Methylene-2-cyclohexenes

Cyclization of 1,6-enynes3 (cf. 13, 91 14, 299). Cyclization of these enynes catalyzed by palladium or nickel complexes generally leads to five-membered ring products. However, cyclization catalyzed by Wilkinson s catalyst generally leads to methylene-2-cyclohexenes. Substitution on either of the terminal groups suppresses this cyclization, which probably involves insertion of Rh(I) in the C—H bond of the terminal alkyne. 

Transition-metal-catalyzed intramolecular cycloisomerization is one of the most useful carbocyclization reactions, and specifically, the Rh-catalyzed cycloisomerization of 1,6-enynes provides a powerful tool in organic synthesis. Cyclization of l,6-en5me 452 catalyzed by Wilkinson s catalyst gave 1-exo-methylene-2-cyclohexene 453 via a 6-exo-trig mode in 83% yield (Scheme 2-50). Terminal substitution on the alkene moiety dramatically suppressed the cyclization, and substitution of the terminal alkyne moiety was detrimental to the reaction as well. 

It is obvious that these compounds have in common an uninterrupted cyclic arrangement of cross-conjugated jr-systems. Compound 5 likewise contains the maximum number of exocyclic double bonds at a perimeter consisting only of sp2-hybridized carbon atoms. Thus, our definition allows one to call it a radialene, i.e. naphtharadialene on the other hand, it excludes hydrocarbons such as 6 [3,4,5,6-tetrakis(methylene)cyclohexene]. Although in the latter molecule all carbon atoms are indeed sp2-hybridized, the number of exocyclic double bonds has not reached its maximum. In 5, however, the number of double bonds cannot be increased further. 

Vinylbuta-1,3-diene produces the 1,2- and 3,4-mono-insertion adducts with dichlorocarbene in a 4 1 ratio [37]. A similar preference in reactivity is observed with 3-methylene-cyclohexenes [90]. 1,2-Dienes react with two equivalents of dichlorocarbene to form spiropentanes [21, 90] (Scheme 7.6). Spiropentanes (50-95%) are also obtained from methylenecyclopropanes [36,106] and by the reaction of electron-deficient alkenes with an excess of chloroform [31] (see Scheme 7.12)... 

Methylene cyclohexenes are other products obtained by cyclopropanation of the carbene gold intermediate over a second olefin unit [148]. 

Conjugated Dienes. General Observations, The catalytic hydrogenation of conjugated dienes by pentacyanocobaltate(II) is completely selective, yielding only monoolefin, which cannot be reduced further (19, 20, 21), Lack of reduction of 2,5-dimethyl-2,4-hexadiene indicates that dienes in which the 5-cis conformation is sterically hindered might not be catalytically hydrogenated (21). However, reduction of 3-methylene cyclohexene, a diene with a fixed s-trans conformation, disproves this and indicates that other steric factors are probably involved (24). 

Das mit Limonen isomere 4-Isopropenyl-l-methylen-cyclohexen(l7,8-Menthadien) (zwei endstandige disubstituierte Doppelbindungen) kann mit Platin nicht partiell hydriert werden ... 

Even 4,5-di(methylene)cyclohexene, which isomerizes extremely readily, can be prepared by pyrolysis of the corresponding diacetate 25... 

Nitrogen is passed through a vertical borosilicate combustion tube (20 x 300 mm) that is filled with solid masses and heated at 500°. 4,5-Cyclohexenedi(methyl acetate) (300 g, 1.33 moles) is added in portions at 1.5-min intervals. The product is condensed, washed with water, and dried over potassium carbonate. Thorough fractionation provides 4,5-di-(methylene)cyclohexene (68.1 g) together with starting material and the (2-methylene-4-cyclo-hexenyl)methylacetate that results from partial pyrolysis. 

Problem 9.3L As noted in Problem 9.30, the product with the endocyclic double bond (1-methylcyclohexene) is more stable than its exocyclic isomer (methylene-cyclohexene). Using any computational program at your disposal and a small basis set (to conserve machine time), determine the difference in energy between these isomers. Presume that they can be brought into equilibrium. Predict the equilibrium constant. 

The photolysis of cis- and trans- isomers of 3-methyl-5-phenyl dicyano-methylene cyclohexene gives rise to a (1, 3] benzylic shift, leading to cis- and irons- 6, 6-dicyano-3-methyl-5-phenylmethylene cyclohexanes, respectively. 

The photo-NOCAS reaction with 2,6-dimethyl-l,6-heptadiene gave two major cyclic aryl-methoxy adducts (a cyclohexane and a cycloheptane) as well as an acyclic heptene adduct Variation in concentration of the nucleophile, methanol, and co-donor, biphenyl, has been shown to affect the product ratios. Further applications of the photo-NOCAS S 2Ar reactions with the aUeyl-4-enols, a-terpineol, limonene, 2-methyl-2-butene, 2,3-dimethyl-2-butene, (3-myrcene, and 1,4-bis (methylene) cyclohexene have been reported.The aryl-methoxy adduct product ratios have been investigated and discussed in terms of the stability of radical intermediates and the factors controlling the regiochemistry of reaction with the nucleophiles alcohols, cyanide, and fluoride attempts to justify the results by ab initio molecular orbital calculations have been made. The photo-NOCAS reaction with 2-methylpropene in the absence of methanol and a donor molecule has shown that solvent acetonitrile can act as a nucleophile. Under these conditions a tetrahydroisoquinoline product is formed, prior to HCN elimination, in high yield as illustrated in Scheme 8. The adduct product formation was rationalized on the basis of the relatively high oxidation potential of the olefin. 

Trifluoro- l-hydroxy-l-methylethyl)cyclohexene is monooxidized at both allylic positions in the nng with different agents. The major product is the rearranged product C when oxidation is accomphshed with chromiuin trioxide m methylene chlonde [42] (equation 34). 

Cyclohexene, purification of, 41, 74 reaction with zinc-copper couple and methylene iodide, 41, 73 2-CyclohEXENONE, 40,14 Cydohexylamine, reaction with ethyl formate, 41, 14... 

Melhylenecydobutane-l,2-dicar-boxylic anhydride, 43,27 Methylenecydobutanes by addition of allenes to alkenes, 43, 30 Methylenecyclohexane, 40, 66 Methylene iodide, reaction with zinc-copper couple and cyclohexene, 41, 73... 

Zinc-copper couple, 41, 72 reaction with methylene iodide and cyclohexene, 41, 73... 

Wenn die C=C-Doppelbindungen durch eine Methylen-Gruppe getrennt sind, verlaufen beide Reaktionen parallel. Allyl-cyclohexen liefert so z.B. 15% d.Th. Propyl-cyclohexan und 55% d.Th. Allyl-cyclohexan ... 

Similarly, the direct [Eq. (11.24)] and photosensitized [Eq. (11.25)] decomposition of diazomethane in the presence of cyclohexene yielded product distributions indicative of greater selectivity in the triplet methylene addition ... 

Py-GC/MS of Laropal K80 results in a number of intense oligomeric fragments, while smaller fragments are minor pyrolysis products [71]. Cyclohexene, cyclohexanol, cyclohexanone, methyl-cyclohexanone and methylene-cyclohexanone can be attributed to secondary pyrolysis products of the monomeric units (Figure 12.7). The most intense... 

Figure 12.7 Pyrogram of Laropal K80. Peak assignments 1, cyclohexene 2, cyclohexanol 3, cyclohexanone 4, 2 methyl cyclohexanone 5, 2 methylene cyclohexanone 6, 2 methyl 6 methylene cyclohexanone 7, 2,6 dimethylene cyclohexanone 8, cyclohexenyl cyclohex anol 9, cyclohexenylmethyl cyclohexanone 10, cyclohexanolmethyl cyclohexanone 11,2 methylene 6 cyclohexenylmethyl cyclohexanone 12,13, 2,2 methylenebiscyclohexa none 14,15, 2,2 methylenebiscyclohexanol. Peaks labelled with letters, and having mass spectra with molecular ions m/z—302 and m/z—304, have been assigned to tricyclic cyclohexanone based molecules...

A conveniently prepared amorphous silica-supported titanium catalyst exhibits activity similar to that of Ti-substituted zeolites in the epoxidation of terminal linear and bulky alkenes such as cyclohexene (22) <00CC855>. An unusual example of copper-catalyzed epoxidation has also been reported, in which olefins are treated with substoichiometric amounts of soluble Cu(II) compounds in methylene chloride, using MCPBA as a terminal oxidant. Yields are variable, but can be quite high. For example, cis-stilbene 24 was epoxidized in 90% yield. In this case, a mixture of cis- and /rans-epoxides was obtained, suggesting a step-wise radical mechanism <00TL1013>. 

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