New reactions of 1-alkynes catalyzed by transition metal complexes

Recently discovered catalytic reactions with ruthenium and lanthanide metal complexes have extended the scope of 1-alkynes as useful reagents. The specific formation of aryl-substituted (Z)-1,3-enynes via the dimerization of HC≡CR ¹ (R ¹ = aryl) has been attained using dimeric lanthanide complexes, the catalytic activity of which appears to be unaffected by time. The dimerization of HC≡CR ² (R ² = t-Bu, SiMe ₃) catalyzed by Ru(cod)(cot)/PR ₃ or RuH ₂(PPh ₃) ₃ produces a good yield of butatrienes (Z)-R ²CH=C=C=CHR ² with a high degree of selectivity. Under certain conditions, HC≡C-SiMe ₃ dimerizes to yield exclusively (Z)-M ₃Si-C≡C-CH= CH-SiMe ₃. The hydration of HC≡CR ³ (R ³ = alkyl, aryl) catalyzed by RuCl ₂/PR′ ₃ or CpRuCl(PR″ ₃) ₂ has realized the first example of anti-Markovnikov regioselectivity in an addition reaction of water that produces aldehydes R ³CH ₂-CHO. The application of this reaction to propargylic alcohols has lead to their formal isomerization to α,β-unsaturated aldehydes. In contrast, the addition of amines R ⁴-NH ₂ (R ⁴ = aryl) to HC≡CR ⁵ (R ⁵ = alkyl, aryl) conforms to Markovnikov's rule to produce ketimines R ⁵-(C=NR ⁴)-CH ₃ when catalyzed by a Ru ₃(CO) ₁₂/additive. Since the reaction can be performed in air without the need for any solvents, it enables the practical synthesis of aromatic ketimines, which are difficult to prepare by conventional methods. The synthesis of indoles using deactivated anilines is one practical application of this reaction. The mechanisms of some of these reactions have been analyzed in detail with the aid of theoretical calculations.