Carbene Center Research Articles

Homoaromaticity in carbene intermediates

Analyses of isodesmic reactions for singlet 7-carbenanorbornene (4S), 8-carbena-endo-tricyclo[3.2.1.0(2,4)]octane (5S), 3-carbenabicyclo[3.1.0]hexane (3S), 2-carbenanorbornene 6S, and 2-carbenabicyclooctadiene 7S at the B3LYP/6-311+G(3df, 2p)//B3LYP/6-31G level provide stabilization energies of 13.83, 13. 50, 3.00, -2.22, and -3.01 kcal/mol, respectively. The C7 carbene in 4S and the C8 carbene center in 5S are strongly bent toward the double bond and cyclopropane ring, respectively, in contrast to their related triplets, 4T and 5T and parent hydrocarbons. The geometric change for 3S compared to 3T or to parent bicyclo[3.1. 0]hexane is minimal. Comparison of the stability of 6S with 2-carbenanorbornane and the geometry of 6S with that of 6T and also with the singlet and triplet 2-carbenanorbornane suggests very modest bridging. The stabilization energy and geometry of 2-carbenabicyclooctadiene 7 resemble antihomoaromatic bicyclooctadienyl cation 9 rather than the related homoaromatic bicyclooctadienyl anion 8. The diamagnetic susceptibility exhaltations (Deltachi) of 3, 4, 5, and 7, calculated at the B3LYP/6-311+G(2d,p)//B3LYP/6-31G level, are -0.7, 22.7, 26.0, and -10.3 cgs-ppm, respectively. The singlet-triplet energy differences, DeltaE(TS), for carbenacyclohexane, carbenacyclopentane, 3, 4, 5, 6, 7, and 2-carbenabicyclo[3.2.1]oct-3-ene at the B3LYP/6-311+G(3df, 2p)//B3LYP/6-31G level are 3.4, 10.3, 8.8, 27.1, 25.9, 12.7, -4.0, and -0.9 kcal/mol, respectively.

A cyclic carbanionic valence isomer of a carbocation: diphosphino analogs of diaminocarbocations

Diaminocarbocations (or amidinium salts) feature a three-center 4pi electron system with an open planar structure. Their 2pi electron three-membered cyclic valence isomers, in which the carbon atom bears a negative charge, are predicted to be about 541 kilojoules per mole higher in energy than the open form. This isomer has not been identified yet. In contrast, the attempted synthesis of a diphosphorus analog of amidinium salts leads to the cyclic carbanionic form. There is no precedent for such a transformation of a carbocationic center into a carbanionic center, but with the help of heavier main-group elements, numerous examples can be imagined. This approach will enable the preparation of many unknown structural moieties that are difficult or even impossible to access in the corresponding carbon and nitrogen series.

Nucleophilic Reactivity of the Multiply Bonded Carbon Center in Group 4−Pincer Bis(iminophosphorano)methanediide Complexes

The bis(iminophosphorano)methanediide (carbene) complexes [MCl2{C(Ph2PNSiMe3)2-κC,κ2N,N‘}] (M = Hf (1), Zr (2)) form Lewis acid−base adducts with THF, nitriles, and isonitriles and undergo 1,2-addition reactions with amines, alcohols, and alkyl iodides and [2 + 2]-cycloaddition reactions with heteroallenes. The halide complexes can be alkylated with lithium alkyl reagents without attack at the carbene center. All these reactions are consistent with high-valent metal alkylidene character.

A new route to functionalized 3-aminopyridazines by ANRORC type ring transformation of 1,2,4-triazines with carbon nucleophiles

The reaction of 3-chloro-6-phenyl-1,2,4-triazine 1a with carbon nucleophiles 2ad bearing a cyano substituent at a carbanionic center has been studied. In all reactions the formation of the corresponding 3-aminopyridazines 3ad takes place via ANRORC mechanism involving addition of the nucleophile at position 5 in compound 1a, ring opening with breaking of the N4C5 bond and intramolecular ring closure of the resulting open-chain intermediate. A 15N study with labeled phenylacetonitrile 2a* has shown that the nitrogen atom of the exocyclic amino group of 3-amino-4,6-diphenylpyridazine 3a was originally present in phenylacetonitrile.

New Transformations from a 3-Silyloxy 2-Aza-1,3-diene: Consecutive Zr-Mediated Retro-Brook Rearrangement and Reactions with Electrophiles

A one-pot procedure for the transformation of the title compound to α-functionalized (silylated) and α,β-unsaturated secondary amides was described. The following steps were involved: a Zr-mediated retro-Brook rearrangement, selective deprotonation with n-BuLi on the organometallic intermediate, and trapping with electrophiles including alkyl and acyl halides and aldehydes. The electrophilic addition step occurred at a stabilized α-silyl carbanion center without affecting the near transition metal residue. In the case of aldehydes, the Peterson alkenation reaction took place on the transition metal complex in a highly stereoselective way.

Enantiocontrolled Macrocycle Formation by Catalytic Intramolecular Cyclopropanation

Stereoselectivity in intramolecular cyclopropanation reactions resulting in cyclopropane fusion with ten- and larger-membered rings has been examined using chiral copper(I) and dirhodium(II) catalysts. The influence of alkene structure and catalyst has been obtained using the 1,2-benzenedimethanol linker between the allylic double bond and diazoacetate. Control features in the addition reaction, especially those for diastereoselectivity and enantioselectivity, have been elucidated, and they are associated with the metal itself or its attendant ligands that influence the trajectory of the alkene to the carbene center. The influence of ring size, from five- to twenty-membered rings, on stereoselectivity has been determined with selected copper(I) and dirhodium(II) catalysts, and the changes in stereocontrol as a function of ring size can be understood as being due to a change in the olefin trajectory to the carbene center. Hydride abstraction from a benzylic position accompanies addition when dirhodium cata...

Tetrakis Fischer-Type Carbene Complexes:1 Synthesis, Molecular Structure, and [3 + 2 + 1]-Benzannulation

Tetrakis{pentacarbonyl[(methyleneoxy)phenylcarbene]chromium}methane (3) and tetrakis{pentacarbonyl[(methyleneoxy)(2-methoxyphenyl)carbene]chromium}methane (4) have been synthesized from tetramethylammonium acylchromate precursors 1 and 2 and pentaerythritol by an acylation/alcoholysis sequence. The X-ray structure analysis of 3 established a distorted-tetrahedral structure, whereas an averaged regular tetrahedral arrangement is indicated for 3 and 4 by NMR spectroscopy in solution. Upon reaction with 3-hexyne the phenyl complex 3 underwent complete benzannulation at all four carbene centers. The reaction was only moderately diastereoselective, and resultedafter in situ silylationin at least seven inseparable stereoisomers of tetrakis-(CO)3Cr-coordinated tetrakis[1-((tert-butyldimethylsilyl)oxy)-2,3-diethyl-4-(methyleneoxy)naphthyl]methane. Oxidative demetalation gave an 80% yield of tetrakis[1-((tert-butyldimethylsilyl)oxy)-2,3-diethyl-4-(methyleneoxy)naphthyl]methane (5). Benzannulation is blocked by inc...

The influence of protecting the hydroxyl group of β-oxy-α-diazo carbonyl compounds in the competition between Wolff rearrangement and [1,2]-hydrogen shift. Density functional theory study and topological analysis of the charge density

The influence of protecting the hydroxyl group of a β-oxy-α-diazo carbonyl compound on the competition between the Wolff rearrangement (WR) and the [1,2]-hydrogen shift (HS) was investigated theoretically. Stationary points on the potential-energy surface were located with the B3LYP density functional and the 6-31G** basis set. For the basic system geometry optimisations at B3LYP/6-311+G** were performed to validate the reliability of the B3LYP/6-31G** calculations. Single-point energy calculations were carried out at the B3LYP/6-311+G** level on the B3LYP/6-31G**-optimised geometries. Further insight into the processes was achieved with the aid of the theory of “atoms in molecules” of Bader. The calculated energy barriers qualitatively predicted the yields of HS and WR obtained experimentally. In order to rationalise the calculated energy barriers, it was necessary to take into account not only the electronegativity of the protective groups but also the alignment of the migrating groups with the depletion sites at the carbene centre. Further, when the hydroxyl group was not protected the existence of an intramolecular hydrogen bond played an important role in both HS and WR.

A new approach to the synthesis of porphyrin-fullerene dyads.

A new synthetic approach to porphyrin-fullerene dyads utilizing tetraphenylporphyrin (TPP) and three novel reactions (vicarious nucleophilic substitution, metathesis, and [4+2]-cycloaddition to fullerene) is described. 5-(4-Nitrophenyl)-10,15,20-triphenylporphyrin zinc complex reacts with the carbanion of chloromethyl para-tolyl sulphone (which bears a leaving group Cl at the carbanionic center), affording the nucleophilic substitution of hydrogen product containing a CH2SO2Tol group. The product obtained when alkylated with an alkyl halide (bearing an acetylenic function in the terminal position), followed by the cross-enyne metathesis reaction (with the use of an efficient ruthenium catalyst), resulted in the formation of a TPP-diene building block. Its Diels-Alder reaction with C60-fullerene led to the title dyad--a new artificial photosynthetic model. This method opens up the possibility for synthesis of dyads with variable distances between chromophores, depending on the type of the alkyl halides used.

Lithiated α-cyanophosphonates: self-assembly of two-dimensional molecular sheets composed of interconnected twenty-four membered rings †

Reaction of the phosphonates (RO)2P(O)CH2CN (R = Et I or PriII) with one molar equivalent of LiNPri2 in THF gave the metallated complexes [(RO)2P(O)CHCNLi·THF]∞ (R = Et 1 or Pri2). Crystallographic analyses of them reveal that they form dimeric Li2(O=P)2 units, which further aggregate through inter-dimer association via the nitrile of the phosphonate to form cross-linked, polymeric network structures. These sheets align in the crystals to form a ‘patchwork’ arrangement of molecular channels. The carbanionic centres of the phosphonates are devoid of Li–C contacts and almost perfectly planar. In solution, the dimeric units most likely remain intact but the nitrile is involved in chelation to a metal centre via intra-dimer association. Theoretical calculations show that chelation of the nitrile unit is a viable mode of bonding.

Crowded organometallic compounds of the alkali metals with diphenylphosphino substituents in the organic group

The lithium compound [LiC(SiMe2CH2PPh2)3] 1, obtained by metallation of the precursor HC(SiMe2CH2PPh2)3I with LiMe, crystallised from benzene as a solvate 1·1.5C6H6 that both in solution and in the solid state has an unusual tricyclic structure, with lithium bound to the carbanionic centre and the three phosphorus atoms. Lithiation of the related precursor HC(SiMe3)2(SiMe2CH2PPh2) II under similar conditions gave the dilithium compound [Li(thf)C(SiMe3)2{SiMe2CH[Li(thf)2]PPh2}] 2, which has a fluxional structure in solution. Metallation of the precursor HC(SiMe3)2(SiMe2PPh2) III gave the compounds MC(SiMe3)2(SiMe2PPh2) (M = Li 3 or Na 4). Compound 3 is fluxional in benzene with interchange of methyl groups between SiMe2 and SiMe3 fragments. Compound 4, in contrast, has a non-fluxional molecular structure. In the solid the molecules of 4 form chains in which the sodium is bound intramolecularly to the carbanionic centre and to phenyl and intermolecularly to phenyl and phosphorus. Attempts to make the potassium analogue of 3 or 4 led to cleavage of the P–Si bond and formation of KPPh2. This has a complicated polymeric crystal structure in which molecules are linked by potassium–phenyl interactions.

Small-ring carbenes carrying a positive charge: the effects of substituting by CN in c-C 4H 3+

Calculations at B3LYP/6-31+G( d) show c-C 4H 3 + to have a singlet ground state, with the triplet being 19.5 kcal mol −1 higher in energy. Singlet c-C 4H 3 + has a puckered ring structure ( C s symmetry) with an out-of-plane angle of 51.4°. The barrier to inversion via the planar ring structure ( C 2 v symmetry) is 29.1 kcal mol −1. In the puckered ring the cross–ring distance between CH groups is 1.793 Å and the Wiberg bond order is 0.436, indicating considerable interaction. The stability of the singlet state then is attributed to homoaromaticity. Ion c-C 4H 3 + in its triplet state is planar, but with alternate bonds being long and short, reminiscent of the geometries of c-C 4H 4 and c-C 4H 4 +. Substitution of H atoms by CN groups stabilises the triplet state relative to the singlet, and for C 4(CN) 3 + the triplet is only 8.4 kcal mol −1 higher. Substitution also reduces the out-of-plane angle of the ions in the singlet state, going from 51.4° in the unsubstituted ion c-C 4H 3 + to 43.4 kcal mol −1 in C 4(CN) 3 +. A CN group adjacent (α) to the carbene centre reduces the barrier by ∼7 kcal mol −1 whereas substitution at the opposite carbon (β position) reduces it by ∼2 kcal mol −1. These effects are roughly additive, with C 4(CN) 3 + having a barrier to inversion of only 12.9 kcal mol −1.

IR spectra and structure of (4-nitrophenyl)acetonitrile and of its carbanion: experimental and ab initio studies

The structures of (4-nitrophenyl)acetonitrile and of its carbanion were studied on the basis of both quantitative IR spectra and ab initio force field calculations. The spectral and structural changes, which take place in the course of the conversion of the parent molecule into the carbanion, are essential and spread over the whole species. In agreement between theory and experiment, the conversion studied causes strong frequency decreases (down to 136cm−1) and intensity increases (up to 90-fold) of the cyano and nitro stretching bands. The molecule→carbanion conversion is accompanied by both quinoidization of the phenylene ring and a change in the configuration of the methylenic carbon atom: from tetrahedral in the molecule it becomes planar in the carbanion. The carbanionic charge is delocalized over the carbanionic center (0.40e−), phenylene (0.24e−), nitro (0.21e−) and cyano (0.15e−) groups.

Diaminocarbenes; Calculation of Barriers to Rotation about Ccarbene−N Bonds, Barriers to Dimerization, Proton Affinities, and 13C NMR Shifts

Barriers to rotation about the Ccarbene−N bonds in diaminocarbene (H2N)2C 1a, bis(dimethylamino)carbene ((CH3)2N)2C 1b, the related formamidinium ions (H2N)2CH+ 2a and ((CH3)2N)2CH+ 2b, and the Li+ complexes (H2N)2CLi+ 3a and ((CH3)2N)2CLi+ 3b have been calculated using density-functional theory in order to study the extent of π-bond stabilization of the carbene center. Experimental barriers from DNMR are reported for 1b and 2b and compared with those for bis(diisopropylamino)carbene 1c and the N,N,N‘,N‘-tetraisopropylformamidinium ion 2c; rotational barriers computed for 1b and 2b including thermal corrections compare well with experiment. The dimerization of 1a and 1b have been studied with (full) geometry optimization up to the levels QCISD(T)/cc-pVDZ//MP2/cc-pVDZ and B3LYP/cc-pVDZ//B3LYP/cc-pVDZ, respectively. The minimum-energy path for the dimerization of 1a has been computed using the BPW91/cc-pVDZ method. It is shown that the transition state geometries for the dimerizations of 1a and 1b have C2 a...

(−)-Sparteine-Mediated Stereoselective Intramolecular Carbolithiation of 4-Substituted 5-Hexynyl Carbamates. Synthesis of Enantiopure 1,3-Difunctionalized Alkylidene Cyclopentanes

The stereoselective carbolithiation of alkynes with external chiral induction has been achieved for the first time by fusing the concepts of the asymmetric deprotonation (A) with s-BuLi/(−)-sparteine (s-BuLi/1) and the intramolecular carbolithiation (B). Several 4-functionalized 5-hexynyl carbamates with different terminal substituents have been prepared and efficiently cyclized providing enantiopure highly substituted alkylidene cyclopentanes. The presence of a sterically demanding substituent in the propargylic position is the essential feature of these cyclizations in order to suppress the abstraction of the remaining propargylic proton. Furthermore, in dependence on the terminal substituent, the 6-phenyl-substituted precursors undergo an intramolecular carbolithiation whereas for the 6-trimethylsilyl-substituted alkynes a subsequent migration of the O-carbamoyl group onto the vinylic carbanionic center follows.

Carbene-Lithium Interactions

Abstract The syntheses and characterizations are described for carbene•lithium complexes in which the lithium center is ligated only by carbon atoms. A single crystal X-ray diffraction structure determination on one adduct confirms the coordination environment. The C-Li distance at the carbene center is 215.5 pm. All other coordination to the lithium center originates from a substituted η5-cyclopentadienyl ligand with an average C–Li distance of 225.4 (7) pm.

The geometrical effect on the chemical reactivity of nucleophilic carbenes. A theoretical study

The reaction mechanism for the insertion of the nucleophilic carbenes into the C–H bond of methane has been investigated using the B3LYP/6-31G ∗ level of theory. It is found that the singlet–triplet splitting of a carbene can be used as a diagnostic tool for the prediction of its reactivity. Our theoretical findings also suggest that stronger p(π) donor atoms bonded the carbene atom, and a more bent bond angle at the carbene center should enhance stability of carbenes. Theoretical data for 5- and 4-membered ring carbenes are compared.

Base-promoted 1,2-shifts in α-benzyloxy ketones

Deprotonation of representative α- p-methoxybenzyl ketones results in the formation of rearranged α-benzylated α-hydroxy ketonic products. In the present examples, formal inversion of configuration at the carbanionic center is observed.

H ax vs. H eq migrations in tetrahydropyranylidenes generated by Bamford-Stevens reactions. Temperature dependence and non-stereoselective activation by ring oxygen

For 1,2-shifts in singlet carbenes, a tetrahydropyranyl oxygen strongly activates its adjacent H's. To determine whether this activation is stereoselective for H ax or H eq, we studied d-labeled analogs of 17a-oxa-D-homosteroidal C- 16 ketones ( 6a). A putative carbene center was generated at C-16 by thermal and photic Bamford-Stevens reactions, and the relative proportions of H and D shift from C-17 were assayed. After correction for primary H/D isotope effects, the H ax/H eq migration ratios in thermolysis were 2.2 at 170°C, 3.3 at 120°C, and 4.1 at 95°C; and in photolysis it was 4.4 at −70°C. The ratio at 170°C is virtually the same as for ordinary cyclohexylidenes under comparable thermal conditions, so the ring oxygen activates H ax and H eq non-stereoselectively. Graphical analysis of the temperature dependence in thermolysis gave ΔE a (or Δ(ΔH ‡)) = 2.7 k cal/mol (in favor of H ax) and Δ(ΔS ‡) 4.5 e.u. (in favor of H eq). Extrapolation indicates the H ax/H eq ratio would be ca. 10 at room temperature, corresponding to Δ(ΔG ‡) = ca. 1.35 kcal/mol.

Experimental and Computational Determination of the Effect of the Cyano Group on Carbon Acidity in Water

Rate constants kDO (M-1 s-1) for the deprotonation of cyanoalkanes by deuterioxide ion in D2O at 25 °C were determined by following the appearance of the deuterium-labeled cyanoalkanes by 1H NMR. These data were evaluated to give the following pKa's in water: CH3CN, 28.9; CH3CH2CN, 30.9; NCCH2CH2CN, 26.6. High level ab initio calculations on cyanoalkanes and α-cyano carbanions and combined QM/Monte Carlo calculations of their free energies of solvation were carried out. The interaction between a carbanionic center and an α-cyano substituent is concluded to be largely polar. The 5.1-fold difference in α-cyano and β-cyano substituent effects on carbon acidity in water which, nominally, is consistent with significant resonance stabilization of α-cyano carbanions is attributed to the differential solvation of cyanoalkanes and cyanocarbanions. The free energy change for the highly unfavorable tautomerization of acetonitrile to ketenimine in water was computed as ΔGT = 30.7 kcal/mol. We propose that the large ...