Platinum(0)-Catalyzed Diboration of Alkynes with Tetrakis(alkoxo)diborons:  An Efficient and Convenient Approach to <i>cis</i>-Bis(boryl)alkenes

Abstract

Tetrakis(methoxo)- or bis(pinacolato)diboron [(RO)2BB(OR)2; (RO)2 = (MeO)2 (4a) and Me4C2O2 (1)] added to both terminal and internal alkynes in the presence of a catalytic amount of Pt(PPh3)4 to provide stereodefined cis-bis(boryl)alkenes (3) in excellent yields. Because reagents and reaction conditions were sufficiently mild, the procedure was readily extended to various functionalized alkynes. Mechanistic study revealed that the oxidative addition of bis(pinacolato)diboron (1) to Pt(PPh3)4 generates cis-Pt(BO2C2Me4)2(PPh3)2 (5), whose structure was fully characterized by multinuclear NMR spectroscopies as well as single-crystal X-ray diffraction analysis. Complex 5 exhibited high reactivity for insertion to the alkyne giving 3 in high yields, thus implying that the oxidative addition of the B−B bond to a Pt(0) complex is an initial step in the platinum(0)-catalyzed diboration of alkynes.