Arylation organoboron compounds

An aryl-organoboron compound is prepared from an organolithium compound and... 

Palladium-catalyzed carbon-carbon bond forming reactions like the Suzuki reac-tion as well as the Heck reaction and the Stille reaction, have in recent years gained increased importance in synthetic organic chemistry. In case of the Suzuki reaction, an organoboron compound—usually a boronic acid—is reacted with an aryl (or alkenyl, or alkynyl) halide in the presence of a palladium catalyst. 

The application of in situ-generated (alkoxy)palladium(II) species (Scheme 14.23) can be extended to reactions of a-carbonates with organoboron compounds. Crosscouplings of allenes 108 with aryl (or alkenyl) boron acids or their esters catalyzed by a palladium(O) complex afforded the 2-aryl(alkenyl)-l,3-butadienes 109 in excellent yields (Scheme 14.24) [53], The coupling reactions of 9-BBN-derived intermediates such as ester 111 can be accelerated by applying K3P04 as additive (Eq. 14.15). 

The overall mechanism is closely related to that of the other cross-coupling methods. The aryl halide or triflate reacts with the Pd(0) catalyst by oxidative addition. The organoboron compound serves as the source of the second organic group by transmetala-tion. The disubstituted Pd(II) intermediate then undergoes reductive elimination. It appears that either the oxidative addition or the transmetalation can be rate-determining, depending on reaction conditions.134 With boronic acids as reactants, base catalysis is normally required and is believed to involve the formation of the more reactive boronate anion.135... 

The recent emergence of the Suzuki coupling as the method of choice for the cross-coupling of aryl halides,2 3 due to its functional group tolerance and the stability of organoboron compounds, is clearly reflected by the proportion of these transformations in the literature. 

Palladium catalysts have been found which are effective in the Suzuki coupling reaction of arylboronic acids with aryl chlorides carrying electron-withdrawing groups.73 Biaryls may also be synthesized by cross-coupling of arylboronic acids with arenediazonium salts.74,75 There has been a report of the polymer-bound palladium-catalysed Suzuki coupling of aryl triflates with organoboron compounds.76 Arylbor-onates may themselves be synthesized by the palladium-catalysed reactions of... 

Some of the most useful organoboron compounds are ethers of boric acid, trialkyl(aryl)borates. There are several techniques for producing these ethers ... 

Among a variety of organometallics for this purpose, organoboron compounds were first used by Kojima for the synthesis of alkyl aryl ketones [167]. The action of Zn(acac)2 in this reaction (Scheme 2-61) is ascribed to the formation of a RCOPd (acac) species which undergoes transmetallation without assistance of bases (Scheme 2-20). 

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