Transition metal catalyzed alkyne hydroamination catalyst

TRANSITION-METAL-CATALYZED HYDROAMINATION OF OLEFINS AND ALKYNES Table 16.6. Effect of catalyst precursor on the intramolecular hydroamlnation In Equation 16.61. 

The hydroamination of alkynes catalyzed by group 4 complexes were some of the first transition-metal-catalyzed hydroamination reactions. One example of these reactions is shown in Equation 16.84. The reactions only occur with hindered amines and are slow. Nevertheless, the reactions occur m high yield and, in the absence of air, the catalysts are stable indefinitely. An early intramolecular reaction catalyzed by CpTiClj to form a cyclic enamine is shown in Equation 16.85. Reactions with internal alkynes occur to form products with Markovnikov regiochemistry. ° As described in more detail below, tliese reactions occur by [2+2] additions of the alkyne to an intermediate metal-imido complex. 

The excellent ability of late transition metal complexes to activate alkynes to nucleophilic attack has made them effective catalysts in hydroamination reactions. The gold(l)-catalyzed cyclizations of trichloroacetimidates 438, derived from homopropargyl alcohols, furnished 2-(trichloromethyl)-5,6-dihydro-4f/-l,3-oxazines 439 under exceptionally mild conditions (Equation 48). This method was successfully applied to compounds possessing aliphatic and aromatic groups R. With R = Ph, cyclization resulted in formation of 439 with complete (Z)-stereoselectivity <2006OL3537>. 

The hydroamination of alkenes and alkynes has been of longstanding interest in organometallic chemistry [26]. Much of the early work in this area focused on early transition metal or lanthanide metal catalyst systems. However, much recent progress has been made in late-metal catalyzed hydroamination chemistry, and several interesting hydroamination reactions that afford nitrogen heterocycles have been developed using palladium catalysts. 

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