Reactions of Cycloheptatriene Complexes

In addition to benzene rings, cycloheptatriene is activated or protected by forming the stable if complex 300. An example of the strong stabilization effected by coordination is shown by isolation of the optically active l,3,5-cycloheptatrien-3-ols 301, 304 and 305 as their enol forms. l,3,5-Cycloheptatrien-3-ol was isolated as complex 301 by hydrolysis of silyl enol ether 300. The triene system is stabilized by coordination, 

The cycloheptatriene complex undergoes cycloadditions with various unsaturated bonds, such as monoenes, dienes and alkynes, under mild conditions to afford interesting polycyclic compounds. In particular, the intramolecular version offers efficient methods for the construction of strained polycyclic skeletons, which are difficult to synthesize by other means [72]. 

Thus [6+2] cycloaddition of alkene with complex 306, bearing an optically active side chain, under irradiation at room temperature afforded the bicyclic compound 307 in 98% de [73]. According to the Woodward-Hoffmann rule, the [6+2] cycloaddition proceeds by irradiation, and is thermally forbidden. However, the cycloheptatriene complex 308 underwent 1,5-hydride shift, followed by [6+2] cycloaddition by heating, to give the tricyclic compound 309 in 90% yield [74], The cycloaddition was applied to the synthesis of /f-cedrene [75]. 

The bicyclo[4.4.1]undecane derivative 311 was obtained by [6+4] cycloaddition of 310 with a conjugated diene [72]. The [6+4] cycloaddition is a thermally allowed reaction, and free cycloheptatriene undergoes the [6+4] cycloaddition by heating. However, the [6+4] cycloaddition of cycloheptatriene coordinated by Cr(CO)3 proceeds at 0°C under irradiation. These results show the profound effect of coordination of Cr(CO)3 on reactivity. 

The [6+4] cycloaddition reaction was applied to the construction of a taxane skeleton [76], Adduct 314 was obtained by the reaction of complex 312 with diene 

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