Bicyclo butanes structure

Figure 7.21. Structure of the oxyallyl radical, and the ORTEP representations from X-ray of the bicyclobutanones 45 and 46. Box Schematic representation of the stretch-hond isomerism of bicyclo[1.1.0]butane. (Adapted from ref. 117.)...

Within the isolobal formalism, the conversion of 47 to 48 is a symmetry-allowed process, if it were to proceed as a concerted reaction (50). Structure 47 represents a transoid-2-meta.Wa-1,3-butadiene. In the bonding description, complex 48 represents formally a 1-metalla-bicyclo[1.1.0]butane. Therefore, the conversion of 47 to 48 represents a thermally allowed, concerted [ 2a + 2S] ring closure, in analogy to the pericyclic ring opening of bicyclo[1.1.0]butanes to give trans,trans-, 3-butadienes. 

The structures of bicyclic and polycyclic small-ring compounds have attracted some interest. The relationship between the C-C-H bond angles at the bridgehead of bicyclo[1.1.0]butane (2) and the angle between its cyclopropane rings has been studied.31 Bicyclobutane and most other cyclopropane derivatives have bonds that are formed from orbitals that are bent in the same direction. However, with some mm -fused bicyclic compounds containing a cyclopropane... 

The 7r-nature of the central bond in bicyclo[1.1.0]bulane is aptly demonstrated by its cycloaddition reaction with benzyne in 1,2-dichloroethane, from which two compounds 29 and 30 were isolated in moderate yield in a ratio of 6 1. The structures of the major and the minor compounds were identified as 3-phenylcyclobutene (29) and benzobicyclo[2.2.1]hex-2-ene (30), respectively.44 Deuterium labeling experiments showed that both products resulted from attack on bicy-clo[1.1.0]butane from the endo side.44... 

Bicyclo[1.1.0]butanes 63 can also be prepared by elimination reactions of cyclobutane derivatives 62. As a result of the nonplanarity of a cyclobutane ring, Cl and C3 are only separated by a distance of approximately 2.1 A. This unique structural feature of cyclobutanes explains the remarkable ease with which they can be transformed into bicyclo[1.1.0]butanes. 

Sections VILA and VII.B cover cyclic derivatives which contain the cyclopropane or cyclo-propene unit in a fused ring system according to scheme 9 or 10. Structures of many simple molecules of this class were determined long ago. Some of them will be mentioned here further structural data and references are given in previous reviews. Bridged derivatives of bicyclo[ 1.1.0]butane are discussed in Sections VII. C. 1 and VII. C.2. 

TABLE 20. Structures of group 14 analogs of bicyclo[1.1.0]butanes, of their cation radicals and of metallabicyclo[2.2.0]hexanes (pm, deg)... 

MO calculations have been carried out on the isomerization of cyclopropane to propene, and the MNDO method has been used to study the reaction pathway and to optimize the structure of reactant, transition structure, and product of the ring opening reaction of bicyclo[1.1.0]butane. Various methods have been employed to estimate the rate constants for ring opening of the 2-cyclopropyl-2-propyl radical. 1-Acceptor-1-sulfenyl-substituted 2-vinylcyclopropanes of the type (430) have been found to afford 6-sulfenyl-a,jS y, -unsaturated carboxylic esters and nitriles (431) upon treatment with acid, by a process which involves C(l)—C(2) bond fission and a novel 1,5-sulfenyl rearrangement (see Scheme 110). It has been shown that the benzophenone-sensitized photolysis of vinyl norcaradiene derivatives, such as 5-(2-methylprop-l-enyl)-3-oxatricyclo[4.4.0.0 ]deca-7,9-dien-4-ones (432), results in the regioselective cleavage of only one of the cyclopropyl c-bonds to afford isochroman-3-one derivatives (433). It has been reported that the major product obtained from the reaction of structurally diverse a-diazo ketones with an electron-rich alkene in the... 

Protonation of bicyclo[1.1.0]butane at C(l) leads to an interesting structure that has been proposed as an intermediate in the solvolysis of cyclopropylcarbinyl and cyclobutyl compounds. It is the subject of the next section. 

Bicyclo[1.1.0]butane, bicyclo[l.l.l]pentane, [l.l.ljpropellane, and spiro[2.2] pentane are other molecules where strain and rehybridization affect molecular properties. The molecules show enhanced reactivity that can be attributed to characteristics of the rehybridized orbitals. The structures are shown in Scheme 1.6, which also shows calculated AIM charge distributions and strain energy. 

The strain in bicyclo[1.1.0]butane results in decreased stability and enhanced reactivity. The strain energy is 63.9kcal/mol the central bond is nearly pure p in character, and it is associated with a relatively high HOMO. These structural features are reflected in enhanced reactivity toward electrophiles. In acid-catalyzed reactions, protonation gives the bicyclobutonium cation (see Section 4.4.5) and leads to a characteristic set of products. 

Within certain structural requirements alkyl halides may cause intramolecular alkylation to give three- and four-membered ring systems. For example, methyl bicyclo[1.1.0]butane-l-carboxylate is obtained in 77% yield from methyl 3-bromocyclobutanecarboxylate and sodium hydride 388... 

In Table 6, which was compiled from work on bicyclo[n.l.O]alkanes, bicyclo[1.1.0]-butanes, bicyclo[2.1.0]hexanes, bicyclo[2.2.1]heptanes , bicyclo[3.1.1]heptanes bicyclo[2.2.2]octanes ", bicyclo[3.3.0]octanes", bicyclo[4.n.0]alkanes, bicyclo[3.3.13-nonanes" spiro[4.5]alkanes" and trans-decalins the chemical shifts of several bicyclic alkanes are tabulated. The values were mainly explained by the steric effects present in the different structural environments, with the exception of fused cyclopropane rings (see below). 

The rather peculiar electronic structure of 49, as compared to cyclopropane and bicyclo[1.1.0]butane 1, is strongly reflected in the correlation diagram of Figure 27, in particular by the fact that the first ionization energy of 49 cannot be obtained by a linear extrapolation of the first ionization energies of cyclopropane and of 1. 

Extended Hiickel calculations on 10 are in substantial agreement with the structure derived from microwave spectroscopic studies Extensive restricted Hartree-Fock calculations have been carried out on the inversion of substituted bicyclo[ 1.1.0] butanes and on the energetics of variation of the flap angle The results indicate that the ring system is surprisingly flexible, with little energy required for variations of up to 20° in the flap angle. These calculations also reveal that inversion of the system involves non-least-motion movement of the substituents at C(d and C ). 

Szeimies and his coworkers have prepared a number of more stable and structurally more complex [4.1.1]propellanes using two related approaches. The bridgehead positions of bicyclo[ 1.1.0] butanes are somewhat acidic, so that 127 can be selectively lithiated and... 

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