Bicyclo pentenes

Since the reverse reaction is thermally disallowed, the products may be unexpectedly stable. Thus cyclopentadiene can be partially converted photochemically into bicyclo [2,1,0] pentene, which in spite of having a strained structure has a half life of about two hours at room temperature. 

The direct irradiation of 1,3,5-cyclooctatriene (184) in ether or hydrocarbon solvents leads to the slow formation of two stable isomers corresponding to disrotatory 47T-electrocyclization (185) and bicyclo[3.1.0]pentene (186) formation along with small amounts of the reduced product 187 (equation 69)279-281. Conventional flash photolysis experiments later showed that, in fact, the main primary photochemical process is the formation of a short-lived stereoisomer (r = 91 ms)282, most likely identifiable as ,Z,Z-184. The transient decays to yield a second transient species (r = 23 s) identified as Z,Z-l,3,5,7-octatetraene (188), which in turn decays by electrocyclic ring closure to regenerate 184282 (equation 70). The photochemistry of 184 has been studied on the picosecond timescale using time-resolved resonance Raman spectroscopy49. 

If all the pentane is removed before pyrolysis, the bicyclo [2.1.0]pentane shows no impurities on vapor phase chromatography with a 20% Dow 710 on 50/60 U Anaprep column. Analysis by n.m.r. also revealed the absence of any traces of cyclo-pentene in the spectrum consisting of three complex multiplets at 0.3-0.8, 1.1-1.7, and 1.9-2.4 p.p.m. (downfield from internal tetramethylsilane reference). 

Mit dem gleichen Reagenz erhalt man z. B. aus N-Propyl-benzaldimin erythro-4-Methyl-5-phenyl-5-propylamino-l-penten (75%) und aus N-Benzyl-benzaldimin 5-Benzylamino-4-methyl-5-phenyl-1-penten (90% erythro threo = 8 92)2 analog erhalt man aus 2-Phenyl-propanal-isopropylimin mit 9-Allyl-9-bora-bicyclo[3.3.1]nonan erythro-4-Isopropylamino-5-phenyl-l-hexen ( 90%)1. 

Valence isomer formation is a feature also of the photochemistry of naphthalenes (3.33) and anthracenes for naphthalenes, as for benzenes, the extent ol steric crowding helps to determine which type of valence isomer predominates, since there is more severe interaction in the bicyclohexadiene products than in the benzvalene products. Amongst five-membered heteroaromalic compounds there are many known ring photoisomerizations that involve conversion of a 2-substituled to a 3-substituted system (e.g. 3.34). In some cases non-aromatic products can be isolated, such as bicyclo[2.1.0]pentene analogues from thiophenes 13.35). or acylcycfopropenes from furans (3.36) related species may be... 

The bicyclic pentafluoro oxo derivative 13 of Dewar benzene (see Section 5.3.4.3.) rearranges to pcntafluorophcnol when photolyzed in solution, in contrast to the desired decarbonylation to form the bicyclo[2.1.0]pentene systems.16... 

FIGURE 10. PMO interaction diagrams for the frontier orbitals of bicyclo[2.1.0]pentene (left) and bicyclo[3.1.0]hexenyl cation (right) according to Jorgensen73. The Walsh orbitals of the cyclopropyl ring are denoted by ws and wa, electrons by dots. Compare with Figure 11... 

FIGURE 14. CC bond orders n and bond ellipticities i of cyclopropyl homoconjugated molecules (a) norcaradiene, (b) bicyclo[2.1.0]pentene, (c) bicyclo[3.1. OJhexenyl cation. On the right, the preferred mode of electron delocalization is indicated by dashed lines. Also given is the number of delocalized electrons as calculated from topological bond orders. See text... 

We assert in this review that, at this point in time, there are several examples of neutral molecules which have been shown to display either bond or no-bond homoaromaticity. These include, in addition to the boranes mentioned above in Section III. B, cyclohepta-triene, norcaradiene, bridged cycloheptatrienes and norcaradienes, semibullvalenes, bar-baralanes, bridged annulenes, etc. Confirmation of the homoaromatic character of these systems comes from thermochemical and spectroscopic studies, and force field and ab initio calculations. In particular, the work of Roth and coworkers must be mentioned in this connection in that they were the first to provide reliable resonance energies of a large number of these neutral molecules225 226. These authors have also demonstrated that systems such as bicyclo[2.1.0]pentene are homoantiaromatic. 

Bicyclo[2.1.0]pentene, 116, can be considered to be the prototype of a neutral homoantiaromatic molecule. The types of structural and bonding effects found for this molecule parallel in many respects those found for the bicyclo[3.1.0]hexenyl cation reported above. Further studies on both of these 4q systems will likely be rewarding in terms of fully understanding the nature of cyclopropyl homoconjugation and homoantiaromaticity. 

When a cyclobutene is so constituted that conrotation cannot occur for steric reasons, then the concerted reaction cannot occur easily. Substances that otherwise might be predicted to be highly unstable often turn out to be relatively stable. An example is bicyclo[2.1.0]-2-pentene, which at first sight might seem incapable of isolation because of the possibility of immediate rearrangement to 1,3-cyclopentadjene. This rearrangement does occur, but not so fast as to preclude isolation of the substance ... 

Exercise 21-22 Unlike the conversion of bicyclo[2.1,0]-2-pentene to 1,3-cyclo-pentadiene, bicyclo[4.1.0]-2,4-heptadiene is transformed to 1,3,5-cycloheptatriene very rapidly at low temperatures by what appears to be a wholly concerted mechanism. Account for this difference. 

Were superstrain absent, the difference of the enthalpies of formation of bicyclo[ . 1.0]alkanes and (n + 2)-membered rings would be independent of n. Accepting the enthalpies of formation of the bicycles from either Reference 16 or 56, we find for n = 2, 3 and 4 values of 130.3 1.5, 115.2 1.2 and 111.0 1.5 kJmol-1. It is thus seen that some of the destabilization of bicyclo[2.1.0]pentene can be ascribed to this superstrain of the bicycle, the majority of the destabilization of the olefin arises from electronic factors. 

Examples Cyclobutenyl Bicycio [2.1.0] Bicyclo [3.1.0] cation pentene hexenyl cation... 

Schemes. Nonplanar bicyclo[2.1.0]-pentene radical cation.

Oxymercuration (2, 265-267 3, 194). Brown and Geoghegan have investigated the relative reactivities of a number of olefins in the oxymercuration reaction in a 20 80 (v/v) mixture of water and THF. The following reactivity is observed terminal disubstituted > terminal monosubstituted > internal disubstituted > internal trisub-stituted > internal tetrasubstituted. Thus steric factors play a major role in the reactivity of olefins. Increased substitution on the double bond and increased steric hindrance at the site of hydroxyl or mercury substituent attachment decrease the rate of reaction. In the case of olefins of the type RCH=CHR, civ-olefins are more reactive than the corresponding trans-olefins. Inclusion of the double bond in ring systems causes a moderate rate increase which varies somewhat with structure cyclohexene > cyclo-pentene cyclooctene norbornene bicyclo[2.2.2]-octene-2. 

Homoantiaromaticity is even less commonly invoked. Homocyclobutadiene 1.32b and the homocyclo-pentadienyl cation 1.33b are close to the transition structures for the interconversion of cyclopentadiene 1.32a and bicyclo[2.1.0]pentene 1.32c and of the cyclohexatrienyl cation 1.33a and the bicyclo[3.1.0]hex-enyl cation 1.33c. However, homoantiaromaticity does show up in these cases, in the sense that, unlike the interconversions in 1.30 and 1.31, neither of these interconversions is rapid. 

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