Pentadienyl cation, electrocyclic ring closure

The pentadienyl cation 126 thus formed undergoes an electrocyclic ring closure that leads, after hydrolysis, to the annulated ketone 127 (Scheme 16) . 

The Nazarov cyclization is an example of a 47r-electrocyclic closure of a pentadienylic cation. The evidence in support of this idea is primarily stereochemical. The basic tenets of the theory of electrocyclic reactions make very clear predictions about the relative configuration of the substituents on the newly formed bond of the five-membered ring. Because the formation of a cyclopentenone often destroys one of the newly created centers, special substrates must be constructed to aUow this relationship to be preserved. Prior to the enunciation of the theory of conservation of orbital symmetry, Deno and Sorensen had observed the facile thermal cyclization of pentadienylic cations and subsequent rearrangements of the resulting cyclopentenyl cations. Unfortunately, these secondary rearrangements thwarted early attempts to verify the stereochemical predictions of orbital symmetry control. Subsequent studies with Ae pentamethyl derivative were successful. - The most convincing evidence for a pericyclic mechanism came from Woodward, Lehr and Kurland, who documented the complementary rotatory pathways for the thermal (conrotatory) and photochemical (disrotatoiy) cyclizations, precisely as predicted by the conservation of orbital symmetry (Scheme 5). 

The pericyclic process comes next and it is a Nazarov reaction (p. 962), a conrotator electrocyclic closure of a pentadienyl cation to give a cyclopentenyl cation. There is r stereochemistry and the only regiochemistry is the position of the double bond at the end of th -. reaction. Here it prefers the more substituted side of the ring. 

Few electrocyclic reactions leading to boron heterocycles have been reported as such some examples can be identified, however. Since it is unlikely that the formation of boroles from stannoles occurs by two simultaneous boron-tin exchanges (36) [Eq. (11)], the presence of an open-chain boron-tin intermediate can be inferred (19). A conrotatory ring-closure of the 1,3-alkadienylborane (analogous to the pentadienyl cation), followed by elimination of MeaSnClz, would yield the borole. 

Figure 15.18 shows several examples of electrocyclic processes. Since the reactions are always allowed in either a conrotatory or disrotatory manner, the key issue is the control of stereochemistry. Electrocyclic reactions provide a good example of the power of pericyclic reactions in this regard. In all cases, the reaction proceeds as predicted from the various theoretical approaches. The restrictions placed by the orbital analysis on the reaction pathway are nicely demonstrated by examples D and E in Figure 15.18 only the stereochemistry given is found. An instructive example of the fact that it is the number of electrons that controls the process, not the number of atoms or orbitals, is the conrotatory ring closure of the four-electron pentadienyl cation prepared by protonation of a divinyl ketone (example G). 

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