The regioselective silylation of C(sp3)–H bonds at the benzylic position in 4-alkylpyridines with hydrosilanes is described. The reaction proceeds in the presence of a catalytic amount of Ir4(CO)12 or Ir(acac)(CO)2, which possess CO as a ligand, or [Ir(OMe)(cod)]2 under 1 atm of CO. After optimizing the reaction conditions, by using other pyridine derivatives, such as 3,5-dimethylpyridine, as additives, the low product yields of 2-substituted 4-methylpyridines were improved markedly.Keywords: 4-alkylpyridine; C−H functionalization; hydrosilane; iridium catalyst; silylation;

The iridium-catalyzed C(sp3)–H silylation of 2-alkylpyridines with hydrosilanes at the benzylic position to afford 2-(1-silylalkyl)pyridines is described. The low product yield was markedly improved by adding 3,5-dimethylpyridine. Norbornene is also an essential additive for the reaction to proceed as a hydrogen scavenger. Carbon monoxide plays an important role in the catalytic cycle as a ligand. Other transition-metal carbonyls such as Rh4(CO)12 and Ru3(CO)12 can also be used as catalysts for this C–H silylation.

The combination of TpRh(C2H4)2 (Tp = tris(pyrazol-1-yl)borate) and P(2-furyl)3 catalyzes the reaction of tertiary alkyl-substituted alkynes with tert-butylhydrazine, leading to the formation of 3,3,3-trisubstituted propionitrile derivatives. This reaction system is applicable to 1,1-disubstituted propargyl alcohols and amines to afford the corresponding β-cyanohydrins and β-amino nitriles, respectively. The catalytic cycle involves the formation of a vinylidenerhodium complex as a key intermediate.

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.

The reaction of terminal alkynes with imines using ReBr(CO)5 as a catalyst results in the production of N-alkylideneallylamines and not the conventional propargylamines. The substituent on the imine nitrogen is important, and a diphenylmethyl group gave the best result. The catalytic cycle of this regioselective C–C bond forming reaction appears to involve the formation of an alkynyl rhenium species and subsequent nucleophilic attack of the alkynyl β-carbon atom on the imine carbon to give a vinylidene rhenium species.

The reaction of terminal alkynes with hydrosilanes and tert-alkyl isocyanides in the presence of Rh4(CO)12 gives (Z)-β-silyl-α,β-unsaturated imines in good yields. On the other hand, the use of aryl isocyanides in place of tert-alkyl isocyanides leads to the formation of E isomers.