Regio‐ and Stereoselective Synthesis of 1,1‐Diborylalkenes via Brønsted Base‐Catalyzed Mixed Diboration of Alkynyl Esters and Amides with BpinBdan
Xiaocui Liu, Wenbo Ming, Xiaoling Luo, Alexandra Friedrich, Jan Maier, Udo Radius, Webster L. Santos, Todd B. Marder
Abstract
The NaO t Bu‐catalyzed mixed 1,1‐diboration of terminal alkynes using the unsymmetrical diboron reagent BpinBdan (pin = pinacolato; dan = 1,8‐diaminonaphthalene) proceeds in a regio‐ and stereoselective fashion affording moderate to high yields of 1,1‐diborylalkenes bearing orthogonal boron protecting groups. It is applicable to gram‐scale synthesis without loss of yield or selectivity. The mixed 1,1‐diborylalkene products can be utilized in Suzuki–Miyaura cross‐coupling reactions which take place selectivly at the C–B site. DFT calculations suggest the NaO t Bu‐catalyzed mixed 1,1‐diboration of alkynes occurs through deprotonation of the terminal alkyne, stepwise addition of BpinBdan to the terminal carbon followed by protonation with t BuOH. Experimentally observed selective formation of ( Z )‐diborylalkenes is supported by our theoretical studies.