Palladium-Catalyzed Controlled Carbopalladation of Benzyne
journal contributionposted on 15.07.2000, 00:00 by Eiji Yoshikawa, K. V. Radhakrishnan, Yoshinori Yamamoto
2-Trimethylsilylphenyl trifluoromethanesulfonate 1a, a benzyne precursor, reacted with the allylic chlorides 2a−f in the presence of CsF (2.0 equiv) and Pd2(dba)3·CHCl3 (2.5 mol %)−dppf (5 mol %) in a 1:1 mixed solvent of CH3CN and THF to produce the phenanthrene derivatives 3 along with their minor regioisomers 4 in good yields (i) and the reaction of 1a with 2a and the internal alkynes 15a,c−e afforded the naphthalene derivatives 16 in moderate yields (ii). The reaction of benzyne precursor 1a with the alkynes 15a−c,f−h in the presence of Pd(OAc)2 (5 mol %)−(o-tolyl)3P (5 mol %) catalyst and CsF (2.0 equiv) in CH3CN gave the phenanthrene derivatives 17 (iii), whereas the reaction of 1a with the alkynes 15a,b,i in the presence of the same catalysts and CsF in CH3CN−toluene gave the indene derivatives 18 in good yields (iv). Detailed mechanistic investigation revealed that the former two reactions i and ii proceed through carbopalladation to free benzyne, while the latter two reactions iii and iv proceed through the nonfree benzyne mechanism, in which the initial step of the catalytic cycle begins with Pd(0) insertion to the Ar−OTf bond of 1.