2- -1 methylenecyclobutan

Free Radical Induced Ring Enlargement of 2-(3-Halopropyl)cyclobutanones and l-(3-Halopropyl)-2-methylenecyclobutanes... 

Representative examples are listed in Table 11. Further examples are reported in refs 215 — 217. Similar ring enlargements were found for l-(3-halopropyl)-2-methylenecyclobutanes under the same reaction conditions.218 Thus, 4 is converted to 5 in refluxing benzene after slow addition of tributyltin hydride and catalytic amounts of 2,2 -azobisisobutyronitrile over a period of several hours. Further examples of this reaction are given in ref 218. 

Thus, 2-isopropylidene-l-methylcyclobutanol (2) was converted to l-isopropylidene-2-methylenecyclobutane (3), quantitatively however, 2 on standing in a sealed tube for seven years underwent the expected C4->C3 ring contraction to 1-acetyl-1-isopropylcyclopropane (4) in 60% yield."" > °... 

There is no evidence for the intermediacy of either of the suggested thermal isomerization intermediates tetraphenylprismane or 1,2,3,4-tetraphenyl-Dewarbenzene." The isomerization of hexamethyl-Dewar-benzene to hexamethylbenzene is catalysed by the usual transition-metal complex catalysts. In the presence of palladium(n) chloride or of [RhCl(CO)2]2, methylenecyclobutane (33) does not undergo isomerization, but gives 77--allyl complexes of the type (34). This reaction may be... 

Oxidative rearrangement takes place in the oxidation of the 1-vinyl-l-cyclo-butanol 31, yielding the cyclopentenone derivative 32[84], Ring contraction to cyclopropyl methyl ketone (34) is observed by the oxidation of 1-methylcyclo-butene (33)[85], and ring expansion to cyclopentanone takes place by the reaction of the methylenecyclobutane 35. [86,87]... 

This method for the preparation of cyclobutanone via oxaspiropentane is an adaptation of that described by Salaiin and Conia. The previously known large-scale preparations of cyclobutanone consist of the reaction of the hazardous diazomethane with ketene, the oxidative degradation or the ozonization in presence of pjrridine of methylenecyclobutane prepared from pentaerythritol, or the recently reported dithiane method of Corey and Seebach, which has the disadvantage of producing an aqueous solution of the highly water-soluble cyclobutanone. A procedure involving the solvolytic cyclization of 3-butyn-l-yl trifluoro-methanesulfonate is described in Org. Syn., 54, 84 (1974). 

The reverse reaction has also been shown to occur (18). Another example is the reaction of methylenecyclobutane to produce dicyclobutylidene and ethene (18a). 

METHYLENECYCLOBUTANE-l,2-DICARBOXYLATE (3-MethylenecycIobutane-l,2-dicarboxylic acid, dimethyl ester)... 

The crude anhydride is carefully fractionated through a 13-mm. x 1.2-m. Nester still at a pressure of 25 mm. (Note 4) and a reflux ratio of at least 10 1. After a fore-run of maleic anhydride, b.p. 50-100°/25 mm., and a small intermediate fraction, there is obtained 75-90 g. (22-26%) of 3-methylenecyclobutane-l,2-dicar-boxylic anhydride b.p. 155-159°/25 mm. 1.4935-1.4952 (Note 5). This material is of sufficient purity for most uses, but it contains approximately 2-5% of propargylsuccinic anhydride. Redistillation through the Nester still gives 65-80 g. (19-23%) of 3-methylenecyclobutane-l,2-dicarboxylic anhydride b.p. 155°/25 mm. 1.4946-1.4955. 

B. Dimethyl 3-methylenecyclobutane-l,2-dicarboxylate. One liter of methanol is added cautiously with occasional shaking to 276 g. (2.00 moles) of 3-methylenecyclobutane-l,2-dicarboxylic anhydride ( d 1.4946-1.4955 Note 6) and 5 g. of />-toluenesulfonic acid in a 2-1. three-necked flask fitted with a thermometer, a condenser, and a dropping funnel. Refluxing starts after about two-thirds of the methanol has been added. The remainder is added at a rate that maintains vigorous boiling. The solution is refluxed for 30-40 hours with the pot temperature increasing... 

The first step of this procedure illustrates a general reaction, the addition of allenes to alkenes to form methylenecyclobutanes. The reaction has been reviewed recently.7... 

Since 3-methylenecyclobutane-l,2-dicarboxylic anhydride is easily converted to 3-methyl-2-cydobutene-l,2-dicarboxylic acid, it is an intermediate to a variety of cyclobutenes. The dimethyl ester of 3-methylenecyclobutane-l,2-dicarboxylic acid is also a versatile compound on pyrolysis it gives the substituted allene, methyl butadienoate, and on treatment with amines it gives a cyclobutene, dimethyl 3-methyl-2-cyclobutene-l,2-di-carboxylate. ... 

The pyrolysis tube is flushed with nitrogen, the lower section is heated to 600° and the upper section to 300° (Note 1), and the pressure is regulated at 25 mm. Then 184 g. (1.00 mole) (Note 2) of dimethyl 3-methylenecyclobutane-l,2-dicarboxylate2 is admitted over a period of 3 hours (Note 3). The product, which amounts to 172-177 g., collects in the traps. It is distilled through a 13-mm. x 1.2-m. Nester spinning-band still.3 First... 

Wittig product (57). The methylenecyclobutanes (60) were formed from the same phosphoranes and the methylenecyclopropene (59). The formation of the pyran-2-ones may involve the intermediate cyclobutenones (56) as shown. 

The palladium(II)-catalyzed hydroxylation of the methylenecyclobutane 255 in the presence of water results in ring enlargement to the corresponding cyclopen-tanones 256 [145]. (Scheme 101)... 

Except for the still thermochemically uninvestigated 1,2-bismethylenecyclopropane44, 57, all bismethylenecycloalkanes can further be divided into two categories—those in which the exo-methylene groups are on adjacent carbons and those further apart. The two isomeric bismethylenecyclobutanes have been studied. Roth presents an enthalpy of formation for the 1,2-isomer, 58, of 204.2 kJmol-1. In the absence of any additional strain-induced destabilization or conjugative/delocalization-induced stabilization, we would expect the disproportionation reaction 27 of methylenecyclobutane (59)... 

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

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... 

Cyclobutane has not been polymerised cationically (or by any other mechanism). Thermochemistry tells us that the reason is not thermodynamic it is attributable to the fact that the compound does not possess a point of attack for the initiating species, the ring being too big for the formation of a non-classical carbonium ion analogous to the cyclopropyl ion, so that there is no reaction path for initiation. The oxetans in which the oxygen atom provides a basic site for protonation, are readily polymerizable. Methylenecyclobutane polymerises without opening of the cyclobutane ring [72, 73]. 

Concerning the structure, the cyclopropane derivatives 524—526 deviate from the generally observed cycloadducts of cyclic allenes with monoalkenes (see Scheme 6.97 and many examples in Section 6.3). The difference is caused by the different properties of the diradical intermediates that are most likely to result in the first reaction step. In most cases, the allene subunit is converted in that step into an allyl radical moiety that can cyclize only to give a methylenecyclobutane derivative. However, 5 is converted to a tropenyl-radical entity, which can collapse with the radical center of the side-chain to give a methylenecyclobutane or a cyclopropane derivative. Of these alternatives, the formation of the three-membered ring is kinetically favored and hence 524—526 are the products. The structural relationship between both possible product types is made clear in Scheme 6.107 by the example of the reaction between 5 and styrene. 

It is reasonable to assume that 5 and styrene generate the diradical 527, which collapses to give 528 at 30 °C under kinetic control. At temperatures of 75 °C and above, the last step is reversible and an equilibrium is established in that 528 and the methylenecyclobutane derivative 529 maintain a ratio of 1 6 [215]. The [2 + 2]-cydoadduct 530 of 5 to phenylacetylene should be formed analogously via a diradical of the type 527 and is closely related in its structure to the cephalosporin derivatives 437 and 438 (R= Ph, Scheme 6.89). In addition to 530, 2-phenylindene was obtained, which has to be considered as the product of the thermal rearrangement of 530 [216]. Akin to such a process, 526 [194] and 529 [215] were converted into indane derivatives on heating. 

As exemplified in Eq. 8.38, thermal [2 + 2] cycloadditions of 4-vinylidene-2-oxazoli-dinone 287 and alkynes such as phenylacetylene result in the formation of 3-phenyl-substituted methylenecyclobutene 288 [149]. The authors confirmed by NMR analysis that only the Z-configuration isomer was formed. It is worth noting that the [2 + 2] cycloaddition of allenes 287 is not restricted to alkynes even olefins such as acrylic esters or silyl enol ethers furnish the corresponding methylenecyclobutanes... 

On the other hand, 4-vinylidene-l,3-oxazolidin-2-one 26 undergoes a facile [2 + 2]-cycloaddition with electron-deficient olefins regioselectively at the terminal double bond to furnish methylenecyclobutane derivatives [25]. 

The highly reactive hexafluoro-2-butyne reacted with 1,1-dimethylallene to afford methylenecyclobutanes [63]. 

Reactions of 3-methylthio-4-trimethylsilyl-l,2-butadiene with electron-poor monosub-stituted and disubstituted alkenes were promoted by a catalytic amount of ethylaluminum dichloride, affording the corresponding methylenecyclobutanes with high selectivities and with yields ranging from 37% for methyl crotonate to 97% for methacrylonitrile15. 

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