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Enhanced Electrophilicity of Heterobimetallic BiRh Paddlewheel Carbene Complexes: A Combined Experimental, Spectroscopic and Computational Study Lee R. Collins, Maurice van Gastel, Frank Neese, and Alois Fürstner J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08384 • Publication Date (Web): 14 Sep 2018 Downloaded from http://pubs.acs.org on September 16, 2018
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Enhanced Electrophilicity of Heterobimetallic Bi-Rh Paddlewheel Carbene Complexes: A Combined Experimental, Spectroscopic and Computational Study Lee R. Collins, Maurice van Gastel, Frank Neese,* and Alois Fürstner* Max-Planck-Institut für Kohlenforschung, Mülheim/Ruhr 45470, Germany ABSTRACT Dirhodium paddlewheel complexes are indispensable tools in modern organometallic catalysis for the controlled decomposition of diazo-compounds. Tuning the reactivity of the thus-formed transient carbenes remains an active and dynamic field of research. Herein, we present our findings that the distal metal center plays an as yet underappreciated role in modulating this reactivity. Replacement of one rhodium atom in the bimetallic core for bismuth results in the formation of a significantly more electrophilic carbene complex. Bismuth-rhodium catalysts thereby facilitate previously unknown modes of reactivity for α-diazoester compounds, including the cyclopropanation of alkenes as electron deficient as trichloroethylene. While dirhodium paddlewheel complexes remain the catalysts of choice for many carbene-mediated transformations, their bismuth-rhodium analogues exhibit complimentary reactivity and show great potential for small molecule and solvent activation chemistry. DFT calculations highlight the importance of metal-metal bonding interactions in controlling carbene electrophilicity. The paucity of these interactions between the 4d orbitals of rhodium and the 6p orbitals of bismuth results in weaker π-back-bonding interactions for bismuth-rhodium carbene complexes compared to dirhodium carbene complexes. This leads to weakening of the rhodium-carbene bond and to a more carbene-centered LUMO, accounting for the observed enhancement in bismuth-rhodium carbene electrophilicity. These findings are supported by a detailed spectroscopic study of the 1
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‘donor-donor’ carbene complexes Rh2(esp)2C(p-MeOPh)2 (19) and BiRh(esp)2C(p-MeOPh)2 (20), employing a combination of UV-Vis and resonance Raman spectroscopy. The results reveal that carbene chemoselectivity in MRh(L)4 catalysis can be modulated to be previously unrecognized extent by the distal metalloligand. INTRODUCTION The discovery nearly fifty years ago that Rh2(OAc)4 (A, R = Me) catalytically decomposes diazo compounds opened a broad and exciting field of research.1 The electrophilic character of the thus formed carbene intermediates has since been exploited in a wide range of catalytic reactions, including cyclopropanation, C-H functionalization, cycloadditions, carbon-heteroatom insertions and ylide-type chemistry.2 Replacement of acetate with more functionalized bridging ligands has led to the development of a plethora of (chiral) catalysts whose properties and selectivities provide the modern-day synthetic chemist with a diverse tool-box from which to choose.2,3 Decades later, research continues toward understanding the nature of these electrophilic carbene intermediates and the rationalization of their reactivity and selectivity patterns. Before direct spectroscopic evidence became available for these highly reactive intermediates, numerous experimental and computational studies were conducted.4 These studies converge on a frontier orbital description of dirhodium carbenes as consisting of a singlet carbene interacting in a σ-donating and π-accepting manner with the dirhodium core, in analogy to Fischer carbenes. However, in contrast to Fischer carbenes, dirhodium carbenes display enhanced electrophilic reactivity, being able to activate nucleophiles as weak as C-H bonds. While this is part due to the absence of heteroatom stabilization,5 work by the groups of Nakamura and Berry has identified
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the importance of σ and π 3c4e bonding interactions in making dirhodium carbenes ‘superelectrophilic’.4c,6
Figure 1. Prototype paddlewheel complexes to be discussed herein; L = kinetically labile two-electron donor ligand
Recently, dirhodium carbene complexes bearing one (B) or two electron donating groups (C) were characterized spectroscopically and crystallographically for the first time.7,8 These data provide clarity to the role of the carboxylate framework and carbene substituents in determining the trajectory of an incoming nucleophile and thus help to rationalize the stereoselectivity profiles observed in catalysis. While variation of the bridging ligands remains the dominant method by which Rh(II) carbene reactivity and selectivity are tuned, increasing attention is being paid to the possible role of axial ligands.9 Incorporation of a Lewis base (either as solvent, co-solvent or stoichiometric additive) can have dramatic consequences on chemo-, diastereo- and enantioselectivity.10 For example, the Jessop and Davies groups reported that coordinating solvents or addition of methyl benzoate increase enantioselectivity in donor-acceptor cyclopropanation reactions of styrene.10d,e These effects may be rationalized by invoking axial coordination. A conclusive explanation, however, is difficult to reach due to the lability of axial 3
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ligands and the implicit uncertainty over what is actually coordinated in-situ to the distal metal at the instant of carbene formation and/or its encounter with the reaction partner (Figure 2).11
Figure 2. Possible equilibria involved in Rh(II) carbene formation and reactivity. L = neutral, two electron donor ligand.
Alternative modifications to the dirhodium core include oxidation to mixed valence Rh2(II,III) complexes,9,12 or to even replace the distal rhodium atom itself with a different metal. The only such heterobimetallic MRh(L)4 complexes reported in the literature are those where M = Bi (D), first prepared by Dikarev in 2005.13 These complexes were found to display Lewis acidic behavior only at the rhodium face, thus providing a system with no ambiguity over the nature of the distal metal center and its state of ligation.13a,13b,14 Despite the arguably rather dramatic change that the incorporation of a main group element in lieu of the second Rh atom might entail, such bismuth-rhodium complexes were described to exhibit qualitatively similar reactivity to their dirhodium counterparts in a series of cyclopropanation and C-H activation reactions with donor-acceptor type carbenes; the most significant difference was the much reduced rate of diazo decomposition (by a factor of ~103).15 Seeming to offer no advantage over more readily available dirhodium complexes, no follow up studies of bismuth-rhodium carbene chemistry have been reported, to date. 4
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Figure 3. Complexes employed as catalysts in this study. M = Rh or Bi.
During our research into controlling carbene mediated chemistry,8,16 we have revisited these heterobimetallic complexes and their reactivity with acceptor-type α-diazoesters (Figure 3). In addition to the kinetic differences observed before,15 our results very clearly show that changing the bimetallic core actually results in strikingly divergent reactivity profiles. Bismuth-rhodium based acceptor carbene complexes of type E (Figure 4, M = Bi) are even more electrophilic than their already ‘superelectrophilic’ dirhodium analogues.6 This extreme character opens new vistas in solvent activation and the functionalization of small molecules with very little precedent and therefore offers a complimentary reactivity profile to established dirhodium complexes (E, M = Rh). Furthermore, BiRh(esp)2 is found to be a remarkably robust catalyst for these purposes, providing high isolated yields with
