Metal Bound or Free Ylides as Reaction Intermediates in Metal-Catalyzed [2,3]-Sigmatropic Rearrangements? It Depends

Abstract

Density functional theory calculations were applied to study four previously published metalcatalyzed [2,3]-rearrangements from onium ylide intermediates, in pursuit of generalizations about when, during these types of reactions, catalysts dissociate. Our results corroborate past studies where free ylide mechanisms were proposed to be operative. Calculations on case studies predict that the origin of metal-catalyst 'falling off' (dissociation) can be attributed primarily to the steric bulkiness of functional groups adjacent to the carbene carbon.

Melding of Experiment and Theory Illuminates Mechanisms of Metal-Catalyzed Rearrangements: Computational Approaches and Caveats

“…By delving into such previously published experimental studies, we set out to (a) confirm that our theoretical approaches could provide results consistent with experiments and (b) determine the physical factors that impact whether metal catalyst remains bound during the rearrangement step, thereby setting the stage for future reaction design. 41 In the four reactions examined, three with Rh and one with Au, we found that our calculations supported the conclusions from control experiments. Here, we utilized the (U)B3LYP/LANL2DZ[6-31G(d)] level of theory to investigate the mechanism (single-point calculations with dispersion correction and with other functionals produced qualitatively similar results).…”

“…161 In discussing the nature of the oxonium, sulfonium, and selenonium ylides in our study -wherein we found metalbound oxonium ylides and free sulfonium and selenonium ylides -we made the point that our conclusions "should not be generalized to all similar ylides undergoing [2,3]-rearrangements." 41 The work by Dang's group bolsters our observation that the nature of the ylide is system-dependent. Enabled by the work done by experimental (and other computational) groups, we seem to have converged on two key factors that determine whether an ylide intermediate is free or metal-bound: (a) steric bulk directly attached to the carbene carbon 41 and (b) the electronic nature of the heteroatom directly bound to the carbene carbon.…”

“…75,76 The choice of DFT methods is generally a practical one; 77-80 systems of the size (number of electrons) of interest here generally cannot be modeled in a reasonable amount of time with currently available post-Hartree-Fock WFT methods. [39][40][41][42][43][44][45] Much has been written on what recipe of functional and basis set is best to achieve 'chemical accuracy' (<1 kcal mol -1 ) for particular types of organometallic reactions, and we recommend several recent reviews for a more in-depth discussion on this topic. 63,71,81,82 We provide several examples below that highlight the importance of (a) knowing which functionals and basis sets have been validated for particular metals (here, mainly Rh, Cu, and Au) undergoing particular types of reactions (e.g., closed shell versus open shell processes), (b) which functionals and basis sets are affordable but sufficient for addressing conformational complexity and variability of ligand arrangements, and (c) which models of solvation, both implicit and explicit, are appropriate for answering the mechanistic questions at hand.…”

“…3,39 In addition, it is sometimes unclear which steps in the path to products actually involve a covalently bound metal. 3,40,41 Freeing such details from the shadows presents an opportunity for discovery.…”

1,3‐Difunctionalization of Imino‐Carbenes via Rhodium‐Catalyzed Reactions of Triazoles with Acyl Selenides

“…The reaction of organoselenium compounds with carbenes constitutes an important transformation to rapidly build‐up molecular complexity via sigmatropic rearrangement reactions [1–7] . In 1995, Uemura initially described the reaction of allyl selenides with aryldiazoacetates in the presence of copper catalysts [3] .…”