Radical Cascade Transformations of Tris(o-aryleneethynylenes) into Substituted Benzo[a]indeno[2,1-c]fluorenes

Tethering Three Radical Cascades for Controlled Termination of Radical Alkyne peri-Annulations: Making Phenalenyl Ketones without Oxidants

Although Bu3Sn-mediated radical alkyne peri-annulations allow access to phenalenyl ring systems, the oxidative termination of these cascades provides only a limited selection of the possible isomeric phenalenone products with product selectivity controlled by the intrinsic properties of the new cyclic systems. In this work, we report an oxidant-free termination strategy that can overcome this limitation and enable selective access to the full set of isomerically functionalized phenalenones. The key to preferential termination is the preinstallation of a "weak link" that undergoes C-O fragmentation in the final cascade step. Breaking a C-O bond is assisted by entropy, gain of conjugation in the product, and release of stabilized radical fragments. This strategy is expanded to radical exo-dig cyclization cascades of oligoalkynes, which provide access to isomeric π-extended phenalenones. Conveniently, these cascades introduce functionalities (i.e., Bu3Sn and iodide moieties) amenable to further cross-coupling reactions. Consequently, a variety of polyaromatic diones, which could serve as phenalenyl-based open-shell precursors, can be synthesized.

Dual gold-catalyzed regioselective synthesis of benzofulvenes via 5-endo dig cyclization

An efficient dual gold-catalyzed regioselective synthesis of benzofulvenes has been developed from substituted allyloxy 1,5-diynes via 5-endo dig cyclization. In this intramolecular organic transformation a new C-C bond formation occurs and moderate to very good yields are obtained in one pot.

Zwitterionic Bergman cyclization triggered polymerization gives access to metal-graphene nanoribbons using a boron metal couple

With the rapid growth in artificial intelligence, designing high-speed and low-power semiconducting materials is of utmost importance. This investigation provides a theoretical basis to access covalently bonded transition metal-graphene nanoribbon (TM-GNR) hybrid semiconductors whose DFT-computed bandgaps were much narrower than the commonly used pentacene. Systematic optimization of substrates containing remotely placed boryl groups and the transition metals produced the zwitterions via ionic Bergman cyclization (i-BC) and unlocked the polymerization of metal-substituted polyenynes. Aside from i-BC, the subsequent steps were barrierless, which involved structureless transition regions. Multivariate analysis revealed the strong dependence of activation energy and the cyclization mode on the electronic nature of boron and Au(I). Consequently, three regions corresponding to radical Bergman (r-BC), ionic Bergman (i-BC), and ionic Schreiner-Pascal (i-SP) cyclizations were identified. The boundaries between these regions corresponded to the mechanistic shift induced by the three-center-three-electron (3c-3e) hydrogen bond, three-center-four-electron (3c-4e) hydrogen bond, and vacant p-orbital on boron. The ideal combination for cascade polymerization was observed near the boundary between i-BC and i-SP.

Design principles of the use of alkynes in radical cascades

One of the simplest organic functional groups, the alkyne, offers a broad canvas for the design of cascade transformations in which up to three new bonds can be added to each of the two sterically unencumbered, energy-rich carbon atoms. However, kinetic protection provided by strong π-orbital overlap makes the design of new alkyne transformations a stereoelectronic puzzle, especially on multifunctional substrates. This Review describes the electronic properties contributing to the unique utility of alkynes in radical cascades. We describe how to control the selectivity of alkyne activation by various methods, from dynamic covalent chemistry with kinetic self-sorting to disappearing directing groups. Additionally, we demonstrate how the selection of reactive intermediates directly influences the propagation and termination of the cascade. Diverging from a common departure point, a carefully planned reaction route can allow access to a variety of products.

A new approach to 10-arylated 5H-dibenzo[b,f]azepines using syn-selective hydrohalogenation of ethynylaniline

A new synthetic method for 10-arylated dibenzo[b,f]azepines was developed. The pseudo-intramolecular hydrohalogenation of 2-(2'-bromophenyl)ethynylaniline, which proceeded in a syn-selective manner without forming any detectable over-addition product, was a crucial step. All attempts of subsequent arylation via Suzuki-Miyaura cross coupling and construction of a seven membered ring via Ullmann-type intramolecular coupling were unsuccessful because of dehydrohalogenation or other side reactions. This problem was overcome by the N-acetylation of the amino group, which facilitated the abovementioned coupling reactions to afford the desired 10-arylated dibenzoazepines.

Phenalenannulations: Three-Point Double Annulation Reactions that Convert Benzenes into Pyrenes

3-Point annulations, or phenalenannulations, transform a benzene ring directly into a substituted pyrene by "wrapping" two new cycles around the perimeter of the central ring at three consecutive carbon atoms. This efficient, modular, and general method for π-extension opens access to non-symmetric pyrenes and their expanded analogues. Potentially, this approach can convert any aromatic ring bearing a -CH₂ Br or a -CHO group into a pyrene moiety. Depending upon the workup choices, the process can be directed towards either tin- or iodo-substituted product formation, giving complementary choices for further various cross-coupling reactions. The two-directional bis-double annulation adds two new polyaromatic extensions with a total of six new aromatic rings at a central core.

Visible-Light-Mediated [2+2+1] Carbocyclization Reactions of 1,7-Enynes with Bromofluoroacetate to Form Fused Monofluorinated Cyclopenta[c]quinolin-4-ones

Herein, we describe a new protocol for photoinduced radical [2+2+1] carbocyclization reactions of 1,7-enynes with bromofluoroacetate. These reactions, which proceed via a cascade involving fluoroalkylation, 6-exo-dig and 5-endo-trig cyclizations, H-transfer step, and oxidative dehydrogenation, provide an efficient and general route to a variety of fused monofluorinated cyclopenta[c]quinolin-4-one derivatives.

Alkyne benzannulations in the preparation of contorted nanographenes

Nanographenes are a popular area of research due to their promising properties for electronics. Over the last twenty years there has been a significant increase in interest in the development of contorted nanographenes. While many top-down techniques are employed in the synthesis of these planar nanographenes, the use of alkynes in bottom-up syntheses allows for easy functionalization and the development of contorted nanographenes. The syntheses of contorted nanographenes with a focus on utilizing alkynes is reviewed here.

Quantum Interference and Substantial Property Tuning in Conjugated Z-ortho-Regio-Resistive Organic (ZORRO) Junctions

Coherence is a significant factor in nanoscale electronic insulator technology and necessitates an understanding of the structure-property relationship between constructive and destructive quantum interference. This is particularly important in organic dielectric circuitry, which is the subject of this work. It is known that molecular wires composed of (i) meta-substituted phenylene rings, (ii) cross-conjugated double bonds (orthogonal to the molecular long axis), and (iii) single bonds can dramatically reduce electrical transmission. Here we add to these tools the use of an unexplored molecular shape to create strong and counterintuitive interference: a fully conjugated molecular wire with a structure that is forced back on itself in a Z shape, thereby exhibiting remarkably low conductance (G = 0.43 × 10^-9 S) even though the phenylene arrangements are ortho- rather than meta-disposed. We call these Z-shaped molecules having ultralow conduction Z-ortho-regio-resistive organics (ZORROs). Here we analyze a series of ZORRO molecules and find them to have significant insulating properties in the coherent electron-transport regime due to interfering transmission pathways in the phenylene rings. Importantly, we find that both electron-withdrawing (fluorine) and electron-donating (methoxy) substituents enhance the transmission, which is not desirable. The former is due to the suppression of the destructive quantum interference at the F site, thereby enhancing the overall transmission, much like a Büttiker probe. The latter is due to a methoxy unit resonance additive effect, akin to oxygen doping, and positively contributes to the transmission. We then examine the effects of replacing the phenylene rings with 4,5- and 3,4-disubstituted thiophenes and how this ZORRO modification further reduces the transmission. An ultralow conductance of 0.13 × 10^-9 S and a relatively high dielectric constant (ε_r) of ∼5 are predicted for the 3,4-thiophene ZORRO derivative, which closely resembles two cross-conjugated units, making it an intriguing candidate for a gate dielectric material.

Nanographene and Graphene Nanoribbon Synthesis via Alkyne Benzannulations

The extension of π-conjugation of polycyclic aromatic hydrocarbons (PAHs) via alkyne benzannulation reactions has become an increasingly utilized tool over the past few years. This short review will highlight recent work of alkyne benzannulations in the context of large nanographene as well as graphene nanoribbon synthesis along with a brief discussion of the interesting physical properties these molecules display.

Alkyne Origami: Folding Oligoalkynes into Polyaromatics

Do not bend the triple bonds! This familiar undergraduate mantra must be disobeyed if the alkyne group is used as a building block in molecular construction. This Account will describe our exploits in "alkyne origami", that is, folding oligoalkynes into new shapes via cyclization cascades. This research stems from a set of guidelines for the cyclizations of alkynes that we suggested in 2011 ( Gilmore Chem. Rev. 2011 , 111 , 6513 ; Alabugin J. Am. Chem. Soc. 2011 , 133 , 12608 ). The guidelines blended critical analysis of ∼40 years of experimental research with computations into the comprehensive predictions of the relative favorability of dig-cyclizations of anions and radicals. In this Account, we will show how this new understanding has been instrumental in building polyaromatics. In particular, we illustrate the utility of these stereoelectronic models by developing a toolbox of practical, selective, and efficient synthetic transformations. The high energy and high carbon content render alkynes the perfect precursors for the preparation of polyaromatic ribbons and other carbon-rich materials with precisely controlled structure and reactivity. Still, the paradox of alkyne reactivity (alkynes store a lot of energy but are protected kinetically by their relatively strong π-bonds) requires precise use of stereoelectronic factors for lowering the activation barriers for alkyne cyclizations. These factors are drastically different in the "all-exo" and the "all-endo" cyclization cascades of oligoynes. This Account will highlight the interplay between the stereoelectronics of bond formation and topology of acyclic precursor "folding" into a polycyclic ribbon. The topology of folding is simpler for the endo cascades, which are compatible with initiation either at the edge or at the center. In contrast, the exo cascades require precise folding of an oligoalkyne ribbon by starting the cascade exactly at the center of the chain. These differences define the key challenges in the design of these two types of alkyne cyclization cascades. For the endo processes, the folding is simple, but these processes require a strategy ("LUMO Umpolung") for inverting the usual stereoelectronic requirements of alkyne cyclizations. We also show how alkenes can be used as alkyne equivalents in cyclizations coupled with fragmentations and how one can make endo cyclization products without ever going through an endo cyclization. In contrast, each elementary step of the exo cascades benefits from the inherent exo preference for the radical attack, but these cascades require precise initiation by starting exactly at the central alkyne unit of the oligoyne. This strict selectivity requirement led to the development of traceless directing groups capable of supramolecular assistance to the initiation step and self-terminating departure at the end of the cascade. With attention to electronic effects that can stop radical cascades, oligoalkynes can be selectively converted into precisely shaped and functionalized polyaromatic products. The generality of these concepts is further illustrated by the development of radical "peri annulations" at the zigzag edge of acenes.

Radical cascade cyclization of 1,n-enynes and diynes for the synthesis of carbocycles and heterocycles

This review highlights recent advances in radical cascade cyclization of 1,n-enynes and diynes for the construction of carbo- and heterocycles.

Noncovalent Dimerization after Enediyne Cyclization on Au(111)

We investigate the thermally induced cyclization of 1,2-bis(2-phenylethynyl)benzene on Au(111) using scanning tunneling microscopy and computer simulations. Cyclization of sterically hindered enediynes is known to proceed via two competing mechanisms in solution: a classic C(1)-C(6) (Bergman) or a C(1)-C(5) cyclization pathway. On Au(111), we find that the C(1)-C(5) cyclization is suppressed and that the C(1)-C(6) cyclization yields a highly strained bicyclic olefin whose surface chemistry was hitherto unknown. The C(1)-C(6) product self-assembles into discrete noncovalently bound dimers on the surface. The reaction mechanism and driving forces behind noncovalent association are discussed in light of density functional theory calculations.

Diindenopyrenes: Extended 1,6- and 1,8-Pyrenoquinodimethanes with Singlet Diradical Characters

Diindenopyrene 4b and its diastereomer, which are extended homologues of 1,6- and 1,8-pyrenoquinodimethanes fused by indene units, respectively, were synthesized by reaction of 1,4,5,8-tetrakis(mesitylethynyl)naphthalene (7) with bis(2,4,6-trimethylpyridine)iodine(I) hexafluorophosphate via cationic cyclization mechanisms at both centers of reaction. Unexpectedly, reaction of 7 with iodine, a reagent that typically gives products of cationic cyclization, gave cycloheptapentalenoindenophenalene derivative 9b and its diastereomer incorporating an azulene unit at one end of the π framework, via two different modes (radical and cationic) of cyclization at each reaction site. The physical properties of the products are presented, and the dual modes of cyclization of 7 and its model compound with only one reaction center are discussed.

A metal-free yne-addition/1,4-aryl migration/decarboxylation cascade reaction of alkynoates with Csp3–H centers

Reactions of alkynoates with different types of radical precursors under metal-free conditions affording single-site-addition poly-substituted alkenes were reported.

Lewis acid-catalyzed tandem synthesis of 9-sulfonylamino- and 9-arylfluorenes

A Lewis acid-catalyzed three-step tandem synthesis of 9-arylfluorene was developed by simply heating a mixture of 2-formyl biphenyl, TsNCO and an arene.

Conformational flexibility of fused tetracenedione propellers obtained from one-pot reductive dimerization of acetylenic quinones

Reductive dimerization of acetylenic anthraquinones provides synthetic access to flexible nonplanar polyaromatics with a tetracenedione core. In solution, these nonplanar, contorted polycycles exist as equilibrating mixtures of two symmetric conformers. The fused tetracenediones are easily reduced and exhibit rich electrochemical behavior.