Trans -Cycloheptene adducts

When an attempt was made to produce rrcm.v-cycloheptene from trans-1,2-cycio-heptenethionocarbonate (8) by the same procedure, only cfy-cycloheptene was obtained. However, the reaction of trans-1,2-cycloheptenethionocarbonate (8) with trimethyl phosphite in the presence of 2,5-diphenyl-3,4-isobenzofurane (9) as trapping agent gave an adduct (10), m.p. 150°, isomeric with the known adduct of the benzofurane with cii-cycloheptene, m.p. 185°. It is apparent, therefore, that trans-cycloheptene is the primary product from trans- 1,2-cyclohcptenethionocarbonate. 

Irradiation of cyclohexene in the presence of CuOTf produces the dimers 14 (49%) and 15 (8%), along with the cyclohexylcyclohexene 16 (24%) (Scheme 5)T The stereochemical outcome in Cu(I)-catalyzed dimerization of cyclohexene may be the result of cis-tram isomerization on irradiation of the initially formed Cu(I)-cyclohexene complex to the trans-cyclohexene-CuOTf complex 13, followed by a concerted ground state 2 + 2 cycloaddition of the highly strained tram-cyclohexene to another cyclohexene. Cycloheptene, on the other hand, produces the aU tra s-fused trimer 17 (Scheme 6) as the sole product. A 1 3 CuOTf-trans-cycloheptene complex has been proposed to be the precursor of this product. No dimerization reaction has been observed for cyclooctene and acychc olefins.However, mixed photocycloaddition occurs with cyclooctene if the other olefin is sufficiently reactive. Thus, cyclooctene adds to norbornene to produce the cyclobutane derivative 18 in 40% yield. GDdimerization was also observed when a mixture of cyclohexene and cycloheptene was irradiated in the presence of CuOTf to yield the adduct 19 (Scheme 6). ... 

In cyclic alkenes such as cyclohexene and cycloheptene trans addition was also highly stereoselective [7]. However, cis cyclooctene afforded both cis and trans adducts in the ratio 1 3 in the presence of oxygen 1,4-dichlorides were also formed [8]. In cyclodecenes, both cis and trans, the main product was the allylic chloride, with some formation of transannular dichlorides [9]. In some substrates with an exocyclic double bond, such as methylene cyclobutane, only (dichloroiodo)benzene was suitable for a clean addition [10]. 

Diels-Alder reactions continue to be of value for the interception of highly reactive dienes or dienophiles. Thus, irradiation of 2-cyclo-octen-l-one or of 2-cyclohepten-l-one in the presence of excess cyclopentadiene affords the norbornene derivatives (173 n = 5 or 4 respectively), which undergo retro-[4 -I- 2] addition on distillation. Presumably the trans-2-cycloalken-l-ones are reactive intermediates in these processes. Elimination of acetic acid from bis(trifluoromethyl)cyclopentadienylcarbinyl acetate yields the reactive 6,6-bis(trifluoromethyl)fulvene, which rapidly dimerizes by [4 + 2] cycloaddition the fulvene intermediate can be intercepted by similar addition to cyclopentadiene. The adduct (174) is formed on reaction of cyclopentadiene with the imine produced by iodine-catalysed rearrangement of NN-... 

Among cycloalkenes, cyclohexenes, cycloheptenes, and cyclooctenes and their 1-methyl derivatives imdergo ci -tram-isomerization on irradiation. Trans-isomers of cyclohexenes and cycloheptenes are veiy unstable because of high ring strain and hence are isolated by trapping in hydroxyhc solvents like methanol [4]. For example, the photo-induced isomerization of cis-l-methylcyclohexene 2 to transisomer is trapped in methanol as methanol adduct [4]. 

Evidence has also been obtained for the intermediacy of the (E)-cycloheptene (E)-7 in the formation of methyl ethers 8 and 9 on direct, p-xylene-sensitized and methyl benzoate-sensitized irradiation of the 3,7-disilacycloheptene (Z)-7 in acidified methanol. Sensitized irradiation of (Z)-7 in methylcyclohexane solution at -75°C followed by siphoning of the irradiation mixture into acidified methanol similarly afforded ethers 8 and 9, showing that a long-lived intermediate is involved. Addition of cyclopentadiene to the cold irradiation mixture in the dark afforded a Diels-Alder adduct shown to have a trans ring juncture. 

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