Intermolecular cycloadditions benzene derivatives

The benzene derivative 401 by the intermolecular insertion of acrylate[278], A formal [2 + 2+2] cycloaddition takes place by the reaction of 2-iodonitroben-zene with the 1,6-enyne 402. The neopentylpalladium intermediate 403 undergoes 6-endo-lrig cyclization on to the aromatic ring to give 404[279],... 

Gilbert, A. (2004) Intra- and intermolecular cycloadditions of benzene derivatives, in CRC Handbook of Organic Photochemistry and Photobiology,... 

The tandem cyclization/intermolecular cycloaddition procedure is also feasible for the synthesis of 9-oxabicyclo[4.2.1]nonane derivatives via the generation of seven-membered cyclic carbonyl ylides. The Rh2(OAc)4-catalyzed decomposition of 719 in the presence of DMAD or methyl propio-late in benzene gives 720 together with the product of a formal carbenoid... 

A complex prepared in situ from [Rh(cod)2]OTf and P[OCH-(CF3)2] efficiently catalyzes the intermolecular [4+2] cycloaddition reaction between a vinylallene and an alkyne. The transformation proceeds under mild conditions, affording the corresponding substituted benzene derivatives (eq 9). ... 

Although ruthenacyclopentadiene intermediates are able to coordinate a third molecule of alkyne to afford benzene derivatives, catal3Aic intermolecular [2+2+2] cycloadditions involving three nontethered a]k3mes are seldom because of their low selectivity. For example, substituted o-phtalates could be chemoselectively obtained by reaction of two equivalents of terminal alkynes with dimethylacetylenedicarboxylate (DMAD) [11] or by cycloaddition of an internal alkyne, a terminal alkyne and DMAD [12] [Eq. (8)] in the presence of the Cp RuCl(cod) complex. 

The reactions that yield benzene rings can be categorized further into the following types according to the substrates involved (1) intermolecular cycloaddition of three alkynes (cyclotrimerization), (2) partially intramolecular cycloaddition ofdiynes with alkynes, and (3) fully intramolecular cyclotrimerization of triynes. In the next section, the synthetic routes to benzene derivatives using ruthenium-catalyzed cycloaddition are surveyed according to these classifications. Classic examples of [2 + 2 + 2] alkyne cycloadditions using stoichiometric ruthenium mediators are included since they provide useful information on the further development of ruthenium catalysis. 

In 1999, Hashimoto and coworkers demonstrated the first successM examples of the intermolecular cycloadditions of carbonyl ylides derived from a-diazo ketones with dimethyl acetylenedicarboxylate (DMDA) using fV-benzene-fitsed phthaloyl-(5)-valine-derived Rh catalyst, Rh2(5-BTPV)4 with high enantioselectivity (up to 92% ee) (Scheme 7.21) [57]. 

Fused aromatic compounds such as polyacenes have attracted much attention as organic conductive materials. However, established methods are very limited. Lack of general and convenient synthetic methods for fused aromatic compounds and their very poor solubility in organic solvents are the most serious problems that control further advances in this very important field. Taka-hashi and coworkers have recently developed a synthetically useful method for preparation of fused aromatic compounds, by using the zirconocene-mediated aromatization of alkynes. In order to solve the solubility problem, alkyl substituents are introduced into to the skeletons. In principle, two types of synthetic protocols have been used. Type I protocol is via the homologation starting from a functionalized benzene derivative (Scheme 3) [75] the Type II protocol is via the intermolecular cycloaddition of two alkynes to an arene (Scheme 4) [76]. 

Drawn from these examples it is apparent that controlling the chemose-lectivity in inter-intermolecular Heck-Diels-Alder reactions of two different alkenes can be tedious if the alkenes show comparable reactivities. Nevertheless, the stepwise approach was realized in several other cases. In a synthesis of a derivative of cephalostatin 1 containing a central benzene instead of the pyrazine ring, Winterfeldt et al. linked two steroidal systems by a Heck coupling and subsequently performed high pressure Diels-Alder reactions of the conjugated diene with electron-deficient alkynes [34], Another example, reported by Hayashi et al., involves a selective Heck reaction of a bromoglu-cal with ethylene or acrylic acid derivatives followed by cycloadditions with maleic anhydride or N-phenylmaleimide [35]. 

Intermolecular.- cis-trans-Isomerization of the enone (31) occurs on irradiation and this is followed by formation of the adducts (32) and (33) by a thermal cycloaddition of the trans-isomer of (31) to a ground state cis-isomer. Such a mechanism involving cis-frans-isomerization is supported by the fact that the reaction does not show a solvent dependency and can be brought about equally efficiently in t-butanol or benzene.The enone 31) does not undergo 1,2-phenyl migrations of the type associated with the di-Tc-methane rearrangement of 4,4-diphenyl eye 1 ohexenone derivatives. The regiospecific... 

In contrast to the preferred meta mode of intramolecular photoaddition of 5-phenyl-l-pentenes, where the alkene and benzene groups are separated by three atoms, irradiation of the styrene (64) yields a single stereoisomer of the ortho adduct (65). In (64), not only are the reacting units separated by 4 atoms, but also it is the styrene rather than the benzene which is excited. Comparable photoreactivity is seen for phenanthrene-styrene systems such as (66) which yield 2+2 adducts (67) along with products derived from competing Paterno-Btichi reaction of the ester carbonyl with the alkene side chain. The photochemical cycloaddition also proceeds in an intermolecular fashion between the ester of 9-phenanthrene carboxylic acid and para-methoxy-0-methylstyrene. The mechanism of this reaction is shown to involve addition of the styrene to the singlet excited state of the phenanthrene derivative. °... 

In the intermolecular reaction of the nitrene that is derived from tosyl azide with benzene, little of the A -tosylazepine (5) is obtained at normal pressures, but the yield is markedly increased when the reaction takes place under a high pressure of nitrogen for example, to 48% at ca 90 atm.In a similar reaction with dimethyl terephthalate, it is suggested that the reaction involves an initial [1,3] cycloaddition of the azide to the aromatic ring to give (6), rather than a nitrene reaction. 

---