Experimental and theoretical investigation of reversible interconversion, thermal reactions, and wavelength-dependent photochemistry of diazo Meldrum's acid and its diazirine isomer, 6,6-dimethyl-5,7-dioxa-1,2-diaza-spiro[2,5]oct-1-ene-4,8-dione

The photochemical or thermal decomposition of diazo Meldrum's acid (1) in methanolic solutions yields ketoester 3a, the product of the Wolff rearrangement, while products produced from the singlet carbene were not detected. This observation, combined with the analysis of activation parameters for the thermal decomposition of 1, as well as with the results of DFT B3PW91/6-311+G(3df,2p) and MP2/aug-cc-pVTZ//B3PW91/6-311+G(3df,2p) calculations, allows us to conclude that the Wolff rearrangement of 1 is a concerted process. The outcome of the photolysis of diazo Meldrum's acid depends on the wavelength of irradiation. Irradiation with 254 nm light results in an efficient (Phi(254) = 0.34) photo-Wolff reaction, while at 355 nm, the formation of diazirine 2 becomes the predominant process (Phi(350) = 0.024). This unusual wavelength selectivity indicates that Wolff rearrangement and isomerization originate from different electronically excited states of 1. The UV irradiation of diazirine 2 leads to the loss of nitrogen and the Wolff rearrangement, apparently via a carbene intermediate. This process is accompanied by a reverse isomerization to diazo Meldrum's acid. Triplet-sensitized photolysis of both isomers results in the formation of Meldrum's acid, the product of a formal reduction of 1 and 2. Mild heating of diazirine 2 produces quantitative yields of diazo Meldrum's acid. The activation parameters for thermal reactions of diazo 1 and diazirino 2 isomers were determined in aqueous and dioxane solutions.     

Single-Nitrogen Atom Incorporation into B=B Bond via the N=N Bond Splitting of Diazo Compound and Diazirine

Triboraazabutenyne 3 is synthesized by the reaction of diboraazabutenyne 1 with aryl boron dibromide followed by the reduction. The ligand exchange to replace phosphine on the terminal sp² B atom with carbene furnishes 4 . ¹¹B NMR, solid-state structures, and computational studies disclose that 3 and 4 feature the extremely polarized B=B bond. 4 readily splits the N=N bond of both diazo compound and diazirine under ambient conditions, whereby one nitrogen atom is incorporated into the B=B moiety leading to a neutral diboraazaallene 6 . The mechanism of the reaction between 4 and diazo compound is extensively investigated by density functional theory (DFT) calculations, as well as the isolation of an intermediate.     

Experimental and theoretical analysis of the photochemistry and thermal reactivity of ethyl diazomalonate and its diazirino isomer. The role of molecular geometry in the decomposition of diazocarbonyl compounds

The photochemical or thermal decomposition of ethyl diazomalonate (1) or ethyl 3,3-diazirinedicarboxylate in methanol solutions yields the O-H insertion product 6, while products of the Wolff rearrangement were not detected in both cases. The analysis of temperature-dependent (13)C NMR spectra and the results of DFT B3LYP/6-311+G(3df,2p) and MP2/aug-cc-pVTZ//B3LYP/6-311+G(3df,2p) calculations allow us to conclude that diazodiester 1 predominantly exists in the Z,Z-conformation. In contrast, photolysis of the cyclic isopropylidene diazomalonate (3), which also has a Z,Z-configuration of the diazodicarbonyl moiety, results in a clean Wolff rearrangement. These observations allow us to conclude that the direction of the photodecomposition of diazomalonates is not controlled by the ground-state conformation. The quantum-mechanical analysis of the potential energy surfaces for the dediazotization of 1 and 3 suggests that the formation of a carbene as a discrete intermediate is controlled by the ability of the latter to adopt a conformation in which carbonyl groups are almost orthogonal to the carbene plane. The outcome of the photolysis of ethyl diazomalonate depends on the wavelength of irradiation. Irradiation with 254 nm light results in the loss of nitrogen and the formation of dicarboethoxycarbene (5, Phi(254) = 0.31), while at longer wavelengths, diazirine 2 becomes an important byproduct (Phi(350) = 0.09). This observation suggests that the formation of carbene 5 and isomerization to diazirine proceed from different electronically excited states of ethyl diazomalonate.     

Time resolved spectroscopy of carbenepyridene ylides: Distinguishing carbenes from diazirine excited states

The photochemistry of diazirines and diazo compounds is not as simple as nitrogen extrusion and carbene formation. The C-H bonds adjacent to the diazo and diazirine moieties can migrate in the excited state and produce stable products without the benefit of a relaxed carbene intermediate. Additionally, cyclobutyl substituted systems exhibit carbon migration. It is unfortunate that the products of photochemical rearrangement of precursor excited states are identical to the products of thermal rearrangement of carbenes. This has prevented accurate measurement of the yield and absolute reactivity of alkylcarbenes. That pyridine reacts selectively with carbenes and not with the excited states of their nitrogenous precursors has allowed the separation of these two pathways and an appreciation of their relative importance with structural variation.     

Metallacarbenes from diazoalkanes: an experimental and computational study of the reaction mechanism

PCP ligand (1,3-bis-[(diisopropyl-phosphanyl)-methyl]-benzene), and PCN ligand ([3-[(di-tert-butyl-phosphanyl)-methyl]-benzyl]-diethyl-amine) based rhodium dinitrogen complexes (1 and 2, respectively) react with phenyl diazomethane at room temperature to give PCP and PCN-Rh carbene complexes (3 and 5, respectively). At low temperature (-70 degrees C), PCP and PCN phenyl diazomethane complexes (4 and 6, respectively) are formed upon addition of phenyl diazomethane to 1 and 2. In these complexes, the diazo moiety is eta(1) coordinated through the terminal nitrogen atom. Decomposition of complexes 4 and 6 at low temperatures leads only to a relatively small amount of the corresponding carbene complexes, the major products of decomposition being the dinitrogen complexes 1 and 2 and stilbene. This and competition experiments (decomposition of 6 in the presence of 1) suggests that phenyl diazomethane can dissociate under the reaction conditions and attack the metal center through the diazo carbon producing a eta(1)-C bound diazo complex. Computational studies based on a two-layer ONIOM model, using the mPW1K exchange-correlation functional and a variety of basis sets for PCP based systems, provide mechanistic insight. In the case of less bulky PCP ligand bearing H-substituents on the phosphines, a variety of mechanisms are possible, including both dissociative and nondissociative pathways. On the other hand, in the case of i-Pr substituents, the eta(1)-C bound diazo complex appears to be a critical intermediate for carbene complex formation, in good agreement with the experimental results. Our results and the analysis of reported data suggest that the outcome of the reaction between a diazoalkane and a late transition metal complex can be anticipated considering steric requirements relevant to eta(1)-C diazo complex formation.     

Reactions of Diazo Compounds with Alkenes Catalysed by [RuCl(cod)(Cp)]: Effect of the Substituents in the Formation of Cyclopropanation or Metathesis Products

The reaction of diazo compounds with alkenes catalysed by complex [RuCl(cod)(Cp)] (cod=1,5‐cyclooctadiene, Cp=cyclopentadienyl) has been studied. The catalytic cycle involves in the first step the decomposition of the diazo derivative to afford the reactive [RuCl(Cp){?C(R¹)R²}] intermediate and a mechanism is proposed for this step based on a kinetic study of the simple coupling reaction of ethyl diazoacetate. The evolution of the Ru–carbene intermediate in the presence of alkenes depends on the nature of the substituents at both the diazo N₂?C(R¹)R² (R¹, R²=Ph, H; Ph, CO₂Me; Ph, Ph; C(R¹)R²=fluorene) and the olefin substrates R³(H)C?C(H)R⁴ (R³, R⁴=CO₂Et, CO₂Et; Ph, Ph; Ph, Me; Ph, H; Me, Br; Me, CN; Ph, CN; H, CN; CN, CN). A remarkable reactivity of the complex was recorded, especially towards unstable aryldiazo compounds and electron‐poor olefins. The results obtained indicate that either cyclopropanation or metathesis products can be formed: the first products are favoured by the presence of a cyano substituent at the double bond and the second ones by a phenyl.     

Conformations and reactions of bicyclo[3.2.1]oct-6-en-8-ylidene

Bicyclo[3.2.1]oct-6-en-8-ylidene (1) can assume either the conformation of "classical" carbene 1a or that of foiled carbene 1b in which the divalent carbon bends toward the double bond. Oxadiazoline precursors for the generation of 1 were prepared, followed by photochemical and thermal decomposition as well as flash vacuum pyrolysis (FVP) of a tosyl hydrazone sodium salt precursor, to give a number of rearrangement products. Matrix isolation experiments demonstrate the presence of a diazo intermediate and methyl acetate in all photochemical and thermal precursor reactions. The major product from rearrangements of "classical" bridged carbene 1a is bicyclo[3.3.0]octa-1,3-diene as a result of an alkyl shift, while dihydrosemibullvalene formed from a 1,3-C-H insertion. In contrast, thus far unknown strained bicyclo[4.2.0]octa-1,7-diene formed by a vinyl shift in foiled carbene 1b. The experimental results are corroborated by density functional theory (DFT), MP2, and G4 computations.     

A nonspectroscopic method to determine the photolytic decomposition pathways of 3-chloro-3-alkyldiazirine: carbene,diazo and rearrangement in excited state

C(60) acts as a mechanistic probe for the formation of carbene, diazo compound, and for the rearranged product via the excited state in the photolysis of 3-chloro-3-isopropyldiazirine and 3-chloro-3-chloromethyldiazirine. The carbene adds to C(60) to form methanofullerene, whereas the diazo compound adds to C(60) to form fulleroid. The olefin product arises as a result of the rearrangement in the excited state.     

Inter- and innermolecular reactions of chloro(phenyl)carbene

Supramolecular photolyses of 3-chloro-3-phenyl-3H-diazirine (8) were performed within cyclodextrin (CyD) hosts to determine whether these toroidal inclusion compounds could alter the reactivity of the ensuing carbene reaction intermediate, chloro(phenyl)carbene (9). Remarkably, no intramolecular products stemming from carbene 9 could be detected. Instead, modified CyDs were formed via so-called innermolecular reactions. Hence, diazirine 8 was photolyzed in various conventional solvents to gauge the intermolecular reactivity of carbene 9. Relevant results were used to rationalize the CyD innermolecular reaction products.     

Chemiluminescence from arylcarbene oxidation: Phenylchlorocarbene and (2-chlorophenyl)carbene

Chemiluminescence is observed in the thermal reaction of phenylchlorocarbene or (2-chlorophenyl)carbene and O₂, matrix-isolated in Ar. The chemiluminescence spectra closely match the phosphorescence of the corresponding carbonyl compounds. The reactivity of both carbenes towards O₂ is very different. Singlet carbene phenylchlorocarbene reacts thermally only slowly with O₂ up to 60 K. The oxidation products phenylchloroformate, benzoyl chloride and O(³P) are mainly formed photochemically on irradiation of the diazirine precursor. Triplet carbene (2-chlorophenyl)carbene reacts readily with O₂ at cryogenic temperatures to give mostly 2-chlorobenzaldehyde-O-oxide. The carbonyl-O-oxide is photochemically easily cleaved to give 2-chlorobenzaldehyde and O(³P). The reaction step leading to carbonyl compounds in their excited states is in both carbene oxidations the recombination of the free carbene and O(³P).     

tert-butylcarbene from 1,1-diiodoneopentane

When 1,1-diiodoneopentane is passed through a hot tube containing methyllithium-coated Pyrex chips, 1,1-dimethylcyclopropane and 2-methyl-2-butene are produced in near quantitative yield. The ratio of products indicates that the intermediate carbene is the same as is produced from thermal or photosensitized decomposition of tert-butyldiazomethane but different from that formed by direct irradiation of the diazo compound.     

Glycosylidene Carbenes. Part 2. Synthesis of O-Aryl Glycosides

Phenol, 4-methoxyphenol, 4-nitrophenol, methyl orsellinate ( 1 ), and 2,6-di(tert-butyl)-4-methylphenol (BHT; 2 ) have been glycosylated by thermal reaction (20–60°) with various glycosylidene-derived diazirines. 4-Methoxyphenol reacted with the D-glucosylidene-derived diazirine 3 to give O-glucosides ( 4 and 5 , 69%, 3:1) and C-glucosides ( 6 and 7 , 16%, 1:1). Similarly, phenol yielded O-glucosides ( 10 and 11 , 70%, 4:1) and C-glucosides ( 12 and 13 , 13%, 1:1). 4-Nitrophenol gave only O-glycosides, 3 leading to 14 and 15 (75%, 3:2; Scheme 1), and the D-galactosylidene-derived diazirine 17 to 22 and 23 (52% (from 16 ), 65:35; Scheme 2). The reaction of phenol with 17 yielded 58% (from 16 ) of the O-galactosides 18 and 19 (4:1) and 14% of the C-galactosides 20 and 21 (1:1). From the D-mannosylidene-derived diazirine 25 , we predominantly obtained the α-D-configurated 26 (38 % from 24 ). These results are interpreted by assuming that an intermediate (presumably a glycosylidene carbene) first deprotonates the phenol to generate an ion pair which combines to give O- and - with electron-rich phenolates - also C-glycosides. A competition experiment of 3 with 4-nitro- and 4-methoxyphenol gave the products from the former ( 14 and 15 ) and the latter phenol ( 4-7 ) in almost equal amounts. Differences in the kinetic acidity of OH groups, however, may form the basis of a regioselective glycosidation, as evidenced by the reaction of 3 with methyl orsellinate ( 1 ) yielding exclusively the 4-O-monoglycosylated products 27 and 28 (78%, 85:15), although diglycosidation is possible ( 27 → 31 and 32 ; 67%, 4:3; Scheme 3). Steric hindrance does not affect this type of glycosidation; 3 reacted with the hindered BHT ( 2 ) to afford 33 and 34 (81 %, 4:1). The predominant formation of 1,2-trans -configurated O-aryl glycosides is rationalized by a neighbouring-group participation of the 2-benzyloxy group.     

Reaction of carbethoxycarbene with 2-phenyloxirane and 2-phenyloxetane

Thermal or photochemical decomposition of ethyl diazoacetate in 2-phenyloxirane gave a complex mixture, the analysis of which indicated that the reaction of carbethoxycarbene had proceeded according to the scheme summarized in Fig. 1. The reaction probably involves intermediate formation of an oxygen-ylide (IV), thus accounting for the observed oxygen-transfer and oxetane formation. Copper-catalysed thermal decomposition resulted in a more selective distribution of products, secondary reactions as well as the tar-formation being drastically reduced. The action of other carbenes such as dihalo-, phenyl- and diphenyl-carbene yielded less or no oxygen-transfer products and the formation of oxetane was not observed. In some cases, especially in the reaction of bis(benzene-sulphonyl)carbene, the major product was cyclic dimers of 2-phenyloxirane.

2-Phenyloxetane reacts more selectively with carbethoxycarbene to produce a mixture of cis and trans isomers of 2-carbethoxy-3-phenyltetrahydrofuran in 72–80% yield.     

Oxorhenium complexes as aldehyde-olefination catalysts

Several oxorhenium compounds in the formal oxidation states V and VII are examined as catalysts for the aldehyde-olefination starting from diazo compounds, phosphines, and aldehydes. Of these, [ReMeO2(eta2-alkyne)] complexes provide the simplest catalysts to study, although [ReOCl3(PPh3)2] still remains the most efficient rhenium catalyst for aldehyde-olefination described to date. Prior to the reaction with the Re catalysts the phosphine and the diazo compound react to form a phosphazine. No catalytic reaction occurs in cases where no phosphazine formation is observed. The first step of the catalytic cycle involves the formation of a carbene intermediate by the reaction of phosphazine and catalyst under extrusion of phosphine oxide and dinitrogen. In a second step the carbene reacts with aldehyde under olefin formation and catalyst regeneration. Excess of alkyne as well as the presence of ketones slows down the catalytic reaction. The olefination of 4-nitrobenzaldehyde with diazomalonate is possible with these Re catalysts. In contrast, this reaction does not take place either in the classical Wittig fashion from Ph3P=C(CO2Et)2 and aldehyde or by use of all other catalysts for aldehyde olefination reactions reported to date. Catalytic ylide formation from diazo compounds seems therefore not to be the only pathway through which catalytic aldehyde-olefination reactions can proceed.     

Elevated Catalytic Activity of Ruthenium(II)–Porphyrin‐Catalyzed Carbene/Nitrene Transfer and Insertion Reactions with N‐Heterocyclic Carbene Ligands

Bis(NHC)ruthenium(II)–porphyrin complexes were designed, synthesized, and characterized. Owing to the strong donor strength of axial NHC ligands in stabilizing the trans M?CRR′/M?NR moiety, these complexes showed unprecedently high catalytic activity towards alkene cyclopropanation, carbene C? H, N? H, S? H, and O? H insertion, alkene aziridination, and nitrene C? H insertion with turnover frequencies up to 1950 min^?1. The use of chiral [Ru(D₄‐Por)(BIMe)₂] ( 1 g ) as a catalyst led to highly enantioselective carbene/nitrene transfer and insertion reactions with up to 98 % ee. Carbene modification of the N terminus of peptides at 37 °C was possible. DFT calculations revealed that the trans axial NHC ligand facilitates the decomposition of diazo compounds by stabilizing the metal–carbene reaction intermediate.     

Silver(I)-carbene complexes/ionic liquids: novel N-heterocyclic carbene delivery agents for organocatalytic transformations

[reaction: see text] N-Heterocyclic carbene (NHC) complexes with silver were investigated as sources of unsaturated NHC carbene catalysts via thermal decomposition. The NHC complex (1-ethyl-3-methylimidazol-2-ylidene)silver(I) chloride is an ionic liquid, and was found to catalyze the ring-opening polymerization of lactide at elevated temperatures to give narrowly dispersed polylactide of predictable molecular weight. Silver-carbene complexes can also be used for the catalysis of small molecule transesterification reactions. Thermolysis of the silver complexes in the presence of CS(2) yielded the zwitterionic CS(2) adducts of the carbene, implicating the intermediacy of the free carbene in these reactions.     

61-甲基-61-[12-(N,N-四氯邻苯二甲酰基)脱氢枞胺]-1,2-亚甲基富勒烯[60]的合成

以脱氢枞胺为原料,经氨基酰化、12位乙酰化、与对甲苯磺酰肼形成对甲苯磺酰腙衍生物,再通过卡宾中间体与C60进行[2+1]环加成反应合成了C60-脱氢枞胺衍生物。目标化合物经IR,UV-Vis,1H NMR,13C NMR,MALDI-TOF MS表征,所得化合物为[6,6]闭环结构C60加成产物。     

1,2-migration in rhodium(II) carbene transfer reaction: remarkable steric effect on migratory aptitude

A series of diazo carbonyl compounds bearing different substituents have been prepared in order to investigate the steric effect in 1,2-migration reaction of rhodium(II) carbene. Through the investigation on the diazo decomposition of these compounds with Rh2(OAc)4, it was found that the steric effect could dramatically influence the migratory aptitude. In many cases, the steric effect could override the inherent electronic effect of the substituent.     

Copper–Carbene Intermediates in the Copper‐Catalyzed Functionalization of OH Bonds

Copper–carbene [Tp^xCu?C(Ph)(CO₂Et)] and copper–diazo adducts [Tp^xCu{η¹‐N₂C(Ph)(CO₂Et)}] have been detected and characterized in the context of the catalytic functionalization of O?H bonds through carbene insertion by using N₂?C(Ph)(CO₂Et) as the carbene source. These are the first examples of these type of complexes in which the copper center bears a tridentate ligand and displays a tetrahedral geometry. The relevance of these complexes in the catalytic cycle has been assessed by NMR spectroscopy, and kinetic studies have demonstrated that the N‐bound diazo adduct is a dormant species and is not en route to the formation of the copper–carbene intermediate.     

Direct observation of carbene and diazo formation from aryldiazirines by ultrafast infrared spectroscopy

Ultrafast laser flash photolysis (lambda(ex) = 270 nm) of phenyldiazirine produces transient infrared absorptions at 2040 and 1582 cm(-1). The first band is assigned to phenyldiazomethane, and the second is assigned to singlet phenylcarbene. This assignment is consistent with DFT calculations. Diazo band integration reveals that photoisomerization from diazirine to diazo occurs within a few picoseconds of the laser pulse. The majority of carbene produced is also formed instantaneously.