Crotyltri-n-butyltin

Crotyltri-n-butyltin, CH3CH=CHCH2Sn(C4H9-n)3 (1). (n-Bu)3SnLi reacts with (E)-or (Z)-crotyl chloride in THF at — 20 to form (E)- or (Z)-crotyltri-n-butyltin with complete retention of the stereochemistry of the double bond. 

Addition of crotyltri-n-butyltin (5 11, 143) to chiral a-alkoxy aldehydes (6) presents a more complicated situation, since four products are possible. Products 7 and 8 result from chelation-controlled diastereofacial selectivity products 9 and 10 are products of Cram-Felkin control. In the reaction catalyzed by BF, etherate the major products are 7 and 9 in the ratio 67 33. Use of TiCl4 or MgBr, results in formation of only 7 and 8. With the former catalyst the 7/8 ratio is 63 37 with the latter, 92.5 7.5. The almost exclusive formation of 7 is consistent with the known ryn-stereoselectivity in the reaction of 5 with achiral aldehydes. 

The sterochemistry of additions of allyltri-n-butyltin (1) and crotyltri-n-butyltin (5) with 3-alkoxy aldehydes is more complex. In the allyltin reactions, useful diastereofacial selectivity is observed only with TiCU (4.6 1) or SnCU (9 1). Optimum stereoselectivity in the reaction of the p-alkoxy aldehyde 11 with 5 is observed in the reaction catalyzed by BF, etherate, which results mainly in the product of Cram-Felkin control (equation I). The highest chelation-controlled selectivity in reactions of 5 with the p-benzyloxy aldehyde 12 is obtained with MgBr, (equation II). 

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