Methylenecyclohexanes

The 1.3-allylic diacetate 135 can be used for the formation of the methy-lenecyclopentane 137 with the dianionic compound 136(86]. The cyclohexa-none-2-carboxylate 138 itself undergoes a similar annulation with the 1,3-allylic diacetate 135 to form the methylenecyclohexane derivative 139(90]. The reaction was applied as a key step in the synthesis of huperzin A[91]. On the other hand. C- and 0-allylations of simple J-dikctones or. 1-keto esters take place, yielding a dihydropyran 140(92]. 

As a further application of the reaction, the conversion of an endocyclic double bond to an c.xo-methylene is possible[382]. The epoxidation of an cWo-alkene followed by diethylaluminum amide-mediated isomerization affords the allylic alcohol 583 with an exo double bond[383]. The hydroxy group is eliminated selectively by Pd-catalyzed hydrogenolysis after converting it into allylic formate, yielding the c.ro-methylene compound 584. The conversion of carvone (585) into l,3-disiloxy-4-methylenecyclohexane (586) is an example[382]. 

B. Methylenecyclohexane. A 500-ml. three-necked round-bottomed flask is fitted with a reflux condenser, an addition funnel, a mechanical stirrer, and a gas inlet tube. A gentle flow of nitrogen through the apparatus is maintained throughout the reaction. An ethereal solution of -butyllithium (0.10 mole, about 100 ml., depending on the concentration of the solution) and 200 ml. of anhydrous ether is added to the flask. The solution is stirred and 35.7 g. (0.10 mole) of triphenylmethylphosphonium bromide is added cautiously over a 5-minute period (Note 2). The solution is stirred for 4 hours at room temperature (Note 3). 

The methylenecyclohexane was analyzed by vapor-phase chromatography and found to be better than 99% pure. 

The procedure is a modification of that published. Methylenecyclohexane has been prepared by the pyrolysis of N,N-dimethyl-l-methylcyclohexylamine oxide, N,N,N-trimethyl-l-methylcyclohexylammonium hydroxide, N,N-dimethylcyclo-hexylmethylamine oxide, and N,N,N-trimethylcyclohexylmethyl-ammoniumhydroxide. It has also been obtained from the pyrolysis of cyclohexylmethyl acetate and of cyclohexylideneacetic acid and from the dehydrohalogenation of cyclohexylmethyl iodide. ... 

The effect of introducing -hybridized atoms into open-chain molecules was discussed earlier, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are noted when sp centers are incorporated into six-membered rings. Whereas the fiee-energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecyclohexane and to 4.9 kcal/mol in cyclohexanone. ... 

Consider the conformations possible for 3-substituted methylenecyclohexanes. Dc you expect typical substituents to exhibit larger or smaller preferences for the equatorial orientation, as conpared to the same substituent on a cyclohexane ring ... 

Regioselectivity in opening of a-functionalized epoxides by treatment with trimethylamine dihydrofluoride has been observed. Thus, cw-isophorol epoxide gives exclusively 3-fluoro-l,2-diol, whereas from the trans isomer, 2-fluoro-l,3-diol is obtained as the main product together with 3-methylenecyclohexane-l,2-diol. This behavior has been discussed m terms of the influence of a-substituents on the transition state conformations [7 J] (equations 11 and 12). 

Methylenecyclohexane A 500-ml, three-necked, round-bottom flask is fitted with a mechanical stirrer, a reflux condenser leading to a mercury filled U-tube, a pressureequalizing dropping funnel, and a gas inlet tube. The system is flushed with nitrogen... 

Freshly distilled cyclohexanone, 10.8 g (0.11 mole), is added toO. 10mole of methylene-triphenylphosphorane, and the reaction mixture is stirred at room temperature for 30 minutes followed immediately by distillation under reduced pressure to give about 8 g (85%) of methylenecyclohexane, bp 42" (105 mm), which is collected in a cold trap. 

Solvents can have a large influence on the extent of double-bond migration (6). The factors involved are complex as shown in the hydrogenation of methylenecyclohexane, 3-methylcyclohexene, and 4-methylcyclohexene to methylcyclohexane in benzene-ethanol, in peniane, and in ethanol over 5% Pd, 5% Pt, and 5% Rh-on-carbon. The amount of isomerized 2-methylcy-clohexene was measured ai 25% completion and, depending on the system,... 

Here s an example how might we prove that E2 elimination of an alkyl halide gives the more highly substituted alkene (Zaitsev s rule, Section 11.7) Does reaction of 1-chloro-l-methylcyclohexane with strong base lead predominantly to 1-methyl cyclohexene or to methylenecyclohexane ... 

Methylcyclohexene will have five sp3-carbon resonances in the 20 to 50 6 range and two s/ -carbon resonances in the 100 to 150 6 range. Methylenecyclohexane, however, because of its symmetry, will have only three sp3-carbon... 

---