Ligand properties of N-heterocyclic and Bertrand carbenes: A density functional study

In order to probe the ligand properties we have examined a series of Cr(CO)₅L and Ni(CO)₃L complexes using density functional theory (DFT). The ligands included in our study are N-heterocyclic carbenes (NHCs) and Bertrand-type carbenes. Our study shows that the carbene–metal bonds of imidazol-2-ylidenes (1), imidazolin-2-ylidenes (2), thiazo-2-ylidenes (3), and triazo-5-ylidenes (4) are significantly stronger than those of Bertrand-type carbenes (5–7). The force constants of C–O in complexes are related to the property of isolated carbenes such as proton affinity (PA), electronegativity (χ), and charge transfer (ΔN). NHCs and Bertrand-type carbenes are identified as nucleophilic, soft ligands. Carbene stabilization energy (CSE) computations indicate that carbenes 1 and 4 are the most stable species, while 2 and 3 are less stable. In contrast to NHCs, CSE of carbenes 5–7 are much smaller, and their relative stabilities are in the order (amino)(aryl) carbenes 7e–7g > (amino)(alkyl) carbenes 7a–7d > (phosphino)(aryl) 6d–6e, and (phosphino)(silyl) carbenes 5a–5c > (phosphino)(alkyl) carbenes 6a–6c.     

Novel chromeno[2,3-c]pyrroles synthesized via intramolecular rhodium(ii) carbene trapping

Rhodium(ii) carbenes generated from (E)-3-diazo-4-(2-hydroxybenzylidene)-1-phenylpyrrolidine-2,5-dione under- went a facile transformation into novel succinimide-fused 2H-chromenes, chromeno[2,3-c]pyrrole-1,3(2H,3aH)-diones. The process presumably involves an intramolecular O-H insertion reaction.     

Substituted six-membered ring carbenes: the effects of amino and cyclopropyl groups through DFT calculations

DFT calculations are employed to compare and contrast six-membered ring carbenes including 1,3-dimethyltetrahydropyrimidin-2-ylidene (1a), 1-methyl-3-cyclopropyltetrahydropyridine-2-ylidene (2a), and 1,3-dicyclopropylcyclohexane-2-ylidene (3a) as well as their unsaturated analogues 1b, 2b, 3b, and 2c. The amino groups exert singlet-triplet energy separation (?E_s−t) of 60.9 kcal/mol to 1a while cyclopropyls induce a ?E_s−t of 14.8 kcal/mol to 3a. The simultaneous presence of amino and cyclopropyl in 2a leads to a ?E_s−t of 43.3 kcal/mol. Unsaturation slightly increases the ?E_s−t of 1a and 3a but not that of 2a. Our thermodynamic, kinetic, and reactivity results are compared with those of synthetic five-membered ring N-heterocyclic carbenes.     

Diastereoselective carbenes: Stereoelectronic control of bent singlet trans-2′-substituted cyclobutylcarbenes

The UM06-2X/aug-cc-pVTZ//UM06-2X/6-311G(d) theoretical models of representative singlet trans-2′-substituted cyclobutylcarbenes were computed. The :CH-group of carbenes with an ERG is bent toward the c-C₄H₇-ring’s proximal C1′–C2′ single bond. The :CH-group of carbenes with an EWG is bent toward the c-C₄H₇-ring’s distal C1′–C4′ single bond. The contrasting stereoelectronic effect was computationally tested using trimethylsilyl vs. bromo substituents to determine if exo-1- and exo-2-substituted bicyclo[2.1.0]pentanes would be produced in diastereomeric excess.     

Heterocyclic carbenes of diverse flexibility: A theoretical insight

A systematic density functional investigation has been carried out on the structure, stability and reactivity of heterocyclic carbenes of diverse flexibility, i.e., carbenes with different modes of binding specially normal and remote mode of binding. Ligating properties of these carbenes have been assessed by virtue of their HOMO energies and verified further by inspection of the symmetric CO frequencies of their respective palladium carbonyl complexes. All the remote carbenes were found to have higher σ-donating abilities compared to their normal analogs. N-heterocyclic carbenes 1 and 5 are found to be electrophilic in nature while the remote carbene 3 and P-heterocyclic carbene 6 are found to be nucleophilic. Quantum theory of atoms in molecules (QTAIM) reveals significant covalent character in the C_carbene-Pd bonds.     

Unconventional nucleotide analogues: Reaction of carbenes with uracil and uridine derivatives

Carbenes 2a-d (:CCl₂, :CBr₂, :CFC1, :CHCOOC₂H₅) add to the 5,6-double bond of uracil derivatives la-e to give adducts in modest to good yields The unsymmetrically substituted carbenes :CFC1 2c and CHCOOC₂H₅, 2d lead, as expected, to the formation of mixtures of endo and exo-isomers 3c-n which were separated and identified via their ¹H NMR spectra. Reaction of 2d with pyrimidine derivatives 5a,b resulted in products which can be rationalized in terms of addition at either the 5,6 or the 3,4 double bond. Addition of carbenes 2a-c to uridine derivatives gave diastereomeric adducts 12-16 which were isolated and identified. The two diastereomeric series, referred to as A and B, have been correlated and assigned the configurations 5S, 6S and 5R, 6R, respectively, on the basis of X-ray structure analysis of 12B The adducts of 2a,c and the uridine derivatives 11a,b have been deprotected to give 7,7-dihalocyclothymidines.     

Multiple fluorescences. Different triplets from N-methyl-2-N-phenylamino-6-naphthalenesulfonate and its C-protonated form by laser pulse photolysis

The triplets generated by laser pulse excitation of N-methyl-2-N-phenylamino-6-naphthalenesulfonate (1M) and its C-protonated form, the N-methyl betaine of 6-sulfonato-3β-tetralenone N-phenylimine (2M) are distinct and apparently do not interconvert at 25°C either in dioxane-water or in glycerol, for which single pulses must be used to avoid conversion of 1M to 2M. Some parallel results are reported for the N-hydroxyethyl derivatives (1HEand2HE).     

The electronic structure of 5-methylhexa-1,2,4-triene-1,3-diyl, the first representative of highly delocalized triplet ethynylvinylcarbenes, from ESR spectroscopy data and quantum chemical calculations

The ESR spectrum of the first representative of highly conjugated triplet ethynylvinylcarbenes, 5-methylhexa-1,2,4-triene-1,3-diyl (1), was recorded in solid argon matrix. The zero-field splitting (ZFS) parameters of carbene 1 (D = 0.5054±0.0006 cm^?1 and E = 0.0045±0.0002 cm^?1) determined from the experimental ESR spectrum are in between the corresponding parameters of ethynylcarbene C3H2 (2) and vinylcarbene C₃H₄ (3): D(3) < D(1) < D(2) and E(2) < E(1) < E(3). Quantum chemical calculations of the ZFS parameters of 1, 2, and 3 have been carried out for the first time using two DFT-based approaches, RODFT and UDFT. An analysis of the experimental and theoretical ZFS parameters shows that carbene 1 is characterized by a greater extent of delocalization of the spin density of unpaired electrons than carbenes 2 and 3. The characteristic structural fragments of carbene 1 possess the principal features of the electronic structure of both ethynylcarbene (2) and vinylcarbene (3), respectively. Magnetic spin-spin interactions are identical in carbenes 1 and 2. The dominant contribution to D in 1 and 2 results from the one-center spin-spin interactions on carbon atoms in the propynylidene group, which are subjected to strong spin polarization.     

The photodecomposition and photoracemization of methyl (+)-O-methylmandelate

Irradiation at 254 nm of the bichromorphoric molecule methyl (+)-O-methylmandelate ((+)-3) in methanol has been found to lead to photodecomposition and photoracemization. The major monomeric product isolated, methyl phenylacetate (1b), is shown to arise by a mechanism totally different from that responsible for racemization. The high efficiency of the direct irradiation, the very short singlet lifetime of 3 (ca 1 nsec), and the inability to sensitize the formation of 1b with acetone triplets implicate the singlet state of 3 as the reactive species. Mechanistic models to account for the products are discussed.     

Generation of allenylthioketene S,S-dioxides through [3,3] sigmatropic rearrangement of alkynyl propargyl sulfones

Thioketene S,S-dioxides 2 were successfully generated through [3,3] sigmatropic rearrangement of alkynyl propargyl sulfones 1 and the formation of 2 was confirmed by trapping experiment using cyclohexene or allyltrimethylsilane affording a [2+2] cycloadduct 5. In situ generated thioketene S,S-dioxides 2 underwent facile conversion into allenynes 3 in moderate yields, via formation and subsequent [1,2] shift of vinylidene carbenes.     

Synthese et proprietes chimiques de deux diazoalcanes α-thiophosphores. observation d'un couplage 4jpp

Transfer of diazo and/or azide groups to the methylene of phosphine sulfides 1 Ph₂P(S) CH₂R, leads to the diazoalkanes Ph₂P(S)CN₂R 2b and 2c and azides Ph₂P(S)CHN₃R 3a and 3c in good yields. Thermolysis and photolysis in various media are studied. The 1,3-dipolar addition of 2c on trans dibenzoyléthylène and ethylfumarate is followed by Ph₂P(S) group migration. The corresponding carbenes give very complex reactions causing P-C bond breaking. New coupling constants ⁴J_PP and J_PC are observed.     

Synthesis,structure and electronic properties of N-dialkylamino- and N-alkoxy-1,2,4-triazol-3-ylidene ligands

4-Amino- and 4-alkoxy-1,2,4-triazoles 4a–d and 6 were readily obtained from the reaction of N,N-dimethylformamidazin dihydrochloride 3 with hydrazines 2 and hydroxylamine 5. Alkylation of compounds 4a–d and 6 by MeOTf or MeI afforded azolium salts 9–11, which in turn were transformed into Rh(I) carbene complexes 13–15, Ag carbenes 16, and cationic Rh(I) bis-carbenes 17. Additionally, complexes 13 and 15 were transformed into dicarbonyl derivatives 18 and 19, and the carbonyl stretching frequencies of these compounds were used to evaluate the effect of the amino and alkoxy groups in the σ-donor ability of these 1,2,4-triazol-3-ylidenes.     

Acceptor substituted N-heterocyclic carbenes and their Rh(I)complexes: Synthesis, structure and properties

Rhodium(I) complexes of acceptor substituted N-heterocyclic carbenes were obtained either by transmetalation from the corresponding Ag(I) complexes or by thermal decomposition of corresponding pentafluorobenzene carbene adducts. All complexes were fully characterized by means of NMR- and mass spectroscopy. Compounds 5, 6, 7 and 11 were although characterized by single-crystal X-ray analysis. The relative σ-donor/π-acceptor strength of the NHC ligands was determined by means of IR spectroscopy. Dimerisation behaviour of Rh carbonyl complexes was studied.     

Electronic structure and reactivity of propellanes : Photoelectron spectra of thia[4.4.3]propellanes

The photoelectron spectra of 12-thia[4.4.3]propell-3-ene (), 12-thia[4.4.3]propell-3-ene-12-oxide (2), 12-thia[4.4.3]propella-3,8-diene-12-oxide (3), 12-thia[4.4.3]propella-3,8-diene-12-dioxide (4), 12 thia[4.4.3]propella-2, 4-diene-12-oxide (5), 12-thia(4.4.3]propella-2,4,7,9 tetraene-12-oxide (6) and 12-thia[4.4.3]propella-2,4,7,9 tetraene-12-dioxide (7) have been investigated and the first bands have been interpreted. For compounds 3 and 4 the endo-endo conformation could be excluded. For 6 and 7 the interaction of the two butadiene moieties is of the same order as that of the π-orbitals in norbornadiene. The electronic structure of 3 and 4 allows us to rationalize and to predict the direction of the addition of cations and carbenes.     

Cyclic (alkyl)(amino)carbene gold(I) complexes: A synthetic and structural investigation

A series of mono- and dicarbene gold(I) complexes of types Au(CAAC)(Cl) [CAAC = cyclic (alkyl)(amino)carbene] (1) and [Au(CAAC)₂]⁺[X]^? (X = Cl, AuCl₂) (2) have been prepared through reaction of AuCl(SMe₂) with free carbenes a–e, and structurally characterized by single X-ray diffraction studies (1a, 1b, 2d, 2e). In addition two new free cyclic (alkyl)(amino)carbenes (c and e) have been synthesized.     

Highly efficient trialkylsilylcyanation of aldehydes, ketones and imines catalyzed by a nucleophilic N-heterocyclic carbene

The synthetic utility of N-heterocyclic carbenes was demonstrated by the trialkylsilylcyanation of aldehydes, ketones and imines. In the presence of a catalytic amount of 3a, the reactions with Me₃SiCN proceeded smoothly to give the corresponding cyanohydrin trimethylsilyl ethers or amino nitrile derivatives in good to excellent yields.     

α,β-(Phosphino)(aminocarbene) and α,ω-(phosphino)(oxyaminocarbene): new bidentate ligands for transition metal complexes

Direct complexation of (amino)(phosphino)carbene 1a and (amino)(oxy)carbene 1b featuring a phosphino group in position-6 to the carbene with [Rh(CO)₂Cl]₂ has been studied. With the 1,2-bidentate ligand 1a, an original cationic complex 2 featuring two (amino)(phosphino)carbenes η²-bonded to the metal has been isolated in 79% yield. In the case of the 1,6-bidentate ligand 1b, a rhodium(I) complex 3 in which the carbene is in trans position relative to the CO ligand was obtained in 85% yield. Both compounds were fully characterized including X-ray diffraction studies.     

Functionalized enamines—XVII : Ring expansion of α,β-unsaturated cyclic ketones by reaction of pyrrolidine dienamines with dihalocarbenes; Synthesis of A-homo-steroids

Reaction of pyrrolidine dienamine 2b with carbenes 3a and 3b leads to the formation of ring expanded ketones 4a and a mixture of 4a and 4b, respectively. The ring-expansion reaction has been utilized in a two-step conversion of Δ⁴-cholestenone and testoterone acetate into the A-homo-steroids 7a-c. Structure assignments and the mechanism of formation of the reaction products are discussed.     

Palladium complexes of amido-functionalized N-heterocyclic carbenes as effective precatalysts for the Suzuki-Miyaura C-C cross-coupling reactions of aryl bromides and iodides

A series of air-stable, robust and highly active palladium based precatalysts of amido-functionalized N-heterocyclic carbenes for the Suzuki-Miyaura C-C cross-coupling reaction has been designed. In particular, the [1-R-3-{N-(benzylacetamido)imidazol-2-ylidene]₂PdCl₂ [R = i-Pr (1c) and CH₂Ph (2c)] complexes efficiently carried out the Suzuki-Miyaura coupling of the aryl bromide and iodide substrates with phenyl boronic acid in good to excellent yields in air at 90 °C in 12 h. Quite interestingly, of these palladium precatalysts, the i-propyl derivative (1c) exhibited superior activity as compared to the benzyl derivative (2c). The density functional theory (DFT) studies carried out on the 1c and 2c complexes revealed the strong σ-donating nature of the NHC ligand as reflected in their high d/b ratio [i.e. forward σ-donation (d) to backward π-donation (b)] of these complexes and, thus, point towards greater stability of the Pd-NHC interaction in these complexes.     

[4+2] Cyclization reactions of chiral Cβ-substituted Fischer alkenyl carbene complexes: efficient synthesis of enantiopure cyclohexenone and norbornene derivatives

Chiral alkenyl carbene complexes of tungsten(0) 1 and 2, readily available from the chiral pool, undergo the [4+2] cycloaddition with the Danishefsky diene to provide enantiopure 4-alkenyl-2-cyclohexenone adducts 5 and 7 with high stereoselectivity after treatment of the primary cycloadducts 4 and 6 with TBAF. Cyclopentadiene also cycloadds to carbenes 1 and 2 affording the expected norbornene metal carbene complexes 10 and 12 with remarkable diastereo and face selectivity. Oxidative removal of the metal pentacarbonyl fragment leads to the ester derivatives 11 and 13. The X-ray structure analysis of two cycloadducts derived from carbenes 1 and 2 has been performed.