Rhodium(III)-Catalyzed C–H Activation/Annulation with Vinyl Esters as an Acetylene Equivalent

Palladium-catalyzed annulation of N-alkoxy benzsulfonamides with arynes by C–H functionalization: access to dibenzosultams

Palladium-catalyzed C–H/N–H functionalization of N-alkoxy benzsulfonamides with arynes provides dibenzosultams via C–C/C–N bond formation, accompanied by an unexpected N–O bond cleavage.

Substituted Dihydroisoquinolinones by Iodide-Promoted Cyclocarbonylation of Aromatic α-Amino Acids

Imidazolidinone derivatives of a range of aromatic α-amino acids, on treatment with phosgene and potassium iodide, undergo a mild Bischler-Napieralski-style cyclocarbonylation reaction that generates a tricyclic lactam by insertion of a C═O group between amino acid nitrogen and the ortho position of the aryl substituent. Regio- and diastereoselective functionalization of the lactam generates a library of substituted dihydroisoquinolinones and their congeners in enantioenriched form.

Rh(III)-Catalyzed [4 + 2] Self-Annulation of N-Vinylarylamides

An efficient rhodium(III)-catalyzed self-annulation of N-vinylarylamide has been developed. This reaction features a simple system and good reactivity with complete regioselectivity. The protocol provides easy access to an aminal incorporated dihydroisoquinolinone, which proved to be a versatile synthetic synthon. The kinetic isotope effect experiments showed that C-H activation is the rate-limiting step, and competition studies revealed the annulation exhibits a strong self-recognition mode. In addition, a seven-membered rhodacycle species was isolated and established as the key reaction intermediate.

Methanol-Assisted Phthalimide Ring Opening: Concerted or Stepwise Mechanism?

The opening of the five-membered ring is the essential step for phthalimide and its derivatives to be used as the reactants in many chemical synthetic routes. Reportedly, such ring opening follows the concerted mechanism in methanol solvent, which, however, has an unreasonably high energy barrier (36.3 kcal mol^-1 at the M06-2X/6-311++G(d,p) level of theory). By density functional theory calculations, we report that this ring opening prefers the alternatively stepwise mechanism. The stepwise mechanism has a much lower energy barrier (21.0 kcal mol^-1 at the same level of theory) and thus is much more completive than the concerted one. The stepwise mechanism should be considered as the dominant mechanism responsible for the phthalimide ring opening when studying the kinetics of the relevant synthetic reactions in the future.

Rh(iii)-Catalyzed regio- and stereoselective bisindolylation of vinyl acetate: an efficient approach toward (E)-1,2-bis(2-indolyl)ethenes

An efficient Rh(iii)-catalyzed regio- and stereoselective cross-coupling between vinyl acetate andN-(2-pyridiyl)indoles providing a series of (E)-2,2′-bis(indolyl)ethenes has been developed.

Rhodium-catalyzed C–H bond activation alkylation and cyclization of 2-arylquinazolin-4-ones

An efficient method for the synthesis of isoquinolino[1,2-b]quinazolin-8-one derivatives and 12-methyl-12H-isoindolo[1,2-b]quinazoline-10-one derivatives is described herein.

Rhodium- and Non-Metal-Catalyzed Approaches for the Conversion of Isoxazol-5-ones to 2,3-Dihydro-6H-1,3-oxazin-6-ones

Two approaches were developed for the conversion of isoxazol-5-ones to 2,3-dihydro-6H-1,3-oxazin-6-ones. The first involves dirhodium-catalyzed reaction of aryl diazoacetates, leading to rhodium carbene intermediates that undergo insertion into the N-O bond of isoxazol-5-ones. The second approach involves conversion of the aryldiazoacetates to the corresponding tosylates, followed by reaction with the isoxazol-5-one in a metal-free one-pot procedure. These studies illustrate an alternative method to achieve a carbene-like transformation without requiring a metal catalyst.

Heptamethylindenyl (Ind*) enables diastereoselective benzamidation of cyclopropenes via Rh(iii)-catalyzed C-H activation

The diastereoselective coupling of O-substituted arylhydroxamates and cyclopropenes mediated by Rh(iii) catalysis was successfully developed. Through ligand development, the diastereoselectivity of this reaction was improved using a heptamethylindenyl (Ind*) ligand, which has been rationalized using quantum chemical calculations. In addition, the nature of the O-substituted ester of benzhydroxamic acid proved important for high diastereoselectivity. This transformation tolerates a variety of benzamides and cyclopropenes that furnish cyclopropa[c]dihydroisoquinolones with high diastereocontrol, which could then be easily transformed into synthetically useful building blocks for pharmaceuticals and bio-active molecules.

Correlating Reactivity and Selectivity to Cyclopentadienyl Ligand Properties in Rh(III)-Catalyzed C-H Activation Reactions: An Experimental and Computational Study

CpXRh(III)-catalyzed C-H functionalization reactions are a proven method for the efficient assembly of small molecules. However, rationalization of the effects of cyclopentadienyl (CpX) ligand structure on reaction rate and selectivity has been viewed as a black box, and a truly systematic study is lacking. Consequently, predicting the outcomes of these reactions is challenging because subtle variations in ligand structure can cause notable changes in reaction behavior. A predictive tool is, nonetheless, of considerable value to the community as it would greatly accelerate reaction development. Designing a data set in which the steric and electronic properties of the CpXRh(III) catalysts were systematically varied allowed us to apply multivariate linear regression algorithms to establish correlations between these catalyst-based descriptors and the regio-, diastereoselectivity, and rate of model reactions. This, in turn, led to the development of quantitative predictive models that describe catalyst performance. Our newly described cone angles and Sterimol parameters for CpX ligands served as highly correlative steric descriptors in the regression models. Through rational design of training and validation sets, key diastereoselectivity outliers were identified. Computations reveal the origins of the outstanding stereoinduction displayed by these outliers. The results are consistent with partial η5-η3 ligand slippage that occurs in the transition state of the selectivity-determining step. In addition to the instructive value of our study, we believe that the insights gained are transposable to other group 9 transition metals and pave the way toward rational design of C-H functionalization catalysts.

Recent advances in positional-selective alkenylations: removable guidance for twofold C–H activation

Recent advances in transition-metal catalyzed positional-selective alkenylations via twofold C–H activation directed by removable or traceless directing groups are reviewed.

Iron-catalyzed C–H/N–H activation by triazole guidance: versatile alkyne annulation

Iron-catalyzed C–H/N–H functionalizations were achieved by the aid of modular triazole amides.

Design and synthesis of dihydroisoquinolones for fragment-based drug discovery (FBDD)

This study describes general synthesis aspects of fragments for FBDD, as illustrated by the dihydroisoquinolones 1-3. Previous Rh(III) methodology is extended to incorporate amines, heteroatoms (N and S), and substituents (halogen, ester) as potential binding groups and/or synthetic growth points for fragment-to-lead elaboration.

A Simple and Versatile Amide Directing Group for C-H Functionalizations

Achieving selective C-H activation at a single and strategic site in the presence of multiple C-H bonds can provide a powerful and generally useful retrosynthetic disconnection. In this context, a directing group serves as a compass to guide the transition metal to C-H bonds by using distance and geometry as powerful recognition parameters to distinguish between proximal and distal C-H bonds. However, the installation and removal of directing groups is a practical drawback. To improve the utility of this approach, one can seek solutions in three directions: 1) Simplifying the directing group, 2) using common functional groups or protecting groups as directing groups, and 3) attaching the directing group to substrates via a transient covalent bond to render the directing group catalytic. This Review describes the rational development of an extremely simple and yet broadly applicable directing group for Pd(II) , Rh(III) , and Ru(II) catalysts, namely the N-methoxy amide (CONHOMe) moiety. Through collective efforts in the community, a wide range of C-H activation transformations using this type of simple directing group have been developed.

Recent developments in transition metal catalysis for quinazolinone synthesis

Quinazolinones are an important class of heteroaromatic compound, and they are prevalent in a variety of important bio-active scaffolds. Traditionally, their synthesis has been achieved via classical condensation procedures, however over recent years catalytic methodologies have emerged as effective alternatives. This review examines recent developments in the employment of catalytic approaches towards this highly important heterocyclic motif.

Expeditious Synthesis of Isoquinolones and Isocoumarins with a Vinyl Borane as an Acetylene Equivalent

An innovative and simple expeditious synthesis of 3,4-unsubstituted isoquinolones and isocoumarins starting from safe and easy to handle two-carbon acetylene equivalent was developed. The synthetic potential of this new method was further demonstrated in the facile total synthesis of two naturally occurring alkaloids: corydaldine and doryanine.

Convergent Synthesis of Diverse Nitrogen Heterocycles via Rh(III)-Catalyzed C-H Conjugate Addition/Cyclization Reactions

The development of Rh(III)-catalyzed C-H conjugate addition/cyclization reactions that provide access to synthetically useful fused bi- and tricyclic nitrogen heterocycles is reported. A broad scope of C-H functionalization substrates and electrophilic olefin coupling partners is effective, and depending on the nature of the directing group, cyclic imide, amide, or heteroaromatic products are obtained. An efficient synthesis of a pyrrolophenanthridine alkaloid natural product, oxoassoanine, highlights the utility of this method.

Enantiospecific Alkynylation of Alkylboronic Esters

Enantioenriched secondary and tertiary alkyl pinacolboronic esters undergo enantiospecific deborylative alkynylation through a Zweifel-type alkenylation followed by a 1,2-elimination reaction. The process involves use of α-lithio vinyl bromide or vinyl carbamate species, for which application to Zweifel-type reactions has not previously been explored. The resulting functionalized 1,1-disubstituted alkenes undergo facile base-mediated elimination to generate terminal alkyne products in high yield and excellent levels of enantiospecificity over a wide range of pinacolboronic ester substrates. Furthermore, along with terminal alkynes, internal and silyl-protected alkynes can be formed by simply introducing a suitable carbon- or silicon-based electrophile after the base-mediated 1,2-elimination reaction.