Cyclobutane, isomerization

A which is not observed in individual solutions of the two enones at the same concentrations and may thus be indicative of a complex formation. However, the ratio of isomeric cyclobutane products resulting from such photocycloadditions is generally seen to be a quite sensitive function of steric effects and of the properties of the reaction solvent, of the excited state(s) involved (in some cases two different excited triplet states of the same enone have been found to lead to different adducts) and of the substituents of the excited enone and substrate. No fully satisfactory theory has yet been put forth to draw together all the observations reported thus far. 

In a similar manner diethyl maleate (actually diethyl fumarate since the basic enamine catalyzes the maleate s isomerization upon contact) forms unstable 1,2 cycloadducts with enamines with hydrogens at temperatures below 30°C (37). At higher temperatures simple alkylated products are formed (41). Enamines with no )3 hydrogens form very stable 1,2 cycloadducts with diethyl maleate (36,37,41). The two adjacent carboethoxy groups of the cyclobutane adduct have been shown to be Irons to one another (36,37). 

Crotonaldehyde, hydrogenation of, 43-48 Cubane, isomerization of, 148 Cyclic dienes, metathesis of, 135 Cyclic polyenes, metathesis of, 135 Cycloalkenes, metathesis of, 134-136 kinetic model, 164 ring-opening polymerization, 143 stereoselectivity, 158-160 transalkylation, 142-144 transalkylidenation, 142-144 Cyclobutane configuration, 147 geometry of, 145, 146 Cyclobutene, metathesis of, 135 1,5,9-Cyclododecatriene, metathesis of, 135... 

When the substituent groups in the polyphosphazenes were azobenzene [719] or spiropyran [720] derivatives, photochromic polymers were obtained, showing reversible light-induced trans-cis isomerization or merocyanine formation, respectively. Only photocrosslinking processes by [2+2] photo-addition reactions to cyclobutane rings could be observed when the substituent groups on the phosphazene backbone were 4-hydroxycinnamates [721-723] or 4-hydroxychalcones [722-724]. 

With two y,8 double bonds, two a,/3 double bonds, and the possibilities of cis and trans ring fusions with syn and anti configurations, 20 isomeric dimers are possible. Surprisingly, only one product is formed in a head-to-tail fashion. The sole product of the irradiation of the 3,5-diene-7-ketosteroid (76), however, is the head-to-head dimer. The specificity and mode of addition arise presumably through the effect of the specific environment of the chromaphore. The dimerization of (75) is believed to involve the addition of the a,fi double bond of a photoexcited molecule to the less hindered y,8 double bond of a ground state molecule. The photocondensation of (76) with cyclopentene, in which steric hindrance should not be a controlling factor, was found to yield a cyclobutane product involving the a,/ bond of the steroid in contrast to dimerization across the y,8 bond. 

Several other types of photochemical reactions involving unsaturated carbohydrates have been reported. One of these is38 photochemical, E -Z isomerization of the groups attached to a double bond (see Scheme 5). A second is the internal cycloaddition between two double bonds connected by a carbohydrate chain.39-41 Although the carbohydrate portion of the molecule is not directly involved in this cycloaddition, its presence induces optical activity in the cyclobutane derivatives produced photochemically. Finally, a group of acid-catalyzed addition-reactions has been observed for which the catalyst appears to arise from photochemical decomposition of a noncarbohydrate reactant.42-44... 

With even-membered rings certain molecules show only geometric isomerism. Examples are afforded by 1, 3 disubstituted cyclobutanes. 

The crystalline state of 193 was irradiated with sunlight at 5 °C (equation 93) to afford the cyclobutanes 194 and 195 in a 3 1 ratio117. Compound 195 obviously arose from the dimerization of the c/s-isomer of 193. The disordered crystal structure of 193 permits isomerization of 193 to the (. A-isomer which photolytically reacted with 193 to give 195. Interestingly, the crystalline state of compound 196 and 198 was photolysed to 197 and 198, respectively (equations 94 and 95), but /i-nitro-p-methylstyrene was photostable. 

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). 

Many other ion-molecule reactions involving highly unsaturated hydrocarbon ions and neutral olefins or the equivalent strained cycloalkanes have been studied by mass spectrometry98. For example, we may mention here the addition of ionized cyclopropane and cyclobutane to benzene radical cations giving the respective n-alkylbenzene ions but also isomeric cyclodiene ions such as ionized 8,9-dihydroindane and 9,10-dihydrotetralin, respectively. Extensive studies have been performed on the dimerization product of charged and neutral styrene4. 

Tetramethylene-ethane (TME), or 2,2/-bis-allyl diradical 81, was suggested as an intermediate in the thermal dimerization of allene, as well as in the interconversions of 1,2-dimethylenecyclobutane 82, methylenespiropentane 83, bis-cyclopropylidene 84 and other bicyclic systems (equation 30)45. The isolation of two different isomeric dimethylene cyclobutanes 87 and 88 (in a ca 2 I ratio) after the thermal rearrangement of the deuteriated 1,2-dimethylene cyclobutane 85 suggests that the rearrangement proceeds via a perpendicular tetramethyleneethane diradical (2,2/-bisallyl) 86 (equation 31)45. 

Calculations based on this second model give the observed value for the entropy of activation. In addition, this model may be used to account for the observed isotope effect (Benson and Nangia, 1963). If the tetra-methylene biradical is involved then it is to be expected that appropriately substituted cyclobutanes might undergo cis-trans isomerization reactions. This will be referred to again later. One final point should be mentioned in connection with biradical intermediates in both cyclopropane and cyclobutane reactions. This concerns the absence of any effect of radical inhibitors on these systems, when it might be expected that they would interact with the biradicals. In fact calculations show that, under the conditions of formation, the biradicals have extremely short lifetimes sec) and hence, unless radical inhibitors are... 

The values of these Arrhenius parameters contrast dramatically with those obtained for the bicyclo[2,2,0]hexane isomerization. In this compound there is no weak bridgehead bond, and hence the reaction path is more closely akin to that for cyclobutane itself. The similarity of the A factors for this reaction and that for other simple cyclobutanes supports this contention. If this is so, then the lowering of the energy of activation in this bicyclic compound by some 7 kcal mole from that observed in the alkylcyclobutanes is to be attributed to extra strain energy in this molecule. 

The thermal isomerizations of other bicyclic systems containing a cyclobutane ring appear not to have been investigated kinetically in detail, with the exception of a-and )3-pinene. These isomerizations all probably proceed through allylically stabilized biradicals, but the systems are complex and the studies were carried out well before the advent of modem analjrtical techniques of gas analysis. It is doubtful therefore whether a detailed discussion is worth while before more precise data are available (see Trotman-Dickenson, 1955). 

The very low value of the energy of activation for this isomerization is of considerable interest. Comparison with the decomposition of cyclobutane shows a reduction of 30 kcal mole caused by the presence of the double bond. If a similar transition state were involved in both reactions, then this difference would be a measure of the extra strain energy of the cyclobutene. This is quite unrealistically high. Thus we eliminate the possibility that the reaction path is as shown below ... 

These reactions of 1-Me resemble that of (dichloromethylene)cyclopropane [31] and radicophilic alkenes with a capto-dative substitution pattern [32]. Thus, it is not surprising that 1-Me reacts with a-ferf-butylthioacrylonitrile (18), yielding the two isomeric cyclobutane derivatives 19a, b (ratio 2.2 1) as a mixture of two diastereomers each [29] (Scheme 5), and this reaction occurs under milder conditions than the [2-1-2] cycloaddition of 18 onto methylenecyclopropane. 

The mechanism of this reaction is still not clear, but the key steps are probably a cyclopropylcarbinyl to cyclobutyl ring enlargement [45] with subsequent ring enlargement of the cyclobutane derivative 46. In fact, such cyclobutane derivatives 46c,d could easily be prepared in 86 and 94% yield, respectively, by stirring dichloromethane solutions of 42c,d in the presence of AI2O3 at 20 °C, and 46 c, d quantitatively isomerized into 47 c, d upon heating in DMSO at 100 °C for 2 h. 

Apparently the fusion of a benzocyclopropene to two cyclobutanes represents the upper limit for isolable benzocyclopropenes. Several attempts to synthesize dicyclopropabenzenes failed. Thus, attempted aromatization of the carbene adducts of the isomeric cyclohexadienes, 99 and 101 afforded none of the expected 100 and 102, respectively, and attempted aromatization of 103 resulted only in uncharac-terizable decomposition products without any evidence for the intermediacy of 104. ... 

Irradiation of the dienone shown generates three isomeric saturated ketones, all of which contain cyclobutane rings. Postulate reasonable structures. 

Cyclobutanes disubstituted in the 1,2-positions should favor strucmre-type C or a related distonic structure with one broken C—C bond. Calculations [QCISD-(T)/ 6-31G //UMP2/6-31G ] suggest a trapezoidal structure for frawi-1,2-dimethyl-cyclobutane radical cation.This expectation is born out by experimental results such as the ET induced cis-trans-isomerization of 1,2-diaryloxycyclobutane (Ar = aryl), leading to IS " ", and likely involving the distonic radical cation (14 +) formed via a type C ion. ... 

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