Intramolecular polar [4(⊕)+2] cycloadditions of aryl-1-aza-2-azoniaallene salts: unprecedented reactivity leading to polycyclic protonated azomethine imines

The Nature of Azo-Substituted Carbocations: N-N π-Electron Stabilization versus Nitrogen Nonbonding Electron Stabilization

Computational and experimental studies reveal two different modes of cation stabilization by the phenylazo group. The first mode involves a relatively weak conjugative interaction with the azo π-bond, while the second mode involves an interaction with the nitrogen nonbonding electrons. The 4-phenylazo group is slightly rate-retarding in the solvolysis of cumyl chloride and benzyl mesylate derivatives but rate-enhancing in the solvolysis of α-CF₃ benzylic analogs. The phenylazo group can become a potent electron-donating group in cations such as [Me₂C-N═N-Ph]⁺. Nonbonding electron stabilization can be strong enough to offset the very powerful γ-silyl stabilization. In aromatic cyclopropenium and tropylium cations, the demand for stabilization is quite low, and the mode of phenylazo stabilization reverts back to the less-effective π-stabilization. The solvolysis of cis-4-phenylazo benzyl mesylate is faster than that of trans-4-phenylazo benzyl mesylate. Products formed suggest a stepwise ionization, cation isomerization, and nucleophile capture mechanism. Computational studies indicate a vanishingly small barrier for the isomerization of the cis-cation intermediate to the trans-cation.

Synthesis of new tricyclic 5,6-dihydro-4H-benzo[b][1,2,4]triazolo[1,5-d][1,4]diazepine derivatives by [3+ + 2]-cycloaddition/rearrangement reactions

Two new series of tricyclic heterocycles, namely 5,6-dihydro-4H-benzo[b][1,2,4]triazolo[1,5-d][1,4]diazepinium salts 10 and the related neutral, free bases 13 were synthesized from 4-acetoxy-1-acetyl-4-phenylazo-1,2,3,4-tetrahydroquinolines 8 and nitriles 9 in the presence of aluminium chloride by the [3⁺ + 2]-cycloaddition reaction of the in situ generated azocarbenium intermediates 14 followed by a ring-expansion rearrangement. In the rearrangement reaction, the phenyl substituent in the initially formed spiro-triazolium adducts 16 underwent a [1,2]-migration from C(3) to the electron-deficient N(2). This led to the ring expansion from 6-membered piperidine to 7-membered diazepine furnishing the tricyclic 1,2,4-triazole-fused 1,4-benzodiazepines.

Two-Step Sequence of Cycloadditions Gives Structurally Complex Tetracyclic 1,2,3,4-Tetrahydrocinnoline Products

Intramolecular [4 + 2]-cycloaddition of heteroallene salts gives polycyclic tetrahydrocinnoline structures that contain an N-aminoiminium motif. Deprotonation with mild base gives the corresponding azomethine imines, which readily participate in 1,3-dipolar cycloaddition reactions. The structurally complex tetracyclic 1,2,3,4-tetrahydrocinnoline products typically form as a mixture of two separable diastereomers. The major diastereomer is generally formed by reaction of the dipolarophile with the convex face of the dipole.

Synthesis of Triazolodiazepinium Salts: Sequential [3++2] Cycloaddition/Rearrangement Reaction of 1-Aza-2-azoniaallenium Cation Intermediates Generated from Piperidin-4-ones

The bicyclic 1-aza-2-azoniaallenium salt intermediates, generated from the azoester species upon treatment with a Lewis acid, have been demonstrated to participate in Huisgen-type cycloaddition with nitriles to result in the formation of fused 6,7,8,9-tetrahydro-5 H-[1,2,4]triazolo[1,5- d][1,4]diazepinium salts. This transformation is interpreted as a regular [3⁺+2] cycloaddition between intermediates as the reactive 1,3-monopole reactants and nitriles as the nucleophilic reagents followed by spontaneous [1,2]-cationic rearrangement. The azoester precursors were easily accessible via oxidation of the corresponding hydrazones using hypervalent iodine oxidant PhI(OAc)₂ under mild conditions. The [1,2,4]triazolodiazepine compounds represent a class of N-containing biologically important heterocycles with a new type of scaffold.

(2+1)-Cycloaddition Reactions Give Further Evidence of the Nitrenium-like Character of 1-Aza-2-azoniaallene Salts

Cationic 1-aza-2-azoniaallene salts react with structurally constrained alkenes in intramolecular reactions by C-H insertion at the allylic position, or by (2+1)-cycloaddition with the alkene followed by ring opening. The latter reaction gives further evidence of the nitrenium-like character of 1-aza-2-azoniaallene salts. DFT calculations show that alkene addition is intrinsically more favorable, but that predistortion can lead to C-H insertion.

Intramolecular (4 + 2) cycloaddition of aryl-1-aza-2-azoniaallene salts: A practical approach to highly sterically-congested polycyclic protonated azomethine imines

We report an improved two-step reaction sequence that gives tricyclic protonated azomethine imine products containing a 1,2,3,4-tetrahydrocinnoline scaffold in high yield. This sequence involves the oxidation of aryl hydrazones with TFAA-activated DMSO to give the corresponding α-trifluoroacetoxyazo products, which react readily with TMSOTf to give 1-aza-2-azoniaallene salt intermediates that undergo intramolecular (4+2) cycloadditions with pendent alkenes. This reaction sequence is more general, more practical and more environmentally friendly than our initially reported method. The cycloaddition provides exceptionally sterically-hindered products in high yield.

Unprecedented Intramolecular [4 + 2]-Cycloaddition between a 1,3-Diene and a Diazo Ester

Diazo compounds that are well-known to undergo [3 + 2]-cycloaddition provide the first examples of the previously unknown [4 + 2]-cycloaddition with dienes that occur thermally under mild conditions and in high yields. Reactions are initiated from reactants prepared from propargyl aryldiazoacetates that undergo gold(I)-catalyzed rearrangement to activated 1,3-dienyl aryldiazoacetates. These reactions proceed to mixtures of both [4 + 2]-cycloaddition and the 1,3-dienyl aryldiazoacetate after long reaction times. At short reaction times, however, both E- and Z-1,3-dienyl aryldiazoacetates are formed and, after isolation, thermal reactions with the E-isomers form the products from [4 + 2]-cycloaddition with ΔH(‡)298 = 15.6 kcal/mol and ΔS(‡)298 = -27.3 cal/(mol·°C). The Z-isomer is inert to [4 + 2]-cycloaddition under these conditions. The Hammett relationships from aryl-substituted diazo esters (ρ = +0.89) and aryl-substituted dienes (ρ = -1.65) are consistent with the dipolar nature of this transformation.

Mechanism and Dynamics of Intramolecular C-H Insertion Reactions of 1-Aza-2-azoniaallene Salts

The 1-aza-2-azoniaallene salts, generated from α-chloroazo compounds by treatment with halophilic Lewis acids, undergo intramolecular C-H amination reactions to form pyrazolines in good to excellent yields. This intramolecular amination occurs readily at both benzylic and tertiary aliphatic positions and proceeds at an enantioenriched chiral center with retention of stereochemistry. Competition experiments show that insertion occurs more readily at an electron-rich benzylic position than it does at an electron-deficient one. The C-H amination reaction occurs only with certain tethers connecting the heteroallene cation and the pendant aryl groups. With a longer tether or when the reaction is intermolecular, electrophilic aromatic substitution occurs instead of C-H amination. The mechanism and origins of stereospecificity and chemoselectivity were explored with density functional theory (B3LYP and M06-2X). The 1-aza-2-azoniaallene cation undergoes C-H amination through a hydride transfer transition state to form the N-H bond, and the subsequent C-N bond formation occurs spontaneously to generate the heterocyclic product. This concerted two-stage mechanism was shown by IRC and quasi-classical molecular dynamics trajectory studies.