Organoboronic ester

In 2006, Ukai et al. proposed an interesting alternative with a rhodium(I)-catalyzed carboxylation of aryl- and alkenylboronic esters proceeding under mild conditions, and leaving ancillary reactive functional groups such as carbonyl- and cyano unreacted [51] (Scheme 5.13). Considering that organoboronic esters are easily available, and that various functional groups are tolerated, this reaction appeared to be particularly useful for the preparation of functionalized arylcarboxylic acids, such as benzoic and cinnamic acid derivatives. 

Micalizio and Schreiber [47] developed a key reaction, the transesterification of unsaturated boronic esters with allylic esters or propargylic alcohols. This reaction transiently provided mixed organoboronic esters that could be trapped by using... 

Although other transition metals have been found to catalyze hydroborations with HBcat, early studies have shown that rhodium complexes are the most effective for reactions of simple alkenes. The catalytic cycle proposed resembles one suggested previously for the rhodium-catalyzed addition of carborane B-H bonds to the C=C unit in acrylate esters. The reaction is believed to proceed via initial loss of phosphine and oxidative addition (see Oxidative Addition) of the B-H bond of HBcat to the coordinatively unsaturated (see Coordinative Saturation Unsaturation) rhodium center. Coordination ofthe alkene (seeAlkene Complexes) and subsequent insertion (see Insertion) into the Rh-H bond and reductive elimination (see Reductive Elimination) of the B-C bond then generates the organoboronate ester product(s) (Scheme 1). 

The cross-coupling reaction of 1 with palladium catalyst provides a convenient method for the synthesis of organoboronic esters from organic electrophiles (Scheme... 

This method is an adaption of the U. S. Borax Research group s procedure that Illustrates a practical and efficient method for the synthesis of tetra(alkoxo)diborons. Several (alkoxo)diborons, such as tetra(methoxo)-,3 bis(catecholato)-, and bis(pinacolato)diboron (1). are synthesized from tetrakis(dimethylamino)diboron. The diborons are excellent reagents for the synthesis of various organoboronic esters via the transition metal-catalyzed addition and cross-coupling reactions. ... 

Coupling reactions. The Ru complex catalyzes replacement of the amino group of o-aminoaryl ketones with the carbon residue of an organoboronic ester. Direct activation of a C—H bond ortho to the carbonyl group is also possible. ... 

Evans and co-workers developed rhodium catalyzed hydroboration reactions, which enable the use of catechol borane and other boronate esters as hydroborating agents to afford organoboronic esters as products.9 These products have increased stability and can be used directly in palladium cross-coupling applications. However, the rhodium-catalyzed method is most effective for the hydroboration of monosubstitued olefins (i.e., 16), as lower reactivity is observed with more substituted alkenes. 

The photochemistry of silylated derivatives continues to be intensively investigated. A typical example of a synthetically useful process is the generation of silyloxycarbenes by photoisomerization of acylsilanes. In the presence of organoboronic esters these intermediates are trapped via formal B-C bond insertion. The adducts then rearrange to a boradioxepan that can be hydrolyzed to a ketone. The overall process offers a convenient, transition metal free method for the intermolecular carbon-carbon bond formation (see Scheme 22). ... 

Preparation of alkenylzinc reagents by boron-zinc transmetalation from stable organoboronic esters and dialkylzinc is hampered by the instability of the evolved alkenylzinc mixtures of the two reagents turned black and the reaction of the evolved alkenylzinc on remaining pinacol alkenylboronic ester was speculated on the basis of the structure of by-products. Conversely, this boron to zinc exchange can lead to good results if the evolved alkenylzinc is trapped in situ by an electrophile. The reaction of an alkenylboronic ester... 

On the other hand, the fact thatthe reaction of organoboronic esters takes place in the absence of water might account for the transmetalation under anhydrous conditions between organoboronic esters and 7 however, the experimental results do not support this process (Scheme 8). The addition of the phoiylboronic ester of diethylene glycol to 1,3-diphenylpropen-l-one (calcon)... 

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