Combining Bidentate Lewis Acid Catalysis and Photochemistry: Formal Insertion of o-Xylene into an Enamine Double Bond

Lewis Acid-Catalyzed Domino Inverse Electron-Demand Diels-Alder/Thermal Ring Expansion Reaction for the Synthesis of Arene-Annulated Eight-Membered Nitrogen Heterocycles

A domino inverse electron-demand Diels-Alder reaction/thermal ring expansion sequence was developed to enable the one-step synthesis of arene-annulated eight-membered nitrogen heterocycles from readily available aromatic 1,2-diazines. A boron-based, bidentate Lewis acid catalyst facilitates the initial cycloaddition of Boc-protected 2-azetine with various electron-poor and electron-rich phthalazines. The subsequent electrocyclic ring expansion furnishes azocines fused to differently substituted aromatics, a structural motif that holds vast potential for further derivatization.

Donor-free 9,10-dihydro-9,10-dialuminaanthracenes

Despite their promising potential, e.g., as ditopic, cooperatively binding Lewis acids, 9,10-dihydro-9,10-dialuminaanthracenes (DAA-R2; R: terminal Al-bonded substituent) have remained unexplored for long due to the challenges in synthesizing the ligand-free species. We demonstrate that DAA-Me2 is accessible via the reaction of 1,2-(Me3Sn)2C6H4 with AlMe3, producing volatile SnMe4 as the sole byproduct. In non-coordinating solvents and in the solid state, DAA-Me2 exists as a dimer (DAA-Me2)2. Treatment of (DAA-Me2)2 with 4 equiv. AlBr3 cleaves the dimer, leads to quantitative Me/Br exchange, and forms the double AlBr3 adduct DAA-Br2·(AlBr3)2. Removal of AlBr3 with 2,2'-bipyridine gives free DAA-Br2, which also dimerizes in the absence of bases to form (DAA-Br2)2. (DAA-Me2)2 and (DAA-Br2)2 readily react with mono- (e.g., pyridine) or ditopic Lewis bases (e.g., potassium pyrazolide) to afford trans-diadducts or triptycene-type frameworks. Upon addition of [nBu4N]Br, DAA-Br2·(AlBr3)2 undergoes selective cleavage of Al-C bonds to produce the Br- chelate complex of 1,2-(Br2Al)2C6H4, a valuable synthon for 1,2-dideprotonated benzenes.

Highly electron-deficient 3,6-diaza-9-borafluorene scaffolds for the construction of luminescent chelate complexes

The synthesis and characterization of two fluorinated 3,6-diaza-9-hydroxy-9-borafluorene oxonium acids featuring improved hydrolytic stability and the strong electron-deficient character of the diazaborafluorene core is reported. These boracycles served as precursors of fluorescent spiro-type complexes with (O,N)-chelating ligands which revealed specific properties such as delayed emission, white light emission in the solid state and photocatalytic performance in singlet oxygen-mediated oxidation reactions.

Synthesis, bridgehead functionalization, and photoisomerization of 9,10-diboratatriptycene dianions

9,10-Diboratatriptycene salts M2[RB(μ-C6H4)3BR] (R = H, Me; M+ = Li+, K+, [n-Bu4N]+) have been synthesized via [4 + 2] cycloaddition between doubly reduced 9,10-dihydro-9,10-diboraanthracenes M2[DBA] and benzyne, generated in situ from C6H5F and C6H5Li or LiN(i-Pr)2. [HB(μ-C6H4)3BH]2- reacts with CH2Cl2 to form quantitatively the bridgehead-derivatized [ClB(μ-C6H4)3BCl]2-, while twofold H- abstraction with B(C6F5)3 in the presence of SMe2 leads cleanly to the diadduct (Me2S)B(μ-C6H4)3B(SMe2). Photoisomerization of K2[HB(μ-C6H4)3BH] (THF, medium-pressure Hg lamp) provides facile access to diborabenzo[a]fluoranthenes, a little explored form of boron-doped polycyclic aromatic hydrocarbons. According to DFT calculations, the underlying reaction mechanism consists of three main steps: (i) photoinduced di-π-borate rearrangement, (ii) "walk reaction" of a BH unit, and (iii) boryl anion-like C-H activation.

Orthogonal Catalysis for an Enantioselective Domino Inverse-Electron Demand Diels-Alder/Substitution Reaction

An enantioselective domino process for the synthesis of substituted 1,2-dihydronaphthalenes has been developed by the combination of chiral amines and a bidentate Lewis acid in an orthogonal catalysis. This new method is based on an inverse electron-demand Diels-Alder and a subsequent group exchange reaction. An enamine is generated in situ from an aldehyde and a chiral secondary amine catalyst that reacts with phthalazine, activated by the coordination to a bidentate Lewis acid catalyst. The absolute configuration of the product is controlled by chiral information provided by the amine. The formed ortho-quinodimethane intermediate is then transformed via a group exchange reaction with thiols. The new method shows a broad scope and tolerates a wide range of functional groups with enantiomeric ratios up to 91 : 9. All-in-all, this enantioselective synthesis tool provides an easy access to complex 1,2-dihydronaphthalenes starting from readily available phthalazine, aldehydes and thiols in a combinatorial way.

Transition-Metal-Free Addition of Dialkyl Phosphites to Phthalazin-2-ium Bromide: Synthesis of α-Aminophosphonate Analogues

α-Aminophosphonate analogues containing a phthalazine skeleton were efficiently obtained by a new transition-metal-free addition of dialkyl phosphites to phthalazin-2-ium bromide under mild conditions. A mechanistic study using isotope labeling and radical inhibition experiment revealed that the present transformation passes through a nucleophilic addition of dialkyl phosphates, rather than an insertion of P-H to carbenes.

Synthesis of Medium-Sized Carbocycles via a Bidentate Lewis Acid-Catalyzed Inverse Electron-Demand Diels-Alder Reaction Followed by Photoinduced Ring-Opening

The combination of a Lewis acid-catalyzed inverse electron-demand Diels-Alder (IEDDA) reaction with a photoinduced ring-opening (PIRO) reaction in a domino process has been established as an efficient synthetic method to access medium-sized carbocycles. From readily available electron-rich and electron-poor phthalazines and enamines, respectively, as starting materials, various 9- and 11-membered carbocycles were prepared. This versatile transition-metal-free tool will be valuable for broadening the structural space in biologically active compounds and functional materials.