Alkynes Fujiwara hydroarylation

Another C—H bond functionaUzation reaction that has been intensively investigated using complexes with poly-NHC Ugands is the so-called Fujiwara hydroarylation, namely the intermolecular addition of an aryl C—H bond across the triple bond of an alkyne, generally producing the thermodynamically less stable ds-arylalkene (Fig. 19). 

Hydroarylation, (addition of H-Ar, Ar = aryl), of alkynes, catalysed by Pd(OOCCH3)2 or Pd(OOCCFj)j in acetic acid, is an atom-economic reaction, giving rise to substituted c/i-stilbenes (Fujiwara reaction). Catalytic conversions and improved chemoselectivity to the mono-coupled product under mild conditions can be achieved by modification of the metal coordination sphere with NHC ligands. Hydroarylation of mesitylene by ethylpropiolate (Scheme 2.19) catalysed by complex 107 (Fig. 2.18) proceeds in good conversions (80-99%, 1 mol%) under mild conditions at room temperature. 

Hydroarylations of alkynes are catalyzed by gold complexes and these bear some resemblance to the Fujiwara Pd-catalyzed reaction. In general, when using gold chemistry, better Z/E selectivities are observed compared with palladium, lower catalyst loadings and milder conditions (neutral not TFA) are used. The mechanism involves the attack of ArH on the Au-coordinated alkyne. Flowever, electron-poor acetylenes only appear to work with palladium chemistry (Equations (75) and (76)).72... 

Fujiwara Reaction Palladium-catalyzed Hydroarylations of Alkynes and Alkenes... 

Arenes, especially electron-rich ones, and five-membered-ring heteroaromatic compounds have been found to react with alkynes in the presence of Pd(II) or Pt(II) species in acidic solvents to give the corresponding hydroaryl-ation products (Eqs. 68 and 69) [155-158]. The mechanism proposed for the reaction involves aromatic palladation to form ArPd(II) species, as in the Fujiwara-Moritani reaction [10], insertion of an alkyne,and protonolysis of the... 

Pd-catalyzed hydroarylation of alkynes via aromatic C—bond activation has been reported by Fujiwara. The reaction offers a good synthetic method of arylalkenes, which are usually prepared by Heck reaction of alkenes. Electron-rich arenes bearing more than one OH, OMe, and alkyl groups react with electron-deficient alkynes [6], The reaction proceeds with a catalytic amount of Pd(OAc)2 in trifluoroacetic acid (TEA) at room temperature. Hydroarylation of ethyl propiolate (15) with / -dimethoxybenzene yielded ethyl (Z)-3-(2,5-dimethoxyphenyl)-2-propenoate (16). Mesitylene (17) is an active arene and the reaction of 17 with 3-butyn-2-one (18) afforded the imsaturated ketone 19 which has f-form [7]. 

Fujiwara, Y. and Kitamura, T. (2005) Fujiwara reaction palladium-catalyzed hydroarylations of alkynes and alkenes, in Handbook of C-H Transformations (ed. G. Dyker), Wiley-VCH Verlag GmbH, pp. 194-202. 

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