Olefin metathesis over a Re2O7/Al2O3 metathesis catalyst: Mechanism for initial metallacarbene formation

Rhenium(VII) Oxide/Aluminum Oxide: More Experimental Evidence for an Oxametallacyclobutane Intermediate and a Pseudo‐Wittig Initiation Step in Olefin Metathesis

“…[19] Later, no degenerate metathesis was observed when using a mixture of 13 C-di-labelled and unlabelled ethene: the non-reactivity of ethene inferred that it is necessary to have an allylic hydrogen in order to generate the active species. [7] More recent studies have shown that rhenium alkylidene complexes can be generated from the corresponding glycolate through an oxarhenacyclobutane, suggesting that the pseudoWittig pathway can indeed be a viable pathway of initiation. [15][16][17] Here below, a combined reactivity study of various olefins with Re 2 O 7 /Al 2 O 3 shows that this catalyst contains only 2 % of active sites, and the propagating alkylidene does not need to be formed through a p-allyl intermediate, so that the pseudoWittig reaction is a preferred initiation mechanism.…”

“…[15][16][17] Here below, a combined reactivity study of various olefins with Re 2 O 7 /Al 2 O 3 shows that this catalyst contains only 2 % of active sites, and the propagating alkylidene does not need to be formed through a p-allyl intermediate, so that the pseudoWittig reaction is a preferred initiation mechanism. , [7] and shows that propene can generate the propagating carbene, while ethene cannot [Eqs. (1) and (2) First, the formation of 2-pentenes (0.018 equiv.…”

“…[2][3][4] The mechanism of olefin metathesis is now well-accepted, [5] and, despite years of research, the nature of the active site of Re 2 O 7 /Al 2 O 3 is still unknown, probably because of the low content of active Re (2 %), which has precluded a molecular understanding of this phenomenon. [6] In fact, several possible initiation steps have been proposed to explain the formation of the necessary propagating carbene species (Scheme 1): [7] a) H-assisted alkylidene formation, [8] …”

“…) À H bond followed by a s-vinyl-alkylidene transposition, [9,10] c) C À H activation of an allylic C À H bond followed by the transformation of the p-allyl hydride intermediate into a metallacyclobutane (p-allyl mechanism) [7,11,12] and d) a pseudo-Wittig reaction. [13][14][15][16][17] Note that cases a, b and c require one to start with a reduced Re species.…”

“…[7,18,19] For example, in 1980 the degenerate metathesis of ethene, observed from a C 2 D 4 /C 2 H 4 mixture, on Re 2 O 7 /Al 2 O 3 was described, thus inferring a pseudo-Wittig pathway. [19] Later, no degenerate metathesis was observed when using a mixture of 13 C-di-labelled and unlabelled ethene: the non-reactivity of ethene inferred that it is necessary to have an allylic hydrogen in order to generate the active species.…”