The formation of highly enantioenriched boronic esters through both stoichiometric and catalytic methods has received much attention over the past decade. Accordingly, the transformations of the boronic ester moiety into other functional groups is of considerable interest in synthesis. Specifically, transformations which retain the high enantioenrichment of the starting boronic ester, either through a stereoretentive or a stereoinvertive pathway, lead to the formation of new C-C, C-O, C-N, C-X, or C-H bonds at stereogenic centres. This feature article summarises the current state of the art in stereospecific transformations of both secondary and tertiary boronic esters into other functionalities and groups, whilst considering critically the transformations that are currently unattainable and would represent future advances to the field.

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The conversion of widely available carboxylic acids into versatile boronic esters would be highly enabling for synthesis. We found that this transformation can be effected by illuminating the -hydroxyphthalimide ester derivative of the carboxylic acid under visible light at room temperature in the presence of the diboron reagent bis(catecholato)diboron. A simple workup allows isolation of the pinacol boronic ester. Experimental evidence suggests that boryl radical intermediates are involved in the process. The methodology is illustrated by the transformation of primary, secondary, and tertiary alkyl carboxylic acids as well as a diverse range of natural-product carboxylic acids, thereby demonstrating its broad utility and functional group tolerance.

Diazo compounds are useful synthetic intermediates in organic synthesis but, due to their toxicity and unpredictable explosive behaviour, their unique reactivity has not been fully exploited and their use on large scale has been avoided. We have developed a reliable method that generates diazo compounds in situ. Our approach is based on the BamfordStevens reaction, which utilizes tosylhydrazone salts as diazo precursors. In the presence of phase-transfer-catalysts (PTC), we found that tosylhydrazone salts can be cleanly converted to diazo compounds under mild reaction conditions and in a wide range of solvents. These diazo compounds can then be