HYDRATED SIGMA-BONDED ORGANOMETALLIC CATIONS IN ORGANIC-SYNTHESIS .2. ALLYLSTANNATION OF ALDEHYDES BY CROTYLTIN CHLORIDES IN ACID-MEDIA

Allylstannation of three aldehydes RCHO (RC2H5, (CH3)2CH, and (CH3)3C) with crotyltin chlorides Bu3-nClnSnCH2CHCHCH3, n = 1, 2, and 3, has been carried out in the presence of aqueous 0.1–4.0 M HClO4. The stereochemical course of these reactions, to give the threo / erythro alcohols RCH(OH)CH(CH3)CHCH2 and/or the E/Z alcohols RCH(OH)CH2CHCHCH3, depends on the number, n, of the chlorine atoms bonded to tin and on the concentration of perchloric acid. In 0.1 M HClO4, the threo / erythro isomer pair is obtained. With increasing acid concentration, the threo / erythro pair is gradually replaced by the E/Z isomer pair, which is the sole product in 4 M HClO4. However, in most of the examined cases the ratio anti / syn = (threo + E)/(erythro + Z) remains constant for any given aldehyde and crotyl compound. Stereoselectivity, which is fairly good in some cases, is determined by the nature of the aldehyde: syn-convergence occurs with pivalaldehyde, and anti-convergence with isobutyraldehyde. No stereoselection is found in the case of propanaldehyde.