Bicyclo butanes, formation

More recent results have provided additional detail on the conformational requirements for bicyclo[1.1.0]butane formation from conjugated dienes158. Hopf and coworkers have shown that high yields of the isomeric bicyclobutane 78 are obtained from irradiation of... 

Aliphatic dienes undergo three main photochemical pericyclic processes, whose individual efficiencies depend largely on the torsional angle about the central bond in the specific diene conformer which is excited. These are (a) cyclobutene formation, (b) bicyclo[ 1.1.0] butane formation and (c) [l,5]-hydrogen migration. A fourth process, methylcyclopropene formation, has also been observed in minor amounts in several cases. 

It was concluded from the formation of a dimer that 598 emerged by a DMS reaction in addition to a bicyclo[1.1.0]butane derivative [248]. The azacydoheptatetraene 599, trapped in an argon matrix kept at -261 °C, was observed by IR spectroscopy after photolysis of 3- and 4-diazomethylpyridine [249]. According to quantum-chemical calculations, the protodebromination of the respective bromodihydrodiazepi-nium ions is believed to proceed via the l,4-diaza-5,6-cycloheptadienes 600 and 601 as intermediates [169, 250],... 

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... 

A suggested mechanism for this reaction involves formation of the lA4-thionia-bicyclo[1.1.0]butane ion, which as a result of attack by phenoxide anion at the 3-position forms the desired 3-phenoxythietane (Scheme 26) <2003RJ0226>. 

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. 

It is well known that the thermal decomposition of a bicyclo[1.1.0]butane gives the corresponding 1,3-butadiene probably via the Woodward-Hoffmann allowed concerted pathway, while photolysis of cyclobutene also provides 1,3-butadiene as a major product. A remarkable difference in the reaction modes between the silicon and carbon analogs of the bicyclo[1.1.0]butane/cyclobutene system would partially be a consequence of the fact that the formation of the silicon analog of a conjugated diene is highly undesirable. 

This is explained in terms of dimerisation of the carbene, or a related carbenoid. However, formation and rearrangement of a bicyclo(1.1.0)butane related to (188) must also be considered. 

Although some carbenes are reported not to add to cyclopropenes207, there are several examples of inter- and intra-molecular addition leading initially to the formation of bicyclobutanes. l,2-Diphenylcyclopropene-3-carboxylates are converted to a mixture of three stereoisomeric bicyclo[1.1.0]butanes by reaction with ethoxy-carbonylcarbene generated from the thermolysis of ethyl diazoacetate an additional product is the diene (278) which is apparently formed by rearrangement of an intermediate zwitter ion208). It should be noted, however, that cyclopropenes readily undergo addition to diazo-compounds, and that subsequent transformations may then lead to bicyclobutanes (see Section 8), and that a free carbene may therefore not be involved in the above process. 

Bicyclo[1.1.0]butane is usually a side product of the photocyclization of butadiene to cyclobutene (Srinivasan, 1963) in isooctane, the quantum yield ratio is I 16 (Sonntag and Srinivasan, 1971). It becomes the major product in systems in which the butadiene moiety is constrained near an s-trans conformation and bond formation between the two terminal methylene groups that leads to cyclobutene is disfavored. An example is the substituted diene 88 in Scheme 30, for which the bicyclobutane is the major product a nearly orthogonal conformation should result from the presence of the 2,3-di-r-bu-tyl substituents (Hopf et al., 1994). 

Two-electron reduction [58a] or oxidation [224] of 1,3-dimethylidenecyclobutanes yields bicyclo[1.1.0]butanes. Cyclic voltammetry of 2,4-di-9//-fluoren-i-yliden-1,1,3,3-tet-ramethylcyclobutane in DMF at low temperatures demonstrated that bond formation proceeds via an EEC mechanism the rate constant was found to be 20 s [58a]. 

We also know the enthalpies of formation of the triene, l,6-(butane-l,4-diyl)-tropilidene (bicyclo[4.4.1]undeca-l,3,5-triene, 88) as well as of the related tetraene [ 1,6-(2-butene-1,4-diyl)-tropilidene, 89] and pentaene [l,6-( 1,3-butadiene-1,4-diyl)-tropilidene, 90], respectively. Choosing Roth s suggested value for the enthalpy of formation of the parent tropilidene so that all four species are taken from the same primary source, we find that attachment of these varying 4-carbon chains increase the enthalpy of formation by —40, 74 and 136 kJ mol respectively. Upon affixing these same 4-carbon chains to benzene to form tetralin, 1,4-dihydronaphthalene and naphthalene (91, 92 and 7, respectively) the corresponding enthalpy of formation changes by —57, 51 and 68 kJmoU. ... 

The cyclopropanation of a cyclopropene to produce a bicyclo[1.1.0]butane has often been achieved by dipolar addition to give an isolated 2,3-diazabicyclo[3.1.0]hexane followed by loss of nitrogen in a second step to produce the second cyclopropane. The formation of the diazabicyclo[3.1.0.]hexanes is discussed in Section 1.1.6.1.5.3.1. The decomposition of these to produce bicyclobutanes is described in Section 4.2.1.1.2. This section will only discuss those reactions in which a cyclopropene is converted directly into a bicyclo[1.1.0]butane, by thermal or photochemical methods (Table 20). Metal-catalyzed processes are discussed in Section 1.1.6.3.1.2. 

The second approach involves several examples of intramolecular reactions which employ cyclopropenes with a diazoketone substituent. In these cases, copper and rhodium catalysts effectively promote the formation of bridged bicyclo[1.1.0]butanes. ... 

Dimethyl 3-oxotricyclo[2.1.0.0 ]pentan-l,5-dicarboxylate (8) gives trimethyl bicyclo-[1.1.0]butane-l,2,3-tricarboxylate (9) in 76% yield upon dissolution in methanol and standing for 3 hours.When oxo diester 8 was dissolved in methanol-d, stereospecific formation of the corresponding endo-deuterated triester was observed. 

Formation of Cyclopropylmethyl Derivatives from Bicyclo[1.1.0]butanes... 

Electrophilic ring opening of bicyclo[l. 1.0]butanes generally leads to mixtures of cyclobutyl, cyclopropylmethyl and allylmethyl compounds, among which the first two products dominate.The parent bicyclo[1.1.0]butane (1) is converted into cyclobutanol and cyclo-propanemethanol, without formation of any but-3-en-l-ol, upon treatment with 0.001 N sulfuric acid. Acetolysis of bicyclo[1.1.0]butane affords a mixture of all three acetates. In general, the product ratio is highly dependent on the substitution pattern of the bicyclic system. 

Substitution involving C-N cleavage has been achieved by thermolysis and photolysis of various 2,3-diazabicyclo[3.1.0]hex-2-enes, which result in nitrogen evolution and concomitant formation of a bicyclo[1.1.0]butane derivative. For details, see Section 4.2.1.1. and refs 515-519. 

The mechanism is not certain, but could involve formation and subsequent isomerization of a bicyclo[l. 1.0]butane however, in one case a dihydropyridazine was also isolated. Tetramcthyl-cyciopropene does not give any allene-containing product. 

With acyclic dienes, the quantum yield for cyclobutene formation (cb) rarely exceeds ca 0.1, the expected result of the fact that the planar s-trans conformer normally comprises the bulk (96-99%) of the conformer distribution at room temperature. However, cb is often significantly larger than the mole fraction of s-cis form estimated to be present in solution. For example, 1,3-butadiene, whose near-planar (dihedral angle 10-15° 05,i06 s-cw con fonner comprises ca 1% of the mixture at 25 °C, yields cyclobutene with <1>CB = 0.04 " , along with very small amounts of bicyclo[1.1.0]butane. A second well-known example is that of 2,3-dimethyl-l,3-butadiene (23 ca 4% gauche s-cis at 25 °C ), which yields 1,2-dimethylcyclobutene (25) with cB = 0.12 (equation 16) ". Most likely, these apparent anomalies can be explained as due to selective excitation of the s-cis conformers under the experimental conditions employed, since it is well established that s-trans... 

---