Highly Regioselective and Stereoselective Hydrostannylation of (Z)-2-Ethoxycarbonyl-1,3-Enynes Leading to (1E,3E)-2-Ethoxycarbonyl-3-Stannyl-1,3-Dienes

Abstract

The Stille coupling of (E)-α-stannyl-α,β-unsaturated esters with alkynyl bromides in DMF in the presence of Pd(PPh3)4 and CuI gave (Z)-2-ethoxycarbonyl-1,3-enynes in good yields. The latter underwent the palladium-catalysed hydrostannylation with tributyltin hydride in THF at room temperature to afford (1E,3E)-2-ethoxycarbonyl-3- stannyl-1,3-dienes in good yields with high regio- and stereoselectivities.

Hydrostannation of Alkynes

“…Inverting ester and alkyl groups on the double bond of enynes had no influence on the α-regioselectivity as (1E,3E)-2ethoxycarbonyl-3-tributylstannyl-substituted 1,3 dienes were obtained as single isomers (Scheme 42). 84 A similar trend in α-regioselectivity was obtained with conjugated enynes bearing a sulfonyl substituent on the double bond, affording 1-sulfonyl-3-tributylstannyl-substituted 1,3dienes. 85 Other conjugated enynes also function well in regioselective hydrostannations.…”

“…Inverting ester and alkyl groups on the double bond of enynes had no influence on the -regioselectivity as (1E,3E)-2ethoxycarbonyl-3-tributylstannyl-substituted 1,3 dienes were obtained as single isomers (Scheme 42). 84  Scheme 42. Pd-catalyzed hydrostannation of internal enynes.…”

“…In transition-metal-catalyzed reactions, palladium complexes are the catalyst of choice for hydrostannation of alkynes if E -alkenylstannanes are desired. In addition to the most widely used palladium complexes Pd­(PPh 3 ) 4 and PdCl 2 (PPh 3 ) 2 , many other palladium sources, with or without phosphine ligands, that have been successfully employed including Pd­(OAc) 2 /PPh 3 , Pd­(OH) 2 /C, PdCl 2 (dppe), Pd 2 (dba) 3 /PAr 3 , or Pd 2 (dba) 3 /Cy 3 PHBF 4 . Contrary to early work, the type of ligand used dramatically affects the regioselectivity of Pd-catalyzed hydrostannation of terminal alkynes .…”