Phosphino carbenes

The most recent theoretical study, by Alhrichs and co-workers, deals with the di(phosphino)carbene Id and (phosphino) (phosphonio)carbenes Ie,f.16 The optimized geometry of the di(phosphino)carbene Id is weakly bent (PCP angle 160.5°) and highly unsymmetrical Only one of the phosphorus centers (P1) is in a planar environment, and it is much more closely bonded to the carbenic center than the other one (P1C 1.533 and P2C 1.765 A). The atomic charges (P1 +1.0, C -0.8, P2 +0.6) indicate that the short P bond is a double bond reinforced by Coulombic attraction, while the nature of the molecular orbitals revealed a slight delocalization of the carbene lone pair into the low-lying a (P-N) orbitals of the two phosphino substituents. The distortion from the symmetrical structure can be viewed as a second-order Jahn-Teller effect. 

Deprotonation of the stable (phosphino)(phosphonio)carbene 2d gives rise to the transient bis(phosphino)carbene 2g.31 Obviously, this method is extremely restricted, but this is certainly the only example of the preparation of a carbene from a carbene ... 

Vacuum thermolysis (160°C) of the hemiaminal 11 generates the azaphos-phetane 65 in 85% yield.32 This product clearly results from the intramolecular insertion of the transient (amino) (phosphino)carbene 2h into the C-H bond of a diisopropylamino group bonded to phosphorus. Note that the four-membered heterocycle 65 is formed exclusively in spite of the ratio of six methyl-CH bonds to one methine-CH bond, and that only one of the two possible diastereomers is detected. The same regio- and diastereo-selectivity have already been observed with the di(phosphino)carbene 2g,74 but is in marked contrast to the exclusive formation of five-membered rings... 

An exchange reaction occurs when nitrosylchloride is reacted with (phos-phino)(trimethylsilyl)diazomethane la to yield an unstable ce-nitroso a-phosphinodiazo derivative lq and chlorotrimethylsilane.75 Spontaneous loss of N2 leads to the transient (nitroso)(phosphino)carbene 2q, which can also be regarded as the phosphorus vinyl ylide 2q or even better as the A3-... 

The amino(phosphino)carbene 12a with a pyramidalized phosphino substituent behaves as a,p-bidentate ligand to give highly stable Rh(I) [53] and Pd(II) [54] complexes. 

Several unique synthetic strategies for bidentate(amino)(oxy)- and (amino)(aryl) carbenes have been described (Scheme 10). For the former, the reaction of the amino(phosphino)carbene with an ort/io-quinone leads to the transient formation of a zwitterionic species featuring both a phosphonio nucleofuge and an aryloxide nucleophile that allow for a subsequent intramolecular substitution process. The... 

We first investigated the synthesis of a (phosphino)carbene featuring an electron-rich aryl group. The (phosphino)(mesityl)carbene (XlVa) was generated by photolysis at —50 °C of a THF solution of the corresponding diazo compound. The and NMR chemical shifts of XlVa ( P 8 -23.5 5 151.1) compare... 

We have recently shown that instantaneous and quantitative formation of the corresponding phosphorus ylides occurred when one equivalent of trimethylphosphine was added at low temperature to carbenes la and Xlla (Scheme 8.20). The formation of phosphorus ylides in these reactions is of particular significance since it is certainly the most striking evidence for the presence of an accessible vacant orbital at the carbene center of (phosphino)carbenes, as expected for singlet carbenes. 

Interestingly, highly regioselective reactions were also observed by addition of Lewis acids to the (amino)(phosphino)carbene (XVd). Indeed with one equivalent of BF3 OEt2 the quantitative formation of the carbene-BF3 complex was observed, whereas the softer Lewis acid BH3 interacts selectively with the phosphorus lone pair to afford a new stable carbene XVId (Scheme 8.25). ... 

Figure 8.17. Molecular structure of the (amino)(phosphino)carbene rhodium complex XVdRH.

Up to 2000, the number and variety of stable carbenes have been limited by the perceived necessity for two strongly interacting substituents. The preparation of stable or persistent (aryl)- or (alkyl)-(phosphino)carbenes as well as (aryl)(amino)-carbenes demonstrates that a single electron-active substituent allows the spectroscopic characterization of singlet carbenes under standard laboratory conditions. It has been shown that an amino substituent is more efficient than a phosphino substituent to stabilize a carbene center and that the steric bulk of the spectator substituent can be decreased even to the size of a methyl group in the phosphino series, so that a broad range of observable carbenes will be readily available. 

BC NMR Shifts (S Values, CDC13) and IR Frequencies (cm-1) of Selected Alkoxy(2-aminoalkenyl)carbene (= 4-Amino-1-chroma-1,3-dienes) and Alkoxy(2-PHOSPHINO)CARBENE COMPLEXES (= 4-PHOSPHINO-l-CHROMA-1,3-DIENEs)... 

Figure 3.79 Synthesis of a phosphino carbene with a chiral backbone.

A Rh—P Experimental and theoretical studies of complexes based on (amino)(phosphino)carbenes 2004JOM1431, 2003CEJ5858, 2002JA11834... 

A M-P M = Ni, Pd Complexes based on (amino)phosphino)carbenes 2005JOM5541... 

R=/Pr2N 25 R=iPr2N,E = H 26 R=iPr2N,E = SnMe3 Synthesis of phosphino carbenes... 

Igau A, Grutzmacher H, Baceiredo A et al. (1988) Analogous.i/.,a -bis-carbenoid, triply bonded species synthesis of a stable X3-phosphino carbene-X5-phosphaacetylene. J Am Chem Soc 110 6463-6466... 

Addition reactions of low coordinate phosphorus compounds have proved to be an interesting route to novel acyclic phosphazenes. Phosphino carbenes, R2PCSiMej, exhibit typical multiple bond behavior in 2+3 cycloaddition reactions with trimethylsilylazide. The cycloadduct easily rearranges to the diazo derivatives R2P(=NSiMe3) C (=N2) SiMej. The addition of the... 

Baceiredo and Bertrand combined the two privileged coordination moieties to an (amino)(phosphino)carbene ligand [117], after which the palladium complex formed by reaction of this ligand and PdCl2(cod) was isolated and structurally characterized. Whilst the results of the amination experiments were not... 

Bis(phosphino)diazomethane is obtained as air-stable orange crystals by reaction (6.657). It looses nitrogen to produce a phosphaalkene, presumably by rearrangement of the bis (phosphino) carbene. 

While Bertrand and co-workers described in 1988 the stable k -phosphino-carbene 8, which did not act as a ligand, Arduengo et al. prepared in 1991 the first free and stable bottleable N-heterocyclic carbene 9 by deprotonation of the corresponding imidazolium salt (Scheme 1.4). This deprotonation method was later supplemented by Kuhn, who introduced the reductive desulfurization of thiones for the preparation of stable imidazol-2-ylidenes. ... 

Direct complexation of an (amino)(phosphino)carbene 296 and an (amino)(oxy)carbene 297 featuring a phosphino group in position-6 to the carbene with [RhCl(CO)2]2 provided two new Rh(l) carbene complexes, 298 and 299. ... 

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