Cross-coupling reactions proceeding through eta(1)- and eta(3)-propargyl/allenyl-palladium(II) intermediates

The efficiency of cross-coupling between propargyl electrophiles and organo tin(IV) or zinc(II) compounds catalyzed by phosphine-palladium complexes ha s been examined as a function of PR3/Pd ratio, the nature of the organometa llics, and the nature of the leaving group of the electrophile. The couplin g between RC=CCH2Cl and PhSnBu3 proceeded much more smoothly by use of a ca talyst precursor having a PPh3/Pd ratio of 1/1 rather than 4/1, while the r ate of the coupling using PhC=CSnBu3, instead of PhSnBu3, was less sensitiv e to the PPh3/Pd ratio. The regiochemistry of the coupling (formation of al kyne or allene) was also sensitive to the PPh3/Pd ratio in the case of the reaction between RC=CCH2Cl and PhZnCl, but was much less sensitive in the r eaction using PhSnBu3. These results have been rationalized by a general me chanistic scheme involving not only eta(1)-propargyl or eta(1)-allenylpalla dium(II) which was considered in previous studies but also eta(3)-propargyl palladium(II) species as a new reactive intermediate candidate. The carbona tes, RC=CCH2OCOC2H5, failed to undergo as clean a cross-coupling as was fou nd with the chlorides, but instead provided products arising from the coupl ing of two RCCCH2 units without any participation of organotin(IV) reagents . (C) 1999 Elsevier Science S.A. All rights reserved.