Hexatrienes conformation

A. M. Brouwer, J. Cornelisse, and H. J. C. Jacobs, /. Photochem. Photobiol. A Chem., 42, 313 (1988). Photochemistry of 2,5-Dimethyl and 2,5-di-tert-Butyl-l,3,5-hexatrienes— Conformation and Reactivity—A Quantitative Study. 

Table III. Relative Energies of Perfluoro-l,3,5>hexatriene Conformers...

Notice molecular symmetry at work. The Huckel rr-electron model is in many ways a blunt instrument, because we would get exactly the same answers for either of the following possible conformers of hexatriene (Figure 7.3). 

We will now consider the studies devoted to the next two linear polyenes 1,3,5-hexatriene and 1,3,5,7-octatetraene. First, we will present the results corresponding to geometries and conformational energies computed for these compounds. We will then discuss the computed frequencies and force fields. 

The most stable conformation of both hexatriene and octatetraene is the al -s-trans one. Figure 2 represents these structures schematically. 

Unstable conformers of trans- and cis-hexatriene have been detected by means of the combination of matrix-isolation infrared spectroscopy and photoexcitation (or the high-temperature nozzle technique)84. Ab initio MO calculations at the HF/6-31G level have been performed for several conformers of 1,3,5-hexatriene93. The observed infrared bands of unstable conformers have been attributed to the gTt (major species) and gTg (minor species) conformers of /raw.s -hexalricne and the gCt conformer of cw-hexatriene93. It is noted that, in the previous paper93, the notation c is used for twisted structures for the sake of simplicity. The calculated torsional angles around C—C bonds for the gTt, gTg and gCt conformers are in the range between 32° and 45°. The observed and calculated vibrational frequencies of gTt and gCt are reported in Reference 93. 

We now consider the relative stability of 1,3,5-hexatriene in its aU-fnzns and all-cis planar conformations. 1,3,5-hexatriene can be viewed, theoretically, as the result of the union of a central ethylenic fragment A and two ethylenic fragments B and... 

On the basis of the above analysis, we predict that the relative electronic stabilization of the various conformations of 1,3,5-hexatriene will vary in the order... 

It should be emphasized that this analysis is based on the assumption of some appreciable 2—5 overlap in all-c/s and cis-gauche 1,3,5-hexatriene. Furthermore, it should be pointed out that our analysis of the conformational preference of 1,3,5-hexatriene is aiming at revealing electronic patterns. In reality, the all-c/s conformation of 1,3,5-hexatriene is unfavorable due to repulsive interactions between the two methylene groups, i.e. conformational preference varies in the order all-frans > cis-gauche. 

An alternative approach for determining the relative pi electronic stabilization of two torsional isomers utilizes a molecular dissection into two open shell radical fragments. This approach is illustrated by examining torsional isomerism in butadiene and 1,3,5-hexatriene. The ir MO s of the conformational isomers of 1,3,5-hexatriene can be constructed from the union of the n MO s of two formal allyl radicals. The two regiochemical modes of union of interest will be designated cis and tram ... 

Photochemical d.s jraw.v-confonnational interconversion is also known to occur in larger polyenes. Brouwer and Jacobs have reported the results of irradiation of E- and Z-2,5-dimethyl-l,3,5-hexatriene (14) in argon matrices at 10 K108. Irradiation of the -isomer gives rise to various retainers, while irradiation of the Z-isomer results only in ,Z-isomerization. Photochemical translcis conformer interconversion has also been observed for , -1,3,5,7-octatricnc in matrices at temperatures below 10 K32. 

It should be noted that products like 443 and 447 are the normal products of photochemical reactions of acyclic 1,3,5-hexatrienes, as well as of divinyl aromatics, but are quite unusual for thermal transformations of such substrates. Presumably, the electrostatic repulsion between CF2 groups prevents the formation of conformation 450 which is necessary for the electrocyclic ring closure (i.e. 438 — 439 and 445 -> 446). Instead, it leads to conformation 451 which is favorable to generate the diradical and then the fused vinyl-cyclopropane intermediates 452 (equation 170). Note that the rearrangement 452 —> 453... 

Fig. II. (a) Schematic representation of hydrocarbons adsorbed on the [111] plane of platinum. Intersections of the lines of triangular net denote positions of the centers of platinum atoms. (1) Cyclohexane (2) all-cis conformation of cij-l,3,5-hexatriene (3) transoid conformations of cis- and trans-1,3,5-hexatriene (S4). (b) Adsorption configurations of acetylene and ethylene found most probable according to LEED studies 141).

All the 311 molecules adopt a P-helical conformation in the central hexatriene part (this crystal is named (P)-311 NpF). Single crystals with the opposite chirality, (M)-311 NpF, in which 311 is in an M-helical conformation, were also obtained from the same sample batch. The chiral cocrystals underwent photochromism. Upon irradiation with 365 nm light, the colorless crystal turned blue. 

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... 

Upon irradiation at 366 nm, a diastereomeric ratio of 95/5 for the dosed form is reached, while subsequent irradiation at 495 nm regenerates the open form (as a mixture of conformers (P)-31E /(P)-31Ef> = 57/43). From Scheme 17 it can be seen that only (P)-31Ea adopts the right geometry for the hexatriene component to undergo photocyclization... 

As an application, let us compare the energies of two isomers of hexatrienes. The linear s-trans conformation can be described as a resonance between the canonical structure 15 and long-bond structures 16-18 (Scheme 3.7), where one short bond is replaced by a long one. On the other hand, the branched isomer is made of only structures 19-21, since it lacks an analogous structure to 18. 

There are two isomers of hexatriene a cis form and a trans form. The name refers to the geometry about the central double bond. The two isomers have different chemical and physical properties. Rotation is still possible about the single bonds (although slightly more difficult than around a normal single bond) and there are three different planar conformations possible for each isomer. Keeping the central black double bond the same, we can rotate about each of the green o bonds in turn. Each row simply shows different ways to draw the same compound. 

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