Carbene Rearrangement Research Articles

Samarium reagent-promoted formation of benzoins from diarylmethanones and DMF via a carbene rearrangement reaction

Prompted by samarium metal in DMF with TMSCl or iodine as an activator, or by SmI 2/THF system, diarylmethanones react readily with DMF to afford benzoins in moderate to good yields via rearrangement of aryl groups. As for asymmetric diarylmethanones, products resulting from the migration of either aryl group were obtained, where the migration of aryl groups shows certain priority.

Preparation and Structure of Di(2-azulenyl)ketene Adducts. g-Lactone and b-Lactam Derivatives

Di(2-azulenyl)ketene (1) generated by thermal decomposition of diazodi(2-azulenyl)ethanone (2) via Wolff rearrangement of carbonyl carbene (8), and the reactive intermediate were confirmed by trapping reagents, such as azobenzene and cyclopentadienes. The reaction of 1 with tropone or imines afforded the corresponding γ -lactone (12) and β -lactams (15), respectively.

Synthesis and thermal cyclization of an enediyne-sulfonamide

The synthesis of an enediyne sulfonamide by alkylidene carbene rearrangement is reported. The compound cyclizes thermally to give the Bergman product, which was prepared independently for comparison. Like other σ-acceptor substituents at the enediyne alkyne termini, such as fluoride, oxonium or ammonium groups, the sulfonamide moiety enhances the reactivity for thermal Bergman cyclization as shown by the cyclization kinetic of the title compound.

Engineering Carbene Rearrangements in Crystals: From Molecular Information to Solid‐State Reactivity

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Thermally Induced Cyclopropene—Carbene Rearrangements: An Overview

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Engineering carbene rearrangements in crystals: from molecular information to solid-state reactivity.

Although severely restricted, reactions in crystals have delivered some of the most impressive examples of chemical control and provided an ideal medium for detailed mechanistic studies. However, to take full advantage of their potential and avoid their shortcomings, a systematic approach for the reliable design of reaction in crystals is required. To that end, strategies based on structural and energetic information contained at the molecular level have been explored with reactions involving carbene rearrangements.

Metallacyclopentatriene−Butadienyl Carbene Rearrangement in the Oxidative Coupling of Alkynes Mediated by [RuCp(SbR3)(CH3CN)2]+ (R = Ph, n-Bu)

The oxidative coupling of alkynes is investigated as promoted by the half-sandwich compound [RuCp(SbR3)(CH3CN)2]PF6 (R = Ph, n-Bu). There are various ways by which the initially formed electrophilic metallacyclopentatriene complex rearranges to a stable product. In the case where the alkyne disposes of an α-alkyl substituent, intramolecular 1,2 hydrogen migration leads to a butadienyl carbene complex as obtained by using 2,8-decadiyne or HC⋮CCH2R‘ (R‘ = n-Pr, Ph, OH). A conceivable mechanism for this reaction sequence is established by means of DFT/B3LYP calculations. In line with experiment, the existence of two isomeric butadienyl carbene complexes is proposed. In the case of R‘ = Ph and OH the butadienyl carbene is not the final product but rearranges to give η3-allyl-vinyl and η3-allyl-acyl complexes. In the absence of an α-alkyl substituent as for HC⋮CC6H9, HC⋮CC6H10OH, and HC⋮CCMe2OH, η3-butadienyl-vinyl complexes are formed. Finally, from the alkynes HC⋮CPh, HC⋮CBut, and HC⋮CSiMe3 no tractable materials are obtained.

Thermally induced cyclopropene-carbene rearrangements: an overview.

Thermally induced cyclopropene-carbene rearrangements: an overview.

Density functional (DFT) study of acyloxy carbene–carbene rearrangements

The rearrangement of the acyloxy carbene, XCOOC:H, to the carbene XC:OCHO, via a shift of the carbonyl oxygen from one carbon to another, has been investigated by the DFT method using the B3LYP potential. Different substituents taken for the study are X=H (S 0 and T 1), CH 3, B, F and Cl. It has been found that the reaction proceeds in two steps and the intermediate is a four-centered structure, which is the formally neutral equivalent of the 6π-electron cyclobutadienyl dianion. The activation energy for the first step is not affected very much with change in the substituent (X), but the second step is very sensitive to the nature of the substituent. This has been explained on the basis of the charge transfer effects. The thermodynamic and kinetic parameters for the reaction have also been discussed.

The 1-norbornene skeleton by carbene rearrangement.

4-Bromo-1-(dibromomethyl)bicyclo[2.1.1]hexane (18) was synthesized by formation of the dienolate of dimethyl cyclopentanedicarboxylate 15 b, which is then transformed into 16. Reaction of 16 with diiodomethane gives the diester 14 b, and selective saponification leads to the half-ester 14 c. Degradation of 14 c to methyl 4-bromobicyclo[2.1.1]hexane-1-carboxylate (17 a), reduction of the ester to the corresponding carbinol 17 b, oxidation of 17 b to the aldehyde 17 c, and conversion of the aldehyde with triphenyl phosphite/bromine gives compound 18. Reaction of 18 with NaN(SiMe(3)) in diethyl ether in the presence of diphenylisobenzofuran afforded a 3.3:1 mixture of the Diels-Alder adducts 22 and 23, indicating the presence of 2,4-dibromobicyclo[2.1.1]hept-1-ene as a reactive intermediate generated by rearrangement of carbene 19.

Singlet-triplet splittings and barriers to Wolff rearrangement for carbonyl carbenes.

High-level ab initio calculations at the G3(MP2)//B3-LYP level have been used to study carbomethoxychlorocarbene and related halogenocarbenes and carbonyl carbenes. Initial calculations at the more accurate W1' level on the subset CH(2), HCCl, HCF, CCl(2), and CF(2) provide support for the reliability of G3(MP2)//B3-LYP for this type of problem. The W1' calculations also suggest that the experimental S-T splitting is slightly underestimated for HCCl and CF(2) and substantially underestimated for CCl(2), in keeping with other recent high-level studies. Whereas the parent carbonyl carbenes, namely formylcarbene, carbohydroxycarbene, and carbomethoxycarbene, are all predicted to have triplet ground states, their chloro and fluoro derivatives are predicted to have singlet ground states. In particular, carbomethoxychlorocarbene is predicted to have a singlet ground state, with the singlet-triplet splitting estimated as -16.0 kJ mol(-)(1). The barriers to Wolff rearrangement of the singlet carbonyl carbenes generally (but not always) correlate with the exothermicity accompanying the production of ketenes. In the case of the parent carbonyl carbenes, for which the rearrangement reaction is most exothermic, the barriers lie between about 10 and 30 kJ mol(-)(1), whereas for the less exothermic rearrangements of the chloro- and fluoro-substituted carbonyl carbenes, the Wolff rearrangement barriers increase significantly to between 58 and 75 kJ mol(-)(1). The calculated barrier for carbomethoxychlorocarbene is 58.2 kJ mol(-)(1).

An experimental and computational study of 1,2-hydrogen migrations in 2-hydroxycyclopentylidene and its conjugate base.

Thermal decomposition of alpha-hydroxydiazirine 2 gives primarily cyclopentanone and some allylic alcohol, in similar amounts as the known cyclohexyl analogue 1. Calculations (B3LYP/6-31+G) also show cyclopentanone to be the major product of this carbene rearrangement. Diazirine 2 and the lithium salt of the corresponding conjugate base 3 were decomposed by photolysis. The proportion of ketone formed increases with deprotonation, a trend also found computationally. In comparison, the base-induced isomerization of cyclopentene oxide, which proceeds via alpha-elimination to a carbenoid intermediate similar to that obtained from 3, yields primarily allylic alcohol rather than ketone; neither ring size nor charge thus accounts for the unusual product distribution observed. Interestingly, the calculations reveal that in the gas phase with no counterion, the singlet, oxyanionic carbene, and the alpha-deprotonated epoxide are the same, rather than discrete structures. This intramolecular complexation stablilizes the oxyanionic carbene by 20-25 kcal/mol.

Matrix Isolation and Photochemistry of α-Diazo Sulfoxides: Formation of α-Oxo Sulfine as an Intermediate

The photochemistry of the α-diazo sulfoxides 1a and 1b has been investigated in solid argon at 10 K by IR and UV/Vis spectroscopy. Both diazo compounds react to form sulfine 2a by photochemically induced hetero Wolff rearrangement of sulfinyl carbene 3, which itself cannot be detected in solid argon. Irradiation at λ > 320 nm results in decomposition of the sulfine 2a to give 2-oxa-4-thiabicyclo[4.4.0]decane-3.5-dione (5) and 7-oxabicyclo[4.2.0]octan-8-one (6) under abstraction of COS. The matrix-isolated species have been identified by comparison of experimental and calculated IR spectra.

The reaction of benzene with a ground state carbon atom, C(3Pj)

The reaction between benzene and a single ground state carbon atom, C(3Pj), which yields a C7H5 radical without a barrier in the exit channel, has been studied using density functional theory (B3LYP), Møller–Plesset perturbation theory, and the G2(B3LYP/MP2) and complete basis set (CBS) model chemistries. Comparing the computed reaction energies for the formation of various C7H5 radicals with experimental data suggests that the 1,2-didehydrocycloheptatrienyl radical (15) is observed in crossed-beams experiments at collision energies between 2 and 12 kcal mol−1. The carbon atom attacks the π-electron density of benzene and forms without entrance barrier a Cs symmetric complex (17T) in which the carbon atom is bound to the edge of benzene. From 17T, the insertion of the C atom into a benzene CC bond to yield triplet cycloheptatrienylidene (9T) is associated with a much lower barrier than the insertion into a CH bond to give triplet phenylcarbene (7T). As both steps are strongly exothermic, high energy vinyl carbene rearrangements on the triplet C7H6 potential energy surface provide pathways between 9T and 7T below the energy of separated reactants. In addition, intersystem crossing in the vicinity of 17T and 9T might give rise to singlet cycloheptatetraene (12S). The monocyclic seven-membered ring compounds 9T or 12S are precursors of the 1,2-didehydrocycloheptatrienyl radical: the dissociation of a CH bond α to the divalent carbon atom proceeds without an exit barrier, in agreement with experiment. In contrast, a direct carbon–hydrogen exchange reaction pathway analogous to the aromatic electrophilic substitution followed by rearrangement of phenylcarbyne (13) to 15 involves high barriers (39 kcal mol−1 with respect to separated reactants) and is thus not viable under the experimental conditions.

Formation of Anti-Bredt Olefins from Bridgehead Carbene Precursors: A Computational Study

Density functional theory (DFT) calculations are employed to examine the energetics of the rearrangement of bridgehead carbenes to anti-Bredt olefins. These C-migrations are found to have very small activation barriers. For instance, rearrangement of bicyclo[2.2.2]octylcarbene has a transition-state energy that lies 3.8 kcal/mol above the carbene. These rearrangements are also quite exothermic, despite the instability of the bridgehead alkene. Rearrangement of bicyclo[1.1.1]pentylcarbene to bicyclo[2.1.1]hex-1-ene is exothermic by 18.4 kcal/mol. The greater thermodynamic stability of the anti-Bredt olefin products, as well as the low reaction barrier, demonstrates that bridgehead carbenes are a viable source of these strained alkenes.

Flash vacuum pyrolysis of N-alkenylbenzotriazoles and N-alkenylisoxazolones

The flash vacuum pyrolysis of 1-(2-ethoxycarbonylethenyl)benzotriazole has been reinvestigated, and the processes leading to the formation of ethyl indole-3-carboxylate and 2-ethoxyquinolin-4(1H)-one elucidated. Similar pathways are followed in the flash vacuum pyrolysis of the corresponding benzisoxazolone. Some N-ethenylisoxazolones have been pyrolysed to form pyrroles, but rearrangement of the intermediate carbenes may be observed. Photolysis of the isoxazolones gives carbenes that may be captured by solvent or give pyrroles.

ChemInform Abstract: Carbene Rearrangements. Part 50. 1‐Bromobicyclo[1.1.0]butane as an Easily Obtainable C4‐Building Block: A Novel Route to Cyclobutanone.

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Carbene−Bridgehead Olefin−Carbene Rearrangement: Formation of the Tetracyclo[4.3.0.02,8.04,7]nonane Skeleton by Carbene CH Insertion1

The sequence of rearrangements of carbenes 6 has been studied. Precursors for 6 were 1-halo-3-(n-halomethyl)bicyclo[1.1.1]pentane derivatives 11, which were obtained by addition of polyhalomethanes to the central bond of [1.1.1]propellane derivative 13 via radical chain processes. Reaction of compounds 11 with MeLi or sodium bis(trimethylsilyl)amide generates carbenes 6, which are stabilized by a carbene (6)−bridgehead olefin−carbene (9) rearrangement, followed by an intramolecular CH insertion reaction of carbenes 9 to afford tetracyclononane derivatives 15 in good yields. The carbenes 6 show only slight preferences in the rearrangement to alkenes 7 and 8. Both could be trapped with α-methylstyrene as ene adducts 16 and 17. The strongly twisted bridgehead olefin 7 has only the option to rearrange to carbene 9, whereas 8 could give 9 or 10, but prefers to give 9, in accordance with results of DFT calculations. The tetracyclononanes 15c and 15d contain halide atoms in the 1,3-positions; on treatment with t...

A Laser Flash Photolysis Study of 2-Naphthyl(carbomethoxy)carbene

Photolysis of methyl 2-diazo(2-naphthyl)acetate releases singlet 2-naphthyl(carbomethoxy)carbene. The singlet carbene relaxes to the lower energy triplet state within 350 ps−1 ns. Singlet to triplet carbene intersystem crossing is much faster than Wolff rearrangement to the corresponding ketene. The barrier to Wolff rearrangement of the spin-equilibrated carbene is 3.4 kcal/mol in hexafluorobenzene. In this system, ketene is formed from the carbene and is not formed to a significant extent from an excited state of the diazo compound.

The case of the missing acetylene. The mechanism of an intramolecular SN(V) reaction and a new route to 1-methylbenzo[de]quinolines

1-Halogeno-2-(1-naphthyl)alkenes with a dimethylamino group in the peri position are smoothly converted to 1-methylbenzo[de]quinoline under mild conditions, in a process involving the loss of MeBr rather than HBr. The Z-bromide is 45 times more reactive than its E-isomer, and 7 times more reactive than the Z-chloride. These systems were designed to show efficient elimination of HBr, but acetylene is not a likely intermediate. There is good evidence to support both of two alternative addition–elimination mechanisms: a 6-endo-dig route which would involve a “normal” addition–elimination process in an unusual setting; and a tandem 5-exo-dig Michael addition–carbene rearrangement. The evidence so far does not permit a final choice between the two.