Novel Catalyst System for Hydrostannation of Alkynes

Abstract

A catalyst system was developed for the highly regio- and stereoselective hydrostannation of a range of alkynes with tributylstannane under mild conditions. The active catalytic species was generated from a stable diruthenium complex by illuminating household fluorescent light (30 W) at room temperature.

Regioselective Formation of (E)‐β‐Vinylstannanes with a Topologically Controlled Molybdenum‐Based Alkyne Hydrostannation Catalyst

“…This observation illustrates that the electronic properties of the alkyne substrate can alter the regioselectivity governed by the inherent topological features of the catalyst system. It is also important to note that sterically biased propargyl alcohols can be converted into b-(E)-vinylstannanes with high regioselectivity (entries [14][15][16], as has been reported previously for other hydrostannation catalysts. [4] Accordingly, the scope of the regioselective alkyne hydrostannation to (E)b-vinylstannanes using complex 1 is expanded, rather than compromised, relative to other catalyst systems.T othis end, it is noteworthy that 1 produces the (E)-b-vinylstannane isomer of mifepristone (Scheme 2) with 2:98 a/(E)-b regioselectivity and in 98 %yield, thereby demonstrating that afair degree of molecular complexity can be accommodated by the system Scheme 2.…”

“…It is also important to note that sterically biased propargyl alcohols can be converted into b-(E)-vinylstannanes with high regioselectivity (entries [14][15][16], as has been reported previously for other hydrostannation catalysts. Of these latter examples,the conversion of 2-hexyne into the corresponding (E)-bvinylstannane with 9:91 a/(E)-b regioselectivity is particularly noteworthy (entry 12), as this result indicates that the molecular topology accessible from 1 can finely discriminate between methyl and propyl groups on opposing ends of an unsymmetrically substituted dialkyl alkyne.H owever,w hen electron-withdrawing substituents were present, as in the case of 4-cyano-1-butyne (entry 13), the regioselectivity of the hydrostannation process was again diminished.…”

“…[3,4] However,despite substantial progress in the development of alkyne hydrostannation catalysts,precise control over the hydrometalation regioselectivity continues to be an important challenge.S everal catalysts have now been reported that reliably provide a-vinylstannane products by catalyst-controlled hydrometalation (Scheme 1, top). [4,[12][13][14][15][16] Indeed, the only systems known to provide reasonable regioselectivities for (E)-b-vinylstannanes are highly encumbered palladium/phosphine catalysts that operate through af ormal Pd 0 /Pd II cycle. [4,[12][13][14][15][16] Indeed, the only systems known to provide reasonable regioselectivities for (E)-b-vinylstannanes are highly encumbered palladium/phosphine catalysts that operate through af ormal Pd 0 /Pd II cycle.…”

“…[3][4][5][6][7][8][9][10][11] In contrast, catalysts that deliver (E)-b-vinylstannanes by regioselective syn Sn À Haddition have been particularly difficult to develop for alkyne substrates that lack steric or electronic directing groups (Scheme 1, top). [4,[12][13][14][15][16] Indeed, the only systems known to provide reasonable regioselectivities for (E)-b-vinylstannanes are highly encumbered palladium/phosphine catalysts that operate through af ormal Pd 0 /Pd II cycle. [3,4,12,14,15] However,t he regioselectivity achieved with these systems is not general over awide scope of substituted alkynes, [4,15] and the systems lack the predictability associated with the most regioselective catalysts for the production of a-vinylstannanes.Ac ommonality among the Pd catalysts that show selectivity for (E)-b-vinylstannanes is the formation of as quare-planar cis-[Pd(C-alkenyl)(SnR 3 )L 2 ]i ntermediate during the hydrometalation process (Scheme 1, middle).Thes quare-planar coordination geometry of this intermediate positions ancillary ligands both cis and trans to the key C-alkenyl fragment that is generated upon initial hydride transfer to the alkyne substrate.Ithas been argued that when these catalyst systems are sufficiently encumbered, the regioselectivity for (E)-b-vinylstannanes originates from steric hindrance between the cis-ancillary ligand and the a-vinylstannane-producing C-alkenyl unit (Intermediate A Pd ; Scheme 1).…”

Hydrostannation of Alkynes

“…In contrast to all other transition-metal-catalyzed hydrostannation reactions documented in the literature, it was recently reported that ruthenium complexes 16-18 [29][30][31] (Scheme 9) provide unique antiselectivity across various types of internal alkyne substrates to afford (Z)-alkenylstannane product (Scheme 10). A mechanistic hypothesis explaining this unique transhydrostannation of symmetrical and unsymmetrical internal alkynes in the presence of a transition metal has been reported by Fürstner (Scheme 11).…”