2 + 2] cross coupling of benzene and tropylium ligands in reductively activated piano stool complexes of Mn,Cr, and W

We have established cation/anion coupling reactions between the tropylium ligand in [M(eta7-C7H7)(CO)3]+ (M = Cr, W) and the reductively activated eta4-benzene ligand in [Mn(eta4-C6H6)(CO)3]- (3-) to form [M(CO)3(mu2-eta6:eta5-C7H7-C6H6)Mn(CO)3]; [Cr(CO)3(mu2-eta6:eta5-C7H7-C6H6)Mn(CO)3] can be further reduced to [Cr(CO)3(mu2-eta5:eta4-C7H7-C6H6)Mn(CO)3]2-, in which the tropylium and benzene ligands have undergone a [2 + 2] cross coupling reaction.     

Mechanistic Study of the Rhodium‐Catalyzed [3+2+2] Carbocyclization of Alkenylidenecyclopropanes with Alkynes

A systematic theoretical study has been performed on the recently reported Rh^I‐catalyzed [3+2+2] carbocyclization reactions between alkenylidenecyclopropanes (ACPs) and alkynes. With the aid of theoretical calculations, two possible mechanisms, that is, alkene‐carbometalation‐first and alkyne‐carbometalation‐first mechanisms, are examined in this study. In the oxidative addition step, the possibility of reaction on either the distal or proximal C? C bond of the cyclopropane group has been evaluated. The calculations indicate that the alkene‐activation‐first mechanism is more favored for the overall catalytic cycle. This mechanism involves four steps, that is, oxidative addition of the distal (rather than the proximal) C? C bond of cyclopropane group, alkene carbometalation, alkyne carbometalation, and reductive elimination. The rate‐determining step in the overall catalytic cycle is the carbometalation of the alkyne (i.e., the alkyne‐insertion step) and this step also determines the regioselectivity. Finally, the origin of the regioselectivity is determined by the steric effect (i.e., the steric crowding between the electron‐withdrawing group on alkyne and other ligands on the rhodium center) in the alkyne‐insertion step.     

The enantiomeric scaffold approach to highly functionalized 1-oxadecalines: enantio- and regiocontrolled [4 + 2] cycloadditions of 5-alkenyl-eta3-pyranylmolybdenum complexes

TpMo(CO)2(5-alkenyl-eta-2,3,4-pyranyl) diene complexes function as excellent chiral scaffolds for the efficient regio- and enantiocontrolled synthesis of highly functionalized 1-oxadecaline derivatives through a novel transition metal-mediated Diels-Alder reaction. Very good to excellent yields and excellent levels of endo selectivity are obtained, and the reaction gives products with complete retention of enantiomeric purity when carried out with chiral, nonracemic scaffolds. A subtle structural modification on the diene (replacement of an H by a trans-CH3 group) leads to a complete change of regiochemistry, which is discussed from a mechanistic point of view. The role of the eta3-coordinated TpMo(CO)2 moiety is also critical to the further functionalization of the [4 + 2] cycloadducts, as illustrated by the preparation of 20 variously functionalized 1-oxadecaline derivatives (>98% ee when carried out with high enantiopurity scaffolds).     

A theoretical study on the complete catalytic cycle of the hetero-Pauson-Khand-type [2+2+1] cycloaddition reaction of ketimines,carbon monoxide and ethylene catalyzed by iron carbonyl complexes

The [2+2+1] cycloaddition reaction of 1,4-diazabutadienes, carbon monoxide and ethylene catalyzed by iron carbonyl complexes produces pyrrolidin-2-one derivatives. Only one of the two imine moieties is activated during the catalysis. The mechanism of this cycloaddition reaction is studied by density functional theory at the B3LYP/6-311++G(d,p) level of theory. In accordance with experimental results, a [(diazabutadiene)Fe(CO)(3)] complex of square-pyramidal geometry is used as the starting compound S of the catalytic cycle. Based on experimental experience, the reaction with ethylene is considered to take place before any interaction with carbon monoxide. According to the computational results, the reaction does not proceed by ligand dissociation followed by addition of ethylene and subsequent intramolecular activation steps but by the approach of an ethylene molecule from the base of the square-pyramidal complex. This reaction yields an intermediate I(4) in which ethylene is coordinated to the iron centre and a new C-C bond between ethylene and one of the imine groups is formed. The insertion of a terminal carbon monoxide ligand into the metal-carbon bond between ethylene and iron produces the key intermediate I(7). The reaction proceeds by metal-assisted formation of a lactam P. The catalytic cycle is closed by a ligand-exchange reaction in which the diazabutadiene ligand substitutes P with reformation of S. This reaction pathway is found to be energetically favored over a reductive elimination. It leads to the experimentally observed heterocyclic product P and a reactive [Fe(CO)(3)] fragment.     

The origin of the ligand-controlled regioselectivity in Rh-catalyzed [(2 + 2) + 2] carbocyclizations: steric vs. stereoelectronic effects

Density functional theory calculations demonstrate that the reversal of regiochemical outcome of the addition for substituted methyl propiolates in the rhodium-catalyzed [(2 + 2) + 2] carbocyclization with PPh₃ and (S)-xyl-binap as ligands is both electronically and sterically controlled. For example, the ester functionality polarizes the alkyne π* orbital to favor overlap of the methyl-substituted terminus of the alkyne with the p_π-orbital of the alkenyl fragment of the rhodacycle during alkyne insertion with PPh₃ as the ligand. In contrast, the sterically demanding xyl-binap ligand cannot accommodate the analogous alkyne orientation, thereby forcing insertion to occur at the sterically preferred ester terminus, overriding the electronically preferred orientation for alkyne insertion.     

Mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines

A theoretical study of the mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines by using density functional theory with the B3LYP functional leads us to discard the so far accepted nitrogen-triggered mechanism, in which the isomerization occurs on N-bonded intermediates and transition states, in favor of a variation of the classical allylic mechanism for olefin isomerization. The modified allylic mechanism consists of four main steps: 1) N-coordination of the allylamine to Rh(I); 2) intramolecular isomerization from kappa(1)-(N)-coordination to eta(2)-(C,C)-coordination of the allylamine; 3) oxidative addition of C(1)--H to form a distorted octahedral eta(3)-allyl complex of Rh(III); and 4) hydrogen transfer to C(3) (reductive C(3)--H elimination). The two hydrogen transfer steps (oxidative addition and reductive elimination) have the highest barriers of the overall process. The oxidative addition barrier, which includes solvent effects, is 28.4 kcal mol(-1). For the reductive elimination, the value in solvent is 28.6 kcal mol(-1), very similar to the oxidative addition barrier.     

Density functional theory analysis of the reaction pathway for methane oxidation to acetic acid catalyzed by Pd2+ in sulfuric acid

Density functional theory has been used to investigate the thermodynamics and activation barriers associated with the direct oxidation of methane to acetic acid catalyzed by Pd2+ cation in concentrated sulfuric acid. Pd2+ cations in such solutions are ligated by two bisulfate anions and by one or two molecules of sulfuric acid. Methane oxidation is initiated by the addition of CH4 across one of the Pd-O bonds of a bisulfate ligand to form Pd(HSO4)(CH3)(H2SO4)2. The latter species will react with CO to produce Pd(HSO4)(CH3CO)(H2SO4)2. The most likely path to the final products is found to be via oxidation of Pd(HSO4)(CH3)(H2SO4)2 and Pd(HSO4)(CH3CO)(H2SO4)2 to form Pd(eta2-HSO4)(HSO4)2(CH3)(H2SO4) and Pd(eta2-HSO4)(HSO4)2(CH3CO)(H2SO4), respectively. CH3HSO4 or CH3COHSO4 is then produced by reductive elimination from the latter two species, and CH(3)COOH is then formed by hydrolysis of CH3COHSO4. The loss of Pd2+ from solution to form Pd(0) or Pd-black is predicted to occur via reduction with CO. This process is offset, though, by reoxidation of palladium by either H2SO4 or O2.     

Theoretical investigation of the reactivity of copper atoms with OCS: comparison with CS2 and CO2

The reaction mechanism of the Cu atom with OCS and CO2 has been studied by means of density functional method (B3LYP). The overall energetics has been refined at the CCSD(T) level. In the case of the Cu + OCS reaction, the CS insertion route is found much more favorable than the CO insertion one. This later reaction is direct and involves an activation energy of 83.3 kcal/mol and is endothermic by 50.0 kcal/mol at the CCSD(T) level. The insertion into the CS bond proceeds through the eta1s and eta2cs coordination species as intermediates and is found exothermic by about 20 kcal/mol. The highest transition structure along this route is only 11.5 kcal/mol higher in energy than the reactant's ground states. In the case of the Cu + CO2 reaction, the insertion route into the CO bond is also found direct but with a lower endothermicity (30.6 kcal/mol) and smaller activation energy (61.1 kcal/mol) than that into the CO bond of OCS. In all cases, the insertion mechanism proceeds simultaneously with electron transfer from the Cu atom to OCS (or CO2) molecule.     

Easily prepared chiral scorpionates: tris(2-oxazolinyl)boratoiridium(i) compounds and their interactions with MeOTf

Optically active C 3-symmetric monoanionic ligands are uncommon in organometallic chemistry. Here we describe the synthesis of readily prepared tris(4 S-isopropyl-2-oxazolinyl)phenylborate [To (P)] and fluxional, zwitterionic four- and five-coordinate iridium(I) compounds [Ir(To (P))(eta (4)-C 8H 12)] ( 4) and [Ir(To (P))(CO) 2] ( 5). The highly fluxional nature of 4 and 5 makes structural assignment difficult, and the interaction between the iridium(I) center and the [To (P)] ligand is established by solid-state and solution (15)N NMR methods that permit the direct comparison between solution and solid-state structures. Although iridium cyclooctadiene 4 is a mixture of four- and five-coordinate species, the dicarbonyl 5 is only the five-coordinate isomer. The addition of electrophiles MeOTf and MeI provides the oxazoline N-methylated product rather than the iridium methyl oxidative addition product. N-Methylation was unequivocally proven by through-bond coupling observed in (1)H- (15)N HMBC experiments.     

Rhodium(I)-Catalyzed Three-Component [4+2+1] Cycloaddition of Two Vinylallenes and CO

Transition metal-catalyzed [4+2+1] reactions of dienes (or diene derivatives such as vinylallenes), alkynes/alkenes, and CO (or carbenes) are expected to be the most straightforward approach to synthesize challenging seven-membered ring compounds, but so far only limited successes have been realized. Here, an unexpected three-component [4+2+1] reaction between two vinylallenes and CO was discovered to give highly functionalized tropone derivatives under mild conditions, where one vinylallene acts as a C₄ synthon, the other vinylallene as a C₂ synthon, and CO as a C₁ synthon. It was proposed that this reaction occurred via oxidative cyclization of the diene part of one vinylallene molecule, followed by insertion of the terminal alkene part of the allene moiety in another vinylallene, into the Rh−C bond of five-membered rhodacycle. Then, CO insertion and reductive elimination gave the [4+2+1] cycloadduct. Further experimental exploration of why ene/yne-vinylallenes and CO gave monocyclic tropone derivatives instead of 6/7-bicyclic ring products were reported here.     

Rh(I)-catalyzed formal [5 + 1]/[2 + 2 + 1] cycloaddition of 1-yne-vinylcyclopropanes and two CO units: one-step construction of multifunctional angular tricyclic 5/5/6 compounds

A novel Rh(I)-catalyzed formal [5 + 1]/[2 + 2 + 1] cycloaddition of 1-yne-vinylcyclopropanes and two CO units for the construction of multifunctional angular tricyclic 5/5/6 skeletons with one or two adjacent bridgehead quaternary all-carbon stereocenters in one step has been developed. Preliminary density functional theory calculations have been carried out to investigate the reaction mechanism and the substituent effects.     

2+2] versus [3+2] addition of metal oxides across C=C double bonds: toward an understanding of the surprising chemo- and periselectivity of transition-metal-oxide additions to ketene

The peri-, chemo-, stereo-, and regioselectivity of the addition of the transition-metal oxides OsO4 and LReO3 (L = O-, H3PN, Me, Cp) to ketene were systematically investigated using density-functional methods. While metal-oxide additions to ethylene have recently been reported to follow a [3+2] mechanism only, the calculations reveal a strong influence of the metal on the periselectivity of the ketene addition: OsO4 again prefers a [3+2] pathway across the C=C moiety whereas, for the rhenium oxides LReO3, the [2+2] barriers are lowest. Furthermore, a divergent chemoselectivity arising from the ligand L was found: ReO4- and (H3PN)ReO3 add across the C=O bond while MeReO3 and CpReO3 favor the addition across the C=C moiety. The calculated energy profile for the MeReO3 additions differs from the CpReO3 energy profile by up to 45 kcal/mol due to the stereoelectronic flexibility of the Cp ligand adopting eta5, eta3, and eta1 bonding modes. The selectivity of the cycloadditions was rationalized by the analysis of donor-acceptor interactions in the transition states. In contrast, metal-oxide additions to diphenylketene probably follow a different mechanism: We give theoretical evidence for a zwitterionic intermediate that is formed by nucleophilic attack at the carbonyl moiety and undergoes a subsequent cyclization yielding the thermodynamically favored product. This two-step pathway is in agreement with the results of recent experimental work.     

One-dimensional polymers based on [{CpMo(CO)2}2(mu,eta2-P2)]: solid-state conformation analysis by NMR spectroscopy and DFT calculations

Reaction of the complex [{CpMo(CO)2}2(mu,eta2-P2)] (1) with CuI halides leads to the quantitative formation of the novel one-dimensional linear polymers [CuX{Cp2Mo2(CO)4(mu,eta2:eta1:eta1-P2)}](infinity) (X=Cl (4), Br (5), I (6)). The same products 4 and 5 were obtained when 1 was treated with CuCl2 and CuBr2, respectively. The solid-state structures are compared and their remarkable influence on the respective (31)P magic angle spinning (MAS) NMR spectra is interpreted with the help of density functional theory (DFT) calculations on the model compounds [{(CuX)2{Cp2Cr2(CO)4(mu,eta(2):eta1:eta1-P2)}2}3] (X=Cl (4 a), Br (5 a)) in which the molybdenum atoms are replaced by their lighter homologue chromium.     

Rhodium(I)-catalyzed [2+2+1] cycloadditions of 1,3-dienes, alkenes, and CO

Initial examples of a Rh(I)-catalyzed [2+2+1] reaction of diene-enes and CO are described. This method allows for the facile, efficient, and diastereoselective construction of a variety of alkenyl cyclopentanones in good to excellent yields. Control studies show that the diene moiety is required for this process as bis-enes do not give the [2+2+1] products under the same conditions.     

Regio- and enantioselective intermolecular rhodium-catalyzed [2+2+2] carbocyclization reactions of 1,6-enynes with methyl arylpropiolates

Transition metal-catalyzed [m+n+o] carbocyclization reactions provide powerful methods for the construction of complex polycyclic systems that are generally not accessible through classical pericyclic reactions. We have developed the first regio- and enantioselective crossed intermolecular rhodium-catalyzed [2+2+2] carbocyclization of carbon- and heteroatom-tethered 1,6-enynes with unsymmetrical 1,2-disubstituted alkynes. This study clearly delineates the ligand requirements for obtaining excellent regio- and enantioselectivity. Furthermore, the ability to utilize various electron-withdrawing groups, and to introduce quaternary carbon stereogenic centers, provides the level of versatility necessary for its application to target-directed synthesis. Additional studies on the development and application of this novel methodology to the total synthesis of natural products are currently underway.     

Kinetic Study of the Oxidative Addition Reaction between Methyl Iodide and [Rh(imino-β-diketonato)(CO)(PPh)3] Complexes,Utilizing UV–Vis,IR Spectrophotometry,NMR Spectroscopy and DFT Calculations

The oxidative addition of methyl iodide to [Rh(β-diketonato)(CO)(PPh)₃] complexes, as modal catalysts of the first step during the Monsanto process, are well-studied. The β-diketonato ligand is a bidentate (BID) ligand that bonds, through two O donor atoms (O,O-BID ligand), to rhodium. Imino-β-diketones are similar to β-diketones, though the donor atoms are N and O, referred to as an N,O-BID ligand. In this study, the oxidative addition of methyl iodide to [Rh(imino-β-diketonato)(CO)(PPh)₃] complexes, as observed on UV–Vis spectrophotometry, IR spectrophotometry and NMR spectrometry, are presented. Experimentally, one isomer of [Rh(CH₃COCHCNPhCH₃)(CO)(PPh₃)] and two isomers of [Rh(CH₃COCHCNHCH₃)(CO)(PPh₃)] are observed—in agreement with density functional theory (DFT) calculations. Experimentally the [Rh(CH₃COCHCNPhCH₃)(CO)(PPh₃)] + CH₃I reaction proceeds through one reaction step, with a rhodium(III)-alkyl as the final reaction product. However, the [Rh(CH₃COCHCNHCH₃)(CO)(PPh₃)] + CH₃I reaction proceeds through two reaction steps, with a rhodium(III)-acyl as the final reaction product. DFT calculations of all the possible reaction products and transition states agree with experimental findings. Due to the smaller electronegativity of N, compared to O, the oxidative addition reaction rate of CH₃I to the two [Rh(imino-β-diketonato)(CO)(PPh)₃] complexes of this study was 7–11 times faster than the oxidative addition reaction rate of CH₃I to [Rh(CH₃COCHCOCH₃)(CO)(PPh₃)].     

Diastereoselective synthesis of five- and seven-membered rings by [2+2+1], [3+2], [3+2+2], and [4+3] carbocyclization reactions of beta-substituted (alkenyl)(methoxy)carbene complexes with methyl ketone lithium enolates

beta-Substituted alkenylcarbene complexes react with methyl ketone lithium enolates to give different carbocyclization products depending on the structure of the lithium enolate, on the metal of the carbene complex, and on the reaction media. Thus, the reactions of aryl and alkyl methyl ketone lithium enolates with beta-substituted alkenyl chromium and tungsten carbene complexes in diethyl ether afford 1,3-cyclopentanediol derivatives derived from a formal [2+2+1] carbocyclization reaction. However, the lithium enolates of acetone and tungsten complexes furnish formal [3+2+2] carbocyclization products. In the case of alkynyl methyl ketone lithium enolates, competitive formal [2+2+1] and [3+2] carbocyclization reactions occur and 1,3-cyclopentanediol and 3-cyclopentenol derivatives are formed. Conversely, alkenyl methyl ketone lithium enolates react with alkenylcarbene complexes under the same reaction conditions to form 2-cycloheptenone derivatives by a formal [4+3] carbocyclization reaction. Finally, when the reaction was performed in the presence of a coordinating medium, the [3+2] carbocyclization pattern was observed independently of the nature of the methyl ketone lithium enolate used.     

Electronic structure of [(CpCr)[(CO)3M]]mu-Cot (M = Cr, Fe): a theoretical study

The electronic structure of two cyclooctatetraene-bridged dinuclear first-row transition metal complexes of the type [(CpM)[(CO)3M']]mu-Cot (M = Cr; M' = Fe (1), Cr (2)) was investigated by complete active space self-consistent field (CASSCF) calculations. In this context the differences in the binding capabilities of the complex fragments CpM and (CO)3M are discussed on the basis of extended Huckel molecular orbital (MO) calculations. The geometries used for the CASSCF calculations for complex 1 were obtained from the crystal structure. For 2 a model structure was established by geometry optimization using density functional methods. The CASSCF results agree well with the experimental findings and provide insight into the binding situation of the two compounds. Complex 1 can be regarded as being composed of a chromocene-like subunit CpCr(eta5-C5H5) and the fragment (CO)3Fe(eta3-C3H3). A direct metal-metal bond is found, involving one initially singly occupied orbital of each fragment, leading to a doublet ground state for 1 with the remaining unpaired electron localized at the chromium center. For 2 no such direct metal-metal bond can be recognized. A very weak direct metal-metal interaction is induced by electron donation from the Cot2- ligand into a formally unoccupied metal-metal binding orbital combination. In the quartet ground state all three unpaired electrons are localized at the chromium center of the formally doubly positive charged CpCr unit, on which complex fragment [(CO)3Cr(eta5-Cot)]2- acts like a cyclopentadienyl ligand. The coordination sphere of the chromium center of the CpCr unit resembles that of a metallocene metal center and its metal 3d occupation scheme corresponds to that of vanadocene.     

Theoretical study of the [2+2+2+1] cycloaddition mechanism of enediynes and carbon monoxide catalyzed by rhodium

The [2+2+2+1] cycloaddition mechanism of enediynes and carbon monoxide catalyzed by the [Rh(CO)2Cl]2 rhodium dimer has been studied using density functional theory, comparing this multistep process with the two-step reaction in the absence of a catalyst. According to our results, the multistep mechanism agrees with that previously suggested. The great selectivity of this reaction and the influence of the chosen solvent in this selectivity were also analyzed.     

Theoretical insights into enantioselective catalysis: the mechanism of the Kharasch-Sosnovsky reaction

The mechanism of the Kharasch-Sosnovsky reaction has been investigated using B3 LYP/6-31G* calculations on a chiral reaction model [cyclohexene+tert-butyl perbenzoate-->cyclohex-2-enyl benzoate+tert-butyl alcohol, catalyzed by a chiral bisoxazoline-copper(I) complex]. Although two previous reaction mechanisms have been considered, the results are consistent with a new mechanistic pathway. This path involves ligand exchange between the catalyst-cyclohexene complex with tert-butyl perbenzoate to give a catalyst-perester complex, which undergoes an (either one- or two-step) oxidative addition reaction to yield a copper(III) complex. The limiting step of the Kharasch-Sosnovsky reaction consists of an intramolecular step involving the abstraction of an allylic hydrogen from cyclohexene [which is pi-bound to the copper(III) complex]. The resulting allyl-copper(III) complex (subsequent to the loss of tert-butanol) can undergo a haptotropic rearrangement by means of an eta1-allyl/eta3-allyl equilibrium, leading to scrambling between vinylic and allylic positions when an isotopically labeled substrate is used. The allyl-copper(III) ion undergoes a stereospecific reductive elimination involving the pi-bond migration to yield a reaction product-catalyst complex, which can regenerate the alkene-copper(I) complex by ligand exchange. The proposed reaction mechanism is consistent with all known experimental results (including enantioselectivity data).