TRANSFORMATION OF A DIALKYNYL KETONE TO A ETA(1)-(EXO-ALKYLIDENE)CYCLOBUTENONE LIGAND BY PT-H ADDITION AND REARRANGEMENT REACTIONS

The cationic Pt-H complex, [trans-PtH(PPh3)(2)(THF)](+), and O=C(C=CCM e3)(2) undergo reaction in CH2Cl2 solution at -78 to 25 degrees C to f orm, following a chloride quench of the reaction solution, a neutral P t(II)(PPh3)(2)Cl complex containing a eta(1)-(exo-alkylidene)-cyclobut enone ligand. This carbocyclic ligand is formed through addition of th e Pt-H bond to one of the alkynyl substituents of the dialkynyl ketone followed by an overall Pt-mediated insertion of the second alkynyl su bstituent into the resulting Pt-C(alkenyl) bond. A proposed mechanism for this reaction is presented along with supporting experimental data . The X-ray structure of this new Pt complex, trans-Pt[C=C(CMe3)C(O)C= C(CMe3)(H)](PPh3)(2)(Cl). 1/2CH(2)Cl(2) . H2O, has been determined: sp ace group, Pnma (No. 62); a=18.829(1), b=15.848(2), c=17.019(2) Angstr om; Z=4. (C) 1997 Elsevier Science S.A.