Alkyl group 1,2 -shift with retention

When an alkyl group migrates, there is an additional stereochemical feature to consider. The shift can occur with retention or inversion at the migrating center. The analysis of sigmatropic shifts of alkyl groups is illustrated in Fig. 11.7. The allowed processes include the suprafacial 1,3-shift with inversion and the suprafacial 1,5-shift with retention. Sigmatropic rearrangements of order [3,3] are very common ... 

In all these reactions the migrating group moves with retention of configuration. It also explains why concerted [1, 2] shifts to electron rich centres will be unfavourable or why 1, 2 shifts of hydrogen or alkyl groups in radicals or anions are rare. 

Patterson and co-workers113 observed that the thermal rearrangement of N-alkylpyrroles led to a mixture of 2- and 3-alkyl derivatives and suggested that a 2JF/-pyrrole was an intermediate. Photolysis of the same class of compounds, using a low-pressure Hg lamp, led to isolable 2/f-pyrroles as well as 3-alkylpyrroles (Scheme 42).19,20 A chiral group migrates with some retention of optical activity,19 and an /V-(a-methallyl) derivative gave a mixture of a-methallyl and cis- and trans-crotyl products.20 The 3-isomer appears to arise from a direct [l,3]-shift rather than via the 2f/-pyrrole.20... 

When an allyl group migrates, an additional stereochemical feature arises because of p-orbital of migrating carbon of alkyl group. In this case, the thermal [l,3]-suprafacial alkyl shift is an allowed process with inversion of configuration of migrating alkyl carbon and the thermal suprafacial [l,5]-alkyl shift is also allowed process with retention of configuration of alkyl carbon (Pig. 4.5) [1, 2],... 

In Summary Hydroboration-oxidation constitutes another method for hydrating alkenes. The initial addition is syn and regioselective, the boron shifting to the less hindered carbon. Oxidation of alkyl boranes with basic hydrogen peroxide gives anti-Markovnikov alcohols with retention of configuration of the alkyl group. 

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