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Showing papers in "Journal of the American Chemical Society in 1975"


Journal ArticleDOI
TL;DR: In this article, a unique mean plane is defined for a general monocyclic puckered ring, which is described by amplitude and phase coordinates which are generalizations of those introduced for cyclopentane by Kilpatrick, Pitzer, and Spitzer.
Abstract: A unique mean plane is defined for a general monocyclic puckered ring. The geometry of the puckering relative to this plane is described by amplitude and phase coordinates which are generalizations of those introduced for cyclopentane by Kilpatrick, Pitzer, and Spitzer. Unlike earlier treatments based on torsion angles, no mathematical approximations are involved. A short treatment of the four-, five-, and six-membered ring demonstrates the usefulness of this concept. Finally, an example is given of the analysis of crystallographic structural data in terms of these coordinates. Although the nonplanar character of closed rings in many cyclic compounds has been widely recognized for many years, there remain some difficulties in its quantitative specification. An important first step was taken by Kilpatrick, Pitzer, and Spitzer in their 1947 discussion of the molecular structure of cyclopentane.' Starting with the normal modes of out-of-plane motions of a planar regular pentagon,* they pointed out that displacement of the j t h carbon atom perpendicular to the plane could be written 2 112 zj = (/'SI 4 COS (2+ + 4 n ( j 11/51 (11 where q is a puckering amplitude and $ is a phase angle describing various kinds of puckering. By considering changes in an empirical potential energy for displacements perpendicular to the original planar form, they gave reasons to believe that the lowest energy was obtained for a nonzero value of q (finite puckering) but that this minimum was largely independent of $. Motion involving a change in fi at constant q was described as pseudorotation. Subsequent refinement of this work has involved models in which constraints to require constant bond lengths are imposed3q4 and extensions to larger rings5-' and some heterocyclic systems are considered.* Although the correctness of the model of Kilpatrick, et a f . , I and the utility of the (q. $) coordinate system is generally accepted, application to a general five-membered ring with unequal bond lengths and angles is not straightforward. Given the Cartesian coordinates for the five atoms (as from a crystal structure), determination of puckering displacements z, requires specification of the plane z = 0. A least-squares choice (minimization of Zz i2) is one possibility, but the five displacements relative to this plane cannot generally be expressed in terms of two parameters q and $ according to eq 1. An attempt to define a generalized set of puckering cordinates which avoids these difficulties was made by Geise, Altona, Romers, and S~ndara l ingam.~l ' Their quantitative description of puckering in five-membered rings involves the five torsion angles 0, rather than displacements perpendicular to some plane. These torsion angles are directly derivable from the atomic coordinates and are all zero in the planar form. They proposed a relationship of the form\

6,526 citations




Journal ArticleDOI
TL;DR: In this paper, the effect of geometrical distortions, electronegativity, and variation of substituents on the magnetic interaction in dimeric systems is examined in detail for singly bridged L,M-X-ML, (n = 3, 4, 5); Cu~C16~ and other doubly bridging species where the bridging ligands are halogens, OR, pyridine N-oxides, oxalate, squarate; and the acetate bridged dimers C u ~ (R C 0 0 ) 4.
Abstract: A molecular orbital analysis shows that the antiferromagnetic contributions to magnetic coupling, favoring a lowspin ground state for a dimer containing two weakly interacting metal centers, can be analyzed in terms of pairwise interactions of dimeric molecular orbitals, with the square of the splitting in energy between the members of a pair being a measure of the stabilization of the low-spin state. The effect of geometrical distortions, electronegativity, and variation of substituents on the magnetic interaction in dimeric systems is examined in detail for singly bridged L,M-X-ML, ( n = 3 , 4 , 5); Cu~C16~and other doubly bridged species where the bridging ligands are halogens, OR, pyridine N-oxides, oxalate, squarate; and the acetate bridged dimers C u ~ ( R C 0 0 ) 4 . The emphasis is on d9 Cu(I1) dimers, but other transition metal systems are also analyzed. Transition metal complexes containing more than one metal atom with unpaired electrons can generally be categorized according to their magnetic behavior into three main groups depending on the strength of the metal-metal interaction. In the noninteracting type the magnetic properties of the dimer (or polymer) a re essentially unchanged from the paramagnetic monomer. In the strongly interacting type formation of relatively strong metal-metal bonds occurs, and the molecule will display simple diamagnetic behavior (for even numbers of electrons). In this paper the properties of weakly interacting metal ions will be investigated. In such compounds this weak coupling between the electrons of the two metal ions leads to low-lying excited states of different spin which can be populated a t thermal energies (SI000 cm-I). The resulting magnetic behavior will be antiferromagnetic or ferromagnetic, depending on whether the low spin (spins paired) or high spin (spins parallel) state is the ground state, respectively. These interactions-often termed superexchange because of the large distances involved (3-5 A) between the metal ions-have been observed in a wide variety of compounds. I 5 In experimental studies the magnetic interaction between spins SA and Sg for atoms A and B is usually written in a form suggested originally by Heisenberg, Dirac, and Van

1,224 citations






Journal ArticleDOI
TL;DR: In this paper, a-phenyl-seleno ketones were converted to enolates by selenenylation and selenoxide elimination, followed by a-bromination-dehydrobromination.
Abstract: The scope and limitation of the transformation of ketones to enones by selenenylation followed by selenoxide elimi- nation have been examined. Several procedures for the preparation of a-phenylseleno ketones have been developed. The most useful are direct selenenylation of ketone enolates using PhSeBr and the reaction of enol acetates with electrophilic selenium species such as benzeneselenenyl trifluoroacetate. Several oxidants (ozone, hydrogen peroxide, sodium metaperiodate) and reaction conditions are described to allow optimization of the yield obtained in the transformation of a-phenylseleno ketones to enones. The reaction is quite general for acyclic carbonyl compounds and for tertiary selenides. Difficulties in achieving high yields may be anticipated when very strained double bonds are introduced, when the a-phenylseleno ketone is cyclic and has an a-hydrogen, or when the product is extremely reactive. Qualitative mechanistic studies have revealed two types of side reactions: (1) Pummerer-like transformations to give a-diketones and (2) reactions between the enolate or enol of a-phenyl- selenino ketones and selenenylating species formed during the disproportionation of benzeneselenenic acid. Reaction condi- tions which minimize these side reactions have been developed. The utility of benzeneseleninyl chloride as a seleninylating agent has been explored. One pot transformations of ketones to enones using this reagent can be achieved in satisfactory yield, but the procedure is prone to side reactions because of the sensitivity of the selenoxide function. The many synthetic transformations originating from a,P-unsaturated carbonyl compounds have made their prep- aration a long standing important synthetic problem. The most straightforward method is the dehydrogenation of car- bonyl compounds. There are a number of methods for per- forming this the most important of which is the a-bromination-dehydrobromination method.' Orienta- tional control is difficult to achieve in direct bromination of ketones, but Stotter and HillId have recently shown that bromination of cyclohexanone enolates can be carried out in high yield, and also that dehydrobromination can be per- formed without loss of regiospecificity. Isomerization of a- bromo ketones under conditions of the debrominations has been frequently reported,Ie however, particularly for bro- mides of 0-dicarbonyl compounds.If,g The vigorous reaction conditions (frequently temperatures in excess of 120') also severely limit this method because of the sensitivity of many enones. Direct dehydrogenations can be performed by a number of reagents, including selenium dioxide,2a%b dichlorodicyan- oquinone,2c periodic acid,2d oxygen in the presence of tran- sition metal catalysts,2e and pyridine N-oxide-acetic anhyd- ride.2f The first two methods have been studied in great de- tail, and some excellent procedures have been developed, but yields vary greatly, and effective control of regioselecti- vity is frequently a problem. The discovery by Jones, Mundy, and Whitehouse3 that selenoxides undergo clean syn elimination to form olefins at or below room temperature suggested that this reaction may offer a solution to the problem discussed above. We re- port here full details of our ~ork~~-~ on the conversion of ketones to enones using the selenoxide elimination (eq 1). have explored the reaction for the dehydrogenation of ketone^,^^^^^^^,^^ e~ters,~~,~~ lac- tone~,~~,'~ and nitriles.8 Selenium stabilized anions9 have been used for formation of new carbon-carbon bonds, with subsequent selenoxide elimination to give aJ-unsaturated e~ters,~~,"~ olefin^,^^,^' allyl alcohol^,^^^,^^^ and dienes.4d Sulfoxide eliminations have also been explored for the in- troduction of unsaturati~n.~'~ ~~~'~

610 citations


Journal ArticleDOI
TL;DR: An accurate spectrophotometric method of determining relative equilibrium acidities of carbon acids in DMSO has been developed as discussed by the authors, and the pK of fluorene, formerly arbitrarily taken as 20.5, has been raised to an absolute value of 22.6.
Abstract: An accurate spectrophotometric method of determining relative equilibrium acidities of carbon acids in DMSO has been developed. The pK scale in DMSO has been anchored by comparisons of values obtained by the spectrophotometric method with those obtained potentiometrically in the 8 to 11 pK range. As a result, the pK of fluorene, formerly arbitrarily taken as 20.5, has been raised to an absolute value of 22.6. The pK’s of other carbon acids previously reported, including nitromethane, acetophenone, acetone, phenylacetylene, dimethyl sulfone, acetonitrile, and the corresponding indicator pK‘s must also be raised. The pK’s have been found to be correlated with heats of deprotonation in DMSO by potassium dimsyl, and evidence is presented to show that pK measurements in DMSO are free from ion association effects. Data are presented which indicate a pK of 35.1 for DMSO. In the methane carbon acids, CH3EWG, the order of acidities is NO2 >> CH3CO > CN, CH3S.02. The differences amount to 12.2 and 6.8 kcal/mol, respectively, which are believed to be of a comparable magnitude to gas-phase substituent effects. Carbon acids wherein the charge on the anion resides mainly on oxygen, such as ketones and nitroalkanes, are found to be weaker acids in DMSO than in water by 5 .5 to 9.6 pK units. On the other hand, carbon acids wherein the charge on the anion is delocalized over a large hydrocarbon matrix, such as in the anion derived from 9-cyanofluorene, are stronger acids in DMSO than in water. Factors that may contribute to this reversal are discussed. The scale of pK’s for 9-substituted fluorenes in DMSO is shown to be expanded when compared to the earlier pK scale determined by the H method. A rationale is presented. The apparent relative acidities of fluorenes and phenylacetylene differ by 6 and 1 I pK units, respectively, for cyclohexylamine (CHA) vs. DMSO solvents and benzene vs. DMSO solvents. Similarly, in benzene, acetophenone is a stronger acid than fluorene by ca. 6 pK units, whereas in DMSO acetophenone is a weaker acid by 3.2 pK units. These differences result from ion association effects that occur in solvents of low dielectric constant (benzene, ether, CHA, etc.) causing relative acidities to be dependent on the reference base, as well as the solvent. This is not true in strongly dissociating solvents of high dielectric constant, such as DMSO. A list of 13 indicators covering the pK range 8.3 to 30.6 in DMSO is presented. Equilibrium acidities of weak (i.e., pK 1 15) carbon acids have been measured by a variety of methods3 in a variety of solvents including ether,4a benzene,4b d i g l ~ m e , ~ cyclohexylamine (CHA),6 mixtures of dimethyl sulfoxide (DMSO) with ethanol, methanol, or ~ a t e r , ’ . ~ . ~ and pure Journal of the American Chemical Society / 97:24 1 November 26,1975 DMSO.’O We have chosen DMSO for our studies because i t allows accurate measurements to be made spectrophotometrically for many different types of carbon acids over a wide range of pK (ca. 30 pK units) with apparently little or no interference from ion association effects.’ Furthermore,

492 citations










Journal ArticleDOI
TL;DR: In the light of evidence based on ’H and I3C nuclear magnetic resonance experiments, and on electron impact and field desorption mass spectrometry, the structure of the antibiotic echinomycin has been revised.
Abstract: In the light of evidence based on ’H and I3C nuclear magnetic resonance experiments, and on electron impact and field desorption mass spectrometry, the structure of the antibiotic echinomycin has been revised. The hitherto accepted structure incorporated a dithian cross-link (i) which is now modified to a thioacetal cross-link (ii). Since the structures of quino-