Permutational Isomerization Research Articles

Synthesis, molecular and crystal structure, and stereochemical non-rigidity of (O→Ge)-Bischelate bis[1-(2-oxoperhydroazepinyl)methyl]bromogermanium iodide and triflate

Reactions of (O→Ge)-bischelate dibromogermane (L7CH2)2GeBr2 (4c) with trimethyliodosilane and trimethylsilyl triflate proceed with the substitution of only one bromine atom and yield (O→Ge)-bischelate monobromogermanes (L7CH2)2Ge(Br)X (8c, X = I; 9c, X = OTf; LnCH2 = lactamomethyl C,O-chelate ligand; n = lactam ring size). According to X-ray crystal analysis data, triflate 9c is characterized by nearly ionic structure with the monodentate ligand and anion in trans-positions. The configuration of the germanium atom in the complex is intermediate between trigonal bipyramidal and square pyramidal. Quantum chemical calculations show the absence of additional coordinate bonding between the germanium atom and anion.Dynamic 1H NMR spectroscopy (1H DNMR) was used to study the stereochemical non-rigidity of monobromides 8c and 9c. Activation parameters for the inversion of the germanium atom configuration were calculated in various solvents. Based on the 1H DNMR data and quantum chemistry calculations, possible mechanisms of permutational isomerization are discussed.

Synthesis, structures and stereodynamic behavior of pentacoordinate (O→Si)-Chelate Difluoro(methyl)silylmethyl derivatives of amides and imides

The structures and stereodynamic behavior of (O→Si)-chelate methyldifluorosilanes in solutions were studied by multinuclear (1H, 13C, 19F, 29Si) and 2D (COSY, HETCOR 1H, 13C) NMR spectroscopy. The range of studied complexes included amides RC(O)N(R')CH2SiMeF2 [R = Ph, R' = H (1a); R = R' = Me (1b); R = Me, R' = CHMePh (1c)], lactams LCH2SiMeF2 (L = 2-oxoazepan-1-yl (2a), 2,2-dimethyl-4-oxo-2H-benzo[1,3]oxazin-3-yl (2b) or 4-methyl-2-oxoquinolin-1-yl (2c)] and the six-membered imide Im6CH2 [2,6-dioxopiperidin-1-yl (3)]. The pentacoordination of silicon atoms in all studied compounds was confirmed by NMR study in solutions and single-crystal X-ray studies of complexes 1c, 2a, 2c and 3. The temperature dependency observed for the 19F NMR signals in solution was explained by the ligand exchange in the coordination environment of silicon atoms (permutational isomerization) and used for determining the activation parameters of this process by 19F DNMR. Quantum-chemical calculations for various isomers of complexes 1c, 2a, 2c and 3 suggested the "turnstile rotation" as the most likely mechanism for the observed stereodynamic processes.

Dynamic Permutational Isomerism in a closo-Cluster.

Permutational isomers of trigonal bipyramidal [W2RhIr2(CO)9(η(5)-C5H5)2(η(5)-C5HMe4)] result from competitive capping of either a W2Ir or a WIr2 face of the tetrahedral cluster [W2Ir2(CO)10(η(5)-C5 H5)2] from its reaction with [Rh(CO)2(η(5)-C5HMe4)]. The permutational isomers slowly interconvert in solution by a cluster metal vertex exchange that is proposed to proceed by Rh-Ir and Rh-W bond cleavage and reformation, and via the intermediacy of an edge-bridged tetrahedral transition state. The permutational isomers display differing chemical and physical properties: replacement of CO by PPh3 occurs at one permutational isomer only, while the isomers display distinct optical power limiting behavior.

Synthesis, Structures, and Stereodynamic Behavior of Novel Pentacoordinate Fluorosilanes: Fluorosilyl Derivatives of Proline

The (O→Si)-chelate N′-(dimethylfluorosilylmethyl))-N′-methyl-N-(organosulfonyl)prolinamides RSO2-Pro-N(Me)CH2SiMe2F (2a–f, R = Me (a), Ph (b), 4-MeC6H4 (c), 4-ClC6H4 (d), 4-BrC6H4 (e), 4-NO2C6H4 (f)) were synthesized from the corresponding disiloxanes 1a–f using Et2O·BF3. According to the NMR and IR data, the extent of dimerization of fluorosilanes 2a–f in solution is negligible, while the O→Si coordination in solution is weaker than that in the solid state. Comparative CP/MAS NMR and X-ray diffraction studies revealed that in solution the coordination Si–O bond length varies in a narrow range (2.22–2.24 Å) that is 0.02–0.11 Å longer than in the crystalline state. Dynamic NMR (DNMR) studies of the fluorides revealed a fine structure of the 19F signals in the 0–20 °C temperature range, which was related to the structural features of the coordination set in these complexes. The temperature dependence of the SiMe2 signals in the 1H DNMR spectra was attributed to a permutational isomerization process involving a positional exchange of equatorial ligands. The narrow range of activational barriers of the process (23–24 kcal mol–1 and more) and high negative values of the entropy of activation are similar to those observed earlier for Si-substituted N-(dimethylsilylmethyl) and N-(methylphenylsilylmethyl) amides and lactams, which suggests similar permutational processes in all cases. Gas-phase quantum chemical studies demonstrate that the solvation of F– reduces the activation barrier.

Synthesis, structure and dynamic stereochemistry of (O → Si)-chelate N-(trifluorosilylmethyl)-[ N-( S)-(1-phenylethyl)]acetamide and 1-(trifluorosilylmethyl)-2-oxoperhydroazepine: Retention of the O → Si coordination in the adduct with KF and 18-crown-6

The novel compounds, N-(trifluorosilylmethyl)-[ N-( S)-(1-phenylethyl)]-acetamide ( 1a) and 1-(trifluorosilylmethyl)-2-oxoperhydroazepine ( 1b) have been prepared from the corresponding NH-compounds using ClCH 2SiCl 3/Et 3N or ClCH 2SiCl 3/(Me 3Si) 2NH followed by methanolysis or hydrolysis of the reaction mixture in the presence of Lewis bases, and then BF 3 etherate. Potassium-(18-crown-6)-(2-oxoperhydroazepinomethyl)tetrafluorosilicate ( 2) was synthesized by reaction of the trifluoride ( 1b) with KF in the presence of 18-crown-6. Using 19F, 29Si NMR and X-ray diffraction techniques it was established that the silicon atom is pentacoordinate in the trifluorides ( 1a, b) and hexacoordinate in the adduct 2. Thus the internal coordination of the O → Si bond present in the trifluoride ( 1b) is retained in the adduct 2. The stereochemical non-rigidity of the trifluorides ( 1a, b) and the N-(trifluorosilylmethyl)- N-methylacetamide ( 1c) was investigated using dynamic 19F NMR spectroscopy. The activation barriers for permutational isomerization are in the range 9.5–10 kcal mol −1. Lower values of Δ G # for permutation of trifluorides ( 1a– c) compared to the monofluorides with the coordination core OSiC 3F together with small negative values for the activation entropy implies a non-dissociative mechanism. Quantum-chemical analysis suggests a mechanism involving a turnstile rotation.

Structures and stereochemical non-rigidity of Si-substituted N-(dimethylsilylmethyl)- and N-(methylphenylsilylmethyl)amides and -lactams

Eleven new silicon-substituted N-(dimethylsilylmethyl)- and N-(methylphenylsilylmethyl)amides and -lactams bearing a chiral carbon in the amide or lactam fragment, and containing the OSiC 3X (X = Hal, OTf) coordination fragment have been synthesized and their structures determined in solution by spectroscopic means. These structures are consistent with the hypervalency model. Quantum chemical calculations adequately reflect correlations between the type of monodentate ligand X and the geometric parameters of the N–C–O–Si–X fragments. The activation parameters ( Δ G 298 # , Δ H # , Δ S # ) for enantiomerization and diastereomerization in these new compounds and the other related compounds were determined by the dynamic NMR (DNMR) method using full line-shape analysis. The free activation energy values ( Δ G 298 # ) in the absence of external nucleophiles vary from 9 to 27 kcal mol −1. The entropies of activation (Δ S #) are negative (−20 to −50 cal mol −1 K −1) in all cases except for the chloride derivatives of 4-phenyl-2-pyrrolidone and 4-oxazolidinone that have weaker intramolecular O → Si coordination. Irregular mechanisms of permutational isomerization were proposed on the basis of the DNMR data and the results of quantum-chemical calculations carried out by ab initio (HF) and DFT (PBE, B3PW91, 6-311++G(d,p)). Depending on the coordination environment at silicon, the mechanisms proposed involve either the dissociation of the Si–X bond followed by the Berry pseudorotation or similar in the intermediate or the cleavage of intramolecular O–Si bond with subsequent inversion at the silicon atom. The apparently simple pseudorotation mechanism involving only the pentacoordinate structures 1– 21 does not appear to be favoured in any of the examples studied.

Combining smart darting with parallel tempering using Eckart space: Application to Lennard–Jones clusters

The smart-darting algorithm is a Monte Carlo based simulation method used to overcome quasiergodicity problems associated with disconnected regions of configurations space separated by high energy barriers. As originally implemented, the smart-darting method works well for clusters at low temperatures with the angular momentum restricted to zero and where there are no transitions to permutational isomers. If the rotational motion of the clusters is unrestricted or if permutational isomerization becomes important, the acceptance probability of darting moves in the original implementation of the method becomes vanishingly small. In this work the smart-darting algorithm is combined with the parallel tempering method in a manner where both rotational motion and permutational isomerization events are important. To enable the combination of parallel tempering with smart darting so that the smart-darting moves have a reasonable acceptance probability, the original algorithm is modified by using a restricted space for the smart-darting moves. The restricted space uses a body-fixed coordinate system first introduced by Eckart, and moves in this Eckart space are coupled with local moves in the full 3N-dimensional space. The modified smart-darting method is applied to the calculation of the heat capacity of a seven-atom Lennard-Jones cluster. The smart-darting moves yield significant improvement in the statistical fluctuations of the calculated heat capacity in the region of temperatures where the system isomerizes. When the modified smart-darting algorithm is combined with parallel tempering, the statistical fluctuations of the heat capacity of a seven-atom Lennard-Jones cluster using the combined method are smaller than parallel tempering when used alone.

Some further applications of discrete path sampling to cluster isomerization

The discrete path sampling approach is applied to analyse the dynamics of several atomic and molecular clusters. Permutational isomerization rates are first calculated for icosahedral atomic clusters containing 13 and 55 atoms. The transformation between decahedral and icosahedral morphologies of a 75-atom cluster is then investigated, for which the potential energy surface has double funnel character. The final system considered is a cluster of twenty water molecules treated using a rigid molecule pair potential. Detailed analysis of the database of stationary points produced by the initial sampling is used to investigate the accuracy of the two-state description in each case. A clear deviation from two-state behaviour occurs for (H2O)20, where low-lying intervening minima exist.

Discrete path sampling

A theoretical framework is developed for the calculation of rate constants by sampling connected pathways composed of local minima and transition states that link them together. The theory is applicable to two-state or effective two-state systems and is applied to permutational or morphological isomerization in a two-dimensional cluster of seven Lennard-Jones atoms, water clusters containing eight and nine molecules, and a cluster of 38 Lennard-Jones atoms, which exhibits a double funnel energy landscape.

Topological specification of ensembles of molecules as a basis of stereochemical considerations

A basis for a definite notation of ensembles of molecules is found. It also considers dynamically stereochemical and conformational properties. Mathematical methods introduced by permutational isomerism allow the consistent description of atoms of any valency. A concept for modelling of stereochemical reactions is created.

Angular Spectroscopy: Rapid Visualization of Three-Dimensional Substructure Dissimilarity Using Valence Angle or Torsional Descriptors

A simple method, termed ''angular spectroscopy'', is developed for the rapid visual assessment of 3D shape diversity (conformations, metal coordination geometries) that is exhibited by a specific chemical substructure as observed in a number of different crystal structures. If there are q = 1 --> N-s instances of the substructure in 3D and each conformation is defined by i = 1 --> N-t torsion angles, then we can calculate a set of dissimilarity coefficients D-pq(n) that relate each of the q instances to some fixed reference conformation p. The Minkowski metric, adapted to take account of permutational isomerism and enantiomorphic inversions, is used to calculate city-block (n = 1) or Euclidean (n = 2) dissimilarities. The N-s values of D-pq(n) provide a unidimensional representation of the multivariate parameter space and can be plotted as a simple histogram. Multiple peaks in the histogram, or torsional spectrum, indicate the presence of multiple conformations in the dataset. Dissimilarity calculations based on valence angle descriptors can be used to assess the different coordination geometries that may exist around; a metal of fixed ligancy. The reduction in dimensionality of the representation, i.e., from N-t to unity, can lead to information loss and to the accidental overlap of peaks due to different conformations. To combat this problem, two simple modifications of the Minkowski metric have been investigated which generate multiplicative (M(pq)(n) and cumulative (C-pq(n)) dissimilarities, respectively. When all three types of coefficient are applied to a variety of example substructures, then the known conformational or configurational diversity in these datasets is clearly revealed. It is found that the multiplicative coefficient, M(pq)(n) is generally effective in minimizing peak overlap.

Homolytic Substitution at Phosphorus: An Ab initio Study of the Reaction of Hydrogen Atom and Methyl Radical With Phosphine and Methylphosphine

Homolytic substitution reactions of hydrogen atom and methyl radical at the phosphorus atom in phosphine and methylphosphine have been examined by high-level ab initio molecular orbital theory. MP4SDTQ/6-31G**//MP2(FC)/6-31G** calculations predict that free-radical attack at the phosphorus atom in phosphines is facile, with energy barriers of 14-33 kJ mol-1 and likely to involve hypervalent phosphoranyl radical intermediates. These intermediates, in turn, are found to have dissociative energy barriers of 10-31 kJ mol-1, depending on leaving group, and are unlikely to undergo pseudorotation prior to dissociation. MP5/6-31G**//MP2/6-31G** calculations indicate that permutational isomerism of phosphoranyl radical is likely to involve barriers of 145 and 127 kJ mol-1 for mechanisms involving transition states of D4h and C4v symmetry respectively.

The permutational isomerism of Jahn-Teller wheels

Molecular rearrangements based on a Jahn-Teller mechanism are characterized by a specific permutational isomerism. This is illustrated for the metallacyclobutane rearrangement, proposed by Wilker and Hoffmann, and for the butterfly isomerisation of Fe(CO) 4. It is shown that in both cases a full experimental conformation of the proposed Jahn-Teller mechanisms requires the resolution of enantiomeric reaction cycles.

Synthesis, crystal and molecular structures, and steroechemical nonrigidity ofN-(chlorodimethylgermylmethyl)-andN-(bromodimethylgermylmethyl)-N-[(S)-1-phenylethyl]acetamides andN-(chlorodimethylgermylmethyl)-4-phenyl-2-pyrrolidone

RacemicN-(chlorodimethylgermylmethyl)-4-phenyl-2-pyrrolidone and the first optically active amide derivatives containing the asymmetrical carbon atom and the five-coordinate germanium atom,viz.,N-(chlorodimethylgermylmethyl)- andN-(bromodimethylgermyl-methyl)-N-[(S)-1-phenylethyl] acetamides, were synthesized. Their structures were established by X-ray diffraction analysis. The geometric characteristics of the trigonal-bipyramidal valence environment about the germanium atoms are compared with those of analogous enantiomeric silicon compounds and the related five-coordinate germanium compounds. The barriers to permutational isomerization of the title compounds were determined by dynamic1H NMR spectroscopy. It was found that these barriers are higher than those of the corresponding silicon analogs.

Hexacoordinate silicon species: a possible model for reaction intermediates. 2. Nondissociative permutational isomerization in solution

Hexacoordinate silicon species: a possible model for reaction intermediates. 2. Nondissociative permutational isomerization in solution

Nondissociative permutational isomerization of an octahedral derivative of a nonmetallic element, a 12-Te-6 species

Mise en evidence d'une isomerisation intramoleculaire, non dissociative de pertellurane tous-trans (OC-6-12) en pertellurane cis-(OC-6-22). Etude par diffraction RX

Amine base promoted .beta.-elimination in .alpha.-bromo ester substrates. Evidence for permutational isomerism in the TBP carbon intermediate

Reactions d'elimination β realisees sur le α-bromo benzenepropionate d'ethyle et le α-bromo α-methyl benzenepropionate de methyle

Organo-nonmetallic chemistry: Pentacoordinate, ten-electron (10-X-5) compounds of silicon, phosphorus and sulfur

Abstract We have shown that ligands to nonmetals can be designed to stabilize five-coordinate species, relative to four- or six-coordinate ones. Nondissociative permutational isomerization of identically substituted hypervalent 10-Si-5 siliconate anions and neutral 10-P-5 phosphoranes is faster for the isoelectronic isostructural silicon species. Nucleophile mediated stereochemical inversion in silanes which are substituted by ligands which stabilize trigonal bipyramidal species is shown to proceed by pseudorotation of an intermediate 10-Si-5 species.

PATTERNS OF THERMAL REARRANGEMENTS INVOLVING 3RD ROW ELEMENTS

Abstract The thio-Claisen rearrangement of allyl aryl sulfides was found to be catalyzed by nucleophiles exerting only a small fraction of their nucleophilic abilities. A mechanism of nucleophilic triggering of concerted sigmatropic rearrangement was proposed and successfully tested against the rival proposal of cyclization-induced rearrangement on the basis of substituent-rate and secondary D-isotope effects. On the basis of the evidence presented the thioallylic rearrangement of allyl aryl sulfides under light-free, anaerobic conditions was shown to be a 2-step cyclic process in which a zwitterionic thiacyclobutane intermediate intervenes, involving octet expansion of the central sulfur. This is transformed to its rearranged isomer via permutational isomerism. By contrast, the corresponding silaallylic rearrangement, however, follows a concerted pericyclic pathway in which a 3p orbital of silicon bridges the allylic framework with complete inversion of configuration and unattended by any racemization. The rearrangement of β-silylketones to siloxyalkenes, however, shows an entirely different pattern one quite similar to the thioallylic arrangement in which silicon experiences octet expansion through formation of an intramolecular, apical Si[dbnd]O bond in the structure of a trigonal bipyramid intermediate. The occurrence of permutational isomerism in this TBP intermediate is confirmed by the observation of retention and racemization of the configuration of an initially chiral silicon reactant.

Permutational isomerization of spirosulfuranes derived from catechols

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPermutational isomerization of spirosulfuranes derived from catecholsBenjamin A. Belkind, Donald B. Denney, Dorothy Z. Denney, Y. F. Hsu, and G. Edwin Wilson Jr.Cite this: J. Am. Chem. Soc. 1978, 100, 20, 6327–6329Publication Date (Print):September 1, 1978Publication History Published online1 May 2002Published inissue 1 September 1978https://doi.org/10.1021/ja00488a005RIGHTS & PERMISSIONSArticle Views41Altmetric-Citations7LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (383 KB) Get e-Alerts Get e-Alerts