Question

how do you explain the nmr trend of thiocarbonyls and carbonyls, the carbon shifts downfield while...

how do you explain the nmr trend of thiocarbonyls and carbonyls, the carbon shifts downfield while neighbouring protons shift upfield? Also in case of C=S the LUMO is lower than that of C=O then, C=S should be a better dieneophile for diels alder reaction right? can anyone explain this better??

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In NMR the thiocarbonyl carbon appears downfield to that of the carbonyl carbon. This is because the sulfur pulls the electrons less strongly than oxygen due to its lower electronegativity. Thereby the carbon is more shielded and appears downfield to that of carbonyl carbon. The sulfur also has lone pair of electrons which are easily given off to neighbouring atoms making the neighbouring protons shift upfield. In diels alder reaction, the dienophile always has an electron wthdrawing group attached to it. So moving from more electronegative C=O to less electronegative C=S would not be beneficial for the diels alder reaction.

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