Sulfur Inversion Research Articles

Solution behavior of half sandwich ruthenium(II) complexes ligated by sym N,N'-diarylthiourea: Structural aspects and temperature dependent NMR

The bridge-splitting reaction of [(η6-C10H14)RuCl(μ-Cl)]2 (η6-C10H14 = η6-p-cymene) with sym N,N′-diarylthiourea (2 equiv.), (ArNH)2C=S in toluene at room temperature afforded [(η6-C10H14)RuCl2{(S)C(NHAr)2}]⋅xH2O (sym = symmetrical; Ar = C6H5 (1), 2-MeC6H4 (2), 2,4-Me2C6H3 (3), 2,5-Me2C6H3 (4), 2,6-Me2C6H3 (5), and 2,6-iPr2C6H3 (6); x = none (1–2, and 4–6) and 1 (3)) in 86%−95% yields. However, the aforementioned reaction afforded [(η6-C10H14)RuCl{(S)C(NHAr)2}2]+[(ArNH)2C(S)⋅Cl]–⋅toluene (Ar = 2,6-Me2C6H3; 7) in 90% yield when carried out with four equiv of (ArNH)2C=S. The mode of coordination in all the Ru(II) complexes was confirmed by spectroscopic methods and X-ray crystallography. In solution, complexes 2 and 7 exhibited two isomers in about 2:1 and 1:0.15 ratios respectively and confirmed by a temperature dependent 1H NMR spectral titration. The spectral pattern was attributed to the rotation through C−N(H)Ar single bond of thiourea in 2 and sulfur inversion of one of the coordinated thiourea in 7. Various reasons that are responsible for the presence of two isomers for 2 and 7 in solution have been discussed.

Reversible crystal-to-crystal phase transition of an octahedral zinc(II) complex with six dimethylsulfoxide

Reversible crystal-to-crystal phase transition was observed for a zinc(II) complex [Zn(dmso)6][BPh4]2 [hexakis(dimethylsulfoxide-κO)zinc(II) bis(tetraphenylborate)] (dmso: dimethylsulfoxide) at 230 K. The reversibility was ascertained by a single-crystal X-ray method, and the phase transition temperature was determined by the differential scanning calorimetry (DSC). In the crystal at 293 K, the complex cation was centrosymmetric, and the six dmso moieties were disordered, while in the crystal at 90 K, reported previously, the symmetry around the zinc(II) ion was lower, and only one dmso moiety was disordered. It was concluded that the sulfur inversion motion of the dmso moieties was taking place in the high-temperature phase and that the motion stopped in the low-temperature phase. From the conformational analysis, the driving force of the phase-transition was attributed to the crystal packing effect of the bulky tetraphenylborate anions, which imposed the volumetric compression of the flexible complex cations.

Synthesis, characterization, and catalytic application in aldehyde hydrosilylation of half-sandwich nickel complexes bearing (κ1-C)- and hemilabile (κ2-C,S)-thioether-functionalised NHC ligands.

Neutral nickel-N-heterocyclic carbene complexes, (κ1-C)-[NiCpBr{R-NHC-(CH2)2SR'}] [Cp = η5-C5H5; R-NHC-(CH2)2SR' = 1-mesityl-3-[2-(tert-butylthio)ethyl]- (1a), 1-mesityl-3-[2-(phenylthio)ethyl]- (1b), 1-benzyl-3-[2-(tert-butylthio)ethyl]- (1c), 1-benzyl-3-[2-(phenylthio)ethyl]-imidazol-2-ylidene (1d)], which bear a N-bound thioether side arm, were prepared by the reaction of nickelocene with the corresponding imidazolium bromides [R-NHC-(CH2)2SR'·HBr] (a-d), via conventional or microwave heating. The 1H NMR spectra of the benzyl-substituted species 1c and 1d showed signals for diastereotopic NCH2CH2S protons at room temperature. However, structural studies established the absence of coordination of the sulphur atom in the solid state, and solvent DFT calculations showed that bromide displacement by sulphur is an unfavourable process (ΔG = +13.5 kcal mol-1 for 1d), thereby suggesting that the observed disatereotopicity is more likely due to significant steric congestion rather than to a possible C,S-chelation in solution. Treatment of these complexes with KPF6 in tetrahydrofuran (THF) led to bromide abstraction to afford the cationic complexes [NiCp{R-NHC-(CH2)2SR'}](PF6) (2a-c). Alternatively, 2a-c could also be prepared by the direct reaction of nickelocene with the corresponding imidazolium hexafluorophosphate salts [R-NHC-(CH2)2SR'·HPF6]. Inversely to the neutral species, whereas X-ray crystallography established C,S-chelation in the solid state, the 1H NMR spectra (CDCl3, CD2Cl2, or thf-d8) at room temperature showed no diastereotopic NCH2CH2S protons, thus suggesting the possible displacement of the sulphur atom by the respective solvents and/or very fast sulphur inversion. DFT calculations established a low energy inversion process in all cases (+9 ≤ΔG‡≤ +13 kcal mol-1) as well as a favourable solvent coordination process (ΔG‡≈ +11 kcal mol-1; ΔG≈-7 kcal mol-1) with a solvent such as THF, thus suggesting that sulphur inversion and/or solvent coordination can both account for the absence of diastereotopy at room temperature, depending on the solvent. While all complexes catalysed the hydrosilylation of benzaldehyde in the absence of any additive, the cationic C,S-chelated complexes 2 proved more active than the sterically constrained neutral species 1. In particular, 2c proved to be the most active pre-catalyst and its catalytic charge could be lowered down to 2 mol% with PhSiH3 as the hydrogen source.

Quantum mechanical study on the different fluxional mechanisms in a [Pd(η2-olefin) complex

In this work, using quantum mechanics, different fluxional mechanisms in a Pd(η2-olefin) complex including sulfur inversion, olefin pseudo rotation, Pd–olefin bond dissociation, propeller-like olefin rotation and a bimolecular process have been investigated theoretically in a solvent environment using the polarisable continuum model (PCM). The activation energy, activation enthalpy and activation Gibbs free energy of the different pathways have been calculated and compared with each other and with experimental results. These activation energies for olefin pseudo rotation, Pd–olefin bond dissociation and propeller-like olefin rotation are greater in comparison with those for the sulfur inversion and bimolecular process, which demonstrates that these latter mechanisms are the dominating paths and make the main contributions in the reaction. The computed activation enthalpies of the sulfur inversion and bimolecular process pathways are in good agreement with the experimental values. All of the calculations have been performed using a hybrid density functional method (B3LYP) in the solution phase.

A Dimorphic Os(IV) Complex with Pyramidal Sulfur Inversion in a Sulfanyl Chelating Ligand

Coordination compound [Os(SC6F4H-4)2(SC6F3H-4(SC6F4H)-2)(C6H5)] is obtained during the thermolysis of [Os(SC6F4H-4)4(PPh3)] in non-dried toluene, as a product of C–F bond cleavage. The complex presents the expected trigonal–bipyramidal coordination geometry, with a thiolate-sulfanyl κ2 S,S’-ligand chelating the metal center in axial and equatorial sites. The complex was crystallized in the centrosymmetric space group C2/c and in the non-centrosymmetric space group Pna21. In both forms, the crystal is a RS racemate, with the chiral center being the axially coordinated sulfanyl S atom. In the case of the non centrosymmetric space group, the asymmetric unit retained for the refinement consists of two independent molecules related by a pyramidal sulfur inversion at the stereogenic center. Cell parameters: form I, C2/c, a = 20.8459(14) A, b = 12.2646(12) A, c = 25.253(3) A, β = 91.582(7)°; form II, Pna21, a = 16.208(2) A, b = 16.500(3) A, c = 23.604(3) A. An Os(IV) coordination complex bearing a thiolate-sulfanyl κ2 S,S’-ligand was crystallized in two spaces groups, C2/c with one molecule in the asymmetric unit, and Pna21 with two enantiomeric molecules in the asymmetric unit.

Bacterial dioxygenase- and monooxygenase-catalysed sulfoxidation of benzo[b]thiophenes

Asymmetric heteroatom oxidation of benzo[b]thiophenes to yield the corresponding sulfoxides was catalysed by toluene dioxygenase (TDO), naphthalene dioxygenase (NDO) and styrene monooxygenase (SMO) enzymes present in P. putida mutant and E. coli recombinant whole cells. TDO-catalysed oxidation yielded the relatively unstable benzo[b]thiophene sulfoxide; its dimerization, followed by dehydrogenation, resulted in the isolation of stable tetracyclic sulfoxides as minor products with cis-dihydrodiols being the dominant metabolites. SMO mainly catalysed the formation of enantioenriched benzo[b]thiophene sulfoxide and 2-methyl benzo[b]thiophene sulfoxides which racemized at ambient temperature. The barriers to pyramidal sulfur inversion of 2- and 3-methyl benzo[b]thiophene sulfoxide metabolites, obtained using TDO and NDO as biocatalysts, were found to be ca.: 25-27 kcal mol(-1). The absolute configurations of the benzo[b]thiophene sulfoxides were determined by ECD spectroscopy, X-ray crystallography and stereochemical correlation. A site-directed mutant E. coli strain containing an engineered form of NDO, was found to change the regioselectivity toward preferential oxidation of the thiophene ring rather than the benzene ring.

Carbon−Sulfur Bond Activation of Dibenzothiophenes and Phenoxythiin by [Rh(dippe)(μ-H)]2 and [Rh2(dippe)2(μ-Cl)(μ-H)

The rhodium dimer [Rh(dippe)(μ-H)]2 (1) reacts with dibenzothiophene to form the C−S cleavage product [Rh2(dippe)2(μ-SC12H9)(μ-H)] (3). Complex 1 also reacts with 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene to form [Rh2(dippe)2(μ-S-MeC12H8)(μ-H)] (4) and [Rh2(dippe)2(μ-S-Me2C12H7)(μ-H)] (6), respectively. Reaction with phenoxythiin forms [Rh2(dippe)2(μ-S-C6H4OC6H5)(μ-H)] (8). In 3, rhodium addition to the ortho C−H bond of the β-ring of the biphenylthiolate substituent does not occur at 100 °C. The crystal structures of [Rh2(dippe)2(μ-Cl)(μ-H)] (2), 4, 6, and 8 indicate that each complex contains a nearly planar Rh2(μ-X)(μ-H) core. The 1H NMR spectra of 3, 4, and 6 suggest that the pyramidal geometry at the sulfur atom is maintained in solution and that sulfur inversion is absent. Unreacted 1 catalyzes H/D exchange between C6D6 and the biphenylate substituent of 3 faster than it cleaves the remaining C−S bond to make free biphenyl. Complex 3 is unreactive toward excess dibenzothiophene, whil...

Fluxional rearrangement in congested ruthenium(II) complexes containing pyrimidine- and/or pyridine-based dithioether ligands

The solvento species obtained by the treatment of cis-RuCl2(N,N-L)2 [L = di-2-pyridyl sulfide (dps), di-2-pyrimidyl sulfide (dprs)] with AgPF6, reacted with dithioethers L′ [L′ = 2,6-bis(2-pyridylthiomethyl)pyridine (pytmp), 2,6-bis(2-pyrimidylthiomethyl)pyridine (prtmp) and 2,6-bis{2-(4-methyl)pyrimidylthiomethyl} pyridine (mprtmp)] to afford the compounds [Ru(N,N-L)2(N,S-L′)][PF6]2. The 1H NMR spectra indicate that L′ is chelated through S and N atoms with the formation of a four-membered ring. As a consequence, the ruthenium and sulfur atoms are stereogenic centers with ∆ and Λ and (R) and (S) configurations, respectively. NMR spectra, at low temperatures, show that two invertomers, of similar abundance, as enantiomeric couples ∆S, ΛR and ∆R, ΛS are present. In the methylene region, four AB systems are observed that in both the species contain two non-equivalent methylene groups. Variable-temperature NMR spectra and EXSY experiments show that the sulfur inversion produces an exchange between the invertomers. The one-dimensional band-shape analysis of the exchanging methylene signals showed that the energy barriers for the process are in the 43–52 kJ mol−1 range. The possible mechanisms of the sulfur inversion are discussed.

Electron Exchange Involving a Sulfur-Stabilized Ruthenium Radical Cation

Half-sandwich Ru(II) amine, thiol, and thiolate complexes were prepared and characterized by X-ray crystallography. The thiol and amine complexes react slowly with acetonitrile to give free thiol or amine and the acetonitrile complex. With the thiol complex, the reaction is dissociative. The thiolate complex has been oxidized to its Ru(III) radical cation and the solution EPR spectrum of that radical cation recorded. Cobaltocene reduces the thiol complex to the thiolate complex. The 1H and 31P NMR signals of the thiolate complex in acetonitrile become very broad whenever the thiolate and thiol complexes are present simultaneously. The line broadening is primarily due to electron exchange between the thiolate complex and its radical cation; the latter is generated by an unfavorable redox equilibrium between the thiol and thiolate complexes. Pyramidal inversion of sulfur in the thiol complex is fast at room temperature but slow at lower temperatures; major and minor conformers of the thiol complex were observed by 31P NMR at -98 degrees C in CD2Cl2.

Experimental and theoretical studies on inversion dynamics of dichloro( l-difluoromethionine- N, S)platinum(II) and dichloro( l-trifluoromethionine- N, S)platinum(II) complexes

Fluorinated analogues of methionine such as l- S-(difluoromethyl)homocysteine ( l-difluoromethionine; DFM) and l- S-(trifluoromethyl)homocysteine ( l-trifluoromethionine; TFM) have been demonstrated to be interesting analogues for incorporation into peptides and proteins. The presence of the fluorine nucleus adjacent to the sulfur atom in the side chain not only serves to alter the nucleophilicity and electron density of the sulfur atom but it can function as an important NMR spectroscopic ( 19F) probe. Additional information on the properties of these fluorinated amino acid analogues was obtained by studying their interactions with dipotassium tetrachloroplatinate (K 2PtCl 4). The resulting complexes, dichloro( l-difluoromethionine- N, S)platinum(II) and dichloro( l-trifluoromethionine- N, S)platinum(II) were investigated with respect to their sulfur inversion rates utilizing dynamic NMR methods. Inversion barriers for the DFM– and TFM–platinum complexes were experimentally determined to be 16.4 ± 0.2 and 18 ± 1 kcal/mol, respectively. Density functional calculations at the B3LYP/SDD level were also performed to model the structures and energies of the ground and transition states for these complexes.

Hemilabile Thioether Ligands Based on Pyrimidine and/or Pyridine Derivatives that Interconvert between N,S‐ and N‐Coordination in Congested Ruthenium(II) Complexes

AbstractThe thioether ligands L [L = 2‐pyridylmethyl 2'‐pyridyl sulfide (L1), 2‐pyridylmethyl 2'‐pyrimidyl sulfide (L2) and 2‐pyridylmethyl 2'‐(4‐methylpyrimidyl) sulfide (L3)] react with cis‐[Ru(N,N‐diimine)2Cl2] {diimine = 2,2'‐bipyridine (bipy), di‐2‐pyrimidyl sulfide (dprs), 2,2'‐bis(5‐ethylpyrimidyl) sulfide (5edprs)} to give compounds [Ru(N,N‐diimine)2L][PF6]2 {diimine = bipy, L =L1 (1), L2 (2), L3 (3); diimine = dprs, L = L1 (4); diimine = 5edprs, L = L1 (5); diimine = dprs, L = L2 (6), L = L3 (7); diimine = 5edprs, L = L2 (8), L = L3 (9)}. NMR investigations show that these potentially tridentate ligands act as N,S‐bidentate species, to form a five‐membered RuSCCN(Ru–N) ring, and in certain cases, as N‐monodentate species coordinated to the ruthenium through the 2‐pyridylmethyl group. The N,S‐chelated species contain chiral sulfur and ruthenium atoms with (R) and (S), and Δ and Λ configurations, respectively. Two invertomers and two sets of NMR signals in the slow‐exchange region are expected. However, the low‐temperature 1H NMR spectra show that sulfur inversion is fast. The variable‐temperature 1H NMR spectra allow two species to be observed (6a + 6b, 7a + 7b, 8a + 8b and 9a + 9b), which exhibit different abundances. In the minor species (6b, 7b, 8b, and 9b), L2 or L3 exhibits an N‐monodentate coordination, while in the major species, the usual N,S‐coordination. At low temperatures, the population ratio is about 85:15, while when the temperature increases, the abundance of the minor species grows rapidly. The one‐dimensional band‐shape analysis of the exchanging methylene proton signals shows that the energy‐barrier for the interchange process (ΔG#298) for 6a + 6b, 7a + 7b, 8a + 8b, and 9a + 9b is practically the same (ca. 59.5 kJ·mol–1), while the ΔS# values are negative or near to zero. The possible mechanisms for the process are discussed. The NMR spectroscopic findings strongly support the formation of an N,N‐chelated labile intermediate. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Cationic Iridium Complexes with Chiral Dithioether Ligands: Synthesis, Characterisation and Reactivity under Hydrogenation Conditions

AbstractA series of cationic IrI complexes containing chiral dithioether ligands have been prepared in order to study the influence of the sulfur substituents and the metallacycle size on the acetamidoacrylate hydrogenation reaction. In the case of complexes 6, 7 and 10, a mixture of diastereomers is observed in solution due to the sulfur inversion processes. In contrast, this fluxional behaviour is efficiently controlled by using bicyclic ligands which inhibit the S‐inversion in complexes 8 and 9. The solid‐state structure of complex 10b shows only one diastereomer with the sulfur substituents in a relative anti disposition and in an overall configuration of SCSCSSSS at the coordinated dithioether ligand. Iridium complexes containing seven‐ and six‐membered metallacycles (6b–d, 7b,c, 10a,b) react with the substrate through S‐ligand substitution, and the rate of this substitution is related to the position of the fluorine atom on the aromatic ring. On the contrary, complexes containing a bismetallacycle (8 and 9) are not displaced by the substrate. The catalytic hydrogenation activity of complexes 8 and 9 is analysed in terms of the high stability of the corresponding dihydride complexes (13 and 14). In both cases, only two of the four possible diastereomeric dihydride species are formed in solution. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Restricted Phenyl Rotation in Pyridyl Thioether Ligands N,S‐Chelated to Congested Diiminoruthenium Cores

AbstractThe thioethers 2‐pyridylmethyl p‐tolyl sulfide (a) and p‐chlorophenyl 2‐pyridylmethyl sulfide (b) react with the precursors cis‐[RuCl2(N,N‐diimine)2] [diimine = di‐2‐pyridyl sulfide (dps), 1; 2,2′‐bis(4‐methylpyridyl) sulfide (4mdps), 2; 2,2′‐bis(5‐methylpyridyl) sulfide (5mdps), 3; di‐2‐pyrimidyl sulfide (dprs), 4; and 2,2′‐bis(5‐ethylpyrimidyl) sulfide (5edprs), 5] in the presence of NH4PF6 to give the complexes [Ru(N,N‐diimine)2(N,S‐a)][PF6]2 and [Ru(N,N‐diimine)2(N,S‐b)][PF6]2, respectively. As a consequence of the N,S chelation all the complexes contain a five‐membered RuSCCN(Ru−N) ring, the sulfur and ruthenium atoms of which are stereogenic centres, with (R) and (S) and Δ and Λ configurations, respectively. Furthermore, the coordinated thioethers contain anisochronous methylene protons and phenyl protons which are sensor nuclei for pyramidal sulfur inversion and rotation of the pendant phenyl ring, respectively. In the low‐temperature 1H NMR spectra of the complexes a single AB system for the methylene protons, in agreement with a fast sulfur inversion, is observed. Well‐separated signals of the two ortho‐ as well as the meta‐phenyl protons, indicating restricted phenyl rotation, are also observed. At higher temperatures the fast exchange of the two halves of the phenyl ring leads to averaged signals for the phenyl protons. The rates and activation energies of this fluxional process were measured by one‐dimensional band‐shape analysis. Certain trends were immediately apparent. The rotation barrier values (ΔG#298) of the dps, 4mdps and 5mdps complexes were ca. 4.0 kJ·mol−1 lower than those of the dprs and 5edprs complexes. On the contrary, substitution of H4 or H5 dps protons with methyl groups and H5 dprs protons with ethyl groups, as well as substitution of a with b, leaves the rotation‐barrier values practically unchanged. This effect can be correlated to the steric hindrance produced by the two ligands cis to the rotating group. The crystal structure of the enantiomeric couple ΔS/ΛR of [Ru(dps)2(2‐pyridymethyl 2‐pyridyl sulfide)][PF6]2 (1c), in which the 2‐pyrimidylmethyl 2‐pyridyl sulfide (c) ligand acts in an N,S‐bidentate mode forming a five‐membered RuSCCN(Ru−N) chelate ring, is also reported. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Structural properties and inversion mechanisms of [Rh(dippe)(μ-SR)] 2 complexes

Several new bis-thiolate complexes of the type [Rh(dippe)(μ-SR)] 2 where R=H, methyl, cyclohexyl, o-biphenyl, and phenyl, or (SR) 2SCH 2CH 2CH 2S have been synthesized and characterized by NMR spectroscopy and single crystal X-ray diffraction. All [Rh(dippe)(μ-SR)] 2 complexes except [Rh(dippe)(μ-SPh)] 2 exhibit bent geometries, while the orientation of the thiolato substituents changes with increasing steric bulk. 1H and 31P NMR spectroscopies indicate that both ring inversion and sulfur inversion occur among the members of the series, which allows them to access several isomeric forms when they are in solution. 31P NMR spectroscopy indicates that sulfur inversion in [Rh(dippe)(μ-SH)] 2, [Rh(dippe)(μ-Sbiphenyl)] 2, and [Rh(dippe)(μ-SPh)] 2 is a non-dissociative process.

Kinetics and Mechanisms of Reactions of ReO(κ2-edt)(κ2-edtMe): Phosphane Displacement of the Thioether Group and Inversion of the Thioether Sulfur

Phosphanes (PZ3) convert ReO(κ2-edt)(κ2-edtMe) (1) into ReO(κ2-edt)(κ1-edtMe)(PZ3) (2)1,2 in equilibrium reactions that have been studied in benzene at 25.0 °C. The equilibrium constants and rate constants were evaluated by NMR spectroscopy and stopped-flow studies. The equilibrium constants were correlated by the Hammett equation, giving ρK = −1.59, which indicates an electronic effect on the equilibrium that is in agreement with reaction constants for the kinetics, ρfor = −0.07 and ρrev = 1.21, for forward and reverse reactions, respectively. The small reaction constant for the forward reaction and large value for the reverse reaction can be explained by proposing an early transition state of the substitution reaction. In other words, the Re−P bond is not substantially made at the point where the Re−SMe bond is broken to a considerable extent. The kinetics of inversion of the thioether sulfur was investigated by determining the temperature profile of the NMR spectra, from which ΔH⧧ = 24 ± 1 kJ mol-1. Fr...

Cyclometalated products of [(COE)(2)RhCl](2) and 1,3-(RSCH(2))(2)C(6)H(4) (R = (t)Bu, (i)Pr) Are Dimeric. Synthesis, molecular structures, and solution dynamics of [mu-ClRh(H)(RSCH(2))(2)C(6)H(3)-2,6](2).

Two tridentate thioether pincer ligands, 1,3-(RSCH(2))(2)C(6)H(4) (R = (t)()Bu, 1a; R = (i)()Pr, 1b) underwent cyclometalation using [(COE)(2)RhCl](2) in air/moisture-free benzene at room temperature. The resultant complexes, [mu-ClRh(H)(RSCH(2))(2)C(6)H(3)-2,6](2) (R = (t)Bu, 2a; R = (i)Pr, 2b) are dimeric both in the solid state and in solution. A battery of variable-temperature one- and two-dimensional (1)H NMR experiments showed conclusively that both complexes undergo dynamic exchange in solution. Exchange between two dimeric diastereomers of 2a in solution occurred via rotation about the Rh-C(ipso) bond. The dynamic exchange of 2b was significantly more complex as an additional exchange mechanism, sulfur inversion, occurred, which resulted in the exchange between several diastereomers in solution.

Evidence for a nonradical pathway in the photoracemization of aryl sulfoxides.

Photolysis of (R(S),S(C))-1-deuterio-2,2-dimethylpropyl p-tolyl sulfoxide provides mainly (S(S),S(C))-1-deuterio-2,2-dimethylpropyl p-tolyl sulfoxide at low conversion, though the other two stereoisomers are formed to smaller extents. Thus, the predominant process leading to sulfur inversion yields only sulfur inversion, without inversion of the adjacent CHD stereogenic center. This is taken as evidence for a mechanism for photochemical epimerization of sulfoxides that does not involve homolytic alpha-cleavage chemistry.

(±)-3,3-Diethoxycarbonyl-2- p-tolylsulfinylacrylonitrile as dienophile: relationship between the barrier for pyramidal inversion of vinyl sulfoxides and the electronic effects of the substituents at the double bond

The dienophilic behavior of (±)-3,3-diethoxycarbonyl-2- p-tolylsulfinylacrylonitrile ( 6 ) is reported and compared with that of trialkoxycarbonyl sulfinylethylene ( 3 ). The change of a CO 2R group by a CN one increases both the reactivity and the stereoselectivity of the cycloaddition. It seems that the linear structure of the CN group increases the planarity of the system reinforcing its electron-withdrawing power and therefore improving its dienophilic behavior. Otherwise, the low configurational stability at room temperature of the sulfoxide 6 , which is obtained as a racemic from its optically pure precursor, must also be a consequence of this strong electronic demand of the substituents which results in an unusually small energetic barrier for pyramidal inversion of the sulfinyl sulfur in compound 6 (Δ G ‡=16.3 kcal mol −1 at T c=25°C, determined by dynamic NMR spectroscopy). This is the first evidence of the existing relationship between racemization barrier of vinyl sulfoxides and the electron-withdrawing power of the groups joined to the double bond.

Substitution reactivity of the hydrido sulfido bridged dirhenium complex [Re2(µ-H)(µ-Snaph)(CO)8] (naph = 2-naphthyl)

The substitution reactivity of the hydrido sulfido bridged dirhenium complex [Re2(μ-H)(μ-Snaph)(CO)8] 1 (naph = 2-naphthyl) has been investigated. The former complex can be easily substituted with one equivalent of tmno (trimethylamine-N-oxide) followed by addition of a ligand L giving complexes of the general formula [Re2(μ-H)(μ-Snaph)(CO)7L] [L = NCMe (2a), NC(But) (2b), pyridine (2c), PH3 (2d), PMe3 (2e), H2P(p-C6H4OMe) (2f), PPh3 (2g), P(OMe)3 (2h)]. L is always perpendicularly co-ordinated to one of the Re atoms with respect to the Re–(μ-SR)–Re plane. In case of ligands with small cone angles two isomers are found. They differ in the relative orientation of L with respect to the substituent R attached to the sulfido bridge (syn and anti isomer). Temperature dependent 2D-EXSY NMR experiments with 2a dissolved in CDCl3 proved that both isomers are in a dynamic equilibrium, which is based on the pyramidal inversion of the bridging sulfur. The thermodynamic data of this process at 298.15 K were determined to be: ΔG‡ = 68.6 ± 1.2 kJ mol−1, ΔH‡ = 49.0 ± 4.2 kJ mol−1 and ΔS‡ = −66 ± 21 J K−1 mol−1. Owing to intramolecular sterical hindrance ligands with large cone angles give only the anti isomer. Disubstituted complexes [Re2(μ-H)(μ-Snaph)(CO)6L2] [L = NCMe (3a), pyridine (3b), PPh3 (3c)] and [Re2(μ-H)(μ-Snaph)(CO)6(μ-L–L)] [L–L = dppm (4a), dppe (4b)] were prepared in a similar way. The reaction of [Re2(μ-H)(μ-SH)(CO)8] 5 with one equivalent of tmno and PPh3 gave [Re2(μ-H)(μ-SH)(CO)6(PPh3)2] 6 in 38% yield. Compound 6 is rigid on the timescale of 31P NMR spectroscopy. The molecular structures of 2a, 2c, 2g, 2h, 3a, 4b and 6 were confirmed by X-ray structure analyses.

A Congested Ru(dps)2 or Ru(dprs)2 Core (dps = di-2-pyridyl sulfide; dprs = di-2-pyrimidinyl sulfide) Promotes Sulfur Inversion ofN,S-Chelate Thioethers Containing CH2R and 2-Pyridyl or 2-Pyrimidinyl Groups

The thioethers L [L = 4-methylbenzyl 2-pyridyl sulfide (L1), 4-chlorobenzyl 2-pyridyl sulfide (L2), 3-chlorobenzyl 2-pyridyl sulfide (L3), 1,4-bis(2-pyridylthiomethyl)benzene (L4), 4-methylbenzyl 2-pyrimidinyl sulfide (L5), and 4-chlorobenzyl 2-pyrimidinyl sulfide (L6)], containing a CH2R group bonded to the sulfur atom, were prepared and characterized. Compounds L1, L2, L3, and L4 reacted with cis-Ru(N,N-dprs)2Cl2 or cis-Ru(N,N-dps)2Cl2 (dprs = di-2-pyrimidinyl sulfide, dps = di-2-pyridyl sulfide) leading to the complexes [Ru(N,N-dprs)2(N,S-L)][PF6]2 and [Ru(N,N-dps)2(N,S-L)][PF6]2. Similar products were obtained from [Ru(N,N-dps)2(NO2)(NO)][PF6]2 and L5 or L6. As a consequence of the L ligand N,S-chelation, all the complexes contain the four-membered ring RuSCN(Ru−N). Since the ruthenium and sulfur atoms are stereogenic centres, with Δ and Λ, and R and S configurations, respectively, they led to four isomers, including the enantiomers. NMR investigations show that the sulfur inversion produces an exchange between the diastereoisomers ΔR and ΔS, as well as ΛS and ΛR. The one-dimensional band-shape analysis of the exchanging methylene proton signals showed that the inversion barriers (ΔG‡298 K) for the dprs complexes are in the 54.9−53.8 kJ mol−1 range, with the two invertomers exhibiting similar abundance. Substitution of dprs with dps affects the relative invertomer population leaving the magnitude of ΔG‡298 K (52.0−50.6 kJ mol−1) practically unchanged. Conversely, the substitution of the pyridine thioethers (L1, L2) with pyrimidine thioethers (L5, L6) influences the inversion barriers, and ΔG‡298 K values of 47.5 and 47.0 kJ mol−1 were found for [Ru(N,N-dps)2(N,S-L5)][PF6]2 and [Ru(N,N-dps)2(N,S-L6)][PF6]2, respectively. An intramolecular mechanism without any bond rupture is suggested on the basis of the ΔS‡ values (negative or close to zero) and the NMR spectra, temperature-reversible and concentration-independent. The contemporary presence of the congested Ru(N,N-dps)2 or Ru(N,N-dprs)2 core and sterically demanding N,S-coordinated thioether ligands is invoked to explain the low energy barrier of the process. This hypothesis is also corroborated by the different behaviour observed for the complex [Ru(bipy)2(N,S-L1)][PF6]2.