Organoboron formation

Olefins may be obtained by elimination from the organoboron intermediates formed from enol derivatives and diborane. " As in the reaction with a,jS-unsaturated ketones (section IX), the conversion is carried out in two parts first formation of the diborane adduct and second, decomposition in refluxing acetic anhydride. 

To immobilize such anions as borate, organoboron polymers were reacted with aryllithium reagents.31,32 The reaction of alkylborane polymers with n-BuLi was examined first however, the ionic conductivity of the resulting material was very low. Moreover, complicated peaks were observed in the H-NMR spectrum. Conversely, selective lithium borate formation was observed in the nB-NMR spectrum when PhLi was employed (scheme 6). An ionic conductivity of 9.45 X 10 7Scm 1 was observed at 50°C. The observed ionic conductivity was relatively low because of the decreased number of carrier ions compared with dissolved salt systems. However, the lithium transference number of this polymer was markedly high (0.82 at 30°C). 

Our review of the use of organoboron compounds in radical chemistry will concentrate on applications where the organoborane is used as an initiator, as a direct source of carbon-centered radicals, as a chain transfer reagent and finally as a radical reducing agent. The simple formation of carbon-heteroatom bonds via a radical process is not treated in this review since it has been treated in previous review articles [3,9]. 

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... 

Suzuki reaction A palladium catalyzed reaction that results in the formation of new carbon to carbon bonds by reacting an organohalide or organotriflate with an organoboronic acid. 

Aliphatic amines can be readily oxidized by Pd(II) to imines or iminium salts and hydrido complexes. The latter can transfer hydrogen to alkenes, leading to the formation of alkanes as byproducts of the Heck reaction (last example, Scheme 8.18). Such reactions can be avoided by using alkali carbonates as base instead of aliphatic amines [148]. Treatment of stannanes or organoboron derivatives with electron-deficient alkenes under acidic reaction conditions can also lead to formal products of Michael addition instead of the products of a Heck-type reaction [149, 150] (Scheme8.19). 

The covalent B-G bond of organoboron compounds shows little reactivity toward representative electrophiles, but the presence of an empty low-lying orbital facilitates transmetallation to other metals. The transmetallation to zinc709 and other metal reagents has been extensively studied for carbon-carbon bond formation via addition and coupling reactions of trialkylboranes and alkylboronic acids. 

Addition reactions of organoboronic acids to electron-deficient alkenes were found to be catalyzed by rhodium(i)931 or dicationic palladium(n) complexes.932,933 The reaction proceeding through the transmetallation to a transition metal has been proved to be a general technique for a wide range of selective carbon-carbon bond formation via 1,4-addition to a,/ -unsaturated ketones, aldehydes, esters, and amides, and the 1,2-addition to aldehydes and imines (Equation (217)).934... 

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