Cyclic cross-conjugated systems

Tlie name fulvaleiie was first mentioned by R. Brown (49TFS296), who expanded the class of fulvenes for those compounds containing two cyclic poly-enic systems with a central double bond. Tlius, depending on the ring size, cy-clopropylidenecyclopropene 1, fulvaleiie 2, heptafulvalene 3 and the unsymmetrical hybrid molecules triapentafulvalene (calicene) 4, triaheptaful-valene 5, and pentaheptafulvalene (sesquifulvalene) 6 are members of this class of cyclic cross-conjugated systems (Scheme 1). 

Ketene acetals show a pattern of product formation very similiar to enamines79 Diphenyl-4,4-diacetyl triafulvene is converted to diacetylmethyl cyclopentadiene 529 by S,N-acetals, whilst diphenyl-4,4-dicyano triafulvene undergoes C—C-inser-tion to S,N- and N,N-acetals, e.g. 530/531, resulting in cross-conjugated systems 533/534 by analogy with enamines. Cyclic S,N-acetals 532, however, yield exclusively the bicyclic fulvenes 535 due to additional loss of methyl mercaptan. 

Fulvenes, cyclic molecules with odd number of carbon atoms in the ring, represent one of the fundamental categories of cross-conjugated systems (see Chapters 1 and 6). According to the ring size of these molecules, the compounds are classified astriafulvenes 1, pentafulvenes 2, heptafulvenes 3, and so on (Scheme 7.1). Owing to their dipole moments, fulvenes possess unique reactivity, different from that of open-chain polyolefins and aromatic compounds. In 1989, Neuenschwander... 

While the usual eonsequence of hydration of enamines is eleavage to a secondary amine and an aldehyde or ketone, numerous cases of stable carbinolamines are known (102), particularly in examples derived from cyclic enamines. The selective terminal hydration (505) of a cross-conjugated dienamine-vinylogous amide is an interesting example which gives an indication of the increased stabilization of the vinylogous amide as compared to simple enamines, which is also seen in the decreased nucleophilicity of the conjugated amino olefin-carbonyl system. 

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... 

It is obvious that these compounds have in common an uninterrupted cyclic arrangement of cross-conjugated jr-systems. Compound 5 likewise contains the maximum number of exocyclic double bonds at a perimeter consisting only of sp2-hybridized carbon atoms. Thus, our definition allows one to call it a radialene, i.e. naphtharadialene on the other hand, it excludes hydrocarbons such as 6 [3,4,5,6-tetrakis(methylene)cyclohexene]. Although in the latter molecule all carbon atoms are indeed sp2-hybridized, the number of exocyclic double bonds has not reached its maximum. In 5, however, the number of double bonds cannot be increased further. 

Enolates may be derived from a,/l-unsaturated ketones 16 by base-catalyzed proton abstraction. Under kinetic control the a -proton is abstracted and a cross-conjugated metal dienolate is formed, whereas under thermodynamic conditions the extended dienolate is the major product3,, l. Successful alkylations of dienolates derived from cyclic a,/l-unsaturated ketones have been performed (see Section 1.1.1.3.1.1.2.1.). The related a,/ -unsaturated ester systems have also been investigated22-24. Open-chain structures 16 pose a rather complicated... 

Cross-conjugated mesomeric betaine - Cross-conjugated heterocyclic mesomeric betaines are cyclic mesomeric betaines in which the positive and negative charges are exclusively restricted to separate parts of the Tt-electron system of the molecule <1985T2239>. For examples see Sections 2.2.1.2.2 and 2.3.1.2.1. 

Trimethylenecyclopropane, [3]radialene (169), which is one of the benzene isomers and the smallest member of the radialene family, is characterized by a cyclic arrangement of cross-conjugated Ti-electron systems. The synthesis and properties of radialenes have been reviewed recently by Hopf and Maas The C—C and C=C bond lengths in 169,1.453 (3) and 1.343 (3) A, determined by are similar to those in its hexamethyl derivative... 

Yet, compared to the other x-systems, the acyclic linear polyenes, and the cyclic oligoenes (i.e., the annulenes), cross-conjugated molecules have lived in the shadows so far. Acyclic linear polyenes are encountered widely in fatty acid, polyketide, and terpenoid natural product domains and the ultimate polyene, polyacetylene is a conductive polymer that was the focus of the 2000 Chemistry Nobel Prize. The annulenes played crucial roles in the development of practical and theoretical organic chemistry (aromatic and antiaromatic compounds), and serve as pivotal bulk chemicals in industry (benzene, toluene, the xylenes, styrene, etc.). 

Altogether there are three fundamental classes of oligo-olefins that exhibit cross-conjugation the dendralenes (acyclic and branched), the radialenes (cyclic and branched), and the fulvenes. The fulvenes contain both branched and linear conjugation within the same it-system, and are therefore hybrid systems. As they are widely regarded as cross-conjugated molecules in the chemical literature, we follow this convention here. 

Cross-conjugated cyclic dienones with 6-, 7-, and 8-membered rings exhibit a rich and diverse photochemistry. The deep-seated photochemical rearrangements found in these systems have attracted numerous mechanistic studies. Cydohexadienones have received the most attention and provide the general framework for the mechanistic understanding of the photochemistry for this class of compounds. The key intermediate in these systems is an oxyallyl transient that is formed by ring closure across the P, P -carbons in the dienone s) tems A —> B. 

In 1982 the present author discovered cyclic orbital interactions in acyclic conjugation, and showed that the orbital phase continuity controls acyclic systems as well as the cyclic systems [23]. The orbital phase theory has thus far expanded and is still expanding the scope of its applications. Among some typical examples are included relative stabilities of cross vs linear polyenes and conjugated diradicals in the singlet and triplet states, spin preference of diradicals, regioselectivities, conformational stabilities, acute coordination angle in metal complexes, and so on. 

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