Metal Coordination to the Formal PN Bond of an Iminophosphorane and Charge‐Density Evidence against Hypervalent Phosphorus(V)

Abstract

The iminophosphorane Ph(2)P(CH(2)Py)(NSiMe(3)) (1) was treated with deprotonating alkali metal reagents to give [(Et(2)O)Li[Ph(2)P(CHPy)(NSiMe(3))]] (2), [[Ph(2)P(CH(2)Py)(NSiMe(3))]Li[Ph(2)P(CHPy)(NSiMe(3))]] (3) and [[Ph(2)P(CH(2)Py)(NSiMe(3))]Na[Ph(2)P(CHPy)(NSiMe(3))]] (4). We report their coordination behaviour in solid-state structures and NMR spectroscopic features in solution. Furthermore, we furnish experimental evidence against hypervalency of the phosphorus atom in iminophosphoranes from experimental charge-density studies and subsequent topological analysis. The topological properties, correlated to the results from NMR spectroscopic investigations, illustrate that the formal P=N double bond is better written as a polar P(+)--N(-) single bond. Additionally, the effects of metal coordination on the bonding parameters of the iminophosphorane and the related anion are discussed.

The modern interpretation of the Wittig reaction mechanism

“…However, while a betaine-LiBr complex has been observed by NM R, 65 uncomplexed betaines have never been detected spectroscopically in a Wittig reaction mixture, or in an 15 independent betaine generation experiment. OPAs are also necessarily produced in each of the independent generations of betaine, and they can be detected by NM R in solution in such experiments, and in Wittig reactions.…”

“…There has been much discussion in the literature over the nature of this stabilistion both in ylides, 10,11,35 and in similar bonds in phosphorus stabilised carbanions, 12 phosphine oxides, 11,13,14 and iminophosphoranes. 15 There seems to be a developing consensus that the P=O and P=N bonds are composed of a polar covalent  bond combined with an electrostatic interaction. However for the ylides the evidence 40 suggests that there may be negative hyperconjugation of the lone pair on the ylide -carbon into the * orbitals of the bonds of phosphorus to its other substituents, albeit that the P-C bond is still heavily polarised towards carbon.…”

“…22 -New reaction conditions. 25,28,29 -One-pot reaction from either phosphine 19,20,29,30,31 or alcohol 32 15 -Synthetic studies. 31,33 -Catalytic variants.…”

“…10 Wittig reactions of the most commonly encountered semistabilised stabilised ylides, derived from triphenylphosphine, are generally not very selective for one isomer or another. Thus allylidene or benzylidene triphenylphosphoranes (Chart 1 (e)) generally react to give approximately equal proportions of E and 15 Z alkene, while semi-stabilised alkyldiphenylphosphine-derived ylides (Chart 1 (f)) show moderate and in some cases very high E-selectivity. 4 Semi-stabilised ylides with two or more of the Pphenyl groups replaced by alkyl groups (Chart 1 (g)) 4 and also those derived from methyldibenzophosphole (Chart 1 (h)) 60 show 20 exceptionally high E-selectivity.…”

“…10 Rebuttal and counter-evidence: The betaine mechanism requires reversible formation of betaine and/or OPA in reactions of stabilised ylides in order to explain the observed selectivity -it is unable to account for E-selectivity under kinetic control. It has been shown that deprotonation of erythro-β-HPSs derived from 15 ester-stabilised ylides decompose stereospecifically to Z-alkene (presumably this necessarily occurs through erythro-betaine and then cis-OPA). 59,60 Since neither the betaine nor the OPA precursor of Z-alkene underwent reversion, it follows that the highly E-selective reactions of these ester-stabilised ylides are 20 under kinetic control.…”