Generation and spectroscopic observation of a tungsten-carbene-alkene intermediate in cyclopropane formation

Article abstract of DOI:10.1021/om00123a021

The alkoxide-catalyzed reaction of but-3-en-1-ol with (CO)₅W=C(OCH₃)C₆H₄-p-CH₃ gives the (butenyloxy)carbene complex (CO₅)W=C(OCH₂CH₂CH=CH₂)C ₆H₄-p-CH₃, 1, which possesses a carbene ligand and an uncomplexed alkene. Upon heating at 38°C in benzene-d₆, 1 is converted to the cyclopropane 1-(4-methylphenyl)-2-oxabicyclo[3.1.0]hexane, 10. During the conversion of 1 to 10, an intermediate tetracarbonyltungsten-carbene-alkene complex 15 was detected by ¹H NMR and IR spectroscopy. Intermediate 15 reacted with CO to regenerate 1 and with PEt₃ to give cis-(CO)₄(PEt₃)W=C-(OCH₂CH ₂CH=CH₂)C₆H₄-p-CH₃,18. The kinetics of the conversion of 1 to 10 are very complex. After an initial induction period, an accelerating autocatalytic decomposition of 1 occurs. A detailed mechanism for the autocatalysis is proposed in which the key steps are decomposition of tungsten-carbene-alkene intermediate 15 which reacts successively with two molecules of starting material 1 to generate W(CO)₆ and 2 equiv of tungsten-carbene-alkene intermediate 15.