Allenes as Carbon Nucleophiles in Intramolecular Attack on (π‐1,3‐Diene)palladium Complexes: Evidence for trans Carbopalladation of the 1,3‐Diene

Abstract

Reaction of allene-substituted cyclohexa- and cyclohepta-1,3-dienes with [PdCl(2)(PhCN)(2)] gave eta(3)-(1,2,3)-cyclohexenyl- and eta(3)-(1,2,3)-cycloheptenylpalladium complexes, respectively, in which C-C bond formation between the allene and the 1,3-diene has occurred. Analysis of the (pi-allyl)palladium complexes by NMR spectroscopy, using reporter ligands, shows that the C-C bond formation has occurred by a trans carbopalladation involving nucleophilic attack by the middle carbon atom of the allene on a (pi-diene)palladium(II) complex. The stereochemistry of the (pi-allyl)palladium complexes was confirmed by benzoquinone-induced stereoselective transformations to allylic acetates.

Palladium‐Catalyzed Oxidative Carbocyclizations

“…[82] External trans attack by p nucleophiles on p-coordinated unsaturated hydrocarbons is rare, but has been reported to take place with allenes as p nucleophiles. [83] It is worth noting that the observed trans carbopalladation in Schemes 82 and 83 is different from the indole annulation achieved by a cis carbopalladation process reported by Stoltz (see Scheme 17). [24a,b] Furthermore, protection of the indole N À H group is not necessary in the reactions in Widenhoefers procedure (Schemes 82 and 83) in contrast to Stoltzs approach (Scheme 17).…”

Scope and Mechanism of the PdII‐Catalyzed Arylation/Carboalkoxylation of Unactivated Olefins with Indoles

“…In comparison, 2-(5-hexenyl)-1-methylindole (13) undergoes palladium-catalyzed 7-exo-trig cyclization to form 14 in 74 % yield (Table 2, entry 14). It is noteworthy that significant amounts of a chlorinated intermediate, [9] presumably 3-chloro-2-(5-hexenyl)-1-methylindole (15), are generated early in the palladium-catalyzed cyclization of 13; compound 15 is then converted slowly into 14. More specifically, after 15 min at room temperature 63 % of 13 had been con- www.chemeurj.org sumed to form a 9:1 mixture of 15 and 14, which together account for 82 % of the reaction products.…”

“…The anti stereochemistry of the carbopalladation of an olefin with a p-nucleophile has previously been observed in the Pd II -catalyzed addition of allenes to 1,3-dienes. [15] The C(3) position of an indole is more nucleophilic than the C(2) position. For this reason, we also considered a mechanism for the stereospecific cyclization of (E)-and (Z)-31 involving outer-sphere attack of the C(3) carbon atom of the indole on a Pd-complexed olefin to form the spirocyclic iminium ion intermediate IVa, followed by stereospecific 1,2-migration of the primary alkyl group to the C(2) carbon atom of the indole (Scheme 3).…”

Palladium-catalyzed approach to the synthesis of five-membered O-heterocycles

“…As this nucleophile attacks the face lying opposite to that of the palladium atom, the result is a trans-carbopalladation of the diene, hence, a cisring junction. The use of different reaction conditions can control the stereochemical outcome of the second nucleophilic attack, brought about by AcOH [158]. Numerous external nucleophiles such as carboxylic acids, alcohols, phenols, and thiophenols were used to introduce a variety of substituents at the C-4 position of the bicyclic system.…”