C-H Donors

The reasoning applied above to 0 0 interactions can be extended to weak C-H 0 bonds. The presence of a short C-H 0 distance is not necessarily indicative of an overall attractive interaction, if the C-H donor and the O acceptor groups belong to anions, as illustrated in the following example. The structure of the mixed-valent species [(ri6-C6H6)2Cr] + [Cr03(0CH3)] [34] is based on columns of methoxychromate anions and of columns of bis-benzene chromium... 

The intramolecular kinetic isotope effect determined in reaction of BTNO with p-MeO-C6H4CH(D)0H gave a h/ d ratio of 5.6 in MeCN , consistent with a rate-determining H-abstraction step. Additional determinations gave a h/ d of 7 with PhCH(D)OH, and 12 for the intermolecular competition of fluorene vs. 9,9-dideuteriofluorene. The latter value supports the contribution of tunnelling already commented on for reaction of PINO with various C—H donors ( h/ d values in the 11-27 range) . ... 

It should be mentioned that in the publications of Cristiano et al. < 1996J(P 1) 1453, 1997J(P2)489, 1997JCM164, 2006JOC3583>, apart from the thermal process, an alternative reaction pathway was considered which is Pd/C, H-donor hydrogenolysis (Equation 80). 

Bifurcated acceptors are common, since most organometallic crystals have an excess of C-H donors over OC(M) acceptors. 

In comparison with the rich chemistry of a radical anions the synthetic utility of a radical cations is quite restricted, because pure a donors are mostly limited to Si-Si, C-Si, C-Ge, and C-Sn functionalities (Scheme 18). Only strained carbocyclic compounds [119] have low ionization energies (/ ) that make them readily accessible (t(C-C) and cr(C-H) donors, but they are of limited synthetic use. In contrast, Me3M-MMe3 (M = Sn, Ge, Si lEs ranging from 8.0 to 8.3 eV) [5] and Me4M (M = Sn, Ge, Si lEs ranging from 8.9 to 9.8 eV) [5] are, in general, better donors... 

Steiner, T., Starikov, E. B. and Tamm, M. (1996) Weak hydrogen bonding. Part 3. A benzyl group accepting equally strong hydrogen bonds from O-H and C-H donors. 5-Ethynyl-5-H-dibenzo[a,d]cyclohepten-5-ol, J. Chem. Soc. Perkin Trans. 2, 67-71. 

Novoa, J.J., Whangbo, M.-H., and Williams, J.M. (1990) Ab initio computational study of the C H- -donor and C-H- - -anion contact interactions in organic donor salts. Mol Cryst. Liquid Cryst. 181, 25-42. 

Figure 5. H -O frequency distributions in crystals for hydrogen bonds from C=C-H donors to (top) C=0 acceptors, n = 33, and (bottom) C-OH acceptors, n = 31. Database analysis perform for this article [CSD, June 1997 update with 167797 entries, ordered and error-free crystal structures with R < 0.10, normalized H-atom position based on a linear C=C-H group, and a C-H bond distance of 1.08 A neither donor nor acceptor group directly bonded to a metal atom, H -O < 2.95 A for three-center hydrogen bonds, only the short component is considered).

Halide ions are among the strongest hydrogen bond acceptors, and hydrogen bonds of terminal acetylenes to halides are frequently observed. The distribution of H" Cr distances in crystals is shown in Figure 12 in comparison with related distributions for other C-H donors. For chloride ions, the mean H - Hal" distance is 2.56(4) A, and for bromide ions, it is 2.70(6) A [the mean C—Hal distances are 3.58(4) and 3.72(5) A, respectively] [5/]. 

Figure 12. Distributions of H -CI separations for hydrogen bonds (H- -CI < 3.0 A) from three different types of C-H donors. Drawn using published data [57].

In essence, the above observations show that in hydrogen bond networks, the terminal alkyne group can play the same role as the hydroxyl group, and is in many instances found involved in mixed networks where hydroxyl and alkynyl groups are isofunctional. This is a behavior that distinguishes terminal alkynes from all other C-H donors and Ji-acceptors. 

The effect of alkyl substiments on the stabilities of carbenium ions provides the electronic basis of the textbook Markovnikov s rule. The stabilizing effect of positive hyperconjugation increases for stronger o-donors. For example, the stabihzing effect of a silyl substituent in p-silylethyl cation is calculated to be ca. 38kcal/mol stronger than a C-H donor of the ethyl cation in the gas phase (see Section 6.3). The effects of Ge, Sn, and Hg are also substantial. For example, hyperconjugative activation by a Sn-C bond can accelerate a reaction by a factor of >10 . ... 

Martinez-Mayorga, K., Juaristi, E., Cuevas, G. (2004). Manifestation of Stereoelectronic Effects on the Calculated Carbon-Hydrogen Bond Lengths and One-Bond 17C-H NMR Coupling Constants. Relative Acceptor Ability of the Carbonyl (C = 0), Thiocarbonyl (C = S) and Methylidene (C = CH2) Groups toward C-H Donor Bonds. Journal of Organic Chemistry, 69, 7266-7276. 

A persistent theme in MDPU, DNPU and PNPU crystal structures is the absence of the urea tape network but recurrence of the intramolecular C-H- - -Ourea synthon (Figure 5.18). A slight activation of the proximal C-H donor by an electron-withdrawing group is necessary. The C-H- - -O interaction occurs on the pyridyl side but not the phenyl side in unsymmetrical urea [23]. Similarly, the N02-Ph moiety is co-planar with the urea group but the X-Ph side adopts different conformations in PNPU structures [27]. The recurrence of the intramolecular C-H- - -Ourea interaction was also noted by Steed and co-workers [28] in closely related pyridyl urea systems (Figure 5.19). 

Davies and co-workers found that C-H donor/acceptor substituted carbenoids were capable of undergoing highly regioselective insertions. Thus, this strategy has been shown to be an aldol reaction equivalent, " Claisen condensation equivalent, Mannich reaction equivalent, and other important carbon-carbon bonding forming reaction equivalents. ... 

---