Synthesis of 3-fluoro-3-methyl-3-silatetrahydropyran and its conformational preferences in gas and solution by GED,NMR and theoretical calculations

The 3,3-disubstitued 3-silaheterocyclohexane with an electronegative substituent at silicon, 3-fluoro-3-methyl-3-silatetrahydropyran 1, was synthesized, and its molecular structure and conformational properties studied by gas-phase electron diffraction (GED) and low temperature ¹³C and ¹⁹F NMR spectroscopy. Quantum-chemical calculations were carried out both for the isolated species and H-complexes in gas and in polar medium. The predominance of the 1-F_eqMe_ax conformer (1-F_eq:1-F_ax ratio of 65:35, ΔG°?=?0.37?kcal/mol) determined from GED is close to the theoretically estimated conformational equilibrium, especially at the DFT level. In solution, low temperature NMR spectroscopy showed no decoalescence of the signals in ¹³C (down to 95?K) and ¹⁹F NMR spectra (down to 123?K). However, the calculated ¹⁹F chemical shift of ?173.6?ppm for the 1-F_eqMe_ax conformer practically coincides with the experimentally observed value (?173 to ?175?ppm) as distinct from that for the 1-F_axMe_eq conformer (?188.8?ppm), suggesting compound 1 to be anancomeric in solution, in compliance with its theoretical and experimental preference in the gas phase.     

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-Ph_ax conformer (1-Ph_eq:1-Ph_ax 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 C_ipso 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-Ph_eq conformer was found to be the preferred one [(1-Ph_eq: 1-Ph_ax?=?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 Ph_ax conformer in the gas phase and of the Ph_eq conformer in solution is confirmed.     

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 103 K. Predominance of the equatorial conformer of compound 1 (Ph_eq/Ph_ax=78%:22%) is much less than in its carbon analog, phenylcyclohexane (nearly 100% of Ph_eq). And in contrast to 1-methyl-1-phenylcyclohexane, the conformers with the equatorial Ph group are predominant for compounds 3 and 4: at 103 K, Ph_eq/Ph_ax 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) kcal mol^?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.     

1-Methylthio-1-phenyl-1-silacyclohexane: Synthesis,conformational preferences in gas and solution by GED,NMR and theoretical calculations

1-Methylthio-1-phenyl-1-silacyclohexane 1, the first silacyclohexane with the sulfur atom at silicon, was synthesized and its molecular structure and conformational preferences studied by gas-phase electron diffraction (GED) and low temperature ¹³C and ²⁹Si NMR spectroscopy (LT NMR). Quantum-chemical calculations were carried out both for the isolated species and solvate complexes in gas and in polar medium. The predominance of the 1-MeS_axPh_eq conformer in gas phase (1-Ph_eq:1-Ph_ax = 55:45, ΔG° = 0.13 kcal/mol) determined from GED is consistent with that measured in the freon solution by LT NMR (1-Ph_eq:1-Ph_ax = 65:35, ΔG° = 0.12 kcal/mol), the experimentally measured ratios being close to that estimated by quantum chemical calculations at both the DFT and MP2 levels of theory.     

Conformational rivalry of geminal substituents in silacyclohexane derivatives: 1-Phenyl vs. 1-OR,R=H or Me

Molecular structure and conformational equilibria of 1-methoxy- 1 and 1-hydroxy-1-phenylsilacyclohexanes 2 were studied by quantum chemical (QC) calculations and combined gas electron diffraction/mass spectrometry (GED/MS). Both molecules may exist in 5 or 6 forms, differing from each other by the substituents' position: (i) axial or equatorial and (ii) rotational orientation relative to the six-membered ring frame. The contribution of axial forms of both compounds varies from 35 to 60% depending on the theoretical method applied. From the GED data, the summarized molar fractions of the conformers were found to be Ph_ax:Ph_eq = 70(15):30(20) and 50(20):50(20)% which corresponds to ΔG = G_ax–G_eq = ?0.55(46) and 0.00(56) kcal/mol, for compound 1 and 2, respectively. The concentration of the Ph_ax forms of 1-phenyl-1-(X)-silacyclohexanes (X = H, HO, Me, MeO and Me₂N) increases with the size of the second substituent at the silicon atom: 38(10)<50(20)<58(15)<70(15) <80(15)%.     

Conformational Analysis of Marine Polyhalogenated β-Chamigrenes Through Temperature-Dependent NMR Spectra

Temperature-dependent ¹H- and ¹³C-NMR spectra reveal that polyhalogenated marine β-chamigrenes or synthetic derivatives thereof which are trans-diequatorially, substituted at C(8) and C(9), such as rogiolol ((?)- 2 ), obtusol ((+)- 3 ), and their acetates (+)- 1 and (?)- 4 , undergo slow ring- A chair-chair inversion. Conformational equilibria and kinetics are investigated with the aid of synthetic model compounds and molecular-mechanics calculations. Thus, steric repulsions between Br_ax–C(2) and H_eq–C(7) are seen to disfavour thermodynamically conformers 1b , 2b , 3b , and 4b , which can only be detected through cross-saturation transfer, while additional steric repulsions between Me_ax–C(1) and OH_ax–C(3) make conformer 8b of obtusol epimer so scarcely populated that it can not be detected. In agreement, with (+)- 9 and (+)- 10 , which have a trigonal C(2), two conformers can be directly observed by NMR. The kinetic barriers, which are seen to arise mainly from steric repulsions between H_ax–C(14) and the axial H or halogen atoms at C(8) and C(10), are calculated and discussed with respect to well documented exocycliemethylidene-substituted cyclohexane(ene) systems. This helps to rationalize why in rogiolol acetate ((+)- 1 ) ring B is unusually inert towards Zn/Et₂O/AcOH which causes bromohydrine-group elimination from ring A.     

Spectroscopic and structural characterization of O,O′-(diphenylphosphineoxide)amidate and acetylacetonate complexes of pentacoordinate nickel(II)

Heteroleptic nickel pentacoordinate complexes with the macrocyclic ligands 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene (Me₃-mcN₃) or its 9-methyl derivative (Me₄-mcN₃), as ancillary ligands, and O,O′-(diphenylphosphineoxide)amidate ligands, [RC(O)NP(O)Ph₂]^¯ (R = C₆H₆ (1), C₅H₄N (2), C₄H₃S (3)), have been prepared as well as related acetylacetonate derivatives. The complexes have been studied by spectroscopic methods (IR, UV-Vis and ¹H NMR). In acetone solution, the complexes exhibit isotropically shifted ¹H NMR resonances. The full assignment of these resonances has been achieved using one- and two-dimensional ¹H NMR techniques. The single-crystal structures of {(Me₄-mcN₃)Ni[OP(Ph₂)NC(Tf)O]}[PF₆] (9) and {(Me₃-mcN₃)Ni(acac)}[PF₆] (10) have been established by X-ray diffraction.     

Conformational effects in compounds with 6-membered rings—XIII : Detection of twist conformers using 13C NMR chemical shifts

¹³C NMR spectra of derivatives of cyclohexane, piperidine, and thian in chair and twist eonformers, and of model compounds, lead to estimates of deshielding (Δδ = 3.6 ± 0.2 ppm) for axial C$\text{Me}$₃ on a cyclohexane ring and shielding (Δδ = ?0.2 to ?0.6 ppm) for ψe-C$\text{Me}$₃ in twist conformers, relative to equatorial C$\text{Me}$₃. Ring carbon atoms are considerably shielded in twist conformers relative to chair eonformers. The value of ¹³C chemical shifts in the study of chair-twist equilibria is exemplified by variable temperature measurements on diastereomeric pairs of compounds (11 and 13; 38 and 50).     

Neutral penta-coordinated diorganotin(IV) complexes derived from ortho-aminophenol Schiff bases: Synthesis, characterization and molecular structures

The one pot reactions carried among ortho-aminophenol, R₂SnO (R = Me or Ph) and acetyl acetone, 2-hydroxyacetophenone and 2-hydroxy-3-methylacetophenone led to six new diorganotin(IV) compounds Me₂SnL1 (1), Ph₂SnL1 (2), Me₂SnL2 (3) Ph₂SnL2 (4), Me₂SnL3 (5) and Ph₂SnL3 (6) (H2L1 = 2-(3-hydroxy-1-methyl-but-2-enylideneamino)-phenol, H2L2 and H2L3 = 2-[1-(2-hydroxyaryl)alkylideneamino]-phenol) in good yields. Combination of IR, ¹H, ¹³C and ¹¹⁹Sn NMR and X-ray diffraction techniques along with elemental analyses evidenced the formation of penta-coordinated monomeric species. The crystal structures of ligand H2L1 and complexes 1, 3 and 4 were determined by single crystal X-ray diffraction study. In the solid state, the ligand H2L1 exists as keto-enamine tautomeric form. There are N-H…O intra-molecular hydrogen bonds between amine and carbonyl groups. Diorganotin(IV) complexes 1, 3 and 4 are monomers with TBP (trigonal bipyramidal) geometry surrounding the tin atom. The O, N, O- tridentate ligand places its two oxygen donating atoms in the axial positions, and the nitrogen atom occupies one equatorial position. The two R groups attached to tin occupy the other two equatorial positions. The solution structures were predicted by ¹¹⁹Sn NMR spectroscopy.     

Application of J(C,H) coupling constants in conformational analysis

Conformational equilibria for a number of methyl substituted 1,3-dioxanes 1, 1,3-oxathianes 2 and 1,3-dithianes 3 were calculated at the HF and DFT levels of theory. In addition to the chair conformers also the energetically adjacent twist conformers were considered and the positions of the corresponding conformational equilibria estimated. On the basis of the global energy minima of conformers, participating in the conformational equilibria, the ¹J_C,Hax,equ coupling constants were calculated using the GIAO method and compared with the experimental values obtained from ¹³C,¹H coupled ¹³C NMR spectra. The Perlin effect, the influence of the solvent and the suitability of this NMR parameter for assigning the conformational equilibria present are critically discussed.     

Bis(diphenylphosphino)methylamine as chelate ligand in pentamethylcyclopentadienylrhodium(III) and iridium(III) complexes.: Crystal structure of [(η5-C5Me5)RhCl{η2-P,P′-(PPh2)2NMe}]BF4

Reactions of the dimers [{(η⁵-C₅Me₅)MCl(μ-Cl)}₂] (M=Rh, Ir) with the ligand NMe(PPh₂)₂ in 1:2 molar ratio afford the mononuclear cationic complexes [(η⁵-C₅Me₅)MCl{η²-P,P′-(Ph₂P)₂NMe}]Cl (M=Rh 1, Ir 2). Similar iodide complexes, [(η⁵-C₅Me₅)MCl{η²-P,P′-(Ph₂P)₂NMe}]I (M=Rh 3, Ir 4), can be prepared by N-functionalization of co-ordinated dppa ligand in complexes [(η⁵-C₅Me₅)MCl{η²-P,P′-(Ph₂P)₂NH}]BF₄. The tetrafluoroborate derivatives, [(η⁵-C₅Me₅)MCl{η²-P,P′-(Ph₂P)₂NMe}]BF₄ (M=Rh 5, Ir 6) are prepared by reaction of complexes 1–4 with AgBF₄ in acetone. All the compounds described are characterised by microanalysis, IR and NMR (¹H, ³¹P{¹H}) spectroscopy. The crystal structure of complex 5 is determined by X-ray diffraction methods. The complex exhibits a pseudo-octahedral molecular structure with a C₅Me₅ group occupying three co-ordination positions and a bidentate chelate P,P′-bonded ligand and a chloride atom completing the co-ordination sphere.     

Tetraphenylphosphonium carboxylates and sulfonates. Synthesis and structure

The reaction of the pentaphenyphosphorus solvate Ph₅P·1/2PhH (I) with carboxylic and sulfonic acids was used to synthesize tetraphenylphosphonium carboxylates Ph₄POC(O)R, R = C₆H₄(2-OH) (II), C₆H₄ (2-COOH) (III), H (IV), Me (V), CCl₃ (VI), Ph (VII), PhCH=CH (VIII), CH₂CH₂C(O)OH (IX), CH=CHC(O) OH(X), and CH₂C(O)OH (XI) and tetraphenylphosphonium sulfonates Ph₄POSO₂Ar, Ar = Ph (XII), C₆H₄Me₄ (XIII), and C₆H₃(-COOH)(4-OH) (XIV). Compound XII was also prepared from compound I and SO₃ in benzene. According to X-ray diffraction data, the crystals of I contain two types of crystallographically independent molecules with a slightly distorted trigonal-bipyramidal configuration [Ia, CaxPCax 178.44(8)°, P- C_ax 1.985(2), 1.987(2) Å, P-C_eq 1.854(2), 1.846(2), 1.840(2) Å; Ib, C_axPC_ax 178.45(9)°, P-C_ax 1.980(2), 1.975 (2) Å, P-C_eq 1.840(2), 1.846(2), 1.854(2) Å]. In the cations of compounds II, III and XIV, the coordination of the phosphorus atom is tetrahedral [CPC angle: II, 106.2(2)?111.6(1)°; III, 104.01(6)?113.03(6)°; XIV, 107.54 (6)?112.79(6)°]; the anions contain intramolecular O-H?O hydrogen bonds between the hydroxyl hydrogen atom and carboxyl oxygen atom (II, 1.34; III, 1.23; and XIV, 1.83 Å).     

Enamine chemistry—XXII: The preparation of 4-acylaminopent-3-en-2-ones and the corresponding enamino-thiones

Acylation of 4-aminopcnl-3-en-2-one, 5, with different acid chlorides, RCOCl(R = Me, Me₂CH, Me₃C, C₆H₅, CICH₂) gives only N-acylated products, 6, in high yields. The N-acylated enaminones, 5, are shown only to exist m the Z-s-Z form in solution. The UV spectra of 6 are discussed and an additional increment to the Dabrowski-Table²¹ is given. The reaction of 6 with 2,4-bis(4-methoxyphenyl)-1,2,3,4-dithiadiphosphetane 2,4-disulfide, 2, at room temperature in DME gives the corresponding enamino-thiones, 7, in good yields. The ¹H and ¹³C NMR spectra of 6 and 7 are discussed.     

Properties of η-pentamethylcyclopentadienyl rhodium(III) and iridium(III) complexes with quinolin-8-ol and their cytostatic activity

Reaction of dimers [M(η⁵-C₅Me₅)Cl₂]₂ (M-Rh, Ir) with quinolin-8-ol in molar ratio 1:2 leads to formation of monomer complexes [Rh(η⁵-C₅Me₅)Cl(qol)] (1) and [Ir(η⁵-C₅Me₅)Cl(qol)] (2) (qol = quinolin-8-olate). Compounds 1 and 2 have been characterized with elemental analysis and spectroscopic methods. ¹H NMR spectra revealed that quinolin-8-olate is coordinated via oxygen and nitrogen atoms. The ¹H NMR and ¹³C NMR spectra showed that carbon and hydrogen atoms of pentamethylcyclopentadienyl ligand are equivalent. The structure of rhodium complex has been calculated using DFT B3LYP method. The calculated geometry of complex 1 agrees very well with data found for rhodium complexes containing Cl, C₅Me₅ and qol ligands. Both complexes are active antitumor and antibacterial agents.     

Metallcarbonyl-synthesen: XIII. Eine mangan—mangan-dreifachbindung

Photolysis of the manganese half-sandwich complex (η⁵-C₅Me₅)Mn(CO)₃ (1) in tetrahydrofuran (THF) yields cleanly the solvent complex (η⁵-C₅Me₅)(Mn(CO)₂THF (2). Compound 2 undergoes spontaneous elimination of carbon monoxide to give at ambient temperature in approx. 20 h, or upon removal of solvent in vacuo, the novel dinuclear derivative (η⁵-C₅Me₅)₂Mn₂(μ-CO)₃ (3). Elemental analyses and infrared and mass as wel as the ¹H and ¹³C NMR spectra unequivocally demonstrate this compound to adopt a triply carbonyl-bridged structure containing the first example of a manganesemanganese multiple bond.     

Zur Reaktion von Metall-koordinierten Kohlenmonoxid mit Yliden XXII. Bis(diphenylphosphino)methanid-Eisen-Komplexe Cp(L)Fe(Ph2PCHPPh2) (L = CO,Me3P)

The reaction of [Cp(CO)(dppm)Fe]BF₄ (1a) with the phosphorus ylide Me₃PCH₂ yields the novel bis(phosphino)methanideiron complex Cp(CO)Fe(Ph₂PCHPPh₂) (2), which upon photolysis in the presnece of Me₃P is converted into Cp(Me₃P)Fe(Ph₂PCHPPh₂ (3). Reaction of 2 with MeOSO₂CF₃ gives a mixture of the iron salts [(Cp(CO)Fe(Ph₂PCR(R′)PPh₂)]CF₃SO₃ (R = R′ = H (1b), R = R′ = Me (6) and R = H, R′ = Me (syn/anti-4)).     

Improper hydrogen-bonded cyclohexane C-Hax?Yax contacts: experimental evidence from H NMR spectroscopy of suitable axial cyclohexane models

B3LYP/6-31+G(d,p) calculations predicted the presence of improper hydrogen-bonded C-H_ax?Y_ax contacts of different strength in cyclohexane derivatives;¹ it was predicted that the addition of an appropriate bridging fragment X_ax between an axial substituent Y₁ and a cyclohexane carbon would strengthen the improper hydrogen-bonded contact C-H_ax?Y₁ when the X_ax-Y₁ bond vector bisects the cyclohexane ring. To support the theoretical predictions with experimental evidence for this effect, several 2-substituted adamantane analogues with suitable improper H-bonded C-H_ax?O contacts of different strength were synthesized, as models of the corresponding cyclohexane derivatives, and their ¹H NMR spectra were recorded at 298 K. The ¹H NMR signal separation within the cyclohexane ring γ-CH₂s is increased when the B3LYP/6-31+G(d,p)-calculated strength of the H-bonded C-H_ax?O=C_ax contact interaction is increased.     

Syntheses, structures and intermolecular interactions of tetraorganoammonium, -phosphonium and -stibonium dimethyl- and diphenyltetrahaloantimonates

The syntheses and spectroscopic (NMR, MS) investigations of the antimonates [Ph₄P]⁺[Me₂SbCl₄]⁻ (1), [Me₄Sb]⁺[Me₂SbCl₄]⁻ (2), [Et₄N]⁺[Ph₂SbCl₄]⁻ (3), [Bu₄N]⁺[Ph₂SbCl₄]⁻ (4), [Me₄Sb]⁺[Ph₂SbCl₄]⁻ (5), [Et₃MeSb]⁺[Ph₂SbCl₄]⁻ (6), [Et₄N]⁺[Ph₂SbF₄]⁻ (7) and [Et₄N]⁺[Ph₂SbBr₄]⁻ (8) are reported. Halogen scrambling reactions of Et₄NBr or Ph₄EBr (E = P, Sb) with R₂SbCl₃ (R = Me, Ph) produce mixtures of compounds from which crystals of [Et₄N]⁺[Ph₂SbBr_1.24Cl_2.76]⁻ (9), [Et₄N]⁺[Ph₂SbBr_2.92Cl_1.08]⁻ (10) or [Ph₄Sb]⁺[Me₂SbCl₄]⁻ (11) were isolated. The crystal and molecular structures of 1 and 3-11 are reported.     

Reactions of ketenes with ethoxyalkynes: Synthesis of 2,2,4-trialkylcyclobutane-1,3-diones

Treatment of dimethyl ketene with ethoxyacetylene 1a, 1-ethoxyoct-1-yne 1b, and 1-ethoxytetrade-1-yne 1c afforded the 3-ethoxycyclobutenones 2a–c. Hydrolysis of 2a–c with dilute hydrochloric acid gave the cyclobutane-1,3-diones 3a–c. The ¹H NMR spectra of these compounds indicate that in CDCl₃ solution 2,2-dimethylcyclobutane-1,3-dione 3a exists as the diketone, whereas the 2,2,4-trialkylcyclobutane-1,3-diones 3b and 3c exist as the monoenols.     

Comparative reactivity of substituted 1,3-and 1,4-thiasilinane <Emphasis Type="Italic">S</Emphasis>-oxides in the sila-Pummerer rearrangement and inversion of the thiocarbonyl group

The thermal sila-Pummerer rearrangement of diastereomeric 2,3,3-trimethyl-1,3-thiasilinane S-oxides was studied. Introduction of the methyl group in the 2 position of 3,3-trimethyl-3-thiasilinane S-oxide slows down the rearrangement. When heated in CCl₄, the trans isomer (2-Me_eq, SO_eq) converts into the cis isomer (2-Me_eq, SO_ax) which rapidly rearranges into 2,2,7-trimethyl-1,6,2-oxathiasilepane. On the contrary, the isomeric 2,4,4-trimethyl-1,4-thiasilinane S-oxide is thermally stable up to 160°C in DMSO. The inversion at the sulfur atom in 2,3,3-trimethyl-1,3-thiasilinane S-oxides and 2,4,4-trimethyl-1,4-thiasilinane S-oxides under the action of triethyloxonium tetrafluoroborate was studied. The trans isomer of 2,3,3-trimethyl-1,3-thiasilinane S-oxide (2-Me_eq, SO_eq) forms with Et₃O⁺BF ₄ ^? a salt which decomposes in two ways. The first involves recovery of the starting sulfoxide due to Sn2 substitution at the carbon atom of the ethoxy group, and the second, convertion into the cis isomer (2-Me_eq, SO_ax) which rearranges into 2,2,7-trimethyl-1,6,2-oxathiasilepane. Under the same conditions, the cis isomer of 2,3,3-trimethyl-1,3-thiasilinane S-oxide (2-Me_eq, SO_eq) decomposes to form siloxanes. trans-2,4,4-Trimethyl-4-thiasilinane S-oxide (2-Me_eq, SO_eq) under the action of Et₃O⁺BF ₄ ^? convers into the cis isomer (2-Me_eq, SO_ax). The B3LYP/6-311G(d,p) theoretical analysis showed that the thermal inversion at the sulfur atom in the compounds studied has a high energy barrier.