Benzylation reactions aryl-benzyl coupling

There are very few reports on the aryl-allyl or aryl-benzyl coupling reaction, but these combinations are very popular in other organometalHcs (Scheme 38). Arylation of allyl acetates or carbonates was catalyzed by NiCl2(PPh3)2 or NiCl2(dppf) with net inversion of the stereochemistry of the C-0 bond for... 

Specifically, diphenylmethane was prepared in >90% yield by the reaction of either Ph4Sn or PhjSnMe with BrCH2Ph in the presence of PhCH2Pd(PPh3)2Cl.t The synthesis of polychlorinated diaryhnethanes shown in Scheme 26 is noteworthy despite the modest product yields. Most of the other currently known examples of Pd-catalyzed aryl-benzyl coupling pertain to the synthesis of heteroarenes (Scheme 27). 

Scheme 3.48 Synthesis of allylarenes by Pd-catalyzed allyl-aryl, aryl-allyl, benzyl-alkenyl, and alkenyl-benzyl coupling reactions.

In an early example of this reaction aryl and benzyl zinc halides were coupled with aryl halides (equation 40) (102). Alkynyl zinc compounds cross-couple with aryl halides (iodides or bromides) in the presence of a Pd catalyst (equation 41) (103) or alkynyl iodides can be coupled with aryl and alkenyl zinc compounds (equation 42) (100). 

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

Palladium-catalyzed aminations of aryl halides is now a well-documented process [86-88], Heo et al. showed that amino-substituted 2-pyridones 54 and 55 can be prepared in a two-step procedure via a microwave-assisted Buchwald-Hartwig amination reaction of 5- or 6-bromo-2-benzyloxypyri-dines 50 and 51 followed by a hydrogenolysis of the benzyl ether 52 and 53, as outlined in Fig. 9 [89]. The actual microwave-assisted Buchwald-Hartwig coupling was not performed directly at the 2-pyridone scaffold, but instead at the intermediate pyridine. Initially, the reaction was performed at 150 °C for 10 min with Pd2(dba)3 as the palladium source, which provided both the desired amino-pyridines (65% yield) as well as the debrominated pyridine. After improving the conditions, the best temperature and time to use proved... 

Scheme 16 sp-sp C-C bond formation via Fe(OTf)3ATfOH-cocatalyzed coupling reaction of terminal aryl alkynes with benzylic alcohols... 

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