On the Mechanism of Trimethylphosphine-Mediated Reductive Dimerization of Ketones

Abstract

High-level single-reference calculations reveal that trimethylphosphine-mediated reductive dimerization of properly substituted (e.g., CF) ketones proceeds via initial formation of an oxaphosphirane intermediate, with the oxygen atom occupying an equatorial position at phosphorus. In the "oxirane route", this oxaphosphirane intermediate loses a trimethylphosphine oxide unit, thus behaving as a carbene transfer agent toward a second carbonyl molecule and giving rise to a carbonyl ylide that cyclizates to the corresponding oxirane. This in turn transfers the carbene unit to a second phosphine molecule, with loss of acetone, affording a phosphorane. The latter undergoes typical Wittig reaction to the final homocoupling product through the oxaphosphetane intermediate. The alternative direct conversion of oxaphosphirane into phosphorane constitutes the lowest energy path as it skips the highest barrier oxirane → phosphorane conversion. The oxirane route is favored by the use of polar solvents and electron deficient carbonyl components. The lowest barrier most exergonic process from oxaphosphirane is the pericyclic cycloaddition of the acetone C═O bond along the endocyclic P-C bond, furnishing the stable 1,3,2-dioxaphospholane product.

Quantum Chemical Calculations on CHOP Derivatives—Spanning the Chemical Space of Phosphinidenes, Phosphaketenes, Oxaphosphirenes, and COP− Isomers

“…For comparative purposes, local correlation schemes of type LPNO (Local Pair Natural Orbital) for high level single reference methods, such as CEPA (Coupled Electron-Pair Approximation) [70,71], here the slightly modified NCEPA/1 version [72] implemented in ORCA, was used, as well as the spin-component scaled second-order Möller–Plesset perturbation theory (SCS-MP2) level [73,74] and the double-hybrid-meta-GGA functional PWPB95 [75,76], together with the D3 correction (PWPB95-D3) (see the SI). Due to the unavailability of unrestricted formalism for DLPNO/CCSD(T) calculations in the working version of ORCA, the corresponding values in the case of triplet electronic states were taken from the LPNO/NCEPA1 level, making use of the reported small differences between the results in these two levels for closed-shell systems [77]. Figure 2, Figure 3 and Figure 7 were drawn with VMD [78].…”

Between Oxirane and Phosphirane: The Spring‐loaded Oxaphosphirane Ring

“…On the contrary, “high‐coordinate” oxaphosphiranes received early attention, and indeed σ 5 λ 5 ‐oxaphosphiranes IV were proposed in 1988 as reaction intermediates, without full spectroscopic characterization, [17] and later on as a final stable product although not fully confirmed by X‐ray analysis [18] . They have been also theoretically studied [19] and proposed as key intermediates in the Perkow reaction of α‐haloketones with phosphites [20] and in phosphane‐mediated reductive dimerization of ketones to alkenes [21] . Regarding σ 4 λ 5 ‐oxaphosphiranes V , they were prepared by [2+1] cycloaddition reactions between iminophosphanes and fluorinated ketones (E=NR), [22] or proposed to participate in ring expansion reactions (E=O) [23] .…”

“…[18] They have been also theoretically studied [19] and proposed as key intermediates in the Perkow reaction of α-haloketones with phosphites [20] and in phosphane-mediated reductive dimerization of ketones to alkenes. [21] Regarding σ 4 λ 5 -oxaphosphiranes V, they were prepared by [2 + 1] cycloaddition reactions between iminophosphanes and fluorinated ketones (E = NR), [22] or proposed to participate in ring expansion reactions (E = O). [23] Labile P-complexation with boranes VI ([M] = BR 3 ) was fully explored by theoretical means as a way to protect the phosphorus lone pair and potentially opening up the access to still unknown "low-coordinate" σ 3 λ 3 -oxaphosphirane III.…”

Addition reactions of a phosphorus triamide to nitrosoarenes and acylpyridines

“…Phosphorus adducts formed with various unsaturated compounds. [17][18][19][20][21][22] Similarly, oxazaphosphiranes V have been suggested to arise from [2+1] addition of phosphines to nitrosoarenes. σ 5 ,λ 5 Dioxophospholenes with 1-to-1 stoichiometry are formed via formal [4+1] addition to 1,2-dicarbonyl compounds.…”

“…[23,24] The synthetic importance of these intermediates are underscored by the numerous reports in which the addition of phosphine to carbonyl or nitroso substrates leads to net deoxygenation with transfer of a carbene or nitrene equivalent, respectively. [19,[25][26][27] We have recently been investigating the reactivity of bicyclic phosphorus triamide 1. [28][29][30] Compound 1 and related species have been shown to undergo a number of addition reactions with polarized single bonds (O-H, N-H, B-H).…”