Carbenes proton affinity

Alternatively, enthalpies of formation of carbenes and biradicals can be measured by using the approach shown in Eq. 5.4b. The key to the measurement is the determination of the proton affinity of the substrate, PA(R), which can be obtained... 

The reactivities of carbenes toward alkenes have been correlated with the inductive and resonance effects of the carbene substituents, log k — a Eat + fcEaR+ + c.m Analogous correlations cannot be obtained for the reaction rates of carbenes with alcohols, neither with the substituent parameters used by Moss,109 nor with related sets.110 In particular, the substituent parameters do not describe the strong, rate-enhancing effect of aryl groups. For a detailed analysis, see the discussion of proton affinities (Section V.A). 

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). 

Table 6. Experimental Gas Phase Basicities (GB) and Proton Affinities (PA) of Carbenes and Reference Compounds (298 K)...

The proton affinity of a carbene can be derived by means of Eq. 9 (M = RR C ) if AHf of the carbene and of the corresponding carbocation have been estimated independently (Table 7). Appearance potentials (AP) are convenient, although sometimes inaccurate, sources of AHf (RR CH+).129 Adiabatic ionization energies (IEa) of free radicals, in combination with dissociation enthalpies... 

Figure 4. Proton affinities (PA) of carbenes from Table 7 (circles) and Table 8 (squares) as a function of Taft parameters for proton transfer equilibria.110,150 The coefficients for Ecra, Ectf, and Ectr+ were optimized, with PA( CH2) = 206 kcal/mol constant (r2 = 0.897).

The heats and rates of reaction of carbenes with substituted pyridines to form ylides have been measured and used to calculate the ylides heats of formation.54 The heats of reaction of methylchloro- and phenylchlorocarbene with were found to correlate well with the pXa s and proton affinities of the pyridines. However, the correlation is not good for sterically demanding... 

A more general method for preparing carbenes often involves the a elimination of halides from carbanions.1-57 PAC can be used to examine the rates and energetics of the reverse reactions, the complexation of halides with carbenes (Fig. 5).58 Plots of A//com versus the proton affinities (PA) of the halides are linear for the two carbenes studied. Although the slopes of the plots are similar, complexation of the halides with phenylchlorocarbene is more exothermic than phenylfluorocarbene. This indicates that fluoro substitution stabilizes the carbene relative to the carbanion more than chloro substitution. The rate of complexation of carbenes with salts has also been examined by nanosecond absorption spectroscopy.59... 

Carbocations have similar electronic structures to carbenes. The P-protonated derivative of phosphinine should also be similar to 23. Indeed, while investigating the proton affinity of 3. the most preferred protonation site was phosphorus and not carbon, whereby the cyclic jt system would be interrupted. ... 

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. 

Singlet carbenes can be viewed as the conjugate bases of secondary carbo-cations, and so the pKgS and proton affinities of diaminocarbenes are of fundamental interest. As mentioned in section 1.2, we were able to measure the pKg of 2,4-diisopropyl-3,4-dimethylimidazol-2-ylidene on the DMSO scale as 24. Unfortunately attempts to measure the pKg of non-aromatic diaminocarbenes on the same scale have not been successful, due to side reactions (see Scheme 1). 

However Denk et al. [74] recently reported that carbenes 34 and 11, R = i-Bu, are inert towards dry (triplet) oxygen. When exposed to moist air, they only produce the hydrolysis products 35 and 36 (Scheme 11). Hydrolysis of 11, R = i-Bu is instantaneous, but for 34, hydrolysis takes days to become noticeable This suggests a significant difference in pKg between these two carbenes, contrary to what might be expected from the proton affinities for related carbenes described in Section 2.3.3. 

Lee and Houk calculated gas phase proton affinities of orotate and depro-tonated uracil, which suggest that 0-4 rather than 0-2 is the favorable site of protonation for substrate OMP [24]. On the basis of these findings, Lee and Houk proposed a carbene-based mechanism that involves protonation at 0-4 by either an active-site acidic residue or a site-bound water molecule (Fig. 3c) [24, 25]. In this mechanism, the formation of a neutral carbene at C-6 is stabilized by an active site environment that displays a low dielectric constant. The recent determination of the crystal structures of ODCase (see below) questions the plausibility of this mechanism. These structures reveal a highly charged active site, one that might be poorly suited for stabilization of an uncharged carbene. The structures also demonstrate the lack of an acidic residue near the 0-4 atoms of bound ligands. 

Deprotonation of the species MCHj (M = Fe, Co) with a series of reference bases has been monitored, yielding the proton affinities of the corresponding carbene MCH2 species. Detailed kinetic studies have also been reported for the hydrogenation of the osmium alkyl bond in the complex (55) (L = PMea) [see Eq. (16)] which was shown to proceed by an acid-catalyzed path. ... 

Formally this molecule now contains a silicon(O) central atom and could be interpreted as a silaallene if there were two Si=C double bonds (analogue a in Fig. 25). The substituents, however, make a difference. If the adjacent substituents are carbenes, then the molecule should rather be called a carbodicarbene or a Carbone (b in Fig. 25). They have been predicted from theory by Gemot Frenking et al. to be bent (C-C-C a 135°) and show two remarkable proton affinities (292 and 155 kcal/mol) [89, 90]. A year later those molecules were synthesized and employed in metal coordination by Guy Bertrand et al. and Alois Fiirstner et al. [91-94]. Remarkably even the trisilaallene synthesized by Kira et al. shows a bent stmcture of 136° at the central silicon atom (c in Fig. 25) [95]. 

---