Silyloxyarenes are demonstrated to be a versatile substrate class in a variety of nickel-catalyzed coupling processes. The C(sp2)–O bond of aryl silyl ethers is directly transformed into C–H or C–Si bonds using Ti(O-i-Pr)4 or trialkylsilanes as reagents using nickel catalysts with N-heterocyclic carbene (NHC) ligands. Paired with the useful characteristics of silyl protecting groups, these methods enable protected hydroxyls to directly participate in high-value bond-forming steps rather than requiring deprotection-activation strategies that conventional approaches require. These processes of silyloxyarenes provide reactivity complementary to widely used phenol derivatives such as aryl pivalates, carbamates, and methyl ethers, thus enabling a powerful strategy for sequential chemoselective derivatization of complex substrates without protecting group and activating group manipulations.Keywords: catalysis; cross-coupling; C−O bond activation; nickel; silane; silyloxyarenes;

An exceptionally hindered class of enantiopure NHC ligands has been developed. While racemic forms had previously been utilized, a scalable and practical route to the enantiopure form of this ligand class is described utilizing a Buchwald–Hartwig N,N-diarylation in a highly sterically demanding environment. Using this newly accessible ligand class, nickel-catalyzed enantioselective reductive coupling reactions of aldehydes and alkynes have been developed. These studies illustrate that the newly available NHC ligands are well suited for simultaneous control of regio- and enantioselectivity, even in cases with internal alkynes possessing only very subtle steric differences between two aliphatic substituents. The steric demand of the new ligand class enables a complementary regiochemical outcome compared with previously described enantioselective processes. Using this method, a number of allylic alcohol derivatives were efficiently obtained with high regioselectivity (up to >95:5) and high enantioselectivity (up to 94% ee). The reaction conditions can also be extended to the reaction of aldehydes and allenes, providing silyl-protected allylic alcohol derivatives possessing a terminal methylene substituent. Computational studies have explained the origin of the exceptional steric demand of this ligand class, the basis for enantioselectivity, and the cooperative relationship of the aldehyde, alkyne, and ligand in influencing enantioselectivity.

The diversification of late stage synthetic intermediates provides significant advantages in efficiency in comparison to conventional linear approaches. Despite these advantages, accessing varying ring scaffolds and functional group patterns from a common intermediate poses considerable challenges using existing methods. The combination of regiodivergent nickel-catalyzed C–C couplings and site-selective biocatalytic C–H oxidations using the cytochrome P450 enzyme PikC addresses this problem by enabling a single late-stage linear intermediate to be converted to macrolactones of differing ring size and with diverse patterns of oxidation. The approach is made possible by a novel strategy for site-selective biocatalytic oxidation using a single biocatalyst, with site selectivity being governed by a temporarily installed directing group. Site selectivities of C–H oxidation by this directed approach can overcome positional bias due to C–H bond strength, acidity, inductive influences, steric accessibility, or immediate proximity to the directing group, thus providing complementarity to existing approaches.

A cascade cyanation/diborylation of terminal allenes proceeds efficiently with copper catalysis using bis(pinacolato)diboron (B2Pin2) and N-cyano-N-phenyl-p-methylbenzenesulfonamide (NCTS) as reagents. Mechanistic studies suggest that the process proceeds through cyanoborylation of the substituted π-system of the allene followed by hydroboration of the remaining π-component. A wide array of product derivatives may be accessed through site-selective cross-couplings and N-bromosuccinimide-promoted heteroarylations as well as standard oxidative and reductive conversions of the initially obtained adducts.

Copper-catalyzed functionalization of terminal or 1,1-disubstituted alkenes with bis(pinacolato)diboron and methanol provides formal hydroboration products with exceptional regiocontrol favoring the branched isomer. Pairing this procedure with photocatalytic cross-couplings using iridium and nickel cocatalysis provides an effective, highly regioselective procedure for the hydroarylation of terminal alkenes.

The control of regiochemistry is a considerable challenge in the development of a wide array of catalytic processes. Simple π-components such as alkenes, alkynes, 1,3-dienes, and allenes are among the many classes of substrates that present complexities in regioselective catalysis. Considering an internal alkyne as a representative example, when steric and electronic differences between the two substituents are minimal, differentiating among the two termini of the alkyne presents a great challenge. In cases where the differences between the alkyne substituents are substantial, overcoming those biases to access the regioisomer opposite that favored by substrate biases often presents an even greater challenge.Nickel-catalyzed reductive couplings of unsymmetrical π-components make up a group of reactions where control of regiochemistry presents a challenging but important objective. In the course of our studies of aldehyde–alkyne reductive couplings, complementary solutions to challenges in regiocontrol have been developed. Through careful selection of the ligand and reductant, as well as the more subtle reaction variables such as temperature and concentration, effective protocols have been established that allow highly selective access to either regiosiomer of the allylic alcohol products using a wide range of unsymmetrical alkynes. Computational studies and an evaluation of reaction kinetics have provided an understanding of the origin of the regioselectivity control. Throughout the various procedures described, the development of ligand–substrate interactions plays an essential role, and the overall kinetic descriptions were found to differ between protocols. Rational alteration of the rate-determining step plays a key role in the regiochemistry reversal strategy, and in one instance, the two possible regioisomeric outcomes in a single reaction were found to operate by different kinetic descriptions. With this mechanistic information in hand, the empirical factors that influence regiochemistry can be readily understood, and more importantly, the insights suggest simple and predictable experimental variables to achieving a desired reaction outcome.These studies thus present a detailed picture of the influences that control regioselectivity in a specific catalytic reaction, but they also delineate strategies for regiocontrol that may extend to numerous classes of reactions. The work provides an illustration of how insights into the kinetics and mechanism of a catalytic process can rationalize subtle empirical findings and suggest simple and rational modifications in procedure to access a desirable reaction outcome. Furthermore, these studies present an illustration of how important challenges in organic synthesis can be met by novel reactivity afforded by base metal catalysis. The use of nickel catalysis in this instance not only provides an inexpensive and sustainable method for catalysis but also enables unique reactivity patterns not accessible to other metals.

An inhibitory role of 1,5-cyclooctadiene (COD) in nickel-catalyzed C–H functionalization processes was identified and studied. The bound COD participates in C–H activation by capturing the hydride, leading to a stable off-cycle π-allyl complex that greatly diminished overall catalytic efficiency. Computational studies elucidated the origin of the effect and enabled identification of a 1,5-hexadiene-derived pre-catalyst that avoids the off-cycle intermediate and provides catalytic efficiencies that are superior to those of catalysts derived from Ni(COD)2.

The nickel-catalyzed coupling of enones or enals with alkynes in the presence of silane and titanium alkoxide reductants provides direct access to skipped diene products. The process involves a net four-electron reductive coupling and proceeds with deoxygenation of the starting enone or enal. A new class of well-defined nickel(0) precatalysts bearing an unhindered N-heterocyclic carbene ligand, which was developed in optimization of the process, is essential for the efficiency of the transformation. The strategy allows the high reactivity of α,β-unsaturated carbonyl substrates to be utilized in couplings with simultaneous extrusion of the oxygen atom, thus enabling a traceless strategy for alkene installation.

A new method for the intramolecular glycosylation of alcohols is described. Utilizing carbohydrate-derived silanes, the catalytic dehydrogenative silylation of alcohols is followed by intramolecular glycosylation. Appropriate combinations of silane position and protecting groups allow highly selective access to β-manno, α-gluco, or β-gluco stereochemical relationships as well as 2-azido-2-deoxy-β-gluco- and 2-deoxy-β-glucosides. Intramolecular aglycone delivery from the C-2 or C-6 position provides 1,2-cis or 1,2-trans glycosides, respectively. Multifunctional acceptor substrates such as hydroxyketones and diols are tolerated and are glycosylated in a site-selective manner.

A general protocol for the highly exo-selective macrocyclization of ynals using a nickel/N-heterocyclic carbene catalyst system has been developed. A series of 10- to 21-membered macrocycles bearing an exomethylene substituent was synthesized in good yields with excellent regioselectivity (exo/endo >95:5). Very high levels of long-range diastereocontrol can also be achieved for some classes of macrocycles. Complementary to previously reported endo-selective macrocyclizations, this method provides accesses to exoalkylidene macrocycles from simple ynals in high selectivity.

Highly regioselective remote hydroxylation of a natural product scaffold is demonstrated by exploiting the anchoring mechanism of the biosynthetic P450 monooxygenase PikCD50N-RhFRED. Previous studies have revealed structural and biochemical evidence for the role of a salt bridge between the desosamine N,N-dimethylamino functionality of the natural substrate YC-17 and carboxylate residues within the active site of the enzyme, and selectivity in subsequent C–H bond functionalization. In the present study, a substrate-engineering approach was conducted that involves replacing desosamine with varied synthetic N,N-dimethylamino anchoring groups. We then determined their ability to mediate enzymatic total turnover numbers approaching or exceeding that of the natural sugar, while enabling ready introduction and removal of these amino anchoring groups from the substrate. The data establish that the size, stereochemistry, and rigidity of the anchoring group influence the regioselectivity of enzymatic hydroxylation. The natural anchoring group desosamine affords a 1:1 mixture of regioisomers, while synthetic anchors shift YC-17 analogue C-10/C-12 hydroxylation from 20:1 to 1:4. The work demonstrates the utility of substrate engineering as an orthogonal approach to protein engineering for modulation of regioselective C–H functionalization in biocatalysis.

Combinations of ligand, reducing agent, and reaction conditions have been identified that allow alteration in the rate- and regioselectivity-determining step of nickel-catalyzed aldehyde–alkyne reductive couplings. Whereas previously developed protocols involve metallacycle-forming oxidative cyclization as the rate-determining step, this study illustrates that the combination of large ligands, large silanes, and elevated reaction temperature alters the rate- and regiochemistry-determining step for one of the two possible product regioisomers. These modifications render metallacycle formation reversible for the minor isomer pathway, and σ-bond metathesis of the metallacycle Ni–O bond with the silane reductant becomes rate limiting. The ability to tune regiocontrol via this alteration in reversibility of a key step allows highly regioselective outcomes that were not possible using previously developed methods.

The mechanism of nickel(0)-catalyzed reductive coupling of aldehydes and alkynes has been studied. Extensive double-labeling crossover studies have been conducted. While previous studies illustrated that phosphine- and N-heterocyclic carbene-derived catalysts exhibited differing behavior, the origin of these effects has now been evaluated in detail. Many variables, including ligand class, sterics of the ligand and alkyne, temperature, and ring size being formed in intramolecular versions, all influence the extent of crossover observed. A computational evaluation of these effects suggests that dimerization of a key metallacyclic intermediate provides the origin of crossover. Protocols that proceed with crossover are typically less efficient than those without crossover given the thermodynamic stability and low reactivity of the dimeric metallacycles involved in crossover pathways.

Advances in hydroarylation have been achieved by the development of a one-pot regioselective allene hydrosilylation/Pd(0)-catalyzed cross-coupling protocol. The regioselectivity is primarily governed by N-heterocyclic carbene (NHC) ligand identity in the hydrosilylation step and is preserved in the subsequent cross-coupling reaction. This methodology affords streamlined access to functionalized 1,1-disubstituted alkenes with excellent regiocontrol.

Regioselective methods for allene hydrosilylation have been developed, with regioselectivity being governed primarily by the choice of metal. Alkenylsilanes are produced via nickel catalysis with larger N-heterocyclic carbene (NHC) ligands, and allylsilanes are produced via palladium catalysis with smaller NHC ligands. These complementary methods allow either regioisomeric product to be obtained with exceptional regiocontrol.

Strategies for the reductive cycloaddition of enals or enoates with alkynes have been developed. The enal–alkyne cycloaddition directly affords cyclopentenols, whereas the enoate–alkyne cycloaddition affords the analogous cyclopentenones. The mechanism of these processes likely involves formation and protonation of a metallacyclic intermediate. The general strategy provides a straightforward entry to five-membered ring products from simple, stable π-systems.

The regioselectivities of N-heterocyclic carbene (NHC) ligands in Ni-catalyzed alkyne–aldehyde reductive coupling reactions with silane reducing agents are investigated using density functional theory. Reversal of regioselectivity can be achieved by varying the steric bulkiness of the ligand. The steric influences of NHC ligands are highly anisotropic. Regioselectivity is primarily controlled by the steric hindrance at the region of the ligand close to the alkyne. Analysis of 2D contour maps of the NHC ligands indicates that the regioselectivities are directly affected by the shape and orientation of the N-substituents on the ligand.

The mechanism of nickel-catalyzed, silane-mediated reductive cyclization of ynals has been evaluated. The cyclizations are first-order in [Ni] and [ynal] and zeroth-order in [silane]. These results, in combination with the lack of rapid silane consumption upon reaction initiation, are inconsistent with mechanisms involving reaction initiation by oxidative addition of Ni(0) to the silane. Silane consumption occurs only when both the alkyne and aldehyde are present. Mechanisms involving rate-determining oxidative cyclization to a metallacycle followed by rapid reaction with the silane are consistent with the data obtained.

The reductive coupling of enones or enals with alkynes, followed by olefin oxidative cleavage and Paal–Knorr cyclization, provides a versatile entry to a variety of pyrrole frameworks. A number of limitations of alternate entries to the requisite 1,4-dicarbonyl intermediate are avoided. Classes of pyrroles that are accessible by this approach include 2,3-, 2,4-, 1,2,3-, 1,2,4-, 2,3,5-, and 1,2,3,5-substituted monocyclic pyrroles as well as a number of fused-ring polycyclic derivatives.

A nickel-catalyzed cycloisomerization coupling an allylic alcohol with an alkyne has been developed. Mechanistic insights gained through deuterium-labeling crossover studies and stereochemical probes illustrate that oxidation of the allylic alcohol to a metal-free enone is not involved in the cyclization pathway. The intermediacy of a metallacycle directly derived from oxidative cyclization of Ni(0) with the allylic alcohol and alkyne is consistent with the results obtained. The simple mechanistic probes employed could be useful in the study of many classes of cycloisomerization processes.

Strategies for the reductive coupling of enones or enals with alkynes have been developed. The reducing agents employed include organozincs, organoboranes, organosilanes, and methanol. The latter of these strategies is simple, cost-effective, and tolerant of many functional groups. Isotopic labeling strategies have provided supporting evidence for the mechanistic proposals.

The nickel-catalyzed reductive coupling of propargyl alcohols and alkynes proceeds with excellent regiochemical control with an underlying electronic preference that can be supplemented by ligand size effects. The products obtained may be readily converted to substructures that are not directly available by an aldehyde−alkyne reductive coupling. A simple model for how steric and electronic factors are both important in governing regiochemistry in couplings of this type is presented, along with examples of how the effects can combine in either a constructive or destructive manner.

Regio- and stereoselective oxidation of an unactivated C–H bond remains a central challenge in organic chemistry. Considerable effort has been devoted to identifying transition metal complexes, biological catalysts, or simplified mimics, but limited success has been achieved. Cytochrome P450 mono-oxygenases are involved in diverse types of regio- and stereoselective oxidations, and represent a promising biocatalyst to address this challenge. The application of this class of enzymes is particularly significant if their substrate spectra can be broadened, selectivity controlled, and reactions catalyzed in the absence of expensive heterologous redox partners. In this study, we engineered a macrolide biosynthetic P450 mono-oxygenase PikC (PikCD50N-RhFRED) with remarkable substrate flexibility, significantly increased activity compared to wild-type enzyme, and self-sufficiency. By harnessing its unique desosamine-anchoring functionality via a heretofore under-explored “substrate engineering” strategy, we demonstrated the ability of PikC to hydroxylate a series of carbocyclic rings linked to the desosamine glycoside via an acetal linkage (referred to as “carbolides”) in a regioselective manner. Complementary analysis of a number of high-resolution enzyme-substrate cocrystal structures provided significant insights into the function of the aminosugar-derived anchoring group for control of reaction site selectivity. Moreover, unexpected biological activity of a select number of these carbolide systems revealed their potential as a previously unrecorded class of antibiotics.

A strategy for catalyst-controlled regioselectivity in aldehyde−alkyne reductive couplings has been developed. This strategy is the first where either regiochemical outcome may be selected for a broad range of couplings, without relying on substrate biases or directing effects. The complementary use of small cyclopropenylidene carbene ligands or highly hindered N-heterocyclic carbene ligands allows the regiochemical reversal with unbiased internal alkynes, aromatic internal alkynes, conjugated enynes, or terminal alkynes.

A new method for the intramolecular glycosylation of alcohols is described. Utilizing carbohydrate-derived silanes, the catalytic dehydrogenative silylation of alcohols is followed by intramolecular glycosylation. Appropriate combinations of silane position and protecting groups allow highly selective access to β-manno, α-gluco, or β-gluco stereochemical relationships as well as 2-azido-2-deoxy-β-gluco- and 2-deoxy-β-glucosides. Intramolecular aglycone delivery from the C-2 or C-6 position provides 1,2-cis or 1,2-trans glycosides, respectively. Multifunctional acceptor substrates such as hydroxyketones and diols are tolerated and are glycosylated in a site-selective manner.