Facile and controllable synthesis of multiply substituted benzenes via a formal [3+3] cycloaddition approach

Multicomponent benzannulation of allylic P-ylides with isocyanates or aldehydes for construction of anilines and biaryls

The reactivity of allylic phosphorus ylides generated in situ from alkoxycarbonylmethylenephosphoranes and propiolates is investigated toward isocyanates and aromatic aldehydes, which leads to one-pot multicomponent benzannulations for efficient synthesis of polysubstituted anilines and biaryls, respectively. The mechanism may involve a tandem [2+2] cycloaddition/fragmentation/Wittig/cyclization/elimination/aromatization sequence.

Amine-catalyzed and functional group-controlled chemo- and regioselective synthesis of multi-functionalized CF3-benzene via a metal-free process

A novel strategy for the synthesis of CF₃-containing multi-substituted benzenes with high chemo- and regioselectivities under metal-free and air-tolerant conditions was established.

Facile synthesis of substituted diaryl sulfones via a [3 + 3] benzannulation strategy

A metal-free and efficient synthesis of highly substituted diaryl sulfones via a base-mediated [3 + 3] benzannulation strategy.

AlCl3-catalyzed [3 + 3] cycloaddition of chalcones and β-enamine ketones (esters): a highly efficient access to multisubstituted cyclohexa-1,3-dienamines

An efficient protocol was successfully achieved to give 1,3-cyclohexadiene derivatives in good to excellent yields from chalcones and β-enamine ketones (esters) promoted by AlCl₃.

Stereoselective synthesis of highly substituted cyclohexanes by a rhodium-carbene initiated domino sequence

A stereoselective synthesis of cyclohexanes bearing four stereocenters from vinyldiazoacetates and allyl alcohols by a rhodium-carbene initiated domino reaction is described. The reaction cascade features a tandem ylide formation/[2,3]-sigmatropic rearrangement, oxy-Cope rearrangement, and type II carbonyl ene reaction, all of which proceed with a high degree of stereocontrol. The products are routinely isolated with excellent stereocontrol (>97:3 dr, 99% ee).

The [3 + 3]-cycloaddition alternative for heterocycle syntheses: catalytically generated metalloenolcarbenes as dipolar adducts

The combination of two or more unsaturated structural units to form cyclic organic compounds is commonly referred to as cycloaddition, and the combination of two unsaturated structural units that forms a six-membered ring is formally either a [5 + 1]-, [4 + 2]-, [2 + 2 + 2]-, or [3 + 3]-cycloaddition. Occurring as concerted or stepwise processes, cycloaddition reactions are among the most useful synthetic constructions in organic chemistry. Of these transformations, the concerted [4 + 2]-cycloaddition, the Diels-Alder reaction, is by far the best known and most widely applied. However, although symmetry disallowed as a concerted process and lacking certifiable examples until recently, stepwise [3 + 3]-cycloadditions offer advantages for the synthesis of a substantial variety of heterocyclic compounds, and they are receiving considerable attention. In this Account, we present the development of stepwise [3 + 3]-cycloaddition reactions from virtual invisibility in the 1990s to a rapidly growing synthetic methodology today, involving organocatalysis or transition metal catalysis. With origins in organometallic or vinyliminium ion chemistry, this area has blossomed into a viable synthetic transformation for the construction of six-membered heterocyclic compounds containing one or more heteroatoms. The development of [3 + 3]-cycloaddition transformations has been achieved through identification of suitable and compatible reactive dipolar adducts and stable dipoles. The reactive dipolar species is an energetic dipolar intermediate that is optimally formed catalytically in the reaction. The stepwise process occurs with the reactive dipolar adduct reacting as an electrophile or as a nucleophile to form the first covalent bond, and this association provides entropic assistance for the construction of the second covalent bond and the overall formal [3 + 3]-cycloaddition. Organocatalysis is well developed for both inter- and intramolecular synthetic transformations, but the potential of transition metal catalysis for [3 + 3]-cycloaddition has only recently emerged. The key to the rapid development of the transition metal-based methodology has been recognition that certain catalytically generated vinylcarbenes are effective dipolar adducts for reactions with stable dipolar compounds, including aryl and vinyl ylides. In particular, metallo-enolcarbenes that are generated catalytically from conveniently prepared stable enoldiazoacetates or from donor-acceptor cyclopropenes are highly effective dipolar adducts for [3 + 3]-cycloaddition. The electron-donating oxygen of the silyl ether enhances electrophilic ring closure to the metal-bound carbon of the initial adduct from vinylogous addition, and this enhancement inhibits the alternative [3 + 2]-cycloaddition across the carbon-carbon double bond of the vinylcarbene. Catalytically generated metallo-enolcarbenes react under mild conditions with a broad spectrum of compatible stable dipoles, including nitrones, azomethine imines, ylides, and certain covalent precursors of stable dipoles, to form [3 + 3]-cycloaddition products having the β-ketoester functionality (in dihydrooxazines, tetrahydropyridazines, pyrazolidinone and pyraxole derivatives, dihydroquinolines, and quinolizidines, for example) in high yield. Two ways to access these metallo-enolcarbenes, either by dinitrogen extrusion from enoldiazoacetate esters or by rearrangement of donor-acceptor cyclopropenes, enhance the versatility of the process. The [3 + 3]-cycloaddition methodology is a complementary strategy to [4 + 2]-cycloaddition for the synthesis of heterocyclic compounds having six-membered rings. High levels of enantioselectivity are obtained with the use of chiral ligands on transition metal catalysts that include those on dirhodium(II) and silver(I).

[3+3] annulation of allylic phosphoryl-stabilized carbanions/phosphorus ylides and vinyl azides: a practice strategy for synthesis of polyfunctionalized anilines

Tandem Michael addition and Witting or Horner–Wadsworth–Emmons olefination initiated [3+3] annulation between vinyl azides and allylic phosphorus ylides or allylic phosphoryl-stabilized carbanions has been developed.