Bicyclobutanes theoretical studies

As can be seen, both routes lead to the same product. Nevertheless, in compounds where the conformational flexibility of the bicyclobutane frameworks is restricted, 1,3-addition is found to be favored via the diequatorial mode.12,13 This aspect is illustrated in a reaction in which iodine reacted with tricyclo[4.1.0.02,7]heptane in carbon tetrachloride to give 6,7-diiodobicy-clo[3.1. l]heptane (9) in 55% yield.13 In support of this stereochemistry, the majority of results obtained from the dculeration of tricyclo[4.1.0.02,7]heptane suggested that the attack is from the equatorial position.14 Theoretical studies also support the notion that the equatorial approach of an electrophile to the bridgehead of bicyclo[l.1.0Jbutane is thermodynamically favored.14,15... 

One of the major structural differences between bicyclobutane and an analogous olefin lies in the differing symmetries of the two systems. The former lacks the element which is typical of the latter. In other words, unlike ethylene, bicyclobutane has two unidentical faces above and below its central 7c-like bond. This necessarily leads to the question which of the two faces is more reactive Electrophilic reactions are a good vehicle for the analysis of this problem. As will be mentioned later, protonation of bicyclobutane can lead to products that may be regarded as resulting from attack on the central bond as well as on a side bond. Two theoretical studies were addressed to the interesting problem of the preferred direction for proton approach. 

A silicon version of the well-studied electrocyclic interconversion of C4H6 (cyclobutene, bicyclobutane, butadiene, etc.) has been investigated both experimentally23,101 and theoretically.102,103 Irradiation of 51 in 3-methylpentane (X >420nm) gives a 1 9 mixture of 51 and bicyclo[1.1.0]tetrasilane 88 at the photostationary state as determined by UV-vis spectroscopy.23 When the mixture is left for 12 h in the dark at rt, 51 is produced quantitatively [Eq. (58)]. The photochemical conversion of 51 to 88 and the thermal reversion can be repeated more than 10 times without any appreciable side reactions. 

The question of two rings going-on one is also seminal to the study of homoaromaticity. For n odd, the set of [(CH) CH2] ions demonstrate a delicate balance between mono and bicyclic structures (see Ref 133 and numerous references cited therein to both the experimental and theoretical literature). In the case of n = 3, one can imagine a planar cyclobutenyl cation (63) and a markedly non-planar, highly puckered bicyclobutyl cation (64). Both calculational theory on the parent and experiment on derivatives show the latter geometry to be preferred. However, as in the case of the other purported bicyclobutane derivatives characterized by the 2,4-carbons trigonally coordinated that were discussed earlier in this section, formal theory shows there is no 1,3-bond. The ion is not homoaromatic and there is no cyclopropane ring. In the case of n = 5,... 

Most of the basic chemistry of bicyclobutane was uncovered in the exciting competitive atmosphere of the small-ring compound era. Since then, the temperature of the subject has somewhat decreased. An indication to this can be obtained from a plot of the ratio of fast publications (such as communications and letters) to full papers per year, as shown in Figure 1. It is interesting to note that while this ratio declines with time, the total number of papers published on bicyclobutane increases (the noticeable peak in the early seventies which distorts the relatively steady trends results from the impact of transition metal chemistry on bicyclobutane research). Naturally, the current interest in bicyclobutane has somewhat shifted from basic studies towards more specific theoretical and experimental issues aimed at uncovering the secrets of this interesting molecule. 

Theoretical and PES studies on benzvalene and other derivatives where aromatic rings bridge carbons 2 and 4 of bicyclobutane, have indeed confirmed these postulated interactions. ... 

The pathway for conversion of benzvalene to benzene has been studied theoretically with perhaps the best calculations - CISD(T)/DZP//B3LYP/DZP + ZPVE - revealing an unsymmetrical, but concerted transition state nearly resembling a biradical having opposite bicyclobutane bond lengths of 2.32 and 1.704 Further, the heat of formation of this transition state relative to benzene is 100.9 kcal/mol which is not inconsistent with the experimental activation energy. 

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