The Stability of Carbenes

The stability of an unsubstituted carbene is quite low in water. Highly correlated ab initio MO calculations have been used to study the energetics and mechanism governing the reaction between the radical CH2 and H2O in the gas phase and in solution, and it was found that methylene reacts in a barrierless fashion to produce the ylide-like intermediate methyleneoxonium, H2C-OH2, which in turn undergoes a 1,2-hydrogen shift to produce CHaOH. The presence of substituents appears to stabilize carbenes toward water. 

3 The Reaction of Carbenes with Aikenes in Aqueous Medium 

Cyclopropanation reactions involving ethyl diazoacetate and olefins proceed with high efficiency in aqueous media using Rh(II) carboxy-lates. Nishiyama s Ru(II) Py-box and Katsuki s Co(II) salen complexes that allow for highly enantioselective cyclopropanations in organic solvents can also be applied to aqueous cyclopropanations with similar results. In-situ generation of ethyl diazoacetate and cyclopropanation also proceeds efficiently (Eq. 3.33).  

Insertions of dichlorocarbene into tertiary C-H bonds were observed even if these were not activated by neighboring phenyl or ether groups. Yields are 3-29% under the conditions used and this insertion does not require a thermoexcitation of the dichlorocarbene as was assumed earlier. 

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Wanzlick showed that the stability of carbenes is increased by a special substitution pattern of the disubstituted carbon atom [12-16]. Substituents in the vicinal position, which provide n-donor/a-acceptor character (Scheme 2, X), stabilize the lone pair by filling the p-orbital of the carbene carbon. The negative inductive effect reduces the electrophilicity and therefore also the reactivity of the singlet carbene. 

Nevertheless, a more traditional approach to the stabilization of carbenes and the investigation of their spectral properties deals with the direct generation of carbenes in low-temperature matrices, e.g. by the photolysis of diazo-compounds or ketenes. The method allows stabilization of carbenes in their ground electronic state, prevents intramolecular isomerization and also facilitates direct spectroscopic monitoring of their chemical transformations in low-temperature matrices. 

As heavier analogs of carbenes141) stannylenes can be used as ligands in transition-metal chemistry. The stability of carbene complexes is often explained by a synergetic c,7t-effect cr-donation from the lone electron pair of the carbon atom to the metal is compensated by a a-backdonation from filled orbitals of the metal to the empty p-orbital of the carbon atom. This concept cannot be transferred to stannylene complexes. Stannylenes are poor p-a-acceptors no base-stabilized stannylene (SnX2 B, B = electron donor) has ever been found to lose its base when coordinated with a transition metal (M - SnXj B). Up to now, stannylene complexes of transition metals were only synthesized starting from stable monomoleeular stannylenes. Divalent tin compounds are nevertheless efficient cr-donors as may be deduced from the displacement reactions (17)-(20) which open convenient routes to stannylene complexes. 

Of course, bulky substituents kinetically stabilize carbenes, but interestingly, during the course of our study, we realized that the stability of carbenes is often inversely proportional to the stability of the starting diazo compounds,27 as illustrated in Table I. 

Based on this concept and the development of appropriate synthetic methods, many heteroatom-substituted carbenes have been isolated since the first successful attempts by Igau et al. and by Arduengo et alP The stability of carbenes was... 

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. 

Many carbene complexes, especially those bearing one or two hydrogen substituents, are thermally unstable with respect to the formation of alkenes or their complexes (Figure 5.13), usually via bimolecular intermediates. For this reason the thermal stability of carbene complexes can normally be enhanced by inclusion of sterically demanding co-ligands. Often within a triad the stability of carbene complexes increases in the order 4d < 3d < 5d when analogous compounds can be obtained and compared. 

The deprotonation step was deduced from H/D exchange studies and the second stage from a steady-state treatment of the overall rates of hydrolysis. Stabilisation of the intermediate carbanions by halogen follows the order I > Br > Cl > F (Chapter 4, Section VII) and loss of halide ion in stage 2 is in the order of leaving group ability, I > Br > Cl > F. Therefore, it was possible to conclude that the effect of fluorine on the stability of carbenes is in the order F > Cl > Br > 1 [42]. 

Several excellent reviews covering stable carbene chemistry have been published since the first one by Herrmann and Kocher in 1997 [3]. They include the influence of the substituents on the stability of carbenes, the synthetic methods available, structural data, reactivity, coordination behavior, and the catalytic properties of the corresponding complexes. 

In this chapter, we have tried to emphasize general aspects of main-group chemical bonding, with particular emphasis on periodic trends. The periodic table remains the most important classification tool in chemistry, and it is crucial to understand even subtle secondary periodicities if one is to make efficient use of the various elements for different chemical applications. The radial nodal structure of the valence orbitals has been pointed out to account for more of the known trends than most practitioners of chemistry are aware of. For example, the inversion barriers of phosphines or silyl anions, the dependence of the inert-pair effect on the electronegativity of the substituents, the stability of carbene- or carbyne-type species or of multiple bonds between heavy main-group elements are aU intricately linked to hybridization defects of s- and p-valence orbitals of disparate sizes. Even the question of hypervalency is closely connected to the effects of primogenic repulsion . 

Bulky substituents kineticaUy stabilize phosphinocarbenes,but the stability of carbene is often inversely proportional to that of the diazo precursor. 

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