摘要:

The first reagent-controlled asymmetric halolactonizations are described. Complexes of I+withO-acyl- andO-aryldihydroquinidines are used to promote the asymmetric halolactonization of prochiral θ, &dgr;-unsaturated carboxylic acids with low but measurable and reproducible enantioselectivity. Experimental factors affecting the ee's are described.Key words: halocyclization, halogenation, halonium ions, chiral, stereoselective.

ISSN:0008-4042    

DOI:10.1139/v98-153

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction of methanol, 7-methyl-3-methylene-1,6-octadiene ( &bgr;-myrcene,1), and 1,4-dicyanobenzene yields five 1:1:1 adducts:cis-2-(4-cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylenecyclohexane (15),trans-2-(4-cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylenecyclohexane (16), 1-(4-cyanophenylmethyl)-4-(1-methoxy-1-methylethyl)cyclohexene (17), 4-[4-methoxy-3,3-dimethylcyclohex-(E)-1-ylidenyl]methylbenzonitrile (18), and 4-(1-vinyl-4-trans-methoxy-3,3-dimethylcyclohexyl)benzonitrile (19). All of these adducts are cyclic; variation in the product ratio as a function of methanol concentration indicates cyclization is occurring, 1,6-endo, with both the initially formed radical cation and with the intermediate &bgr;-alkoxyalkyl radicals. Evidence based upon comparison of the ionization and oxidation potential of &bgr;-myrcene with model alkenes and with conjugated dienes indicates the initial electron transfer involves the trisubstituted mono alkene moiety; the diene moiety, mono-substituted at a nodal position, has a higher oxidation potential. High-level ab initio molecular orbital calculations (MP2/6-31G*//HF/6-31G*) provide useful information regarding the nature (relative energies and charge and spin distribution) of the intermediate radical cations, which supports the proposed reaction mechanism.Key words: photoinduced electron transfer, radicals, radical cations, &bgr;-myrcene, cyclization.

ISSN:0008-4042    

DOI:10.1139/v98-156

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Reactions of the bifunctional boranet-BuCH=C[B(C6F5)2]2,1, with simple Lewis bases were investigated to probe its Lewis acidity. Ethereal bases (Et2O, THF) bind very weakly, if at all, as no adducts were detectable in solution. Certain ketones, however, were observed to bind to1. Acetone forms an adduct,2, reversibly in solution (Keqat 293 K = 69.1), and the temperature dependence of this equilibrium yield thermodynamic parameters ofH° = -59.5 ± 2.0 kJ mol-1;S° = -167 ± 4 J K-1mol-1. Low temperature NMR spectroscopy suggests that the acetone binds to the more sterically accessible boron centertransto thet-Bu group. Ketone binding is strengthened when 2,3-diphenylcyclopropenone is reacted with1, irreversibly forming an isolable adduct,4, in high yield (95%). The availability of a resonance form incorporating the aromatic cyclopropenyl cation contributes to the added stability of this adduct in comparison to2.Key words: organoboranes, diboranes, ketone comple

ISSN:0008-4042    

DOI:10.1139/v98-170

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The addition of indole and methyl indole at both high and ambient pressures to a series of Michael acceptors under the influence of ytterbium triflate was investigated. Under ambient pressure the more reactive and less sterically hindered electrophiles gave the expected 3-alkylated indoles in good to excellent yields. The more problematic Michael acceptors were subjected to pressures of 13 kbar. In all cases a dramatic reduction in reaction time and a significant improvement in yields was observed. In the cases involving 3-methylcyclohex-2-en-1-one, a by-product was formed and was characterized by single crystal X-ray diffraction. alpha , beta-Unsaturated ketones gave the best yields. Enals tended to polymerize while enoates proved to be much too unreactive. A particularly reactive malonate derived ester and beta-nitrostyrene gave good yields at ambient pressures.Key words: hapalindole, indole, Michael addition, high pressure, ytterbium triflate, Lewis acid.

ISSN:0008-4042    

DOI:10.1139/v98-158

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Based upon the geometrical properties of regular polyhedrons, the possibility of the existence of certain polyhedral molecules composed of only one element is investigated. A very promising candidate - the regular dodecahedron - is selected as the convenient polyhedral structural pattern and phosphorus as the appropriate element. A series of high-level ab initio calculations is performed on the dodecahedral P20molecule, including predictions of its thermodynamic and kinetic stability, natural bond orbital analysis, vibrational analysis, and inclusion of some elements into the molecular skeleton. Due to the potential stabilizing agents that may eventually form stable inclusion compounds and the estimated high kinetic stability, the question of the possible existence of P20is answered in the positive.Key words: ab initio calculations, inclusion compounds, P20, phosphorus clusters, polyhedra.

ISSN:0008-4042    

DOI:10.1139/v98-157

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The31P chemical shift tensor components and anisotropies of the trimethyl- and triphenylphosphine complexes of the group 6 metal pentacarbonyls, M(CO)5PR3(M = Cr, Mo, W and R = Me, Ph), have been measured using solid-state CP-MAS31P NMR spectroscopy. For the trimethylphosphine derivatives, the chemical shift tensors have near axial symmetry and the shift tensor components are in reasonable agreement with the calculated values for the chromium and molybdenum complexes. In the triphenylphosphine complexes, the tensors are asymmetric due to the different torsion angles of the phenyl rings. The trend to higher shielding of the isotropic31P chemical shifts on descending group 6 arises from changes in the perpendicular components of the shift tensor. The one-bond coupling constants,1J(95/97Mo-31P), for the trimethyl- and triphenylphosphine complexes are 129 and 133 Hz, respectively.Key words: chemical shift anisotropy, phosphines, chromium, molybdenum, tungsten.

ISSN:0008-4042    

DOI:10.1139/v98-167

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Rates of ketonization of (Z)-2-methoxy-1,2-diphenylvinyl alcohol were measured in aqueous perchloric acid and sodium hydroxide solutions as well as in formic acid and ammonium ion buffers, and the results were used to construct a rate profile for this reaction. These data show this substance to be a remarkably stable enol with a lifetime of 3.6 h at the bottom of its rate profile and a hydrogen ion catalytic coefficient 660 000 times less than that for the enol of acetophenone. Comparison with simple models allows partition of this rate factor into a 440-fold retardation for the &bgr;-phenyl substituent and a 1500-fold retardation for the &bgr;-methoxy substituent. A global rate of enolization of the keto isomer producing bothZandEenol isomers was also measured, and this leads to pKE> 5.4 as the lower limit of the keto-enol equilibrium constant for theZenol. This result could be dissected into an enol-content enhancing effect of &dgr; pKE= 3.2 for the &bgr;-phenyl group and a surprising enol-content diminishing effect of &dgr; pKE> 0.6 for the &bgr;-methoxy group.Key words: enol ketonization, enol stability, keto-enol equilibrium, &bgr;-phenyl and &bgr;-methoxy substituent effects in keto-enol systems.

ISSN:0008-4042    

DOI:10.1139/v98-163

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

After hydrogenation and hydrolysis, the two adducts formed from the Diels-Alder reaction of acryloyl chloride and 4,5,6,6a-tetrahydropentalene were originally assigned 7-exo-and 7-endo-carboxylic acid structures3and4. Based on unexpected results in using these acids as starting materials for a superacid carbocation study, a reinvestigation of these structural assignments was carried out. From an extensive series of 2D NMR experiments, we conclude that4is correctly assigned, but that the other isomer is the 9-endocarboxylic acid8. The formation of these two isomers (out of eight possible modes of addition) can be rationalized. Finally, an X-ray study of8was carried out as a final proof of structure.Key words: Diels-Alder reaction, facial selectivity, regioselectivity, X-ray structure, MO calculation.

ISSN:0008-4042    

DOI:10.1139/v98-168

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Bodor and Buchwald (8) have produced a two-parameter equation for calculating the logarithm of the partition coefficient (logP) of an organic solute betweenn-octanol and water. They computed the first parameter, a van der Waals volume of the of the solute molecule, using an algorithm combining analytical and numerical techniques. This parameter can be obtained more simply by addition of increments for the various atoms making up the molecule. The second parameterN,an integer, is obtained by summation of the integralNvalues associated with the different polar groups present in the molecule, as long as these polar groups are not too close to each other. TheNterm results in a decrease in logP.TheNparameter is thought to measure the total number of H-bonds between the hydrogen of water and H-acceptor bonding sites of the various polar groups of the solute molecule. The detailed nature of the hydrogen bonds between a polar group and water can be elucidated by calculations involvingNandV0, the partial molal volume in water of a solute molecule containing this group.Key words: partition coefficient, molecular volume, hydrogen bonding, van der Waals volume.

ISSN:0008-4042    

DOI:10.1139/v98-162

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The reduction ofcis-2,6-dimethylcyclohexanone with NaBH4in methanol is shown to produce predominantly the axial alcohol, an unexpected result based upon prior reports and current paradigms for similar cyclohexanone reductions. This finding prompted a careful and systematic investigation of the NaBH4and LiAlH4reductions ofcis- andtrans-2,6-dimethylcyclohexanones in various solvents, with additional results contrary to literature reports. Possible explanations for these discrepancies are given, an unusual solvent effect is noted, the rate of epimerization versus reduction is examined, molecular modeling results are reported, and an important caveat is offered for future stereochemical studies of this nature.Key words: cyclic ketone reduction, stereochemistry, molecular modeling.

ISSN:0008-4042    

DOI:10.1139/v98-154

出版商:NRC Research Press    

年代:1998

数据来源: NRC