Cycloheptatriene cycloaddition reactions

The reaction of alkenylcarbene complexes and alkynes in the presence of Ni(0) leads to cycloheptatriene derivatives in a process which can be considered as a [3C+2S+2S] cycloaddition reaction [125]. As shown in Scheme 77, two molecules of the alkyne and one molecule of the carbene complex are involved in the formation of the cycloheptatriene. This reaction is supposed to proceed through the initial formation of a nickel alkenylcarbene complex. A subsequent double regioselective alkyne insertion produces a new nickel carbene complex, which evolves by an intramolecular cycloprop anation reaction to form a nor-caradiene intermediate. These species easily isomerise to the observed cycloheptatriene derivatives (Scheme 77). 

It has been shown how alkenylcarbene complexes participate in nickel(0)-me-diated [3C+2S+2S] cycloaddition reactions to give cycloheptatriene derivatives (see Sect. 3.3). However, the analogous reaction performed with alkyl- or aryl-carbene complexes leads to similar cycloheptatriene derivatives, but in this case the process can be considered a [2S+2S+2S+1C] cycloaddition reaction as three molecules of the alkyne and one molecule of the carbene complex are incorporated into the structure of the final product [125] (Scheme 82). The mechanism of this transformation is similar to that described in Scheme 77 for the [3C+2S+2S] cycloaddition reactions. 

The metal-mediated and metal-catalyzed [6 + 2]- and [6 + 4]-cycloaddition reactions, pioneered by Pettit and co-workers105 106 and Kreiter and co-workers,107 respectively, involve the cycloaddition of metal-complexed cyclic trienes with 7r-systems such as alkenes, alkynes, and dienes. The [6 + 2]-reactions produce bicyclo[4.2.1]nonadiene derivatives and the [6 + 4]-reactions produce bicyclo[4.4.1]undecatrienes (Scheme 32). Trienes complexed to chromium, which can be prepared on large scale (40 g) as reported by Rigby and co-workers,108 react with 7r-systems upon thermolysis or irradiation.109-111 Chromium and iron-catalyzed [6 + 2]-reactions of cycloheptatrienes and disubstituted alkynes... 

Although disubstituted alkynes are used successfully as two-carbon components in chromium-mediated and -catalyzed [6 + 2]-reactions, the use of terminal alkynes produces a [6 + 2 + 2]-reaction (Section 10.13.3.7). Buono and co-workers have discovered that when a cobalt catalyst is employed, several monosubstituted alkynes can be used in [6 + 2]-cycloadditions with cycloheptatriene (Scheme 35). The use of a chiral BINOL-phosphoramidite cobalt complex affords an enantioselective [6 + 2]-cycloaddition reaction (Equation (18)).121... 

Mechanistic studies of the metal catalyzed cycloaddition reactions establish the intermediacy of a species bearing cyclopolyenyl or cyclopolyene ligand and a n -bound alkene or alkyne. Low-temperature solution studies established that coupling takes place in two steps, often with rearrangement of the alkene or alkyne fragments. This process is illustrated for alkyne addition to a cycloheptatriene ligand in Scheme 17. 

Various unsaturated cyclic 7t-ligands undergo, within a transition-metal complex, cycloaddition reactions and rearrangements with simultaneous formation of cyclopropane subunits. This is observed with cycloheptatriene ehromium and iron complexes such as 4 which give cycloaddition products, e.g. 

Diazoketones also undergo macrocyclic aromatic cycloaddition reactions. Decomposition of 108 with rhodium(II) prefluorobutyrate yields aromatic cycloaddition products 109 and 110 in 30% and 9% yield, respectively. When cycloheptatriene 109 is exposed to neutral alumina, isomerization to 111 occurs. It is interesting that the 1,4-isomer 110 is inert to rearrangement on both silica and alumina. The ability to formally connect a carbene to a remote aromatic ring provides new opportunities for the construction of macrocyclic compounds. 

Alkenes.- Hexa-2,4-dienes are formed in the catalytic codimerisation of butadiene with ethylene in the presence of [Ti(n -l-methallyl)Cp2]f with 99% regioselectivity. The Ziegler catalyst TiCl /EtjAlCl and the Ti(II) complex [Ti(ij -CsHel (AlCli, )2 ] both induce (6+2] cycloaddition reactions of cycloheptatriene with dienes and acetylenes . [Mo(allyl) ] reacts with HCl to give [M02 (U-CD2 (allyl)e ] which catalyses the polymerisation of buta-1,3-diene. The cocondensation of Fe... 

Cycloaddition.— Reactions of electrophilic olefins and acetylenes with tricarbonyl-iron complexes of cycloheptatriene and cyclo-octatetraene lead to 1,3-exo-products. Troponetricarbonyliron and tcne, however, have now been found to give complex (20) which results from previously unobserved 1,5-exo-cycloaddition. A dipolar intermediate (21) resulting from initial attack by the electrophile on the hydrocarbon... 

The use of Cr(0)-promoted 6 r - - 2 r-cycloaddition in the synthesis of pharmacologically active natural products has been reviewed. The 6 -I- 3-cycloaddition of dimethylaminofulvene (194) with benzoquinones provides an efficient route to the 3-oxabicyclo[4.3.0]nonane system (195) (Scheme 76). Density-functional theory calculations provide evidence for a feasible concerted antara-antara cycloaddition for the 6-I-4-reaction of c -hexa-l,3,5-triene with buta-1,3-diene. Azulene-1,5-quinones (196) and azulene-l,7-quinones undergo a variety of 6-1-4- and 2-1-4-cycloaddition reactions with cyclic dienes and cycloheptatrienes to produce cycloadducts, e.g. (197) and (198) (Scheme 77) ... 

Both Fe(CO)3 and Cr(CO)3 templates have been used for the cycloaddition reaction between an alkyne and a coordinated >/-l,3,5-cycloheptatriene to give a product that has a complexed triene ... 

Morita and coworkers [76] described a [8+2] cycloaddition of 8-oxoheptafulvene 299, a heptafulvene ketene, with cycloheptatriene iron carbonyl 300 to give tricyclo[8.5.0.0 ]pentadeca-3,5,7,ll,13-pentaene-2-one Fe(CO)3 301 (Scheme 7.66). Usually, ketenes undergo [2+2] cycloaddition reactions (see above), and the observation mentioned here is a special case. 

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