Highly regio- and Stereoselective Diels–Alder Cycloadditions of α-Alkoxyvinyl(ethoxy)carbene complexes with exo-heterocyclic dienes

Abstract

A comparative study of reactivity and selectivity was carried out between the α-alkoxyvinyl(ethoxy)carbene complexes of group 6 metals (CO)5M = C[C(=CH2)(OR)]OEt [1a–c: M = Cr; 6a–c: M = Mo; 7a–c: M = W; a, R = Et; b, R = n–Pr; c, R = n–Bu] and exo-heterocyclic dienes 5a–e in Diels–Alder reactions. The cycloadditions of complexes 1, 6 and 7 with the unsubstituted diene 5a (R1 = H) were performed at 25 °C, proving to be highly regioselective in favor of the para pentacarbonyl cycloadducts 8, 10 and 12. On the other hand, the cycloadditions of complexes 1a–c and 6a–c with the substituted dienes 5b–e (R1 = Me or Et) took place at a higher temperature (50 °C) to afford the novel para-endo C,O-chelated tetracarbonyl cycloadducts 15a–f and 17a–d, respectively, in high regio- and stereoselectivity. DFT calculations of the transition states rationalized their selectivity. Under the same reaction conditions, the W(0) carbene complexes 7a–c produced the para-endo non-chelated pentacarbonyl complex adducts 19a–e. The stereochemical assignment of the cycloadducts was supported by NOE measurements. The C,O-chelated tetracarbonyl cycloadduct 15b and non-chelated cycloadduct 19d were further characterized by single-crystal X-ray diffraction.