Stereospecificity of the 1,2Wittig Rearrangement

Stereospecificity of the 1,2Wittig Rearrangement - J. Am....

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Stereospecificity of the 1,2-Wittig Rearrangement: How Chelation Effects Influence Stereochemical Outcome Robert E. Maleczka, Jr.,* and Feng Geng Department of Chemistry Michigan State Uni V ersity East Lansing, Michigan 48824 Recei V ed May 18, 1998 Since its discovery the rearrangement of R -metalated ethers, particularly the [2,3]-Wittig rearrangement, has been the subject of intensive mechanistic and synthetic investigations. 1 Relative to the [2,3]-shift, the [1,2]-Wittig rearrangement has received relatively little publicity. Most studies of the [1,2]-Wittig have been mechanistic in origin, resulting in the widely accepted theory that the reaction proceeds via a radical pair dissociation - recombination mechanism. 2 Several years ago, Schreiber 3 reported an important observation on the stereospecific nature of this rearrangement (Scheme 1). Deprotonation of 1 resulted in “synthetically useful levels” of the [1,2]-rearrangement product that was heavily biased toward the syn isomer 3 . Schreiber postulated that the product arose from bond reorganization via a diradical transition state in which a lithium tether 2 sets up the syn stereochemistry. Another surprising feature of this rearrangement was the high level of retention (94%) at the migrating center. This observation became more interesting upon Cohen’s 4 and Bru ¨ckner’s 5 recent evidence that [1,2]-Wittig rearrangements proceeded with inversion of the lithium-bearing terminus. Nakai 6 addressed this question and showed clearly that the [1,2]-Wittig rearrangement of enantiodefined R -alkoxylithium species proceeds with retention of the migrating center and with inversion of the lithium-bearing terminus (Scheme 2). In these examples, the stereochemistry of the product alcohols is not the result of chelation control, but rather decided by the configuration of the stannane precursor. While there is little argument with either mechanistic explana- tion for the observed stereochemistries, it is important to note that the enantiodefined stannanes studied by Nakai did not hold the possibility of rearrangement under chelation control. 7 We found it intriguing to consider substrates with an ether oxygen capable of coordinating with the lithium of the stereodefined lithium terminus. In such substrates the appropriate stereochem- ical combination could put Schreiber’s mechanism in stereo- chemical conflict with the results of Nakai, Cohen, and Bru ¨ckner (Scheme 3). We therefore sought to prepare and study such
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Stereospecificity of the 1,2Wittig Rearrangement - J. Am....

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