Application of Fluoride-Catalyzed Silane Reductions of Tin Halides to the in Situ Preparation of Vinylstannanes

Abstract

We have found that either Bu 3 SnCl/PMHS/KF (aq) or the combination of tributyltin fluoride, PMHS, and catalytic quantities of tetrabutylammonium fluoride can serve as in situ sources of tributyltin hydride for both free radical and palladium-catalyzed hydrostannylation reactions. These methods are tolerant of a variety of functional groups, including silyl ethers. Furthermore, Me 3 SnCl is also reduced under these conditions, providing a relatively convenient and safe manner by which Me 3 -SnH can be formed and reacted. We have also observed that the Bu 3 SnCl/siloxane/fluoride combination offers improvements over the existing protocols for transforming 1-bromoalkynes into trans-1-(tributylstannyl)-1-alkenes. Specifically, the KF/PMHS methodology allows the 1-bromoalkyne to be formed and reacted in a single pot and with substoichiometric amounts of tin. Finally, alternative reductants such as Red-Sil are also amenable to the method.

Ni, Co, and Mo-catalyzed alkyne hydrostannations using Bu3SnCl/PMHS/KF/18-crown-6 as an in situ Bu3SnH source

“…7 Our own contributions to these efforts include the establishment of Bu 3 SnCl/polymethylhydrosiloxane (PMHS)/aqueous KF as an in situ source of Bu 3 SnH for radical or palladium catalyzed alkyne hydrostannations. 7d This PMHS mediated hydrostannation protocol was integral to the invention of a one-pot hydrostannation/Stille coupling sequence that was catalytic in tin.…”

“…7 Our own contributions to these efforts include the establishment of Bu 3 SnCl/polymethylhydrosiloxane (PMHS)/aqueous KF as an in situ source of Bu 3 SnH for radical or palladium catalyzed alkyne hydrostannations. 7d This PMHS mediated hydrostannation protocol was integral to the invention of a one-pot hydrostannation/Stille coupling sequence that was catalytic in tin. 9 In conjunction with efforts aimed at developing the next generation of Stille couplings that are catalytic in tin, we became interested in mixed catalysts systems where the hydrostannation and cross-coupling steps were catalyzed by different metals.…”

One-pot Pd-catalyzed hydrostannation/Stille reaction with acid chlorides as the electrophiles

“…Despite the well-established power of the Stille reaction, there are negative issues associated with handling the often unstable and/or toxic organostannanes used in these couplings [2]. To obviate direct manipulation of the stannane coupling partners, our group has developed one-pot Pd-catalyzed hydrostannation/Stille coupling sequences [3] that begin with the in situ generation of triorganotin hydrides [4]. The hydrides so formed react in situ with alkynes to form vinylstannanes, which without isolation undergo Stille cross-coupling reactions (Scheme 1).…”

“…Nonetheless, provided these problems could be defeated, achieving the synthesis of various a,b-unsaturated ketones from alkynes and acid chlorides in a single pot using an organotin salt as the initial tin source, a single load of catalyst, and unpurified vinyltin intermediates would be attractive. In starting our exploration of this putative one-pot sequence, we opted to use an ''anhydrous'' variation for the in situ generation of tributyltin hydride [4]. Thus, Bu 3 SnF, PMHS, and a catalytic amount of TBAF were reacted in the presence of an alkyne and an acid chloride.…”

“…However, for reasons that remain unclear, that substrate did not work well in the hydrostannation/crosscoupling sequence. Another option involved running the hydrostannation step under free radical conditions [4] and then adding the acid chloride along with a Pd-catalysts to carry out the second step. Owing to the volatility [2] of Me 3 SnH, we chose to run the radical hydrostannation with Bu 3 SnH.…”