摘要:

The substitution reactions ofcis-Mn(CO)4LBr (L = PPh3, AsPh3, SbPh3) with diphos, diarsine, and dipy (A—A) take place with the loss of one CO group and L to formfac-Mn(CO)3(A—A)Br. The observed reaction rates in CHCl3solution are first order in substrate but depend on the nature of L and A—A, and in some cases the concentration of A—A as well. When L = PPh3and A—A = diphos, and when L = AsPh3and A—A = diphos, diarsine, or dipy, the rates are independent of [A—A]. For these reactions, an SN1 dissociative mechanism involving the rupture of one of the Mn—CO bonds as the rate-determining step is proposed. The observed positive entropies of activation (∼10 e.u.) for these reactions are supportive of the proposed mechanism. The reactions ofcis-Mn(CO)4(SbPh3)Br with A—A are essentially independent of both the nature and the concentration of A—A. However, for these reactions, an SN1 dissociative mechanism involving the rupture of the Mn—SbPh3bond as the rate-determining step is proposed; the observed positive ΔS≠values (∼12 e.u.) are in accord with such a mechanism. The rates when L = PPh3and A—A = diarsine or dipy are dependent on both the nature and the concentration of A—A. A mechanism involving rapid pre-equilibrium steps between the parent molecule and the intermediates, Mn(CO)3(PPh3)Br and Mn(CO)4Br, is proposed to account for the complex kinetic behaviour found for these reactions. These results are particularly noteworthy because octahedral first-row transition metal complexes generally undergo substitution reactions which are independent of the concentrations of the entering nucleophiles.

ISSN:0008-4042    

DOI:10.1139/v76-401

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

Major inconsistencies which are currently acknowledged in discussions involving atomic charges, inductive effects,13C nmr shifts, and stability of carbonium ions are removed (i) by the use of an appropriate definition of charges and (ii) by the recognition that in alkanes and alkyl carbonium ions any decrease in positive charge at carbon atoms results in a downfield13C shift. Methyl appears to be less electron withdrawing than hydrogen, and stability of carbonium ions is associated with dispersion of positive charge at the carbonium center, in line with the more traditional views which are repeatedly challenged in recent literature.

ISSN:0008-4042    

DOI:10.1139/v76-402

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

Ten acyclic and two cyclic 1,3-bissulfonium bistetrafluoroborates have been synthesized by appropriate modification of the literature procedures for the preparation of such compounds. The acyclic salts are deprotonated readily by one molar-equivalent of potassium hydroxide to give 1,3-bissulfonium ylides. However, under the same conditions, the cyclic salts decompose. Hydrogen exchange experiments reveal that the C2 hydrogens of the cyclic salts are significantly less acidic than the C2 hydrogens of the acyclic salts. The relative acidities of the C2 hydrogens of the cyclic salts depend upon the medium and upon the configuration and conformation of the molecule, and preliminary attempts have been made to associate the kinetic and thermodynamic acidities of these hydrogens with their specific stereochemical environments.Deprotonation of a 1,3-bissulfonium salt is accompanied by a decrease of at least 5 kcal/mol in the barrier to pyramidal inversion at sulfur. However, attempts to establish the static and dynamic behaviour at the carbanionic centre were not successful.

ISSN:0008-4042    

DOI:10.1139/v76-403

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

A synthesis of the fungal metabolite pencolide from citraconic anhydride and threonine is described. Evidence is presented which corroborates theZconfiguration recently assigned to the metabolite.

ISSN:0008-4042    

DOI:10.1139/v76-404

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

Bromohydrin acetates from sodium borohydride reduction of di- and tribrominated 17β-hydroxy-5α-androstan-3-one and 17β-hydroxy-5α-estran-3-one have been prepared and their structures determined by spectroscopic methods. Some 220 MHz proton magnetic resonance data are reported for structure determination. The formation of cyclosteroid derivatives by 1,3-elimination of bromine with zinc–copper couple is discussed.

ISSN:0008-4042    

DOI:10.1139/v76-405

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

The bi-1-cycloalken-1-yl compoundsreact with arsines and phosphines R2EH to yieldn= 2, R2E = (CH3)2As,1;n= 2, R2E = (C6H5)2P,2;n= 3, R2E = (CH3)2As,3; andMethyl diphenylphosphite affordsThe ditertiary phosphine2is photochromic in the solid state. It reacts with M(CO)6(M = Cr, Mo, W) to give (L—L)M(CO)4. Similar compounds are obtained from the ditertiary arsines1and3. The solid state structure of the Mo(CO)4derivative of3has been determined from three-dimensional single crystal data. The compound crystallizes in the orthorhombic space groupPbcnwitha = 16.26(1) Å,b = 11.55(1) Å,c = 13.34(1)Å, and there are four molecules in the unit cell. The coordinates of the heavy atoms were determined by vector space methods. All other atomic parameters were obtained by full matrix least-squares refinement to a finalRfactor of 10.1% for 715 reflections. The ligand is chelated to the molybdenum atom and the resulting seven-membered ring is considerably puckered. The As—Mo—As angle is 89.6(0.2) 

ISSN:0008-4042    

DOI:10.1139/v76-406

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

To facilitate the interpretation of isotope effects on the ionization of weak acids in water and deuterium oxide, a comparison of these quantities with that for a standard acid is proposed. Therefore, a quantityKrhas been defined by the equationKr = (kH/kD)/(kH′/KD′). The equilibrium constantsKHandKDrefer to the ionization constants in water and deuterium oxide respectively. The equilibrium constantKX′(X = H, D) refers to the acid used as a reference. An equation is derived from which it may be concluded, that for a series of acids closely similar in geometry and force field around the acidic group, a value ofKrclose to unity should be obtained. To test this prediction the values ofKH/KDfor a series ofn-alkanoic acids (N = 3–6) are compared experimentally with the value ofKH/KDof acetic acid (25 °C). An emf method has been used in these measurements (quinhydrone electrode). In agreement with the theory a value forKrclose to unity has been observed.

ISSN:0008-4042    

DOI:10.1139/v76-407

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

INDO–MO calculations have been performed for all the proton–proton coupling constants in ethylene, propylene, andcis- andtrans-2-butene, both at their equilibrium geometry and at geometries deviating from equilibrium, thus resulting inparameters for all coupling constants as a function of all valence angles. It is shown that such valence angle effects can be appreciable, particularly for two- and three-bond couplings. The valence angle effects are demonstrated to be independent and additive. The calculated data, in combination with the exact geometrical data and experimental values for the coupling constants allow the separation of (inductive) substitution and rehybridization effects. It is also shown that for three-bond couplings the effects ofin-path C—C—H bond angle changes are minor in comparison to theexo-path C=C—C valence angle effects. The new data provide a consistently good model to explain certain empirical trends in two- and three-bond coupling constants as a function of the size of the substituents, assuming a simple steric hindrance induced rehybridization mechanism. On the other hand it is also found that such a mechanism cannot account for similar trends in the H—C=C—CH3allylic and the CH3—C=C—CH3homoallylic coupling constants.

ISSN:0008-4042    

DOI:10.1139/v76-408

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

Examination of the alkaloids ofLycopodium magellanicum(Palisot de Beauvois) has revealed the presence of five alkaloids of established structure and a previously unreported alkaloid, magellanine (C17H25NO2). The ring System of magellanine, deduced from an examination of its spectroscopic properties, is identical with that of paniculatine. The structure of magellanine has been confirmed, and its relative stereochemistry established, in an X-ray crystallographic study.

ISSN:0008-4042    

DOI:10.1139/v76-409

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

Seven alkaloids have been isolated fromLycopodium paniculatumDesvaux. Three of these, lycopodine, dihydrolycopodine, and acetyldihydrolycopodine are alkaloids of established structure. Of the four new alkaloids the structure of one, paniculatine, has been clarified. Its structure, elucidated through an X-ray crystallographic study of its hydrobromide, is different from other alkaloids of theLycopodiumfamily. The spectral properties of paniculatine are interpreted in the light of its structure.

ISSN:0008-4042    

DOI:10.1139/v76-410

出版商:NRC Research Press    

年代:1976

数据来源: NRC