Rhodium-Catalyzed Asymmetric Cyclodimerization of Oxabenzonorbornadienes and Azabenzonorbornadienes: Scope and Limitations

Asymmetric alkene-alkene reductive cross-coupling reaction via visible-light photoredox/cobalt dual catalysis

The first example of asymmetric alkene-alkene reductive coupling is demonstrated via visible-light-fueled photoredox/cobalt dual catalysis. The desymmetrization reaction provided products (>20 examples) with up to five chiral centers in single-step operation in up to 95% yields with very high relative (>99 : 1 dr) and absolute stereochemistry (up to 98 : 2 er) control. The preliminary mechanistic investigations suggested that the critical mechanistic steps involved light-mediated controlled low-valent cobalt complex generation, oxidative ene-ene cyclization, and protonation.

Palladium-catalyzed stereocontrolled ring-opening of 7-oxabenzonorbornadienes with organic carboxylic acids

A palladium-catalyzed tandem reaction of 7-oxabenzonorbornadienes with organic carboxylic acids that provides cis-1,2-dihydro-l-naphthyl derivatives in moderate to good yields in a highly diastereoselective manner is described. A possible reaction mechanism involving syn acylation followed by insertion and diastereoselective ring-opening with 7-oxabenzonorbornadienes is proposed to account for the reaction.

Iridium-catalyzed hydroacylation reactions of C1-substituted oxabenzonorbornadienes with salicylaldehyde: an experimental and computational study

An experimental and theoretical investigation on the iridium-catalyzed hydroacylation of C1-substituted oxabenzonorbornadienes with salicylaldehyde is reported. Utilizing commercially available [Ir(COD)Cl]2 in the presence of 5 M KOH in dioxane at 65 °C, provided a variety of hydroacylated bicyclic adducts in up to a 95% yield with complete stereo- and regioselectivity. The mechanism and origins of selectivity in the iridium-catalyzed hydroacylation reaction has been examined at the M06/Def2TZVP level of theory. The catalytic cycle consists of three key steps including oxidative addition into the aldehyde C-H bond, insertion of the olefin into the iridium hydride, and C-C bond-forming reductive elimination. Computational results indicate the origin of regioselectivity is involved in the reductive elimination step.

Recent Advances on Diels-Alder-Driven Preparation of Bio-Based Aromatics

The preparation of high value-added chemicals from renewable resources is a crucial approach towards a sustainable economy. One prominent alternative to the production of petroleum-based chemicals from fossil resources is through the sequential Diels-Alder/aromatization reactions of biomass-derived furan platforms. This Concept is focused on the recent boom in bio-based furan DA strategies for aromatization of bio-based platform chemicals, particularly that of furfurals, ranging from indirect use and activation strategies to recent examples of direct DA reaction of these electron-withdrawing biomass-derived furans.

Chemistry of Unsymmetrical C1-Substituted Oxabenzonorbornadienes

Oxabenzonorbornadiene (OBD) is a useful synthetic intermediate, which can be readily activated by transition metal complexes with great face selectivity due to its dual-faced nature and intrinsic angle strain on the alkene. To date, the understanding of transition-metal catalyzed reactions of OBD itself has burgeoned; however, this has not been the case for unsymmetrical OBDs. Throughout the development of these reactions, the nature of C1-substituent has proven to have a profound effect on both the reactivity and selectivity of the outcome of the reaction. Upon substitution, different modes of reactivity arise, contributing to the possibility of multiple stereo-, regio-, and in extreme cases, constitutional isomers, which can provide unique means of constructing a variety of synthetically useful cyclic frameworks. To maximize selectivity, an understanding of bridgehead substituent effects is crucial. To that end, this review outlines hitherto reported examples of bridgehead substituent effects on the chemistry of unsymmetrical C1-substituted OBDs.

Catalytic asymmetric transformations of oxa- and azabicyclic alkenes

This review provides an overview of the fundamental concepts and recent developments in a wide range of enantioselective transformations involving oxa- and azabicyclic alkenes.

Three-Component Cycloaddition To Synthesize Aziridines and 1,2,3-Triazolines

An efficient three-component cycloaddition of oxa(aza)bicyclic alkenes/norbornene in the presence of NaN₃ and arylsulfonyl chlorides was developed, affording the corresponding aziridine products in good yields (up to 82%) with moderate to good endo/exo selectivities (up to >99:1 endo/exo). Further studies showed that the cycloaddition of oxa(aza)bicyclic alkenes in the presence of NaN₃ and chloroalkanes could afford the exo-cycloadduct 1,2,3-triazolines in good to excellent yields (up to 95%). Compared with the existing methodologies, the current protocol demands very simple and mild reaction conditions and is a metal-free catalyzed reaction. In addition, a plausible mechanism for the cycloaddition reaction was also proposed.

Palladium/Lewis Acid Cocatalyzed Ring-Opening Reactions of Unsymmetrical Oxabenzonobornadienes with Oximes

The palladium/Lewis acid cocatalyzed ring-opening reaction of various C₁-substituted unsymmetrical oxabenzonorbornadienes (OBD) with oxime nucleophiles was investigated. The effects of various C₁ substituents were explored. Moderate to excellent yields and excellent regioselectivities were obtained for electron-withdrawing groups. The presence of electron-donating alkyl groups leads to isomerization of the corresponding OBD to afford the substituted naphthol derivatives. Additionally, a mechanism for the formation of C₂ regioisomeric ring-opened products has been proposed.

Recent Advances in Transition Metal-Catalyzed Reactions of Oxabenzonorbornadiene

BACKGROUND

Oxabenzonorbornadiene (OBD) is a useful synthetic intermediate capable of undergoing multiple types of transformations due to three key structural features: a free alkene, a bridged oxygen atom, and a highly strained ring system. Most notably, ring-opening reactions of OBD using transition metal catalysts and nucleophiles produce multiple stereocenters in a single step. The resulting dihydronaphthalene framework is found in many natural products, which have been shown to be biologically active.

OBJECTIVE

This review will provide an overview of transition metal-catalyzed reactions from the past couple of years including cobalt, copper, iridium, nickel, palladium and rhodium- catalyzed reactions. In addition, the recent derivatization of OBD to cyclopropanated oxabenzonorbornadiene and its reactivity will be discussed.

CONCLUSION

It can be seen from the review, that the work done on this topic has employed the use of many different transition metal catalysts, with many different nucleophiles, to perform various transformations on the OBD molecule. Additionally, depending on the catalyst and ligand used, the stereo and regioselectivity of the product can be controlled, with proposed mechanisms to support the understanding of such reactions. The use of palladium has also generated a cyclopropanated OBD, with reactivity similar to that of OBD. An additional reactive site exists at the distal cyclopropane carbon, giving rise to three types of ring-opened products.

Palladium-catalyzed ring-opening reactions of cyclopropanated 7-oxabenzonorbornadiene with alcohols

Palladium-catalyzed ring-opening reactions of cyclopropanated 7-oxabenzonorbornadiene derivatives using alcohol nucleophiles were investigated. The optimal conditions were found to be 10 mol % PdCl₂(CH₃CN)₂ in methanol, offering yields up to 92%. The reaction was successful using primary, secondary and tertiary alcohol nucleophiles and was compatible with a variety of substituents on cyclopropanated oxabenzonorbornadiene. With unsymmetrical C1-substituted cyclopropanated 7-oxabenzonorbornadienes, the regioselectivity of the reaction was excellent, forming only one regioisomer in all cases.

Multi-Pathway Consequent Chemoselectivities of CpRuCl(PPh3 )2 /MeI-Catalysed Norbornadiene Alkyne Cycloadditions

Chemoselectivities of five experimentally realised CpRuCl(PPh₃)₂/MeI‐catalysed couplings of 7‐azabenzo‐norbornadienes with selected alkynes were successfully resolved from multiple reaction pathway models. Density functional theory calculations showed the following mechanistic succession to be energetically plausible: (1) CpRuI catalyst activation; (2) formation of crucial metallacyclopentene intermediate; (3) cyclobutene product (P2) elimination (ΔG_Rel(RDS)≈11.9–17.6 kcal mol⁻¹). Alternative formation of dihydrobenzoindole products (P1) by isomerisation to azametalla‐cyclohexene followed by subsequent CpRuI release was much less favourable (ΔG_Rel(RDS)≈26.5–29.8 kcal mol⁻¹). Emergent stereoselectivities were in close agreement with experimental results for reactions a, b, e. Consequent investigations employing dispersion corrections similarly support the empirical findings of P1 dominating in reactions c and d through P2→P1 product transformations as being probable (ΔG≈25.3–30.1 kcal mol⁻¹).

Syntheses of arnottin I and arnottin II

Short total syntheses of arnottin I and II were accomplished in 5 and 6 steps, respectively. A sesamol-benzyne cycloaddition with a 3-furyl-benzoate followed by regiospecific lactonization provided rapid, large-scale access to the core of arnottin I. Saponification of arnottin I and hypervalent iodide mediated spirocyclization provided an efficient and direct preparation of racemic arnottin II.

4-Hydroxy-1-naphthaldehydes: proton transfer or deprotonation

Proton captured/proton lost: in 4-hydroxy-1-naphthaldehyde concentration dependent deprotonation is observed, while when a piperidine unit is attached the proton is captured.

Rhodium-catalyzed ring-opening reactions of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with arylboronic acids

Rhodium-catalyzed ring-opening reactions of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with arylboronic acids were investigated. The use of [RhCl(COD)](2), (+/-)-BINAP, and NaHCO(3) in MeOH at 60 degrees C was found to be the optimized conditions for the ring-opening reactions to give the 1,2-ring-opened products. Various arylboronic acids were examined, and low to moderate yields were obtained and stereoselectivities (trans/cis) from 50:50 to 100:0 were observed.

Thiol-selective fluorogenic probes for labeling and release

Thiol alkylation is a powerful technique for the labeling of proteins. We report a new class of highly reactive, selective, and fluorogenic probes for thiols in aqueous solution at neutral pH, based on the 7-oxanorbornadiene (OND) framework. The maleate moiety in 7-oxabicyclo[2.2.1]hept-2,5-diene-2,3-dicarboxylic acid esters serves as both a tunable electrophile and an intramolecular quencher of an attached dansyl fluorophore. Thiols have been found to add with high rates (second-order rate constants of 40-200 M(-1) s(-1)) to give adducts that exhibit enhancements of fluorescence intensity up to 180-fold. The resulting adducts are also versatile with respect to cleavage (release) reactions by two mechanisms. First, retro-Diels-Alder fragmentation occurs with half-lives from days to weeks at room temperature, and an epoxide derivative is also reported that is incapable of cycloreversion cleavage. Second, monoamide OND derivatives undergo rapid closure to succinimides upon thiol addition, providing a thiol-triggered mechanism for immediate alcohol release. Peptides and proteins containing free thiol groups were labeled with OND electrophiles with high chemoselectivity. Since the system is so easily assembled from readily accessible modules, various functional groups can be added to OND linkers to allow the attachment of other molecules of interest.