1.5- cyclooctadiene 1,2- cyclobutane

The dimerization of isoprene has been accompHshed by methods other than heating. Thus isoprene has been dimerized by uv radiation in the presence of photosensitizers to give a complex mixture of cyclobutane, cyclohexene, and cyclooctadiene derivatives (36,37). Sulfuric acid reportedly... 

The photosensitized dimerization of isoprene in the presence of henzil has been investigated. Mixtures of substituted cyclobutanes, cyclohexenes, and cyclooctadienes were formed and identified (53). The reaction is beheved to proceed by formation of a reactive triplet intermediate. The energy for this triplet state presumably is obtained by interaction with the photoexcited henzil species. Under other conditions, photolysis results in the formation of a methylcydobutene (54,55). 

In all the latter cases the easier dimerization reaction is connected with the particular stability of the intermediate diradical species. This is also the reason for the recently found facile dimerization of the 1-donor substituted allylidene-cyclopropane 136a (Scheme 66) [127]. Allylidenecyclopropane 136a cyclodimer-izes to the expected cyclobutane 467 in very mild thermal conditions, due to the stabilization of the intermediate 466. At higher temperature (120 °C) both 136a and 467 give a more complex mixture of products, with the cyclooctadiene dimer 468 being the prevailing one (Scheme 66) [127],... 

Rearrangement of the stereoisomers in the cyclobutane series leads to different products, cis-1,2-Diviny IcycIobutane rearranges through a transition state similar to 30 to give ds,ds-l,5-cyclooctadiene [Eq. (4.41)]. The radical transformation of the trans isomer, in contrast, leads to 4-vinylcyclohexene242 [Eq. (4.42)] ... 

In certain cases the formation of dimethylcyclooctadiene occurs only after an induction period of approximately an hour, while the cyclobutane is formed immediately. This strongly suggests that the cyclooctadiene is formed by the nickel-catalyzed rearrangement of the cyclobutane derivative, the necessary reorientation of a trans.trans into a cis,cis configuration occurring by conformational changes in the free cyclobutane [Eq. (57)]. 

Cyclobutanes are produced by using cyclooctadiene as an olefinic substrate via an unusual [4+2] cycloaddition. The mechanism is postulated to proceed... 

The Cope rearrangement was used in the total synthesis of (-)-asterisca-nolide (14), a novel sesquiterpene natural product4 (Scheme 1.4e). Ring-opening metathesis of the cyclobutene 15 with ethylene in the presence of the ruthenium catalyst 165 proceeded smoothly to provide the cyclooctadiene 18 via Cope rearrangement of the intermediate dialkenyl cyclobutane (17). 

Brij BSA Bu CBDTS CD CHIRAPHOS Cn COD Cp Cp CTAB DBA = seep.78 = bovine serum albumin = butyl = tetrasulfonated cyclobutane-DIOP, 37, see p. 17 = cyclodextrin = 2,3-bis(diphenylphosphino)butane = 1,4,7-trimethyl-1,4,7-triazacyclononane = 1,5-cyclooctadiene = r 5-cyclopentadienyl = ti5-CsMe5, r 5-pentamethylcyclopentadienyl = hexadecyltrimethylammonium bromide = 1,5-diphenyl-l, 4,-pentadiene-3-one (dibenzylideneacetone)... 

As well as [2 + 2], [4 + 4] and [4 + 4 + 4] products, the cyclodimerization of conjugated dienes also yields [4 + 2] cycloadducts47Thus, butadiene gives 4-vinyleyelohexene, ci.v-1,2-divinyl-cyclobutane and 1,5-cyclooctadiene. The influence of the catalyst and reaction conditions on the product distribution has been carefully investigated50- 53. Efforts towards asymmetric induction have concentrated on the stereoselective synthesis of 4-vinylcyclohexene as the sole chiral product. 

Fig. 12.6 Catalytic conversion of 1,3-butadiene into cyclic dimers. VCH = 4-vinylcyclohexene COD = as-1,5-cyclooctadiene DVCB = cis-1,2-divinyl-cyclobutane. When L = P(OC,H PH, ), at 60 C, COD = 94.5% L = PPhEt, COD = 38.2% DVCB = 12.6% VCH = 46.0% of total products.

Earlier we used curved arrows to show the flow of electrons that takes place in the process of bond breaking and bond forming in the Diels-Alder reaction. As discussed, these reactions involve a four-carbon diene and a two-carbon dienophile and are termed [4 + 2] cycloadditions. We can write similar electron-pushing mechanisms for the dimerization of ethylene by a [2 + 2] cycloaddition to form cyclobutane and for the dimerization of butadiene by a [4 + 4] cycloaddition to form 1,5-cyclooctadiene. 

Lanthanum(III) atoms have rarely been used to study the photo-reactivity in CPs. Michaelides and coworkers explored the photo-reactivity of muconate ligands bonded to Er(III) and Y(III) atoms. Muconate is another linear spacer ligand with two conjugated C=C bonds in the backbone which presents different possibilities of photoproducts, including mono [2-1-2] cycloaddition or [4-1-4] cycloaddition products. The transformation of cyclobutane to cyclooctadiene by Cope rearrangement is another possibility. Further, it may also result in the formation of a highly strained ladderane structure by double [2-1-2] cycloaddition reaction. 

CuOTf-catalyzed photocycloaddition of the tetraene 96 produces a mixture of the compounds 98,99, and 100 (Scheme 25). The 1,2-divinyl cyclobutanes 97 initially formed from [2 + 2]-addition of the tetraene 96 undergo further reaction on prolonged irradiation in the presence of CuOTf to form these products. The tricyclic compound 100 arises from intramolecular 2jt + 2Jt addition of cyclooctadiene derivative 99. In fact, the transformation of cis,c/s-l,5-cyclooctadiene 101 to the tricychc compound 102 on irradiation in the presence of CuCl was the first example of an intramolecular Cu(I)-catalyzed photocycloaddition reaction. ... 

When the 1,3-dienes are largely or exclusively in the s-trans-conformation, such as with diene 83, pyridone 18 leads to three products, 87, 88, and 89." The two cyclobutane products 87 and 88 derive from Cope rearrangement of the highly strained [4-1-4]-photocycloaddition adducts 84 and 85, a mechanism precedented by photocycloadditions of 1,3-dienes with other aromatic species. Pyridine 89 may be formed from oxetane 86. When the 1,3-diene is delivered intramolecularly (90), the only photoproduct is 91, presumably "via an intermediate relating to 85. In contrast with the chemistry of the cis-pyridone dimers (Scheme 3), cyclobutane 91 thermally rearranges to the cyclooctadiene 92. ... 

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