[2+2] versus [3+2] Addition of Metal Oxides Across CC Double Bonds:  Toward an Understanding of the Surprising Chemo- and Periselectivity of Transition-Metal-Oxide Additions to Ketene

Abstract

The peri-, chemo-, stereo-, and regioselectivity of the addition of the transition-metal oxides OsO4 and LReO3 (L = O-, H3PN, Me, Cp) to ketene were systematically investigated using density-functional methods. While metal-oxide additions to ethylene have recently been reported to follow a [3+2] mechanism only, the calculations reveal a strong influence of the metal on the periselectivity of the ketene addition: OsO4 again prefers a [3+2] pathway across the C=C moiety whereas, for the rhenium oxides LReO3, the [2+2] barriers are lowest. Furthermore, a divergent chemoselectivity arising from the ligand L was found: ReO4- and (H3PN)ReO3 add across the C=O bond while MeReO3 and CpReO3 favor the addition across the C=C moiety. The calculated energy profile for the MeReO3 additions differs from the CpReO3 energy profile by up to 45 kcal/mol due to the stereoelectronic flexibility of the Cp ligand adopting eta5, eta3, and eta1 bonding modes. The selectivity of the cycloadditions was rationalized by the analysis of donor-acceptor interactions in the transition states. In contrast, metal-oxide additions to diphenylketene probably follow a different mechanism: We give theoretical evidence for a zwitterionic intermediate that is formed by nucleophilic attack at the carbonyl moiety and undergoes a subsequent cyclization yielding the thermodynamically favored product. This two-step pathway is in agreement with the results of recent experimental work.

Road Maps for Nitrogen‐Transfer Catalysis: The Challenge of the Osmium(VIII)‐Catalyzed Diamination

“…The calculated reaction free enthalpies of the metalla‐analogous cycloadditions are also presented in Table 1. In contrast to the addition of rhenium( VII ) oxides across CC bonds,9, 10 each activation free enthalpy reported in this work is in agreement with the thermodynamics of the reaction 18. The [2+2] additions are predicted to be endergonic, while the [3+2] additions are exergonic (Table 1).…”

“…Addition of oxo–imido complexes across CC bonds : The mechanism of the initial step of cis ‐dihydroxylation, the addition of osmium tetraoxide across a CC bond, had been controversial for several decades, until quantum‐chemical calculations together with experimental kinetic isotope effects showed a concerted [3+2] addition to be preferred over a stepwise mechanism (Scheme ) 12. 13 While the reaction of (pentamethylcyclopentadienyl)(trioxo)rhenium( VIII )10, 14 and [tris(3,5‐dimethyl pyrazolyl)](borato)(trioxo)rhenium( VIII )15 with CC bonds also follows a [3+2] mechanism, recent quantum‐chemical10 and experimental16 investigations revealed that certain “spectator” ligands L in LReO 3 may invert the relative height of the [3+2] and [2+2] barriers. Little is known about the mechanism if a “reactive” oxo ligand itself in metal oxides, such as osmium tetraoxide, is replaced by an imido ligand.…”

Chemo‐ and Periselectivity in the Addition of [OsO2(CH2)2] to Ethylene: A Theoretical Study

“…First of all we note an unexpected trend in the theoretically predicted activation barriers. The calculated activation energy for the [3+2] addition across the O=Os=O moiety of 1 is rather high (27.5 kcal mol À1 , TS1!2), much higher than the previously calculated barriers for the [3+2] addition across the O=Os=O moieties of OsO 4 (11.8 kcal mol À1 ) [18] and [OsO 2 (NH) 2 ] (8.3 kcal mol…”

Matrix isolation infrared spectroscopic and theoretical study of the transition metal (Mn and Fe) dioxide–ethylene complexes

“…In recent years, tremendous research efforts focused on the high-valence transition metal oxide additions across C@C bond of olefins, since there is considerable industrial interest in the activation of C@C double bonds by readily accessible metal oxides [14,15]. For example, permanganate serves to convert olefins to diols, ketones and carboxylic acids under mild condition [16].…”