Cleavage of a P=P Double Bond Mediated by N‐Heterocyclic Carbenes

Abstract

The reaction of the bulky diphosphenes (Rind)P=P(Rind) (1; Rind=1,1,3,3,5,5,7,7‐octa‐R‐substituted s‐hydrindacen‐4‐yl) with two molecules of N‐heterocyclic carbene (NHC; 1,3,4,5‐tetramethylimidazol‐2‐ylidene) resulted in the quantitative formation of the NHC‐bound phosphinidenes NHC→P(Rind) (2), along with the cleavage of the P=P double bond. The reaction times are dependent on the steric size of the Rind groups (11 days for 2 a (R=Et) and 2 h for 2 b (R=Et, Me) at room temperature). The mechanism for the double bond‐breaking is proposed to proceed via the formation of the NHC‐coordinated, highly polarized diphospehenes 3 as an intermediate. Approach of a second NHC to 3 induces P−P bond cleavage and P−C bond formation, which proceeds through a transition state with a large negative Gibbs energy change to afford the two molecules of 2, thus being the rate‐determining step of the overall reaction with the activation barriers of 80.4 for 2 a and 29.1 kJ mol−1 for 2 b.

N‐Heterocyclic Carbene–Phosphinidene Adducts: Synthesis, Properties, and Applications

“…[15] The groups of Slootweg and Grützmacher have extended this synthetic methodology by showing that carbenes can also be generated in situ, and synthesized 1·PPh, 5·PPh and 6·PPh from the corresponding imidazolium salts, (PPh) 5 and sodium tert-butoxide in an efficient one-pot synthesis in high yields (87-95 %; Scheme 3). [9] 1·PPh [δ 31 P = -49.1; P-C NHC 1.7917 (14) Å, N-C-P-R 48.01(16)°; see Table 1] displays features similar to the structurally related 2·PPh, both in solution and in the solid state. Scheme 3.…”

“…Matsuo and co-workers reacted the bulky diphosphenes (Rind)P=P(Rind) (Rind = 1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl; R = Me, Et) with two equivalents of 1,3,4,5-tetramethylimidazol-2-ylidene (2), which resulted in the quantitative formation of the NHC-phosphinidene adducts 2·P(EMind) (δ 31 P = -70.5) and 2·P(Eind) (δ 31 P = -63.9; Scheme 4). [14] The molecular structure of 2·P(Eind) shows that the coordinated NHC is almost perpendicular to the benzene ring of the Eind group with a central P-C bond of 1.767(3) Å and a N-C-P-R dihedral angle of 3.4(3)°. Ragogna and co-workers discovered that the related phosphinidene selenides (RP=Se) n can be used as phosphorus source and treated the phosphinidene selenide dimer (DmpP= Se) 2 (Dmp = 2,6-dimesitylphenyl) with 4 equivalents of 1,3,4,5tetramethylimidazol-2-ylidene (2) or 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (4) to afford the NHC=PR adducts 2·P(Dmp) (δ 31 P = -76.7) and 4·P(Dmp) (δ 31 P = -75.0) in 44 and 66 % yield, respectively, together with the corresponding selenoureas (Scheme 5).…”

“…In this review, we highlight all synthetic approaches leading to N-heterocyclic carbene-phosphinidene adducts, focus on the nature of the chemical bond between the carbene and phosphinidene moieties and provide an overview of the reactivity of the NHC=PR adducts towards main group compounds, transition-metal complexes and organic electrophiles. All the carbene-phosphinidene adducts discussed in this review are summarized in Table 1, and ordered according to the stabilizing carbene and size of the P substituent, starting with phosphinidene adducts of imidazolylidenes (1)(2)(3)(4)(5)(6)(7)(8)(9), followed by those of imidazolinylidenes (10,11), amido carbenes (12)(13)(14), tetrahydropyrimidinylidenes (15,16), thiazolylidene (17), and finally cyclic (alkyl)(amino)carbenes (CAACs: 18-22). For simplicity, we display all carbene-phosphinidene adducts in this review in resonance form A.…”

A General Synthetic Route to NHC‐Phosphinidenes: NHC‐mediated Dehydrogenation of Primary Phosphines

“…[21] In a similar fashion, the hitherto unknown (iPr 2 Im)PMes (7) was synthesized and isolated as a sticky oil. The formation of (iPr 2 Im)PMes (7) was unambiguously verified by 1 H, 13 1).…”

π-Electron systems containing Si=Si double bonds

“…In order to obtain the initial target molecule, ( E )-1,2-di(1-pyrenyl)disilene 28 , we examined the photoreaction of 29 [ 103 ]. As shown in Figure 16 , after the photolysis ( λ = 530 nm) of 29 in THF at room temperature, the solution color has changed from purple to blue.…”