Detection of Ylide Formation between an Alkylidenecarbene and Acetonitrile by Femtosecond Transient Absorption Spectroscopy

Probing the alkylidene carbene-strained alkyne equilibrium in polycyclic systems via the Fritsch-Buttenberg-Wiechell rearrangement

Strained cycloalkynes are valuable building blocks in synthetic chemistry due to their high degree of reactivity and ability to form structurally complex scaffolds, common features of many pharmaceuticals and natural products. Alkylidene carbenes provide a pathway to the formation of strained cycloalkynes through Fritsch-Buttenberg-Wiechell rearrangements, but this strategy, like other methods of alkyne generation, is believed to depend upon a thermodynamic equilibrium that favors the alkyne over the carbene. Herein three highly strained, polycyclic alkynes, previously thought to be thermodynamically inaccessible, are generated under mild conditions and intercepted through Diels-Alder cycloaddition with a diene trapping agent. The use of a different trapping agent also allows for the interception of the alkylidene carbene, providing the first instance in which both an exocyclic alkylidene carbene and its cycloalkyne Fritsch-Buttenberg-Wiechell rearrangement product have been trapped.

Carbenes from cyclopropanated aromatics

Although a ripe old discipline by now, carbene chemistry continues to flourish as both theorists and experimentalists have shown sustained interest in this area of research. While there are numerous ways of generating carbenes, the thermal and/or photochemical decomposition of diazo compounds and diazirines remains, by far, the most commonly used method of producing these intermediates. There is no disputing the fact that these nitrogenous precursors have served carbene researchers well, but their use is not without problems. They are often sensitive and hazardous to handle and, sometimes, the desired nitrogenous precursor simply may not be available, e.g., for synthetic reasons, to study the particular carbene of interest. Furthermore, there is a legitimate concern that the photochemical generation of carbenes in solution from diazo compounds and diazirines may be contaminated by reactions in the excited states (RIES) of the precursors themselves. As an alternative, several laboratories, including ours, have used cyclopropanated aromatic systems to generate a wide range of carbenes. In each case, the cheleotropic extrusion of carbenes is accompanied by the formation of stable aromatic by-products such as phenanthrene, indane, naphthalene, and 1,4-dihydronaphthalene. The emergence of these "non-traditional" carbene sources, their versatility, and promise are reviewed in this work.

Adamantylidenecarbene: Photochemical Generation, Trapping, and Theoretical Studies

Photolysis of 1-(2-adamantylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene in benzene (or benzene-d6) at ambient temperature produces adamantylidenecarbene. The carbene undergoes dimerization to a cumulene and may also be trapped in a stereospecific fashion by cis- and trans-4-methyl-2-pentene. No products attributable to 4-homoadamantyne, resulting from ring expansion of the carbene, could be detected. Coupled cluster/density functional theory calculations place the singlet carbene ∼49 kcal/mol below the triplet and show that the former must overcome a barrier of ∼13.5 kcal/mol to rearrange into 4-homoadamantyne.

Formation of 3-Oxa- and 3-Thiacyclohexyne from Ring Expansion of Heterocyclic Alkylidene Carbenes: A Mechanistic Study

The rearrangement pathways of two alkylidene carbenes appended to an oxa or thiacyclopentane into the corresponding heterocyclohexynes were elucidated using 13C-labeling experiments. Both carbenes exhibited a preference for migration of the allylic carbon bound to the heteroatom. Anomeric interactions involving a heteroatom lone pair and antibonding orbital of the migrating bond and inductive destabilization of the minor migratory pathway are discussed as plausible reasons for the observed trends.

Aromatic Homo-Nazarov-Type Cyclization of Benzonorcaradienes: Stereoselective Synthesis of Hydrochrysenes

Formal homo-Nazarov cyclization of benzonorcaradienes produced by intramolecular hydroarylation of arylated alkynylcyclopropanes promoted by TfOH has been described, providing stereoselective access to highly substituted hydrochrysenes. An unprecedented 1,2-acyl migration occurred for the 2-heteroaroyl substrates, thus giving the same products as their 3-heteroaroyl analogs. Moreover, these products could be readily oxidized by air to fully π-conjugated chrysenes after decarboxylation.

Direct Observation of the Dynamics of Ylide Solvation by Hydrogen-bond Donors Using Time-Resolved Infrared Spectroscopy

The photoexcitation of α-diazocarbonyl compounds produces singlet carbene intermediates that react with nucleophilic solvent molecules to form ylides. The zwitterionic nature of these newly formed ylides induces rapid changes in their interactions with the surrounding solvent. Here, ultrafast time-resolved infrared absorption spectroscopy is used to study the ylide-forming reactions of singlet carbene intermediates from the 270 nm photoexcitation of ethyl diazoacetate in various solvents and the changes in the subsequent ylide-solvent interactions. The results provide direct spectroscopic observation of the competition between ylide formation and C-H insertion in reactions of the singlet carbene with nucleophilic solvent molecules. We further report the specific solvation dynamics of the tetrahydrofuran (THF)-derived ylide (with a characteristic IR absorption band at 1636 cm-1) by various hydrogen-bond donors and the coordination by lithium cations. Hydrogen-bonded ylide bands shift to a lower wavenumber by -19 cm-1 for interactions with ethanol, -14 cm-1 for chloroform, -10 cm-1 for dichloromethane, -9 cm-1 for acetonitrile or cyclohexane, and -16 cm-1 for Li+ coordination, allowing the time evolution of the ylide-solvent interactions to be tracked. The hydrogen-bonded ylide bands grow with rate coefficients that are close to the diffusional limit. We further characterize the specific interactions of ethanol with the THF-derived ylide using quantum chemical (MP2) calculations and DFT-based atom-centered density matrix propagation trajectories, which show preferential coordination to the α-carbonyl group. This coordination alters the hybridization character of the ylidic carbon atom, with the greatest change toward sp2 character found for lithium-ion coordination.

Tuning the Cyclopropane Ring-Opening Reaction over Electronic Bias by Fusion to a Pre-Aromatic Ring: TfOH-Promoted Aromatization of Dibenzonorcaradienes to Dibenzo[f,h]isocoumarins

Fusion of the cyclopropane ring bearing two vicinal acceptors to the pre-aromatic dihydrophenanthrene ring, which is constructed by the Pd-catalyzed cross-coupling between the vicinal aromatic rings, is found to effectively direct the cleavage of the electronically unfavored cyclopropane bond between the vicinal acceptors. Consequently, a modular method for the rapid synthesis of dibenzo[f,h]isocoumarins from methyl ketones, aryl aldehydes, and α-keto esters via a reaction cascade of aldol condensation, Kukhtin-Ramirez cyclopropanation, Pd-catalyzed direct arylation, and acid-promoted aromatization has been realized.