Six-membered cycle

Indole and carbazole are characterized by the enhanced Tr-electron density within the six-member cycles, which allows one to predict r] coordination as the preferential coordination route. In contrast to pyrrole, indole, in accord with experimental (62JA2534 64JA3796 71JA5102) and theoretical (82T3693) data, has... 

The 1 R,6R,7R,8S-as-fused structure and conformation of 102 were elucidated on the basis of their NMR spectroscopic data. The observed formation of only one sulfonium salt in this cyclization reaction was remarkable in that either sulfur atom might have been expected to participate in tosylate displacement. The H NMR spectrum of salt 102 shows a large three-bond scalar coupling of 10.6 Hz between H-6 (5 = 4.736) and H-7 (5 = 4.606) this indicates that they have an almost antiperiplanar relationship. The equatorial orientation of H-6 and the 3C6 conformation of its six-membered cycle are consistent with the strong NOEs observed between H-7 and both H-2axja and and... 

The idea behind this survey of nonbonded interactions is to get rid of them elegantly as explicit terms requiring separate calculations by means of Eq. (10.3). We shall examine to what extent nonbonded Coulomb-type interactions are at least approximately additive. The formulation of additivity is presented here for C H2 +2-2m hydrocarbons [208], where m is the number of six-membered cycles. 

To help develop a familiarity with Eq. (10.37), we examine a general formula for saturated hydrocarbons, C H2 +2-2m> containing n carbon atoms and m chair or boat six-membered cycles. These alkanes contain (n — 1 + m) CC bonds and (2n + 2 — 2m) CH bonds hence... 

Six-membered cycles are formed in alcohol-terminated cyclizations in high yields, in contrast to the reaction in the absence of alcohol, as shown, for example, in Scheme 9. ... 

Multiple carbonylative cyclizations can take place with substrates containig several double bonds. Up to three consecutive cyclizations can take place in good overall yields and high diastreoselectivity. All five- and six-membered cycles are formed in x (9-processes (Scheme 19). ... 

Formation of six-membered cycles is also possible and best achieved using a copper catalyst (Equation (46)). 

In a marked difference with respect to complexes of o-hydroxyazo compounds containing a six-member cycle of the type shown by 427, the existence of five- [751,857-862] or five- and six-member [751,863] cycles, shown by structures 482 and 483, respectively, is characteristic for coordination compounds obtained from analogous 2-mercapto derivatives ... 

There are five-member metal-cycles in the chelates 589, six-member metal-cycles in 590 and 591, and both five- and six-member cycles in 592. 

Another six-membered cycle with two isolated Si-Si linkages was formed by Schmidbaur554) while investigating the stability of ylids with disilanyl groups. They found a high tendency to the formation of cyclic ylids ... 

Free energy of activation for ring inversion in six-membered cycles [110]... 

The latter determination performed on a single crystal shows a rhenium(I) center in an octahedral environment with a facial arrangement of the three carbonyl ligands. The diphenylbis[(phenylthio)methyl] silane Ic forms a six-membered cycle with the rhenium center, in which the phenyl rings of the thioether groups are both orientated in the same direction as the bromide ligand. 

While 2,2 -bipyridine or phenanthroline adducts are likely to contain an approximately planar, five-membered -M-N-C-C-N moiety, upon coordination of a RR 2C(pz)2 a six-membered cycle is formed for which a boat conformation is predicted. Nevertheless, both the internal and external angles of the formally related M-(N-N)2-E moieties (where E is not carbon), are known to be able to undergo wide variations. Indeed, X-ray studies carried out on several / ,-pyrazolato-/V,/V -derivatives showed that the six-membered ring is not always in the boat conformation. This may undergo a severe folding as in bis(3,5-dimethylpyrazolyl)borato-7V,/V ](//3-cyclohepLatrienyl) (dicarbonyl)molybdenum,195 or may even approximate a chair, or at least, a distorted chair as in dimeric bis(cyclopentadienyltitanium)(//-pyrazolato-/V,7V ) (Scheme 10a).196... 

Wider valence angles at N result in a more planar structure of (Me2SiNH)3 as compared to cyclohejtane. The six-membered cycle in this compound is characterized by the NNN/NSiN dihedral angle of 20°. 

During the early years of development of the multiplet theory, attention was paid chiefly to the correspondence of the structure of reacting molecules and catalyst, especially in relation to the sextet model of dehydrogenation of six-membered cycles on metal catalysts. This work permitted the determination of the group of metals that can act as catalysts for the dehydrogenation of cyclohexane (the so-called Blandin s square of activity ) and the prediction of catalytic activity, e.g., for Re which was unknown as a catalyst for this reaction. 

An ester enolate attacks one of the ketones, giving a second six-membered cycle. 

For example, pyrrolidine enamines are more nucleophilic than piperidine and morpholine-based enamines. This difference in reactivity of cyclic amines originates from variations in pyramidalization at nitrogen. The nitrogen atom in a five-membered cycle is much more flat than the nitrogen atom in a six-membered cycle. 

Similar effects apply to aromatic ethers and related compounds. Chein and Corey discovered an interesting stereoelectronic preference for heteroaromatic ethers to orient the OR group in a way that minimizes lone pan-repulsion of the exocyclic substituent o-lone pair and the lone pair of nitrogetf but maximizes the anomeric interactions between OR lone pairs and endocyclic C-X bond. The antiperiplanar interactions of this type are likely to contribute as well, more so in the six-membered cycles where the interacting orbitals can easily adopt the ideal parallel geometry (Figure 6.121). 

Indeed, a heteroatom embedded into a ring certainly affects its c- and TC-electron system. Such incorporation effects can be evaluated by CT-electron-donor-acceptor descriptor [sEDA(II)] and ir-Electron-donor-acceptor descriptor [pEDA(II)] characteristics, which have been developed especially for unsaturated five- and six-membered cycles with heteroatoms (2012JOC2608), or their later modification for fused heterocycles, sEDA(III) andpEDA(III) (2015JP0290). 

Aside from the common formation of five- and six-membered cycles, the intramolecular Mizoroki-Heck reaction also proved to be a reliable tool for the formation of medium-sized rings. 

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