C-H-0 Interactions

The HMQC spectrum of podophyllotoxin shows heteronuclear crosspeaks for all 13 protonated carbons. Each cross-peak represents a one-bond correlation between the C nucleus and the attached proton. It also allows us to identify the pairs of geminally coupled protons, since both protons display cross-peaks with the same carbon. For instance, peaks A and B represent the one-bond correlations between protons at 8 4.10 and 4.50 with the carbon at 8 71.0 and thus represent a methylene group (C-15). Cross-peak D is due to the heteronuclear correlation between the C-4 proton at 8 4.70 and the carbon at 8 72.0, assignable to the oxygen-bearing benzylic C-4. Heteronuclear shift correlations between the aromatic protons and carbons are easily distinguishable as cross-peaks J-L, while I represents C/H interactions between the methylenedioxy protons (8 5.90) and the carbon at 8 101.5. The C-NMR and H-NMR chemical shift assignments based on the HMQC cross-peaks are summarized on the structure. 

The selectivity in the Heck reaction of allylic alcohol 111 is interesting, and the factors that lead to the observed preference for (3-hydride elimination toward nitrogen in this system are unclear, although a combination of steric effects and stereoelectronic factors (i.e., alignment of C-H and C-Pd bonds, nN a c H interactions) is likely involved. Examination of related examples from the literature (Scheme 4.20) reveals no clear trend. Rawal and Michoud examined substrate 115, which lacks the influence of both the amine and hydroxyl substituents and also seems to favor (3-hydride elimination within the six-membered ring over formation of the exocyclic olefin under standard Heck conditions [18a]. However, under... 

The characteristic tendency of hypovalent transition metals to interact in a side-on (T-shaped) manner with nearby alkyl C—H bonds has been designated as the agostic effect.45 Agostic M- C—H interactions can also be identified with general 3c/2e donor-acceptor interactions of 2c— lc type. Availability of suitable lc acceptor orbitals is a signature of hypovalent early transition metals. 

Figure 4.55 The optimized structure of YH2(C2H5), showing the T-shaped geometry of the agostic Y C—H interaction.

Therefore, there is a certain equilibrium distance r , at which the dispersive-attractive and the repulsive forces balance and the system achieves minimum energy at the minimum of potential curve y(r ). The van der Walls radius, r, for the C-H interaction can be assumed to be about 0.16 nm. 

From a conformation point of view, one of the relevant systemsto examine is the ( + )-(Coen3)3+ ion. Assuming the ion adopts the kkk (lei) conformation (D3), the theoretical C-proton spectrum should be three equivalent sets of A2B2 lines, ignoring for the moment the N—H and C—H interactions. In fact only a broad line (21 cps, Fig. 15) is observed, which narrows slightly (18 cps) when the NH2 groups are deuterated (32). 

The N-(bicycloalkenyl)nitrone (37) afforded the tetracyclic isoxazolidine (38) in 67% yield none of the regioisomeric isoxazolidine was observed (Scheme 9).17 The authors attributed this to non-bonded C—H interactions as shown, but the observed isoxazolidine was also entropically favored, forming via a six-membered, as opposed to a seven-membered, carbocyclic transition state. The N-(bicycloalkenyl)ni-trone formed from (39) and furfural cyclized in 45% yield predominantly to the tetracyclic product (40b), but some (41b) was also produced (95 5 ratio).18 Reaction of (39) and formaldehyde gave a mixture of (40a) and (41a) (62 38 ratio). The authors attribute the somewhat higher regioselectivity for (40b) in part to non-bonded interaction of the 2-furyl substituted methylene with the C-8 endocyclic C—H bond. 

Which crystal structure is adopted is based on the optimisation of C---C interactions, which are at a maximum between parallel molecules stacked in an offset face-to-face fashion at van der Waals separation (glide interactions), and C---H interactions, which are optimum in inclined molecules as a result of their electrostatic nature (stack interactions). The y- and / -structures are adopted in cases... 

The effective ionization and characteristic radii for the heteronuclear C-H interaction, Re = /rc th, are used to calculate AE = 460x0.01/1.2 = 3.83 kJmol-1. Using the same dissociation energy an isolated bond length of 1.03 A is calculated, compared to the observed 1.08 A. Using this difference to calculate... 

Zero-order interaction in a heteroatomic pair is calculated in essentially the same way. The C- H interaction serves to illustrate the method. Starting from the first-order interaction d = 1.08 A, d = 0.90, be = 0.1377, D = 0.3983. Hence... 

Moursi, A. M., Globus, R. K., and Damsky, C. H., Interactions between integrin receptors and fibronectin are required for calvarial osteoblast differentiation in vitro. J. Cell Sci. 110, 2187-2196 (1997). 

Bartell has pointed out the C-H and C-C interactions also play an important role. As we shall see, more important than the inclusion of the C-C, C-H interaction is the overall barrier for the planar conformation. BartelFs potential is of the form ... 

Given this structure, it is natural to envision four individual C—H interactions (we call them bonds). The energy of stabilization of CF14 is divided equally among the four bonds to give an average C—H bond energy per mole of C—H bonds ... 

Typical examples are the Ag(I) complexes that are synthesized by mixing the hydrocarbon with the soluble silver salt in HjO. The anion influences the stability of the Ag(I)-C=C-h interaction, and this decreases [BFJ < [004] < [N03] . Unsaturated molecules that coordinate to cationic metal-IB centers include ... 

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