Chelate-directed alkenylation

Besides Pd, Rh and Ru catalysts have also been employed for the Fujiwara-Moritani reaction [10]. Such catalyst systems have been foimd to be applicable for chelate-directed alkenylation reactions (see later discussion). 

The Murai reaction (Scheme 4), the replacement of an ortho-CH on an aromatic ketone by an alkyl group derived from a substrate olefin, is catalyzed by a variety of Ru complexes. This C bond formation occurs via chelate directed C-H bond activation (cyclometalation) in the first step, followed by alkene insertion into RuH and reductive elimination of the alkylated ketone. In a recent example of the use of a related cyclometalation in complex organic synthesis, Samos reports catalytic arylation (Suzuki reaction) and alkenylation (Heck reaction) of alkyl segments of a synthetic intermediate mediated by Pd(II). 

Even though not (yet) among the typically employed reactions for the production of materials, C-H olefinations of arenes are a valuable, more sustainable alternative for the synthesis of olefin-substituted arenes. C-H olefinations are among the first C-H functionalizations ever reported [65], and the originally stoichiometric modifications of simple arenes with olefins in the presence of Pd" have initiated broad research efforts. To this day, C-H alkenylations are more common using aromatic substrates without directing groups [18], and these reactions will be addressed in the next chapter of this book. However, several examples of chelate-assisted C-H olefinations are known [30, 66-74], which exhibit excellent ortho-selectivity. 

In addition, oxazole-4-carboxylate 10 can be subjected to direct Pd-catalyzed alkenylation, benzylation, and alkylation in the 2-position [257]. Recently, direct 2-C-H-functionaUzation (alkylation and arylation) has been reported for oxazoles (as well as thiazoles and benzazoles) with free 2-position utihzing chelated Ni/Cu-catalysts derived from 2,2 -bis(dimethylamino)diphenylamine [258] or chelated Cu/Pd-complexes derived from Xantphos ]259] in the presence of a base. 

Kakiuchi reported that alkenyl acetates can function as alkenyl sources in the chelation-assisted C-H bond functionalization reaction, which is catalyzed by Ru (cod)(cot) [Eq. (18)] [78]. In this reaction, two normally unreactive bonds, i.e., C-H and C-OAc, are activated within the catalytic cycle. OxazoUnes, tetrazoles, and thiazoles were also effective directing groups in this reaction. 

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