Methylene cyclobutane

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. 

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). 

The transformation of spiropentane to methylenecyclobutane can be regarded as a ring enlargement of a cyclopropane derivative (Section 7. A.3.3.), while the transformations of methylene-cyclobutanes can be classified as a rearrangement from one cyclobutane to another (Section 1.A.5.2.4.2.). The mechanistic aspects of these rearrangements were first studied in 1961 by Frey7y and Chesick.80... 

Exocyclic unsaturation can stabilize small ring heterocycles. In three-membered rings it is difficult to separate the contributions from increased angle strain and from electronic interactions between the unsaturation and the heteroatom. In four-membered rings such separation has been done (74PMH(6)199, p. 235). The CRSEs change "from oxetane (106 kJ mol-1) by -11 kJ mol-1 to oxelan-2-one (95 kJ mol-1) (corrected for electronic effects) and 4-methyleneoxetan-2-one (95 kJ mol" ). In contrast, an increase of 10 kJ mol 1 over the value for cyclobutane (111 kJ mol-1) is observed on going to both methylenecyclobutane and l,3-bis(methylene)cyclobutane. 

The quenching cross section for cyclopropane is considerably smaller than those for olefins.497 However, the fact that cyclopropane-de is, if anything, a more effective quencher of triplet mercury than cyclopropane-/ 6497 favors an energy transfer process over a hydrogen abstraction process. Recently Gunning has reported that methylene-cyclobutane is the major high-pressure product in the mercury-sensitized decomposition of spiropentane.509 This result provides very compelling support for the intermediacy of a biradical species. 

In cyclic alkenes such as cyclohexene and cycloheptene trans addition was also highly stereoselective [7]. However, cis cyclooctene afforded both cis and trans adducts in the ratio 1 3 in the presence of oxygen 1,4-dichlorides were also formed [8]. In cyclodecenes, both cis and trans, the main product was the allylic chloride, with some formation of transannular dichlorides [9]. In some substrates with an exocyclic double bond, such as methylene cyclobutane, only (dichloroiodo)benzene was suitable for a clean addition [10]. 

Table 11. Synthetic route for l,l,3,3-Tetramethyl-l,3-disila-2(methylene)-cyclobutane...

The syntheses of l,l,3,3-tetramethyl-l,3-trisilacyclopentene 87 and of 1,1,3,3-tetramethyl-l,3-disila-4-methylene-cyclobutane 88 are described in Section C. By adding Br2 or HBr, the ring system is cleaved as a first reaction step (35). For syntheses of carbosilanes, information on the corresponding six- or seven-membered ring systems was required. Therefore, the synthesis of the compounds 179 and 180 was investigated (54). 

Cyclopenten l-Methyl-3-oxo-2-(l-pentenyl)- E17e, 506 l-OSiR3 — 1 -[CH(OR) — en] — 2-methylen-cyclobutan/H + ... 

Allene and its decomposition products increase the rate of ketene consumption and that of CO formation, while they hardly influence the rate of CO2 formation. A similar effect was caused by addition of methylene cyclobutane, which decomposes into allene and ethylene. The decomposition of ketene is not inhibited significantly by the usual chain inhibitors, nor is it initiated by biacetyl. Thus, chains, if any, are very short in the thermal decomposition. 

Wavelength dependence of the photolytic behaviour of the bicyclobutane (116) has been studied in inert solvents. At 254 nm and 300 nm the irradiation affords the three products (117), (118), and (119) but the ratio of products is different at the different wavelengths. The authors believe that the reactions arise from the ic-ic state. Irradiation at 300 nm in diisopropyl ether or in toluene yields only the methylene cyclobutane (117). 

Synthesis of a cyclobatane derivative. A du Pont groups synthesized 3-methylene-cyclobutane-1,2-dicarboxylic anhydride (3) by charging a cooled (—70°) autoclave with maleic anhydride, 645 ml. of benzene, 0.25 g. of hydroquinone, and allene. The... 

Fig. 2.6. Variation of the A rotational constants (in MHz) with ring-puckering vibrational state for cyclobutanone and methylene-cyclobutane.

It is well known that cyclobutanones can be transformed to cyclopentane derivatives directly with diazomethane or indirectly via the cyanohydrins218> 228), or with tris(phe-nylthio)methyllithium 219,220), but in many cases the reactions are not regioselective and lead to different regioisomers. Cyclobutyl ketones lead to substituted cyclopentanones in highly acidic media 226 and the synthesis of cyclopentanones from methylene-cyclobutanes requires a palladium catalyst 227). 

An example in which an adjacent cyclopropyl moiety participates in the rearrangement is the vapor-phase pyrolysis of methylenespiropentane (21). When heated for 20 hours at 300 °C 21 yielded 1,2-bis(methylene)cyclobutane (22, 15.8%) and l,3-bis(methylene)cyclobutane (23, 19.8%), along with 60.6% of unreacted starting material. 

Like ketenes, allenes generally undergo [2 + 2] cycloadditions with alkenes affording methylene cyclobutanes . In reactions with 1,3-butadienes, both Diels-Alda- adducts and [2 -F 2] cycloadducts are formed. Cyclopentadiene, however, has been reported to react with several allenes to give exclusively Diels-Alder adducts. From the sevraal possible mechanisms by which [2 -F 2] cycloaddition reactions of allenes could occur,... 

FIGURE 4 Plot of the A rotational constant of methylene cyclobutane against puckering vibration quantum number. (Reproduced by permission from Scharpen and Laurie.29)... 

In the presence of Et3Al/TiCl4, polymerization of methylene cyclobutane gives a polymer having exomethylene units (P9b) representing about 40% of the structure, 60% being partially cyclized. 

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