Arenes 4 + 3 cycloaddition reactions

Another recent development in the field of palladium-catalyzed reactions with alkynes is a novel multicomponent approach devised by the Lee group. Starting from a-bromovinyl arenes and propargyl bromides, the assembly ofeight-membered car-bocycles can be realized via a cross-coupling/[4+4] cycloaddition reaction. The authors also presented the combination of a cross-coupling and homo [4+2], hetero [4+2], hetero [4+4] or [4+4+1] annulation leading to various cyclic products [147]. 

Organometals and metal hydrides as electron donors in addition reactions 245 Oxidative cleavage of carbon-carbon and carbon-hydrogen bonds 253 Electron-transfer activation in cycloaddition reactions 264 Osmylation of arene donors 270... 

Making elegant use of the intramolecular arene-olefin meta-cycloaddition reaction, Wender and Howbert have achieved a total synthesis of ( + )-cedrene (575) Irradiation of 569 led to an approximately equal mixture of 570 and 571 which... 

While both hydrogenation and epoxidation reactions of (7) (and substituted forms) occur on the oxepin valence tautomer, cycloaddition reactions proceed more readily on the arene oxide form (where the diene is closer to planarity). Thus the dienophiles DM AD and maleic anhydride (MA) readily yielded [4 + 2] cycloadducts with (7) as shown in Scheme 22 (67AG(E)385). A similar type of singlet oxygen cycloaddition reaction gave an unstable endoperoxide (106) which upon heating yielded trans-benzene trioxide quantitatively (equation 14). (75JOC3743). 

The presence of substituents at the 2,7-positions of (7) results in an almost total preference for the oxepin form and this may explain why the arene oxide forms of oxepins (98) (78TL2999) and (108) (79JOC468) apparently did not participate in cycloaddition reactions. Oxepins (16) and (108) thus adopted the role of dienes in [4 + 2] cycloaddition reactions with azo compounds (equation 16). 

Substituted l,2,4-triazoline-3,5-diones are excellent dienophiles which react rapidly at room temperature with oxepins, but particularly with the arene oxide valence tautomer. A similar [4+2] cycloaddition reaction between the episulfide tautomer of thiepin (44) and 4-phenyl-l,2,4-triazoline-3,5-dione has been reported (74AG(E)736>. Benzene episulfide (the valence tautomer of thiepin 44) was generated in situ by thermal decomposition of the diepisulfide (151) at 20 °C and trapped as a cycloadduct at the same temperature (equation 34). A 1,3-dipolar cycloaddition reaction between thiepin (152) and diazomethane has been reported (56CB2608). Two possible cycloadduct products are shown since the final structure has not been unequivocally established (equation 35). 

Arene oxide-oxepin systems have also been reported to undergo [2 + 4] or [4 + 6] pericyclic cycloaddition reactions with heterocyclic dienes like the tetrazine 279 and the triazine 280. 65 Thus 86 96 reacts with 279 and 280 to yield the dihydrooxepino [4,5-d] pyridazine 281 and the oxepino [4,5-c] pyridine 282, respectively, via a [2 + 4] cycloaddition as well as the phthalazine 283 and isoquinoline 284, respectively, probably via a [6 + 4] cycloaddition reaction. However, 157 gives only 285 and 286 arising from a [2 + 4] cycloaddition reaction. 

Cycloaddition reactions, characteristics, 10, 628 [4+2]-Cycloaddition reactions with (arene)tricarbonylchromium substituents, 5, 243 via silver catalysts, 9, 566-567 [4+3]-Cycloaddition reactions, examples, 10, 616 [4+4]-Cycloaddition reactions, characteristics, 10, 618 [4+2+l]-Cycloaddition reactions, characteristics, 10, 626 [4+2+2]-Cycloaddition reactions, characteristics, 10, 633 [4+4+l]-Cycloaddition reactions, characteristics, 10, 636 [5+2]-Cycloaddition reactions... 

The first photochemical [3 + 2] cycloadditions of arenes with aikenes were reported by two groups in 1966 [76, 77]. This is one of three types of cycloaddition reaction ortho, meta, and para) of arenes with aikenes, and is referred to as the meta-cydoaddition reaction. There are many combinations of arenes and aikenes and, indeed, a large number of reports and reviews have been made on the regiochemistry and stereochemistry [78]. Because this reaction can be used easily to construct a tricyclo[3.3.0.02,8]oct-3-ene framework, the organic syntheses of many natural compounds using this method have been attempted [79], in which the photochemical [3 + 2] cydoaddition plays a key role. 

When compounds (7) were heated with alkyne in excess, two types of complexes, both involving alkyne coupling, are formed. A compound with the stoichiometry Co2(CO)4(C4R2CO)2, formed mainly from terminal alkynes having one bulky substituent R, represents derivatives of Co2(CO)g where two CO groups at either metal are replaced by a cyclopentadienone ligand. This compound type represents one of the many instances where alkynes combine with CO in the presence of a transition metal fragment to yield mostly cyclopentadienones, often complexed to the metal this cycloaddition reaction is similar to the Pauson-Khand scheme except for the use of an alkyne in place on an alkene (see also Section 5.1.4 and Scheme 26). The reaction eventually proceeds further to liberate an arene. Thus, from the use of t-BuC=CH, the alkyne trimerization product 1,2,4-tri-f-Bu-benzene was isolated. 

The NS2+ cation is an important reagent in S-N chemistry, especially in thermally allowed cycloaddition reactions with organic nitriles and alkynes that give quantitative yields of heterocyclic cations (see Scheme 3). The dominant orbital interaction in these cycloadditions is between the LUMO of S2N+ and the HOMO of the alkyne or nitrile (see Figure 2). Cycloaddition reactions also occur with alkenes, and colored charge-transfer complexes are formed with arenes. 

Diene cycloaddition reactions have frequently been used to characterize relatively unstable arene oxide-oxepins. The reactions of 1 with maleic anhydride or dimethyl... 

The cycloaddition reactions of arene oxides-oxepins already discussed have all been done with dienophilic reagents that is, [4+2] tr cycloadditions. The reaction of oxepin 1 with a dienone provides a further example within this category, but the oxepin acts as a dienophile. When 2,5-dimethoxycarbonyl-3,4-diphenylcyclo-... 

The unusual reaction of 1 with trichloro(nitroso)ethene to form cis-epoxyepimino-1,3-cyclohexadiene may result from an initial [4+2]7t cycloaddition reaction between the arene oxide and the nitroso group followed by a spontaneous rearrange-... 

Various Ni and Co complexes catalyze cyclotrimerization reactions of alkynes to give arenes. These reactions are formally [2 + 2 + 2] cycloadditions. The cyclotrimerization of PhC=CPh using CpCo(CO)2 as catalyst is illustrative. 

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