Endocyclic double bond

In this section we compare the enthalpy of formation of isomeric pairs of cyclic olefins, one with an exocyclic double bond and the other with the double bond endocyclic. We start with 1-methylcycloalkenes and related methylenecycloalkanes, species genetically 18a and 18b, respectively. From prior experience with cyclopropanes and the thermochemical consequences of replacement of sp3 carbons by sp2 carbons in three-membered rings52, we expect 1-methylcyclopropene to be considerably less stable than methylenecyclopropane because the former compound has two trigonal carbons within the ring while the latter has but one. And so it is found56 the former has a gas-phase enthalpy of formation 43.1 2.2 kJ mol-1 more positive than the latter. 

Cyclization that eventualy places the resulting double bond endocyclic also occurs, as in the annulation of indoles and the formation of spirocyclic furanones and pyrrolones which involves a rearrangement process. ... 

Use the same method to calculate the dipole moment of cyclopentadienone,. Assume, for the calculation, that the endocyclic double bonds are parallel and the angle at carbon 2 is the same as in eyelopropenone. 

As a further application of the reaction, the conversion of an endocyclic double bond to an c.xo-methylene is possible[382]. The epoxidation of an cWo-alkene followed by diethylaluminum amide-mediated isomerization affords the allylic alcohol 583 with an exo double bond[383]. The hydroxy group is eliminated selectively by Pd-catalyzed hydrogenolysis after converting it into allylic formate, yielding the c.ro-methylene compound 584. The conversion of carvone (585) into l,3-disiloxy-4-methylenecyclohexane (586) is an example[382]. 

The most rational way to classify structures related to pyrazole is to take into account the number of double bonds present, either endo- or exo-cyclic. An endocyclic double bond can formally be considered part of a benzene ring in this context. 

Two double bonds. This is the most important class which includes the aromatic compounds pyrazole (3), indazole (4) and isoindazole (5), their non-aromatic isomers, pyrazolenines (or 3iL-pyrazoles 6), isopyrazoles (or 4JT-pyrazoles 7) and 3JT-indazoles (8), and the carbonyl derivatives of pyrazolines with the endocyclic double bond in positions 1, 2 or 3, i.e. (9), (10) and (11), respectively. The indazolones (12) and the pyrazolidinediones (13) and (14) also belong to this group. 

The structures and nomenclature for the common pine resin acids based on the abietane skeleton (abietic-type acids) are given in Fig. 8. The abietic, neoabietic, palustric and levopimaric acids differ only in tbe location of tbeir two double bonds. All double bonds are endocyclic, except in the neoabietic acid in which one is exocyclic. 

Tertiary pyrrolines (49, = 1) and piperideines (49, = 2) (if R = H and the enamine can exist in the monomeric form or if R = aryl) evidently possess an endocyclic -double bond (79,155,156). The stretching frequency of the double bond can be lowered to 1620-1635 cm by conjugation with an aromatic substituent. The double bond of an analogous compound with aliphatic substituents in position 2 may occupy either the endo or the exo position. Lukes and co-workers (157) have shown that the majority of the five-membered-ring compounds, traditionally formulated with the double bond in a position, possess the structure of 2-alkylidene derivatives (50) with an exocyclic double bond, infrared absorption at 1627 cm . Only the 1,2-dimethyl derivative (51) is actually a J -pyrroline, absorbing at 1632 cm . For comparison, l,3,3-trimethyl-2-methylene pyrrolidine (52) with an unambiguous exocyclic double bond has been prepared (54). 

More complex compounds containing enamine grouping, e.g., holarrhena alkaloids such as conkurchine and conessidine, possess an endocyclic rather than exocyclic double bond (159). On the other hand, 1 -methyl-2-alky Ipiperi-... 

The HOMO is an out-of-phase combination of the n orbitals of the exo- and endocyclic double bonds. According to the orbital mixing rules, the n orbital of the exocyclic double bond has the low-lying exocyclic a orbital out of phase with the... 

Observed geometrical features or stabilities of small ring molecules containing an endocyclic double bond have been explained in terms of the Ji-a interactions [137, 138]. The n relaxation is predicted to stabilize such small ring unsaturated molecules of heavy atoms recently prepared [139-142],... 

This method provides a convenient synthesis of alkenes with the double bond in a relatively unstable position. Thus reduction of the p-toluenesulfonylhydrazones of a,(3-unsaturated aryl ketones and conjugated dienones gives rise to nonconjugated olefins. Unsaturated ketones with endocyclic double bonds produce olefins with double bonds in the exocyclic position. The reduction of p-toluenesulfonylhydrazones of conjugated alkynones furnishes a simple synthesis of 1,3-disubstituted allenes. ... 

Evidently, these or closely related intermediates are accessible and reactive, since the synthesis was successfully achieved as outlined in Scheme 13.28. In addition to the key cationic cyclization in Step D, interesting transformations were carried out in Step E, where a bridgehead tertiary alcohol was reductively removed, and in Step F, where a methylene group, which was eventually reintroduced, had to be removed. The endocyclic double bond, which is strained because of its bridgehead location, was isomerized to the exocyclic position and then cleaved with Ru04/I04. The enolate of the ketone was then used to introduce the C(12) methyl group in Steps F-3 and F-4. 

The colored form of spironaphthopyran 32 absorbs at A,max of ca. 450 m,70 and the closed spiro form is colorless, which has no absorption band above 400nm. Bulky substituent group is especially important for photochromic sunglass. Introduction of the spiroadamantane or spirobi-cyclo[3.3.1]heptane into the 2-position of naphthopyran increases the resistance to photo-fatigue reaction, since endocyclic double bond induced by 1,7-hydrogen shift in the colored form cannot be formed in 2-adamantyl or 2-bicycloheptanyl group. 

Exo- and Endocyclic C=C Double Bond with Highly Mesoporous Carbon Supported Pd... 

We have recently investigated X-ray structure of 3-methoxycarbonyl-[l,4,2] diazaphospholo[4,5-a]pyridine as first example of molecular structure determination for [l,4,2]diazaphosphole ring [84], The ester substituent lies strictly in the molecular plane with carbonyl group in the trans orientation with the formal C=P bond. Endocyclic P-N and P-C bonds are averaged between respective single and double bond lengths. 

Similar cyclic ammonium systems with a variety of double bonds, triple bonds, and aromatic rings have been shown to undergo similar reactions. Endocyclic alkenes can also be utilized to give tetracyclic azoniaspiroalkanes, such as 115 (Equation 24) <1996KGS328>. 

Only one report mentions the cyclopropanation with diazodiphenylmethane in the presence of a group VIII metal catalyst. Remarkably enough, the selectivity of the reaction with 5-methylene-bicyclo[2.2.1]hept-2-ene (8) can be reversed completely. With Rh2(OAc)4 as catalyst, the exocyclie double bond is cyclopropanated exclusively (>100 1), whereas in the presence of bis(benzonitrile) palladium(II) chloride the endocyclic C=C bond is attacked with very high selectivity (>50 1)47). 

As an explanation, if was suggested that the degree of charge development in the transition state determines the preferred site of cyclopropanation A transition state with little charge development should prefer the endocyclic double bond (Pd catalysis), whereas one with much charge development should favor the exocyclic bond (Rh catalysis). 

For INOR1, INOR2 and SNOR a significant pyramidalization of the endocyclic double bonds can be observed by all methods. The out-of-plane deviations appear to be around 1.9° to 3.1° (see Table 14). 

Exocyclic double bonds at cyclic systems, which contain cross-conjugated double bonds, cannot be considered as a subgroup of radialenes and shall therefore be treated separately, although many of the structural features are comparable. However, in these systems the exocyclic and endocyclic double bonds are competing with each other as sites for Diels-Alder reactions, cycloadditions and electrophilic attacks. The double bond character of both, as measured by its distance, can provide some evidence for the selec-tivities. If no strain and conjugation are expected, the double bonds should be comparable... 

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