Evidence for Carbene Protonation

Considering the abundant evidence for carbene protonation, some quantitative estimate for the base strength of carbenes is clearly desirable. The conventional spectrometric or potentiometric methods of determining the pKa in solution are not applicable, with the exception of some onium ions 1 and their conjugate bases 2 (Section V.B). In favorable cases, equilibria of carbenes with the conjugate carbenium ions have been studied in the gas phase. Proton affinities of various carbenes can be obtained from their enthalpies of formation, and by ab initio computation (Section V.A). Kinetic data have been evaluated to obtain the pKa of carbenes in solution (Section V.B). 

Diazoalkanes are u.seful is precursors to ruthenium and osmium alkylidene porphyrin complexes, and have also been investigated in iron porphyrin chemistry. In an attempt to prepare iron porphyrin carbene complexes containing an oxygen atom on the /(-carbon atom of the carbene, the reaction of the diazoketone PhC(0)C(Ni)CH3 with Fe(TpCIPP) was undertaken. A low spin, diamagnetic carbene complex formulated as Fe(TpCIPP)(=C(CH3)C(0)Ph) was identified by U V-visible and fI NMR spectroscopy and elemental analysis. Addition of CF3CO2H to this rapidly produced the protonated N-alkyl porphyrin, and Bit oxidation in the presence of sodium dithionitc gave the iron(II) N-alkyl porphyrin, both reactions evidence for Fe-to-N migration processes. ... 

The metal protonated adducts (100) could not be detected directly because they are very acidic (p < ) This means they cannot be generated at the required pH because the thiolate ions are protonated under these conditions and the corresponding thiols are not reactive enough to add to the carbene complex. However there is kinetic evidence for 100. When 99 that has been generated at high pH is reacted with HCl, the pseudo-first-order rate constant for the conversion of 99 back to 98 shows a non-linear dependence on an+, as shown in Fig. 5 for some representative examples. This dependence is consistent with H -catalyzed... 

Conclusive evidence for the cation intermediate was obtained by detection of the absorption spectra of several diarylmethyl cations following nanosecond or picosecond laser flash photolysis of their respective diazo, diphenylazi-ridinylimine, or 3H-indazole precursors in acidic media [107-112], Photochemically generated vinyl carbenes were recently shown to similarly protonate by deuterium labeling experiments to give allylic cations that were detected by transient absorption spectroscopy [113],... 

The mechanism for the addition of singlet carbenes to alcohols has been studied in some detail (Bethell et al, 1971 Kirmse et al, 1981). By and large, the evidence supports two routes. The first, more common, sequence features initial formation of an ylid. Under some circumstances this reaction is reversible (Zupancic et al., 1985 Liu and Subramanian, 1984 Warner and Chu, 1984). Next, proton transfer, either intramolecularly, which may be slowed by symmetry constraints, or by a pair of intermolecular protonation and deprotonation steps, gives the ether. These reactions are outlined in (7). 

While it is intellectually pleasing to find self-consistencies in this mechanism, such as the fact that XII, XIII, and XV would be the Grignard species formed anew that compete against II for proton abstraction in the first step (see experimental evidence, (2)), and that the allenic carbene approach is also attractive, there are still other alternative interpretations to be considered. First, the Sn2 pathway that accounts for II is supported by experimental evidence (3). The authors of the original paper contend, however, that this route is disfavored in the light of their extended studies about the transition metal salt catalyzed Grignard reactions of propargyl halides, wherefrom allenes invariably result. 

Regarding the conversion of 135 into 162, there are two possible pathways, stepwise or concerted (Scheme 17). The stepwise pathway entails rate-limiting protonation on the metal followed by rapid reductive elimination while the concerted pathway involves protonation on the carbene carbon which is simultaneous with bond cleavage between the metal and the carbene carbon. For the reactions with tertiary amines, e.g., pyridine, in the absence of water, the stepwise analog of Scheme 17, i.e., equation (106), has been advocated although no experimental evidence has been cited to exclude a concerted pathway. 

Labeling studies provided some evidence that aldehydes are not intermediates in the formation of heptanol from 1-octene but a hydroxy carbene-like intermediate (Scheme 5.50) [63]. The latter derives from the protonation of the relevant Rh-acyl complex by ethanol and benefits from the high electron density at the metal center, which is caused by trialkylphosphines. The reaction with hydrogen (here D2) produces the alcohol. A similar mechanism was suggested for the tandem reaction with 2-propen-l-ol as a substrate [64]. 

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