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Four-membered Rings (four-membered + ring)
Selected AbstractsRing Expansion of 2-(,-Hydroxyalkyl)azetidines: A Synthetic Route to Functionalized PyrrolidinesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 19 2008François Durrat Abstract A series of 2-(,-hydroxyalkyl)azetidines synthesized from enantiomerically pure ,-amino alcohols and presenting various patterns both on the four-membered ring and on the adjacent hydroxy group were treated with either thionyl chloride or methanesulfonyl chloride in the presence of triethylamine. The thus-prepared 2-(,-chloro- or ,-methanesulfonyloxyalkyl)azetidines were shown to rearrange stereospecifically into 3-(chloro- or methanesulfonyloxy)pyrrolidines. When this rearrangement is conducted in the presence of an added nucleophile (NaN3, KCN, KOH, or NaOAc), the produced pyrrolidine incorporates the added nucleophile at C-3 stereospecifically. The relative configuration of the substituents in the formed pyrrolidines is consistent with a mechanism involving the formation of an intermediate bicyclic aziridinium ion, which is opened regioselectively at the bridgehead carbon atom. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Cationic P,S,X cages (X=Br, I)CHEMISTRY - A EUROPEAN JOURNAL, Issue 6 2006Marcin Gonsior Dr. Abstract The first condensed-phase preparation of ternary P,Ch,X cations (Ch=O,Te, X=F,I) is reported: [P5S3X2]+, [P5S2X2]+, and [P4S4X]+ (X=Br, I). [P5S3X2]+ is formed from the reaction of the Ag+/PX3 reagent with P4S3. The [P5S3X2]+ ions have a structure that is related to P4S5 by replacing PS by P+X and S in the four-membered ring by P(X). We provide evidence that the active ingredient of the Ag+/PX3 reagent is the (H2CCl2)Ag,X,PX2+ cation. The latter likely reacts with the HOMO of P4S3 in a concerted HOMO,LUMO addition to give the P5S3X2+ ion as the first species visible in situ in the low-temperature 31P NMR spectrum. The [P5S3X2]+ ions are metastable at ,78,°C and disproportionate at slightly higher temperatures to give [P5S2X2]+ and [P4S4X]+, probably with the extrusion of 1/n,(PX)n (X=Br, I). All six new cage compounds have been characterized by multinuclear NMR spectroscopy and, in part, by IR or Raman spectroscopy. The [P5S2X2]+ salts have a nortricyclane skeleton and were also characterized by X-ray crystallography. The structure of the [P4S4X]+ ion is related to that of P4S5 in that the exo -cage PS bond is replaced by an isoelectronic P+X moiety. [source] Hetero-,-systems from 2 + 2 cycloreversion, part 2.1Ab initio thermochemical study of heterocyclobutanes 2 + 2 cycloreversion to form heteroethenes H2C=X (X=NH, O, SiH2, PH, S),HETEROATOM CHEMISTRY, Issue 7 2007Leonid E. Gusel'nikov Ab initio and DFT thermochemical study of diradical mechanism of 2 + 2 cycloreversion of parent heterocyclobutanes and 1,3-diheterocyclobutanes, cyclo -(CH2CH2CH2X), and cyclo -(CH2XCH2X), where X = NH, O, SiH2, PH, S, was undertaken by calculating closed-shell singlet molecules at three levels of theory: MP4/6-311G(d)//MP2/6-31G(d)+ZPE, MP4/6-311G(d,p)//MP2/6-31G (d,p)+ZPE, and B3LYP/6-311+G(d,p)+ZPE. The enthalpies of 2 + 2 cycloreversion decrease on going from group 14 to group 16 elements, being substantially higher for the second row elements. Normally endothermic 2 + 2 cycloreversion is predicted to be exothermic for 1,3-diazetidine and 1,3-dioxtane. Strain energies of the four-membered rings were calculated via the appropriate homodesmic reactions. The enthalpies of ring opening via the every possible one-bond homolysis that results in the formation of the corresponding 1,4-diradical were found by subtracting the strain energies from the central bond dissociation energies of the heterobutanes CH3CH2,CH2XH, CH3CH2,XCH3, and HXCH2,XCH3. The latter energies were determined via the enthalpies of the appropriate dehydrocondensation reactions, using C,H and X,H bond energies in CH3XH calculated at G2 level of theory. Except 1,3-disiletane, in which ring-opening enthalpy attains 69.7 kcal/mol, the enthalpies of the most economical ring openings do not exceed 60.7 kcal/mol. The 1,4-diradical decomposition enthalpies found as differences between 2 + 2 cycloreversion and ring-opening enthalpies were negative, the least exothermicity was calculated for , CH2SiH2CH2CH2. The only exception was 1,3-disiletane, which being diradical, CH2SiH2CH2SiH2, decomposed endothermically. Since decomposition of the diradical containing two silicon atoms required extra energy, raising the enthalpy of the overall reaction to 78.9 kcal/mol, 1,3-disiletane was predicted to be highly resisting to 2 + 2 cycloreversion. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:704,720, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20377 [source] Anionic ring-opening polymerization of small phosphorus heterocycles and their borane adducts: An ab initio investigationHETEROATOM CHEMISTRY, Issue 2 2007Michelle L. Coote The kinetics and thermodynamics of anionic ring-opening reactions of phosphiranes, phosphetanes, and phospholanes and their borane adducts have been studied by high-level ab initio procedures. For the free heterocycles, model propagation reactions involving nucleophilic attack by Me2P, at the ring ,-carbon were found to be feasible for the three- and four-membered rings, but not for the five-membered ring. For the borane adducts, nucleophilic attack by Me2(BH3)P, was only facile for the three-membered ring, despite an increased thermodynamic tendency toward ring opening for the borane adducts of both the three- and four-membered rings. The formation constants of the borane adducts of methylphosphirane and methylphosphetane were K = 2.6 × 1013 L mol,1 and K = 1.2 × 1020 L mol,1, respectively. A Marcus analysis of the ring-opening reactions of methylphosphirane, methylphosphetane, and their borane adducts showed that the release of ring strain and an "additional factor" contribute to rate enhancement compared with their strain-free analogues. The additional factor was larger for the three-membered rings than for the four-membered rings and was larger in the free heterocycles than in their borane adducts. The additional factor is complex in origin and appears to reflect an increase in the separation between the bonding and antibonding orbitals of the breaking bond on going from the three-membered rings to the four-membered rings, and on going from the free heterocycles to the borane adducts. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:118,128, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20323 [source] Ring Currents as Probes of the Aromaticity of Inorganic Monocycles : P5,, As5,, S2N2, S3N3,, S4N3+, S4N42+, S5N5+, S42+ and Se42+CHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2004Frank De Proft Prof. Abstract Current-density maps were calculated by the ipsocentric CTOCD-DZ/6-311G** (CTOCD-DZ=continuous transformation of origin of current density-diamagnetic zero) approach for three sets of inorganic monocycles: S42+, Se42+, S2N2, P5, and As5, with 6 , electrons; S3N3,, S4N3+ and S4N42+ with 10 , electrons; and S5N5+ with 14 , electrons. Ipsocentric orbital analysis was used to partition the currents into contributions from small groups of active electrons and to interpret the contributions in terms of symmetry- and energy-based selection rules. All nine systems were found to support diatropic , currents, reinforced by , circulations in P5,, As5,, S3N3,, S4N3+, S4N42+ and S5N5+, but opposed by them in S42+, Se42+ and S2N2. The opposition of , and , effects in the four-membered rings is compatible with height profiles of calculated NICS (nucleus-independent chemical shifts). [source] | ||||||||||