2 -ylidene, calculated

Irradiation of matrix-isolated imidazole-2-carboxylic acid gave the 2,3-dihydro-imidazol-2-ylidene-C02 complex (31) characterized by IR spectroscopy and calculated to lie 15.9 kcal mol above the starting material. A series of non-aromatic nucleophilic carbenes (32) were prepared by desulfurization of the corresponding thiones by molten potassium in boiling THF. The most hindered of the series (32 R = Bu) is stable indefinitely under exclusion of air and water and can be distilled without decomposition. The less hindered carbenes slowly dimerize to the corresponding alkenes. Stable aminoxy- and aminothiocarbenes (33 X = O, S) were prepared by deprotonation of iminium salts with lithium amide bases. The carbene carbon resonance appears at 260-297 ppm in the NMR spectrum and an X-ray structure determination of an aminooxycarbene indicated that electron donation from the nitrogen is more important than that from oxygen. These carbenes do not dimerize. 

In accord with calculations performed by Cavell et al. [110], the oxidative addition of C2-X functionalized azolium cations (X = halogen) to metal centers proceeds faster and with a more favorable reaction enthalpy than the oxidative addition of the C2-H substimted imidazolium cations [118, 119]. The former reaction was applied successfully for the preparation of nickel and palladium complexes bearing a variety of different ylidene ligands [119]. 

Similarly to imidazol-2-ylidene (IV), the calculated value for the energy of dimerization of Enders-type carbene VII is only 9.5 kcal/mol. " These remarkably small values, at least partially due to the loss of aromaticity in the carbene dimers IV 2 and VII 2, highlight the difficulty of dimerization of such carbenes. In contrast, in the case of the parent acyclic diaminocarbene IX, Heinemann and Thiel found a dimerization energy of 45 kcal/mol. This poses another question What is the value of the energy barrier for the dimerization Nowadays, the dimerization of singlet carbenes is believed to follow a nonleast motion pathway that involves the attack of the occupied in-plane ct lone pair of one singlet carbene center on the out-of-plane vacant orbital of a second carbene (Fig. 8.13). Calculations... 

Interestingly, Arduengo observed that Villa was stable with respect to dimerization in the absence of a Brpnsted or Lewis acid catalyst." Similarly, in the absence of an acid catalyst, dimerization of IXb is extremely slow and is first order in carbene. Therefore, the observed formal dimerization of Villa and IXb,c does not involve the coupling of two carbenes, but the nucleophilic attack of one carbene upon its conjugate acid, followed by proton elimination, as already suggested by Chen and Jordan (Scheme 8.13). It is important to keep in mind that even A,A-dialkylimidazolium ions have pXa values of 24 in DMSO, and based on the calculated proton acidity, Alder estimated the pXa values for acyclic diamino-carbenes to be from 2 to 6 pXa units higher than for imidazol-2-ylidenes. " ... 

The carbene l,3-di-l-adamantylimidazol-2-ylidene is additionally stabilized by aromaticity of the 5-membered ring and by steric protection by the neighboring adamantyl substituents. Similar carbenes with the adamantyl groups replaced by aryl groups are also stable [162]. An alkyl group may be regarded as a weak X -type substituent. The So state of dime thy lcarbene, (CH3)2C, is predicted to be only 5.9 kJ/mol lower than T by ab initio calculations [163]. 

In a detailed investigation of the kinetic behaviour of bases generated from (fluoren-9-ylidene)methane derivatives the problem has been overcome by computer simulation of current-time transients expected for the extended mechanism (including reaction 5), The program used for the comparison of simulated and experimental curves allows both kp and k to vary independently until the RMS deviation between the two i/t curves is minimised. The equilibrium constant for reproportionation (kf/kj) is calculable from values of Ep, (1) and Ep (2). It is important to realise that there may be any number of pairs of values of k and k which can give a good fit between experimental and simulated i/t curves. 

Azacyclohexatriene-2-ylidene (3), the 2-isomer of pyridine, has been generated by one-electron oxidation of the corresponding radical cation in neutralization-reionization mass spectrometry.17 It was determined by ab initio H-F calculations that the charge polarization of the radical fonned by H-abstraction from pyrazine can be... 

The dihydrothiazol-2-ylidene (4) was generated by photolysis of matrix-isolated thiazol-2-carboxylic acid.12 Calculations suggested that the barrier to isomerization to thiazole is about 42.3 kcal mol 1 and that the carbene resembles the related imidazol-2-ylidene in structure. An ab initio study of hydroxyoxiranone predicted that the decarboxylation of the zwitterion (5) to form hydroxycarbene (6) would be favourable in vacuo but not in water.13 A theoretical study showed that dihalosulfenes (X2C=S02) are best viewed as dihalocarbenc-SO complexes with a carbon-sulfur bond order of approximately zero.14 hi a study directed at the elusive thionformic acid (7), tandem mass spectrometric methods were applied to isomeric ethyl thioformates.15 The results suggest that the radical cations generated have the carbene structure [(HS)C(OH)]+ ... 

Most recently, stable crystalline l,3-di(weo-pentyl)-2-silabenzimidazol-2-ylidene, 79a, and its related saturated 2,2-dihydro-l,3-di(weo-pentyl)-2-silabenzodiaminosilole, 80a, were synthesized99. 79a can be stored under nitrogen for more than a year at ambient conditions and it can be distilled or sublimed without significant decomposition". 79a was characterized by its X-ray and by the UV photoelectron spectrum which was assigned with the help of MP2/6-31G //MP2/6-31G calculations for 79b"°. 

The rearrangements of 3,3-diphenylcyclobutylidene (88) and 2,2-diphenylcyclobut-ylidene (89) are revealed to be affected significantly by the diphenyl substitution.70 The former undergoes a 1,2-hydrogen shift whereas the latter prefers a 1,2-carbon shift. The data were acquired by steady-state and LFP experiments and the mechanism was probed by MRMP2//MP2 calculations. 

Leszczynski et al. have shown interest in comparing experimental (matrix isolation) and calculated (B3LYP) IR data (frequency and intensity) to discuss the tautomerism of benzo-annelated pyridonone, pyrazinone, and pyrimidinone (144-148). These equilibria were well reproduced by theoretical calculations carried out at the QCISD and QCISD(T) levels. The combined experimental and theoretical results reveal links between aromaticity and tautomerism. Moreover, a UV-induced phototautomeric reaction transforming the oxo forms into the hydroxy tautomers was observed for all (except 3-hydroxyisoquinoline) studied compounds [144], The interest of Leszczynski in problems related to tautomerism, aromaticity, and proton transfer is also apparent in a study of (lH-aza-hetero-2-ylidene)-acetaldehyde and 2-azahetero-2-yl-ethanol tautomeric pairs [145],... 

The affinity of l-ethyl-3-methylimidazol-2-ylidene for protons in the gas phase was measured at 251.3 kcalmol-1 by an extended version of the kinetic method.83 DFT calculations were performed and gave a result which differed by only 9.5 kcalmol-1, which confirms this compound as one of the strongest bases reported. 

The gas-phase proton affinity of the IV-heterocyclic carbene l-ethyl-3-methyl-imidazol-2-ylidene has been determined as 251.3 4 kcalmol-1 using the kinetic method, a value which makes the carbene one of the strongest bases reported thus far.160 Density functional theory calculations have been carried out at the B3FYP/ 6-31+G(d) level to compare the high experimental value with that estimated theoretically. 

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