Coupling across carbonyl groups

As with the mono-ketones, coupling between groups on opposite sides of the carbonyl carbon is usually not observed. Similarly, coupling across the enolized carbonyl group (-C(OH)=CH) is not observed. 

INDO FPT calculations have been performed by Pachter and WesseP to study the dependence of Vhc and Vhc couplings on a nitrogen lone-pair and carbonyl group, and the obtained relationships have been used to study the spatial relationships in a diazepam molecule. Juc couplings across a sulfur atom in two series of compounds, both including a sulfide, a sulfoxide and a sulfone have been measured and calculated by Kleinpeter and co-workers by the use of the DPT method. 

Figure 5.58. The COLOC pulse sequence allows long-range couplings (e.g., between NH and CH proton across the carbonyl group in the peptide) to be observed.

Surprisingly, methyl esters are also suitable substrates whereby intramolecular cyclization occurs with concomitant loss of methyl iodide. Larock used internal alkynes as coupling partners for lactone synthesis (Scheme 2.33) [74]. The proposed mechanism involves oxidative addition of Pd(0) to the aryl iodide, followed by addition across the alkyne and cyclization of the carbonyl O of the ester to form an oxonium ion. Reductive elimination followed by loss of the methyl group then yields the product [74]. Shen and coworkers also reported a variant utilizing o-2,2-dibromovinylbenzoates (Scheme 2.34) [79]. 

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