Alkylboronates, rearrangements

When 18 is treated with acetone, boronic ester 20 with a cyclooctatriene structure is cleanly obtained indicating the valence tautomerism between 18 and 19. In fact, 19 appears to be the kinetic product in the borylation of lithium or potassium cyclononatetraenide with alkylboron dihalides which rearranges quickly to 18 due to the comparatively high thermodynamic stability of the latter.1819 Acetaldehyde,... 

Boronic esters (8) react with dichloromethyllithium to form intermediate borate complexes which then undergo rearrangement to form 2-haloboronic esters (9). When treated with a Grignard reagent, the 2-haloboronic esters (9) rearrange to secondary alkylboronic esters (10 Scheme 3). 

Boronic esters are not as reactive as triorganylboranes towards nucleophiles however, rearrangement does take place if the boronic ester is treated with an a-halocarbanion (equation 54). In this case, the I-chloroethylboronate (36) can be obtained in 95% de due to the diastereofacial influence of the chiral pinanediol. Similarly, chloro- or bromo-alkylboronic esters react with Grignards reagents, alkyllithiums and enolates leading to rearranged products. ... 

A series of borabarbaralanes 54 was prepared by the reaction of K2C8H with appropriate alkylboron dichlorides in hexane at -50 °C (Scheme 2.32) [51,52]. Similarly to their carbon analogs, borabarbaralanes exhibit fast Cope rearrangement yielding averaged proton spectra at ambient temperature (Scheme 2.33). This rearrangement has been studied by line-shape analysis and 2D EXSY (activation parameters are listed in the Scheme 2.34). 

A stereoselective boronate rearrangement (Aggarwal s rearrangement) is a powerful way to generate alkylboronates. The classic version of this reaction uses sec-BuLi at low temperatures (-78 °C) to promote the process however, a modification to this procedure enabled the development of a rearrangement process that occurs without the need for such... 

Yoshida, M., Shoji, Y., and Shishido, K. (2010) Fnantioselertive formal total synthesis of aplysin utilizing a palladium-catalyzed addition of an alkylboronic acid to an aHenic alcohol—Eschenmoser/Claisen rearrangement. Tetrahedron, 66, 5053-5058. 

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