Heptatrienyl anion

In contrast to pentadienyl anions, heptatrienyl anions have been found to cyclize readily to cycloheptadienyl anions. The transformation of heptatrienyl anion to cyclo-heptadienyl anion proceeds with a half-life of 13 min at — 13°C. The Woodward-Hoffinann rules predict that this would be a conrotatory closure. ... 

Dipoles with a,/3 y,8-unsaturation are isoelectronic with the hepta-trienyl anion 1 and, as such, would be expected to undergo a 1,7-electrocy-clization (8tt electron process) analogous to the heptatrienyl 1-cyclohepta-dienyl 2 rearrangement (Scheme 2). Examination of the HOMO of the heptatrienyl anion, F4,3 shows that this ring closure must proceed in a con-... 

Allyl, pentadienyl, and heptatrienyl anions can in principle undergo electrocyclic rearrangements (81). The thermal conversion of a pentadienyl into a cyclopentenyl anion is predicted to be a disrotatory process. The cyclooctadienyl anion cyclizes to the thermodynamically stable isomer of the bicyclo[3.3.0]octenyl ion having cis fused rings (52,82,83). The acyclic pentadienyl anions, however, do not normally cyclize. On the other hand, heptatrienyl anions cyclize readily at — 30°C by a favorable conrotatory thermal process (41,84). This reaction sets a limit upon the synthetic utility of such anions. 

Add the appropriate number of electrons to the MO energy level diagram for the cy-cloheptatrienyl cation in Figure 16.3. Is this ion aromatic or antiaromatic The cyclo-heptatrienyl anion has two more electrons than the cation. Do you expect this anion to be a stable species Explain. 

Apparently, the pentadienyl anion portion moved along the carbon atom backbone until a heptatrienyl anion was formed, which then underwent the expected cyclization (see Sect. C.I) to a heptadienyl anion, which is of greater stability due to its extra carborn carbon single bond. Finally, while the 3-methylcycloheptadienyI anion was found to be thermally stable, it could be completely converted to the 2-methylcycloheptadienyl anion photochemically. 

With substituted conjugated systems, such as heptatriene (314) and butadiene nitrile (319), a similar kind of selectivity is shown with maleic anhydride (315)259 and isoprene (318)260 respectively, they give the lower energy adducts (316 and 320), rather than the alternatives (313 and 317). Once again a frontier orbital treatment, in its simplest form, has little to offer. We need the coefficients in the two compounds (314 and 319), but in neither case do we have them to hand. To avoid doing a calculation for the triene (314), we could add a little of the coefficients of the HOMO of the heptatrienyl anion (p. 123) to those of the HOMO of hexatriene. Similarly, for butadiene nitrile (319), we could... 

Claisen rearrangements introduce the complication of oxygen lone pairs within the rearranging system, rather than as substituents on the perimeter. They may be ignored and the transition structure treated as benzene-like, or they may interrupt the conjugation, and the transition structure is then like a heptatrienyl anion.915 Predictions based on the simple theory above, whichever of these models is taken, match most of the substituent effects, and more elaborate treatments with calculations account for the anomalous accelerating effect of a donor substituent at C-6.916... 

Unlike heptatrienyl anions, which cyclize to cycloheptadienyl anions even at — 30°C, nonatetraenyl anion is stable at room temperature and cyclization is not observed even at higher temperatures. In fact, (284) reacts with n-butyl-lithium to give (285) which opens to (286 R = Et) even at — 30°C 7-vinyl-cycloheptatriene reacts similarly to give (286 R = Bu"). ... 

The hydrogen transfer energy (Table 2) for cycloheptatrienyl anion is very small, suggesting that it is essentially unstabilized. The same conclusion may be reached using the data in Table 6. Here, the formation of the anion is 23 kcal/mol less exothermic than for the heptatrienyl anion. The formation of the latter anion is 17 kcal/mol more exothermic than for the allyl anion, and if this ion has 10 kcal/mol stabilization, the heptatrienyl anion will be stabilized by about 25 kcal/mol and leads to the conclusion that there is no significant. stabilization or destabilization for the cycloheptatrienyl anion. This presumably is at least in part a result of the puckered geometry of the anion. The cycloheptatrienyl radical has some stabilization. 

---