David A. Petrone

Modern Transition-Metal-Catalyzed Carbon–Halogen Bond Formation

The high utility of halogenated organic compounds has prompted the development of a vast number of transformations which install the carbon-halogen motif. Traditional routes to these building blocks have commonly involved multiple steps, harsh reaction conditions, and the use of stoichiometric and/or toxic reagents. In this regard, using transition metals to catalyze the synthesis of organohalides has become a mature field in itself, and applying these technologies has allowed for a decrease in the production of waste, higher levels of regio- and stereoselectivity, and the ability to produce enantioenriched target compounds. Furthermore, transition metals offer the distinct advantage of possessing a diverse spectrum of mechanistic possibilities which translate to the capability to apply new substrate classes and afford novel and difficult-to-access structures. This Review provides comprehensive coverage of modern transition metal-catalyzed syntheses of organohalides via a diverse array of mechanisms. Attention is given to the seminal stoichiometric organometallic studies which led to the corresponding catalytic processes being realized. By breaking this field down into the synthesis of aryl, vinyl, and alkyl halides, it becomes clear which methods have surfaced as most favored for each individual class. In general, a pronounced shift toward the use of C-H bonds as key functional groups, in addition to methods which proceed by catalytic, radical-based mechanisms has occurred. Although always evolving, this field appears to be heading in the direction of using starting materials with a significantly lower degree of prefunctionalization in addition to less expensive and abundant metal catalysts.

A Conjunctive Carboiodination: Indenes by a Double Carbopalladation–Reductive Elimination Domino Process†

Something gained, nothing lost: A Pd(0)-catalyzed domino intermolecular/intramolecular process terminated by carbohalogenation is reported. In this reaction, two new C-C bonds, one new C-I bond and one five-membered ring are formed in a single step, and all of the atoms in the starting materials are incorporated into the product.

Catalytic Hydroacylation as an Approach to Homoaldol Products

A method has been developed for the intermolecular hydroacylation of homoallyl alcohols with salicylaldehydes to furnish homoaldol products in 50-98% yields. The method also applies to the hydroacylation of 2-hydroxystyrenes. This work highlights the use of hydroacylation as a unified approach to both aldol and homoaldol products.

Additive Effects in the Palladium-Catalyzed Carboiodination of Chiral N-Allyl Carboxamides†

The use of Pd catalysis as a means to synthesize organic halides has recently received increased attention. Among the reported methods is the Pd-catalyzed carboiodination, which uses extremely bulky ligands to facilitate carbon-halogen reductive elimination from Pd(II) as the key catalytic step. When approaching substrates exhibiting low stereoselectivity, catalyst troubleshooting becomes difficult as there are few ligands known to promote the key reductive elimination. Herein, we present our finding that tertiary amines act as weakly coordinating ligands which significantly enhance diastereoselectivity in the Pd/QPhos-catalyzed carboiodination of chiral N-allyl carboxamides. This methodology allows efficient access to enantioenriched and densely functionalized dihydroisoquinolinones, and has been applied toward the asymmetric formal synthesis of (+)-corynoline.

Harnessing Reversible Oxidative Addition: Application of Diiodinated Aromatic Compounds in the Carboiodination Process

An I for an I: Conditions for the intramolecular carboiodination and the simultaneous convergent intramolecular carboiodination/intermolecular Heck reaction of various diiodoarenes were developed. The ability of the Pd(0)/QPhos catalyst/ligand combination to undergo reversible oxidative addition allows these reactions to proceed well, thus increasing both the appeal and utility of this class of substrates in site-selective cross-coupling reactions.

Synergistic Steric Effects in the Development of a Palladium‐Catalyzed Alkyne Carbohalogenation: Stereodivergent Synthesis of Vinyl Halides

We report our finding that by exploiting the synergistic steric effects between substrate and catalyst, an intramolecular Pd-catalyzed alkyne carbohalogenation can be achieved. This operationally simple method uses the bulky Pd/Q-Phos combination and allows access to tetrasubstituted vinyl halides from the corresponding aryl chlorides, bromides, and iodides. Steric effects in the substrate play a key role by promoting C sp 2-halogen reductive elimination and enabling catalytic turnover. Through a reversible oxidative addition mechanism, a thermodynamically driven isomerization reaction is observed at elevated temperatures. Thus by changing the reaction temperature, both stereoisomers of the reaction become readily accessible.

Dearomative Indole Bisfunctionalization via a Diastereoselective Palladium-Catalyzed Arylcyanation

The first Pd-catalyzed dearomative indole bisfunctionalization via a diastereoselective arylcyanation is reported. This method facilitates the formation of diverse indoline scaffolds bearing congested stereocenters with high levels of diastereoselectivity. This also represents the first example of a cyanation mechanism involving a 2° benzylic Pd(II) intermediate.

Pd(0)‐Catalyzed Dearomative Diarylation of Indoles

We have developed a protocol for a Pd(0)-catalyzed dearomative syn 1,2-diarylation of indoles using readily available boroxines (dehydrated boronic acids) as coupling partners. This reaction proceeds efficiently using PtBu3 as the ligand to divergently access to fused indolines while minimizing the extent of direct Suzuki coupling. The scope of the reaction is remarkably broad and all products are obtained as single diastereomers in moderate to excellent yields. We have also compiled data which parallels the steric and electronic properties of both substrate and boroxine with the propensity to undergo the desired dearomative process over direct Suzuki coupling.

Diastereoselective Palladium-Catalyzed Arylcyanation/Heteroarylcyanation of Enantioenriched N-Allylcarboxamides

A diastereoselective Pd-catalyzed arylcyanation/heteroarylcyanation of chiral N-allylcarboxamides using Zn(CN)2 as the cyanide source is reported. Nitrile-containing dihydroisoquinolinone products are obtained in good to excellent yields with up to >95:5 dr and with full preseveration of enantioenrichment. By circumventing a difficult nucleophilic cyanation of a hindered neopentyl iodide, this approach represents an improvement to the previously reported formal synthesis of (+)-corynoline.

Palladium-Catalyzed Hydrohalogenation of 1,6-Enynes: Hydrogen Halide Salts and Alkyl Halides as Convenient HX Surrogates

Difficulties associated with handling H2 and CO in metal-catalyzed processes have led to the development of chemical surrogates to these species. Despite many successful examples using this strategy, the application of convenient hydrogen halide (HX) surrogates in catalysis has lagged behind considerably. We now report the use of ammonium halides as HX surrogates to accomplish a Pd-catalyzed hydrohalogenation of enynes. These safe and practical salts avoid many drawbacks associated with traditional HX sources including toxicity and corrosiveness. Experimental and computational studies support a reaction mechanism involving a crucial E-to-Z vinyl-Pd isomerization and a carbon-halogen bond-forming reductive elimination. Furthermore, rare examples of C(sp3)-Br and -Cl reductive elimination from Pd(II) as well as transfer hydroiodination using 1-iodobutane as an alternate HI surrogate are also presented.