1.3- Diene central bonds

Ring opening by reductive cleavage of 1,3-diene central bonds... 

Simpler bicyclobutanes can also be converted to dienes, but in this case the products usually result from cleavage of the central bond and one of the edge... 

Intriguingly, the conical intersection model also suggests that E,Z-isomerization of acyclic dienes might be accompanied by conformational interconversion about the central bond, reminiscent of the so-called Hula-Twist mechanism for the efficient ,Z-photo-isomerization of the visual pigment rhodopsin in its rigid, natural protein environment101. A study of the photochemistry of deuterium-labelled 2,3-dimethyl-l,3-butadiene (23-d2) in low temperature matrices (vide infra) found no evidence for such a mechanism in aliphatic diene E,Z -photoisomerizations102. On the other hand, Fuss and coworkers have recently reported results consistent with the operation of this mechanism in the E,Z-photoisomerization of previtamin D3 (vide infra)103. 

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) bicyclof 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. 

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... 

Although it has been reported that dihalocarbenes generated by mild methods add across the central bond of methyl 3-methylbicyclo[1.1.0]butane-l-carboxylate (7) to furnish methyl 2,2-di-halo-3-methylbicyclo[l.l,l]pentane-1-carboxylates 8 and 9 in low yields,12 when the reaction was repeated on 7 using a different dichlorocarbene source, NMR spectrometry confirmed that the product obtained is actually diene 10.13 The structure of 9 has, however, not been challenged. 

In the formation of tetraenes from bicyclo[4.2.0]octa-2,4-dienes, two bonds are broken. This may occur in one concerted reaction which can be regarded as a retro [2 + 2] cycloaddition. It is also possible that the central bond, being part of a cyclohexadiene system, is the first one to break in a thermal, concerted disrota-tory process that leads to a 1,3,5-cyclooctatriene derivative. Ring opening of the cyclooctatriene then might take place photochemically, again disrotatory, to produce a tetraene. This two-step sequence was first observed by Mirbach et al. [114] in their study of the photocycloaddition of the two parent molecules benzene and ethene. The same explanation for the formation of a tetraene was given by Nuss et al. [160] in their report on the intramolecular ortho photocycloaddition of ( )-6-(2-methoxyphenyl)-5,5-dimethyl-2-hexenenitrile (see Scheme 40). 

W. J. Leigh and J. A. Postigo, /. Chem.Soc., Cbem. Commun., 24, 1836 (1993). The Role of Central Bond Torsional Motions in the Direct cis, 2 trans-Photoisomerization of Conjugated Dienes. 

Reduction of butadiene with sodium in liquid ammonia gives more cis-2-butene than fraws-2-butene, typically in a ratio of about 60 40. Identify all the intermediates. Consider the conformational equilibrium in the diene, its barrier to rotation about the central bond, the corresponding barrier to rotation in each of the intermediates and in the product, and hence identify at which stage the configuration was determined. Explain why the higher energy cis-isomer is the major product. 

This unexpected observation was used by Schulman and Fisanick to support their model for the electronic structure of the central bond. Since according to their model, the bond has only 20 % tt character, it is clear that the orbital is not properly oriented toward the incoming diene and therefore fails to partake in Diels-Alder reactions. It is highly likely that even if the central bond has a higher n character, as predicted by other models (see Section II.B), its reactivity as a dienophile is likely to be reduced greatly as compared to olefins. In addition, some steric inhibition to the approach of the diene will be caus by the two bridgehead substituents. 

This interesting result is not unexpected since, as was pointed out in Section II.B, the constituents of the bonding orbital of the central bond are not properly oriented for efficient overlap with the corresponding orbitals on the diene moiety. 

Metal-catalyzed decomposition of diazomethane in the presence of 1,2,3,4,5,6-hexamethyl-bicyclo[2.2.0]hexa-2,5-diene (11, hexamethyl Dewar benzene) has been shown to give the simple or double cycloaddition products with exo configuration. When the reaction was carried out in the absence of a metal catalyst, the products of C-H insertion to allylic C-H bonds were also observed, in addition to cycloaddition to the double bond to an extent of 25%. A C-C insertion reaction at the unusually long central bond of hexamethyl Dewar benzene was not detected. The reaction conditions were designed to generate methylene in the singlet state, and the observed products were those that would be expected for the reaction of singlet methylene with hexamethyl Dewar benzene. 

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