Cyclopropanes from boranes

This topological rule readily explained the reaction product 211 (>90% stereoselectivity) of open-chain nitroolefins 209 with open-chain enamines 210. Seebach and Golinski have further pointed out that several condensation reactions can also be rationalized by using this approach (a) cyclopropane formation from olefin and carbene, (b) Wittig reaction with aldehydes yielding cis olefins, (c) trans-dialkyl oxirane from alkylidene triphenylarsane and aldehydes, (d) ketenes and cyclopentadiene 2+2-addition, le) (E)-silyl-nitronate and aldehydes, (f) syn and anti-Li and B-enolates of ketones, esters, amides and aldehydes, (g) Z-allylboranes and aldehydes, (h) E-alkyl-borane or E-allylchromium derivatives and aldehydes, (i) enamine from cyclohexanone and cinnamic aldehyde, (j) E-enamines and E-nitroolefins and finally, (k) enamines from cycloalkanones and styryl sulfone. 

The stability of alkyl alanes which are substituted in the 3-position depends very much on the nature of the substituent. Mention has already been made of 1,3 elimination of AlX from 3-halogenoalkyl alanes. Since the compounds are readily formed from the corresponding stable 3-halogeno-alkyl boranes by alkyl exchange, the decomposition to AlX and cyclopropane hydrocarbons ... 

Phenylacetic acids have been prepared from 2,2-dichlorostyrenes by sequential hydroboration and oxidation (Cr03-H2S04) of the intermediate trialkyl borane, and benzoyl benzoic acids (8), not readily available by other routes, have been obtained" in good yield by treatment of phthalic anhydride with aryl-lithiums at — lOO C. Syntheses of some bicyclo [1,1,0] butane-l-carboxylic acids (9) and a range of cyclopropane-1-carboxylic acids [(10) and (II)] have been described. 1-Hydroxy-cyclopropanecarboxylic acids (13) have been prepared from the hydroxy-cyclobutanone derivatives (12), following bromination and hydrolysis. 

A mixture of -propyldiboranes is obtained along with tri- -propyI-borane from the reaction of diborane with cyclopropane at about 95 C. Apparently the banana bonds of cyclopropane can provide sufficient electron density to form the expected pi-complex intermediate (124). 

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