Allenes, cycloaddition hydroamination

The metallocene complex 27 containing a M=X double bond undergoes overall [2 + 2] cycloaddition with an internal alkynes to give heterometallacyclobutenes (28) [77], A formal [2 + 2] cycloaddition of CpjZr (=N Bu)(thf) with imine affords a 2,4-diazametallacyclobutane, whose further reaction with imines results in an imine metathesis reaction [78] azametallacyclobutene is an intermediate in the Cp2Zr(NHR)2-assisted hydroamination of alkynes and allene [79],... 

Cycloadditions are among the more common reactions of imido and 0x0 groups in catalytic transformations. Both [2+2] cycloadditions and [3+2] cycloadditions are well established. The [2+2] cycloaddition chemistry occurs during hydroaminations of alkynes, allenes, and strained olefins catalyzed by imido compounds. - The [3+2] cycloaddition reactions appear to occur during dihydroxylation and aminohydroxylation of olefins catalyzed by osmium complexes. ... 

Examples of the [2+2] cycloadditions and the mechanisms of these processes were presented in detail in Chapter 13 on complexes containing metal-ligand multiple bonds. In short, coordination of the alkyne or allene precedes the [2+2] cycloaddition. This cycloaddition is thermodynamically favorable for aikynes and allenes, but is thermodynamically disfavorable for reactions of alkenes. Studies on the regioselectivity of the stoichiometric [2+2] cycloaddition and of the regioselectivity of zirconocene-catalyzed hydroamination revealed that the [2+2] process is reversible during the hydroaminations catalyzed by zir-conocene complexes. Moreover, it has been shown that addition of an alkyne to an isolated zirconocene azametallacyclobutene leads to exchange. 

In the hydroamination of unsaturated carbon-carbon bonds, gold catalysts play an important role. Intermolecular hydroamination of alkenes [177], 1,3-dienes [204], terminal and internal alkynes [205], and allenes [206] are known to proceed smoothly in the presence of PhsP AufI) or AuCls catalyst. In addition, amino olefins also efficiently undergo intramolecular hydroamination using similar gold catalysts. He and coworkers have developed the catalytic cycloaddition of tosylated amino olefins [207], A representative example is shown in Scheme 18.35. When N-tosylated y-amino olefin (97) is exposed to a mixture of PhsP AuCl and AgOTf (5 mol% each) in toluene at 85 °C, pyrrolidine (98) is obtained in 96% yield. The gold(I)-catalyzed intramolecular hydroamination is applicable to N-alkenyl carbamates [208], N-alkenyl carboxamides [209], and N-alkenyl ureas [210], The use of microwave irradiation results in completing the hydroamination in a much shorter time than that required under thermal reaction conditions [211], The... 

The mechanism of the group 4 metal-catalyzed hydroamination of alkynes (Scheme 4) and allenes has been thoroughly investigated in detailed kinetic and mechanistic [41 6] as well as computational studies [47, 48]. The catalytically active species is believed to be a metal imido complex, which undergoes a reversible, rate-determining [2 + 2]-cycloaddition with an alkyne, respective allene, to yield an azametallacyclobutene species. Subsequent protonolysis leads to the imine hydroamination product. Isolation and characterization of intermediate azacyclobutene species, which are catalytically competent themselves, is a strong support of the mechanism [41,43,49-51]. Although no direct experimental [52, 53]... 

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