Resonance carbene stabilization

Clearly, by the appropriate choice of substituents, carbenic stability, reactivity, and philicity can be simultaneously varied, while the delicate interrelations of these properties can be understood in empirical and, more precisely, in theoretical terms. The kinetic range of the carbene reactions that we have considered is enormous the rate constants span 9 or more orders of magnitude. From this perspective, it is remarkable that the classical tools of physical organic chemistry, resonance and inductive effects, and Hammett relationships, provide such a satisfactory qualitative rationalization of the entire picture. Augmented by modem experimental methods such as LFP, and theoretical tools (FMO and computational methods), we are now able to understand and manipulate carbenic philicity in an intellectually satisfying and synthetically useful manner. 

In a similar way, resonance stabilization of dissociated species can also be used to make C=C double bonds weaker. Instead of radicals formed from a single bond, dissociation of a double bond produces carbenes. Approaches to carbene stabilization can involve a variety of resonance patterns. For example, the C=C... 

The mobility of the proton in position 2 of a quaternized molecule and the kinetics of exchange with deuterium has been studied extensively (18-20) it is increased in a basic medium (21-23). The rate of exchange is close to that obtained with the base itself, and the protonated form is supposed to be the active intermediate (236, 664). The remarkable lability of 2-H has been ascribed to a number of factors, including a possible stabilizing resonance effect with contributions of both carbene and ylid structure. This latter may result from the interaction of a d orbital at the sulfur atom with the cr orbital out of the ring at C-2 (21). 

The diamagnetic ylide complexes 34 have been obtained from the reaction of electron-deficient complexes [MoH(SR)3(PMePh2)] and alkynes (HC=CTol for the scheme), via the formal insertion of the latter into the Mo-P bond. The structural data show that 34 corresponds to two different resonance-stabilized ylides forms 34a (a-vinyl form) and 34b (carbene ylide form) (Scheme 17) [73]. Concerning the group 7 recent examples of cis ylide rhenium complexes 36 cis-Me-Re-Me) have been reported from the reaction of the corresponding trans cationic alkyne derivatives 35 with PR" via a nucleophilic attack of this phosphine at the alkyne carbon. 

The term metalla-/3-diketonate refers to an anionic diacyl complex, such as L M(RCO)(R CO) , in which the two acyl ligands occupy adjacent coordination sites of the metal atom. Resonance stabilization of the negative charge by 7r-delocalization onto the two acyl ligands gives Lewis structures 1 and 2. The two acyl ligands acquire an acyl/carbene... 

Spectator substituents, bonded to the carbene s migration terminus (Ci), directly influence the lifetime and philicity of the carbene, but they do not primarily alter the migratory aptitudes of migrants on C2. Oxa spectator substituents stabilize singlet carbenes by electron donation to the vacant carbenic p orbital (LUMO) cf. resonance hybrid 69. 

Bertrand et al. demonstrated with the stable cyclic (aIkyl)(amino)carbenes (CAACs) 16 that only one nitrogen atom is sufficient for the stabilization of the carbene center. The resonance of the carbene carbon atom in carbenes of type 16 is... 

In contrast to methylene, carbenes which contain an oxygen atom bonded to the carbenic center are known to exist as singlet ground states. The rational for this is that the lone pair electrons on the adjacent oxygen atom have a resonance stabilizing effect on the electron deficient carbenic center ... 

The first nitrile ylide stable enough to be isolated (i.e., 1) has been prepared by the carbene/nitrile method (1). For this dipole, the anionic component is stabilized by electron delocalization and the nitrilium component by the steric bulk of the adamantyl group to such an effect that it has a melting point of 230 °C. The X-ray structure showed that the nitrile ylide moiety is close to linear and much like the resonance structure shown below. 

Even the F for Cl change transmuting CeHsCCl into CeHsCF is accompanied by a diminution in reactivity CeHsCF reacts about one-half as rapidly as CeHsCCl with the alkenes of Table 7.5. This lowering of reactivity can be attributed to greater stabilization of CeHsCX when X=F, as opposed to X=C1. Fluorine is a better resonance electron donor to the vacant carbene 2p orbital than chlorine, and this factor (rather than the greater electronegativity of fluorine) appears to be dominant. ... 

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. 

It seemed possible that treatment of cation 233a with a strong proton abstracting base could led to a resonance stabilized carben 234a (R = H). Pohl by... 

The effect of fluorination on the structure, stability, and reactivity of carbenes has been the subject of several reviews (also sec Houben-Weyl. Vol. E19b, p 1461).13,119 121 a-Fluorinated carbenes are ground-state singlet rather than triplet species as a consequence of resonance stabilization.122,123... 

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