Organoboranes are organic molecules featuring an sp 2hybridised BR 2 group. This BR 2 fragment, with its welldocumented s basicity and p acidity, lends the C À B bond a unique reactivity that has been impressively exploited by organic chemists. In the early 20th century, organoboranes featured in organic chemistry merely as placeholders for hydroxy groups, the oxidation of a CÀBR 2 bond to CÀOH being a reliable method for the synthesis of alcohols. However, it was only after the discovery of the Suzuki-Miyaura cross-coupling of organoboranes with aryl halides that this functional group became widely used in organic chemistry. [1] This method has since developed into one of the most versatile synthetic methods available for CÀC bond formation and it was the subject of the Nobel Prize in Chemistry in 2010. [2] Consequently, the need for convenient and economical methods for the installation of the BR 2 group in organic molecules has fuelled research into organoborane chemistry for some time.
The hydroboration of unsaturated organic substrates developed by Brown and co-workers in the 1950s brought organoborane chemistry out of obscurity and added boron to the organic chemists repertoire. [3] For a long time, simple hydroboration remained the method of choice for the synthesis of organoboronates. In the 1980s, the groups of Sneddon, Marder, and Nçth reported the transition-metalcatalyzed hydroboration reaction, thus providing access to borane products with alternative chemo-and regioselectivity to complement those prepared by classical hydroboration. [4] Furthermore, catalytic diboration of unsaturated compounds [5] as well as the borylation of arenes [6] and alkanes [7] gave access to a much broader variety of organoboronates.
An indispensable starting material in many of these reactions is a tetraalkoxydiborane(4) of the general formula (RO) 2 B À B(OR) 2 . The two most commonly used reagents of this class of compounds are bis(catecholato)diborane (B 2 Cat 2 ; 1) and bis(pinacolato)diborane (B 2 Pin 2 ; 2), which are currently synthesized by a method established by Brotherton in 1960, [8] which has been modified several times over the intervening decades. [9] Starting from boron tribromide, bromobis(dimethylamino)borane is synthesized in two steps. Subsequently, formation of the boron-boron bond is achieved by reductive coupling with sodium to yield B 2 (NMe 2 ) 4 (3), which is further converted into 1 and 2 upon reaction with the corresponding diols (Scheme 1). [9a,d, 10] Alternatively, 1 can be