FLCs with a five-membered ring in the mesogenic core

A study has been undertaken of the structure-activity relationship of eight new chiral compounds having either a pyrazole (series P) or an isoxazole (series I) ring as a central bridge in the mesogenic core. The presence of dimers in the pyrazole compounds accounts for their lower P s values in comparison with the isoxazole analogues. The corresponding four beta-diketone precursors have also been studied and these, as expected given their bent molecular shape, show much worse mesomorphic and ferroelectric behaviour. In order to complete the study, the molecular dipoles of the three types of derivative have been determined using AM1 calculations. Two types of chiral tail have been incorporated into the compounds: alkoxy and alkanoyloxy. The latter tail gives rise to the best mesomorphic and ferroelectric properties. A study of the tail conformations by MM2 calculations provides an explanation of these results. The highest P s value (137 nC cm ) has been obtained for the isoxazole derivative with the (2S)-2-butyloxypropanoyloxy chiral tail (compound I4). The potential of all twelve compounds as chiral dopants for FLC mixtures has been evaluated by a study of 10 mol% binary mixtures in a standard host system.     

Cobalt-mediated two-carbon ring expansion of five-membered rings. Electrophilic carbon-carbon bond activation in the synthesis of seven-membered rings

Electrophilic cobalt(III) mediates an unprecedented two-carbon ring expansion of coordinated five-membered rings, leading to a remarkably general new strategy for the synthesis of seven-membered carbocycles from readily available five-membered ring substrates. The reaction, a metal-mediated [5 + 2] cyclopentenyl/alkyne cycloaddition, proceeds via initial protonation of a cobalt(I) cyclopentadiene complex, followed by rearrangement to an agostic eta3-cyclopentenyl intermediate. The cyclic eta3-allyl residue then undergoes migratory coupling with alkyne followed by carbon-carbon bond activation of the unstrained five-membered ring and recyclization to the ring expanded product, although the order of events and intimate mechanism has not been conclusively established. The reaction is highly selective with respect to which five-membered ring ligand undergoes activation, presumably a consequence of rapid cobalt-mediated interannular hydride transfer and kinetic preference for alkyne insertion into the less substituted cyclopentenyl ring. The alkyne insertion is itself highly regioselective, proceeding via migration to the sterically smaller end of the alkyne. The reaction is sensitive to both the cobalt counterion and the ancillary eta5-cyclopentadienyl substituent but proceeds for a considerable range of alkyl-, aryl-, and trialkylsilyl-substituted terminal and internal alkynes.     

Catalytic dehydrocyclization of diethylamine with the formation of a five-membered heterocyclic ring

Summary It was found that when diethylamine is passed over platinized carbon, pyrrole and n-butylamine are formed, i.e., dehydrocyclization takes place which is analogous to the C₅-dehydrocyclization of paraffins.     

Theoretical studies of five-membered ring ketene acetals

Cyclic ketene acetals are a class of organic molecules characterized by a nucleophilic exo-methylene carbon attached to a carbon with two adjacent O, N, or S atoms. We have carried out a systematic computational study of a series of five-membered cyclic acetals like 2-methylene-1,3-dioxolane and its OS, SS, NO, NS, and NN analogs as well as all the protonated species. The calculations were performed at the MP2 level using a triple zeta plus polarization basis set. The nucleophilicity was discussed in terms of geometrical factors, calculated atomic charges, calculated chemical shifts, and proton affinities. All the six neutral species were strong nucleophiles. The NN analog was predicted to be the strongest and the SS analog the weakest nucleophile.     

The special five-membered ring of proline: An experimental and theoretical investigation of alkali metal cation interactions with proline and its four- and six-membered ring analogues

The interaction of the alkali metal cations, Li+, Na+, and K+, with the amino acid proline (Pro) and its four- and six-membered ring analogues, azetidine-2-carboxylic acid (Aze) and pipecolic acid (Pip), are examined in detail. Experimentally, threshold collision-induced dissociation of the M+(L) complexes, where M = Li, Na, and K and L = Pro, Aze, and Pip, with Xe are studied using a guided ion beam tandem mass spectrometer. From analysis of the kinetic energy dependent cross sections, M(+)-L bond dissociation energies are measured. These analyses account for unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Ab initio calculations for a number of geometric conformations of the M+(L) complexes were determined at the B3LYP/6-311G(d,p) level with single-point energies calculated at MP2(full), B3LYP, and B3P86 levels using a 6-311+G(2d,2p) basis set. Theoretical bond energies show good agreement with the experimental bond energies, which establishes that the zwitterionic form of the alkali metal cation/amino acid, the lowest energy conformation, is formed in all cases. Despite the increased conformational mobility in the Pip systems, the Li+, Na+, and K+ complexes of Pro show higher binding energies. A meticulous examination of the zwitterionic structures of these complexes provides an explanation for the stability of the five-membered ring complexes.     

Novel five-membered ring intermediates in gas phase reactions

This review considers the role of closed-ring intermediates in chemistry, in general, and focuses on recent experimental and theoretical works that provide compelling evidence for a fivemembered ring intermediate in the gas phase. The reactions of oxygen atoms with ethyl iodide and larger alkyl iodides produce HOI and a corresponding ethylenic compound via attack of the oxygen atom at the iodine and closure of the five-membered ring between the oxygen and a hydrogen on the beta-carbon. Detailed results are gathered from infrared chemiluminescence and molecular beam scattering, as well as new kinetics and product yield experiments. The theoretical calculations fully support and characterize the novel reaction mechanism. National Research Council Research Associate, National Institute of Standards and Technology     

Conformational studies of helicenes—VIII1: Synthesis and conformation of two hexahelicene analogues having a non-aromatic,five-membered ring

Phenanthro[3,4-c]fluorenone (5) and phenanthro[3,4-c]fluorene (1) have been prepared from hexahelicene-5,6-quinone. The effect of the presence of a five-membered ring on the helical conformation has been studied by comparison of NMR data of 1 and 5 with those of hexahelicene. Force field calculations of the structure of 1 agree with the results of the NMR analysis.     

Three types of reactions with intramolecular five-membered ring compounds in organic synthesis

There are three types of reactions with intramolecular five-membered ring compounds in organic syntheses: The first type is reactions involving intramolecular five-membered ring compounds which are utilized for the ease of synthesis of these compounds and the stability of the products. The second is reactions performed via intramolecular five-membered ring intermediates, because such intermediates are very reactive and labile compounds. The third is the metal-catalyzed reactions with the intramolecular five-membered ring compounds because these metal compounds have catalytic activities. The third type reactions involving intramolecular five-membered ring pincer compounds are also provided.The first type reactions include carbonylations, alkenylations, alkynylations, acylations, isocyanations, Diels-Alder reactions, etc. The second type reactions include carbonylations, cross-coupling reactions, hydroacylations, ring expansion reactions, carbocyclizations, etc. The third type reactions include cross-coupling reactions, rearrangements, metatheses, reductions, Michael reactions, dehydrogenations, Diels-Alder reactions, etc.     

Reactions of arylhydrazines with α-formyl derivatives of six-and seven-membered ring lactams

Reactions of -formyl derivatives of N-methyl--valerolactam and N-methyl--caprolactam with arylhydrazines lead to the formation of 3,4,5,10-tetrahydroazepino [3, 4-b] indole-1 (2H)-one and 2,3,4,5,6,11-hexahydro-1H-azocino [3,4-b] indol-1-one derivatives. As the size of the lactam ring is increased the role of competing reactions, such as dealkylation at the indole nitrogen atom and the formation of 5-pyrazolone via reaction with -unsubstituted phenylhydrazine, also increases.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1398–1403, October, 1988.     

Core-modified, meso-alkylidenyl porphyrins and their expanded analogues: a new family of nonplanar porphyrinoids

New core-modified, meso-alkylidenyl porphyrinoids bearing multiple exocyclic double bonds were synthesized and characterized. The synthesis was accomplished using a typical "3 + 1"-type condensation approach. Stable exocyclic tautomers bearing double bonds at the meso positions, as well the corresponding endocyclic tautomers, were isolated in the case of both thiabenziporphyrin and thiapyriporphyrin products prepared in the course of this study. On the other hand, only the exocyclic tautomer was isolated in the case of the congeneric oxapyriporphyrin and oxabenziporphyrin. Expanded analogues of the exocyclic forms of oxabenziporphyrin and thiabenziporphyrin were also isolated as minor products. A single-crystal X-ray diffraction analysis of the expanded thiabenziporphyrin (20) revealed that all four pyrrole rings displayed an inverted geometry, presumably reflecting the strong hydrogen-bonding extant between the pyrrole N-H proton and the carbonyl group of the malonate moiety in the solid state. On the other hand, the expanded oxabenziporphyrin (14) was found to possess a severely distorted geometry with only one pyrrole ring being inverted. Careful analysis of the structure revealed that the solid-state geometry of the expanded macrocycles correlates well with the internal angle defined by the 2- and 5 substituents and the centers of the furan (14) or thiophene (20) subunits.     

Derivatives of cyclooctatetraenecyclopentadienyl-titanium containing substituents in the five-membered ring

The syntheses of some substituted cyclooctatetraenecyclopentadienyl-titanium compounds are described, viz: (h⁸-C₈H₈)(h⁵-R)Ti with R = C₅H₄CH₃, C₅H₄C(CH₃)₃, C₅H₄Si(CH₃)₃, indenyl (= Ind) and fluorenyl (= Flu). The compounds have been prepared by reaction of [(h⁸-C₈H₈)TiCl·THF]₂ with RNa in ether solution. The paramagnetic compounds are thermally stable to ca. 350°, but they are sensitive to air and water. The IR spectra and dipole moments of the compounds are given. The mass spectra of the complexes (h⁸-C₈H₈)(h⁵-C₅H₅)Ti, (h⁸-C₈H₈)(h⁵-Ind)Ti and (h⁸-C₈H₈)(h⁵-Flu)Ti indicate weakening of the Tih⁵R bond-strength in this sequence.     

Organoaluminium assisted rearrangements of five-membered ring enol ethers with vinyl substituents

Such organoaluminium reagents as ⁱBu₃A1, PhC=CA1Et₂, and Et₂A1SPh mediate the title reactions in three different directions. (1) [3,3]sigmatropic rearrangement producing 7-membered rings. (2) isomerization to form vinylcyclopropane derivatives and (3) S_N2 type reaction with phenylthio anion via oxolane ring opening.     

The magnetic circular dichroism of five-membered ring heterocycles

The MCD spectra of pyrrole, furan, thiophene, selenophene and teburophene and some of their derivatives are reported and the corresponding energies, oscillator strengths, transition moment directions, and MCD terms are calculated from semi-empirical quantum mechanical calculations hi the π-electron approximation. The MCD spectrum of thiophene is only slightly perturbed by substituents, and this is also expected to be true of the quite similar MCD spectra of selenophene and tellurophene. These molecules can then be classified as “hard” chromophores. On the other hand, pyrrole and furan have different and much weaker MCD spectra which change shape considerably when substituents are introduced. The implications of these observations are further discussed.     

A stable cation of a CSi3P five-membered ring with a weakly coordinating chloride anion

Don't count on counterions: The cyclic five-membered CSi(3)P cation 1 is synthesized in the reaction of benzamidinato-stabilized chlorosilylene and methyl phosphaalkyne. The presence of four π?electrons in 1 means it can be considered as a formal, heavier analogue of the cyclopentadienyl cation. Surprisingly the small counteranion (Cl(-)) does not contribute to the ring stability.     

Synthesis of seven-membered ring glycals via endo-selective alkynol cycloisomerization

[reaction: see text] Alkynyldiols 1 undergo cycloisomerization to the corresponding seven-membered cyclic enol ethers 2 under tungsten carbonyl catalysis. This novel transformation proceeds with good yields and virtually complete regioselectivity for all diastereomers of 1, favoring the product 2 resulting from endo-mode cyclization. The unexpected regioselectivity may be dependent on the presence of the dioxolane structure tethering the terminal alkyne and diol functional groups.