Synthesis of Z- or E-Trisubstituted Allylic Alcohols and Ethers by Kinetically Controlled Cross-Metathesis with a Ru Catechothiolate Complex

Cobalt-Catalyzed Chemo- and Stereoselective Transfer Semihydrogenation of 1,3-Dienes with Water as a Hydrogen Source

(Z)-1,2-Disubstituted, trisubstituted, and tetrasubstituted alkenes are not only important units in medicinal chemistry, natural product synthesis, and material science but also useful intermediates in organic synthesis. Development of catalytic stereoselective transformations to access multisubstituted alkenes with various substitution patterns from easily accessible modular starting materials and readily available catalysts is a crucial goal in the field of catalysis. Water is an ideal hydrogen source for catalytic transfer hydrogenation despite of the high difficulty to activate water. Here, we report a cobalt-catalyzed protocol for regio- and stereoselective transfer semihydrogenation of 1,3-dienes to construct a broad scope of (Z)-1,2-disubstituted, (Z)-, (E)-trisubstituted, and tetrasubstituted alkenes in high stereoselectivity with H2O as the hydrogen source. Mechanistic studies revealed that the reactions proceeded through a unique Co(I)/Co(III) cycle and involved a 1,4-cobalt shift process, which is an unprecedented reaction pathway, providing a new platform for modular synthesis of multisubstituted alkenes as well as opportunities for designing novel reaction modes and pushing forward the advancement in organocobalt chemistry.

Taking Olefin Metathesis to the Limit: Stereocontrolled Synthesis of Trisubstituted Alkenes

ConspectusIn this Account, we share the story of the development of catalytic olefin metathesis processes that efficiently deliver a wide range of acyclic and macrocyclic E- or Z-trisubstituted alkenes. The tale starts with us unveiling, in collaboration with Richard Schrock and his team, the blueprint in 2009 for the design of kinetically controlled Z-selective olefin metathesis reactions. This paved the way for the development of Mo-, W-, and Ru-based catalysts and strategies for synthesizing countless linear and macrocyclic Z-olefins. Six years later, in 2015, we found that abundant Z-alkene feedstocks, such as oleic acid, can be directly transformed to high-value and more difficult-to-access alkenes through a cross-metathesis reaction promoted by a Ru-catechothiolate complex that we had developed; the approach, later coined stereoretentive olefin metathesis, was extended to the synthesis of E-alkenes.It was all about disubstituted alkenes until when in 2017 we addressed the challenge of accessing stereodefined Z- and E-trisubstituted alkenes, key to medicine and materials research. These transformations can be most effectively catalyzed by Mo monoaryloxides pyrrolide (MAP) and chloride (MAC) complexes. A central aspect of the advance is the merging of olefin metathesis, which delivered trisubstituted alkenyl fluorides, chlorides, and bromides with cross-coupling. These catalytic and stereoretentive transformations can be used in various combinations, thereby enabling access to assorted Z- or E-trisubstituted alkene. Ensuing work led to the emergence of other transformations involving substrates that can be purchased with high stereoisomeric purity, notably E- and Z-trihalo alkenes. Trisubstituted olefins, Z or E, bearing a chemoselectively and stereoretentively alterable F,Cl-terminus or B(pin),Cl-terminus may, thus, be easily and reliably synthesized. Methods for stereoretentive preparation of other alkenyl bromide regioisomers and α,β-unsaturated carboxylic and thiol esters, nitriles, and acid fluorides followed, along with stereoretentive ring-closing metathesis reactions that afford macrocyclic trisubstituted olefins. Z- and E-Macrocyclic trisubstituted olefins, including those that contain little or no entropic support for cyclization (minimally functionalized) and/or are disfavored under substrate-controlled conditions, can now be synthesized. The utility of this latest chapter in the history of olefin metathesis has been highlighted by applications to the synthesis of several biologically active compounds, as well as their analogues, such as those marked by one or more site-specifically incorporated fluorine atoms or more active but higher energy and otherwise unobtainable conformers.The investigations discussed here, which represent every stereoretentive method that has been reported thus far for preparing a trisubstituted olefin, underscore the inimitable power of Mo-based catalysts. This Account also showcases a variety of mechanistic attributes─some for the first time, and each instrumental in solving a problem. Extensive knowledge of mechanistic nuances will be needed if we are to address successfully the next challenging problem, namely, the development of catalysts and strategies that may be used to synthesize a wide range of tetrasubstituted alkenes, especially those that are readily modifiable, with high stereoisomeric purity.

E- and Z-trisubstituted macrocyclic alkenes for natural product synthesis and skeletal editing

Many therapeutic agents are macrocyclic trisubstituted alkenes but preparation of these structures is typically inefficient and non-selective. A possible solution would entail catalytic macrocyclic ring-closing metathesis, but these transformations require high catalyst loading, conformationally rigid precursors and are often low yielding and/or non-stereoselective. Here we introduce a ring-closing metathesis strategy for synthesis of trisubstituted macrocyclic olefins in either stereoisomeric form, regardless of the level of entropic assistance. The goal was achieved by addressing several unexpected difficulties, including complications arising from pre-ring-closing metathesis alkene isomerization. The power of the method is highlighted by two examples. The first is the near-complete reversal of substrate-controlled selectivity in the formation of a macrolactam related to an antifungal natural product. The other is a late-stage stereoselective generation of an E-trisubstituted alkene in a 24-membered ring, en route to the cytotoxic natural product dolabelide C.

Stereodefined alkenes with a fluoro-chloro terminus as a uniquely enabling compound class

Trisubstituted alkenyl fluorides are important compounds for drug discovery, agrochemical development and materials science. Despite notable progress, however, many stereochemically defined trisubstituted fluoroalkenes either cannot be prepared efficiently or can only be accessed in one isomeric form. Here we outline a general solution to this problem by first unveiling a practical, widely applicable and catalytic strategy for stereodivergent synthesis of olefins bearing a fluoro-chloro terminus. This has been accomplished by cross-metathesis between two trisubstituted olefins, one of which is a purchasable but scarcely utilized trihaloalkene. Subsequent cross-coupling can then be used to generate an assortment of trisubstituted alkenyl fluorides. The importance of the advance is highlighted by syntheses of, among others, a fluoronematic liquid-crystal component, peptide analogues bearing an E- or a Z-amide bond mimic, and all four stereoisomers of difluororumenic ester (an anti-cancer compound).

Dehydrogenative alcohol coupling and one-pot cross metathesis/dehydrogenative coupling reactions of alcohols using Hoveyda–Grubbs catalysts

Grubbs catalysts are shown to catalyze one-pot cross-metathesis/dehydrogenative alcohol coupling reactions in an efficient manner.

Cross-Metathesis of Methallyl Halides: Concise Enantioselective Formal Total Synthesis of (-)-Presphaerene

The cross-metathesis (CM) of methallyl halides catalyzed using four different ruthenium-based complexes-Grubbs catalyst, Grubbs second-generation catalyst, Hoveyda-Grubbs second-generation catalyst, and Stewart-Grubbs catalyst-was investigated. When methallyl chloride or bromide was reacted with a model substrate containing a benzyl ether group, the Grubbs catalyst, and Grubbs second-generation catalyst did not promote the reaction well. However, the Hoveyda-Grubbs second-generation catalyst and Stewart-Grubbs catalyst afforded the corresponding products in moderate to good yield with moderate E/Z selectivity. Accordingly, several functionalized methallyl halides were prepared by CM. Various functional groups were well-tolerated in this system when the Stewart-Grubbs catalyst was used. To demonstrate the practical utility of our method, methallyl halide CM was successfully employed for the formal total synthesis of a natural product (-)-presphaerene, in which the precursor of the key cyclopentanecarboxylate intermediate was efficiently prepared in three steps.

A Relay Strategy Actuates Pre-Existing Trisubstituted Olefins in Monoterpenoids for Cross-Metathesis with Trisubstituted Alkenes

A retrosynthetic disconnection-reconnection analysis of epoxypolyenes-substrates that can undergo cyclization to podocarpane-type tricycles-reveals relay-actuated Δ^6,7-functionalized monoterpenoid alcohols for ruthenium benzylidene catalyzed olefin cross-metathesis with homoprenyl benzenes. Successful implementation of this approach provided several epoxypolyenes as expected (E/Z, ca. 2-3:1). The method is further generalized for the cross-metathesis of pre-existing trisubstituted olefins in other relay-actuated Δ^6,7-functionalized monoterpenoid alcohols with various other trisubstituted alkenes to form new trisubstituted olefins. Epoxypolyene cyclization of an enantiomerically pure, but geometrically impure, epoxypolyene substrate provides an enantiomerically pure, trans-fused, podocarpane-type tricycle (from the E-geometrical isomer).

Synthesis of Stereodefined Trisubstituted Alkenyl Silanes Enabled by Borane Catalysis and Nickel Catalysis

Regioselective and stereoselective synthesis of trisubstituted alkenyl silanes via hydrosilylation is challenging. Herein, we report the first β-anti-selective addition of silanes to thioalkynes with B(C₆F₅)₃ as the catalyst. The reaction shows broad substrate scope. The products were proven to be useful intermediates to other trisubstituted alkenyl silanes by Ni-catalyzed stereoretentive cross-coupling reactions of the C-S bond. A mechanism study suggests that nucleophilic attack of thioalkyne to an activated silylium intermediate might be the rate-determining step.

Traceless Protection for More Broadly Applicable Olefin Metathesis

An operationally simple in situ protection/deprotection strategy that significantly expands the scope of kinetically controlled catalytic Z- and E-selective olefin metathesis is introduced. Prior to the addition of a sensitive Mo- or Ru-based complex, treatment of a hydroxy- or a carboxylic-acid-containing olefin with commercially available HB(pin) or readily accessible HB(trip)2 (pin=pinacolato, trip=2,4,6-tri(isopropyl)phenyl) for 15 min is sufficient for efficient generation of a desired product. Routine workup leads to quantitative deprotection. A range of stereochemically defined Z- and E-alkenyl chlorides, bromides, fluorides, and boronates or Z-trifluoromethyl-substituted alkenes with a hydroxy or carboxylic acid group were thus prepared in 51-97 % yield with 93 to >98 % stereoselectivity. We also show that, regardless of whether a polar functional unit is present or not, a small amount of HB(pin) may be used to remove residual water, significantly enhancing efficiency.

Delayed catalyst function enables direct enantioselective conversion of nitriles to NH2-amines

Accessing enantiomerically enriched amines often demands oxidation-state adjustments, protection and deprotection processes, and purification procedures that increase cost and waste, limiting applicability. When diastereomers can be formed, one isomer is attainable. Here, we show that nitriles, largely viewed as insufficiently reactive, can be transformed directly to multifunctional unprotected homoallylic amines by enantioselective addition of a carbon-based nucleophile and diastereodivergent reduction of the resulting ketimine. Successful implementation requires that competing copper-based catalysts be present simultaneously and that the slower-forming and less reactive one engages first. This challenge was addressed by incorporation of a nonproductive side cycle, fueled selectively by inexpensive additives, to delay the function of the more active catalyst. The utility of this approach is highlighted by its application to the efficient preparation of the anticancer agent (+)-tangutorine.

A tutorial review of stereoretentive olefin metathesis based on ruthenium dithiolate catalysts

Stereoretentive olefin metathesis based on ruthenium dithiolate complexes has become a very active field of research within the past years. This unique catalyst class is able to kinetically produce both Z- and E-alkenes in high stereochemical purity (typically >95:5) starting from stereochemically pure Z- or E-alkenes. The aim of this tutorial review is to organize the reported information concerning ruthenium dithiolate catalysts in a logic manner, thus providing an "operators handbook" for chemists who wish to apply this methodology in synthesis.