Migration, alkyl group

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. 

Winstein also found that the rate is highly dependent on the electron-donating ability of R—much more so than the rates of other intramolecular rearrangements (see Table 6.3). He therefore postulated that the migrating alkyl group provides anchimeric assistance to the departure of the nitrobenzoate anion and that the transition state for the rearrangement can be represented by a bridged structure such as 130. 

Only one alkyl group is transferred from boron to carbon in the reaction and so generation of the required dialkylborane from thexylborane prevents wastage of more valuable alkyl groups (Equation B4.2). Note that the migrating alkyl group involved in the reaction sequence depicted in Equation B4.2 retains its stereochemistry. 

The rate enhancements by Lewis acids are presumably due to interaction with both the terminal carbonyl group receiving the migrating alkyl group and the newly found acyl group. 

The review problems below present a few additional examples of organoborane reactions. You ll find many more in standard advanced organic texts (see Appendix B). The common feature of all the reactions is that a nucleophile adds to the tricoordinate boron of the organoborane to yield an anionic tetracoordinate boron center, from which the migrating alkyl group takes off. In the chapters to follow, we ll encounter a variety of other platforms, based on other relatively electropositive p-block elements, from which alkyl and aryl groups may migrate. 

Migratory insertion occurs by a concerted process and, therefore, proceeds with retention of configuration at the a-carbon of a migrating alkyl group. 

The reactions of some optically active benzyl halides have provided further evidence for the nucleophilicities of the [Pd(PR3)j] species (Lau et al., 1974, 1976 Wong et al., 1974 Stille and Lau, 1976). The salient features of these studies are summarised in Scheme 6. The stereochemistry of the addition is established by reaction of the palladium-alkyl complex with carbon monoxide (this insertion is known to take place by an intramolecular migration process, with retention of configuration in the migrating alkyl group), and subsequent formation of an ester from this acyl complex. 

During such a reaction, the rearranging cation must pass through an intermediate where the migrating alkyl group R is half-way between the carbon atom it is leaving and the carbon atom to which it is going. It must then be attached to both carbon atoms by a three-center bond, formed from one AO of each carbon atom and one of R. Thus,... 

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