The Sila-Wittig Rearrangement

The Sila-Wittig Rearrangement - J. Am. Chem. Soc. 1997,...

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Communications to the Editor The Sila-Wittig Rearrangement Atsushi Kawachi, Noriyuki Doi, and Kohei Tamao* Institute for Chemical Research, Kyoto Uni V ersity Uji, Kyoto 611, Japan Recei V ed September 3, 1996 The [2,3]-Wittig rearrangement of R -alkoxy carbanions has been extensively studied because the rearrangement offers useful methodologies for regio- and stereoselective C - C bond forma- tion (Scheme 1). 1,2 Recently, the aza-[2,3]-Wittig rearrangement of R -amino carbanions has been also studied due to its potential utility. 3 In contrast to these carbanions, little attention has been paid to the analogs of other group 14 elements. On the basis of our recent sudies of the R -heteroatom-substituted silyl anions, 4 we now report the first examples of silicon analogs to the [2,3]- Wittig rearrangements, that is, [2,3]-sila-Wittig and aza-sila- Wittig rearrangements, which involve intramolecular migration of an allyl group from an oxygen or nitrogen to silicon in [(allyloxy)silyl]lithium or [(allylamino)silyl]lithium (Scheme 1). The term “sila-Wittig” rearrangement is used to differentiate the present reaction from the “silyl-Wittig” rearrangement (reversed Brook rearrangement). 5 A typical example is shown in Scheme 2. [(Allyloxy)silyl]- stannane 1 , a precursor of [(allyloxy)silyl]lithium 2 , was readily prepared from (chlorosilyl)stannane 3 and tertiary allyl 6 alcohol 4 . A solution of 1 in THF was treated with n -butyllithium (2.0 equiv) at - 78 ° C for 3 h. The reaction mixture was stirred at room temperature for 2 h, and the reaction was quenched with Me 3 SiCl to give the rearrangement product, allylsilane-contain- ing disiloxane 5 , in 68% isolated yield. No [1,2]-rearrangement product was detected. The intermediate [(allyloxy)silyl]lithium 2 could be trapped with Me 3 SiCl at - 78 ° C to afford the corresponding disilane 6 in 51% yield, together with the rearrangement product 5 in 21% yield. 7,8 Thus, 2 undergoes the [2,3]-sila-Wittig rearrangement to form lithium allylsilanolate 7 quantitatively at room temperature and to some extent even at - 78 ° C. 9 Furthermore, the rearrangement was greatly enhanced by a crown ether. Treatment of 2 with 12-crown-4 afforded only 5 in 55% yield even at - 78 ° C within 1 h. 10 Some other representative results are summarized in Scheme 3, where the starting materials were prepared from 3 and the corresponding allylic alcohols in 58 - 87% yields, as previously stated. Three points deserve comment. (1) An olefinic stereo- (1) For reviews, see: (a) Marshall, J. A. In Comprehensi V e Organic Synthesis ; Trost, B. M., Fleming, I., Pattenden, G., Eds.; Pergamon Press: Oxford, 1991; Vol. 3, pp 975
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The Sila-Wittig Rearrangement - J. Am. Chem. Soc. 1997,...

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