Low Temperature 13C NMR Spectroscopy Research Articles

Synthesis, conformational preferences in gas and solution, and molecular gear rotation in 1-(dimethylamino)-1-phenyl-1-silacyclohexane by gas phase electron diffraction (GED), LT NMR and theoretical calculations

1-(Dimethylamino)-1-phenyl-1-silacyclohexane 1, was synthesized, and its molecular structure and conformational properties studied by gas-phase electron diffraction (GED), low temperature 13C NMR spectroscopy and quantum-chemical calculations. The predominance of the 1-Phax conformer (1-Pheq:1-Phax ratio of 20:80%, ΔG°(317 K) = −0.87 kcal/mol) in the gas phase is close to the theoretically estimated conformational equilibrium. In solution, low temperature NMR spectroscopy showed analyzable decoalescence of Cipso and C(1,5) carbon signals in 13C NMR spectra at 103 K. Opposite to the gas state in the freon solution employed (CD2Cl2/CHFCl2/CHFCl2 = 1:1:3), which is still liquid at 100 K, the 1-Pheq conformer was found to be the preferred one [(1-Pheq: 1-Phax = 77%: 23%, K = 77/23 = 2.8; −ΔG° = −RT ln K (at 103 K) = 0.44 ± 0.1 kcal/mol]. When comparing 1 with 1-phenyl-1-(X)silacylohexanes (X = H, Me, OMe, F, Cl), studied so far, the trend of predominance of the Phax conformer in the gas phase and of the Pheq conformer in solution is confirmed.

Conformational Preferences of the Phenyl Group in 1-Phenyl-1-X-1-silacyclo-hexanes (X = MeO, HO) and 3-Phenyl-3-X-3-silatetrahydropyrans (X = HO, H) by Low Temperature 13C NMR Spectroscopy and Theoretical Calculations.

New Si-phenyl-substituted silacyclohexanes and 3-silatetrahydropyrans have been synthesized and studied with respect to the conformational equilibria of the heterosix-membered ring by low temperature 13C NMR spectroscopy and quantum chemical calculations. For 1-methoxy-1-phenylsilacyclohexane 1 and 3-phenyl-3-silatetrahydropyran 4 the conformational equilibria could be frozen and assigned. The Phax ⇆ Pheq equilibrium constants at 103 K are 2.21 for 1 and 4.59 for 4. In complete agreement with former studies of similar silicon compounds, molecules 1 and 4 prefer to adopt the Pheq conformation. The conformational equilibria of 1-hydroxy-1-phenylsilacyclohexane 2 and 3-hydroxy-3-phenyl-3-silatetrahydropyran 3 could not be frozen at 100 K and proved to be heavily one-sided (if not anancomeric). Obviously, there is a general trend of predominance of Phax conformer in the gas phase and of Pheq in solution. For the isolated molecules of silanols 2 and 3, calculations allowed to explain the axial predominance of the phenyl group by a larger polarization of the Si-Ph than of the Si-O bond in the Phax conformer and additional destabilization of 3-Pheq conformer by repulsion of unidirectional dipoles of the endocyclic oxygen lone pair and of the highly polar axial Si-O bond.

Stereochemistry of 3-isopropoxy-3-methyl-1,3-oxasilinane–the first 3-silatetrahydropyran with an exo-cyclic RO–Si bond

Molecular structure and conformational behavior of 3-isopropoxy-3-methyl-3-oxasilinane is studied by low temperature 13C NMR spectroscopy and theoretical calculations (DFT, MP2). Two conformers, 1-ROax and 1-ROeq, were found experimentally and located on the potential energy surface. LT 13C NMR spectroscopy gives almost equal population of the two conformers at 98 K with ΔG°98K=0.02 kcal/mol in favor of 1-ROax and ΔG#98K=4.5 kcal/mol. The corresponding DFT calculated values (ΔG°98K=0.03 kcal/mol, ΔG#98K=5.1 kcal/mol) are in excellent agreement with the experiment. Detailed DFT and MP2 calculations of the solvent effect on the conformational equilibrium were performed and highlighted the leveling out of the two conformers when transferred from gas to solution.

Bonding in Complexes of Bis(pentalene)dititanium, Ti2(C8H6)2.

Bonding in the bis(pentalene)dititanium “double-sandwich” species Ti2Pn2 (Pn = C8H6) and its interaction with other fragments have been investigated by density functional calculations and fragment analysis. Ti2Pn2 with C2v symmetry has two metal–metal bonds and a low-lying metal-based empty orbital, all three frontier orbitals having a1 symmetry. The latter may be regarded as being derived by symmetric combinations of the classic three frontier orbitals of two bent bis(cyclopentadienyl) metal fragments. Electrochemical studies on Ti2Pn†2 (Pn† = 1,4-{SiiPr3}2C8H4) revealed a one-electron oxidation, and the formally mixed-valence Ti(II)–Ti(III) cationic complex [Ti2Pn†2][B(C6F5)4] has been structurally characterized. Theory indicates an S = 1/2 ground-state electronic configuration for the latter, which was confirmed by EPR spectroscopy and SQUID magnetometry. Carbon dioxide binds symmetrically to Ti2Pn2, preserving the C2v symmetry, as does carbon disulfide. The dominant interaction in Ti2Pn2CO2 is σ donation into the LUMO of bent CO2, and donation from the O atoms to Ti2Pn2 is minimal, whereas in Ti2Pn2CS2 there is significant interaction with the S atoms. The bridging O atom in the mono(oxo) species Ti2Pn2O, however, employs all three O 2p orbitals in binding and competes strongly with Pn, leading to weaker binding of the carbocyclic ligand, and the sulfur analogue Ti2Pn2S behaves similarly. Ti2Pn2 is also capable of binding one, two, or three molecules of carbon monoxide. The bonding demands of a single CO molecule are incompatible with symmetric binding, and an asymmetric structure is found. The dicarbonyl adduct Ti2Pn2(CO)2 has Cs symmetry with the Ti2Pn2 unit acting as two MCp2 fragments. Synthetic studies showed that in the presence of excess CO the tricarbonyl complex Ti2Pn†2(CO)3 is formed, which optimizes to an asymmetric structure with one semibridging and two terminal CO ligands. Low-temperature 13C NMR spectroscopy revealed a rapid dynamic exchange between the two bound CO sites and free CO.

Molecular structure and conformational analysis of 3-methyl-3-phenyl-3-silatetrahydropyran. Gas-phase electron diffraction, low temperature NMR and quantum chemical calculations

The molecular structure and conformational behavior of 3-methyl-3-phenyl-3-silatetrahydropyran 1 was studied by gas-phase electron diffraction (GED–MS), low temperature 13C NMR spectroscopy (LT NMR) and theoretical calculations. The 1-Pheq and 1-Phax conformers were located on the potential energy surface. Rotation about the Si–CPh bond revealed the phenyl ring orthogonal to the averaged plane of the silatetrahydropyran ring for 1-Pheq and a twisted orientation for 1-Phax. Theoretical calculations and GED analysis indicate the predominance of 1-Phax in the gas phase with the ratio of conformers (GED) 1-Pheq:1-Phax=38:62 (ΔG°307=−0.29 kcal/mol). In solution, LT NMR spectroscopy gives almost the opposite ratio Pheq:1-Phax=68:32 (ΔG°103=0.16 kcal/mol). Simulation of solvent effects using the PCM continuum model or by calculation of the solvent–solute complexes allowed us to rationalize the experimentally observed opposite conformational predominance of the conformers of compound 1 in the gas phase and in solution.

Conformational preferences of Si–Ph,H and Si–Ph,Me silacyclohexanes and 1,3-thiasilacyclohexanes. Additivity of conformational energies in 1,1-disubstituted heterocyclohexanes

The conformational equilibria of 1-phenyl-1-silacyclohexane 1, 3-phenyl-1,3-thiasilacyclohexane 2, 1-methyl-1-phenyl-1-silacyclohexane 3, and 3-methyl-3-phenyl-1,3-thiasilacyclohexane 4 have been studied for the first time by low temperature 13C NMR spectroscopy at 103K. Predominance of the equatorial conformer of compound 1 (Pheq/Phax=78%:22%) is much less than in its carbon analog, phenylcyclohexane (nearly 100% of Pheq). And in contrast to 1-methyl-1-phenylcyclohexane, the conformers with the equatorial Ph group are predominant for compounds 3 and 4: at 103K, Pheq/Phax ratios are 63%:37% (3) and 68%:32% (4). As the Si–C bonds are elongated with respect to C–C bonds, the barriers to ring inversion are only between 5.2–6.0 (ax→eq) and 5.4–6.0 (eq→ax)kcalmol−1. Parallel calculations at the DFT and MP2 level of theory (as well as the G2 calculations for compound 1) show qualitative agreement with the experiment. The additivity/nonadditivity of conformational energies of substituents on cyclohexane and silacyclohexane derivatives is analyzed. The geminally disubstituted cyclohexanes containing a phenyl group show large deviations from additivity, whereas in 1-methyl-1-phenyl-1-silacyclohexane and 3-methyl-3-phenyl-1,3-thiasilacyclohexane the effects of the methyl and phenyl groups are almost additive. The reasons for the different conformational preferences in carbocyclic and heterocyclic compounds are analyzed using the homodesmotic reactions approach.

Conformational analysis of 3‐methyl‐3‐silathiane and 3‐fluoro‐3‐methyl‐3‐silathiane

AbstractThe conformational equilibria of 3‐methyl‐3‐silathiane 5, 3‐fluoro‐3‐methyl‐3‐silathiane 6 and 1‐fluoro‐1‐methyl‐1‐silacyclohexane 7 have been studied using low temperature 13C NMR spectroscopy and theoretical calculations. The conformer ratio at 103 K was measured to be about 5ax:5eq = 15:85, 6ax:6eq = 50:50 and 7ax:7eq = 25:75. The equatorial preference of the methyl group in 5 (0.35 kcal mol−1) is much less than in 3‐methylthiane 9 (1.40 kcal mol−1) but somewhat greater than in 1‐methyl‐1‐silacyclohexane 1 (0.23 kcal mol−1). Compounds 5–7 have low barriers to ring inversion: 5.65 (ax → eq) and 6.0 (eq → ax) kcal mol−1 (5), 4.6 (6), 5.1 (Meax → Meeq) and 5.4 (Meeq → Meax) kcal mol−1 (7). Steric effects cannot explain the observed conformational preferences, like equal population of the two conformers of 6, or different conformer ratio for 5 and 7. Actually, by employing the NBO analysis, in particular, considering the second order perturbation energies, vicinal stereoelectronic interactions between the Si–X and adjacent C–H, C–S, and C–C bonds proved responsible. Copyright © 2010 John Wiley & Sons, Ltd.

Reactive, Dicationic Electrophiles: Electrophilic Activation Involving the Phosphonium Group.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis and Structures of Triorganotelluronium Pseudohalides

The syntheses of [(CH3)3Te]X {X = N3 (1), OCN (2), SCN (3), SeCN (4), [Ag(CN)2] (5)} and [(C6H5)3Te]X {X = N3 (6), SeCN (7), [Ag(CN)2] (8)}, their NMR spectroscopic data, vibrational spectra and single crystal structures are described. Compounds 1−4 are the first trimethyltelluronium pseudohalides, while the known compounds 6 and 7 have been prepared for completion of their analytical and structural properties. The occurrence of intermolecular Te···N, Te···O, Te···S and Te···Se contacts is thoroughly studied. The dicyanoargentates 5 and 8 were obtained in an attempt to prepare telluronium cyanides. Low-temperature 13C NMR spectroscopy of the [Ag(CN)2]− ion in solution has been carried out, with determination of the 13C−107/109Ag coupling. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Dicationic electrophilic systems: the activation of carbocations and carboxonium ions by pyridinium groups and related heterocycles

N-Heterocycles can be alkylated with cinnamyl bromide to give the cationic salts 6– 10 and subsequent reactions with C 6H 6 and superacidic CF 3SO 3H provide the addition products in good yields. Reactions of N-acetonylpyridinium salts give the condensation product from C 6H 6 and CF 3SO 3H and the dicationic intermediate can be directly observed using low temperature 13C NMR spectroscopy.

Reactive, dicationic electrophiles: electrophilic activation involving the phosphonium group

In the following letter, we report our studies of dicationic electrophiles involving the phosphonium group and we present evidence that the cationic phosphonium group may dramatically enhance the reactivities of adjacent electrophilic centers. We describe superacid-catalyzed reactions involving phosphonium-substituted aldehydes, ketones, olefins, and alkynes, and their conversions to products of condensation and addition reactions. We also report the direct observation of a phosphonium–carboxonium dication in FSO 3H–SbF 5–SO 2ClF by low-temperature 13C NMR spectroscopy.

Bidentate phosphines of heteroarenes: effective stabilization of the Co 2(CO) 6 fragment by 4,6-bis(diphenylphosphino) dibenzofuran

Reaction of Co 2(CO) 8 with 4,6-bis(diphenylphosphino)dibenzofuran ( 1) in diethylether gives the dinuclear complex 4,6-bis(diphenylphosphino)dibenzofurandicobalthexacarbonyl ( 2). The solid state structures of 1 and 2 have been established by X-ray crystallography. Low temperature 13C-NMR spectroscopy was used to analyse 2 in solution.

Spectroscopic Characterization of (Iodomethyl)zinc Reagents Involved in Stereoselective Reactions: Spectroscopic Evidence That IZnCH2I Is Not Zn(CH2I)2 + ZnI2 in the Presence of an Ether

We have shown that low-temperature 13C NMR spectroscopy is an extremely powerful technique for characterizing the (iodomethyl)zinc-derived reagents involved in the cyclopropanation reactions. This technique has allowed us to spectroscopically characterize and unambigously differentiate the Furukawa reagent (EtZnCH2I), the Simmons−Smith reagent (IZnCH2I), and the Wittig reagent (Zn(CH2I)2). Unique spectra are obtained for each of these reagents when they are complexed to a chiral diether. We have also demonstrated that IZnCH2I is not converted into Zn(CH2I)2 + ZnI2 in CD2Cl2 in the presence of a chiral complexing agent. Furukawa's reagent “EtZnCH2I”, however, is in equilibrium with Et2Zn and Zn(CH2I)2, and it eventually decomposes into PrZnI and EtZnI at room temperature. The decomposition of Zn(CH2I)2 into IZnCH2I and of IZnCH2I into ZnI2 was monitored by NMR. We have also demonstrated that the general trends observed for the various equilibria involving (iodomethyl)zinc-derived reagents follow those observed with ethylzinc-derived organometallic compounds.

Nitrile exchange reactions of [MI 2(CO) 3(NCMe) 2] (M = Mo or W). X-ray crystal structures of the mixed-ligand seven-coordinate complexes [WI 2(CO) 3(NCR)(AsPh 3)] (R = Et or Ph)

Reaction of [Ml 2(CO) 3(NCMe) 2] (M = Mo or W) with an excess of NCR (for M = W, R = Et, tBu, CH 2Ph or Ph; for M = Mo, R = Ph) in CH 2Cl 2 at room temperature gives the nitrile-exchanged products [MI 2(CO) 3(NCR) 2] ( 1– 5). Equimolar quantities of 1– 5 and L (L = PPh 3, AsPh 3 or SbPh 3) react in CH 2Cl 2 at room temperature to afford the mixed ligand complexes [MI 2(CO) 3(NCR)L] ( 6– 20). X-ray single crystallographic studies were carried out on both the tungsten complexes [WI 2(CO) 3(NCR)(AsPh 3)] (R = Et and Ph). The metal atoms in both complexes have the same seven-coordinate, slightly distorted monocapped trigonal prismatic, coordination geometry, with very similar ligand dispositions. Low temperature 13C NMR spectroscopy was used to correlate the solution and solid-state structures of several of these seven-coordinate complexes. The reactions of the bis(propionitrile) complex [WI 2(CO) 3(NCEt) 2] with neutral monodentate donor ligands are also described.

1,3,7,9,11,15,17,19,23,25,27,34-Dodecaazanonacyclo-[25.5.3.26,9.214,17.222,25.13,7.111,15.119,23.030,34]-tetratetracontane

Pentane-1,3,5-triamine condenses with formaldehyde to give directly and exclusively the nonacyclic title compound, whose conformations have been studied by X-ray analysis and low-temperature 13C NMR spectroscopy.

Solution structures of cobalt-rhodium clusters: The identification of a new hexanuclear cobalt-rhodium species

The thermolysis of Co 2Rh 2(CO) 12 leads to the quantitative formation of Co 3Rh 3(CO) 16. The solution structures of these clusters have been established by IR and low temperature 13C NMR spectroscopy. Co 3Rh 3(CO) 16 reacts with carbon monoxide at moderate pressure to give the equilibrium mixture of Co 2Rh 2(CO) 12 and CoRh(CO) 7.

Konformations-Gleichgewichte von 1,3-Oxathian-3-Imiden

Two 1,3-oxathiane-3-imides and 1,3-oxathiolane-3-p-chlorophenyl-imide have been prepared. The conformational equilibria of 1,3-oxathiane-3-oxide and of the two 1,3-oxathiane-3-imides have been determined by low temperature13C-NMR spectroscopy and compared to data obtained previously2 for other cyclic sulfimides.

Nitrogen‐15 nuclear magnetic resonance spectroscopy. Chemical shifts of methyl substituted cis‐decahydroquinolines

Abstract15N Chemical shifts of cis‐decahydroquinoline, N‐methyl‐cis‐decahydroquinoline, and of 19 methyl substituted NH‐ and NCH3‐cis‐decahydroquinolines are reported. Shift values of conformationally homogeneous compounds can be used to determine the chemical shifts of the possible conformations of the mobile compounds. Equilibrium constants derived from the shifts of the contributing conformations agree with results of low temperature 13C NMR spectroscopy.

Conformational equilibrium in 8‐methyl‐cis‐2‐thiahydrindane and 8‐methyl‐cis‐2‐oxahydrindane by 13C NMR spectroscopy

AbstractThe preferred conformation of 8‐methyl‐cis‐thiahydrindane has been both estimated by 13C NMR chemical shifts and determined by low temperature 13C NMR spectroscopy to be the conformer with the methyl group equatorial with respect to the cyclohexane ring. This result is in disagreement with the interpretation of the temperature dependence of the CD spectra of (+) and (−) 8‐methyl‐cis‐2‐thiahydrindane, whereby the conformation with the methyl group axial with respect to the cyclohexane ring was claimed to be the preferred conformation. The preferred conformation of the related oxygen heterocycle, 8‐methyl‐cis‐2‐oxahydrindane, has been estimated by 13C NMR chemical shifts to be the conformer with the methyl group axial with respect to the cyclohexane ring. Possible reasons for these observations are discussed.

SUBSTITUENT EFFECTS ON CONFORMATIONAL EQUILIBRIA IN THIANE-1-N-ARYL-SULFIMIDES

Abstract Compounds of the general type shown have been synthesized and their conformational equilibria 13 have been established by low temperature 13C nmr spectroscopy.