Generation of 3-borylbenzynes, their regioselective Diels–Alder reactions, and theoretical analysis

Progress toward the Total Synthesis of Nogalamycin Using a Benzyne Cycloaddition Strategy

Nogalamycin (NOG) is a member of the anthracycline glycoside natural products; no total syntheses have yet been reported, and there is minimal understanding of how the aglycone substitution pattern and identities of the A- and D-ring sugars impact the anticancer activity and toxicity. This paper reports progress toward a modular approach to NOG that could enable systematic structure-activity relationship studies. Key steps include a regioselective benzyne cycloaddition and reductive ring-opening to assemble a versatile AB core for analogue synthesis.

Synthesis of Multisubstituted Aromatics via 3-Triazenylarynes

An efficient method for generating 3-triazenylarynes from ortho-iodoaryl triflate-type precursors was developed. The generated arynes reacted with various arynophiles with high regioselectivity because of the triazenyl group. The 3-triazenylaryne precursors functioned as useful intermediates of diverse multisubstituted aromatic compounds through the transformation of the remaining triazenyl group of aryne adducts and triazenyl group-directed ortho-C-H functionalization.

Regioselective transformation of 3-phosphoryl benzyne intermediates to diverse phosphorus-substituted arenes

Pre-functionalized benzyne precursors 5, 6 and 10 bearing a phosphoryl group were efficiently synthesized via a phospho-Fries rearrangement reaction on gram scales, and directly proceed various transformations to poly-substituted organophosphorus arenes in high regioselectivity.

Could London Dispersion Force Control Regioselective (2 + 2) Cyclodimerizations of Benzynes? YES: Application to the Synthesis of Helical Biphenylenes

In recent years, London dispersion interactions, which are the attractive component of the van der Waals potential, have been found to play an important role in controlling the regio- and/or stereoselectivity of various reactions. Particularly, the dispersion interactions between substrates and catalysts (or ligands) are dominant in various selective catalyzes. In contrast, repulsive steric interactions, rather than the attractive dispersion interactions, between bulky substituents are predominant in most of the noncatalytic reactions. Herein, we demonstrate the first example of London dispersion-controlled noncatalytic (2 + 2) cyclodimerization of substituted benzynes to selectively afford proximal biphenylenes in high yields and regioselectivities, depending on the extent of dispersion interactions in the substituents. This method can be applied for the synthesis of novel helical biphenylenes, which would be fascinating for chemists as these compounds are potential skeletons for ligands, catalysts, and medicines.

Lewis pairing and frustration of group 13/15 elements geometrically enforced by (ace)naphthalene, biphenylene and (thio)xanthene backbones

The synthesis, structure, and reactivity of mixed group 13/group 15 compounds (E¹³ = B, Al, Ga, In, Tl; E¹⁵ = N, P, Sb, Bi) featuring a rigid (ace)naphthalene or (thio)xanthene backbone are discussed in this review. The backbone may either enforce or prevent E¹⁵→E¹³ interactions, resulting in Lewis pairing or frustration. The formation of strong E¹⁵→E¹³ interactions is possible upon peri-substitution of (ace)naphthalenes. This gives the opportunity to access and study highly reactive species, as exemplified by P-stabilised borenium salts and boryl radicals. In turn, rigid expanded spacers such as biphenylenes, (thio)xanthenes and dibenzofurans impose long distances and geometrically prevent E¹⁵→E¹³ interactions. Such P-B derivatives display ambiphilic coordination properties and frustrated Lewis pair behaviour towards small molecules, their preorganised structure favouring reversible interaction/activation. Throughout the review, the importance of the scaffold in enforcing or preventing E¹⁵→E¹³ interactions is highlighted and discussed based on experimental data and theoretical calculations.

Aryne Precursors for Selective Generation of 3-Haloarynes: Preparation and Application to Synthetic Reactions

The synthesis and reaction of new 3-haloaryne precursors 2a-2h were studied. The ortho-(trimethylsilyl)aryl triflate precursors 2a-2h were prepared by a simple procedure involving O-trimethylsilylation and migration of a trimethylsilyl group followed by triflation. The remarkable feature of new precursors is the selective generation of 3-haloarynes by suppressing the competitive thia-Fries rearrangement, which is the problem in the reaction using the well-known 3-haloaryne precursors. The advantage of new precursor 2a over a typical precursor 1 was confirmed by the direct comparisons in several reactions. The application of precursors 2a-2h to the syntheses of heterocycles was also reported.

Predicting the regioselectivity of arynes: a simple model based on orbital electronegativity

A simple model based on orbital electronegativity to predict the regioselectivity of arynes is reported; the nucleophiles attack the terminus possessing lower OE.

Quantification of the Electrophilicity of Benzyne and Related Intermediates

The determination of reactivity parameters for short-lived intermediates provides an indispensable tool for synthetic design. Despite that electrophilicity parameters have now been established for more than 250 reactive species, the corresponding parameters for benzyne and related intermediates have not been uncovered. We report a study that has allowed for the quantification of benzyne's electrophilicity parameter. Our approach relies on the strategic use of the diffusion-clock method and also provides electrophilicity parameters E for other substituted arynes.

Thiazolobenzyne: a versatile intermediate for multisubstituted benzothiazoles

Thiazolobenzynes were efficiently generated from ortho-iodoaryl triflate-type precursors, which enabled facile synthesis of various multisubstituted benzothiazoles.

Boryl (Hetero)aryne Precursors as Versatile Arylation Reagents: Synthesis through C-H Activation and Orthogonal Reactivity

(Pinacolato)boryl ortho-silyl(hetero)aryl triflates are presented as a new class of building blocks for arylation. They demonstrate unique versatility by delivering boronate or (hetero)aryne reactivity chemoselectively in a broad range of transformations. This approach enables the unprecedented postfunctionalization of fluoride-activated (hetero)aryne precursors, for example, as substrates in transition-metal catalysis, and offers valuable new possibilities for aryl boronate postfunctionalization without the use of specialized protecting groups.

Trifluoromethanesulfonyloxy-group-directed regioselective (3 + 2) cycloadditions of benzynes for the synthesis of functionalized benzo-fused heterocycles

Highly regioselective (3 + 2) cycloadditions of (trifluoromethanesulfonyloxy)benzynes [(triflyloxy)benzynes] with 1,3-dipoles followed by cross-coupling reactions provided multisubstituted benzo-fused heterocycles. The triflyloxy group at the 3-position of benzynes, and even that at the remote 4-position, greatly affected the regiocontrol of the cycloaddition. These groups also served to install other substituents at their ipso-positions.

Progress and developments in the turbo Grignard reagent i-PrMgCl·LiCl: a ten-year journey

The structural and kinetic perspectives of i-PrMgCl·LiCl help to rationalize the trends of its unique reactivity and selectivity.

The role of aryne distortions, steric effects, and charges in regioselectivities of aryne reactions

The distortion/interaction model has been used to explain and predict reactivity in a variety of reactions where more common explanations, such as steric and electronic factors, do not suffice. This model has also provided new fundamental insight into regioselectivity trends in reactions of unsymmetrical arynes, which in turn has fueled advances in aryne methodology and natural product synthesis. This article describes a systematic experimental and computational study of one particularly important class of arynes, 3-halobenzynes. 3-Halobenzynes are useful synthetic building blocks whose regioselectivities have been explained by several different models over the past few decades. Our efforts show that aryne distortion, rather than steric factors or charge distribution, are responsible for the regioselectivities observed in 3-haloaryne trapping experiments. We also demonstrate the synthetic utility of 3-halobenzynes for the efficient synthesis of functionalized heterocycles, using a tandem aryne-trapping/cross-coupling sequence involving 3-chlorobenzyne.

Enabling the use of heterocyclic arynes in chemical synthesis

Heterocyclic arynes have long been targeted as potential tools for the synthesis of substituted heterocycles. Recent advances have led to an improved understanding of the factors that determine regioselectivity in reactions of these strained intermediates and, in turn, the aryne distortion model. This paper highlights the use of this predictive model to enable the use of heterocyclic arynes, such as indolynes and pyridynes, in chemical synthesis.

ortho-Selective nucleophilic addition of amines to 3-borylbenzynes: synthesis of multisubstituted anilines by the triple role of the boryl group

Nucleophilic addition of amines to 3-[(dan)boryl]benzynes (dan = 1,8-diaminonaphthalene) generated by a fluoride ion proceeded with high ortho-selectivity to give 2-borylaniline derivatives, under conditions that are tolerant to various functional groups. The (dan)boryl group of the adduct was hydrolyzed into a boronic acid under acidic conditions, which could further serve for various C-C, C-O, C-N, and C-H bond-formation reactions. The overall process provides a promising entry for preparing multisubstituted aniline derivatives.