Methylenecycloalkanes

The direct photolysis of methylenecyclopropane, methylenecyclobutane and methylenecyclopentane has recently been studied in the wavelength region 1750-2450 A by Brinton The photolyses of methylenecyclopropane and methylenecyclobutane occur in a simple manner as represented by 

On the other hand the photolysis of methylenecyclopentane does not occur readily. The only detectable photolytic product was a very small amount of hydrogen. 

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For example, the proportion of the axial-equatorial saturated isomers obtained at low pressureis from substituted methylenecycloalkanes is greater than the ratio at high pressures of hydrogen which is consistent with a structure in which the distinction between the alternative points of attachment of the surface site to the cycle is large. In eifect the catalyst acts as a large substituent which assumes that position on a cycle in which the repulsive interactions are minimal (Fig. 17). 

If one of the C —C double bonds of the 1,6-diene is incorporated in or attached to a ring (e.g., cycloalkene, methylenecycloalkane) tricyclic compounds containing a bicyclo[3.2.0]heptane subunit are obtained as products. Thus, photobicyclization of monocyclic diallyl ethers or homoallyl vinyl ethers afford oxatricycloalkanes.6 Similarly, allylic alcohols containing an additional cycloalkenyl ring are converted to oxatricycloalkanes by irradiation in the presence of copper(I) trifluoromethanesulfonate as catalyst.9... 

Methylenecycloalkanes.1 A cycloalkene can be converted into a methylene-cycloalkane by the following steps hydroboration with 9-BBN and carbonylation-reduction2 to give a /i-(cycloalkylmethyl)-9-BBN derivative, which is then allowed to react with an aldehyde1 to form the final product. 

Hypochlorous acid and hypobromous acid react with acyclic alkenes to give Mar-kovnikov products. In striking contrast, exclusive anti-Markovnikov orientation was observed in the transformation of methylenecycloalkanes with HOBr, and mixtures of chlorohydrins were formed with HOC1 146... 

The introduction of carbenes and carbenoids into synthetic organic chemistry revolutionized the synthesis of cyclopropane derivatives21. In particular, cyclopropanation of methylenecycloalkanes became a very useful method for the preparation of SPC. Moreover, since cycloaddition of carbenes to olefins involves a very fast concerted process (i.e. it eliminates any intermediates during the formation of the three-membered ring)21, the method is equally efficient for the preparation of both unstrained and highly strained compounds. 

The Simmons-Smith reaction, which involves cyclopropanation by the Zn CH2I2 system (equation 6), is a very popular variant of this method22. However, an application of the Simmons-Smith reaction to methylenecycloalkanes often requires the use of high excess (up to ten equivalents) of the reagent, which complicates the workup and lowers the yields. 

Rhodium acetate catalyzed cyclopropanation of methylenecycloalkanes with diazocarbonyl compounds (equation 9) provides a direct method for preparation of functionalized SPC27. 

The nature of the ligands on the palladium in ir-allyl complexes can influence the regioselectivity exhibited by soft carbon nucleophiles. ir-Allylpalladium complexes generated from methylenecycloalkanes provide an example of the effect of ligands on regiochemistry. The complexes derived from methylene-cyclopentane and methylenecycloheptane both exhibit exclusive exocyclic addition by the anion of methyl(methylsulfonyl) acetate with triphenylphosphine ligands on the Pd (equation 223). In contrast, the complex derived from methylenecyclohexane yields a 62 38 ratio of exocyclic endocyclic addition (equation 226). 

In contrast to the malonate additions to methylenecycloalkane-derived ir-allyl complexes, where substantial endocyclic addition has been observed with the cyclohexyl analog, essentially exclusive exocy-clic addition occurs for both the five- and six-membered ring cases with PhS02Na (equations 282 and 283)219... 

Single, transient triazoline intermediates are also considered to be formed in the reactions of methylenecycloalkanes with cyanogen azides,190-192 although again, cyclopropylidenecyclohexane gives rise to a mixture of the regioisomeric triazoline intermediates.191... 

IV. EXO- VS EA/DO-CYCLIC DOUBLE BONDS A. Isomeric Methylcycloalkenes and Methylenecycloalkanes... 

In this section we compare the enthalpy of formation of isomeric pairs of cyclic olefins, one with an exocyclic double bond and the other with the double bond endocyclic. We start with 1-methylcycloalkenes and related methylenecycloalkanes, species genetically 18a and 18b, respectively. From prior experience with cyclopropanes and the thermochemical consequences of replacement of sp3 carbons by sp2 carbons in three-membered rings52, we expect 1-methylcyclopropene to be considerably less stable than methylenecyclopropane because the former compound has two trigonal carbons within the ring while the latter has but one. And so it is found56 the former has a gas-phase enthalpy of formation 43.1 2.2 kJ mol-1 more positive than the latter. 

The numerical values of the enthalpies of formation of the cycloalkanones and the corresponding 4o(n) values are found in Table 8. Admittedly, the enthalpies of formation of the desired gas-phase methylenecycloalkanes are only available for n = 3-7. However, while the difference, < 4o( ), clusters between 200-220 kJmol-1, a pattern is disappointingly just as absent for the derived values of < 4o(n) as for those of 8 (n). 

Kinetic investigation of the reaction of (p-anisyl)phenylcarbenium tetrachloroborate with methylenecycloalkanes (ring size 3-12 and 15) exhibits correlation of the second-order rate constants with the solvolysis rates of the corresponding cycloalkyl derivatives382. 

Similar effects were obtained for methylenecycloalkanes 108 with preferential formation of the photo-Conia products 111. Increasing ring size in 110 as well as H/D exchange favored the formation of the Paterno-Buchi products 112. 

The high diastereoselectivities and mildness of these reaction conditions should make this reaction valuable for the synthesis of quaternary carbon centres and exo-methylenecycloalkanes. 

The first report of an aryllithum carbolithiation reaction is due to Woolsey and coworkers14, who reported the cyclization reaction of o-(3-butenyl)bromobenzene via the aryllithium derivative (Scheme 3). More recently, Bailey and coworkers have studied the carbolithiation reaction of aryllithiums tethered to a methylenecycloalkane 291 (Scheme... 

Although many recent improvements in the preparation of the Simmons-Smith reagent might be helpfuP , the authors of this chapter would recommend one to consider an alternative two-step cyclopropanation procedure, which includes cycloaddition of dichloro- or dibromocarbene to methylenecycloalkane followed by reductive dehalo-genation (equation if. The first reaction is usually carried under phase transfer conditions and presents a very simple and efficient procedure. Reduction of gem-dihalocyclopropanes with lithium in rerr-butanol or with sodium in liquid ammonia usually proceeds without complications and with high yield. 

Methylenecycloalkanes. The vinyl anion (a) generated from the trisyl hydrazone (1) of an lo-chloroalkyl methyl ketone with jec-butyllithium (2 equiv.) and TMEDA... 

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