Boron-stabilized alkylation

The mechanism involves the dissociation of the coordinated borane 15 to generate a monoborane intermediate 16. Coordination of the alkene would generate the alkene borane complex. A /3-borylalkylhydride with B-H stabilization is certainly an important resonance structure of 17. An intramolecular reaction would extrude the alkyl boronate ester product and coordination of HBcat would regenerate the monoborane intermediate. 

Alkyl radicals stabilized with boron atom are easily generated and undergo addition and coupling reactions. Such cr-boryl radicals were generated in situ by treatment of a-chloroalkylboronic esters with CrCl2 (Equation (188))706 or with radical initiators such as AIBN (Equation (189)).707... 

In general, all types of boron-stabilized carbanions are readily alkylated by primary alkyl halides. The situation for the i ylation of dimesitylboryl compounds is sununarized in Scheme 2. These reactions represent highly efficient homologation processes. Oxidation of the tertiary all l organoboranes is slow, but use of 4-methoxy-2,6-dimethylphenyl groups instead of mesityl groups renders the products sensitive to solvolysis. ... 

Alkylations of boron-stabilized carbanions have been carried out with primary alkyl halides containing acetal, alkene, alkyne, chloride, cyano, ester and tosylate groups, though a ketone group was not tolerated. ... 

Rathice and Kow first reported the preparation of a boron-stabilized carbanion by direct deprotonation of the carbon acid. They made the important observation that the deprotonation needed a sterically demanding base to prevent its complexation with boron. Thus the anion of B-methyl-9-borabicy-clo[3.3.1]nonane, prepared by deprotonation with lithium 2,2,6,6-tetramethylpiperidine (LiTMP) in benzene, can be alkylated successfully. 

Matteson and coworkers have described the deprotonation and alkylation of bis(l,3,2-dioxaborin-2-yOmethane (11). Deprotonation with LiTMP yields the stabilized anion, which can be alkylated with a variety of primary alkyl halides. Oxidation under basic conditions then unmasks the carbonyl group. Matteson has also described the deprotonation of some substituted boronic esters (12 X = SPh, SiMei). - The alkylated products derived from (12 X = SPh) can be treated with A/-chlorosuccinimide under mild basic conditions to yield either the monothioacetals or the acetals. Boron-stabilized carbanions can also be generated by transmetallation of the organostannanes. ... 

Owing to fast protodeboronation and slow transmetaUation, direct cross-coupling reactions of secondary alkyl boronic acids are notoriously difficult. In order to enhance the stability of boronic acids, various diols can be used to react with boronic acids to form more stable boronic esters. Unfortunately, boron s Lewis acidity dramatically decreases in this case mainly due to the increased steric bulk surrounding the boron atom, making the transmetaUation more difficult. Toward this end, Shibata and coworkers [123] in 2010 designed a system where one of the carbon substituents is another boron pinacolate (Eq. (44), Scheme 2.41). As boron atoms are known to stabilize a-anions due to the empty p orbitals on the boron atom... 

In contrast to arylboronic acids, early reports document the great stability of alkyl-boronic acids under aqueous acidic solutions. For example, various simple alkyl-boronic adds were unaffected by prolonged heating in 40% aqueous HBr or HI [40]. Like arylboronic acids, however, deboronation is observed in hot basic aqueous solutions [76]. Alkenylboronic esters undergo protonolysis in refluxing AcOH [85], and alkynylboronic acids were reported to be quite unstable in basic aqueous solutions (Section 1.3.5). 

Boron-stabilized anions have again been put to use in C—C bond-forming processes, an example of which is provided by the reaction of the anion (74) with electrophiles. Alkyl halides give substituted boronic esters, whereas aldehydes are converted to predominantly ci5-alkeneboronic esters, in contrast to the condensation of diborylmethide ions with aldehydes (mainly trans-). A useful homologation of carbonyl compounds to aldehydes involves reaction with the anion (75), to give alkeneboronic esters (76), followed by oxidation. ... 

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