Mechanism of the Gold-Catalyzed Rearrangement of (3-Acyloxyprop-1-ynyl)oxiranes: A Dual Role of the Catalyst

Abstract

The three competing paths for the rearrangement of 1 (involving 1,2- and 1,3-ester migration with alkyne or oxirane activation) evidence the multifaceted character of gold as a catalyst. The most favorable mechanism for this useful synthetic transformation involves a cascade of more than eight steps. All the functional groups in the substrate play a crucial and synergistic role, and sequential gold coordination to both the pi-system and the lone pairs of oxygen is needed. Exploration of these three paths suggests the use of a nonalkynophilic Lewis acid (BF(3)) as a possible synthetic alternative for this transformation.

Gold(I)-Catalyzed Activation of Alkynes for the Construction of Molecular Complexity

“… 306 Alkynyloxiranes 365 bearing a propargylic ester rearrange to form divinyl ketones 366 (Scheme 115 ). 307 The mechanism of this transformation seems to proceed via anchimeric assistance of the propargyl ester moiety, although DFT calculations predict a complex mechanism involving several equilibriums.…”

Characterization of pericyclic steps in the mechanisms of Gold(I) catalyzed rearrangement of alkynes

“…In other cases where the Au(I) is attached to a Csp 2 atom of the carbon skeleton (Figure 3), the activation energies for the electrocyclic process decrease considerably relative to the nonmetallic counterpart 63. We have also noted this effect in the rearrangement of oxiranyl propargylic esters to divinylketones 32,74. Three different mechanisms were studied (B3LYP/6‐31G(d)‐LANL2DZ with Au(I)PH 3 ) to explain this intriguing cascade process.…”

“…The typical Claisen rearrangement is a [3,3]‐sigmatropic process in which an allyl vinyl ether substructure is thermally converted to a γ,δ‐unsaturated carbonyl derivative via a concerted six‐membered chair‐like transition state (TS) that displays aromatic features. The first step in the Au(I)‐catalyzed transformation of the popular propargylic esters (1,3‐migration in Scheme ) has been characterized as a stepwise process 32,43,47–49. In contrast, the intermolecular carboauraoxidation of an Au(I)‐activated alkyne by an aryl methyl sulfoxide has been proposed to formally evolve through a Claisen‐like rearrangement of very low activation energy (∼4 kcal/mol) 41,46.…”

“…Finally, there are some transformations in which the presence of an Au(I) catalyst acting as a Lewis acid fail to promote the Nazarov reaction at ambient temperatures 32,74 as in the rearrangement of oxiranyl propargylic esters to 2‐acetoxydivinyl ketones after 1,3‐acetate migration (Scheme A), for which an activation energy of 25.9 kcal/mol was computed (which is, however, almost 15 kcal/mol lower than the unpromoted version).…”

“…In addition to the primary process (the 1,3‐rearrangement of propargylic acetates, a [3,3]‐Claisen‐like rearrangement) other pericyclic reactions29–31 have been proposed to follow the initial activation of the alkyne by the Au(I) catalyst. Being in general multistage mechanisms, in particular when densely functionalized substrates are used,32 it is possible to intersect unsaturated intermediates that display the electronic features of a pericyclic array of orbitals. Thus, it has become of undoubtful interest to determine the pericyclic nature of Au(I)‐catalyzed rearrangements, characterize their degree of concert and establish general principles by which Au(I) influences the energetics and selectivity of the rearrangements.…”