Formation of BN Isosteres of Azo Dyes by Ring Expansion of Boroles with Azides

Herein, we present the results of our investigations on the effect of ortho substitution of aryl azides on the ring‐expansion reaction of boroles, five‐membered unsaturated boron heterocycles. These studies led to the isolation of the first 1,2‐azaborinine‐substituted azo dyes, which are bright yellow solids. One of the derivatives, (E)‐2‐mesityl‐1‐(mesityldiazenyl)‐3,4,5,6‐tetraphenyl‐1,2‐azaborinine, was found to be unstable in solution and to transform through a Jacobsen‐like reaction into an indazole and 1‐hydro‐1,2‐azaborinine. DFT calculations were performed to shed light on possible mechanisms to rationalize the unexpected azo‐azaborinine formation and to draw conclusions about the role played by the ortho substituents in the reaction.     

1,2,3‐Diazaborinine: A BN Analogue of Pyridine Obtained by Ring Expansion of a Borole with an Organic Azide

A new pathway for the ring expansion reaction of antiaromatic boroles with organic azides is reported. While the reaction usually leads to 1,2‐azaborinines, it was diverted to the formation of a 1,2,3‐diazaborinine by changing the electronic characteristics of the reagents. The isolable azo‐azaborinine intermediate initially formed from the reaction of 1‐(2,3,4,5‐tetraphenylborolyl)ferrocene with 4‐azido‐N,N‐dimethylaniline gradually decomposed to a 1,2,3‐diazaborinine and benzonitrile. Both the spectroscopic properties and the reactivity of the heteroaromatic compound show analogies to pyridine, to which it is isoelectronic. Density functional theory (DFT) calculations provided insight into the mechanism of this unusual transformation.     

Regioselective Catalytic and Stepwise Routes to Bulky,Functional‐Group‐Appended,and Luminescent 1,2‐Azaborinines

The regioselective syntheses of 1,2‐azaborinines is achieved using an unsymmetrical iminoborane through both catalytic and stepwise modular routes. The 1,2‐azaborinine ring can be selectively functionalized in the 4‐ and/or 6‐position through control of the stepwise reaction sequence, allowing access to vinyl‐functionalized and redox‐active, luminescent, donor‐functionalized 1,2‐azaborinines. The electrochemistry and photochemistry of a tetraarylamine‐substituted 1,2‐azaborinine are studied. Cyclic voltammetry of this compound, relative to a non‐B,N‐substituted reference molecule, showed an additional oxidation wave assigned to the oxidation of the azaborinine ring, while emission spectroscopy indicated that the azaborinine was significantly more fluorescent than the reference.     

Gold‐Catalyzed Ring Expansion of Alkynyl Heterocycles through 1,2‐Migration of an Endocyclic Carbon–Heteroatom Bond

A mild and efficient gold‐catalyzed oxidative ring‐expansion of a series of alkynyl heterocycles using pyridine‐N‐oxide as the oxidant has been developed, which affords highly valuable six‐ or seven‐membered heterocycles with wide functional group toleration. The reaction consists of a regioselective oxidation and a chemoselective migration of an endocyclic carbon–heteroatom bond (favored over C?H migration) with the order of migratory aptitude for carbon–heteroatom bonds being C?S>C?N>C?O. In the absence of an oxidant, polycyclic products are readily constructed through a ring‐expansion/Nazarov cyclization reaction sequence.     

Direct Synthetic Route to Functionalized 1,2‐Azaborinines

A new catalytic synthetic route to functionalized 1,2‐azaborinines has been developed by the [2+2]/[2+4] cycloaddition reactions of di‐tert‐butyliminoboranes and alkynes in presence of a rhodium catalyst. The first examples of ferrocene‐functionalized azaborinines have been synthesized using this strategy. Moreover, the regioselectivity of this reaction can be controlled by the formation of an intermediate rhodium 1,2‐azaborete complex, which results in the isolation of the first azaborinine boronic ester. The isolation of an NH‐containing BN isostere by elimination of isobutene from an N(tBu) group under thermolytic conditions has also been achieved. Theoretical studies give further insight into the formation of 1,2‐azaborinines and the elimination of isobutene from the N(tBu) group.     

1,2‐Phosphaborines: Hybrid Inorganic/Organic P–B Analogues of Benzene

Photolysis of the cyclic phosphine oligomer [PPh]₅ in the presence of pentaarylboroles leads to the formation of 1,2‐phosphaborines by the formal insertion of a phenylphosphinidene fragment into the endocyclic C? B bond. The solid‐state structure features a virtually planar central ring with bond lengths indicating significant delocalization. Appreciable ring current in the 1,2‐phosphaborine core, detected in nuclear independent chemical shift (NICS) calculations, are consistent with aromatic character. These products are the first reported 1,2‐BPC₄ conjugated heterocycles and open a new avenue for B? P as a valence isoelectronic substitute for C? C in arene systems.     

Ring Expansion,Photoisomerization, and Retrocyclization of 1,4,2‐Diazaboroles

Diverse skeletal transformations of 1,4,2‐diazaboroles through ring expansion, photoisomerization, and retrocyclization led to the isolation of various B,N‐dihydroindole (compounds 3 and 6 ), 1,3‐azaborolidin‐2‐imine (compound 7 ), and 1,4,2‐diazaborol‐3‐imine derivatives (compound 11 ).     

Formal [4+2]‐Annulation of Vinyl Azides with N‐Unsaturated Aldimines

Highly functionalized quinolines and pyridines could be synthesized by BF₃?OEt₂‐mediated reactions of vinyl azides with N‐aryl and N‐alkenyl aldimines, respectively. The reaction mechanism could be characterized as formal [4+2]‐annulation, including unprecedented enamine‐type nucleophilic attack of vinyl azides to aldimines and subsequent nucleophilic cyclization onto the resulting iminodiazonium ion moieties.     

Rhodium(II)‐Catalyzed Annulation of Azavinyl Carbenes Through Ring‐Expansion of 1,3,5‐Trioxane: Rapid Access to Nine‐Membered 1,3,5,7‐Trioxazonines

The rhodium(II)‐catalyzed denitrogenative coupling of N‐alkylsulfonyl 1,2,3‐triazoles with 1,3,5‐trioxane led to nine‐membered‐ringed trioxazonines in moderate‐to‐good yields. 1,3,5‐Trioxane, acting as an oxygen nucleophile, reacted with the α‐aza‐vinylcarbene intermediate, giving rise to ylide formation, which was probably the key step in the reaction. Triazoles that contained aryl substituents with various electronic and steric features on the C4 carbon atom were well‐tolerated. The synthesis of trioxazonine derivatives was achieved through a one‐pot, two‐step procedure from 1‐mesylazide and a terminal alkyne by combining Cu^I‐catalyzed 1,3‐dipolar cycloaddition and rhodium‐catalyzed transformations.     

Ring Expansion and 1,2‐Migration Cascade of Benzisoxazoles with Ynamides: Experimental and Theoretical Studies

Demonstrated herein is an Au^I‐catalyzed annulation of sulfonyl‐protected ynamides with substituted 1,2‐benzisoxazoles for the synthesis of E‐benzo[e][1,3]oxazine derivatives. The transformation involves the addition of benzisoxazole to the gold‐activated ynamide, ring expansion of the benzisoxazole fragment to provide an α‐imino vinylic gold intermediate, and 1,2‐migration of the sulfonamide motif to the masked carbene center to deliver the respective ring‐expanded benzo[e][1,3]oxazine of predominant E configuration. A trapping experiment justifies the participation of the α‐imino masked gold carbene. DFT computations also support the hypothesized mechanism and rationalize the product stereoselectivity.     

PPh3‐Catalyzed Ring‐Expansion Reactions of Sulfamate‐Derived Cyclic Imines with Acetylenedicarboxylates

The PPh₃‐catalyzed ring‐expansion reaction of sulfamate‐derived cyclic imines with acetylenedicarboxylates has been developed. The reaction works quite efficiently under very mild conditions to afford benzo[g][1,2,3]oxathiazocine‐4,5‐dicarboxylate 2,2‐dioxide derivatives in high yields.     

An Approach to 1,3,4‐Dioxaphospholane Complexes through an Acid‐Induced Ring Expansion of an Oxaphosphirane Complex: The Problem of Construction and Deconstruction of O,P‐Heterocycles

Treatment of oxaphosphirane complex 1 , triflic acid (TfOH), and various aldehydes yielded 1,3,4‐dioxaphospholane complexes 5a , b – 7a , b after deprotonation with NEt₃. In addition to NMR spectroscopy, IR spectroscopy, and MS data, the X‐ray structures of complexes 5a and 7a were determined. ³¹P NMR spectroscopic monitoring and DFT calculations provided insight into the reaction course and revealed the transient TfOH 1,3,4‐dioxaphospholanium association complex TfOH‐ 5a , b and/or TfOH‐ 5a , b′ as key reactive intermediates. Furthermore, it was observed that the five‐membered ring system was cleaved upon warming and yielded side‐on (E,Z)‐methylenephosphonium complexes 8a , b if deprotonation did not occur at low temperature. Overall, a novel temperature‐ and acid‐dependent construction and deconstruction process of the 1,3,4‐dioxaphospholane ring system is described.     

Gold‐Catalyzed Oxidative Ring Expansion of 2‐Alkynyl‐1,2‐Dihydropyridines or ‐quinolines: Highly Efficient Synthesis of Functionalized Azepine or Benzazepine Scaffolds

A gold‐catalyzed highly regio‐ and chemoselective oxidative ring expansion of 2‐alkynyl‐1,2‐dihydropyridines and its analogues using pyridine‐N‐oxide as the oxidant has been developed. Ring expansion proceeds through exclusive 1,2‐migration of a vinyl or phenyl group, whereas no 1,2‐H and 1,2‐N migration take place. The reaction provides an efficient and attractive route to various types of medium‐sized azepine derivatives in generally high to excellent yields with a broad functional group tolerance. DFT studies indicate that the reaction proceeds through the formation of a cyclopropyl gold intermediate, and no gold carbene species is involved.     

Metal‐Free Decarboxylative Cyclization/Ring Expansion: Construction of Five‐, Six‐, and Seven‐Membered Heterocycles from 2‐Alkynyl Benzaldehydes and Cyclic Amino Acids

A one pot synthesis of 1H‐benzo[g]indoles, tetrahydrobenzo[h]quinolines, and naphtho[1,2‐b]azepines from 2‐alkynyl benzaldehydes and cyclic amino acids is reported. The salient feature of the strategy involves formation of three new bonds (one C? N and two C? C bonds) by a metal‐free decarboxylation/cyclization/one‐carbon ring expansion sequence in one pot.     

Visible‐Light Sensitization of Vinyl Azides by Transition‐Metal Photocatalysis

Irradiation of vinyl and aryl azides with visible light in the presence of Ru photocatalysts results in the formation of reactive nitrenes, which can undergo a variety of C? N bond‐forming reactions. The ability to use low‐energy visible light instead of UV in the photochemical activation of azides avoids competitive photodecomposition processes that have long been a significant limitation on the synthetic use of these reactions.     

Ring Expansions of Boroles with Diazo Compounds: Steric Control of C or N Insertion and Aromatic/Nonaromatic Products

Access to novel imine‐substituted 1,2‐azaborinines, as well as highly arylated boracyclohexa‐3,5‐dienes has been developed by ring expansion of boroles with diazoalkanes with varying degrees of steric bulk. The formation of a diazoalkane intermediate is also discussed for the reaction of ortho‐brominated p‐tolyl‐azide with 1,2,3,4,5‐pentaphenylborole. Structural details as well as UV/Vis spectroscopic and cyclic voltammetric data are provided. These boron‐containing heterocycles have the potential to serve as building blocks for boron‐containing materials.