Bicyclobutanes rearrangement

The rearrangements of 1- and 2-methylbicyclo[2,l,0]pent-2-ene to methylcyclo-pentadienes has been examined in solution and in the vapour phase and a mechanism in which cleavage of the C-1—C-4 bond produces a chemically activated cyclopenta-diene, which then suffers competitive hydrogen shifts and collisional deactivation, has been advanced RRKM calculations support this conclusion. The principle of least motion technique has been applied to this and bicyclobutane rearrangements. ... 

The classic example is the butadiene system, which can rearrange photochemi-cally to either cyclobutene or bicyclobutane. The spin pairing diagrams are shown in Figure 13. The stereochemical properties of this reaction were discussed in Section III (see Fig. 8). A related reaction is the addition of two ethylene derivatives to form cyclobutanes. In this system, there are also three possible spin pairing options. 

Compounds 71 are precursors to the bicyclobutanes, as shown in Scheme 18 heating of the reaction mixture led to their disappearance, with formation of 70 and 70. At the same time rearrangement of 70 to the exo-exo isomer 70 took place. This appears to be the only reaction of disilenes known to occur with inversion of configuration at silicon. 

Heating of thietanes can often lead to a ring contraction. Thermal treatment (150°C, in vacuo) of the tosylhydrazide salt 213 gives the allene epi-sulfide 214, a unique synthesis for this molecule. The mechanism is suggested to proceed either via the intermediate bicyclobutane ylide 215 or the mesomeric structure 216. Dodson et al. have noticed the rearrangement... 

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

In the light of experimental difficulties associated with the identification of intermediates, a MINDO/3 quantum-mechanical study of the singlet state ( a2) of the cyclobutylidene to methylenecyclopropane rearrangement has been carried out. It has been proposed that the whole process is initiated by electrophilic attack from the C3 methylene group of cyclobutylidene at the empty p atomic orbital on the Cl carbene site, so that a shift of electron density towards Cl can take place to give the bicyclobutane-like nonclassical carbene intermediate 4. Finally, the bicyclobutane intermediate 4 undergoes a symmetry-allowed conrotatory bond-fission process to generate methylenecyclopropane. The activation enthalpy calculated for the two steps is 8 kcal mol-1.2... 

Triple-bond compounds1030 react with carbenes to give cyclopropenes, except that in the case of acetylene itself, the cyclopropenes first formed cannot be isolated because they rearrange to allenes.1031 Cyclopropenones (p. 53) are obtained by hydrolysis of dihalocy-clopropenes.1032 It has proved possible to add 2 moles of a carbene to an alkyne to give a bicyclobutane 1033... 

When the carbene or carbenoid resulting from a dihalocyclopropane is unable to rearrange to the al-lene due to steric or other factors, insertion or addition reactions characteristic of carbenes take place. Thus dibromonorcarane on reaction with methyllithium gives a bicyclobutane derivative by insertion of the carbene into a 0-C—H bond (equation 57).178 Allene formation is sterically unfavorable in this case. Similarly, dibromotetramethylcyclopropane gives l,2,2-trimethylbicyclo[1.1.0]butane instead of tetra-methylallene (equation 58).179 181 An example involving a tricyclic dibromocyclopropane is given in equation (59).182... 

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. 

Reaction of 1,2,3-trimethylcyclopropene with trichloromethyl lithium generated from bromotrichloromethane and methyl lithium at —110 °C produces the cyclobutene (279). This may be explained in terms of intermediate formation of the bicyclobutane (280), followed by cyclopropyl-allyl rearrangement, though the dichloro-cyclopropane could not be detected even at —73 °C 209). 

Addition of dimethylvinylidene or a related carbenoid, generated from the reaction of propanone with a diazophosphonate and base, to alkylcyclopropenes leads largely to trienes, eg. (283) from 3,3-dimethylcyclopropene. Apparently the same products are obtained when the carbenoid is generated from 1,1 -dibromo-2-methylpropene and base, but it is not clear whether the reactions involve formation and rearrangement of a bicyclobutane or rather collapse of a polar intermediate such as (284)2U). 

Trimethylsilylcyclopropenes also undergo regioselective addition of diazomethane 248,247) and rearrangement to the diazo-compound 247). Catalytic decomposition of the latter can lead to bicyclobutanes or to isomeric silyldienes, eg. ... 

When the cyclopropene has a single carbomethoxy or aryl substitunt at the 3-position, the predominant stereoisomer of the enone obtained has the substituent cis-to the ketone 260 261). One possible explanation is the formation of a bicyclobutane by peracid attack from the side away from this substituent, followed by a stereocontrolled rearrangement. If the 3-substituent is hydroxymethyl, the reverse stereochemistry is observed in the enone, in agreement with a peracid attack directed by this group to the same face of the cyclopropene, followed again by rearrangement 262). 

Silver s role in the valence isomerization of bicyclobutane to butadiene, however, is most striking and deserves special attention. Masamune et cd., studying the silver perchlorate-catalyzed valence isomerization of exo.exo-and e (7o,ew-2,4-dimethylbicyclobutane, reported largely stereospedfic conversion to trans,trans- and ds,7ra s-2,4-hexadiene, respectively 49). At 5 °C, the isomerization of the ew,e o-bicyclobutane was 77% stereospecific, while the endo,exo isomer was 99% specific. Significantly, both isomers rearranged along the symmetry-forbidden Ya a + a2a] path in preference to alternative, symmetry-allowed [[Pg.85]

Bicychbataaes buUuBenes. Bicyclobutane (I) upon trcabnenl with silver perchlorate at room temperature rearranges to butadiene (2) in about 90% yield. 

The electron-transfer-induced chemistry of bicyclobutane systems offer a rich variety of reactions. Irradiation of naphthalene in the presence of 46 resulted in rapid fluorescence quenching without rearrangement. In contrast, irradiation with either 1-cyanonaphthalene or 9,10-dicyanoanthracene in solutions containing derivatives of 46 resulted in product formation. The product distribution obtained under electron-transfer conditions is compatible with radical cations of structure type 46 +, which is firmly established by ESR and CIDNP results. Nucleophilic capture of the 1,2,2-trimethyl derivative, 50", led to cleavage of the transannular bond. The initial eapture is followed by net addition, producing 51, or dehydrogenation, yielding 52 [179]. 

The most thoroughly studied bicyclobutane system is a bridged derivative, tricy-clo[4.1.0.0 ]heptane (Moore s hydrocarbon, 47), which has revealed many facets of radical cation reactivity. Nucleophilic solvents (CH3OH, H2O) or ionic nucleophiles (CN ) capture 47 +, leading to products formally derived by addition across the transannular bond (53, 54, 55). Significantly, each product is derived by backside attack, as indicated clearly for the monomethyl derivative (54, 55, R = CH3) [180], In the absence of nucleophiles, a (dimeric) rearrangement product (57) was obtained (Scheme 2) [180, 181]. 

The highly-strained bicyclobutane (480) cf. p. 251) rearranged on protonation in aqueous acetic acid to give the 6) ,7j5-methylene-A -unsaturated compound (481)/ The C(7a)-C(8) bond is presumably selected for cleavage by virtue of being the most strained. Hydrogenolysis of the bicyclobutane gave the 8a-methyl-A -olefin (482), as a consequence of attack upon the more exposed bonds of the bicyclobutane. 

The sodium salt of cyclopropanecarboxaldehyde tosylhydrazone (157) behaves similarly on heating to 125 135°C. Labelling experiments using the lithium cyclopropanecarboxaldehyde-p-tosylhydrazone salt has proved the occurrence of a cyclopropylcarbene-cyclobutene C3 - C4 ring expansion instead of a simple intramolecular carbene insertion in one of the four C-H bonds and bicyclobutane ring-opening . For other examples and discussion of such a rearrangement see Ref. 189. In an apparently related reaction, when cyclopropylmethyl tosylate (158) is treated with potassium t-butoxide in dimethyl sulphoxide at room temperature for 1 h, a quantitative mixture of cyclobutene and methylenecyclopropane is produced in equal amounts (equation 110) °. 

As was pointed out earlier, either central bond cleavage or addition to a side bond can account for the formation of the various products obtained in electrophilic addition reactions. It is highly likely that, in reactions where the cyclobutane ring is retained in the product, it is the central bond which is cleaved. However, when cyclopropylcarbinyl derivatives are obtained, both attack on a side bond as well as cleavage of the central bond followed by skeletal rearrangements are conceivable mechanistic pathways. The literature does record one report which purportedly disproves the side bond fission process in the reaction of chlorosulfonyl isocyanate with bicyclobutane The results of this study, however, have been reinterpretedleaving the question moot. 

In light of all this, it is not surprising that Bishop in his review on transition metal promoted rearrangement of small-ring compounds concludes No class of transition metal catalyzed rearrangements has been the subject of more controversy than those of bicyclobutane. ... 

---