Aust. J. Chem., 1987, 40, 987-9

Convenient Preparation of Halogenoborane-Dimethyl Sulfide Complexes

Roger ~ o l t o n , ~ Peter N GatesA, and Steven A. W ones^

A Chemistry Department, Royal Holloway and Bedford New College (University of London), Egham, Surrey TW20 OEX, England.

Author to whom correspondence should be addressed.

Abstracf

Details are reported of the synthesis of mono- and di-halogenoboranes as their dimethyl sulfide complexes. The analysis of such mixtures by "B n.m.r. spectroscopy and some applications of these complexes are described.

Borane-amine complexes react with polyhalogenomethanes or with chloro- triphenylmethane to give the chloroborane-amine derivative.' The low reactivity of the latter complexes limits their use as hydroborating agents but the corresponding complexes with dimethyl sulfide have considerable synthetic potential, apart from being more stable than the previously used diethyl ether complexes. Thus the high regio- and stereo-selectivity of addition gives isomerically pure adducts from which ketones, tertiary alcohol^,^ dienes,, secondary a m i n e ~ , ~ and esters5 may be obtained. Monobromoborane-dimethyl sulfide has been used to hydroborate (Z,Z)-cycloocta- 1,5-diene,~ and the resulting B-bromo-9-borabicyclo[3.3.l]nonane has been reported7 as an ether cleaving reagent which is superior to boron tribromide.

A major disadvantage of these procedures is the tedious preparation of the haloborane complexes. Brown and ~ a v i n d r a n ~ reported the redistribution reaction as a means of obtaining both mono- and di-halogenoboranes:

2BH3 .SMe2 + BX, .SMe2 + 3XBH2.SMe2

We found that such a family of redistribution processes makes the preparation of single borane complexes impossible to realize in the reported yields. Paget and Smith9

' Ryschkevitsch, G. E., and Miller, V. R., J. Am. Chem. Soc., 1973, 95, 2836. Brown, H. C., Ravindran, N., and Kulkarni, S. U., J. Org. Chem., 1979, 44, 2417. Brown, H. C., and Ravindran, N., J. Org. Chem., 1973, 38, 1617. Brown, H. C., Midland, M. M., and Levy, A. B., J. Am. Chem. Soc., 1972, 94, 2114. Brown, H. C., Midland, M. M., and Levy, A. B., J. Am. Chem. Soc, 1972, 94, 3662. Brown, H. C., and Kulkarni, S. U., J. org. Chem., 1979, 44, 2422. Bhatt, M. V., J. Organomet. Chem., 1978, 156, 221. Brown, H. C., and Ravindran, N., Inorg. Chem, 1977, 16, 2938. Paget, W., and Smith, K., J. Chem. Soc., Chem. Commun., 1980, 1170.

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reported the preparation of monochloroborane-dimethyl sulfide through the reaction of borane-dimethyl sulfide with tetrachloromethane or with chlorotriphenylmethane rather than with boron trichloride-dimethyl sulfide as b e f ~ r e : ~ this process was analogous to that reported for the amine complexes.'

Table 1. Yields of halogenoborane-dimethyl sulfide complexes

Organic Reaction Yields (%) determined by "B n.m.r. integral ratios halide conditions XBH2.SMe2 Other BH .X3 - ..SMe2

CC14 reflux, 20 h 70 BH3 24 : BHC12 6 ph3cc lA 0°, 2 h 5 BH3 7 : BHC12 73 : BC13 15 FCBr3 90°, 20 h 84B BH3 3 : BHBr2 13 BrCC13 90°, 20 h 66B BH3 5 : BH2C1 17 : BHBr2 4 : BHC12 3 : BHClBr 5 Br2CC12 90°, 20 h 76B BHBr2 8 : BHBrC16 Ph3CBr 0°, 2 h looC

A Reactant ratio Ph3CC1/BH3.SMe2 2 : 1. At 1 : 1 ratio, 84% BH2Cl formed. X = Br. After several weeks at 4O, redistribution to BH2Br 84%, BHBr2 6% and BH3 10% (all as Me2S

adducts) occurred.

We now report the development and extension of these reactions as sources of these versatile reagents. Table 1 shows the yield of halogenoborane, measured by "B n.m.r. spectroscopy, from the reaction of BH3.SMe2 with a number of organic halides. When triphenylmethyl halides are the source of halogen, the process gives quantitative yields of monobromoborane and very good yields of monochloroborane and dichloroborane. These improved yields are the result of the low reaction temperature: such solutions of halogenoboranes redistribute even at 4' to give mixtures of similar composition to those found at 50-90°, and they should be used soon after their preparation if the selectivity of the abstraction reaction is to be used to full advantage.

Experimental Commercial (Aldrich) BH3.SMe2 solutions were standardized by volumetric measurement of

the hydrogen liberated by hydrolysis. All glassware was oven-dried (120") and assembled hot in a stream of dry nitrogen. 1 1 ~ n.m.r. spectra were measured on a Bruker SM250 spectrometer operating at 80.25 MHz at King's College, London.

(a) Freparation of CIBH2.SMq by using Tetrachloromethane

A two-necked flask (50 cm3) equipped with a tap with rubber septum, a Teflon follower and double-surface condenser fitted with an oil bubbler was assembled hot. The system was flushed with dry nitrogen (by a hypodermic needle inserted through the septum) as the apparatus cooled. Degassed tetrachloromethane (10 cm3, 0.10 mol; BDH AnalaR, stored over molecular sieve) and BH3.SMe2 (0.10 mol) were introduced through the hypodermic needle, and the mixture was heated under reflux for 20 h while a slight positive pressure of nitrogen was maintained.

(b) Preparation of CIBH2 .SMe2 by using Chlorotriphen ylmethane

With the same precautions, chlorotriphenylmethane (20 g, 0 .07 mol) was placed in a three- necked flask (100 cm3) fitted with a thermometer, Teflon follower, septum and condenser, a positive pressure of nitrogen being maintained throughout all operations. Sodium-dried benzene (500 cm3) was added to dissolve the solid halide, and, after cooling in an ice-bath, the stirred solution was treated dropwise with BH3.SMe2 (0.035 mol), the internal temperature being kept below 10". The mixture was stirred for a further 2 h with ice cooling, and 'dichloromethane (10 cm3) was added to dissolve any precipitated C13B.SMe2.

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(c) Preparation of BrBH2 .SMe2

Following the conditions described in (a), but replacing the tetrachloromethane by tribromofluoromethane, bromotrichloromethane or dibromodichloromethane (0.10 mol), and heating the mixture in a bath at 90' for 20 h, gave the desired monobromoborane-dimethyl sulfide complex (Table 1): quantitative yields were obtained by using bromotriphenylmethane and BH3.SMe2 in equimolar ratio with dichloromethane as solvent, as in (b) above.

(d) Application of Monochloroborane-Dimethyl Sulfoxide

Chloro- and bromo-borane complexes prepared in these ways were used directly as hydroborating agents after analysis by "B n.m.r.

Thus, dichloromethane (100 cm3) was added to monochloroborane-dimethyl sulfide (0 .O7 mol), prepared as in (a) from tetrachloromethane (10 cm3) and BH3.SMe2 (0.1 mol), in a flask (250 cm3) cooled in an ice bath. After the mixture had been stirred for 15 min, pent-l- ene (30 cm3, 0.273 mol) was added slowly over 15 min by a septum-capped pressure-equalizing dropping funnel. After removal of the ice bath, the mixture was stirred for a further 2 h, after which volatiles were removed. Fractional distillation gave chlorodipentylborane (89%), b.p. 78-8O016 Torr.

Acknowledgments

We gratefully acknowledge the financial support of a SERC CASE award with Burmah-Castrol, and helpful discussions with Dr B. P. Roberts (University College, London).

Manuscript received 6 October 1986