Turnstile Rotation Research Articles

Synthesis, Molecular Structure, Fluxional Behavior, and Tricarbonyliron Transfer Reactions of (η4-1-Azabuta-1,3-diene)tricarbonyliron Complexes

The (η4-1-azabuta-1,3-diene)tricarbonyliron complexes 10 are easily prepared in high yield by condensation of the corresponding arylamines 7 with the cinnamaldehydes 8 and subsequent ultrasound-promoted complexation of the resulting 1-azabuta-1,3-dienes 9 with nonacarbonyldiiron. The complexes 10 are shown to represent excellent reagents for the transfer of the tricarbonyliron fragment onto cyclohexa-1,3-diene (1a). The structural characterization for the complexes 10 is achieved by IR, 1H-NMR, and 13C-NMR spectroscopy, as well as X-ray crystallography of 10b, 10c, and 10l. Using variable temperature 13C-NMR spectroscopy the fluxionality of the complexes 10a, 10b, 10c, 10e, and 2 is investigated and the activation barrier for the turnstile rotation of the tricarbonyliron fragment is determined. The transfer reaction and the structural factors influencing the transfer of the tricarbonyliron fragment are extensively investigated.

Coordinative interactions in chelated complexes of silicon: XIV. Regular or irregular exchange mechanisms in pentacoordinate silicon compounds: an interpretation based on crystal structures, DNMR-data and molecular modelling

A crystal structure determination on 1-(2-(difluoromethylsilyl))- N, N-dimethylmethanamine (Ib) has revealed a strongly distorted trigonal bipyramidal coordination with N and F at apical positions. A coordinative interaction of 2,346(3) Å is present. Dynamic NMR-data for this complex and related pentacoordinated compounds indicate positional exchange of substituents at silicon with an energy barrier of 7–19 kcal/mol. For the exchange mechanism, regular (pseudorotation, turnstile rotation) and irregular processes, with bond rupture/bond reformation, are discussed. A computer simulation (MNDO-calculations) of the exchange process in 2-((methyl(trifluorosilyl)amino)methyl)pyridine (IIIa), which shows trigonal bipyramidal structure in the crystal and fluxional properties in solution, indicates that any distinction between the mechanisms is unreasonable and can be reduced to the question of which limiting Si-N distance can be regarded as still corresponding to a chemical bond. There is good agreement between the calculated and observed energy barriers. The structure correlation method is used to compare the calculated molecular transformation route with the Berry pseudorotation pathway.

Phosphorus stereochemistry : Mechanistic implications of the observed stereochemistry of bond forming and breaking processes at phosphorus in some 5- and 6-membered cyclic phosphorus esters

The stereochemistry of endocyclic and exocyclic bond forming and breaking processes in 5- and 6-membered cyclic phosphorus esters is summarised and comparisons are made with analogous reactions in acyclic phosphorus esters. The factors that determine which bonds are broken and whether reactions occur with inversion or retention of configuration at phosphorus are complex and usually have more obvious effects for reactions in cyclic than in acyclic phosphorus esters ; in particular conformational effects may be important. The stereochemistry of migration of phosphorus ester groups across 1,3-diols is also described. It is suggested that nucleophilic substitutions at phosphorus are inherently stereospecific in the sense that trigonal bipyramidal reaction intermediates break down either directly or following a single Berry Pseudorotation or Turnstile rotation process. Multiple Turnstile rotations which would lead to racemisation, and which apparently do occur in stable phosphoranes, are insignificant for reactions involving trigonal bipyramidal intermediates.

A mechanism for the hydrolysis of MgATP by actomyosin of skeletal muscle

Based on a model of the active site of myosin (Ramirez, Shukla & Levy, 1978), a chemical mechanism for MgATPase and intermediate oxygen exchange is presented. In this mechanism, oxygen exchange takes place via an oxyphosphorane intermediate that undergoes double turnstile rotation (Ugi, Ramirez, Marquarding, Klusacek & Gillespie, 1971; Ramirez & Ugi, 1974. During hydrolysis by native skeletal muscle myosin, only three [ 18O] atoms from labelled water are rapidly incorporated into the phosphorus that is finally released to the medium as P i; whereas, during hydrolysis by subfragment 1 (S1), which is the head of myosin, four oxygens are labelled rapidly. To explain this difference, we postulate that cleavage of the (S1)-(S2) hinge in the preparation of S1 modifies the interaction of the oxyphosphorane intermediate at the active site. This enables a normally non-exchangeable oxygen to enter the exchange process. This is consistent with our earlier interpretation to the effect that the active site and the hinge in myosin are relatively close to each other Shukla & Levy, 1977b; Shukla & Levy, 1978. We postulate that the major elements of the active site are situated on a 92 amino acid fragment, p 10, isolated by Elzinga & Collins, 1977 from myosin. P 10 is now known to be situated in the region that connects the head to the body of a myosin heavy chain (Lu, Sosinki, Balint & Streter, 1978). An examination of the p 10 fragment for a possible point of proteolytic attack in the region of the hinge which will generate S1 revealed lysine 82. Breaking the protein chain at a point so close to the active site pocket could explain the effect of hinge cleavage on oxygen exchange. Two additional features of the present mechanism are: (1) the protonation of P γ of a MgP α,P γ complex of ATP, which depresses monomeric metaphosphate mediated hydrolysis, and enhances oxyphosphorane formation by addition of water to P γ; (2) the coordination of N τ-methylhistidinet † † N t -methylhistidine is 2-amino-3-(1′methyl-4′-imidazolyl)propanoic acid. of actin with Mg at the active site, which activates the release of the products of hydrolysis.

Large atomic orbital basis SCF and MRD-CI calculations for rearrangements of the PH5 molecule

Configuration interaction calculations in a large atomic orbital basis are reported for PH5 in order to study the relative merits of the Berry pseudo-rotation (BPR) and turnstile rotation (TR) mechanisms for ligand permutation in phosphoranes. A thorough geometrical optimization for the key trigonal bipyramid (D3h), square pyramid (C4v) and C2v-C3v TR (Cs) structures is effected in each case and reaction surfaces connecting these PH5 isomers are constructed. The barrier for the BPR mechanism is only 1.0 kcal (with the C4v structure corresponding to a potential maximum), while that for the TR process is 8.2 kcal if no relative tilting of the PH2(C2v) and PH3(C3v) portions of the molecules is allowed during ligand rotation. A tilted TR structure lying only 3.2 kcal above the optimal D3h conformation is identified, however, and the possible significance of this result is discussed. Speculation concerning which of the BPR and TR mechanisms is preferred for rearrangements of substituted phosphoranes is also made and it is argued that the TR is most likely to be favoured when bulky and relatively electropositive ligands are present in the system.

Intramolecular ligand exchange in phosphoranes. A comparison of Berry pseudorotation and turnstile rotation

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTIntramolecular ligand exchange in phosphoranes. A comparison of Berry pseudorotation and turnstile rotationJulianna A. Altmann, Keith Yates, and I. G. CsizmadiaCite this: J. Am. Chem. Soc. 1976, 98, 6, 1450–1454Publication Date (Print):March 1, 1976Publication History Published online1 May 2002Published inissue 1 March 1976https://doi.org/10.1021/ja00422a028RIGHTS & PERMISSIONSArticle Views157Altmetric-Citations42LEARN 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 (520 KB) Get e-Alerts Get e-Alerts

The apicophilicity of thio-substituents in trigonal bipyramidal phosphoranes

The variable temperature n.m.r. spectra of a number of five-co-ordinate spirophosphoranes having P-phenoxy- and P-phenylthio-groups have given data on the energetics of the pseudorotation processes available to these systems. It is concluded that the apicophilicities of phenoxy- and phenylthio-groups are similar, with the balance varying according to the nature of the other ligands. Further evidence is given for the high apicophilicity of the hydrogen atom. The concept of multiple turnstile rotation processes is discussed with particular reference to spirophosphoranes.

Permutational isomerization of caged polycyclic oxyphosphoranes

The reaction of 1-phospha-2,8,9-trioxaadamantane with hexafluoroacetone gives a crystal-line caged polycyclic pentaoxyphosphorane. The ambient temperature fourier transform 13C NMR spectrum of the phosphorane in CH 2Cl 2 solution, with proton noise-decoupling, shows one singlet for three equivalent methine carbons, and one singlet for three equivalent methylene carbons. Hence, the phosphorane undergoes a rapid permutational isomerization, which is confirmed by the observation of equivalency of the three methine protons and of the three groups of methylene protons in the 1H NMR, and of equivalency of the four CF 3 groups in the 19F NMR. The variable temperature 19F NMR spectra in a vinyl chloride-CHFCl 2 solvent system disclose that the permutational isomerization of this phosphorane cannot be prevented even at − 165°, although there is a progressive line-broadening indicative of a decrease in the exchange rate. The 31P NMR spectrum of the phosphorane has a signal at a higher magnetic field (∂ 31P = +42·4 ppm) than H 3PO 4; the shift is nearly identical in CH 2Cl 2 and in the acidic (CF 3) 2CHOH. The remarkably low energy barrier (< ca 5 kcal/mole) for the permutational isomerization of a compound as constrained as the caged oxyphosphorane is attributed to: (1) A small deviation from the perfect trigonal bipyramidal configuration, which raises the ground state energy. (2) The existence of a barrier configuration which can accommodate the constrains of the caged molecule, and is therefore of relatively low energy. The barrier configuration is deduced from the turnstile rotation mechanism of permutational isomerization.