Synthesis of η¹-α-Phosphinocarbene Complexes of Manganese and Mechanistic Insight into Their Base-Induced Transformations

Abstract

A series of η1-α-phosphinocarbene complexes of manganese, Cp(CO)2MnC(R)PR′R′′ (3; 3a: R = Ph, R′ = H, R′′ = Mes; 3b: R = Me, R′ = H, R′′ = Mes; 3c: R = Ph, R′ = R′′ = Ph; 3d: R = Ph, R′ = R′′ = N(i-Pr)2; 3e: R = Ph, R′ = Me, R′′ = Mes), was generated at low temperature upon nucleophilic addition of the primary phosphine H2PMes and secondary phosphines HPPh2, HP(N(i-Pr)2)2, or (±)-HPMeMes to the cationic carbyne complexes Cp(CO)2Mn+C−R ([1]+; [1a]+: R = Ph; [1b]+: R = Me), followed by deprotonation of the resulting α-phosphoniocarbene intermediates Cp(CO)2MnC(R)P+(H)R′R′′ ([2a−e]+). The η1-α-phosphinocarbene complexes 3c−e derived from secondary phosphines were found to be highly thermolabile, giving upon intramolecular CO insertion the η3-phosphinoketene complexes Cp(CO)Mn(η3-C(O)C(Ph)PR′R′′) (4c: R′ = R′′ = Ph, 4d: i-Pr2N; RR/SS-4e: R′ = Me, R′′ = Mes) in 80−95% yield. The η1-α-phosphinocarbene complexes 3a,b, bearing a mesityl substituent, were isolated in 57−65% yield. The availability of the phosphorus lone pair in such compounds was assessed by their reaction with borane, giving the corresponding adducts Cp(CO)2MnC(R)P(BH3)HMes (5a, R = Ph; 5b, R = Me). Complexes 3a,b slowly rearrange in a nonpolar solvent (benzene) to afford a mixture of the η3-phosphinoketene complexes Cp(CO)Mn(η3-C(O)C(R)PHMes) (4a, R = Ph; 4b, R = Me) and the η1-phosphaalkene complexes Cp(CO)2Mn(η1-P(Mes)C(H)R (6a, R = Ph; 6b, R = Me), and ultimately pure 6a,b as a mixture of E/Z stereoisomers (6a, E/Z 60:40; 6b, E/Z 55:45). By contrast, a stereoselective rearrangement of 3a,b into E-6a,b was observed in a polar solvent (THF), the process being considerably accelerated in the presence of catalytic amounts of base. Deprotonation of 3a by n-BuLi at −80 °C affords the dicarbonyl phosphidocarbene anion [Cp(CO)2MnC(Ph)PMes]Li ([7a]Li), which undergoes an instantaneous CO insertion process upon addition of traces of weak acids (H2O, t-BuOH) to give the cyclic monocarbonylacyl anion [Cp(CO)2 Mn(η2-C(O)C(Ph)PMes]Li ([8a]Li, R = Ph). Besides, deprotonation of 3a,b by t-BuOLi affords directly the cyclic monocarbonylacyl anions [8a,b]Li. The protonation of [8a]Li with various proton sources (HBF4·Et2O, NH4Claq, [Et3NH]Cl) leads to a stereoselective formation of E-6a, illustrating the key role of this anion in the stereoselective 3a → E-6a rearrangement. Alkylation of [8a]Li by MeI resulted in formation of η3-phosphinoketene complex RR/SS-4e as a single pair of diastereomers, whereas its treatment with iodine resulted in thermally unstable η3-phosphinoketene complex Cp(CO)Mn(η3-C(O)C(Ph)PIMes) (11), evolving at room temperature into the η1-phosphaalkene product Cp(CO)2Mn(η1-P(Mes)C(I)Ph) (12) of Z conformation. The solid-state structures of 3b, 4c, 4d, RR/SS-4e, and Z-12 are reported.

A Direct, Modular, and Efficient Construction of the PCP Structural Motif through Coupling of Manganese Carbyne Complexes with Phosphines

“…We first observed that the Mn I phosphoniocarbene complex [MpC(Me)PPh 2 H]BCl 4 [ 2 a ]BCl 4 (here and thereafter, Mp=Cp(CO) 2 Mn) generated in situ upon addition of PPh 2 H to the methylcarbyne precursor [MpCMe)]BCl 4 , [ 1 Me ]BCl 4 at low temperature,1b,d,e can uptake a second molecule of phosphine to give the cationic complex [ 4 aa ‐ H ]BCl 4 featuring a newly formed pendant PCP structural motif containing ligand resulting formally from the coupling of the carbyne fragment and two secondary phosphines (Scheme ). Deprotonation of the latter upon purification on an alumina column afforded a neutral diphosphine complex 4 aa in a nearly quantitative yield.…”

Carbeniophosphines versus Phosphoniocarbenes: The Role of the Positive Charge

“…The method exemplified in the case of manganese shows a high tolerance regarding the nature of substituents of both carbyne complex 55 and phosphine (Scheme ). The deprotonation of α‐phosphoniocarbene complexes 56 derived from primary and secondary phosphines constitute an elegant entry for the preparation of the corresponding phosphinocarbene complexes 57 ,…”

“… Preparation of Mn I complexes 56 and 57 of acyclic phosphonio‐ and phosphinocarbenes, respectively ,…”

Overview of the Synthesis and Catalytic Reactivity of Transition Metal Complexes Based on C═P Bond Systems

“…Additionally, monocarbonyl acyl anion 96A reacted with iodine to afford a thermally unstable η 3 -phosphinoketene complex [Cp(CO) 2 Mn(η 3 -C(O)C-(Ph)PIMes)], which upon warming to room temperature rearranged into the η 1phosphaalkene complex [Cp(CO) 2 Mn(η 1 -P(Mes)�C(I)-Ph)]. 97 Lugan and Valyaev described the syntheses and reactivities of manganese methylene phosphonium complexes 101A-B. Their precursor was obtained from Fischer carbene 97 and Hunig's base.…”

“…Addition of traces of water to a solution of 95A in THF results in its immediate and quantitative isomerization ( 96B ; for all reactions see Scheme ). Additionally, monocarbonyl acyl anion 96A reacted with iodine to afford a thermally unstable η 3 -phosphinoketene complex [Cp(CO) 2 Mn(η 3 -C(O)C-(Ph)PIMes)], which upon warming to room temperature rearranged into the η 1 -phosphaalkene complex [Cp(CO) 2 Mn(η 1 -P(Mes)C(I)Ph)] …”