Electrocyclic processes

We have now considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbitals, orbital correlation diagrams, and transition-state aromaticity. All arrive at the same conclusions about stereochemistiy of electrocyclic reactions. Reactions involving 4n + 2 electrons will be disrotatory and involve a Hiickel-type transition state, whereas those involving 4n electrons will be conrotatory and the orbital array will be of the Mobius type. These general principles serve to explain and correlate many specific experimental observations made both before and after the orbital symmetry mles were formulated. We will discuss a few representative examples in the following paragraphs. 

Fonnation of allylic products is characteristic of solvolytic reactions of other cyclopropyl halides and sulfonates. Similarly, diazotization of cyclopropylamine in aqueous solution gives allyl alcohol. The ring opening of a cyclopropyl cation is an electrocyclic process of the 4 + 2 type, where n equals zero. It should therefore be a disrotatory process. There is another facet to the stereochemistry in substituted cyclopropyl systems. Note that for a cri-2,3-dimethylcyclopropyl cation, for example, two different disrotatory modes are possible, leading to conformationally distinct allyl cations ... 

There are also examples of electrocyclic processes involving anionic species. Since the pentadienyl anion is a six-7c-electron system, thermal cyclization to a cyclopentenyl anion should be disrotatory. Examples of this electrocyclic reaction are rare. NMR studies of pentadienyl anions indicate that they are stable and do not tend to cyclize. Cyclooctadienyllithium provides an example where cyclization of a pentadienyl anion fragment does occur, with the first-order rate constant being 8.7 x 10 min . The stereochemistry of the ring closure is consistent with the expected disrotatory nature of the reaction. 

The photocyclization of N-aryl enamines derived from cyclic or acyclic ketones proceeds under mild conditions to produce 2,3-dihydroindole derivatives (178b). The stereochemistry of the products is predominantly trans, which follows from a photochemical electrocyclic process which should take place in a conrotatory manner (178c,I78d). However, the presence of some cis products is not as easily explained. 

Scheme 2. de Meijere s fourfold Heck coupling-electrocyclization process. 

Sulfur extrusion is the main decomposition pathway, with 1-benzothiepins being more stable than 3-benzothiepins. The required 6it disrotatory electrocyclic process leads to a loss of aromaticity for the [6]annulated benzene ring, whereas the [r/]benzene derivative 4 leads to a thianorcaradiene derivative with less disturbed aromaticity of the benzene ring. 

One of the most important problems that has to be solved in the thiepin chemistry is the mechanism of the sulfur extrusion reaction. Ready loss of sulfur of the simplest thiepins presumably occurs by valence isomerization to the corresponding thianorcaradiene, which requires a [4n -I- 2] disrotatory electrocyclic process leading to a c/s-fused three-membered ring, followed by cheletropic loss of sulfur. A lot of evidence supporting the above mechanism, though inconclusive, is available to date. 

The second type of reactions is an electrocyclic process occurring as the ene reaction (the transition state B in Scheme 3.188). 

Electrocyclic processes are also observed with ionic systems, both cationic and anionic. The decisive factor in the stereochemistry is the number of electrons in the delocalized polyenic system. Thus the mode of ring opening or ring closure depends on the n electrons in the open polyene. 

So in electrocyclic processes the reactant orbitals change into product orbitals through an intermediate transition state and in the most stable transition state the symmetry of the reactant orbitals is conserved while passing to product orbitals. Thus a symmetrical orbital in the reactant must change into an antisymmetric orbital. 

The equilibration of a hexa-1,3,5 triene with a cyclohexa-1,3 diene differs in some respects from the electrocyclic processes discussed above. 

There seems to be no great difference in the free energy between acyclic triene and the cyclic diene. This is because of smaller strain in the six-membered ring as compared with the four-membered one. On the other hand in 6n electron system in electrocyclic process there is more efficient absorption in the near regions of u.v. spectrum. This is why under both thermal and photochemical conditions, the (1, 6) electrocyclic reactions are reversible. Side reactions are more frequent in reversible. Side reactions are more frequent in reversible transformations of trienes than in dienes. The dehydrogenation of cyclic dienes to aromatic compounds may also occur in the thermal process. On heating cyclohexadiene yields benzene and hydrogen. 

Figure 7.9 Electrocyclic processes of a diene and triene by photochemical and thermal pathways...

For so-called electrocyclic processes, which are pericyclic reactions, the photochemical and thermal reactions give different stereoisomers, as shown for the diene and the triene in Figure 7.9. 

The Cope rearrangement mechanism can be also strongly affected by other substituents. Thus, the normal electrocyclic process in the thermal isomerization of divinyl aromatics has been suppressed relative to the thermolysis of l,2-bis(trifluorovinyl)naphthalene 438 (in benzene, at 193 °C, 24 h)231. Three major products 440-442 were isolated from the reaction mixture, but none of them was the expected product 439. Also formed in low... 

Oxidative or reductive coupling of organic ligands attached to a transition metal can often be treated formally as electrocyclic processes. The treatment leads to a simple extension of the Woodward-Hoffmann mles for aich reactions. A number of caveats remain, however. 

Another rhodium vinylidene-mediated reaction for the preparation of substituted naphthalenes was discovered by Dankwardt in the course of studies on 6-endo-dig cyclizations ofenynes [6]. The majority ofhis substrates (not shown), including those bearing internal alkynes, reacted via a typical cationic cycloisomerization mechanism in the presence of alkynophilic metal complexes. In the case of silylalkynes, however, the use of [Rh(CO)2Cl]2 as a catalyst unexpectedly led to the formation of predominantly 4-silyl-l-silyloxy naphthalenes (12, Scheme 9.3). Clearly, a distinct mechanism is operative. The author s proposed catalytic cycle involves the formation of Rh(I) vinylidene intermediate 14 via 1,2-silyl-migration. A nucleophilic addition reaction is thought to occur between the enol-ether and the electrophilic vinylidene a-position of 14. Subsequent H-migration would be expected to provide the observed product. Formally a 67t-electrocyclization process, this type of reaction is promoted by W(0)-and Ru(II)-catalysts (Chapters 5 and 6). 

Various ot,p y,5-unsaturated 1,3-dipoles are known to undergo 1,7-cyclization by a 871-electrocyclization process (329,330), and the corresponding diazo compounds behave similarly. 5-Diazopenta-l,3-diene derivatives such as 285 (Scheme 8.70), generated in situ by thermolysis of the corresponding tosylhydrazone sodium salts, cyclize to form 1,2-diazepines (286) (331). Sharp and co-workers studied the mechanism, scope, and limitations of this transformation. It was found that cis-substitution about the y,8-double bond prevents the 1,7-cyclization and directs the system toward 1,5-cyclization (332,333) (i.e., formation of a 3//-pyrazole), and that the ot,(3-double bond can be part of a phenyl ring (334). In special cases, the y,8-double bond can be incorporated as part of an aromatic [287 288 (335)] or 2- or 3-thienyl ring as well (336). 

The condensation of 6-substituted 4-hydroxypyrones with 1-cyclohexenecarboxaldehydes in the presence of L-proline gives the initial 1,2-addition addition product of the pyrone with the aldehydes, followed immediately by a 67t-electrocyclic process to give substituted oxopyrano[4,3-/ ][l]benzopyrans <1997JOC6888>. 

Early work (B-69MI51600) on 7V-substituted-l//-azepines revealed that they undergo photoinduced ring contraction to bicyclic valence tautomers as indicated in Scheme 1. Subsequently, it has been found that 3H- and 4H- azepines enter into analogous ring contractions, as do some of their oxo and benzo derivatives. These transformations, which parallel those undergone by cycloheptatriene, are often thermally reversible and occur by an orbital symmetry-controlled disrotatory electrocyclic process. 

If the lH-azepine bears a ring substituent then a mixture of two isomeric 2-azabicyclohep-tadienes, (38) and (39), is possible, corresponding to the two allowed electrocyclic processes indicated in Scheme 1 (paths a and b, respectively). In fact, the ratio of the two isomers varies with the position of the substituent. For example, the 3-methylazepine (37 R = 3-Me) affords a 1 1 mixture of the 4- and 7-methyl-2-azabicycloheptadienes, whereas the 4-methylazepine (37 R = 4-Me) yields the 5- and 6-methyl derivatives in the ratio of 1.5 1 (B-69MI5600). 

The electrocyclic process involves a rather complex equilibrating system consisting of a number of open-chain configurational and conformational isomers, e.g., species 460-463 as well as one heterocyclic form 459. Although much information regarding the tautomerism is available, the details of the mechanism shown in Scheme 22 are not completely elucidated. Stereochemical aspects have still not been fully recognized. In some cases the secondary Z-E isomerizations, e.g., 461 ->464 and 463-+464, may be side reactions causing the irreversibility of the process. 

The a,/3-double bond in diazo compounds of the type 57 may also be part of a heterocyclic ring, e.g., the 2,3-bond of thiophene 63 (Scheme 20), but not the 3,4-bond, which does not have sufficient double bond character to allow an electrocyclization process to occur [80JCS(P1)1718],... 

The only example of the generation of a monocyclic heterocycle by y-bond formation is provided by the thermally promoted conversion of the bisallenyl selenide (143) into the selenophene (144) (78TL1493). The cyclization is conveniently formulated as proceeding via an intramolecular ene reaction. Another electrocyclic process appears to be involved in the conversion of allyl phenyl selenide (145) into 2-methyl-2,3-dihydrobenzo[6]seleno-phene (146) (71TL49). The origin of the minor amount of 2-methylbenzoselenophene (147) which is also formed has not been established. Access to 3-keto-2,3-dihy-drobenzo[h]selenophenes (148) (71KGS333) and 2-arylidene-3-keto-2,3-... 

Vitamin D deficiency leads to defects in the structure of bones, a disease known as rickets , common in miners who lived and worked underground during the daytime. They did not have the normal share of sunlight, but it took some time to realize that the lack of vitamin D was related to a photochemical reaction. This reaction is shown in Figure 5.23 in a simplified form, in order to point out the important electrocyclic process. 

A number of photochromic systems have been extensively investigated that undergo cis-trans isomerization (indigos, azo compounds) cleavage (spiropyrans), electrocyclic processes (fulgides, 1,2-diarylethenes) [8.229, 8.244, 8.245], For instance, cis-trans isomerization of a thio-indigo derivative allows the reading of pyrene excimer or monomer fluorescence [8.246]. The 1,2-dithienylethene system presents particularly attractive interconversion properties by photoreversible cyc-litation [8.245],... 

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