Phosphaalkenes with Inverse Electron Density

Abstract

This review gives an account of the syntheses, structures, and properties of phosphaalkenes R1P=CR2R3 with an inverse electron distribution at the P−C double bond (Pδ−Cδ+). The chemical reactivity of compounds of this type is novel and quite surprising compared to the chemistry of “classically polarized” phosphaalkenes (Pδ+Cδ−).

Transition‐Metal Carbonyl Complexes and Electron‐Donating Properties of N‐Heterocyclic‐Carbene–Phosphinidene Adducts

“…The 31 P NMR signal of 1c in C 6 D 6 is found at δ = -51.3 ppm, which is intermediate between the chemical shifts for 1a (δ = -133.9 ppm) and 1b (δ = -18.9 ppm). [2] Initial attempts to isolate the tungsten pentacarbonyl complex [(IPr·PPh)W(CO) 5 ] (4b) involved the photolytically prepared tetrahydrofuran (THF) adduct [(thf )W(CO) 5 ]. The addition of 1b afforded 4b in high purity as a precipitate after several days at room temperature but with low and irreproducible yields (ca.…”

“…Similarly, 4a was obtained in 57 % yield from 1a, whereas bulky 1c gave 4c in only 18 % yield. The corresponding molybdenum complexes were prepared from thermally generated [(thf )Mo(CO) 5 ], [39,40] which provided 5a and 5b as yellow and pale green solids in 67 and 60 % yield, respectively (Scheme 3). Complexes 4a and 5a were isolated as THF solvates, as was also confirmed by X-ray diffraction analysis (vide infra).…”

“…In 1997, Arduengo reported the first N-heterocyclic-carbenephosphinidene adducts (NHC·PR), which were prepared directly through the reactions of free carbenes with cyclic phosphorus(I) compounds such as (PPh) 5 . [1] Alternatively, an extensive series of similar phenylphosphinidene adducts were synthesized by Bertrand and co-workers through the reactions of stable carbenes with PPhCl 2 and subsequent reduction of the resulting phosphorus(III) adducts with 2 equiv.…”

“…[2] These compounds can be represented by structures A-C (Scheme 1), [3] which reveal their nature as inversely polarized phosphaalkenes. [4,5] As a mesomeric shift towards the polarized form B can be expected to afford a more shielded phosphorus atom, the 31 P NMR chemical shifts of these adducts were established as an indicator of the π-accepting properties of the respective carbene ligands. [2] Ph; 4c, R = Mes) were isolated from the reactions of 1a-1c with [(Me 3 N)W(CO) 5 ], whereas the reactions of 1a and 1b with [(thf )Mo(CO) 5 ] (thf = tetrahydrofuran) gave the corresponding molybdenum complexes [(IPr·PR)Mo(CO) 5 ] (5a, R = H; 5b, R = Ph).…”

“…Notably, the intermediate IPr·PSiMe 3 was used as a starting material for the synthesis of carbene-phosphinidyne complexes of the type [(IPr·P)ML n ] with ML n = [(η 6 -cymene)RuCl] or [(η 5 -C 5 Me 5 )-RhCl]. [15] Related transition-metal complexes such as [(η 5 -C 5 Me 5 )Fe(CO) 2 {PC(NMe 2 ) 2 }] were reported by Weber, [5,16] and, more recently, Grützmacher described the synthesis of the mercury(II) complex [Hg(P·IMesH 2 ) 2 ] from the parent phosphinidene adduct IMesH 2 ·PH; the latter was prepared from IMesH 2 by carbene insertion into the P-H bond of PH 3 , followed by dehydrogenation with ortho-quinone. [17] As indicated above, carbene-phosphinidene adducts can be regarded as promising ancillary phosphorus ligands in homogeneous catalysis; [9] therefore, an assessment of their electronic and steric properties is required.…”