Synthesis of α-Silylmethyl-α,β-Unsaturated Imines by the Rhodium-Catalyzed Silylimination of Primary-Alkyl-Substituted Terminal Alkynes

Abstract

In contrast to our previous report on the rhodium-catalyzed reaction of terminal alkynes with equimolar amounts of hydrosilanes and isocyanides leading to (E)- or (Z)-β-silyl-α,β-unsaturated imines A, the addition of an excess molar amount of hydrosilanes relative to isocyanides in the reaction of primary-alkyl-substituted terminal alkynes results in the production of α-silylmethyl-α,β-unsaturated imines B. Various isocyanides bearing tert-butyl and 1-adamantyl groups gave B with good product selectivity. Z isomers were formed stereoselectively in many cases. Regarding the mechanism for this reaction, when the hydrosilane was added to the reaction mixture in two portions, unsaturated imines A were initially formed, which then underwent double-bond isomerization, probably catalyzed by a Rh-H species, to give B.

Dual Palladium/Copper-Catalyzed anti-Selective Intermolecular Allenylsilylation of Terminal Alkynes: Entry to (E)-Silyl Enallenes

“…The alkene difunctionalization reaction is among the most powerful methodologies to economically and rapidly synthesize complex alkene-based structures from readily available feedstocks in synthesis. , Particularly, those reactions that execute intermolecular difunctionalization of alkynes have additional important values because they can bond the seams of three components together in the desired combinations to create CC bond networks and exquisitely control the stereo-, regio-, and chemoselectivity. , Typically, the selective silyl-carbofunctionalization of alkynes that consists of silylation of alkynes with organosilicon reagents followed by carbofunctionalization with carbon donor reagents is attractive and can offer an innovative solution to access versatile vin-2-ylsilane scaffolds, thereby providing the potential to further orient derivatization and increase structural diversity. − Since Pastor pioneered a Rh 4 (CO) 12 -catalyzed silylformylation of alkynes with Me 2 PhSiH and CO (10–30 kg/cm 2 ) in 1989, transition-metal-catalyzed carbosilylation of alkynes with trisubstituted silanes and carbon donors (such as high-pressure CO and alkyl isocyanides) has been well investigated for constructing various functionalized vinylsilanes, mainly through initially manipulating the regioselective silylation of alkynes and then carbofunctionalization (Scheme a). , However, these approaches suffer from unsatisfactory stereoselectivity with a mixture of Z and E isomers and narrow carbon donors. To achieve high stereoselectivity, Murai and co-workers have developed a Pd­(PPh 3 ) 4 catalysis that uses a combination of trisubstituted silyl iodide electrophiles and organometallic nucleophiles (such as organotin and organozinc reagents) to accomplish the syn -selective silyl-carbofunctionalization of terminal alkynes (Scheme b,c). , Very recently, Watson and co-workers found that the ligand effect could control the stereoselectivity of palladium-catalyzed carbosilylation of internal symmetrical alkynes with trisubstituted silyl iodides and primary alkyl zinc iodides.…”

“…Consequently, a possible mechanism was proposed (Scheme ). − Oxidative addition of propargyl acetate 2a with the active Pd(0) species affords the allenyl-PdOAc intermediate A due to its activation by the gem -aryl electronic effect . This is the reason that alkyl-substituted propargyl acetates have no reactivity during the reaction.…”

Versatile Access to Tetrasubstituted 2-Amidoacroleins through Formal Silylformylation of Ynamides

“…In this global context, inspired by Fukumoto’s and Chatani’s work on terminal alkynes, we have decided to develop the formal silylformylation of ynamides using readily available reagents such as silanes and isocyanides to make it as practical as possible. Our choice to avoid carbon monoxide was additionally dictated by the dangerousness of this gas as well as by the need to use expensive specific equipment (high-pressure reactor, detector).…”

“…Based on previous reports on silylformylation reactions, a speculative mechanism can be proposed (Figure ). ,, After an activation step of Rh 4 (CO) 12 precatalyst, the [Si–Rh] newly formed complex could proceed to a silyl-metalation to lead to the intermediate I . The high regioselectivity could be explained by a combined effect of the polarization of the C–C triple bond and a possible coordination of rhodium to the electron-withdrawing group carried by the nitrogen of the ynamides. , Then the insertion of the isocyanide could lead to the acyl-rhodium II which can then react with the silane to form a complex that will undergo a reductive elimination to form the targeted imine 2 while regenerating the active [Si–Rh] catalyst.…”