Trienes conformational equilibrium

The photocyclization of (Z)-1,3,5-hexatriene represents a well-studied example of the influence of ground-state conformational equilibrium on the product composition (Havinga, 1973). The unsubstituted triene exists mainly in the s-trans-s-trans conformation and therefore cannot undergo photocyclization only Z-E isomerization has been observed. With the 2,5-dimethyl derivative the s-cis-s-cis conformation predominates and photocyclization yields dime-thylcyclohexadiene and some dimethylvinylcyclobutene. Finally, with the 2-methyl derivative the s-cis-s-trans conformation is preferred, and a relatively good yield of methylbicyclo 3.l.0]hexene as well as methylvinylcyclobutene and methyIhexatriene-1,2,4 is found. These results are collected in Scheme I. 

FIGURE 7. 2,5-Dimethyl-Z-hexa-1,3,5-triene e - 12,300 (237 nm) ground-state conformational equilibrium and photoproducts. 

The degeneracy of the rapid conformational equilibrium between the non-planar conformations of cycloheptatriene is lifted in 7-deuteriocyclohepta-triene [66]. The equilibrium constant is no longer unity but conformer [66b] with hydrogen syn to the ring is present in higher concentration (Jensen and Smith, 1964). 

Dauben and coworkers reached similar conclusions on the basis of their extensive investigations of the photochemistry of 3-alkyl-6,6,9,9-tetramethyl-A3 5(10 -hexalins (122)217. Direct irradiation of these compounds leads to the formation of 129 and 130 due to secondary irradiation of the initially formed triene 128, which exists as an equilibrium mixture of conformers cZc-128 and cZz-128 (equation 48). Their results showed that as the steric bulk of the 3-substituent increases, the rate of disappearance of starting material decreases and the cyclobutene (129) bicyclo[3.1.0]hex-2-ene (130) product ratio... 

In an alternative mechanism a monodentate intermediate (1) is in rapid equilibrium with reactants and it undergoes at high [XY] rate-determining ring closure. Such a type of mechanism is believed to operate for Ni(trien) + interacting with XY." Reasons for the preferred mechanisms are given. The isomerization may take the form of a conformational change in a metalloprotein. 

Irradiation at 254 nm transforms 2,5-di-fert-butylhexa-l,3,5-triene (14) (a hexatriene with a dominant cZc equilibrium conformation) into the corresponding cyclohexadiene (15) with a 0.54 quantum yield. The reverse reaction transforms 1,4-di-terr-butylcyclohexa-l,3-diene (15) into the corresponding hexatriene (14) with a 0.46 quantum yield.83-85 Consistently, the computed... 

The action of heat on calciferol (Vitamin D2 10), or on pre-calciferol (2), with which it is in chemical equilibrium (see below), gives two further isomers, pyrocalciferol (ii) and isopyrocalciferol (12), both of which have sy i-9,io-configura-tions [ii]. Although these reactions were incomprehensible before the orbital-symmetry principles were developed, we can now see that the formation of sy i-isomers is the consequence of a preference for overlap of C<9) and C io) orbital envelopes on the same side of the molecule. The necessary conformations of the open-chain triene systems for these reactions are considerably strained, but no doubt the elevated temperature (180-200 ) at which cyclisation occurs makes the transition states accessible. 

The subject of macrocyclic conformation is complex and will be illustrated here by reference to Kyba s systems. These 11- " and 14-membered phosphorus macrocycles exist in various conformations. The barrier to inversion for phosphines is approximately 146.4kJmol , but some conformations are remarkably stable. For example, 2,6,10-triphenyl-2,6,10-triphosphabi-cyclo[9.4.0]pentadeca-l l(l),12,14-triene (11) has three conformers (a-c in Figure 1). When a xylene solution of (11) is refluxed (135 °C, 1 h), only (a) is obtained when crystallized. The phenyl groups at the 2-P and 10-P remain cis, although the 6-P does invert, but the crystallization process favours conformer (a) over (b). The equilibrium (a) (b) is displaced to the LHS by removal of (a) as it... 

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