

141

[5J

161

[71

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McKinney, J. A.; Mukai, C. J. Am. Chem. Sot. 1990,112, 5369-5370.

For examples, see: (a) Hopf, H.; Kreutzer, M. Angew. Chem. 1990, 102, 425-426; Angrw. Chem.

Int. Ed. Engl 199&29, 393-395. (b) Yamagucht, M.; Tonsu, K.; Hiraki, K.; Minami, T. Ufem Lett

1990, 2221-2222. (c) Yu, L.; Chen, M., Dalton, L. R. Chem. Mater. 1990,2, 649-659.

(Received in USA 23 April 1991)