Cycloisomerizations enynes, palladium® acetate

Yet another palladium-catalyzed transformation leading to 1,2-dialkyl-idenecycloalkanes was established by Trost et al. when investigating a catalytic Alder-ene reaction (path D in Scheme 12). They showed that two different catalyst systems are capable of cycloisomerizing enynes 92 to either cyclic 1,4-dienes 96—the products of regular Alder-ene reactions— or the 1,3-dienes 95 (Scheme 15) [66-68]. Starting from palladium acetate, the reaction presumably occurs by coordination of both unsaturated moieties (intermediate 93) and subsequent cycloisomerization to the ring-... 

Another useful class of palladium-catalyzed cycloisomerizations is based on the general mechanistic pathway shown in Scheme 13. In this chemistry, a hydridopalladium acetate complex is regarded as the catalytically active species.27b-29 According to this pathway, coordination of a generic enyne such as 59 to the palladium metal center facilitates a hydropalladation reaction to give intermediate 60. With a pendant alkene, 60 can then participate in a ring-form-... 

Equation 26 illustrates the rare enyne cycloisomerization process which is feasible under relatively mild conditions (in acetic acid at 80 °C for 6 h) in the presence of a palladium (0) catalyst. In the presence of EtAlCl2, alkenes undergo ene reactions with 2-(dialkoxyphosphinoyl)propenoic esters (equation 27 " ) the same catalyst, and other Lewis acids, catalysed the intramolecular reactions of 385 and 386 ( = 0 or 1), which are exemplified by reaction 28 . 

A range of cyclization and cycloisomerization reactions of enynes, including enynes with heteroatoms in the skeleton, have been reported for which the possibility of Pd(IV) intermediates has been canvassedJ f t t A typical example is shown in Scheme 16, where supporting ligands at palladium include palladacyclopentadienes, acetate, acetate/PPhj, and acetate/P(o-tol)3. An alternative pathway involving a Pd(0)-Pd(II) cycle has been acknowledged (Scheme 16), and different reaction conditions may favor each pathwayJ Alkylidene complexes as intermediates have also been proposed for some reactions of enynes... 

---