Phosphino carbenes substituent

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. 

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. 

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. 

To date, the (phosphino)(silyl)- and (phosphino)(phosphonio)carbenes are the only stable carbenes that feature a 77-donor and a 77-type-withdrawing substituent. They have a singlet ground state with a planar environment at phosphorus and a short phosphorus-carbon bond distance. However, because of the reluctance of phosphorus to keep this planar geometry, these push-pull carbenes behave in a manner very close to that of most of the transient carbenes. 

Carbenes [74-76], and in particular W-heterocyclic carbenes (NHCs), are today the topics of very intense research [43 8]. Carbenes were originally considered as chemical curiosities before being introduced by Doering in organic chemistry in the 1950s [77], and by Fischer in organometallic chemistry in 1964 [5]. Eater, it was shown that the stability of carbenes could be dramatically enhanced by the presence of heteroatom substituents. After the discovery of the first stable carbene, a (phos-phino)(silyl)carbene, by Bertrand et al. in 1988 [78], a variety of stable acyclic and cyclic carbenes have been prepared. With the exception of bis(amino)cycloprope-nylidenes [79], all these carbenes feature at least one amino or phosphino group directly bonded to the electron-deficient carbenic center. 

Most of the (W,W)-carbenes are predicted to be linear and this substitution pattern results in a polarized two-electron three-center n system. Here also, the C—W bonds have some multiple bond character these (W,W)-carbenes are best described by the superposition of two ylidic structures featuring a positive charge at the carbene carbon atom. The most studied carbenes of this type are the transient dicarbomethoxycarbenes and the masked diborylcarbenes. Since no carbenes of the latter type have yet been isolated, they are not included in this chapter. Lastly, the quasilinear (D,W)-carbenes combine both types of electronic interaction. The D substituent lone pair interacts with the py orbital, while the W substituent vacant orbital interacts with the px orbital. These two interactions result in a polarized allene-type system with DC and CW multiple bonds. Good examples of this type of carbene are given by the transient halogenocarboethoxycarbenes and by the stable (phosphino)(silyl)- and (phosphino)(phosphonio)carbenes (see below). 

Since the discovery of the stable carbene la, many other (phosphino)(silyl) carbenes have been prepared, all of them using diazo precursors,but only a few of them are stable at room temperature. The silyl group at the carbene center can be replaced by an isoelectronic and isovalent phosphonio substituent without dramatic modibcation. Indeed, the stable (phosphino)(phosphonio)carbenes and were synthesized from the corresponding diazo precursors 2a and 2b... 

Figure 8.7. Mesomeric and inductive effects of substituents on (methoxy)(trifluoromethyl)-carbene and (phosphino)(trifluoromethyl)carbene.

Direct observation of singlet (alkyl)carbenes usually requires matrix isolation conditions. " Using the 7i-donor and a-attractor methoxy substituent, Moss and co-workers could characterize the (methoxy)(methyl)carbene (MeOCMe) by ultraviolet (UV) and infrared (IR) spectroscopies, but only in a nitrogen matrix (at 10 K) or in solution thanks to a nanosecond time-resolved LFP technique (fi/2 < 2ps at 20 °C). The remarkable stability of carbene XlVa both in the solid state and in solution (no degradation observed after several weeks at room temperature), prompted us to investigate the preparation of (phosphino)(alkyl)carbenes. 

In all cases, the calculations identify the singlet species as the more stable one at the PMP-4 level, the singlet states of FpP—C—SiFp and (FpNpP—C—SiFp are 5.6 and 13.9 kcalmol-1, respectively, below the triplet states. Thus, all computational evidence points to a description of (R2N)2P—C—SiR3 as a species with a P,C multiple bond, with an emphasis on the ylide structure II in which both charges are stabilized by the respective neighboring substituents. An X-ray structure determination of one of the isolable [bis(dialkylamino)phosphino](silyl)carbenes is not yet available but, as will be shown in... 

A range of (phosphino)(aryl)carbenes (20 R = i-Pr, c-hexyl V, W, X, Y, Z = various combinations of H, F, Me, CF3, NMe2) have been generated by photolysis of their diazo precursors. The substituent effects on carbene stability were investigated both experimentally and theoretically. The presence of an acceptor and also a spectator aryl substituent resulted in stable or at least persistent carbenes, which could be characterized by standard spectroscopic techniques, whereas a donor aryl ring produced a very fleeting carbene. 

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]... 

In summary, the phosphino group clearly acts as Ji-donor substituent, while to some extent the silyl acts as Tt-attractor due to the aptitude of silicon for hypervalency (phosphino)(silyl)carbenes are push-pull carbenes. 

A novel and potent family of metallocarboxypeptidase inhibitors (237), based on oxiranes, bearing a bis(diisopropylamino)thioxophosphoranyl group and aryl- or heteroaryl substituents with (rn .s-stereochemistry, has been synthesized in 56-92% yields by addition of [bis(diisopropylamino)-phosphino](trimethylsilyl)carbene (236) to the corresponding aldehydes and subsequent thiolation with elemental sulfur (Scheme 80). These investigations showed, for the first time, that fully shielded P-moiety might act as an inhibitor of a metal-dependent enzyme. 

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