2-Bromotetradec-1-ene-7,13-diynes 1, 5, 12 with different substituents at the acetylenic terminus under palladium catalysis have been found to either yield an angularly bisannelated benzene derivative 3 (79%), or fulvene derivatives of type 5 (52-74%) as well as monoannelated fulvenes 13b-e (32-48%). The mechanisms for all three reaction modes most probably involve intramolecular 5-exo-trig carbopalladations of an intermediate 1,3,5-hexatrienyl-6-palladium bromide intermediate. Transition-metal-catalyzed C-C-bond forming reactions have been widely explored in the past 15 years. Among them, palladium-catalyzed cross-couplings have attracted considerable attention in both academic and industrial research laboratories.1 Towards the construction of complex organic molecules from relatively simple open chain precursors, the Heck reaction is particularly attractive, as it is compatible with a variety of functional groups and can be incorporated more than once in multistep sequential processes 2 following a number of distinctly different reaction pathways.1,3 Thus, single operation tricyclizations of 2-bromoalk-1-enediynes have been used to prepare carbocyclic as well as heterocyclic angularly bisannelated benzene derivatives in moderate to good yields. 4 All previously reported examples, however, contained either two five-or one five-and one six-membered ring attached to the aromatic central ring. In order to test the accessibility of the octahydrophenanthrene skeleton by this route, we have prepared several 2-bromotetradec-1-ene-7,13-diynes and subjected them to typical Heck cross-coupling conditions. 5Upon treatment of dimethyl 2-bromo-14-(tert-butyldimethylsilyl)-tetradec-1-ene-7,13-diyne-4,4-dicarboxylate (1) with palladium acetate in the presence of triphenylphosphine and potassium carbonate in acetonitrile at 60°C for 20 h, it was smoothly converted into dimethyl (9-tert-butyldimethylsilyl)-1,2,3,4,5,6,7,8-octahydrophenanthrene-2,2-dicarboxylate (3).6 This transformation appears to have proceeded via a 6-endo-trig cyclization or a 6p-electrocyclization of the alkenylpalladium intermediate 2, (Scheme 1), followed by a,b-dehydropalladation. In contrast, dimethyl 2-bromotetradec-1-ene-7,13-diyne-4,4-dicarboxylate (4a) which was obtained from 1 by hydrodesilylation at the acetylenic terminus, under identical conditions did not yield the unsilylated octahydrophenanthrene, but the bisannelated fulvene derivative 5a within 1 h in 74% yield (Scheme 2).7 The 9-methoxy-substituted analog 4b and the heteroanalogous bromoenediyne 4c, under slightly modified conditions, reacted in the same way to give the fulvenes 5b in 65% yield within 45 min, and 5c in 52% yield within 1.5 h, respectively. The structure of the fulvene 5b was rigorously proved by an X-ray crystal structure analysis of the Diels-Alder adduct that was readily formed upon treatment of 5b with dimethyl acetylenedicarboxylate (Figure). 8 The formation of fulvenes 5 may be rationalized in terms of a 5-exo-trig-carbopalladation instead of the 6-endotr...
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.
-059The Dichotomy Between Benzene and Fulvene Formation in Palladium-Catalyzed Domino Tricyclizations of 2-Bromotetradec-1-ene-7,13-diynes.-The mechanism of the Pd-catalyzed tricyclization of enediynes (I), (III), and (V) involves most probably intramolecular 5-exo-trig carbopalladations of an intermediate 1,3,5-hexatrienyl-6-palladium bromide.
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