Hexatriene electrocyclic ring closure

J. Rimmelin and G. Jenner, Tetrahedron, 30, 3081 (1974). A recent measurement of the pressure and temperature dependence of die electrocyclic ring-closure of Z-l,3,5-hexatriene to 1,3-cyclohexadiene in the range of 200 to 2500 bar and 100 to 125 °C does not show a significant temperature dependence of die activation volume (M. K. Diedrich and F. -G. Klarner, unpublished results). 

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... 

The acyclic fluorinated 1,3,5-hexatriene system 454 is also resistant to 6 r-electron electrocyclic ring closure at temperatures up to 200 °C while the analogous hydrocarbons cyclize easily at 160 °C (equation 171)232. 

Fig. 3. Transition structure for the electrocyclic ring closure of 1,3,5-hexatriene to 1,3-cyclo-hexadiene...

Electrocyclic ring closures are particularly important in the formation of six-membered rings many are hetero analogues of the hexatriene-cyclohexadiene transformation 4—>5. As discussed in Section 3.2.1.6.1, they are frequently involved in ring interconversions initiated by nucleophilic attack on a six-membered ring. Further examples are discussed in Sections 4.2.3.6 (preparation of seven-membered rings) and 4.4.8.2.2.2 (formation of bicyclic 6,6 ring systems). 

Thiemann and coworkers [68] sought novel types of steroids with different biological activity, and in doing so prepared areno-annulated compounds such as 6/1-133 (Scheme 6/1.35). This is achieved with a Heck reaction of 6/1-132 with an acrylate, followed by an electrocyclic ring closure of the formed hexatriene. The reaction is then terminated by removal of the nitro group, with formation of the aromatic ring system. 

Figure 3 Comparison of the orbital correlation diagrams for the thennal dis (a) and con (b) 6e electrocyclic ring closures. Note that the electronic configuration is shown in each case only for the ground state of the hexatriene revening to the cyclohexadiene via the dis and con modes.

Another, recently developed method to synthesize a broad spectrum of oligo-and polycyclic aromatics and heteroaromatics in a surprisingly simple manner is likewise based on an electrocyclic - but thermal - hexatriene-cyclohexadiene ring-closure combined with an elimination reaction. This new synthetic method and its scope will be the topic of the following report. 

The proposal that the wavelength dependence of the product yields from irradiation of Z-14 is due largely to selective excitation of specific conformers is supported by the observation that the UV spectrum of the compound can be simulated accurately by a 9 1 combination of the spectra of Z-2-rcrr-butyl- (Z-131) and Z-2,5-di-fert-butyl-l,3,5-hexatriene (Z-15), which serve as model compounds for the cZi and cZc conformers of Z-14, respectively . Both Z-15 and the perfluorinated ,Z, -4,5-dimethyl-2,4,6-octatri-ene derivative (135) are thought to adopt preferred helical cZc conformations, and undergo highly selective electrocyclic ring closure to the corresponding cyclohexadienes 132 and 136, respectively upon irradiation (equations 49 the reaction of Z-15 the... 

The thermal disrotatory [n6] electrocyclization of cis-1,3,5-hexatriene systems has been extensively employed for the synthesis of cyclic hydrocarbons. The average enthalpy of activation is in the range of about 120 kJ mol 1 [36]. The incorporation of two of the hexatriene double bonds in phenyl rings (stil-bene, 1) stabilizes the precursor significantly and necessitates temperatures of 1050°C to obtain a 30% yield of phenanthrene (2, see Scheme 2, [37]). An enthalpy of activation of (250 20) kJ mok was estimated for the conversion of 9,9 -bifluorenylidene (3) to benz[e]indeno[l,2,3-hi]acephenanthrylene (4), a reaction that is accompanied by the radical initiated isomerization of 3 to diben-zo[g,p]chrysene (5, Scheme 2, [38]). It is assumed that both reactions 1 —> 2 and 3 —> 4 are initiated by an electrocyclic ring closure forming a 4 a,4 fr-dihydro-phenanthrene (la) intermediate. 

Electrocyclic ring closure of l-oxa-3,5-diaza-l,3,5-hexatrienes forms 2//-l,3,5-oxadiazines (see Section 6.18.10.1.1), while 4//-l,3,5-oxadiazines arise by (4 - - 2) t cycloaddition of electron-deficient l-oxa-3-aza-1,3-dienes with carbonitriles (see Section 6.18.10.2.2.i) and by cycloaddition of hexa-halogenopropanones (1 equiv.) with dialkylcyanamides (2 equivs.) (see Section 6.18.10.3.l.i). 

Similar arguments hold for the transition state of the electrocyclic ring closure of hexatriene forming 1,3-cyclohexadiene (Figure 13.22). There is a strong diatropic ring current in the transition structure, which is in agreement with the aromaticity concept of pericyclic transition states. 

Thiemann et al. [92] prepared steroid analogues containing an areno-annulation as in 169 (Scheme 8.44). They also used a combination of a Mizoroki-Heck reaction and an electrocyclic ring closure of an intermediately formed 1,3,5-hexatriene. Thus, reaction of... 

There is another approach to predicting the stereochemistry of electrocyclic ring closures. One simply looks at the ends of the HOMO to conclude the proper direction for rotation of the bond by creating in-phase interactions during closure. Show that this method also predicts conrotatory closure for butadiene and disrotatory closure for hexatriene. 

The final cyclization can also proceed in the manner of a fitt-electrocyclic ring closure (67T-ERC) (Scheme 3). After a [l,5]-hydride transfer, the conjugated 1,3,5-hexatriene intermediate I can be produced, which imdergoes subsequent 6tt-ERC to give the cyclized product 4. Thus, the formation of unstable zwitterionic intermediate A (Schemes 1, 2) with charge separation is avoided [52-56]. 

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