Boryl carboxylates

Borinic carboxylic anhydrides Acyl borinates 20 Boryl carboxylates 30... 

Recent optimization studies reveal that the yield of 2-(2-propenyl)-1,3,2-dioxaborolane-4,5-di-carboxylate esters (i.e., the tartrate ester modified allylboronates) is improved by using triiso-propyl borate as the borylating agent1. The improved yields are directly related to the increased efficiency of the preparation of the intermediate allylboronic acid. 

Other boryl esters of dithiocarboxylic acids 69 are obtained by sulfurization of the corresponding carboxylic acids, esters or amides with organoboron sulfides such as (9-BBN)2S andbis(l,5-cyclooctanediylboryl) sulfide (Scheme 16) [124,125]. Methanolysis of the boryl esters 69 gives the corresponding dithiocarboxylic acids. 

Scheme 16 Formation of boryl esters of dithiocarboxylic acids by sulfurization of the corresponding carboxylic acids, esters or amides with an organoboron sulfide...

A catalytic borylation of aryl carbamates has been achieved by Shi s group [Eq. (13)] [62]. Aryl carboxylates also afforded the borylated products albeit with lower yields. 

As described earlier, a variety of transition metal catalysts have been used to promote the addition of B-H bonds across alkynes. The development of metal-free approaches has been challenging to design due to the unique reactivity profile afforded by transition metal catalysts. To this end, a metal-free catalyst system has been devised for the addition of pinacolborane to internal alkynes (Scheme 6.48) [91]. Surprisingly, only a carboxylic acid additive was needed to effect the hydroboration reaction. In addition to common alkynes, an alkynylboronate was successfully borylated using the carboxylic acid catalyst. The functional group tolerance was remarkably broad, and a host of alkynes were successfully functionalized. Given the wide substrate scope, availability of the precursors and catalysts, and the metal-free nature of the chemistry, this is a reasonable approach to selective hydroboration reactions. 

A related theoretical study on palladium-catalyzed borylation of iodobenzene with diboron was reported by Sakaki et al. [73]. Specifically, the mechanism for the transmetalation step between the Pd complexes Pd(X)(Ph)(PH3)2 (X = OH, F) and diboron B2(eg)2 (eg = -OCH2CH2O-) was investigated. Other theoretical studies, but now within the context of organic acids as electrophiles, were reported by Goo/ en et al. [74, 75]. The palladium catalyzed Suzuki reaction of carboxylic anhydrides with arylboronic acids was analyzed, showing that in those cases monophosphine complexes were also involved in the transmetalation process. 

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