Organoboranes, oxidative coupling

Moderate yields of acids and ketones can be obtained by paHadium-cataly2ed carbonylation of boronic acids and by carbonylation cross-coupling reactions (272,320,321). In an alternative procedure for the carbonylation reaction, potassium trialkylborohydride ia the presence of a catalytic amount of the free borane is utilized (322). FiaaHy, various tertiary alcohols including hindered and polycycHc stmctures become readily available by oxidation of the organoborane iatermediate produced after migration of three alkyl groups (312,313,323). 

Oxidative nickel-catalyzed coupling of aldehydes and alkynes to generate allylic alcohols. Intermolecular and intramolecular examples are both effective, and the transmetalating agent (MR" ) may be an organosilane, organoborane, organozinc, or alkenylzirconium. ... 

The in situ synthesis of organoboranes via reaction of alkyl halides with magnesium in the presence of diborane can also be used to prepare coupled products (equations 20 and 21). Oxidation of the reaction mixture with alkaline silver nitrate leads to good yields of dimeric products. The reaction is successful for primary and secondary halides. A related reaction is the coupling of secondary alkyl halides in the presence of catalytic quantities of thallium salts. This procedure fails for primary alkyl halides and gives modest yields for secondary alkyl halides (equation 22). 

Oxidative Cross-coupling. Molecular oxygen has been employed as the bulk oxidant in numerous Pd(0) and Pd(II)-catalyzed oxidation processes. Another example is the cross-coupling of organoboranes with olefins, in an oxidative Heck-type reaction. Both Pd(OAc)2 and Pd2(dba)3 were productive in this process affording the desired product in 85-87% yield. It was found that O2 was crucial for this reaction as very little product was formed under anaerobic conditions. The authors infer that these results suggest that O2 plays a pivotal role in the Pd(II)-catalyzed reaction though the reoxidation of Pd(0) species to Pd(II) (eq 51). ... 

B.i.c. Pd(OAc)2 + nL(n 3) as Precursor of Three-Coordinate Anionic Pd L OAc) (L = PPhj). Pd(OAc)2 + nL] mixtures were initially introduced as catalysts in Heck reactions but are also involved in Suzuki cross-coupling of organoboranes. " In both cases, a palladium(O) is mechanistically required to activate aryl halides C—X bonds. However, the origin of the paUadium(O) center has remained unknown, since no clearly identified reducer is present in the catalytic reactions. A quantitative kinetic study of this system has established that addition of 3 equiv of PPhs to Pd(OAc)2 quantitatively generates a stable palladium(O) complex through an intramolecular reduction of the palladium(II) to paUadium(O), a process by which the phosphine is oxidized to phosphine oxide, (OjPPhj (Scheme 5). 

Organoborane compounds are one of the most commonly used coupling partners for C-C couplings. In 2014, Shi et al. achieved a Pd(ll)-catalyzed selective borylation of methyl C-H bond with O2 as a benign oxidant (Scheme 1.36) [90]. Unambiguously, the produced C-B coupling products possess low reactivity in the presence of palladium catalyst. The direct borylation of unactivated Y-C(sp )-H bond also contribute an efficient access to alcohols, amines, and various functionalized molecules. 

Thiophene rings can be coupled through organoboranes (148). The key step is the oxidative treatment of an ate complex, itself formed by... 

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