A Neutral Borylene and its Conversion to a Radical by Selective Hydrogen Transfer.

A successful selective reduction of X 2 B-Tip (Tip = 1,3,5- i Pr 3 -C 6 H 2 , X = I, Br) with KC 8 and Mg metal, respectively, in the presence of a hybrid ligand (C 6 H 4 (PPh 2 )LSi) leads to a stable low-valent five-membered ring as a boryl radical [C 6 H 4 (PPh 2 )LSiBTip][Br] ( 1 ) and neutral borylene [C 6 H 4 (PPh 2 )LSiBTip] ( 2 ). Compound 2 reacts with 1,4-cyclohexadiene, resulting in hydrogen abstraction to afford the radical [C 6 H 4 (PPh 2 )LSiB(H)Tip] ( 3 ). Quantum chemical studies reveal that compound 1 is a B-centered radical, and compound 2 is a phosphane and silylene stabilized neutral borylene in a trigonal planar environment, whereas compound 3 is an amidinate-centered radical. Although compounds 1 and 2 are stabilized by hyperconjugation and π-conjugation, they display high H-abstraction energy and basicity, respectively.