Understanding the mechanism of the intramolecular stetter reaction. A DFT study

The mechanism of the N-heterocyclic carbene (NHC)-catalyzed intramolecular Stetter reaction of salicylaldehyde 1 to yield chromanone 3 has been theoretically studied at the B3LYP/6-31G** level. This NHC-catalyzed reaction takes place through six elementary steps, which involve: (i) formation of the Breslow intermediate IN2; (ii) an intramolecular Michael-Type addition in IN2 to form the new C-C s bond; and (iii) extrusion of the NHC catalyst from the Michael adduct to yield chromanone 3. Analysis of the relative free energies in toluene indicates that while formation of Breslow intermediate IN2 involves the rate-determining step of the catalytic process, the intramolecular Michael-type addition is the stereoselectivity determining step responsible for the configuration of the stereogenic carbon a to the carbonyl of chromanone 3. An ELF analysis at TSs and intermediates involved in the Michael-type addition allows for the characterization of the electronic changes along the C-C bond-formation.     

New metal-only Lewis pairs: elucidating the electronic influence of N-heterocyclic carbenes and phosphines on the dative Pt-Al bond

The synthesis and full characterization of a new heteroleptic N-heterocyclic carbene (NHC)-phosphine platinum(0) complex and formation of its corresponding alane adduct is reported. The influence of the ligands on the Lewis basic properties was studied via multinuclear NMR-spectroscopy, X-ray analyses, and density functional theory (DFT) calculations. Consistently, the effect of changing the halogens upon the Lewis acid properties of aluminum halides was studied by X-ray analysis and DFT calculations.     

Room Temperature Ring Expansion of N‐Heterocyclic Carbenes and BB Bond Cleavage of Diboron(4) Compounds

We report the isolation and detailed structural characterization, by solid‐state and solution NMR spectroscopy, of the neutral mono‐ and bis‐NHC adducts of bis(catecholato)diboron (B₂cat₂). The bis‐NHC adduct undergoes thermally induced rearrangement, forming a six‐membered ‐B?C?N?C?C‐N‐heterocyclic ring via C?N bond cleavage and ring expansion of the NHC, whereas the mono‐NHC adduct is stable. Bis(neopentylglycolato)diboron (B₂neop₂) is much more reactive than B₂cat₂ giving a ring expanded product at room temperature, demonstrating that ring expansion of NHCs can be a very facile process with significant implications for their use in catalysis.     

Electron microscopic imaging of a single Group 8 metal atom catalyzing C-C bond reorganization of fullerenes

Heating a bulk sample of [60]fullerene complexes, (η(5)-C(5)H(5))MC(60)R(5) (M = Fe, Ru, R = Me, Ph), produces small hydrocarbons because of coupling of R and C(5)H(5) via C-C and C-H bond activation. Upon observation by transmission electron microscopy, these complexes, encapsulated in single-walled carbon nanotubes, underwent C-C bond reorganization reactions to form new C-C bond networks, including a structure reminiscent of [70]fullerene. Quantitative comparison of the electron dose required to effect the C-C bond reorganization of fullerenes and organofullerenes in the presence of a single atom of Ru, Fe, or Ln and in the the absence of metal atoms indicated high catalytic activity of Ru and Fe atoms, as opposed to no catalytic activity of Ln. Organic molecules such as hydrocarbons and amides as well as pristine [60]fullerene maintain their structural integrity upon irradiation by ca. 100 times higher electron dose compared to the Ru and Fe organometallics. The results not only represent a rare example of direct observation of a single-metal catalysis but also have implications for the use of single metal atom catalysis in Group 8 metal heterogeneous catalysis.     

Crystalline Boragermenes

Boragermene 3 featuring a double bond between the Ge and dicoordinate B atoms has been synthesized for the first time by reacting the cyclic (alkyl)(boryl)germylene–PMe₃ adduct 1 with Cl₂BN(SiMe₃)₂ followed by reductive dehalogenation with KC₈. Addition of a Lewis base (^MeNHC) to 3 leads to the formation of the corresponding adduct 4 , which shows double bond character between the Ge and tricoordinate B atoms. Compound 3 undergoes hydrogenation with H₂ concomitant with a complete scission of the Ge=B bond.     

Crystalline Boragermenes

Boragermene 3 featuring a double bond between the Ge and dicoordinate B atoms has been synthesized for the first time by reacting the cyclic (alkyl)(boryl)germylene–PMe₃ adduct 1 with Cl₂BN(SiMe₃)₂ followed by reductive dehalogenation with KC₈. Addition of a Lewis base (^MeNHC) to 3 leads to the formation of the corresponding adduct 4 , which shows double bond character between the Ge and tricoordinate B atoms. Compound 3 undergoes hydrogenation with H₂ concomitant with a complete scission of the Ge=B bond.     

Preparation of [5,6]- and [6,6]-oxahomofullerene derivatives and their interconversion by Lewis acid assisted reactions of fullerene mixed peroxides

[60]Fullerene mixed peroxides C60(O)(OOtBu)4 exhibit chemo- and regioselective reactions under mild conditions. The epoxy moiety is opened by ferric chloride to form vicinal hydroxy chloride C60Cl(OH)(OOtBu)4. BF3 is also effective in opening the epoxy moiety. The O-O bond of the fullerene mixed peroxide is cleaved by aluminum chloride to form both [5,6]- and [6,6]-fullerene hemiketals (oxohomo[60]fullerenes). A Hock-type rearrangement is proposed for the formation of the hemiketals, in which a fullerene C-C bond is cleaved. Lewis acids and/or visible light can initiate isomerization of the hemiketal isomers. Single-crystal X-ray analysis and theoretical calculations confirmed the results.     

Increasing stability of the fullerenes with the number of carbon atoms: the experimental evidence

The values of the molar standard enthalpies of formation, Delta(f)H(o)(m)(C(76), cr) = (2705.6 +/- 37.7) kJ x mol(-1), Delta(f)H(o)(m)(C(78), cr) = (2766.5 +/- 36.7) kJ x mol(-1), and Delta(f)H(o)(m)(C(84), cr) = (2826.6 +/- 42.6) kJ x mol(-1), were determined from the energies of combustion, measured by microcombustion calorimetry on a high-purity sample of the D(2) isomer of fullerene C(76), as well as on a mixture of the two most abundant constitutional isomers of C(78) (C(2nu)-C(78) and D(3)-C(78)) and C(84) (D(2)-C(84), and D(2d)-C(84). These values, combined with the published data on the enthalpies of sublimation of each cluster, lead to the gas-phase enthalpies of formation, Delta(f)H(o)(m)(C(76), g) = (2911.6 +/- 37.9) kJ x mol(-1); Delta(f)H(o)(m)(C(78), g) = (2979.3 +/- 37.2) kJ x mol(-1), and Delta(f)H(o)(m)(C(84), (g)) = (3051.6 +/- 43.0) kJ x mol(-1), results that were found to compare well with those reported from density functional theory calculations. Values of enthalpies of atomization, strain energies, and the average C-C bond energy were also derived for each fullerene. A decreasing trend in the gas-phase enthalpy of formation and strain energy per carbon atom as the size of the cluster increases is found. This is the first experimental evidence that these fullerenes become more stable as they become larger. The derived experimental average C-C bond energy E(C-C) = 461.04 kJ x mol(-1) for fullerenes is close to the average bond energy E(C-C) = 462.8 kJ x mol(-1) for polycyclic aromatic hydrocarbons (PAHs).     

Understanding the cooperative NHC/LA catalysis for stereoselective annulation reactions with homoenolates. A DFT study

The role of Ti(Oi-Pr)(4) Lewis acid (LA) in the cooperative N-heterocyclic carbene (NHC)/LA catalyzed addition of enals to enones to yield cis-cyclopentenes has been investigated using DFT methods at the B3LYP/6-31G** computational level. Ti(IV) effectively catalyzes the reaction by formation of a complex with cinnamaldehyde 1, which favors the nucleophilic attack of NHC 5 on 1, and the subsequent proton abstraction to yield the extended Ti(IV)-Breslow intermediate 21. The nature of the metal involved in the LA catalyst plays a relevant role due to the more basic character of NHCs than aldehydes. Thus, strong LAs, such as Zn(OTf)(2), prevent the catalytic behavior of NHCs to form a very stable complex. The subsequent formation of a complex between chalcone 2 and the extended Ti(IV)-Breslow intermediate 21 favors the cis stereoselective C-C bond-formation. Analysis of the structures of Ti(IV)-complex precursors for the cis and trans C-C bond-formation steps allows for an explanation of the unexpected cis stereoselectivity.     

(NHC)AuI]-catalyzed formation of conjugated enones and enals: an experimental and computational study

The [(NHC)AuI]-catalyzed (NHC=N-heterocyclic carbene) formation of alpha,beta-unsaturated carbonyl compounds (enones and enals) from propargylic acetates is described. The reactions occur at 60 degrees C in 8 h in the presence of an equimolar mixture of [(NHC)AuCl] and AgSbF6 and produce conjugated enones and enals in high yields. Optimization studies revealed that the reaction is sensitive to the solvent, the NHC, and, to a lesser extent, to the silver salt employed, leading to the use of [(ItBu)AuCl]/AgSbF6 in THF as an efficient catalytic system. This transformation proved to have a broad scope, enabling the stereoselective formation of (E)-enones and -enals with great structural diversity. The effect of substitution at the propargylic and acetylenic positions has been investigated, as well as the effect of aryl substitution on the formation of cinnamyl ketones. The presence or absence of water in the reaction mixture was found to be crucial. From the same phenylpropargyl acetates, anhydrous conditions led to the formation of indene compounds via a tandem [3,3] sigmatropic rearrangement/intramolecular hydroarylation process, whereas simply adding water to the reaction mixture produced enone derivatives cleanly. Several mechanistic hypotheses, including the hydrolysis of an allenol ester intermediate and SN2' addition of water, were examined to gain an insight into this transformation. Mechanistic investigations and computational studies support [(NHC)AuOH], produced in situ from [(NHC)AuSbF6] and H2O, instead of cationic [(NHC)AuSbF6] as the catalytically active species. Based on DFT calculations performed at the B3LYP level of theory, a full catalytic cycle featuring an unprecedented transfer of the OH moiety bound to the gold center to the C[triple chemical bond]C bond leading to the formation of a gold-allenolate is proposed.     

An N-heterocyclic carbene adduct of diatomic tin, :Sn[double bond, length as m-dash]Sn:

Reduction of an N-heterocyclic carbene (NHC) adduct of SnCl(2), viz. [(IPr)SnCl(2)] (IPr = :C{N(Dip)C(H)}(2); Dip = 2,6-diisopropylphenyl), with a magnesium(i) dimer, has afforded the first NHC complex of a row 5 element in its diatomic form, [(IPr)Sn[double bond, length as m-dash]Sn(IPr)]; a computational analysis of the complex indicates that it comprises a singlet state, doubly bonded tin(0) fragment, :Sn[double bond, length as m-dash]Sn:, datively bonded by two NHC ligands.     

Insertion of molecular oxygen into a palladium-hydride bond: computational evidence for two nearly isoenergetic pathways

The reaction of a palladiumII-hydride species with molecular oxygen to form palladiumII-hydroperoxide has been proposed as a key step in Pd-catalyzed aerobic oxidation reactions. We recently reported one of the first experimental precedents for such a step (Angew. Chem., Int. Ed. 2006, 45, 2904-2907). DFT calculations have been used to probe the mechanism for this reaction, which consists of formal insertion of O2 into the palladium-hydride bond of trans-(NHC)2Pd(H)OAc (NHC = N-heterocyclic carbene). Four different pathways were considered: (1) hydrogen atom abstraction (HAA) of the Pd-H bond by molecular oxygen, (2) reductive elimination of HX followed by oxygenation of Pd0 and protonolysis of the (eta2-peroxo)-PdII species, (3) oxygenation of palladiumII-hydride with subsequent reductive elimination of the O-H bond from an eta2-peroxo-PdIV center, and (4) formation of a cis-superoxide adduct of the palladium-hydride species followed by O-H bond formation via hydrogen atom migration. The calculations reveal that pathways 1 and 2 are preferred energetically, and both pathways exhibit very similar kinetic barriers. This result suggests that more than one pathway is possible for catalyst reoxidation in Pd-catalyzed aerobic oxidation reactions.     

Preparation of η2-complexes of fullerenes by reduction. Crystal structure and optical properties of {Ni(dppp)·(η2-C70)}·(C6H4Cl2)0.5

The new reduction method for preparation of η(2)-complexes of fullerenes with nickel-1,3-bis(diphenylphosphino)propane has been developed in which Ni(dppp)Cl(2) and C(60)(C(70)) mixtures are reduced with sodium tetraphenylborate. Single crystals of the first η(2)-complex of nickel with fullerene C(70): {Ni(dppp)·(η(2)-C(70))}·(C(6)H(4)Cl(2))(0.5) (1) (C(6)H(4)Cl(2) = o-dichlorobenzene) have been obtained as well as the previously described complex with fullerene C(60): {Ni(dppp)·(η(2)-C(60))}·(Solvent) (2). The crystal structure of 1 has been solved to show the coordination of nickel to the C-C bond of C(70) at the 6-6 ring junction of η(2)-type to form Ni-C(C(70)) bonds of 1.929-1.941(2) ? length, the shortest M-C bonds among those known for η(2)-complexes of fullerenes C(60) and C(70). The length of the C-C bond to which Ni atom is coordinated (1.494(3) ?) is noticeably longer than the average length of these bonds in C(70) (1.381(2) ?). Optical spectra of 1 in the IR- and UV-visible ranges have been analyzed to show the splitting of some C(70) bands due to C(70) symmetry lowering. The complex has a red-brown color in solution and manifests three bands in the visible range at 379, 467 and 680 nm. The solution of 1 is air sensitive since air exposure restores the color and absorption bands of the starting C(70) at 383 and 474 nm.     

Pd-catalyzed synthesis of 3-(diarylmethylene)-2-oxindoles and 3-(arylmethylene)-2-oxindoles

An efficient method for the stereoselective synthesis of 3-(diarylmethylene)-2-oxindoles and 3-(arylmethylene)-2-oxindoles via carbopalladation is described. In this approach, an Ugi-4-component reaction (4-CR) adduct was used as the starting material. A one-pot sequence involving intermolecular carbopalladation C-H activation/C-C bond formation efficiently afforded the oxindole derivatives.     

Change in spin state and enhancement of redox reactivity of photoexcited states of aromatic carbonyl compounds by complexation with metal ion salts acting as Lewis acids. Lewis acid-catalyzed photoaddition of benzyltrimethylsilane and tetramethyltin via photoinduced electron transfer

The lowest excited state of aromatic carbonyl compounds (naphthaldehydes, acetonaphthones, and 10-methylacridone) is changed from the n,pi triplet to the pi,pi singlet which becomes lower in energy than the n,pi triplet by the complexation with metal ions such as Mg(ClO(4))(2) and Sc(OTf)(3) (OTf = triflate), which act as Lewis acids. Remarkable positive shifts of the one-electron reduction potentials of the singlet excited states of the Lewis acid-carbonyl complexes (e.g., 1.3 V for the 1-naphthaldehyde-Sc(OTf)(3) complex) as compared to those of the triplet excited states of uncomplexed carbonyl compounds result in a significant increase in the redox reactivity of the Lewis acid complexes vs uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. Such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C-C bond formation between benzyltrimethylsilane and aromatic carbonyl compounds via the Lewis-acid-promoted photoinduced electron transfer. The quantum yield determinations, the fluorescence quenching, and direct detection of the reaction intermediates by means of laser flash photolysis experiments indicate that the Lewis acid-catalyzed photoaddition reactions proceed via photoinduced electron transfer from benzyltrimethylsilane to the singlet excited states of Lewis acid-carbonyl complexes.     

Development of a Simple Adjustable Zinc Acid/Base Hybrid Catalyst for C−C and C−O Bond‐Forming and C−C Bond‐Cleavage Reactions

A newly designed zinc Lewis acid/base hybrid catalyst was developed. By adjusting the Lewis acidity of the zinc center, aldol‐type additions of 2‐picolylamine Schiff base to aldehydes proceeded smoothly to afford syn‐aldol adduct equivalents, trans‐N,O‐acetal adducts, in high yields with high selectivities. NMR experiments, including microchanneled cell for synthesis monitoring (MICCS) NMR analysis, revealed that anti‐aldol adducts were formed at the initial stage of the reactions under kinetic control, but the final products were the trans‐(syn)‐N,O‐acetal adducts that were produced through a retro‐aldol process under thermodynamic control. In the whole reaction process, the zinc catalyst played three important roles: i) promotion of the aldol process (C?C bond formation), ii) cyclization process to the N,O‐acetal product (C?O bond formation), and iii) retro‐aldol process from the anti‐aldol adduct to the syn‐aldol adduct (C?C bond cleavage and C?C bond formation).     

A Three‐Membered Cyclic Phosphasilene

Small unsaturated phosphacycles are versatile reagents owing to their strain and the added functionality of the double bond and the phosphorus lone pair. Herein we report the synthesis and isolation of the smallest possible cyclic phosphasilene as a stable adduct with an N‐heterocyclic carbene (NHC). First reactivity studies show a) that the PSi₂ ring is a competent ligand to the Fe(CO)₄ fragment via the phosphorus lone pair and b) that the abstraction of the NHC by BPh₃ results in the rapid head‐to‐head or head‐to‐tail dimerization of the PSi₂ unit. The relatively facile NHC cleavage indicates that the P=Si double bond is available for further manipulation.     

Powdered KOH in DMSO: an efficient base for asymmetric cyclization via memory of chirality at ambient temperature

Enolate chemistry has been extensively used for stereoselective C-C bond formation, in which metal amide bases are frequently employed in strictly anhydrous solvents at low temperatures. However, we found that asymmetric intramolecular C-C bond formation via axially chiral enolate intermediates proceeded in up to 99% ee at 20 degrees C using powdered KOH in dry or wet DMSO as a base. The enantioselectivity was even higher than that of the corresponding reactions with potassium hexamethyldisilazide in DMF at -60 degrees C. The racemization barrier of the axially chiral enolate intermediate was estimated to be approximately 15.5 kcal/mol. On the basis of the barrier, the chiral enolate intermediate was supposed to undergo cyclization within approximately 10(-3) sec at 20 degrees C after it is generated to give the product in >or=99% ee. Thus, enolates generated with powdered KOH in DMSO were expected to be extremely reactive.     

Organometallic derivatives of fullerenes: a DFT study of (eta2-Cx)[Pt(PH3)2]n (x = 60, 70, 84; n = 1-6)

To determine the relationship among curvature, patch type, and reactivity of the C-C site, a series of density functional calculations were performed on several substituted fullerenes. [6:6] pyracylene-type sites are the most reactive sites in all analyzed cages: C(60), C(70), and C(84). The binding energy between the Pt(PH(3))(2) unit and fullerene is almost independent of the size of the cage and of the number of metals coordinated on the fullerene surface. Contrarily, curvature and type of carbon-carbon bond are determinant for the coordination strength. The use of relatively large basis sets is necessary to have consistent energies.     

Al/P- and Ga/P-Based Frustrated Lewis Pairs and Electronically Unsaturated Substrates: Ring Cleavage and Ring Closure,C−C and C−N Bond Formation

Al/P- and Ga/P-based frustrated Lewis pairs (FLPs) reacted with an azirine under mild conditions under cleavage of the heterocycle on two different positions. Opening of the C−C bond yielded an unusual nitrile–ylide adduct in which a C−N moiety coordinated to the FLP backbone. Cleavage of a C−N bond afforded the thermodynamically favored enamine adduct with the N atom bound to P and Al or Ga atoms. Ring closure was observed upon treatment of an Al/P FLP with electronically unsaturated substrates (4-(1-cyclohexenyl)-1-aza-but-1-en-3-ynes) and yielded by C−N bond formation hexahydroquinoline derivatives, which coordinated to the FLP through P−C and Al−C bonds. Diphenylcyclopropenone showed a diverse reactivity, which depending on steric shielding and the polarizing effect of Al or Ga atoms afforded different products. An AltBu₂/P FLP yielded an adduct with the C=O group coordinated to P and Al. The dineopentyl derivative gave an equilibrium mixture consisting of a similar product and a simple adduct with O bound to Al and a three-coordinate P atom. Both compounds co-crystallize. The Ga/P FLP only formed the simple adduct with the same substrate. Rearrangement resulted in all cases in C₃-ring cleavage and migration of a mesityl group from P to a former ring C atom by C−C bond formation. Diphenylthiocyclopropenone (evidence for the presence of P=C bonds) and an imine derivative afforded similar products.