Stable Ketenyl Anions via Ligand Exchange at an Anionic Carbon as Powerful Synthons**

Abstract

A (phosphino)diazomethyl anion salt 1 ([[P]−CN2][K(18-C-6)(THF)]) ([P] = [(CH2)(NDipp)]2P; 18-C-6 = 18-crown-6) behaves as a (phosphino)carbyne anion-dinitrogen adduct ([P]−C − ←:N2). Under an atmosphere of carbon monoxide (CO), 1 undergoes a facile N2/CO ligand exchange reaction giving (phosphino)ketenyl anion salt [[P]−CCO][K(18-C-6)] 2. Oxidation of 2 with elemental Se affords (selenophosphoryl)ketenyl anion salt ([P](Se)−CCO][K(18-C-6)]) 3.These ketenyl anions feature a strongly bent geometry at the P-bound carbon and this carbon atom is highly nucleophilic. The electronic structure of 2 is examined by theoretical studies. Reactivity investigations demonstrate 2 as a versatile synthon for derivatives of ketene, enolate, acrylate and acrylimidate moieties.Ketenes of the general formula R 1 R 2 C=C=O are widely used reagents in synthetic chemistry. Due to their polarized cumulated double bonds, ketenes are usually generated in situ as very reactive intermediates, which then ensue (cyclo)additions to provide access to carboxylic derivatives (i.e. acids, esters, anhydrides, amides), cyclobutenones, β-lactams and β-lactones. [1] Despite such utility, synthetic methods toward ketenes are limited and largely rely on transition metal (TM) reagents (e.g. metal carbenes). [2] The development of ketenyl anions/ynolates [RCCO] − (Figure 1a) represents an alternative available material leading to ketenes; however, these anions are generally unstable as well, generated at low temperatures (proposed as ynolates) and consumed as soon as they are produced in chemical syntheses. [3] Recently, low-temperature spectroscopic characterization of Me3Si−C ≡ C−OK has been achieved. [4] Additionally, the group of Stephan spectroscopically characterized Ph2P(S)C(Li(THF)2)CO (A) at room temperature in a reaction mixture of dilithiomethandiide with CO and N2O (Figure 1b). [5] During the preparation of this manuscript, Gessner showed an isolable ketenyl anion Ph2P(S)C(K(18-C-6))CO (B) (18-C-6 = 18-crown-6) via an unprecedented phosphine/CO replacement reaction. [6] This breakthrough allowed the isolation of a series of ketene derivatives via facile ketenylations.

Transition Metal‐Free Activation of Carbon Monoxide: Ketenyl Anions by PPh3/CO Exchange

“…It should be pointed out that both views have their advantages to understand both electronic structure and reactivity. Mechanistically, it appears likely that the PPh 3 /CO exchange reaction proceeds through a mechanism similar to the one we recently reported for the concerted N 2 /CO exchange at a vinylidene carbon center, [10] or the ketenylation reaction by N 2 /CO exchange with metalated lithio‐silyldiazomethane [11, 12] . A reasonable mechanism would be the attack of the metalated carbanion on the LUMO of CO followed by a subsequent (or concerted) elimination of triphenyl phosphine.…”

Synthesis and Reactivity of a Zinc Diazoalkyl Complex: [3+2] Cycloaddition Reaction with Carbon Monoxide

“…Organic or lithiated diazoalkane was reported to react with CO to generate ketene derivatives with the release of N 2 (Scheme 1b). [14] In addition, such a N 2 /CO ligand exchange was also observed for the reactions of diazoalkenes [15] or diazomethyl anions [16] with CO. In these reactions, a C=C double bond is formed on the C atom of CO, which could be considered as an extension of the 1,1-C-addition.…”

“…Remarkably, CO was employed in the [3+2] cycloaddition reaction for the first time, which reflected a new CO activation pattern. In contrast, N 2 /CO ligand exchanges were observed for the previously reported reactions of diazo compounds with CO. [14][15][16] The current reaction profile was also investigated by DFT calculation, which suggested the zinc center played an essential role to cooperatively facilitate the electrophilic CO addition to the terminal nitrogen of the diazo ligand in the initial step. Further studies to synthesize zinc diazoalkyl complexes with different types of ligands and fully explore their reactivities are ongoing in our laboratory.…”

“…As mentioned above, CO was never used in such a cycloaddition, and reactions of diazo compounds with CO usually led to the N 2 /CO ligand exchange. [14][15][16] To our surprise, treatment of complex 3 in toluene under a CO atmosphere afforded a formal [3+2] cycloaddition product 6 as a colorless crystalline solid in 24 % isolated yield (Scheme 5). The solidstructure of dimeric 6 (Figure 4 2) Å].…”

Ring expansion vs. addition – reactivity of a cyclo-P4 complex towards pnictogenium cations

“…in the stabilisation of a singlet phosphinidene, 6 carbyne anions 7 or stable ketenyl anions. 8 Furthermore, the first salts of unstabilised phosphenium and arsenium cations have only been isolated very recently by the group of Beckmann , 9 with the heavier stibenium and bismuthenium cations only known shortly longer. 10 Nevertheless, phosphenium ions and their homologues are key intermediates in the synthesis of extended cationic polyphosphorus frameworks.…”