Carbon-boron bond activation

Important advances in the field of C-H bond activation have involved the photochemical reactions of boryl complexes such as Cp W(CO)3(Bcat) (98, cat = 1,2-02-3,5-Me2C6H2). Transient species derived from these complexes efficiently activate the C-H bonds of alkanes and arenes (see Alkane Carbon-Hydrogen Bond Activation), and they can convert hydrocarbon solvents into alkylboronate esters (equation 27). Experimental and theoretical studies have shown that these reactions proceed via a boron-assisted, a-bond metathesis see a-Bond Metathesis) pathway involving back donation of electron density from the tungsten atom to a formally unoccupied p orbital centered on the boryl ligand. ... 

Schuster and co-workers discovered that 1,4-dicyanonaphthalene solutions containing an alkyltriphenylborate salt, when irradiated, yield one-eleetron oxidation of the alkyltriphenylborate leading to carbon-boron bond cleavage and formation of free alkyl radicals [23]. In Gottschalk s hands [24, 25], it was shown that ionic salt pairs formed from cyanine dyes and alkyltriphenylborates (Figure 1) could be used as photoinitiators [26] that were active in the visible region of the spectrum. 

Alkenylboronates activated with either an ester or a sulfone group have been engaged in [3+2] cycloadditions with methylenecyclopropanes to afford methylenecyclopen-tanes in good yields. The cycloaddition reactions were highly stereoselective and yielded exclusively the corresponding trans-disubstituted products. The carbon-boron bond of the cycloadducts can be further transformed by oxidation to give the corresponding cyclopentanols (Scheme 9.47) [lOOj. 

The Suzuki coupling of aryl halides was also extended to tosylates recently. Benzothiazole 5-tosylate reacted with m-xylene-2-boronic acid (6.13.) to give the coupled product in 94% yield using palladium acetate and a stericly congested biphenyl based phosphine ligand as catalyst.17 Another class of less commonly utilised cross-coupling partners are methyltio derivatives. In the presence of a copper salt, which activates the carbon-sulphur bond, 2-methyltio-benzotiazol coupled readily with a series of arylboronic acids.18... 

The additions of bimetallic reagents such as the silicon-metal and tin-metal compounds to the carbon-carbon multiple bonds have been extensively studied however, there are few reports concerning the metal-boron compounds as shown above. Miyaura and co-workers have examined the addition reaction of tetraafkoxydiboron to alkynes and found that only the platinum complexes such as Pt(PPh3)4 exhibited excellent catalytic activity (Scheme 24). Pd(PPh3)4 and Pd(OAc)2-isocyanide complexes, which have been the best catalysts for the silyl- and stannyhnetallation, were ineffective because their oxidative addition to palladium(0)-phosphine complexes is very slow. ... 

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