Palladium‐Catalyzed Cascade Oligocyclizations Involving Competing Elementary Steps Such as Thermal [1,5]‐Acyl Shifts

Abstract

c ) Current address: Al-Balqa Applied University, As-salt, Jordan Palladium(Pd)-catalyzed oligocyclizations of 2-bromotetradec-1-ene-7,13-diynes with an unsubstituted terminal acetylene moiety like 3 and 5 and 15-bromohexadec-15-ene-3,9-diyn-2-ones like 4 and 6 afforded fulvene derivatives 20 and 21 (Scheme 7) and bis(cyclohexane)-annulated methylenecyclopentene systems 16 and 18 (Schemes 5 and 6), respectively. These transformations constitute cascades of cyclizing carbopalladation steps with ensuing [1,5]-sigmatropic H-atom and acyl shifts, respectively (Scheme 8). In contrast, analogous substrates with one three-atom and one four-atom tether between the unsaturated C,C-bonds, such as 1 and 2, behave differently in that the Pd-substituted hexa-1,3,5-triene intermediates 12 undergo a 6p-electrocyclization instead of a 5-exo-trig carbopalladation followed by bhydride elimination to furnish tricyclic bis-annulated benzene derivatives 13 and 14 (Scheme 4).Introduction. As we previously reported [4], the results of the Pd-catalyzed intramolecular oligocyclization of various 2-bromoalk-1-ene-7,13-diyne substrates initially leading to the formation of two six-membered rings, are highly affected by the lengths of the tethers between each two multiple bonds and the nature of the substituent at the acetylene terminus. As some of these oligocyclizations are followed by the formation of a bis-annulated cyclopentadiene system, [1,5]-sigmatropic rearrangements might become feasible subsequent processes for substrates bearing a terminal group with a high migratory aptitude such as an Ac group or an H-atom. Such bromoalkenediynes would, therefore, lead to yet another type of product, and we here report on the results of a corresponding investigation.

Cyclocarbopalladation as a Key Step in Cascade Reactions: Recent Developments

“…A number of examples implied the 5-exo cyclocarbopalladation. 5,[25][26][27][28][29] Werz et al have developed an elegant intramolecular domino approach to dibenzopentafulvalenes of type 54 with yields ranging from 39-91% by quadruple cyclocarbopalladations: 5-exo-dig/7-exo-dig/5-exo-dig/5-exo-trig (Scheme 15). 30…”

A Study of Palladium Catalyzed Intra/Intermolecular Cascade Cross Coupling/Cyclizations Involving Bicyclopropylidene

“…The so-called Mizoroki-Heck reaction, i.e. , the palladium-catalyzed arylation or alkenylation of alkenes has been applied in a variety of new concepts [5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]. In view of the steadily growing number of oligocyclic compounds with cyclopropyl moieties that show interesting biological activities [27,28,29], it is noteworthy and helpful that the highly reactive building block bicyclopropylidene 10 [30,31] can participate in various palladium-catalyzed cascade reactions to furnish complex skeletons containing cyclopropyl groups [17,18,19,20] A particularly high increase in molecular complexity can be achieved with sequential reactions starting with an intramolecular carbopalladation (the first step of a Heck reaction) of a 2-bromo-1-ene-6-yne and trapping of the formed reactive vinylpalladium halide intermediate by bicyclopropylidene 10 and subsequent transformations [32].…”