2,3,4-Hexatriene

Submitted by Jesse C. H. Hwa and Homer Sims.1 Checked by Virgil Boekelheide and E. A. Caress. 

5-Hexatrietie. In a 500-ml., three-necked, round-bottomed flask fitted with a mechanical stirrer, a thermometer, and a graduated dropping funnel are placed 114 g. (0.42 mole) of phosphorus tribromide (Note 6) and 2 drops of 48% hydrobromic acid. As the contents of the flask are stirred and maintained at 10-15° by means of an ice-water bath, 98 g. (1.00 mole) of 1,5-hexadien-3-ol is added in the course of 1.5 to 1.75 hours. The mixture is allowed to stir at 10-15° for 40 minutes and then to stand at room temperature overnight. The flask is cooled in an ice-salt bath for 20 minutes, and the upper organic layer is decanted from the residue while still cold. The organic layer is successively washed with three 40-ml. portions each of ice water, 5% sodium bicarbonate, and water. The crude bromohexadiene weighs 147-153 g. (91-95%) (Note 7). 

In an assembly similar to that used for the previous reaction, 90 g. (0.67 mole) of dimethylbenzylamine (Note 8), 0.13 g. of hydroquinone, and 500 ml. of water are stirred and heated at 50°. The crude bromohexadiene (107 g., 0.67 mole) is added in the course of 20-40 minutes, and stirring and heating are maintained at 50° for 2-2.5 more hours. The flask is then fitted for downward distillation, and the mixture is distilled at about 40-50° and 30 mm. until no more oil distils with the water. A total of 133-200 ml. of distillate is collected. This is discarded. 

A solution of sodium hydroxide (106 g., 2.7 moles) in 535 ml. of water is placed in a 2-1. flask equipped with a sealed mechanical stirrer and an outlet arranged for downward distillation into an ice-cooled receiver. The aqueous solution of the quaternary bromide is added dropwise to the boiling solution of sodium hydroxide during a period of 2.5-4 hours (Note 9). The hexa-triene and dimethylbenzylamine which form are distilled with the water. Distillation is continued for 10-15 minutes after the final addition of quaternary bromide solution. The clear upper layer of the distillate is separated, cooled to 5-10°, washed with three 170-ml. portions each of cold 2 N hydrochloric add and water, and dried over anhydrous sodium sulfate. The oil is then distilled, 

This method is essentially that of Butz, Butz, and Gaddis except for modified charge ratios to increase the output per batch at some sacrifice in per cent yield. 

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Using Program SCF for ethylene and 1,3,5-hexatriene, list the electron repulsion integrals in the foiiii Yjj, Yj2, and so on. Take the coordinates from Figure 8-6. Try small variations in the atomic coordinates to see what their influence is on Yy. 

The most easily obtained information from such calculations is the relative orderings of the eneigy levels and the atomic coefficients. Solutions are readily available for a number of frequently encountered delocalized systems, which we will illustrate by referring to some typical examples. Consider, first, linear polyenes of formula C H 2 such as 1,3-butadiene, 1,3,5-hexatriene, and so forth. The energy levels for such compounds are given by the expression... 

Carrying out the numerical operations for 1,3,5-hexatriene gives the results shown in Table 1.15. Because the molecule is a six-rr-electron system, j, doubly... 

Table 1.15. Energy Levels and Coefficients for HMOs of 1,3,5-Hexatriene...

Fig. 1.10. Graphic representation of K-molecular orbitals of 1,3,5-hexatriene as combinations of 2p AOs. The sizes of the orbitals are roughly proportional to the coefficients of the Hiickel wave functions.

Repeat your analysis for the LUMO of ethene, 1,3-butadiene, 1,3,5-hexatriene and -carotene, except now focus on each orbital s net antibonding character. (Assume that LUMO energy rises as net antibonding character increases.) What effect does conjugation have on LUMO shape and energy Are your predictions for the HOMO-LUMO energy gap consistent with the experimental data ... 

HOMO and LUMO of 1,3,5-hexatriene show origin and destination of excited electron. 

HOMO of cw-1,3,5-hexatriene anticipates the preferred direction of ring closure. 

Hexatriene has Amax = 258 nm. In light of your answer to Problem 14.46, approximately where would you expect 2,3-dirnethyl-l,3.5-hexatriene to absorb ... 

Figure 30.2 The six n molecular orbitals of 1,3,5-hexatriene. In the ground state, the three bonding MOs are filled. In the excited state,

The Woodward-Hoffmann rules for pericyclic reactions require an analysis of all reactant and product molecular orbitals, but Kenichi Fukui at Kyoto Imperial University in Japan introduced a simplified version. According to Fukui, we need to consider only two molecular orbitals, called the frontier orbitals. These frontier orbitals are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In ground-state 1,3,5-hexa-triene, for example, 1//3 is the HOMO and excited-stale 1,3,5-hexatriene, however, 5 is the LUMO. 

Electrocyclic reaction (Section 30.3) A unimolecular peri-cyclic reaction in which a ring is formed or broken by a concerted reorganization of electrons through a cyclic transition state. For example, the cyciization of 1,3.5-hexatriene to yield 1,3-cyclohexadiene is an electrocyclic reaction. 

Hexamethylphosphoramide, S -2 reaction and. 371 Hexane, 1R spectrum of. 424 mass spectrum of, 413 1,3,5-Hexatriene, molecular orbitals of, 1180... 

Hexatriene, 270 Hydrazine, 109 Hydrogen, 61 Hydrogen cyanide, 76 Hydrogen fluoride, 71 Hydrogen peroxide, 111 2-Hyd roxypropylene, 216 Hyperconjugation, 33, 35, 37, 4 ... 

The alternate approach of Dewar and Zimmerman can be illustrated by an examination of the 1,3,5-hexatriene system.<81,92> The disrotatory closure has no sign discontinuity (Hiickel system) and has 4n + 2 (where n = 1) ir electrons, so that the transition state for the thermal reaction is aromatic and the reaction is thermally allowed. For the conrotatory closure there is one sign discontinuity (Mobius system) and there are 4u + 2 (n = 1) ir electrons, so that the transition state for the thermal reaction is antiaromatic and forbidden but the transition state for the photochemical reaction is aromatic or allowed (see Chapter 8 and Table 9.8). If we reexamine the butadiene... 

The connection of radical and pericyclic transformations in one and the same reaction sequence seems to be on the fringe within the field of domino processes. Here, we describe two examples, both of which are highly interesting from a mechanistic viewpoint. The first example addresses the synthesis of dihydroindene 3-326 by Parsons and coworkers, starting from the furan 3-321 (Scheme 3.79) [128]. Reaction of 3-321 with tributyltin hydride and AIBN in refluxing toluene led to the 1,3,5-hexatriene 3-324 via the radicals 3-322 and 3-323. 3-324 then underwent an elec-trocyclization to yield the hexadiene 3-325 which, under the reaction conditions, aromatized to afford 3-326 in 51 % yield. 

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