New pathways of site selective aromatic alkylation of palladium complexes: fragmentation to arenes vs. ring closure to hexahydromethano-fluorenes or -phenanthrenes

Abstract

Dimeric arylbicycloheptylpalladium halide complexes of type 1 undergo selective alkylation at the aromatic site by reaction with allyl, styryl, and benzyl bromides (RBr) via hexahydromethanopalladafluorenes (2). Ring closure of the resulting palladium complex (7) on sp2 and sp3 C-H bonds of a suitable R group then occurs with formation of hexahydromethanophenanthrene or hexahydromethanofluorene derivatives. Alternatively, substituted arenes derived from bicycloheptene deinsertion are formed. In some cases the latter can be obtained in substantial amounts when methyl isonicotinate is used as ligand.Key words: alkylations, C-H activation, palladium, palladacycles.

Palladium-Catalyzed Domino Reaction for Stereoselective Synthesis of Multisubstituted Olefins: Construction of Blue Luminogens

“…To our delight, the replacement of methyl isonicotine with triphenylphosphine successfully afforded the coupling product trisubstituted olefin Z - 4a in 25% yield along with 5.6% of E isomer 4aa (Table , entry 2). The configurations of Z - 4a and E - 4aa were confirmed by 1 H NMR analysis (see Supporting Information (SI)) . By carefully screening the conditions (see SI, Tables S1 and S2), we found that the phosphine ligand was crucial for the reaction, and no desired product was obtained in the absence of phosphine ligand (Table , entry 4).…”

“…According to the literature , and the above observed results, a plausible mechanism is proposed in Scheme . First, the reaction of aryl iodide with norbornene in the presence of Pd(0) and base produces a five-membered palladacycle A , which coordinates with ( E )-vinyl bromide followed by oxidative addition to generate an arylpalladium species B .…”

“…X-ray single crystal diffraction confirmed the structure of 5n (see SI, Table S4), which resulted from β-H elimination of the alkylpalladium complex, a catalytically active species. This result revealed that the process involved a 5-exo-Heck cyclization reaction . When a 1:1 E / Z mixture of 2,2-disubstituted-1-bromoethenes was subjected to the reaction conditions, the product was obtained as an inseparable E / Z mixture (Scheme , 5w and 5x ).…”

“…We initiated our studies by reacting 1-iodo-2-methoxy-benzene ( 1a , 0.20 mmol) and norbornene ( 3a , 0.6 mmol) directly with ( E )-2-bromovinylbenzene ( 2a , 0.30 mmol) using a catalytic amount of Pd­(OAc) 2 (10 mol %) in the presence of methyl isonicotine (50 mol %) as ligand and K 2 CO 3 (2.0 equiv) as base in DMAc at 80 °C for 12 h. Only a trace amount of the desired product was detected (Table , entry 1). To our delight, the replacement of methyl isonicotine with triphenylphosphine successfully afforded the coupling product trisubstituted olefin Z - 4a in 25% yield along with 5.6% of E isomer 4aa (Table , entry 2).…”

“…Although the Lautens and Perumal groups successively realized the synthesis of norbornene-incorporated tetrasubsituted olefins by Pd-catalyzed reactions of aryl halide, internal alkyne, and norbornene, which involve a sequential insertion of norbornene and internal alkyne into the aryl–palladium bond (Schemea,b), these methods were only limited to synthesize the tetrasubstituted olefins and usually required the tedious multistep synthesis of the internal alkyne. So far, only one example for the synthesis of trisubstituted olefin containing a norbornyl group was reported by Catellani in 2001, wherein a stoichiometric amount of dimeric arylnorbornylpalladium chloride complex was used to react with a large excess of alkenyl bromide (10.0 equiv) . However, the relatively high cost of arylnorbornylpalladium complex and poor stereoselectivity greatly precluded the practicability of this method.…”