3-Methylene-cyclobutanes

Carben.es insert into a C — H bond of a cyclobutane.157 When there is a choice between a cyclopropane and a cyclobutane C — H bond, as in bicyclo[2.1. OJpentane, the insertion of phenylcarbe-ne takes place at a cyclobutane methylene position to give 2-benzylbicyclo[2.1.0]pentane (l).157... 

Next to isobutene, other 1,1-dialkyl substituted ethenes can also be polymerized cationically. Suitable monomers are 2-methyl-1-butene, 2-methyl-1-pentene, and 2,3-dimethyl-1-butene [613] Polymers with very high molecular weights of Mn>300000 are obtained by catalysis with aluminum alkyl halogenides. Also, cyclic hydrocarbons with a methylene group (methylenecyclopropane, methylene cyclobutane, methylene cyclohexane, a-pinene) are suitable monomers [614-619]. 1,1-Disubstituted ethenes with stronger steric hindrance as camphene or 2-methylene-bicyclo-[2.2.1] heptane, however, could not be polymerized cationically [574]. 

Thermal cycloadditions of butadiene to 3-bromo- 133 and 3-methoxy-5-methylene-2(5//)-furanones 220 were studied (95TL749). These systems contain substituents at C3 capable of stabilizing also a possible radical intermediate, influencing hereby the rate and/or the course of the reaction. Thus, the reaction of 133 and 220, respectively, with butadiene at 155°C afforded mixtures of the expected 1,4-cycloadducts 221 and 222, respectively, and of the cyclobutane derivatives... 

Cyelobutanone has been prepared by (1) reaction of diazomethane with ketene,4 (2) treatment of methylenecyclobutane with performic acid, followed by cleavage of the resulting glycol with lead tetraacetate,s (3) ozonolysis of methylenecyclobutane, (4) epoxidation of methylene-cyclopropane followed by acid-catalyzed ring expansion,7 and (5) oxidative cleavage of cyclobutane trimethylene thioketal, which in turn is prepared from 2-(co-chloropropyl)-l,3-dithiane.8... 

An interesting reaction that has been developed over the past decade is the application of (2 + 2)-cycloaddition reactions to the synthesis of cyclophanes49. One of the earliest examples of this is the selective conversion of the bis(arylalkenes) 112 into the adducts 113. The yield of product is dependent to some extent on the chain length separating the aryl groups and the best yield of 41% is obtained when the separation includes four methylene units (n = 4). Lower yields are recorded with the other derivatives. Mixtures of products are formed when the m-isomers 114 are used. This affords 115 and 116. The yields of these are better than those obtained from the p-isomers 11250 51. Nishimura and coworkers52 have examined the ease with which such cyclobutanes, e.g. 115, n = 2,... 

Electron-rich 3-methoxy-4-trimethylsilyl-l,2-butadiene (22) reacted with several electron-poor alkenes in the presence of diethylaluminum chloride to afford methylene cyclobutanes 23. Reactions with alkynes were performed in the presence of methylalu-minum bis(2,4,6-tri-t-butylphenoxide) (equation 7)16. 

The calculations above did not give satisfactory results concerning the structure of even the parent hydrocarbons (cyclobutane and bicydo[2.2.0] hexane), but highly strained cyclobutene, methylene-cyclobutene, Dewar benzene, and so on, were shown to be handled well by MM2 (83a). 

Calculations based on this second model give the observed value for the entropy of activation. In addition, this model may be used to account for the observed isotope effect (Benson and Nangia, 1963). If the tetra-methylene biradical is involved then it is to be expected that appropriately substituted cyclobutanes might undergo cis-trans isomerization reactions. This will be referred to again later. One final point should be mentioned in connection with biradical intermediates in both cyclopropane and cyclobutane reactions. This concerns the absence of any effect of radical inhibitors on these systems, when it might be expected that they would interact with the biradicals. In fact calculations show that, under the conditions of formation, the biradicals have extremely short lifetimes sec) and hence, unless radical inhibitors are... 

In the presence of catalytic amounts of tris(4-bromophenyl)ammoniumyl hexachloroantimonate in methylene chloride at 0°C, trani -anethole is smoothly converted in less than 5 min to the cyclobutane dimer (Bauld et al. 1996 Scheme 7.16). 

Dimers with two methylene bridges and a cyclobutane fusion were synthesized by thermolysis of ethoxycarbonylmethano-l,2-dihydro[60]fullerene. They have a similar structure as 328 [374],... 

Thermolysis of the 1,2,4-dioxathiolane (65) (a thioozonide) in refluxing /-butylbenzene gave a mixture of the thiolactone (66) (31%) and the ketone (67) (65%) (Equation (6)) <9lJCS(Pl)3043>. The former product is consistent with a thermolytic pathway similar to that observed for ozonides of allylic compounds and of methylene cyclobutanes (see earlier in this section). 

If one connects the two double bonded end groups X in form RED of general structure /I (n = 1) with two methylene bridges instead of the usual a-bond, then 4(5red should result. With well adapted X-groups this cyclobutane derivative 6red may... 

Photolysis of the amide 31 in methylene chloride at room temperature results in the formation of the tricyclic lactam 32 in both syn and anti forms <99TL6001>. Treatment of 32 with BF3 etherate brings about cleavage of the cyclobutane ring in the syn isomer only, with the formation of 33. (Scheme 5). The corresponding esters undergo similar transformations, if somewhat less efficiently. 

Chemical shifts of cyclobutane ring carbons are almost insensitive to the configuration of two vicinal methyl or carboxylate groups465. However, cis- and IransA, 2-dimethylcyclobutanc may be easily differentiated by comparing their methyl carbon chemical shifts. In the ci.v-compound the <5 value is 15.4, whereas 5 = 20.5 for the trans-diastereomer465. However, this does not hold for cis- and trauj-l,3-dimethylcyclobutane. In addition, the 2 ppm difference between the methylene carbon chemical shifts is not a safe indicator. Corresponding trends have been found for the 1,2-dimethyl orientations in l-chloro-l,2-dimethylcyclobutane and 1-bromo-l,2-dimethyl-cyclobutane464. 

The decay of the tetramethylene diradical derived from 2,2,5,5-t/4-cyclopenta-none is much slower than seen for the C4Hg diradical. Both principal decay modes, fragmentation to two ethylenes and ring-closure to cyclobutane, may be dependent dynamically on torsional motions of the terminal methylene groups. 

Ketene dimerization is the principal synthetic route to 4-methylene-2-oxetanones. This reaction proceeds very satisfactorily for ketene and methylketene, but disubstituted ketenes dimerize only to cyclobutane-1,3-diones. The cycloaddition reaction of r-butylcyanoketene to ketene and to methylketene gives a-cyanoalkylidene-/3-lactones in about 40% yield in addition to the cyclobutane-1,3-dione dimer of f-butylcyanoketene. A mechanism has been proposed for the formation of both types of dimers from a common zwitterionic intermediate (equation 111), with the relative amount of each product determined by the configurational equilibrium of the intermediate (80JOC4483, 75JOC3417). 

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