Proton carbon from heterocyclic systems

In Scheme 22 another reaction is described in which a C-H bond is formed by proton transfer from the solvent (H2O) to a carbon atom of the substrate. For this reaction to occur, a high electron density is required at the site of protonation. This reaction is the opposite to proton-transfer reactions that electron deficient species (e.g. radical cations) undergo, namely proton transfer from a carbon to the solvent water, as discussed in Section 9.5.1. There are some interesting examples, of biological relevance, of proton transfer to carbon from heterocyclic systems (Schemes 23 and 24) ... 

Figure 23. Correlation of observed and calculated changes in carbon-13 shifts for a number of heterocyclic systems upon protonation. For data and references see Table 12. ° Data from Adam et al. (1969) (Column A) Data from Pugmire and Grant (1968) ignoring changes in A (Column B). x Data from Pugmire and Grant (1968) corrected for changes in A (Column C).

The carbon-boron heterocycle, 3-phenyl-3-benzoborepin, exhibits oxidative stability upon exposure to air, an unusual feature for a trivalent boron compound. In Table XVI are recorded the chemical shift data for the vinyl protons for the benzometallepins of B, Sn, and Si. The PMR spectrum of 3-phenyl-3-benzoborepin exhibits vinyl proton resonances at lower fields than would be expected for an olefinic boron compound (compared to trivinylboron or 4,5-dihydroborepin see Table XV), and also at lower field than the benzostannepin derivative (217). The shift to lower field of 0.4 to 0.8 ppm may be consistent with the presence of a ring current, which would require the participation of the Bp orbital in the 7r-electron system. Support for increased electron density at boron might be provided from B NMR measurements, but such data have not yet been reported. Complexation of boron, which converts the... 

As is the case for substituent effects on proton and carbon, but not phosphorous NMR chemical shifts, nitrogen chemical shifts exhibit deshielding when in the vicinity of substituents that are more electronegative than carbon (Scheme 2.11). Presumably, although there are little data to support this assertion, the effect diminishes rapidly with distance of the substituent from the nitrogen, as is the case for proton and carbon chemical shifts. However, in aromatic heterocycles, such as pyridine, the effect of the substituent is transmitted through the n system, as indicated by the significant influence of para substituents of pyridines (Scheme 2.12).10... 

The attachment of the cyclopentapeptide lactone rings to the carboxy functions at C-1 and C-9 of the actinocin heterocycle B can be deduced from the HMBC plot d Protons 1-H (Sh = 7.35) and S-H Sh = 7.62) of the heterocycle display an AB system in the proton domain. The attached carbon nuclei have been assigned by CH COSY in the literature (5///5c = 7.55/130.33 and 7.(52/125.93, ref p. 426) the other carbon atoms of the benzenoid ring within the phenoxazone are assigned as reported by correlation via Jqh and Vc// coupling detected in d which additionally shows a weak Jqh correlation signal of 8-iTwith C-5a (5///5c = 7.(52/140.53). 

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