Stereoselective bis-functionalizations of arene chromium tricarbonyl complexes via brook rearrangements

[reaction: see text] An efficient method has been developed for the stereoselective bis-functionalization of arene chromium tricarbonyl complexes. Initial nucleophilic addition of organolithium reagents to a carbonyl moiety is followed by a 1,4-carbon to oxygen silyl migration (Brook rearrangement) and alkylation of the resultant aryl anion.     

Intra- and inter-ring haptotropic rearrangements in (naphthalene and anthracene)nickel complexes: a DFT study

Density functional quantum chemical calculations of the mechanisms of metallotropic η²,η²-intra- and η²,η²-inter-ring haptotropic rearrangements (HRs) in 16e zero-valent η²-(naphthalene and anthracene)nickel complexes involving migration of the organometallic group within the same ring or from one aromatic ring to the other were carried out. The structures of the initial complexes, transition states, and intermediates were determined. The intra-ring HRs in these systems proceed via low-energy η⁴-cis-butadiene transition states. The inter-ring HRs proceed along the periphery of the naphthalene and anthracene ligands via high-energy η³-allylic transition states. In contrast to well-investigated η⁶,η⁶-inter-ring HRs in (naphthalene and anthracene)tricarbonylchromium complexes, the activation barriers to the η²,η²-inter-ring HRs in the corresponding nickel complexes are much lower. Transition states of these processes are characterized by higher hapticity compared to the initial complexes. This also distinguishes the nickel complexes from the corresponding Cr(CO)₃ complexes for which the hapticities of transition states of the η⁶,η⁶-inter-ring HRs are lower than those of the initial complexes. The calculated activation barriers to the η²,η²-intra-ring HRs in the (naphthalene and anthracene)nickel complexes as well as the barrier to rotation of the organonickel group in the naphthalene complex are in good agreement with the experimental data. The calculated barriers to the η²,η²-inter-ring HRs in the naphthalene and anthracene complexes are 3–5 kcal mol⁻¹ lower than the experimental values. This is probably due to the competition between two mechanisms of this process, a low-energy intramolecular mechanism and a high-energy intermolecular dissociative mechanism.     

Ligand-induced haptotropic rearrangements in bis(indenyl)zirconium sandwich complexes

Addition of principally sigma-donating ligands such as THF, chelating diethers, or 1,2-bis(dimethyl)phosphinoethane to eta(9),eta(5)-bis(indenyl)zirconium sandwich complexes, (eta(9)-C(9)H(5)-1,3-R(2))(eta(5)-C(9)H(5)-1,3-R(2))Zr (R = alkyl or silyl), induces haptotropic rearrangement to afford (eta(6)-C(9)H(5)-1,3-R(2))(eta(5)-C(9)H(5)-1,3-R(2))ZrL adducts. Examples where L = THF and DME have been characterized by X-ray diffraction and revealed significant buckling of the eta(6) benzo ring, consistent with reduction of the arene, and highlight the importance of the zirconium(IV) canonical form. For the THF-induced haptotropic rearrangements, the thermodynamic driving force for ring migration has been measured as a function of indenyl substituent and demonstrates silylated sandwiches favor THF coordination and the eta(6),eta(5) bonding motif over their alkylated counterparts. In the case of chelating diethers, measurement of the corresponding equilibrium constants establish more stable eta(6),eta(5) adducts with five- over four-membered chelates and with smaller oxygen and carbon backbone substituents. Kinetic studies on both THF and DME addition to (eta(9)-C(9)H(5)-1,3-(SiMe(3))(2))(eta(5)-C(9)H(5)-1,3-(SiMe(3))(2))Zr established a first-order dependence on the incoming ligand, consistent with a mechanism involving direct attack of the incoming nucleophile on the eta(9),eta(5) sandwich. These results further highlight the ability of the indenyl ligand to smoothly adjust hapticity to meet the electronic requirements of the metal center.     

Intramolecular inter-ring haptotropic rearrangement in iridium naphthalene complexes: a DFT study

Quantum chemical simulation of the inter-ring haptotropic rearrangement (IHR) in iridium naphthalene complexes [η⁴-Ir(C₁₀H₈)L₂]⁺ (L = PH₃, PMe₃, PPh₃) involving migration of organometallic group from one of the aromatic ring to the other, was performed using the PBE density functional method with the TZV2p basis set for valence electrons and the relativistic SBK-JC pseudopotentials for the core electrons. The structures of the transition states and intermediates were studied. The IHR proceeds at the periphery of the naphthalene ligand. All transition states have reduced symmetry and hapticity compared with the initial complexes. The calculated thermodynamic parameters of the IHR are in agreement with the NMR data for the related iridium complex of ethylnaphthalene [η⁴-Ir(2-ethylnaphthalene)L₂]⁺A⁻, L = PPh₃, A = SbF₆, k ≈ 6·10⁻⁴ s⁻¹, ΔG ^≠ _{283 K} ≈ 21 kcal mol⁻¹).     

Transition metal complexes with heterocyclic analogs of fluorene. 1. Synthesis, structures, and haptotropic rearrangements of tricarbonylchromium complexes of dibenzothiophene

The complex formation of dibenzothiophene with chromium carbonyl complexes of the general formula L₃Cr(CO)₃ (L = Py, NH₃, or CO) afforded ⁶-C₁₂H₈SCr(CO)₃ (1). In the presence of tetramethylethylenediamine, complex 1 was selectively metallated with BuLi at position 4 of the coordinated ring to form ⁶-4-LiC₁₂H₈SCr(CO)₃ (2). In decane, the tricarbonylchromium group is reversibly and intramolecularly migrates from the unsubstituted to substituted ring due to the inter-ring haptotropic rearrangement (IRHR) at 130 °C for 100 h. In decane, the rate constant of IRHR was estimated experimentally by ¹H NMR spectroscopy and theoretically by the density functional theory (DFT).     

Tuning haptotropic rearrangements of arene-chromium complexes: steric and electronic effects of phosphorus coligands

(Eta6-arene)tricarbonylchromium 2 was synthesised by [3+2+1] benzannulation of the Fischer carbene complex 1 and converted to the thermodynamically more favorable regioisomer 3 by haptotropic metal migration. Photo-induced ligand-exchange reactions in both regioisomers with triphenylphosphine, triphenylphosphite, trimethylphosphine, and trimethylphosphite afforded dicarbonyl(phosphine or phosphite)arene complexes 4-11. The regioisomers were separated by high-performance liquid chromatography (HPLC), and kinetic analyses of the thermo-induced haptotropic metal shift were performed with regioisomers 4, 6, 8, and 10. The kinetic parameters were compared with those obtained for the parent tricarbonyl complex 2 and were discussed in terms of the steric and electronic properties of the phosphorus ligands by applying a quantitative analysis of ligand effects (QALE). The molecular structures of regioisomeric PPh3 and P(OPh)3 complexes 4/5 and 6/7 as well as of P(OMe)3 complex 10 have been established by single-crystal X-ray analysis.     

DFT study of chromium tricarbonyl complexes of coronene and kekulene

A quantum chemical study of the mechanism and determination of the activation barriers of intramolecular η⁶,η⁶-inner-ring haptotropic rearrangements (IHR), consisting in moving a chromium tricarbonyl group Cr(CO)₃ from one six-membered aromatic ring to another, are carried out using the density functional theory (DFT) for the respective η⁶-complexes of coronene I and kekulene II. The stationary states on the potential energy surface, determining the mechanism of η⁶,η⁶-IHR, have a lower hapticity, which is of interest for catalysis because of the possibility of coordinating an additional substrate and reagent around the transition metal during the rearrangement. The processes of η⁶,η⁶-IHR complexes I and II occur with similar energy barriers (ΔG^≠ ≈ 20–25 kcal/mol) that are lower than the barriers (ΔG^≠ ≈ 30 kcal/mol) of similar transformations previously calculated or measured for naphthalene complexes and a number of small polyaromatic hydrocarbons.     

Heterogeneous and homogeneous hydrogenation of styrene and stilbene chromium tricarbonyl complexes

Heterogeneous hydrogenation of the styrene and stilbene chromium tricarbonyl complexes by molecular hydrogen on skeletal nickel and palladium on carbon as catalysts was studied. As compared to styrene and stilbene, their arene chromium tricarbonyl analogs are hydrogenated considerably more slowly, which is related, most likely, to strong adsorption of the π-complexes on the catalyst surface. For the homogeneous hydrogenation of these complexes using a H₂PtCl₆-SnCl₂-LiBr system, styrene and η⁶-styrene chromium tricarbonyl are reduced with a high rate, whereas stilbene and its chromium tricarbonyl complex are hydrogenated very slowly. A possibility of reduction of the unsaturated arene chromium tricarbonyl complexes by sodium borohydride in the presence of cobalt(II) chloride as a catalyst was shown. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 44–47, January, 2007.     

Regioselective lithiation of II-(arene)chromium tricarbonyl complexes

Chromium tricarbonyl complexes of 3-methoxybenzyl alcohol and related compounds were selectively lithiated at the 4-position in contrast to the corresponding metal free arene compounds. The resulting 4-lithio complexes were converted to the 4-substituted arene compounds in moderate to excellent yields through the reactions with proper electrophiles and subsequent demetalation.     

Selective functionalization of crown ethers via arene chromium tricarbonyl complexes

The crown ethers dibenzo-16-crown-4 and dibenzo-18-crown-5 and a diaryl polyether were complexed by the chromium tricarbonyl group for the purpose of selective functionalization. This complexation did indeed permit exclusive functionalization of the complexed ring. CHO and CH₂OH functionalities were introduced ortho to the ether group. It was noted that the nature of the two ether chains had a strong influence on the regioselectivity of the functionalization, which occurred preferentially on the side with the polyether chain. Photochemical decomplexation produced functionalized organic crown ethers.     

Controlled haptotropic rearrangements--towards a stereospecific molecular switch based on chiral arene chromium complexes

A stereospecific molecular switch has been designed based on a reversible thermo- or photo-induced haptotropic shift of a Cr(CO)3 fragment along a naphthohydroquinone skeleton.     

A quantum chemical study of the haptotropic rearrangements of Cr(CO)(3) on naphthalene and phenanthrene systems

First-principles gradient-corrected density functional theory electronic structure calculations of the haptotropic rearrangement of a Cr(CO)(3) unit on naphthalene and phenanthrene derivatives are reported. Coupled-cluster calibration studies of Cr(CO)(3) complexes with benzene and naphthalene derivatives confirm the accuracy of the applied Becke exchange and Perdew correlation functionals. Characteristic points on the energy hypersurface (reactants, products, intermediates, and transition states) were located for various substituents on the aromatic skeleton. It is argued that a -OH/-O(-) substituent may provide a means to steer the haptotropic shift depending on the pH value, i.e., to construct a molecular switch. In addition, the mechanism of the [3 + 2 + 1] benzannulation of chromium pentacarbonyl naphthylcarbene complexes with alkynes was investigated, and the preference of the angular benzannulation leading to phenanthrene complexes of Cr(CO)(3) over the linear benzannulation leading to corresponding anthracene complexes is explained.     

Reversible haptotropic shift in zirconocene-hexapentaene complexes

Low-valent zirconocene species reacted with 1,1,6,6-tetraalkyl-1,2,3,4,5-hexapentaenes to form both 1-zirconacyclopent-3-ynes and eta2-pi-coordinated complexes according to the alkyl groups (R) and the existence of the neutral ligand (L). Haptotropic interconvertion between these two complexes was observed. It was proposed that double insertion of isocyanide into the Zr-C bond of 1-zirconacyclopent-3-ynes takes place via the isocyanide adduct of the eta2-pi-complex as an intermediate.     

Recovery of chromium hexacarbonyl after nucleophilic reactions of arenechromium tricarbonyl complexes

The arenechromium tricarbonyl complexes C₆H₄R₂Cr(CO)₃, (R = H, CHMe₂), obtained from arenes C₆H₄R₂ and Cr(CO)₆, react with nucleophiles (Nu = CH(CH₃)CN, CH(CH₃)CO₂-t-Bu), to form adducts which give, after treatment with CF₃CO₂H under carbon monoxide, the substituted cyclohexadienes C₆H₅R₂Nu (R = H, CHMe₂) and Cr(CO)₆ in good yield.