Spiropentane/methylenecyclobutane

Syntheses of spiropentane, methylenecyclobutane, and bicyclopropyle are described in the appropriate sections. 

The electron transfer spiropentane-methylenecyclobutane rearrangement of 50 to 51 and 52 occurs under a variety of photochemical conditions, but unlike in the direct photolytic reaction, neither 53 nor 54 is formed. Spiropentanes (50a-c) are good electron donors and form colored EDA complexes with TCNS. The EDA complex of the more potent electron donor, 50a (E° = +1.17 V vs. SCE), and TCNE exhibits CT absorption maxima at 398 and 598 nm in dichlo-romethane. The long-wavelength band of the less electron-rich do-... 

An important reaction that is on the spiropentane/methylenecyclobutane energy surface is the 2 + 2 cycloaddition of allene and appropriately substituted ethylenes to give methylenecyclobutane. 

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... 

Noncationic Rearrangements Including Bicyclobutane to Cyclobutene and Spiropentane to Methylenecyclobutane... 

The thermal rearrangement of spiropentanes to methylenecyclobutanes proceeds via two successive bond cleavages. First, a peripheral bond breaks to give a 1,3-diradical 28, and then a radical bond breaks to give a 1,4-diradical 29, until ring closure yields the product(s). 

Despite the demonstration of reductive cyclization of tetrabromopentaerythritol 8 to spiropentane almost a century ago14, the formation of methylenecyclobutane and other side products (equation 4) decreases the preparative importance of the method. 

Kinetic investigations of the spiropentane isomerization to methylenecyclobutane and decomposition to allene and ethylene by Burkhardt and Swinehart 30> and of thermal decomposition of methylenecyclobutane by Chesick 31> led to the recognition that a common intermediate might be involved. Other workers provided additional kinetic data 28,47),... 

In analogy with the spiropentane to methylenecyclobutane rearrangement, one may expect that kinetic studies on 1-methylenespiropentane will be on interest. Thermal reactions of substituted methylenespiropen-tane have just been reported 33>. 

The studies of the thermal stability of substituted spiropentanes demonstrated the possibility of their isomerization via biradical mechanism at temperatures higher than 300 °C. It was estimated that biradical 62 collapses back to spiropentane ten times faster than it undergoes ring opening to methylenecyclobutane. This makes thermal isomerization of substituted triangulanes under controlled conditions synthetically important. For example, cis- [3]triangulane derivative 63 was smoothly converted into the cis-tranS somQv 64 in the gas phase at 370 °C (equation 43) ". ... 

The pyrolysis of spiropentane at 386°C yields methylenecyclobutane (by C3-C4 ring expansion), ethylene and allene (by decomposition). The ratio of the products depends markedly on the total pressure of the system (equation 112). ... 

Under uncontrolled potential electrolysis spiropentane, 2-methyl-1-butene and methylenecyclobutane rather than the dibromo derivative were formed. 

A further addition of diazomethane to the strained exocyclic double bond of methylenecyclopropane gave a mixture of 4,5-dihydro-37/-pyrazoles. Separation of the 4-spiro-sub-stituted isomer by preparative GC and pyrolysis gave spiropentane 56.Note the formation of the methylenecyclobutane. In this case, the alkene-forming side reaction is in part driven by the relief of ring strain. Alternatively, spiropentane derivatives were obtained by treating an allene with an excess of diazoalkane and deazetization of the bisadducts 57 formed, e.g. formation of 1,1,4,4-tetramethylspiropentane (58). ... 

The thermal decompositions of several types of strained hydrocarbons produce diradicals containing the cyclopropylmethyl unit. 1,3-Diradicals of type 2 are formed on thermolysis of spiropentane (1) in the gas phase, in a sealed tube, or in a flow system. Diradical 2 not only ring closes back to spiropentane, but also rearranges by -scission to produce the new delocalized diradical 3, ring closure of which generates methylenecyclobutane 4) A competing reaction which yields allene and ethene is minor, provided the pyrolysis temperature is not allowed to rise higher than ca. 400 °C. 

Monomethyl-, dimethyl-, and trimethylspiropentanes were transformed into rather complex mixtures of products on thermolysis,but spiropentanes containing one or more electron-withdrawing substituents generally gave good yields of a single methylenecyclobutane (Table 15). The remarkable difference in the products formed from the two isomers of 1,1-dicyano-2,3-dimethylspiropentane was indicative of a dramatic change in the mechanism of thermolysis. 

Diradical 2 has, moreover, been generated by thermal decomposition of pyrazolines 5 and 6. In both decompositions the main product at 340 "C was spiropentane (1) methylenecyclobutane (4) was formed in very small quantities. ... 

Polymerization of Spiropentane, Methylenecylobutane, and Bicyclo-propyle. One compound in the spirane hydrocarbon series is especially worthy of attention—namely spiropentane (Afs), which is an isomer of both isoprene and methylenecyclobutane (M9) see Equation 13. From a structural point of view, if one considers that spiropentane is the linear combination of the p orbital and the sp orbital associated with each ring of spiropentane, four equivalent sp hybrid orbitals may be formed. Under these conditions, spiropentane constitutes a highly p-type unsaturated entity that is thus especially suitable for polymerization. 

Spiropentanes undergo a thermal rearrangement to methylenecyclobutanes for the parent system is 54.5 kcal / mol. 

In appropriately substituted systems cis-trans isomerization of substituents on the spiropentane is seen. Furthermore, heating a substituted methylenecyclobutane can lead to scrambling of any substituents on the ring. Write a mechanism for this reaction, propose an intermediate, and include appropriate electron pushing. How might you test your mechanistic postulate ... 

Thermolysis of Spiro-compounds. It is already known that pyrolysis of spiro-pentane gives products whose relative yields depend upon the pressure. Flowers and Gibbons have extended earlier work and shown that RRKM theory accounts for the pressure dependence of the reaction from lO Torr to 1 atm using either of two possible mechanisms. One mechanism involves intervention of a biradical intermediate, the other involves direct transformation of spiropentane into methylenecyclobutane, ethylene, and keten (and... 

Under experimental conditions such that isomerization of the spiropentane to the methylenecyclobutane only occurs, Gajewski and Burka have... 

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