Direct C-H couplings

Information about direct C-H couplings (coupling constants between 120 Hz and 170 Hz) can be seen in the spectrum of the short range C-H chemical shift correlation... 

Figure 12.17. The relation between direct C-H coupling constants and the hybridization of the carbon atom.

Figure 12.36 Selected direct C-H coupling constants calculated at B3LYP/6-3 lC(d,p) level.

Figure 12.40 Conformational effects on direct C-H coupling in esters.

However, the seemingly perfect unison of hyperconjugation and direct C-H coupling constants is not general. More recently, however, Cuevas, Perrin, Juaristi, and coworkers showed the simple picture suggested... 

Figure 12.45 Effect of interactions between bonds and n-systems on the direct C-H coupling constants, calculated at B3LYP/6-3IC(d,p) ieveP...

In 2011, C-H arylations of indoles and pyrroles with aryliodonium salts as coupling partner were reported by Sanford [65] and Ackermarm [66]. As mentioned earlier, previous studies from the Sanford group [52] had shown that palladium catalysts can assist the direct arylation of indoles using aryliodonium salts more recent studies attempted the coupling of aryliodonium salts with more sterically hindered heteroarenes such as 2,5-disubstituted pyrroles. Meanwhile, Ackermann reported a transition-metal-free direct C-H coupling of indoles and aryliodonium salts. 

In 2012, Itami and Yamaguchi discovered the bisoxazoHne-Pd catalysts such as 173 and 174 that enable the synthesis of hindered heterobiaryls by direct C-H coupling. Moreover, they demonstrated the first enantioselective C-H biaryl cross-coupling (Scheme 17.49) [227]. For example, when an n-PrOH solution of thiophene 175 and arylboronic acid 176 was treated with a Pd(OAc)2/biox 174 catalyst and TEMPO at 70 °C under air, (S)-177 was obtained in 41% ee (63% yield). When a more hindered arylboronic add 178 was used, the enantiometric excess of product (S)-179 was increased to 72% at the expense of lower yield. 

Pyridine N-oxides undergo palladium-catalyzed direct C-H coupling with 2-aryl-l, 2,3-triazole N-oxides (Scheme 35) (130L4682). The reaction occurs regioselectively. In addition to tolerating substituents on the pyridine N-oxide, the aryl group of the 1,2,3-triazole can also have a range... 

Scheme 33 Direct C-H coupling of pyridine A/-oxides with alkyl bromides.

By recording the spectrum at very high sensitivity Snyder and Meiboom could also observe the spectra of those protons that are attached to C-isotopes in natural abundance of 1.1%. The analysis of these spectra yields values of all indirect and direct C-H-coupling constants. The complete set of H-H and C—H-coupling constants of cyclohexane is summarized in Table 3. 

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