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) 2 BB(OR) 2; (RO) 2 = (MeO) 2 ( 4a ) and Me 4 C 2 O 2 ( 1 )] added to both terminal and internal alkynes in the presence of a catalytic amount of Pt(PPh 3 ) 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(PPh 3 ) 4 generates cis -Pt(BO 2 C 2 Me 4 ) 2 (PPh 3 ) 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.