Wittig rearrangement complexes

In contrast to the intermediate hydroxystannanes, O-protected stannanes 7 are stable compounds which can be distilled or chromatographed and stored under nitrogen for months. Treatment of 7 with butyllithium in tetrahydrofuran at — 78,JC results in rapid tin/lithium exchange (< 1 min). No products resulting from Wittig rearrangement or formation of an ate complex 8 could be detected9. 

A high level of enantioselectivity in an acyclic system has been reported in the rearrangement of tricarbonylchromium(O) complexes of allyl benzyl ethers using chiral lithium amide base 73 (equation 38) . Upon treatment with 1.1 equivalents of lithium amide 73 and 1 equivalent of LiCl at —78 to —50°C, ether 74 afforded the rearrangement product R)-75 in 80% yield with 96% ee. The effect of substituents on the chemical yields and enantioselectivity of the [2,3]-Wittig rearrangement was also studied (see Table 3). 

Alkylation of benzyl alkyl ethers. The anion of a benzyl alkyl ether is unstable and rearranges to the corresponding alkoxide (Wittig rearrangement). However, coordination of the ether to CrfCO), stabilizes the oi-anion and permits alkylation to give an a-substituted benzyl ether. Complexation also stabilizes the anion of a benzyl alkyl sulfide. The products undergo decomplexation on exposure of ether solutions to air and sunlight (overall yields 65-80%). 

A novel approach to stereocontrol in 2,3-Wittig rearrangements involves the use of chiral chromium tricarbonyl complexes (Table 8a). The reactions, which were carried out with racemic ethers, show excellent diastereoselectivity as a consequence of a preferred transition state in which the benzylic oxygen and the R -substituent and the vinylic grouping and Cr(CO)3 grouping adopt anti orientations (Lc). 

Table 8a 2,3-Wittig Rearrangement of Cr(CO)3-complexed Benzyl Allyl Ethers...

Concerted and multistep radical mechanisms have been proposed (68, 69). Lithium complexation appears to be involved in the aza-Wittig rearrangement (70). 

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