Stereoselective Preparation of (Z)-α,β-Difluorostyrenes and Stereospecific Conversion to (E)-α,β-Difluoro-β-iodostyrenes1
摘要:
Substituted aromatic iodides coupled with (E)-HFC=CFZnI: (E:Z 95:5) under mild conditions, in the presence of catalytic Pd(PPh3)(4) in DMF to give (Z)-alpha,beta-difluorostyrenes in good yields. The coupling reaction was tolerant of a variety of functionalities. Isomerically pure (Z)-alpha,beta-difluorostyrenes were readily converted to the corresponding (E)-alpha,beta-difluoro-beta-iodostyrenes in good yields by two methods; in one method, treatment of the (Z)-alpha,beta-difluorostyrenes with LTMP at low-temperature gave vinyllithium reagents which were captured in situ with Bu3SnCl to form vinylstannanes. The intermediate vinylstannanes could be isolated or treated directly with I-2 to give (E)-alpha,beta-difluoro-beta-iodostyrenes in a one-flask procedure. In a second approach, n-BuLi was utilized to pregenerate vinyllithium reagents which were quenched with I-2 to afford (E)-alpha,beta-difluoro-beta-iodostyrenes.