Thermal C2−C6 Cyclization of Enyne−Carbodiimides: Experimental Evidence Contradicts a Diradical and Suggests a Carbene Intermediate

Hidden intermediate activation: a concept to elucidate the reaction mechanism of the Schmittel cyclization of enyne-allenes

The mechanistic paradigm in which the Schmittel cyclization transitions from one-step to stepwise has been investigated through the stabilization of a full hidden intermediate in the framework of the Diabatic Model of Intermediate Stabilization. Hidden intermediate activation was studied in silico employing quasi-classical trajectories and the Electron Localization Function. The stabilization of hidden intermediates achieved by substituting enyne-allenes with cyano and nitro groups generates the appearance of a partially hidden and an explicit intermediate, leading to one-step asynchronous biradical and stepwise biradical/zwitterionic mechanisms, respectively. The mechanistic feature associated with the activation level of the hidden intermediate arises from the Thornton effect and non-RRKM dynamics, where in the case of the CN-substituted system, despite having a single transition state, 54% of the effective trajectories remain in the intermediate zone after 540 fs, indicating that a mixture of mechanisms is observed.

Development of an Allenyne-Alkyne [4+2] Cycloaddition and its Application to Total Synthesis of Selaginpulvilin A

A new [4+2] cycloaddition of allenyne-alkyne is developed. The reaction is believed to proceed with forming an α,3-dehydrotoluene intermediate. This species behaves as a σπ-diradical to react with a hydrogen atom donor, whereas it displays a zwitterionic reactivity toward weak nucleophiles. The efficiency of trapping α,3-dehydrotoluene depends not only on its substituents but also the trapping agents. Notable features of the reaction are the activating role of the extra alkyne of the 1,3-diyne that reacts with the allenyne moiety and the opposite mode of trapping with oxygen and nitrogen nucleophiles. Oxygen nucleophiles result in the oxygen-end incorporation at the benzylic position of the α,3-dehydrotoluene, whereas with amine nucleophiles the nitrogen-end is incorporated into the aromatic core. Relying on the allenyne-alkyne cycloaddition as an enabling strategy, a concise total synthesis of phosphodiesterase-4 inhibitory selaginpulvilin A is realized.

Preparation of Carbodiimides with One-Handed Axial Chirality

The axial chirality of carbodiimide was proposed in 1932, but the synthesis of carbodiimide with one-handed axial chirality has not been achieved because of the low barrier of racemization. This work presents a strategy to use a conformationally restrained cyclic structure for creating carbodiimides whose biases of the axial chirality (labeled as S_NCN/ R_NCN) are higher than 100:1, as determined by vibrational circular dichroism spectroscopy and density functional theory calculations.

Indium-Catalyzed Annulation of o-Acylanilines with Alkoxyheteroarenes: Synthesis of Heteroaryl[b]quinolines and Subsequent Transformation to Cryptolepine Derivatives

We disclose herein the first synthetic method that is capable of offering heteroaryl[b]quinolines (HA[b]Qs) with structural diversity, which include tricyclic and tetracyclic structures with (benzo)thienyl, (benzo)furanyl, and indolyl rings. The target HA[b]Q is addressed by the annulation of o-acylanilines and MeO–heteroarenes with the aid of an indium Lewis acid that effectively works to make two different types of the N–C and C–C bonds in one batch. A series of indolo[3,2-b]quinolines prepared here can be subsequently transformed to structurally unprecedented cryptolepine derivatives. Mechanistic studies showed that the N–C bond formation is followed by the C–C bond formation. The indium-catalyzed annulation reaction thus starts with the nucleophilic attack of the NH₂ group of o-acylanilines to the MeO-connected carbon atom of the heteroaryl ring in an S_NAr fashion, and thereby the N–C bond is formed. The resulting intermediate then cyclizes to make the C–C bond through the nucleophilic attack of the heteroaryl-ring-based carbon atom to the carbonyl carbon atom, providing the HA[b]Q after aromatizing dehydration.

Solving the puzzling competition of the thermal C(2)-C(6) vs Myers-Saito cyclization of enyne-carbodiimides

The mechanism of the thermal cyclization of enyne-carbodiimides 7a–c has been studied computationally by applying the DFT method. The results indicate that enyne-carbodiimides preferentially follow the C²–C⁶ (Schmittel) cyclization pathway in a concerted fashion although the Myers–Saito diradical formation is kinetically preferred. The experimentally verified preference of the C²–C⁶ over the Myers–Saito pathway is guided by the inability of the Myers–Saito diradical to kinetically compete in the rate-determining trapping reactions, either inter- or intramolecular, with the concerted C²–C⁶ cyclization. As demonstrated with enyne-carbodiimide 11, the Myers–Saito channel can be made the preferred pathway if the trapping reaction by hydrogen transfer is no more rate determining.

Mechanistic Information from Nonstationary Points

The thermal cyclization of enyne-carbodiimides substituted at both the alkyne and carbodiimide terminus showed two curved Hammett correlations (log k/k0 against σp) that were fully reproduced by DFT (density functional theory) computational results. The latter suggest a concerted mechanism, but the transition state (TS) analysis failed to reveal any mechanistic insight about the reason for a curved Hammett correlation. Instead a preTS inspection, i.e., examination of the electronic and steric details on route between reactant and TS, furnished a detailed picture of the mechanism.

An isocyanide insertion approach to substituted pyrrolo[2,3-b]quinolines under metal-free and azide-free conditions

An isocyanide insertion approach to construct the pyrrolo[2,3-b]quinoline skeleton by an I₂/CHP mediated reaction of aminome-thylenecyclo-propanes (amino-MCPs) is developed.

Mechanistic Investigation on the Formation of 2-Halo-3-aryl-4(3H)-quinazoliniminium Halides from Heteroenyne-allenes: A Computational Study

Intramolecular cyclization of the heteroenyne-allene, 2-((phenylimino)methyleneamino)-benzonitrile (1) in the presence of HCl to produce 2-chloro-3-phenyl-4(3H)-quinazoliniminium chloride (Qz) involves the formation of two new bonds: a C-Cl bond and a C-N bond. We propose five pathways for this reaction. Four of these pathways involve chloride capture to form the C-Cl bond prior to the intramolecular nucleophilic attack to form the C-N bond, while one pathway involves ring closure to form the C-N bond prior to C-Cl bond formation. All calculations were carried out at B3LYP and MP2 levels of theory and employed the 6-311G* basis set. The solvent effects were considered using a Polarized Continuum Model with dichloromethane as the solvent. The calculations at both levels show that the mechanism involves initial protonation of 1, preferentially at Nα to give 2 which rapidly captures the chloride ion to form 5. This intermediate is protonated at the -CN group to form 8ROT, which then tautomerizes to its more stable isomer 9ROT. The latter undergoes intramolecular nucleophilic attack from Nβ to the protonated -CN group to form the cyclized intermediate 12, which tautomerizes to its most stable isomer 13. The coordination of Cl(-) ion present in the solution with 13 gives the final product Qz.

A one-pot multistep cyclization yielding thiadiazoloimidazole derivatives

A versatile synthetic procedure is described to prepare the benzimidazole-fused 1,2,4-thiadiazoles 2a–c via a methanesulfonyl chloride initiated multistep cyclization involving the intramolecular reaction of an in-situ generated carbodiimide with a thiourea unit. The structure of the intricate heterocycle 2a was confirmed by single-crystal X-ray analysis and its mechanism of formation supported by DFT computations.