Alkyl migrations inversion

A chiral metal center, as is found in a pseudotetrahedral iron complex with cyclopentudienyl. carbonyl, triphenylphosphine, and ethyl ligands, hus also beer used to address the question of alkyl migration versus carbonyl insertion. Inversion of... 

The inversion of configuration at the iron atom in Eq. (13) is in accord with the proposed alkyl migration mechanism for the decarbonylation reaction22,32,12K... 

Alkyl Migration with Double Inversion of Configuration... 

For alkyl migrations the degree of inversion decreases from 70% (methyl) to 55% (tert-butyl) (Kirmse et al., 1980b). 

B-Me- andB-MeO-9-BBN through this radical bromination under hydrolytic conditions where the facile 1,2-alkyl migration of a ring B-C bond occurs (eq 25). This latter compound serves as a convenient source of 9-oxabicyclo[3.3.1]nonane through base-induced iodination via an SE2-type inversion. ... 

It may be sunilarly deduced that thennochemical [ 1,3] shifts should occur by the supra-antara pathway since the antara-supra process is not geometrically possible. The theory predicts an inversion at the migrating centre, and ain H-shifts are thus precluded. Only one or two alkyl migrations have been observed, reaction (6.90) being the classical example, and the expected inversion at the migrating centre takes place. Steric effects are likely to limit the generality of these processes, and where stabilizing substituents are present, radical mechanisms are likely to be observed. 

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 ... 

Organozinc reagents have been used in conjunction with a-bromovinylboranes in a tandem route to Z-trisubstituted allylic alcohols. After preparation of the vinylborane, reaction with diethylzinc effects migration of a boron substituent with inversion of configuration and exchange of zinc for boron.176 Addition of an aldehyde then gives the allylic alcohol. The reaction is applicable to formaldehyde alkyl and aryl aldehydes and to methyl, primary, and secondary boranes. 

Xlld does not involve the chiral center, so if the reaction takes place by this pathway, the migration of the alkyl group from sulfur to palladium (with the concomitant or subsequent loss of sulfur dioxide) must take place with inversion of configuration at carbon. Inversion of configuration at carbon has been observed in the reverse-type reaction, the sulfur dioxide insertion into a carbon-iron sigma bond (49). Nucleophilic displacement at carbon in compounds of type Xld is unusually difficult, so the reaction via the sulfite intermediate Xlld would appear to be more likely. Conversion of the tosylate of l-phenyl-2,2,2-trifluoroethanol to the corresponding chloride, a reaction which takes place in the presence of tetra- (n-butyl) ajnmonium chloride with inversion of configuration at carbon, requires 100°C for 24 hrs in dimethylsulfoxide. 

This represents an inversion of the charge affinity of the migrating alkyl group from what it is in (107), and raises many interesting speculations for future investigation. However, at the present time the viability of any migratory insertions in alkyl nitrosyls must first be established. 

Another stereospecific route to trisubstituted alkenes is outlined in equation (IV). The synthesis involves migration of an alkyl group with inversion at the migration lermimis and tram-deiodoborona(ion.J... 

Oximes generally have a high barrier to inversion, and accordingly this reaction is envisioned to proceed by protonation of the oxime hydroxyl, followed by migration of the alkyl substituent trans to nitrogen. The N-O bond is simultaneously cleaved with the expulsion of water, so that formation of a free nitrene is avoided. 

Synthesis of symmetrical (Z, >dienes may be performed by the following procedure (Eq, 80) 108). It has been suggested that the migration of an alkyl group proceeds with inversion at the migration terminus and deiodoboration occurs in a trans manner. 

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