Rhodium dinaphthocyclooctatetraene complexes: synthesis, characterization and catalytic activity in [5+2] cycloadditions

Rh COT in the act: a Ni(0)-catalyzed [2+2+2+2] cycloaddition provides a high-yielding, scalable synthesis of the ligand dinaphtho[a,e]cyclooctatetraene (dnCOT). dnCOT complexation with Rh(I) gives [Rh(dnCOT)(MeCN)(2)]SbF(6), an excellent catalyst for [5+2] cycloadditions of vinylcyclopropanes and π-systems with impressive functional group compatibility.     

Enantioselective cyclization/hydrosilylation of 1,6-enynes catalyzed by a cationic rhodium bis(phosphine) complex

[reaction: see text] Reaction of 4,4-dicarbomethoxy-1-octene-6-yne (1) with triethylsilane and a catalytic 1:1 mixture of [Rh(COD)(2)](+) SbF(6)(-) and (R)-BIPHEMP (5 mol %) at 70 degrees C for 90 min gave (Z)-1,1-dicarbomethoxy-3-(1-triethylsilyl)ethylidene-4-methylcyclopentane (2) in 81% isolated yield with 98% de and 92% ee.     

Asymmetric catalysis of the [5 + 2] cycloaddition reaction of vinylcyclopropanes and pi-systems

As part of our studies of metal-catalyzed [m + n (+...o)] cycloadditions, we have previously reported the rhodium-catalyzed [5 + 2] cycloaddition of vinylcyclopropanes (VCPs) and pi-systems. These studies have led to Rh(I) complexes that catalyze these reactions in minutes at room temperature or in water without organic solvents. We describe a comparative evaluation of several chiral catalysts for the [5 + 2] reaction, evaluation of a preferred catalyst, [((R)-BINAP)Rh]+SbF6-, with substrates differing in substitution and tether types-producing enantiomeric excesses >/=95% for several systems. A predictive model for the selectivity is also presented.     

Rhodium-catalyzed asymmetric cyclization/hydroboration of 1,6-enynes

[reaction: see text] Reaction of enyne 1 with catecholborane catalyzed by a 1:1 mixture of [Rh(COD)(2)](+)SbF(6)(-) and (S)-BINAP (5 mol %) followed by Pd-catalyzed arylation with p-IC(6)H(4)CF(3) gave benzylidenecyclopentane 5 in 65% yield with 88% ee. Rhodium-catalyzed asymmetric cyclization/hydroboration followed either by Pd-catalyzed arylation or by oxidation was applied to the synthesis of a number of chiral, nonracemic carbocycles and heterocycles.     

A combined experimental and computational study of dihydrido(phosphinooxazoline)iridium complexes

The reaction of a [(PHOX)Ir(COD)](+) complex (COD = 1,5-cyclooctadiene) with dihydrogen was studied by NMR spectroscopy (PHOX = chiral phosphinooxazoline ligand). A single [(PHOX)Ir(H)(2)(COD)](+) isomer was formed as the primary product at -40 degrees C in THF. Subsequent reaction with H(2) at -40 to 0 degrees C led to a mixture of two diastereomeric [(PHOX)Ir(H)(2)(solvent)(2)](+) complexes with concomitant loss of cyclooctane. The stereochemistry of the three hydride complexes could be assigned from the NMR data. The structures and energies of the observed hydride complexes and the possible stereoisomers were calculated using density functional theory. The substantial energy differences (up to 39 kcal/mol) between the various stereoisomers demonstrate the strong influence of the chiral ligand. The observed stereoselective formation of dihydride complexes can be explained by steric effects of the PHOX ligand combined with a strong electronic influence of the coordinating N and P atoms, favoring addition of a hydride trans to the Ir-N bond.     

Structural studies on the stepwise chelating processes of bidentate 2-(aminomethyl)pyridine and tridentate bis(2-pyridylmethyl)amine toward an acetate-bridged dirhodium(II) center

Several intermediates and final products of the reactions of [Rh(2)(mu-CH(3)COO)(4)(CH(3)OH)(2)] with a tridentate ligand bis(2-pyridylmethyl)amine (bpa) and bidentate 2-(aminomethyl)pyridine (amp) have been isolated, and the chelation processes of these ligands to the dirhodium(II) center are discussed. The reaction of a 2 equiv amount of bpa in chloroform afforded three products, [Rh(2)(mu-CH(3)COO)(2)(eta(1)-CH(3)COO)(bpa)(2)](+) ([1]+), C(2)-[Rh(2)(mu-CH(3)COO)(2)(bpa)(2)](2+) ([2a](2+)), and C(s)-[Rh(2)(mu-CH(3)COO)(2)(bpa)(2)](2+) ([2b](2+)), where C(2) and C(s) denote the molecular symmetry of the two geometrical isomers. X-ray crystallography revealed that [1](+) contains ax-eq chelated bidentate and ax-eq-eq tridentate bpa and that [2a](2+) and [2b](2+) have two ax-eq-eq tridentate bpa ligands (ax denotes the site trans to the Rh-Rh bond, and eq, the site perpendicular to it). The reaction is initiated by almost instantaneous monodentate or inter-Rh(2)-unit bridging coordination of bpa at the ax sites, which is followed by very slow ax-eq chelate formation and then ultimate ax-eq-eq tridentate coordination. The reaction of [Rh(2)(mu-CH(3)COO)(4)(CH(3)OH)(2)] with amp in 1:2 ratio in chloroform initially gives an insoluble polymer in which amp interconnects the ax sites of the dirhodium(II) units. Further reactions afforded [Rh(2)(mu-CH(3)COO)(2)(eta(1)-CH(3)COO)(amp)(2)](+) ([4](+)) and [Rh(2)(mu-CH(3)COO)(2)(amp)(2)](2+) ([5](2)(+)). The X-ray structural studies show that [4](+) has one ax-eq and one eq-eq chelate and [5](2)(+) two eq-eq chelates. More rigid tridentate ligands 2,2':6',2"-terpyridine (tpy) and 4'-chloro-2,2':6',2"-terpyridine (Cl-tpy) have been introduced at ax sites in a monodentate mode ([Rh(2)(mu-CH(3)COO)(4)(tpy)(2)] (8) and [Rh(2)(mu-CH(3)COO)(4)(Cl-tpy)(2)] (9)). While the Rh-Rh distances of these complexes and [Rh(2)(mu-CH(3)COO)(2)(2,2'-bipyridine)(2)(py)(2)](2+) ([7](2)(+)) are practically unchanged (2.56-2.60 A) except for 8 and 9 (2.4 A), the Rh-N(ax) distances range from 2.11 to 2.35 A. Relatively short distances are found for the compounds with ax-eq or ax-eq-eq chelates (<2.22 A). Longest distances (2.32-2.35 A) found for 8 and 9 may be due to the steric effect. The distances of other complexes fall in the normal region. The visible band of the pi*(Rh-Rh) --> sigma*(Rh-Rh) transition in solid-state reflectance spectra shows a red-shift as the Rh[bond]N(ax) distances becomes longer.     

A study of [Co2(alkyne)(binap)(CO)4] complexes (BINAP=(1,1'-binaphthalene)-2,2'-diylbis(diphenylphosphine))

Understanding the interaction of chiral ligands, alkynes, and alkenes with cobaltcarbonyl sources is critical to learning more about the mechanism of the catalytic, asymmetric Pauson-Khand reaction. We have successfully characterized complexes of the type [Co2(alkyne)(binap)(CO)4] (BINAP=(1,1'-binaphthalene)-2,2'-diylbis(diphenylphosphine)) and shown that diastereomer interconversion occurs under Pauson-Khand reaction conditions when alkyne=HC[triple bond]CCO2Me. Attempts to isolate [Co2(alkyne)(binap)(CO)x] complexes with coordinated alkenes led to the formation of cobaltacyclopentadiene species.     

Supramolecular ligand-ligand and ligand-substrate interactions for highly selective transition metal catalysis

The use of non covalent supramolecular ligand-ligand and ligand-substrate interactions in transition metal-catalysed transformations is a new, rapidly emerging area of research. Non-covalent interactions between monodentate ligands such as hydrogen bonding, coordinative bonding, ion pairing, π-π interactions and the formation of inclusion compounds, have been shown to impart higher activity and chemo-, regio-, and stereoselectivity to the corresponding transition metal complexes in a number of catalytic applications. Analogously, supramolecular ligand-substrate interactions, and particularly hydrogen bonding, have been used to direct the regio- and stereochemistry of several metal-catalysed reactions. The catalytic systems relying on supramolecular interactions are generally capable of self-assembling from simpler components in the environment where catalysis is to take place, and are therefore very well-suited for combinatorial catalyst discovery strategies and high-throughput screening.     

Isolable tris(alkyne) and bis(alkyne) complexes of gold(I)

Golden trefoils: Tris(alkyne)gold complex [(coct)(3)Au][SbF(6)] (see picture; 1-SbF(6)) can be synthesized from cyclooctyne (coct) and AuSbF(6) generated in situ. Treatment of AuCl with cyclooctyne led to the bis(alkyne)gold complex [Au(coct)(2)Cl] (2). DFT analysis indicates that the cyclooctyne ligands are net electron donors in 1 but overall electron acceptors in 2. AuSbF(6) is shown to mediate [2+2+2] cycloaddition reactions of alkynes.     

Dirhodium(II) tetra(N-(dodecylbenzenesulfonyl)prolinate) catalyzed enantioselective cyclopropenation of alkynes

Dirhodium tetrakis((S)-N-(dodecylbenzenesulfonyl)prolinate) (Rh(2)(S-DOSP)(4)) is an effective chiral catalyst for the enantioselective cyclopropenation of alkynes by methyl aryldiazoacetates. [reaction: see text]     

Isolation of [Rh(diolefin)X2]- species and their reactions with P- or N-donor ligands,tin(II) halides and carbon monoxide

Anionic [Rh(diolefin)X₂]⁻ species (X = Cl, Br) have been prepared and their reactions studied. The reactions with monodentate ligands led to neutral tetracoordinated complexes, and with N-donor bidentate ligands (Rh : LL = 2 : 1) gave Rh(X)(diolefin)(LL), [Rh(diolefin)(LL)]⁺[Rh(diolefin)X₂]⁻, or [Rh(diolefin)(LL)]X compounds, depending on the nature of LL or X. Reactions with carbon monoxide involved diolefin displacement. A trichlorostannato complex was obtained from the [Rh(COD)Cl₂]⁻ species. Reactions of [Rh(COD)Br]₂ with bidentate N-donor ligands were also studied.     

A pyrimidine thiolate Rh(I) complex: structure, bonding and one-dimensional interactions in solid and in solution

The reaction of [Rh(micro-Cl)(COD)]2 with 4,6-dimethyl-pyrimidinethiolate (Me2-pymt) and subsequent substitution of COD by CO yields [Rh(Me2-pymt)(CO)2]. The stacking pattern found in this compound is in contradiction with previously studied comparable square-planar complexes of type d8-[M(chelate)(monodentate)2] in which each ligand has different pi-acidic character. A theoretical study of the intermolecular interactions and conformation of the title compound has been carried out, combining semi-empirical band calculations on the real chains and ab initio(MP2 level) calculations on a model dimer. The combination of electronic and steric effects determines the rotation of the successive monomers and the deviation from linearity of the one-dimensional stacks. Its behaviour in solution is also special, developing a blue colour and forming micelles, when adding water to acetone solutions.     

Molecular Orbital Calculation and Spectroscopic Study of the Photochemical Generation of Bis(2,2'-bipyridine)rhodium(I) from Bis(2,2'-bipyridine)(oxalato)rhodium(III)

A planar complex, [Rh(bpy)(2)](+) (bpy = 2,2'-bipyridine), was obtained from [Rh(ox)(bpy)(2)](+) (ox = oxalato) by photoirradiation. A rate constant k for the photoreaction was evaluated as 1 x 10(8) s(-1) in simple first-order kinetics, whereas a ligand dissociation, a reorganization of the coordinated bpy, and a two-electron transfer were involved in the reaction. The process of the Rh(I) complex generation was investigated in terms of a discrete variational (DV)-Xalpha molecular orbital calculation on [Rh(ox)(HN=CHCH=NH)(2)](+) instead of [Rh(ox)(bpy)(2)](+). From the calculation, using the transition-state method, it was predicted that a transition of the ox pi orbital to the metal 4d(z)()2 orbital caused the ligand dissociation and the reorganization of the coordinated bpy occurred in the ox pi to Rh 4d(x)()2(-y)2 excited state stabilized by the ox elimination. Upon release of the ligand and a change from octahedral to square-planar geometry, the electron density on the metal increased and the Rh 4d orbital acquired a d(8) electronic configuration.     

Anion-templated self-assembly of highly stable Fe(II) pentagonal metallacycles with short anion-π contacts

The crystal structures of the self-assembled metallapentacycles [{Fe(5)(bptz)(5)(CH(3)CN)(10)} ? 2SbF(6)][SbF(6)](8) (1) and [{Fe(5)(bmtz)(5)(CH(3)CN)(10)} ? SbF(6)][SbF(6)](9) (2) with the π-acidic ligands bptz (3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) and bmtz (3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine), respectively, revealed cationic pentagons templated by [SbF(6)](-) ions. The short anion-π contacts established between the anions and the tetrazine rings play an important role in the stability of the pentagons.     

Theoretical study of rhodium(III)-catalyzed hydrogenation of carbon dioxide into formic acid. Significant differences in reactivity among rhodium(III), rhodium(I), and ruthenium(II) complexes

The title reaction was theoretically investigated, where cis-[RhH(2)(PH(3))(3)](+) and cis-[RhH(2)(PH(3))(2)(H(2)O)](+) were adopted as models of the catalyst. The first step of the catalytic cycle is the CO(2) insertion into the Rh(III)-H bond, of which the activation barrier (E(a)) is 47.2 and 28.4 kcal/mol in cis-[RhH(2)(PH(3))(3)](+) and cis-[RhH(2)(PH(3))(2)(H(2)O)](+), respectively, where DFT(B3LYP)-calculated E(a) values (kcal/mol unit) are given hereafter. These results indicate that an active species is not cis-[RhH(2)(PH(3))(3)](+) but cis-[RhH(2)(PH(3))(2)(H(2)O)](+). After the CO(2) insertion, two reaction courses are possible. In one course, the reaction proceeds through isomerization (E(a) = 2.8) of [RhH(eta(1)- OCOH)(PH(3))(2)(H(2)O)(2)](+), five-centered H-OCOH reductive elimination (E(a) = 2.7), and oxidative addition of H(2) to [Rh(PH(3))(2)(H(2)O)(2)](+) (E(a) = 5.8). In the other one, the reaction proceeds through isomerization of [RhH(eta(1)-OCOH)(PH(3))(2)(H(2)O)(H(2))](+) (E(a) = 5.9) and six-centered sigma-bond metathesis of [RhH(eta(1)-OCOH)(PH(3))(2)(H(2)O)](+) with H(2) (no barrier). RhH(PH(3))(2)-catalyzed hydrogenation of CO(2) proceeds through CO(2) insertion (E(a) = 1.6) and either the isomerization of Rh(eta(1)-OCOH)(PH(3))(2)(H(2)) (E(a) = 6.1) followed by the six-centered sigma-bond metathesis (E(a) = 0.3) or H(2) oxidative addition to Rh(eta(1)-OCOH)(PH(3))(2) (E(a) = 7.3) followed by isomerization of RhH(2)(eta(1)-OCOH)(PH(3))(2) (E(a) = 6.2) and the five-centered H-OCOH reductive elimination (E(a) = 1.9). From these results and our previous results of RuH(2)(PH(3))(4)-catalyzed hydrogenation of CO(2) (J. Am. Chem. Soc. 2000, 122, 3867), detailed discussion is presented concerning differences among Rh(III), Rh(I), and Ru(II) complexes.     

Assessing stereoelectronic effects in dipolar cycloadditions yielding fused thiazolopyridone rings

We report a combined experimental and computational study on the cycloadditions of bicyclic 1,3-thiazolium-4-olates, derived from thiazolidin-2-thiones, with asymmetrically-substituted acetylenes. These results provide further mechanistic insights into the above dipolar cycloadditions and enable an unequivocal characterization by NMR spectroscopy of regiochemical patterns as previous derivatives had substituents at both C-2 (in the dipole) and C-6 (in products). Accordingly, new dihydrothiazolopyrid-2-ones have been obtained from a thioisomünchnone lacking substitution at C-2. With the aim of assessing the steric hindrance as well as the facial stereoselection induced by a bulky group on the Si face (relative to C-7a) of the mesoionic heterocycle, a chiral thioisomünchnone has also been obtained along with the resulting optically active thiazolopyridones. A computational study of these particular cycloadditions, largely based on an NBO analysis, allowed us to evaluate the influence of substituents on intermolecular steric repulsions, charge transfers, as well as solvent effects.     

Cationic carbonyl complexes of rhodium(I) and rhodium(III): syntheses,vibrational spectra,NMR studies,and molecular structures of tetrakis(carbonyl)rhodium(I) heptachlorodialuminate and -gallate, [Rh(CO)4][Al2Cl7] and [Rh(CO)4][Ga2Cl7

Dimeric rhodium(I) bis(carbonyl) chloride, [Rh(CO)(2)(mu-Cl)](2), is found to be a useful and convenient starting material for the syntheses of new cationic carbonyl complexes of both rhodium(I) and rhodium(III). Its reaction with the Lewis acids AlCl(3) or GaCl(3) produces in a CO atmosphere at room temperature the salts [Rh(CO)(4)][M(2)Cl(7)] (M = Al, Ga), which are characterized by Raman spectroscopy and single-crystal X-ray diffraction. Crystal data for [Rh(CO)(4)][Al(2)Cl(7)]: triclinic, space group Ponemacr; (No. 2); a = 9.705(3), b = 9.800(2), c = 10.268(2) A; alpha = 76.52(2), beta = 76.05(2), gamma = 66.15(2) degrees; V = 856.7(5) A(3); Z = 2; T = 293 K; R(1) [I > 2sigma(I)] = 0.0524, wR(2) = 0.1586. Crystal data for [Rh(CO)(4)][Ga(2)Cl(7)]: triclinic, space group Ponemacr; (No. 2); a = 9.649(1), b = 9.624(1), c = 10.133(1) A; alpha = 77.38(1), beta = 76.13(1), gamma = 65.61(1) degrees; V = 824.4(2) A(3); Z = 2; T = 143 K; R(1) [I > 2sigma(I)] = 0.0358, wR(2) = 0.0792. Structural parameters for the square planar cation [Rh(CO)(4)](+) are compared to those of isoelectronic [Pd(CO)(4)](2+) and of [Pt(CO)(4)](2+). Dissolution of [Rh(CO)(2)Cl](2) in HSO(3)F in a CO atmosphere allows formation of [Rh(CO)(4)](+)((solv)). Oxidation of [Rh(CO)(2)Cl](2) by S(2)O(6)F(2) in HSO(3)F results in the formation of ClOSO(2)F and two seemingly oligomeric Rh(III) carbonyl fluorosulfato intermediates, which are easily reduced by CO addition to [Rh(CO)(4)](+)((solv)). Controlled oxidation of this solution with S(2)O(6)F(2) produces fac-Rh(CO)(3)(SO(3)F)(3) in about 95% yield. This Rh(III) complex can be reduced by CO at 25 degrees C in anhydrous HF to give [Rh(CO)(4)](+)((solv)); addition of SbF(5) at -40 degrees C to the resulting solution allows isolation of [Rh(CO)(4)][Sb(2)F(11)], which is found to have a highly symmetrical (D(4)(h)()) [Sb(2)F(11)](-) anion. Oxidation of [Rh(CO)(2)Cl](2) in anhydrous HF by F(2), followed in a second step by carbonylation in the presence of SbF(5), is found to be a simple, straightforward route to pure [Rh(CO)(5)Cl][Sb(2)F(11)](2), which has previously been structurally characterized by us. All new complexes are characterized by vibrational and NMR spectroscopy. Assignment of the vibrational spectra and interpretation of the structural data are supported by DFT calculations.     

First X-ray characterization and theoretical study of pi-alkyne,alkynyl-hydride,and vinylidene isomers for the same transition metal fragment [Cp*Ru(PEt3)2]+

The reaction of the chloro-complex [CpRuCl(PEt(3))(2)] with acetylene gas in methanol gave the pi-alkyne complex [CpRu(eta(2)-HCtbd1;CH)(PEt(3))(2)][BPh(4)] (1), which has been structurally characterized by X-ray analysis. The alkyne complex undergoes spontaneous isomerization even at low temperature, yielding the metastable alkynyl-hydride complex [CpRu(H)(Ctbd1;CH)(PEt(3))(2)][BPh(4)] (2), as the result of the oxidative addition of the alkyne C-H bond. This compound has also been structurally characterized despite it tautomerizes spontaneously into the stable primary vinylidene [CpRu(=C=CH(2))(PEt(3))(2)][BPh(4)] (3). This species has been alternatively prepared by a two-step deprotonation/protonation synthesis from the pi-alkyne complex. Moreover, the reaction of the initial chloro-complex with monosubstituted alkynes HCtbd1;CR (R = SiMe(3), Ph, COOMe, (t)Bu) has been studied without detection of pi-alkyne intermediates. Instead of this, alkynyl-hydride complexes were obtained in good yields. They also rearrange to the corresponding substituted vinylidenes. In the case of R = SiMe(3), the isomerization takes place followed by desilylation, yielding the primary vinylidene complex. X-ray crystal structures of the vinylidene complexes [CpRu(=C=CH(2))(PEt(3))(2)][BPh(4)] (3) and [CpRu(=C=CHCOOMe)(PEt(3))(2)][BPh(4)] (10) have also been determined. Both, full ab initio and quantum mechanics/molecular mechanics (QM/MM) calculations were carried out, respectively, on the model system [CpRu(C(2)H(2))(PH(3))(2)](+) (A) and the real complex [CpRu(C(2)H(2))(PEt(3))(2)](+) (B) to analyze the steric and electronic influence of ligands on the structures and relative energies of the three C(2)H(2) isomers. QM/MM calculations have been employed to evaluate the role of the steric effects of real ligands, whereas full ab initio energy calculations on the optimized QM/MM model have allowed recovering the electronic effects of ligands. Additional pure quantum mechanics calculations on [CpRu(C(2)H(2))(PH(3))(2)](+) (C) and [CpRu(C(2)H(2))(PMe(3))(2)](+) (D) model systems have been performed to analyze in more detail the effects of different ligands. Calculations have shown that the steric effects induced by the presence of bulky substituents in phosphine ligand are responsible for experimentally observed alkyne distortion and for relative destabilization of the alkyne isomer. Moreover, increasing the phosphine basicity and sigma donor capabilities of ligands causes a relative stabilization of an alkynyl-hydride isomer. The combination of both steric and electronic effects, makes alkyne and alkynyl-hydride isomers to be close in energy, leading to the isolation of both complexes.     

Synthesis and computational analysis of densely functionalized triazoles using o-nitrophenylalkynes

Dipolar cylcoadditions with azides using a series of o-nitrophenylethynes and disubstituted alkynes were studied experimentally and computationally. Density functional theory computations reveal the steric and electronic parameters that control the regioselectivity of these cycloadditions. Several new substrates were predicted that would either give enhanced regiocontrol or invert the regiochemical preference. Experimentally, the alkynes were screened in the [3 + 2] cycloaddition with benzyl azide. Of the 11 alkynes screened experimentally, the acetylenes containing halogen substitution directly on the alkyne provided the highest levels of regioselectivity. These haloalkynes were also shown to tolerate variation of the azide moiety with continued good levels of regioselectivity in most cases. Diverse functional groups can be incorporated through the cycloaddition process and their subsequent orthogonal modification was demonstrated.     

Ru-catalyzed highly enantioselective hydrogenation of beta-alkyl-substituted beta-(acylamino)acrylates

Highly enantioselective hydrogenation of beta-alkyl-substituted (E)-beta-(acylamino)-acrylates catalyzed by Ru((R)-Xyl-P-Phos)(C(6)H(6))Cl(2) complex (cat. 1c) was achieved in up to 99.7% ee. Moderate to good enantioselectivities in the hydrogenation of corresponding (Z)-isomers in the presence of [Rh((R)-Xyl-P-Phos)(COD)]BF(4) (cat. 2c) were also obtained. The results demonstrated that the electronic and steric properties of the dipyridylphosphine ligands as well as the different transition metal ions have significant influences on the catalytic properties in the hydrogenation of beta-(acylamino)acrylates.