Stability of carbenes

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. 

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. 

FIGURE 5.11 Proposed mechanisms of participation of N heteroatoms in electrochemical processes on carbon surfaces (a) possible pseudo-Faradaic reaction of the pyridinic group in aqueous medium [95] (b) N-induced stabilization of carbene-like edge carbon atoms, sites responsible for the development of positive charge in acidic solution. 

The influence of substituents on the electronic structure and stability of carbenes has been investigated, and many stable carbenes have been prepared. Bertrand et al. 

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. 

Lee and Wu [20] conducted a model study using diazomethane, which was reacted over various solids including ZSM-5, with the aim of observing possible surface-stabilization of carbene. Although olefins were formed even with "inert" materials such as Vycor, hydrocarbon yields were seen to increase substantially in the presence of acidic surfaces. They proposed the formation of a methyloxonium species via interaction with an acid site. This species reacts with excess CH2N2, forminq an ethvloxonium species by C-H insertion, B-Elimin-ation affords ethylene. 

Theoretical justification for framework-stabilization of carbene was provided by ab initio (MINDU3) calculations of Drenth et al. [14]. The zeolite acid... 

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 . 

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