Bicyclo butane, reactivity

Bicyclo[1.1.0]butane is an example of a molecule in which severe angle strain results in decreased stability and greatly enhanced reactivity. The bicyclo[1.1.0]butane ring has a strain energy of 63.9kcal/mol, and the central bond is associated with a relatively high... 

It can be seen that the HOMO energy of cyclopropane is higher than that of cyclobutane or cyclohexane, and that the much more reactive bicyclo[1.1.0]butane has a much higher HOMO energy, which is close to that of propene. Another important factor is the polarizability, which reflects how easily the electron density may be shifted in the presence of an electric field (such as that developed by a proton). Here again, cyclopropanes have significantly higher polarizability than other cycloalkanes.52... 

In the case of electrophilic addition, bicyclo[1.1.0]butane underwent acetolysis to give acetoxycyclobutane and acetoxymethylcyclopropane both in 50% yield.50 However, bicy-clo[1.1.0]butane is highly reactive and as a result the rate of acetolysis cannot be measured even in the absence of acid catalysis.5... 

Cyclopropane derivatives, including spiropentanc, have proven to be virtually inert towards carbenes,1 For this reason, no literature report that describes cyclobutane synthesis from a C3 and a Cj building block by ring enlargement of cyclopropanes exists. However, due to the partial p character, as well as the increasing reactivity caused by its strain, the central bond of bicyclo[1.1.0]butane (l)2 has been found to react with carbenes.1 Photolysis of diazomethane in the presence of bicyclo[1.1.0]butane (1) at — 50 C provides a mixture of several compounds. The major fraction of the material (80%) was analyzed by means of NMR spectrometry and found to consist of penta-1,4-diene (2, 21%) and bicyclo[l.l.l]pentane (3, 1%), plus several other known compounds as well as some unidentified products.3 The mechanistic pathway for the formation of bicyclo[l.l.l]pentane (3) has not been addressed in detail, but it is believed that a diradical intermediate is involved, as shown below.3... 

Highly strained substrates can be transformed into even more strained isomers by ODPM rearrangement. This has been shown by Murata et al. for the synthesis of a valence isomer of azulene [45]. Albeit the photochemical reaction yielded only 20-25% of the bicyclo[1.1.0]butane derivative 15, the synthesis of the precursor cyclobutene (14) is straight-forward from the bicyclo[3.3.0]octenone 13 (Sch. 18). This substrate has obviously a diverse reactivity pattern when directly excited, however, triplet sensitization reduces these competitive pathways because alkene excitation is excluded. Also benzo-annulated azulene valence isomers were generated by this approach [46]. 

The Si=Si jr-bond is cleaved in practically all reactions of disilenes while the cr-bond remains intact. Even the action of the very reactive oxygen on the Si=Si bond initially results in the formation of the 3,4-disiladioxetanes 119 which, however, rapidly rearrange to give the cyclodisiloxanes 121. The reactions of the disilenes 9, 10 and 14 [R = Mes, 2, 6-Me2C6H3, 4-f-Bu-2,6-Me2CgH2 (Dmt)] with white phosphorus follow a different course and proceed with cleavage of both Si=Si bonds to furnish the bicyclo[1.1.0]butane derivatives 127-129 (equation 30)126 127. 

In contrast to the acyclic disilenes, very little is yet known about the reactivity of the cyclic members of this class of compounds. The photochemically induced isomerization of the cyclotetrasilene 141c to the bicyclo[1.1.0]butane derivative 140b (Section V.A) has already been mentioned. Similarly to the tetrasilyldisilenes69 the cyclotrisilene 151 also reacts spontaneously with tetrachloromethane even at —70°C to furnish the trans-l, 2-dichlorocyclotrisilane 157 (equation 41)137. 

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. 

A prediction of the rearrangement products a priori was not possible, because their formation depends on too many factors. Table 9 summarizes results on the reactivity of bicyclo[1.1.0]butanes . The Ni°-catalysed rearrangement of bicyclo[ 1.1.0]-butane (Table 9 entry la) proceeds via a metal-carbene complex, as was demonstrated by a deuterium-labelling experiment using 65 as a mechanistic probe (equation 33) . ... 

The chemistry of bicyclo[ 1.1.0]butanes is well developed, with particular attention over the last two decades paid to reactivity of the central bond with metal ions and complexes. Earlier work has been reviewed and here we shall emphasize recent studies in this... 

Further examples of the dimerization of allenes to give dimethylenecyclo butanes have been reported. The strained allenes cyclo-octa-l,2,4,6-tetraene and cyclohepta-1,2,5-triene have been generated and trapped. In the absence of trapping agents, the dimers (114) and (115) are formed. The relative reactivities of bicyclo[3,2,l]octa 2,3-diene and cyclohexa-1,2-diene with conjugated dienes and styrenes have been investigated. Both allenes give mainly [2 + 2] adducts. 

Bicyclo[3,2,0]heptane and Bicyclo[4,2,0]octane Derivatives.—The bridged bicyclo-[l,l,0]butanes (281), (282), and (283) rearrange thermally to the bicyclo[3,2,0]hept-enes (284) and (285). The rates of rearrangement are most rapid where radical stabilization by bridging functionality is possible, i.e. (283) reacts more rapidly than (282) which is in turn more reactive than (281). Use of substrates with deuterium specifically incorporated at the 6- and 7-positions showed that these positions become the olefinic positions in the newly formed cyclobutene ring. 

---