Catalyst-directed diastereoselectivity in hydrogenative couplings of acetylene to alpha-chiral aldehydes: formal synthesis of all eight L-hexoses

Correction: Synthesis of vinyl-substituted alcohols using acetylene as a C2 building block

[This corrects the article DOI: 10.1039/D2SC06400F.].

Cu-catalyzed carboboration of acetylene with Michael acceptors

A copper-catalyzed three-component carboboration of acetylene with B2Pin2 and Michael acceptors is reported. In this reaction, a cheap and abundant C2 chemical feedstock, acetylene, was used as a starting material to afford cis-alkenyl boronates bearing a homoallylic carbonyl group. The reaction was robust and could be reliably performed on the molar scale. Furthermore, the resulting cis-alkenyl boronates could be converted to diverse functionalized molecules with ease.

Enantioselective Metal-Catalyzed Reductive Coupling of Alkynes with Carbonyl Compounds and Imines: Convergent Construction of Allylic Alcohols and Amines

The use of alkynes as vinylmetal pronucleophiles in intermolecular enantioselective metal-catalyzed carbonyl and imine reductive couplings to form allylic alcohols and amines is surveyed. Related hydrogen auto-transfer processes, wherein alcohols or amines serve dually as reductants and carbonyl or imine proelectrophiles, also are cataloged, as are applications in target-oriented synthesis. These processes represent an emerging alternative to the use of stoichiometric vinylmetal reagents or Nozaki-Hiyama-Kishi (NHK) reactions in carbonyl and imine alkenylation.

Diverse synthesis of C2-linked functionalized molecules via molecular glue strategy with acetylene

As the simplest alkyne and an abundant chemical feedstock, acetylene is an ideal two-carbon building block. However, in contrast to substituted alkynes, catalytic methods to incorporate acetylene into fine chemicals are quite limited. Herein, we developed a photoredox-catalyzed synthetic protocol for diverse C2-linked molecules via a molecular glue strategy using gaseous acetylene under mild conditions. Initiated by addition of an acyl radical to acetylene, two cascade transformations follow. One involves a double addition for the formation of 1,4-diketones and the other where the intermediate vinyl ketone is intercepted by a radical formed from a heterocycle. In addition to making two new C-C bonds, two C-H bonds are also created in two mechanistically distinct ways: one via a C-H abstraction and the other via protonation. This system offers a reliable and safe way to incorporate gaseous acetylene into fine chemicals and expands the utility of acetylene in organic synthesis.

Although acetylene is an ideal two-carbon building block, very few catalytic methods can be applied to incorporate acetylene into fine chemicals. Here, the authors show photoredox-catalyzed syntheses of C2- linked molecules with gaseous acetylene under mild conditions.

Diastereoselective Alkene Hydroesterification Enabling the Synthesis of Chiral Fused Bicyclic Lactones

Palladium-catalysed diastereoselective hydroesterification of alkenes assisted by the coordinative hydroxyl group in the substrate afforded a variety of chiral γ-butyrolactones bearing two stereocenters. Employing the carbonylation-lactonization products as the key intermediates, the route from the alkenes with single chiral center to chiral THF-fused bicyclic γ-lactones containing three stereocenters was developed.

Conversion of Primary Alcohols and Butadiene to Branched Ketones via Merged Transfer Hydrogenative Carbonyl Addition-Redox Isomerization Catalyzed by Rhodium

The first examples of rhodium-catalyzed carbonyl addition via hydrogen autotransfer are described, as illustrated in tandem butadiene-mediated carbonyl addition-redox isomerizations that directly convert primary alcohols to isobutyl ketones. Related reductive coupling-redox isomerizations of aldehyde reactants mediated by sodium formate also are reported. A double-labeling crossover experiment reveals that the rhodium alkoxide obtained upon carbonyl addition enacts redox isomerization without dissociation of rhodium at any intervening stage.

From Hydrogenation to Transfer Hydrogenation to Hydrogen Auto-Transfer in Enantioselective Metal-Catalyzed Carbonyl Reductive Coupling: Past, Present, and Future

Atom-efficient processes that occur via addition, redistribution or removal of hydrogen underlie many large volume industrial processes and pervade all segments of chemical industry. Although carbonyl addition is one of the oldest and most broadly utilized methods for C-C bond formation, the delivery of non-stabilized carbanions to carbonyl compounds has relied on premetalated reagents or metallic/organometallic reductants, which pose issues of safety and challenges vis-à-vis large volume implementation. Catalytic carbonyl reductive couplings promoted via hydrogenation, transfer hydrogenation and hydrogen auto-transfer allow abundant unsaturated hydrocarbons to serve as substitutes to organometallic reagents, enabling C-C bond formation in the absence of stoichiometric metals. This perspective (a) highlights past milestones in catalytic hydrogenation, hydrogen transfer and hydrogen auto-transfer, (b) summarizes current methods for catalytic enantioselective carbonyl reductive couplings, and (c) describes future opportunities based on the patterns of reactivity that animate transformations of this type.

Feedstock Reagents in Metal-Catalyzed Carbonyl Reductive Coupling: Minimizing Preactivation for Efficiency in Target-Oriented Synthesis

Use of abundant feedstock pronucleophiles in catalytic carbonyl reductive coupling enhances efficiency in target-oriented synthesis. For such reactions, equally inexpensive reductants are desired or, ideally, corresponding hydrogen autotransfer processes may be enacted wherein alcohols serve dually as reductant and carbonyl proelectrophile. As described in this Minireview, these concepts allow reactions that traditionally require preformed organometallic reagents to be conducted catalytically in a byproduct-free manner from inexpensive π-unsaturated precursors.

Correction: Metallacyclopentadienes: synthesis, structure and reactivity

Correction for 'Metallacyclopentadienes: synthesis, structure and reactivity' by Wangyang Ma et al., Chem. Soc. Rev., 2017, 46, 1160-1192.

Synthesis of the C(1)-C(13) Fragment of Leiodermatolide via Hydrogen-Mediated C-C Bond Formation

The C(1)-C(13) fragment of the antimitotic marine macrolide leiodermatolide is prepared in seven steps via hydrogenative and transfer-hydrogenative reductive C-C couplings. A hydrogen-mediated reductive coupling of acetylene with a Roche-type aldehyde is used to construct C(7)-C(13). A 2-propanol-mediated reductive coupling of allyl acetate with (E)-2-methylbut-2-enal at a low loading of iridium (1 mol %) is used to construct C(1)-C(6), which is converted to an allylsilane using Oestereich's copper-catalyzed allylic substitution of Si-Zn reagents. The union of the C(1)-C(6) and C(7)-C(13) fragments is achieved via stereoselective Sakurai allylation.

Achmatowicz Reaction and its Application in the Syntheses of Bioactive Molecules

Substituted pyranones and tetrahydropyrans are structural subunits of many bioactive natural products. Considerable efforts are devoted toward the chemical synthesis of these natural products due to their therapeutic potential as well as low natural abundance. These embedded pyranones and tetrahydropyran structural motifs have been the subject of synthetic interest over the years. While there are methods available for the syntheses of these subunits, there are issues related to regio and stereochemical outcomes, as well as versatility and compatibility of reaction conditions and functional group tolerance. The Achmatowicz reaction, an oxidative ring enlargement of furyl alcohol, was developed in the 1970s. The reaction provides a unique entry to a variety of pyranone derivatives from functionalized furanyl alcohols. These pyranones provide convenient access to substituted tetrahydropyran derivatives. This review outlines general approaches to the synthesis of tetrahydropyrans, covering general mechanistic aspects of the Achmatowicz reaction or rearrangement with an overview of the reagents utilized for the Achmatowicz reaction. The review then focuses on the synthesis of functionalized tetrahydropyrans and pyranones and their applications in the synthesis of natural products and medicinal agents.

Enantioselective Alcohol C-H Functionalization for Polyketide Construction: Unlocking Redox-Economy and Site-Selectivity for Ideal Chemical Synthesis

The development and application of stereoselective and site-selective catalytic methods that directly convert lower alcohols to higher alcohols are described. These processes merge the characteristics of transfer hydrogenation and carbonyl addition, exploiting alcohols and π-unsaturated reactants as redox pairs, which upon hydrogen transfer generate transient carbonyl-organometal pairs en route to products of C-C coupling. Unlike classical carbonyl additions, stoichiometric organometallic reagents and discrete alcohol-to-carbonyl redox reactions are not required. Additionally, due to a kinetic preference for primary alcohol dehydrogenation, the site-selective modification of glycols and higher polyols is possible, streamlining or eliminating use of protecting groups. The total syntheses of several iconic type I polyketide natural products were undertaken using these methods. In each case, the target compounds were prepared in significantly fewer steps than previously achieved.

Divergent de novo synthesis of all eight stereoisomers of 2,3,6-trideoxyhexopyranosides and their oligomers

All eight possible stereoisomers of 2,3,6-trideoxyhexopyranosides are prepared systematically from furan derivatives by a sequence of Achmatowicz rearrangement, Pd-catalysed glycosidation, and chiral catalyst-controlled tandem reductions. This sequence provides access to all possible stereoisomers of naturally occurring rhodinopyranosides, amicetopyranosides, disaccharide narbosine B, and other unnatural oligomeric 2,3,6-trideoxyhexopyranosides. It comprises a unique and systematic strategy for the de novo synthesis of deoxysugars.

A Metallacycle Fragmentation Strategy for Vinyl Transfer from Enol Carboxylates to Secondary Alcohol C-H Bonds via Osmium- or Ruthenium-Catalyzed Transfer Hydrogenation

A strategy for catalytic vinyl transfer from enol carboxylates to activated secondary alcohol C-H bonds is described. Using XPhos-modified ruthenium(0) or osmium(0) complexes, enol carboxylate-carbonyl oxidative coupling forms transient β-acyloxy-oxametallacycles, which eliminate carboxylate to deliver allylic ruthenium(II) or osmium(II) alkoxides. Reduction of the metal(II) salt via hydrogen transfer from the secondary alcohol reactant releases the product of carbinol C-H vinylation and regenerates ketone and zero-valent catalyst.

Paraformaldehyde and methanol as C1 feedstocks in metal-catalyzed C-C couplings of π-unsaturated reactants: beyond hydroformylation

Ruthenium-catalyzed reductive couplings of paraformaldehyde with dienes, alkynes, and allenes provide access to products of hydrohydroxymethylation that cannot be formed selectively under the conditions of hydroformylation. In certain cases, the regioselectivity of the CC coupling can be inverted by using nickel catalysts. With iridium catalysts, methanol engages in redox-neutral regioselective hydrohydroxymethylations.

Catalyst-directed diastereo- and site-selectivity in successive nucleophilic and electrophilic allylations of chiral 1,3-diols: protecting-group-free synthesis of substituted pyrans

The iridium-catalyzed, protecting group-free synthesis of 4-hydroxy-2,6-cis- or trans-pyrans through successive nucleophilic and electrophilic allylations of chiral 1,3-diols occurs with complete levels of catalyst-directed diastereoselectivity in the absence of protecting groups, premetallated reagents, or discrete alcohol-to-aldehyde redox reactions.

Total synthesis of (+)-trienomycins A and F via C-C bond-forming hydrogenation and transfer hydrogenation

The triene-containing C17-benzene ansamycins trienomycins A and F were prepared in 16 steps (longest linear sequence, LLS) and 28 total steps. The C11-C13 stereotriad was generated via enantioselective Ru-catalyzed alcohol CH syn crotylation followed by chelation-controlled carbonyl dienylation. Enantioselective Rh-catalyzed acetylene-aldehyde reductive coupling mediated by gaseous H2 was used to form a diene that ultimately was subjected to diene-diene ring closing metathesis to form the macrocycle. The present approach is 14 steps shorter (LLS) than the prior syntheses of trienomycins A and F, and 8 steps shorter than any prior synthesis of a triene-containing C17-benzene ansamycin.

Iridium-catalyzed allylation of chiral β-stereogenic alcohols: bypassing discrete formation of epimerizable aldehydes

The cyclometalated π-allyliridium 3,4-dinitro-C,O-benzoate complex modified by (R)- or (S)-Cl,MeO-BIPHEP promotes the transfer hydrogenative coupling of allyl acetate to β-stereogenic alcohols with good to excellent levels of catalyst-directed diastereoselectivity to furnish homoallylic alcohols. Remote electronic effects of the C,O-benzoate of the catalyst play a critical role in suppressing epimerization of the transient α-stereogenic aldehyde.

Recent advances in stereoselective synthesis of 1,3-dienes

The aim of this review is to present the latest developments in the stereoselective synthesis of conjugated dienes, covering the period 2005-2010. Since the use of this class of compounds is linked to the nature of their appendages (aryls, alkyls, electron-withdrawing, and heterosubstituted groups), the review has been categorized accordingly and illustrates the most representative strategies and mechanisms to access these targets.

Unlocking Hydrogenation for C-C Bond Formation: A Brief Overview of Enantioselective Methods

Hydrogenation of π-unsaturated reactants in the presence of carbonyl compounds or imines promotes reductive C-C coupling, providing a byproduct-free alternative to stoichiometric organometallic reagents in an ever-increasing range of C=X (X = O, NR) additions. Under transfer hydrogenation conditions, hydrogen exchange between alcohols and π-unsaturated reactants triggers generation of electrophile-nucleophile pairs, enabling carbonyl addition directly from the alcohol oxidation level, bypassing discrete alcohol oxidation and generation of stoichiometric byproducts.

Alkyne-aldehyde reductive C-C coupling through ruthenium-catalyzed transfer hydrogenation: direct regio- and stereoselective carbonyl vinylation to form trisubstituted allylic alcohols in the absence of premetallated reagents

Nonsymmetric 1,2-disubstituted alkynes engage in reductive coupling to a variety of aldehydes under the conditions of ruthenium-catalyzed transfer hydrogenation by employing formic acid as the terminal reductant and delivering the products of carbonyl vinylation with good to excellent levels of regioselectivity and with complete control of olefin stereochemistry. As revealed in an assessment of the ruthenium counterion, iodide plays an essential role in directing the regioselectivity of C-C bond formation. Isotopic labeling studies corroborate reversible catalytic propargyl C-H oxidative addition in advance of the C-C coupling, and demonstrate that the C-C coupling products do not experience reversible dehydrogenation by way of enone intermediates. This transfer hydrogenation protocol enables carbonyl vinylation in the absence of stoichiometric metallic reagents.

Total synthesis of bryostatin 7 via C-C bond-forming hydrogenation

The marine macrolide bryostatin 7 is prepared in 20 steps (longest linear sequence) and 36 total steps with five C-C bonds formed using hydrogenative methods. This approach represents the most concise synthesis of any bryostatin reported, to date.

Mechanism and origins of regio- and enantioselectivities in RhI-catalyzed hydrogenative couplings of 1,3-diynes and activated carbonyl partners: intervention of a cumulene intermediate

The mechanism of the rhodium-catalyzed reductive coupling of 1,3-diynes and vicinal dicarbonyl compounds employing H(2) as reductant was investigated by density functional theory. Oxidative coupling through 1,4-addition of the Rh(I)-bound dicarbonyl to the conjugated diyne via a seven-membered cyclic cumulene transition state leads to exclusive formation of linear adducts. Diyne 1,4-addition is much faster than the 1,2-addition to simple alkynes. The 1,2-dicarbonyl compound is bound to rhodium in a bidentate fashion during the oxidative coupling event. The chemo-, regio-, and enantioselectivities of this reaction were investigated and are attributed to this unique 1,4-addition pathway. The close proximity of the ligand and the alkyne substituent distal to the forming C-C bond controls the regio- and enantioselectivity: coupling occurs at the sterically more demanding alkyne terminus, which minimizes nonbonded interaction with the ligand. A stereochemical model is proposed that accounts for preferential formation of the (R)-configurated coupling product when (R)-biaryl phosphine ligands are used.

Synthesis of the cytotrienin A core via metal catalyzed C-C coupling

A synthetic approach to the C17-benzene ansamycins via metal catalyzed C-C coupling is described. Key bond formations include direct iridium catalyzed carbonyl crotylation from the alcohol oxidation level followed by chelation-controlled Sakurai-Seyferth dienylation to form the stereotriad, which is attached to the arene via Suzuki cross-coupling. The diene-containing carboxylic acid is prepared using rhodium catalyzed acetylene-aldehyde reductive C-C coupling mediated by gaseous hydrogen. Finally, ring-closing metathesis delivers the cytotrienin core.

Formation of C-C Bonds via Iridium-Catalyzed Hydrogenation and Transfer Hydrogenation

The formation of C-C bonds via catalytic hydrogenation and transfer hydrogenation enables carbonyl and imine addition in the absence of stoichiometric organometallic reagents. In this review, iridium-catalyzed C-C bond-forming hydrogenations and transfer hydrogenations are surveyed. These processes encompass selective, atom-economic methods for the vinylation and allylation of carbonyl compounds and imines. Notably, under transfer hydrogenation conditions, alcohol dehydrogenation drives reductive generation of organoiridium nucleophiles, enabling carbonyl addition from the aldehyde or alcohol oxidation level. In the latter case, hydrogen exchange between alcohols and π-unsaturated reactants generates electrophile-nucleophile pairs en route to products of hydro-hydroxyalkylation, representing a direct method for the functionalization of carbinol C-H bonds.

The regio- and stereochemical course of reductive cross-coupling reactions between 1,3-disubstituted allenes and vinylsilanes: Synthesis of (Z)-dienes

In investigations aimed at exploring the potential of disubstituted allenes in stereoselective synthesis, we report studies that explore the reductive cross-coupling reaction of vinylsilanes with a range of substituted allenes. Regiochemical control is attained by employing allenic alkoxides, where the proximal heteroatom dictates the site-selectivity in a process that proceeds by net formal metallo-[3,3] rearrangement (directed carbometalation/elimination). Stereoselectivity in these reactions is complex, with both the nature of allene substitution and relative stereochemistry of the substrate impacting the stereoselective generation of each alkene of a substituted 1,3-diene. 2009 Elsevier Ltd. All rights reserved.

ESI-MS, DFT, and synthetic studies on the H(2)-mediated coupling of acetylene: insertion of C=X bonds into rhodacyclopentadienes and Brønsted acid cocatalyzed hydrogenolysis of organorhodium intermediates

The catalytic mechanism of the hydrogen-mediated coupling of acetylene to carbonyl compounds and imines has been examined using three techniques: (a) ESI-MS and ESI-CAD-MS analyses, (b) computational modeling, and (c) experiments wherein putative reactive intermediates are diverted to alternate reaction products. ESI-MS analysis of reaction mixtures from the hydrogen-mediated reductive coupling of acetylene to alpha-ketoesters or N-benzenesulfonyl aldimines corroborate a catalytic mechanism involving C horizontal lineX (X = O, NSO(2)Ph) insertion into a cationic rhodacyclopentadiene obtained by way of acetylene oxidative dimerization with subsequent Brønsted acid cocatalyzed hydrogenolysis of the resulting oxa- or azarhodacycloheptadiene. Hydrogenation of 1,6-diynes in the presence of alpha-ketoesters provides analogous coupling products. ESI mass spectrometric analysis again corroborates a catalytic mechanism involving carbonyl insertion into a cationic rhodacyclopentadiene. For all ESI-MS experiments, the structural assignments of ions are supported by multistage collisional activated dissociation (CAD) analyses. Further support for the proposed catalytic mechanism derives from experiments aimed at the interception of putative reactive intermediates and their diversion to alternate reaction products. For example, rhodium-catalyzed coupling of acetylene to an aldehyde in the absence of hydrogen or Brønsted acid cocatalyst provides the corresponding (Z)-butadienyl ketone, which arises from beta-hydride elimination of the proposed oxarhodacycloheptadiene intermediate, as corroborated by isotopic labeling. Additionally, the putative rhodacyclopentadiene intermediate obtained from the oxidative coupling of acetylene is diverted to the product of reductive [2 + 2 + 2] cycloaddition when N-p-toluenesulfonyl-dehydroalanine ethyl ester is used as the coupling partner. The mechanism of this transformation also is corroborated by isotopic labeling. Computer model studies based on density functional theory (DFT) support the proposed mechanism and identify Brønsted acid cocatalyst assisted hydrogenolysis to be the most difficult step. The collective studies provide new insight into the reactivity of cationic rhodacyclopentadienes, which should facilitate the design of related rhodium-catalyzed C-C couplings.

Enantioselective iridium-catalyzed carbonyl allylation from the alcohol oxidation level via transfer hydrogenation: minimizing pre-activation for synthetic efficiency

Existing methods for enantioselective carbonyl allylation, crotylation and tert-prenylation require stoichiometric generation of pre-metallated nucleophiles, and often employ stoichiometric chiral modifiers. Under the conditions of transfer hydrogenation employing an ortho-cyclometallated iridium C,O-benzoate catalyst, enantioselective carbonyl allylations, crotylations and tert-prenylations are achieved in the absence of stoichiometric metallic reagents or stoichiometric chiral modifiers. Moreover, under transfer hydrogenation conditions, primary alcohols function dually as hydrogen donors and aldehyde precursors, enabling enantioselective carbonyl addition directly from the alcohol oxidation level.