Mesityl Ring Research Articles

Dimesitylketone O-oxide: spectroscopic characterization, conformation, and reaction modes: OH formation and OH capture.

Dimesitylketone O-oxide 1b was synthesized by photolysis of dimesityldiazomethane dissolved in an oxygen saturated CCl3F solution at 140 K. Conformation and geometry of 1b were determined by comparing measured NMR chemical shifts with the corresponding chemical shifts calculated at the DFT-IGLO level of theory where it had to be considered that the molecule exists in two enantiomeric forms. Measured and calculated 1H chemical shifts agree within 0.1 ppm while the calculated 13C shift of the COO carbon (210.6 ppm) differs by only 0.4 ppm from the measured shift of 211.0 ppm. The two mesityl rings are perpendicular to each other and enclose angles of 40 and 57 degrees with the COO plane. The preferred rearrangement process of 1b is an H migration from one of the ortho-methyl groups to the terminal O atom of the COO unit. The calculated activation enthalpy of this process is 12.7 kcal/mol (B3LYP/cc-pVTZ). In contrast, the activation enthalpy for isomerization to dioxirane is 5 kcal/mol higher. In CCl3F, the activation barrier for the thermal decay was determined to be 13.8 +/- 0.2 kcal/mol and in acetonitrile 13.1 +/- 0.4 kcal/mol. H migration initiates cleavage of the OO bond and the production of an OH and a benzyl radical. Recombination of the latter in the solvent cage leads to the formation of 2-methylhydroxy-pentamethylbenzophenone, while escape of the OH radical from the solvent cage yields a ketone. These results confirm the possibility of OH production from carbonyl oxides in the solution phase.

Conformational studies by dynamic NMR. 79. Dimesityl sulfine revisited: detection of the helical antipodes and determination of their enantiomerization pathways.

By means of low-temperature NMR spectra, it is demonstrated that dimesityl sulfine (Mes2C=SO) adopts in solution the same chiral propeller conformation (C1 symmetry) determined by X-ray diffraction in the crystalline state. With the help of MM calculations, it has been also shown that a correlated rotation (cog wheel effect) of the two mesityl rings reverses the molecular helicity according to an enantiomerization process entailing a one-ring flip pathway with delta G++ = 5.9 kcal mol-1 and a two-ring flip pathway with delta G++ = 13.8 kcal mol-1. On the contrary the Z- and E-isomers of mesityl phenyl sulfine (MesPhC=SO) adopt essentially achiral conformations (Cs symmetry), having the Ph-CSO rotation barriers equal to 5.2 and 5.8 kcal mol-1, respectively, and the mesityl-CSO rotation barriers equal to 21.3 and 15.1 kcal mol-1, respectively.

Conformational studies by dynamic NMR. 78. Stereomutation of the helical enantiomers of trigonal carbon diaryl-substituted compounds: dimesitylketone, dimesitylthioketone, and dimesitylethylene.

The free energies of activation for the enantiomerization of the title compounds (Mes2C = X, Mes = 2,4,6-trimethylphenyl) were determined by dynamic NMR to be 4.6, 6.5, and 9.2 kcal mol-1 for X = O, S, and CH2, respectively. Single-crystal X-ray diffraction showed that the structure of dimesitylketone is that of a propeller (C2 symmetry) with the mesityl rings twisted by 50 degrees with respect to the plane of carbonyl. The same structure was predicted by molecular mechanics calculations, which also produced good agreement between computed and experimental barriers for a dynamic process where a disrotatory one-ring flip pathway reverses the helicity of the conformational enantiomers. Solid-state NMR spectra indicated that the enantiomerization barrier in the crystal must be much higher (at least 19 kcal mol-1) than that in solution. Contrary to the case of dimesitylketone, the calculated barrier of dimesitylethylene agrees better with the experimental value if the enantiomerization process is assumed to be a conrotatory two-ring flip pathway.

ChemInform Abstract: An Investigation of the Solid-State Photochemistry of α-Mesitylacetophenone Derivatives: Asymmetric Induction Studies and Crystal Structure-Reactivity Relationships.

The photochemical conversion of a series of α-mesitylacetophenone derivatives into 2-indanols via δ-hydrogen abstraction has been investigated in the solid state. A correlation between solid-state reactivity and crystal structure has been established for this type of reaction. For the seven compounds whose crystal structures were determined, the average value of d (CO⋯H distance) and L (CO⋯CH3 distance) were 2.77±0.04 A and 3.42±0.06 A, and the value of ω (δ-H out of plane angle), Δ (CO⋯H angle) and θ (C–H⋯O angle) were 59±2°, 80±7° and 123±3°, respectively. These parameters depended mainly on the magnitude of the deviation of the carbonyl group from the fully bisecting position over the mesityl ring, which ranged from 9–14° in the case of in the solid state reactive ketones and approached 0° for the unreactive compounds. Asymmetric induction studies were carried out by providing the reactants with carboxylic acid substituents to which ionic chiral auxiliaries were attached through salt formation with optically active amines. Irradiation of the salts (13 in total) in the crystalline state gave enantiomeric excesses of up to 90%. The crystal structures of three of the salts were determined and on this basis, the reasons for the selectivity in the crystalline state are discussed.

An Investigation of the Solid-State Photochemistry of α-Mesitylacetophenone Derivatives: Asymmetric Induction Studies and Crystal Structure–Reactivity Relationships

The photochemical conversion of a series of α-mesitylacetophenone derivatives into 2-indanols via δ-hydrogen abstraction has been investigated in the solid state. A correlation between solid-state reactivity and crystal structure has been established for this type of reaction. For the seven compounds whose crystal structures were determined, the average value of d (CO⋯H distance) and L (CO⋯CH3 distance) were 2.77±0.04Å and 3.42±0.06Å, and the value of ω (δ-H out of plane angle), Δ (CO⋯H angle) and θ (C–H⋯O angle) were 59±2°, 80±7° and 123±3°, respectively. These parameters depended mainly on the magnitude of the deviation of the carbonyl group from the fully bisecting position over the mesityl ring, which ranged from 9–14° in the case of in the solid state reactive ketones and approached 0° for the unreactive compounds. Asymmetric induction studies were carried out by providing the reactants with carboxylic acid substituents to which ionic chiral auxiliaries were attached through salt formation with optically active amines. Irradiation of the salts (13 in total) in the crystalline state gave enantiomeric excesses of up to 90%. The crystal structures of three of the salts were determined and on this basis, the reasons for the selectivity in the crystalline state are discussed.

Synthesis and Structural Characterization of the Solvent-Free Potassium Derivative of a Terphenyl-Substituted Primary Phosphane: [(2,6-Dimesitylphenyl)P(H)K](4).

The molecular structure of the potassium salt of a bulky primary phosphane, namely (2,6-dimesitylphenyl)phosphane, is reported featuring a centrosymmetric distorted transoid four-step ladder-type arrangement in the solid state which is composed of four KP(H)Dmp units. The potassium atoms are “solvated” to different degrees through π interaction with mesityl rings of the Dmp ligands. No classical Lewis bases are coordinated to the metal centers.

Diorganogallium β-Diketonates and Their Lewis Acid−Base Adducts. Crystal and Molecular Structures of Mes2Ga(hfac) and Me2Ga(hfac)·NC5H5

Compounds of the type R2Ga(bdk) (R = Me, Et, Mes (mesityl); bdk = 2,4-pentanedionato (acac), 1,1,1-trifluoro-2,4-pentanedionato (tfac), 1,1,1,5,5,5-hexafluoro-2,4-penatedionato (hfac), 2,2,6,6-tetramethyl-3,5-heptanedionato (tmhd)) have been prepared by multiple routes, including hydrocarbon elimination, metathesis, and ligand redistribution reactions, and fully characterized by elemental analyses, cryoscopic molecular weight studies in benzene, and IR and multinuclear NMR spectroscopic studies. An X-ray structural study of Mes2Ga(hfac) demonstrated four-coordinate gallium with σ bonds to the two mesityl rings and to the two oxygen atoms of the β-diketonate ligand. The Lewis acidities of the diorganogallium β-diketonate derivatives toward NMe3 and/or pyridine were also investigated. Two adducts, Me2Ga(hfac)·NMe and Me2Ga(hfac)·py, were stable to dissociation at room temperature and thus were fully characterized by elemental analysis, cryoscopic molecular weight studies in benzene, vapor pressure−temperature studies, and IR and multinuclear NMR (including variable temperature) spectroscopic studies. The compound Me2Ga(hfac)·py was also characterized by an X-ray structural study. The structure involved five-coordinate gallium with one Ga−N, two (inequivalent) Ga−O, and two Ga−C bonds.

Raman and UV-VIS study of the conformational polymorphism of solid tetramesityldisilene Mes 2Si [dbnd]SiMes 2

All three modifications of tetramesityldisilene 1 reported to date (orange unsolvated 1a and two yellow 1:1 solvates with toluene 1b and THF 1c) have been shown to readily transform to a new form, yellow unsolvated 1d, for which a quasi-trans conformation with two mesityl rings nearly orthogonal and the other two nearly coplanar to the double bond plane, is tentatively proposed. Form 1d appears to be the most stable conformational polymorph, but can be converted to the orange form upon illumination in the region 514–457 nm.

Structure, Conformation, and Thermochemistry of Dimesityl Methanethial S‐oxide (Dimesityl Sulfine)

AbstractThe structure of dimesityl sulfine in solution and in the solid state was determined by NMR spectroscopy and X‐ray crystallography, respectively. In the crystal the two mesityl groups are twisted by approximately 60°, while in solution an averaged conformation with Cs symmetry is observed, with the anti mesityl ring located in the CSO plane and the syn mesityl ring perpendicular to the CSO plane. From the coalescence temperature Tc at 303 K, the free energy of activation for the rotation of the mesityl rings was estimated to be 14 ± 0.7 kcal/mol. At higher temperatures (140–185°C) dimesityl sulfine yields up to 95% 5,7‐dimethyl‐1‐mesityl‐benzo[c]thiophene.

Trimesitylsilylium cation verification of a free silylium cation in solution by NMR chemical shift calculations

NMR chemical shift calculations at the SOS-DFPT/PW91/[9s6p2d/5s4pld/5s4pld/3s]//B3LYP/6-31G(d) level of theory were used to describe the trimesitylsilylium cation 1, recently synthesized in benzene solution and investigated by NMR spectroscopy. The conformation of cation 1 is characterized by mesityl rings rotated by 47° in a propeller-like form. Contrary to other silylium cations investigated, cation 1 forms a weak Van der Waals complex 3 with benzene rather than a Wheland σ-complex. The calculated 29Si NMR chemical shifts for 1 and 3 are 226 and 227 ppm, compared to the experimental value of 225.5 ppm. The agreement between calculated and measured NMR chemical shifts provides evidence that cation 1 presents the first free silylium cation synthesized.

Dimesityldioxirane

The preparative scale synthesis of dimesityldioxirane (5) via oxidation of dimesitylcarbene (6) is described. The dioxirane was obtained as colorless crystalline material, completely stable at −20 °C and characterized by spectroscopic methods, X-ray crystallography, and DFT calculations (B3LYP/6-31G(d)). The molecule shows approximately local C2 symmetry with the mesityl rings twisted by ca. 60°. The experimental structural parameters, e.g., the O−O distance of 1.503 Å, are in good agreement with the theoretical values. The relief of steric strain caused by the mesityl groups is discussed in terms of the geometric distortion of the three-membered ring and the mesityl groups. Conformational features of 5 in solution are nicely reflected by measured and calculated (SOS-DFPT) NMR chemical shifts.

Substitutional photochemistry of Ru( η6-mes) 22+and Ru( η6-mes)(CH 3CN) 32+ (mes = 1,3,5-trimethylbenzene)

Irradiation of Ru( η 6-mes) 2 2+ (mes is 1,3,5-trimethylbenzene, or mesitylene) in acetonitrile results in the substitution of one mesitylene ring by solvent to yield the half-sandwich complex, Ru( η 6-mes)(CH 3CN) 3 2+. The quantum efficiency of this process exhibits little dependence upon the excitation wavelength or the presence of oxygen, but varies with solvent in the order acetonitrile > water > acetone. Photolysis of Ru( η 6-mes)(CH 3CN) 3 2+ in acetonitrile occurs via two competing pathways: substitution of mesitylene by solvent to produce Ru(CH 3CN) 6 2+, and exchange of coordinated CH 3CN with bulk solvent. Quantum efficiencies for both pathways increase with decreasing excitation wavelength and, at each wavelength, CH 3CN exchange is more efficient than mesitylene substitution. Photochemical mechanisms for these processes are discussed and comparisons with related complexes are presented.

Structural Investigation of Bis(isonitrile)gold(I) Complexes

The reaction of (MeNC)-, (PhNC)- and (MesNC)AuCl in tetrahydrofuran with an equimolar amount of the corresponding isonitrile ligand and one equivalent of Ag+X- (X-= BF- 4 or CF3SO- 3) leads to the formation of the bis(isonitrile)gold(I) complexes (MeNC)2Au+ CF3SO- 3(1), (PhNC)2Au+ BF- 4 (2) and (MesNC)2Au+ BF- 4(3). The crystal structures of 1, 2 and 3 have been determined. In compound 1 there are rod-like cations with a parallel packing into meandering puckered layers, in which the gold atoms have alternating Au-Au contacts of 3.611 and 3.624 Å. In complex 2 the cations form long double-paddles, with the two paddles at an angle of 77.5°. The individual cations are well separated and have no sub-van-der-Waals Au-Au contacts. The crystal structure of 3 is similar, but with a smaller dihedral angle between the planes of the two mesityl rings (56.0°)

Dimesitylketone O-oxide: verification of an unusually stable carbonyl oxide by NMR chemical shift calculations

NMR chemical shift calculations in combination with geometry optimizations both based on density functional theory were used to identify and to describe dimesitylketone O-oxide ( 6), which is the first carbonyl oxide that could be generated in solution at − 78°C and investigated by NMR spectroscopy. The conformation of carbonyl oxide 6 is characterized by two orthogonal mesityl rings and a close O,H contact between one of the H atoms of the mesityl methyl groups and the terminal O atom of the carbonyl oxide. The calculated geometry and conformation of 6 are verified by the agreement of calculated and measured NMR chemical shifts. The unusual rearrangement of 6 to an alcohol is explained.

Synthesis of Group 4 Organometallic Complexes That Contain the Bis(borylamide) Ligand [Mes2BNCH2CH2NBMes2]2-

Titanium and zirconium derivatives of the new chelating bis(borylamido) ligand [Mes2BNCH2CH2NBMes2]2- ([Ben]2-) are prepared by treating MCl4(THF)2 (M = Ti, Zr) with (Ben)Mg(THF)2. Nitrogen−boron π-interactions in (Ben)TiCl2 and (Ben)ZrCl2(THF) result in one mesityl group in each BMes2 unit occupying space roughly above and below the MCl2 plane. (Ben)TiCl2 is smoothly alkylated by Grignard reagents in dichloromethane to give (Ben)Ti(R)Cl (R = CH2Ph, CH2CMe3) and (Ben)TiR2 (R = Me, CH2Ph), while unstable (Ben)ZrMe2 can be prepared from (Ben)ZrCl2(THF) and methyllithium in toluene. An X-ray study of (Ben)Ti(CH2Ph)Cl confirms the proposed ligand conformation and features a highly distorted “η2” benzyl ligand with a Ti−Cα−Cipso angle of only 87.0(5)°. (Ben)MMe2 complexes cleanly decompose by metalation of the ortho methyl groups from mesityl rings on different borons at room temperature (for Zr) or upon heating (for Ti). An X-ray crystal structure of (TwistBen)Zr shows it to be a dimer in which the two zircon...

Stereoelectronic Effects in the Nucleophilic Addition to the sp-Hybridized Carbon of a Ketene and Vinyl Cation: When Is a Mesityl Effectively Smaller than a Phenyl Ring?1

Addition of MesMgBr to mesitylphenylketene 3 yields a ca. 3:1 mixture of the magnesium enolates 5-(E) and 5-(Z), whereas reaction of enol 1-(Z) with MesMgBr results in the exclusive formation of enolate 5-(Z). Acetylation of the configurationally stable 5-(E) and 5-(Z) yields the enol acetates 4-(E) and 4-(Z), respectively. The preference for the formation of the (E)-enolate from 3 indicates that the attack occurs preferentially from the side of the formally bulkier mesityl ring. Ketene 3 and the two diastereomeric transition states obtained by attack of MeLi on its CO were calculated by ab initio calculations. 3 adopts a conformation with a planar Ph-CC moiety while the mesityl is nearly perpendicular to this plane. Since the attack on the CO group occurs in the plane of the CC double bond, the coplanar Ph is effectively bulkier to the approaching nucleophile than the twisted Mes and the preferred attack is from the mesityl side. Similar stereoelectronic effects operate in 1,2-dimesityl-2-phenylvinyl cat...

3,3-Dimethyl-1-(2-pyridylthio)butyl]methyl(2,4,6-trimethylphenyl)phosphine Oxide

In the molecule of [3,3-dimethyl-1-(2-pyridylthio)-butyl]methyl(2,4,6-trimethylphenyl)phosphine oxide, C 21 H 30 NOPS, the P=O bond is nearly coplanar with the mesityl ring and nearly parallel to the pyridyl ring. In the chain connecting the mesityl and pyridyl rings two extended fragments are found, twisted at the C11-S bond; likewise, two extended fragments exist in the chain connecting the methyl atom C10 and the terminal atom C15 of the butyl moiety, twisted at the C11-C12 bond. The structure is stabilized by C11-H111...N intramolecular and C20(py)-H201...O intermolecular hydrogen bonds.

(η6-C6H3Me3)Ru(D-pen)]2Cl2.4H2O

The structure of the title compound, bis[m-D-3-mercaptovalinato(1−)]-1κN,1:2κ 2 S;2κN,1:2κ 2 S-bis[(η 6 -mesitylene)ruthenium] dichloride tetrahydrate, [Ru 2 (C 5 -H 10 NO 2 S) 2 (η 6 -C 9 H 12 ) 2 ]Cl 2 .4H 2 O, contains tridentate anionic amino acid ligands, where the S atoms adopt bridging positions between the two Ru atoms resulting in a near planar Ru−S−Ru−S ring. The coordination at each Ru atom is completed by an amino N atom of the chelating ligand and the η 6 -bonded mesitylene ring

Intramolecular Rotations in 2,2-Dimesityl-1-(9-anthryl)- and -1-(9-triptycyl)vinyl Acetates

2,2-Dimesityl-1-(9-anthryl)vinyl acetate (3) and 2,2-dimesityl-1-(9-triptycyl)vinyl acetate (4) were prepared. The rotation of the mesityl and 9-anthryl rings in 3 and of the mesityl rings and triptycyl substituent in 4 around their bonds to the vinylic carbon were investigated by DNMR. For each system an identical rotational barrier was measured for each substituent, indicating a correlated rotation. The rotational barriers were 16.7±0.1 kcal mol -1 for 3 and 17.0 ±0.1 kcal mol -1 for 4. The suggested rotational mechanism is a three-ring flip for 3 and a correlated (geared) rotation for the three rotors of 4

The effective ‘size’ of the tris(trimethylsilyl)silyl group in several molecular environments

The effective size of the tris(trimethylsilyl)silyl group in several molecular environments has been estimated. 2,2-Dimesityl-1-tris(trimethylsilyl)silylethanol 1g has been prepared and its structure determined by X-ray crystallography. The Mes–CC torsional angles are 59.6 (φ2) and 63.3°(φ2) and the CC–Si bond angle α4 is 133.8°. The two-ring flip barrier for the correlated rotation of the two mesityl rings around the Mes–C bonds is ΔGc‡= 10.2 kcal mol–1. The structures of enols Mes2CC(OH)R, R = H, Me, Et, Pri, But(1a–1e), Me3Si (1f), (Me3Si)3Si (1g) and (Me3Si)3C (1h) and the two-ring flip barriers have been calculated by the MM2* force-field. The calculated and the experimental values are in good agreement, except for somewhat lower calculated α4 for 1b–1e and a shorter C–Si distance in 1g. From the linear correlations between the observed cos φ2 or ΔGc‡ values and Es values for the enols 1a–1e, and the values observed for 1g an average Es value of –1.46 has been calculated for (Me3Si)3Si. MM2* calculations gave an A value for (Me3Si)3Si of 4.89 kcal mol–1. These steric parameters resemble those for the But group (Es=–1.54; A= 4.9 kcal mol–1) indicating a similar effective size for the But and (Me3Si)3Si groups in these specific environments. (Me3Si)3C is significantly larger (A= 13.3 kcal mol–1; estimated Es=–3.7).