Abstract
The NaOtBu‐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 NaOtBu‐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 tBuOH. Experimentally observed selective formation of (Z)‐diborylalkenes is supported by our theoretical studies.
Highlights
Organoboronic acids and their derivatives have become increasingly of interest due to their widespread application in organic synthesis, materials science, and pharmaceuticals.[1]
Alkenylboron compounds have been employed in the stereodefined construction of valuable multisubstituted alkenes including natural products, biologically active molecules, and functional materials.[1h,2] 1,2-Diborylalkenes are well-established and are typically synthesized by catalytic diboration of alkynes using Pt,[3] Pd,[4] Cu,[5] Co,[6] Fe,[7] Zn[8] and metal-free reactions.[9]
1,1-diborylalkenes have emerged as versatile building blocks for the synthesis of multisubstituted alkenes, e.g. the anticancer agent tamoxifen, via selective and stepwise Suzuki–Miyaura couplings.[10]
Summary
Regio- and Stereoselective Synthesis of 1,1-Diborylalkenes via Brønsted Base-Catalyzed Mixed Diboration of Alkynyl Esters and Amides with BpinBdan. Xiaocui Liu,[a] Wenbo Ming,[a] Xiaoling Luo,*[a,b] Alexandra Friedrich,[a] Jan Maier,[a] Udo Radius,*[a] Webster L.
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