Phosphorus-stabilized carbenes

When the scandium and lutetium compounds are triturated with toluene, one THE molecule is lost, resulting in [Ln(CH2Ph)3(THE)2] (Ln = Sc, Lu) of which the scandium compound has a trigonal bipyramidal structure. There is no evidence for [Yb(Bz)3(THF)3] when C6H5CH2K reacts with [Ybl3(THF)3], the mixed-valence compound [Yb°(CH2Ph)(THF)5]+ [Yb (CH2Ph)4(THF)2]- results As a family of a-alkyls obtainable for most lanthanides, the benzyls have promise as synthetic precursors, notably for phosphorus-stabilized carbenes. ... 

Given that the stability of N-heterocyclic carbenes is based on the stabilizing effect of the a-nitrogen atoms, it is surprising that the heavier analogues of nitrogen, such as phosphorus, were initially disregarded for the stabilization of heterocyclic carbenes. Acyclic phosphorus-stabilized carbenes have been... 

Reuclin R, Griinstein D, Le Goff X-F, Le Floch P, Mezailles N (2010) Phosphorus stabilized carbene complexes bisphosphonate dianion synthesis, reactivity and DFT studies of 0 C 0 zirconium(IV) complexes. Dalton Trans 492-499. doi 10.1039/b915468j... 

With phthalic anhydride and trialkyl phosphites, the reaction takes V an entirely different course, the main products being biphthalyl (70%) and trialkyl phosphate. It was suggested that the phosphorus atom of a phosphite ester attacks the oxygen of the anhydride carbonyl, forming an intermediate which undergoes valency expansion to generate in this unique instance a carbene and a phosphate ester (276). Dimerization of this resonance-stabilized carbene w ould furnish the product. 

As well as phosphorus ligands, heterocyclic carbenes ligands 10 have proven to be interesting donor ligands for stabilization of transition metal complexes (especially palladium) in ionic liquids. The imidazolium cation is usually presumed to be a simple inert component of the solvent system. However, the proton on the carbon atom at position 2 in the imidazolium is acidic and this carbon atom can be depro-tonated by, for example, basic ligands of the metal complex, to form carbenes (Scheme 5.3-2). 

The most recent development concerns the heterocyclic (amino)(ylide)carbenes AYC. Such compounds have been known for some years [203] but so far had little impact compared to their diamino stabilized relatives. Both phosphorus ylide (86) and sulfur ylide (87) stabilized AYC ligands have been generated in situ and were stabilized at suitable metal centers (Fig. 27) [204, 205]. The palladium complex 88 with an anionic (amino) [bis(ylide)]carbene is also known [206]. 

If the resonance stabilization of singlet carbenes is increased even more, for example, by replacing the CH3O substituents of (CH30)2C by amino groups [as in (R2N)2C] or phosphorus substiments (13), stable, isolable, nucleophilic carbenes can be obtained. These species are considered in Chapter 8 in this volume. 

The lower stability of (phosphino)(silyl)carbene-group 13 (IIIA) element Lewis acid adducts compared to that of the corresponding imidazol-2-ylidene adducts is easily rationalized by considering the inferior ability of phosphorus to stabilize a positive charge in the ot-position. In other words, once complexed, the carbene... 

These ylides, e.g. 6, can also be described as carbenoids stabilized by coordination to phosphorus.143 Fluorinated carbenes without 7t-electron-donating substituents are typically ground-state triplet species however, perfluoroalkylcarbenes, due to their kinetic stability, can be characterized spectroscopically.13S-139-144... 

In two recent works the phosphorus analogues of pyridones 109-111 (Scheme 58) have been studied computationally [317, 318], 110 is planar, while for 109 and 111 the planar structures are transition states with 11.0 and 19.8 kcal moH above the nonplanar minimum, respectively. The low barriers to inversion indicate that the planar structures have considerable aromaticity, which is shown by the NICS values exhibiting 67%, 51%, and 33% of the benzene value for 109,110, and 111, respectively [317], ELF analysis also indicates a certain aromaticity in these compounds [317], The bonding situation in 109 is similar to the cyclic phosphinocarbene isomer of phosphinine 112 [319] (Scheme 59), which was shown to be planar and aromatic. Stabilization of this carbene can be achieved by incorporating it to an anne-lated ring system 113, and also by the electron donor effect of an ylide formed with an additional o4,l5-P (114) [319],... 

An aminophosphinidene complex Cp Os(CO)2 PCl(N-Tr2) was prepared by reaction of [Cp Os(CO)2] with Tr2NPCl2. As with Fischer carbenes (see Fischer-type Carbene Complexes), the nitrogen group stabilizes the electron-deficient phosphorus atom of the phosphinidene ligand. Based on a crystal structure of the ruthenium analog, there is likely... 

Bertrand s approach to the stable earbene problem has been to seareh for a most delieate balance of stability and reaetivify, such that the essential reaetions expeeted of a earbene are not lost. Carbene 42, for example is a distillable red oil that behaves as a nucleophilie earbene despite its phosphaaeefylene eontributing resonance form it undergoes addition to electron poor olefins (e.g., dimethyl fiimarate). The very eleetron-rieh bis(diisopropylamino)phosphino substituent damps the carbene s electrophliie properties by resonanee donation into the ear-benic p orbital. [115] These phosphinosilylcarbenes are therefore best deseribed by phosphorus vinyl ylide structures with a lone pair on earbon (ef, 42, seeond contributor). [116]... 

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