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Ronald R Sauers

Publications and source records attributed to Ronald R Sauers.

At least 19 recordsLinked to original sources
PubMed2007

Diffuse functions in natural bond orbital analysis.

We show that diffuse function augmentation of Pople basis sets at the 6-311G RHF and B3LYP levels strongly impact conclusions drawn from natural bond orbital (NBO) analysis. The large spatial extent of high quantum number Rydberg orbitals introduced by augmentation contribute importantly to the valence space of neighboring atoms due to the likely inadequacy of the 311 valence functions. In contrast, lesser anomalies are found for augmentation of double zeta type 6-31G. The energetic anomalies found for bond and antibond NBO descriptions, made nonlocal by augmentation, are most serious for molecules with four or more heavy atoms. For these cases augmentation can lead to nonphysical results. NBO results using Dunning-type correlation consistent orbitals exhibit much weaker basis set dependence than those using the Pople basis sets.

Journal Article↗
PubMed2006

Mechanism of thio acid/azide amidation.

A combined experimental and computational mechanistic study of amide formation from thio acids and azides is described. The data support two distinct mechanistic pathways dependent on the electronic character of the azide component. Relatively electron-rich azides undergo bimolecular coupling with thiocarboxylates via an anion-accelerated [3+2] cycloaddition to give a thiatriazoline. Highly electron-poor azides couple via bimolecular union of the terminal nitrogen of the azide with sulfur of the thiocarboxylate to give a linear adduct. Cyclization of this intermediate gives a thiatriazoline. Decomposition to amide is found to proceed via retro-[3+2] cycloaddition of the neutral thiatriazoline intermediates. Computational analysis (DFT, 6-31+G(d)) identified pathways by which both classes of azide undergo [3+2] cycloaddition with thio acid to give thiatriazoline intermediates, although these paths are higher in energy than the thiocarboxylate amidations. These studies also establish that the reaction profile of electron-poor azides is attributable to a prior capture mechanism followed by intramolecular acylation.

Acids↗
PubMed2005

Dichlorodiazirine: a nitrogenous precursor for dichlorocarbene.

The reaction of 3-p-nitrophenoxy-3-chlorodiazirine with a 1.1:1.0:1.6 blend of tetrabutylammonium chloride, cesium chloride, and the ionic liquid 1-butyl-3-methylimidazolium chloride at 40-50 degrees C under vacuum at 1 mm.Hg produced a melt from which dichlorodiazirine distilled into a cold trap containing pentane at -70 degrees C over 6-7 h. Dichlorodiazirine, the first nitrogenous precursor for dichlorocarbene, was reasonably stable in the dark at 25 degrees C and efficiently generated the carbene upon photolysis.

Diazomethane↗
PubMed2005

Endo entry to the nortricyclyl-norbornenyl cation system: stereochemistry in the fragmentation of endo-5-norbornenyl-2-oxychlorocarbene.

Fragmentation of (S)-endo-5-norbornenyl-2-oxychlorocarbene [(S)-8] in cyclohexane-d12 gives approximately 20% (S)-endo-2-chloro-5-norbornene [(S)-7] with approximately 50% ee, 65-70% (R)-exo-2-chloro-5-norbornene [(R)-4] with >95% ee, and approximately 12% (R)-3-nortricyclyl chloride [(R)-5] with approximately 22% ee. (Analogous stereochemical results were also obtained starting with the enantiomeric carbene (R)-8.) The (S)-8 to (S)-7 and (S)-8 to (R)-4 conversions are ascribed mainly to retention and inversion S(N)i transition states, respectively. These have been located by computational methods and are nearly isoenergetic. In more polar solvents (CDCl3 and CD3CN), the fragmentation of (S)-8 increasingly occurs via competitive ion pair pathways in which steroselectivity is diminished, and escape to the norbornenyl-nortricyclyl cation directs the products away from endo-2-chloro-5-norbornene toward exo-chloride 4 and nortricyclyl chloride 5.

Journal Article↗
PubMed2005

Kinetic studies of the cyclization of singlet vinylchlorocarbenes.

[reaction: see text] Vinylchlorocarbenes (R = Me or Cl) cyclize to cyclopropenes with k = (2.4-5.1) x 10(7) s(-1) or (3.1-4.5) x 10(6) s(-1), respectively, in solution at room temperature. The corresponding E(a) values are 6.6 and 5.7 kcal/mol. Excited diazirines are also likely precursors of the cyclopropenes.

Journal Article↗
PubMed2005

Homolytic fragmentation of allyloxychlorocarbene.

The decomposition of allyloxychlorocarbene in hydrocarbon solvents leads via homolysis to allyl and COCl radicals, which recombine to 3-butenoyl chloride or (after scission of (*)COCl to CO and (*)Cl) to allyl chloride. Labeling experiments show that both products are formed in part with allylic rearrangement.

Journal Article↗
PubMed2005

Unprecedented chemistry of an aryloxychlorodiazirine: generation of a dihalodiazirine and diazirinone.

The reaction of p-nitrophenoxychlorodiazirine with tetrabutylammonium fluoride follows three channels: (1) approximately 17% of p-nitrophenoxide/fluoride exchange to chlorofluorodiazirine and p-nitrophenol, (2) approximately 28% of Cl/F exchange to p-nitrophenoxyfluorodiazirine, and (3) approximately 55% of ipso fluoride attack, affording p-nitrofluorobenzene and the previously unknown diazirinone (diazacyclopropenone).

Journal Article↗
PubMed2005

Incorporation of 2-arylhexa-1,5-diene into pentasil zeolite: A distorted 1-arylcyclohexane-1,4-diyl radical cation at room temperature.

Incorporation into a redox-active pentasil zeolite [(Na,H)-ZSM-5] converted 2-arylhexa-1,5-dienes (9a-c; aryl = phenyl, tolyl, anisyl) into 1-arylcyclohexane-1,4-diyl radical cations, 10a-c*+. The ESR spectra of 10a-c*+ (six lines, g = 2.0026; a = 9.0 G) indicated the presence of five essentially equivalent nuclei, indicating limited delocalization of spin and charge into the phenyl group. Sequestered in the pores of ZSM-5, the three species 10a-c*+ are stable at room temperature, in striking contrast to the parent radical cation in cryogenic matrices: cyclohexane-1,4-diyl radical cation is converted to cyclohexene radical cation above 90 K. The structures of radical cation 10a*+ (X = H) and of the unsubstituted parent were probed by density functional theory (DFT) and ab initio calculations.

Journal Article↗
PubMed2004

Solvent-equilibrated ion pairs from carbene fragmentation reactions.

[R(+) OC Cl(-)] ion pairs were generated in methanol/dichloroethane solutions, with R(+) as the 1-bicyclo[2.2.2]octyl, 1-adamantyl, or 3-homoadamantyl cation. Ion pairs were produced either by the direct fragmentation of alkoxychlorocarbenes (ROCCl), with R = 1-bicyclo[2.2.2]octyl, 1-adamantyl, or 3-homoadamantyl, or by the ring expansion-fragmentation of R'CH(2)OCCl, with R' = 1-norbornyl, 3-noradamantyl, or 1-adamantyl. Correlations of the [ROMe]/[RCl] product ratios as a function of the mole fraction of MeOH in dichloroethane showed that the homoadamantyl chloride ion pairs, produced by either the direct or ring expansion-fragmentations, were identical, solvent- and anion-equilibrated, and precursor independent. Laser flash photolysis experiments gave 20-30 ps as the time required for solvent equilibration and precursor independence. Methanol/chloride selectivities of the (less-stable) 1-adamantyl chloride and 1-bicyclo[2.2.2]octyl chloride ion pairs were not independent of their ROCCl or R'CH(2)OCCl precursors. Computational studies provided transition states for the fragmentations and for the structures of the ion pairs.

Journal Article↗
PubMed2004

Carbon tunneling in the ring expansion of noradamantylchlorocarbene.

Irradiation of 3-(3-noradamantyl)-3-chlorodiazirine produced the corresponding noradamantylchlorocarbene, which could be detected in solution with laser flash photolysis via its pyridinium ylide, and in N2 or Ar matrixes at 9 K with IR and UV/vis spectroscopy. At ambient temperatures, or on irradiation in the cryogenic matrixes, the carbene rearranged to 2-chloro-1-adamantene, which could be trapped in solution and which was characterized by IR and UV/vis at low temperatures. In the dark at 9 K, the carbene also slowly ring-expanded to the chloroadamantene, at a rate ca. 10111 times faster than predicted by its B3LYP-calculated activation barrier. It is proposed that the low-temperature rearrangement occurs through carbon quantum mechanical tunneling.

Journal Article↗
PubMed2004

Rearrangements concerted with fragmentation of cyclopropylmethoxychlorocarbene and cyclobutoxychlorocarbene in hydrocarbon solvents and Ar matrices.

Fragmentations of cyclopropylmethoxychlorocarbene (6) and cyclobutoxychlorocarbene (10) lead to rearrangments that afford mixtures of cyclopropylmethyl chloride (7), cyclobutyl chloride (8), and 3-butenyl chloride (9). Isotopic substitution studies show that these rearrangments are accompanied by partial exchange of the methylene groups within 6 and 10. Surprisingly, these processes that are typical of carbocations persist in hydrocarbon solvents such as pentane and cyclohexane-d(12). Quantum chemical calculations reveal that the cis-conformers of the incipient oxychlorocarbenes C(4)H(7)OC(..)Cl decay to C(4)H(7)Cl + CO via transient hydrogen bonded C(4)H(7)(delta)(+)...Cl(delta-) complexes which possess significant ion pair character, even in the gas phase or in nonpolar solvents. In contrast to benzyloxychlorocarbene, no free radicals are formed upon generation or photolysis of 6 or 10 in Ar matrixes, although acid chlorides (the recombination products of these radical pairs) are observed. The IR spectra obtained in these experiments show the presence of several conformers of the two C(4)H(7)OC(..)Cl.

Journal Article↗
PubMed2004

Nortricyclyl-norbornenyl cation system accessed by carbene fragmentation.

Fragmentation of nortricyclyloxychlorocarbene 5 in pentane occurs by an S(N)i-like process which yields nortricyclyl chloride 3g. In more polar solvents, fragmentation leads to nortricyclyl cation chloride anion pairs (9) that give mainly 3g, accompanied by approximately 10% of exo-2-norbornenyl chloride 4g. From exo-2-norbornenyloxychlorocarbene 6 in hydrocarbon solvents, "S(N)i" reactions lead mainly to exo- (4g) and endo-2-chloro-5-norbornenes (4g'). Leakage to ion pairs adds approximately 16% of nortricyclyl chloride 3g. In more polar solvents, the main product remains chloride 4g, but increasing quantities of 3g appear due to enhanced participation of ion pairs. Fragmentations of 5 and 6 in MeOH afford chlorides 3g and 4g as well as the corresponding methyl ethers 3b and 4b. Nortricyclyl cation and norbornenyl cation chloride anion pairs and methanol-solvated nortricyclyl cations are invoked to rationalize the results.

Journal Article↗
PubMed2002

A duality of mechanisms for the fragmentation of substituted benzyloxychlorocarbenes.

Substituted benzyloxychlorocarbenes (X-PhCH2OCCl) were generated photochemically at 25 degrees C in dichloroethane from appropriate diazirine precursors. Fragmentations of the carbenes produced were determined by laser flash photolysis. The data (X, kfrag in s-1) were: p-Me, 2.6 x 105; p-Ph, 8.3 x 104; H, 6.0 x 104; p-Cl, 5.2 x 104; m-Cl, 1.3 x 105; p-F3C, 2.1 x 106; p-O2N, 6.3 x 106. A Hammett correlation of log kfrag versus sigma+ was parabolic. The curvature was taken to imply the gradual change of the fragmentation mechanism from predominantly heterolytic for X-PhCH2CCl with electron-donating X (with developing positive charge on the benzylic carbon in the transition state) to predominantly homolytic for carbenes with electron-withdrawing X. This idea was supported by computational studies.

Journal Article↗
PubMed2002

The carbene fragmentation-ring expansion route to bridgehead carbocations.

[reaction: see text]The products, kinetics, and activation parameters were determined for the fragmentation-ring expansions of 1-norbornylmethyloxychlorocarbene (11) and 3-noradamantylmethyloxychlorocarbene (17). Products from 11 (in dichloroethane) included 1-chlorobicyclo[2.2.2]octane (9, 56%) and 1-chlorobicyclo[3.2.1]octane (10, 37%); from 17, we obtained 1-chloroadamantane (15, 68%) and protoadamantyl chloride (16, 28%).

Journal Article↗