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],... 

The diazepines 13 react with dimethyl acetylenedicarboxylate to yield mixtures of the pyrazole 19 and the benzene derivatives 18. The reaction proceeds by cycloaddition to yield 14, followed by valence isomerization to the 1,2-diazonines 15, a further valence isomerization to 16, a Second cycloaddition to give 17 and, finally, fragmentation."... 

Treatment of 1,2,4-triazines 91a-91e with the electron-deficient die-nophile dimethyl acetylenedicarboxylate gave products, depending on the substituents [77LA( 10) 1718]. Pyrrolo-[2, -/][ ,2,4]triazines 92 were obtained via [4 + 2]-cycloaddition [77LA(9)1413, 77LA( 10)1718] with 91, but interaction with 91b in the absence of solvent gave, in addition to 92, the pyrido[2,l-/][l,2,4]triazine 93 and [l,3]oxazino[2,3-/][l,2,4]-triazine 94. In case of 91a pyridine and benzene derivatives were also formed in addition to 92 (Scheme 23). 

Cycloaddition of furans followed by a subsequent transformation is still adopted as a useful strategy to prepare fluorine-containing benzene derivatives and isoquinoline compounds <00SL550>. The cycloaddition adduct can also be converted to a trifluoromethyl substituted cyclohexanone compound via hydrogenation and hydrolysis. Examples of these transformations are illustrated below. 

Sometimes the formation of benzene derivatives can be explained by cycloaddition rather than by ketolization. Thus the excellent preparation of substituted benzophenones 553 from 2//-pyrans 552 on heating with acetic anhydride253 is interpreted as in Scheme 30. 

Incidentally, a 1,3-cycloaddition of benzyne to thiophene has been postulated (81CC124) to account for the small, but reproducible amount of benzo[ >]thiophene formed in this reaction (Scheme 79). Reactive alkynes have been successfully used as dienophiles to cycloadd with thiophenes benzene derivatives are obtained after extrusion of sulfur from the adduct (Scheme 80) (72TL605, 72TL1909, 73CB674). 

Cycloadditions. This rhodium complex effects trimerization of a 1,6-heptadiyne with a terminal alkyne to form a benzene derivative (equation I). The paper reports one example of an intramolecular [2 + 2 + 2]cycloaddition (equation II). 

Ortho photocycloadditions of benzene derivatives to maleic anhydride have been tabulated in Table 1. Only the structures of the primary ortho adducts are given, but these are not the isolated adducts They always undergo endo [2 + 4] cycloaddition with maleic anhydride, yielding 1 2 adducts. An interesting feature to be seen from Table 1 is that substituents on the benzene (alkyl, phenyl, or halogen) always turn up at the position most remote from the site of addition. In view of the different nature of these substituents, it seems that steric rather than electronic factors are responsible for this regioselectivity. 

Benzene and cyclooctatetraene (COT) derivatives are formed by [2+2+2] and [2+2+2+2] cycloadditions of alkynes. At first the metallacyclopropene 107 and metallacyclopentadiene 108 are formed. Benzene and COT (106) are formed by reductive elimination of the metallacycloheptatriene 109 and the metallacyclononate-traene 110. Formation of benzene by the [2+2+2] cycloaddition of acetylene is catalysed by several transition metals. Synthesis of benzene derivatives from... 

Free 1,2,5-triphospha Dewar benzene derivative 95 is formed in two steps from 3 mol of 44a by oxidative decomplexation of the triphosphacyclohexa-l,4-diyl-2,5-diene ligand of the Hf complex 96 <1997CB1491>. An organometallic [2+1+1] cycloaddition reaction between the phosphinidene complex 97, 44a, and a coordinated CO gives access to the 1,2-diphosphacyclobutenone complex 98 (Scheme 31) <1998CEJ1917>. 

The [2 + 2] cycloaddition of benzene derivatives with alkenes was also carried out using photosensitization. Maleimide 36 was added to benzene in high yields (Scheme 5.8, reaction 16) [42]. In this case, the sensitizer acetophenone 37 transfers its triplet energy to 36, after which the cycloadduct VII reacts immediately with an... 

Many examples of the intramolecular [2 + 2] photocydoaddition of alkenes to benzene derivatives have been reported. The acetophenone derivative 41 undergoes an efficient [2 + 2] photocydoaddition, leading to the cyclobutane derivative 42 (Scheme 5.9, reaction 18) [46, 47]. It was shown that, in this case, a nn triplet state is involved. The presence of a nitrile group in compound 43 induces a [2 + 2] cycloaddition at position 1,2 of the aromatic moiety, leading to intermediate VIII (reaction 19) [48]. Following tautomerization, the final product 44 is formed. 

De Keukeleire, D., Bako, P., and Van der Eycken, E. (2000) Intramolecular ortho and meta photocycloadditions of 4-phenoxybut-l-enes substituted in the arene residue with carbomethoxy, carbomethoxymethyl, and 2-carbo-methoxyethyl groups. Journal of Photochemistry and Photobiology A Chemistry, 133, 135-146 (c) Wender, P.A. and Dore, T.M. (1995) Intra and inter-molecular cycloadditions of benzene derivatives, in CRC Handbook of Organic Photochemistry and Photobiology (eds W.M. Horspool and P-.S. Song), CRC Press, Boca Raton, pp. 280-290. 

X. Photochemical cycloaddition of maleimide to certain benzene derivatives. Zhournal Organicheskoi Khimii, 14, 1522-1529 (b) Zhubanov, B.A., Almabekov, O.A., and Ismailova, Zh.M. (1981) Photoinitiated addition of maleic anhydride to monoalkylbenzenes. Zhournal Organicheskoi Khimii, 17, 996-999. 

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

Recent results revealed that the ortho photocycloaddition frequently occurs concomitantly with the meta photocycloaddition even in cases where only meta derivatives are obtained. Especially in the case or electron-donor substituted benzene derivatives, the competitive ortho cycloaddition is less stereo- or regioselective and the resulting products are less stable. Aside the meta adducts as main products, complex mixtures of ortho products or products resulting from rearrangements of these primary photoproducts have been obtained [15,16], Improved separation techniques recently enabled a better characterization of these products. Furthermore, using particular conditions like an acidic reaction medium, the intermediates resulting from ortho photocycloaddition could be transformed selectively in more stable final products [17]. 

In a study of rhodium-catalyzed [2 + 2 + 2] cycloadditions of alkynes <88JCS(P1)1357>, the intramolecular [2 + 2 + 2] cycloaddition of4,9-dioxadodeca-l,6,12-triyne catalyzed by Wilkinson s catalyst [(PPh3)3RhCl] over a prolonged period of time gave the dihydropyrano- and tetrahydrofuro-fused 6-6-5 tricyclic benzene derivative (67) in moderate yield (Equation (38)). It should be noted that an analogous bis-tetrahydrofuro-fused 5-6-5 tricyclic compound could be prepared (74%) under similar conditions, but after only 3 h at room temperature. 

Some efforts have also been done to induce chirality in the intramolecular [2 + 2] photocycloaddition of benzene derivatives using a chiral auxiliary [88]. 141b was isolated with a diastereoselectivity of 17% from the intramolecular photocycloaddition of the salicylic acid derivative 140b (Scheme 35) [89]. After an initial [2 + 2] cycloaddition, reversible thermal and photochemical rearrangements took place. These equilibria can be displaced by an acid catalyzed and irreversible addition of methanol to the intermediate P. 

Rh(l)-catalyzed [2-I-2-I-2] cyclotrimerization of 1,6-diynes (e.g., 1391 and 1394) with monoynes (e.g., 1392) in combination with stereospecific Ag(i)-catalyzed aldimine (metallo)azomethine ylide — cycloaddition cascades affords rapid access to complex heterocyclic benzene derivatives 1393 and 1395 in one-pot processes with the generation of five new bonds, four stereocenters and three rings (Schemes 266 and 267) <2000T8967>. 

Acetylenes are well known to undergo facile trimerizations to derivatives of benzene in the presence of various transition metal catalysts 23). A number of mechanisms for this process have been considered including the intervention of metal-cyclobutadiene complexes 24). This chemistry, however, was subjected to close examination by Whitesides and Ehmann, who found no evidence for species with cyclobutadiene symmetry 25). Cyclotrimeri-zation of 2-butyne-l,l,l-d3 was studied using chromium(III), cobalt(II), cobalt(O), nickel(O), and titanium complexes. The absence of 1,2,3-trimethyl-4,5,6-tri(methyl-d3) benzene in the benzene products ruled out the intermediacy of cyclobutadiene-metal complexes in the formation of the benzene derivatives. The unusual stability of cyclobutadiene-metal complexes, however, makes them dubious candidates for intermediates in this chemistry. Once formed, it is doubtful that they would undergo sufficiently facile cycloaddition with acetylenes to constitute intermediates along a catalytic route to trimers. 

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