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Electronic differentiations in Pd-catalyzed allylic substitutions are assessed computationally from transition structure models with electronically modified phospha-benzene-pyridine ligands. Although donor/acceptor substitutions at P and N ligand sites were expected to increase the site selectivity, i.e. the preference for "trans to P" attack at the allylic intermediate, acceptor/acceptor substitution yields the highest selectivity. Energetic and geometrical analyses of transition structures show that the sensitivity for electronic differentiation is crucial for this site selectivity. Early transition structures with acceptor substituted ligands give rise to more intensive Pd-allyl interactions, which transfer electronic P,N differentiation of the ligand more efficiently to the allyl termini and hence yield higher site selectivities.

The processes leading to the fragment ions formed from alkylthiophene molecule ions by benzylic cleavage without and with transfer of one hydrogen from the side‐chain to the ring and the influence of additional methyl groups on the relative importance of these two fragmentation reactions were investigated.

It is shown that thiophene-1,l-dioxides carrying an alkyl chain in the 2-and +positions can be readily distinguished by the characteristic fragmentation mode of the latter, viz. benzylic cleavage of the side-chain with back-transfer of two hydrogen atoms (I M --CnHIn-, 1 '). By deuterium labelling it is shown that the migrating hydrogens come very specitically from the 7-and &positions. Since alkylated thiophenes are important geological markers, a ready differentiation of the two types by gas chromatogaphy/mass spectrometry is possible in this way.