Dienes from the thermolysis of dicobalthexacarbonyl-complexed enynes: Mechanistic insight

Synthesis of 1,2,3,4-Tetrahydroisoquinoline-3-carboxylic Acid (Tic) Derivatives by Cycloaddition Approaches

“…Despite the large number of literature reports concerning the synthesis of carbocycles, very few nitrogen‐containing heterocycles have been prepared by isomerization methodologies. In this regard, Krafft and co‐workers have reported the thermolysis of the hexacarbonyldicobalt complex of a 1,7‐enyne to yield a monocyclic 1,3‐diene 15. Having prepared the dienes 23 and 24 , we turned our attention to the preparation of silylated exocyclic dienes.…”

Pauson-Khand Reactions of Alkenyl Sulfones and Alkenyl Sulfoxides: Applications in Asymmetric Synthesis

“…Thus, it was widely accepted since the seminal work of Pauson and Khand, that electron-deficient alkenes, such as a,b-unsaturated aldehydes, ketones, esters and nitriles, were not suitable substrates in PKR. 23 In these cases, it was assumed that the key cobaltacycle intermediate preferentially underwent a b-H elimination process, leading to the observed conjugated 1,3-diene, rather than the carbonyl insertion step required for the formation of the cyclopentenone product 24 (Scheme 2). This result is in sharp contrast with the PKR of alkenes with electron-donating substituents (like alkyl 25 or ether groups 26 ) or strained C-C double bonds (like allenes 27 or cyclopropene groups 28 ), which afford the expected cyclopentenone products.…”

Cobalt mediated cyclisations

“…Krafft has also reported the cycloisomerisation of enyne substrates to conjugated dienes through their cobalt alkyne complexes. 44 Thermolysis of 1,6-or 1,7-enynes gave monocyclic 1,3-dienes. However, deuterium labelling studies have shown that the products come from an α-elimination pathway, as opposed to the expected β-elimination.…”