A novel stereocontrolled synthesis of 1,2-trans cyclopropyl ketones via suzuki-type coupling of acid chlorides with cyclopropylboronic acids

Metal-catalysed C-C bond formation at cyclopropanes

Cyclopropanes are important substructures in natural products and pharmaceuticals. Although traditional methods for their incorporation rely on cyclopropanation of an existing scaffold, the advent of transition-metal catalysis has enabled installation of functionalized cyclopropanes using cross-coupling reactions. The unique bonding and structural properties of cyclopropane render it more easily functionalized in transition-metal-catalysed cross-couplings than other C(sp³) substrates. The cyclopropane coupling partner can participate in polar cross-coupling reactions either as a nucleophile (organometallic reagents) or as an electrophile (cyclopropyl halides). More recently, single-electron transformations featuring cyclopropyl radicals have emerged. This Review will provide an overview of transition-metal-catalysed C-C bond formation reactions at cyclopropane, covering both traditional and current strategies, and the benefits and limitations of each.

Stereospecific Acylative Suzuki–Miyaura Cross-Coupling: General Access to Optically Active α-Aryl Carbonyl Compounds

A novel strategy for the stereospecific Pd-catalyzed acylative cross-coupling of enantiomerically enriched alkylboron compounds has been developed. The protocol features an extremely high level of enantiospecificity to allow facile access to synthetically challenging and valuable chiral ketones and carboxylic acid derivatives. The use of a sterically encumbered and electron-rich phosphine ligand proved to be crucial for the success of the reaction. Furthermore, on the basis of experimental and computational studies, a unique mechanism for the transmetalation, assisted by the noncovalent interactions of the C(sp³)-based organoboron reagent, has been identified.

Rhodium-catalysed diastereo- and enantio-selective cyclopropanation of α-boryl styrenes

A rhodium-catalyzed diastereo- and enantio-selective cyclopropanation of α-boryl styrenes with α-diazoarylacetates was established. Rh₂(S-PTTL)₄ (0.2 mol%) was found to be effective for the conversion, and 21 diastereopure cyclopropylboronates were prepared in high yields with excellent enantioselectivity (ee up to 99%).

Stereoselectivity in Pd-catalysed cross-coupling reactions of enantioenriched nucleophiles

Advances in Pd-catalysed cross-coupling reactions have facilitated the development of stereospecific variants enabling the use of configurationally stable, enantioenriched, main-group organometallic nucleophiles to form C(sp ³)-C(sp ²) bonds. Such stereospecific cross-coupling reactions constitute a powerful synthetic approach to attaining precise 3D control of molecular structure, allowing new stereogenic centres to be readily introduced into molecular architectures. Examples of stereospecific, Pd-catalysed cross-coupling reactions have been reported for isolable enantioenriched alkylboron, alkyltin, alkylgermanium and alkylsilicon nucleophiles. In these reactions, a single, dominant stereospecific pathway of transmetallation to palladium is required to effect efficient chirality transfer to the cross-coupled product. However, the potential for competing stereoretentive and stereoinvertive pathways of transmetallation complicates the stereochemical transfer in these processes and impedes the rational development of new stereospecific cross-coupling variants. In this Review, we describe the use of enantioenriched organometallic nucleophiles in stereospecific, Pd-catalysed cross-coupling reactions. We focus on systems involving well-defined, isolable, enantioenriched nucleophiles in which a clear stereochemical course of transmetallation is followed. Specific modes of electronic activation that influence the reactivity of alkylmetal nucleophiles are described and presented in the context of their impact on the stereochemical course of cross-coupling reactions. We expect that this Review will serve as a valuable resource to assist in deconvoluting the many considerations that potentially impact the stereochemical outcome of Pd-catalysed cross-coupling reactions.

Tandem Wittig Reaction-Ring Contraction of Cyclobutanes: A Route to Functionalized Cyclopropanecarbaldehydes

An original tandem reaction consisting of a Wittig reaction-ring contraction process between α-hydroxycyclobutanone and phosphonium ylides has been developed. Highly functionalized cyclopropanecarbaldehydes are obtained in good to high yield.

Visible Light Photoredox Cross-Coupling of Acyl Chlorides with Potassium Alkoxymethyltrifluoroborates: Synthesis of α-Alkoxyketones

A visible-light, single-electron-transfer (SET), photoredox cross-coupling for the synthesis of α-alkoxyketones has been developed. In this method, various aliphatic and aromatic acyl chlorides were successfully coupled with structurally diverse potassium alkoxymethyltrifluoroborates, producing the corresponding α-alkoxyketones with high yields. In this operationally simple and mild cross-coupling protocol, the desired ketones are obtained in one step from bench stable starting materials by a bond connection that is unique to both alkylboron chemistry and photoredox/Ni catalysis.

Copper-catalyzed regio- and enantioselective aminoboration of alkylidenecyclopropanes: the synthesis of cyclopropane-containing β-aminoalkylboranes

A Cu-catalyzed aminoboration of alkylidenecyclopropanes with B₂pin₂ and hydroxylamines produced cyclopropane-containing β-aminoalkylboranes in moderate to good yields.

Eco-friendly Suzuki–Miyaura coupling of arylboronic acids to aromatic ketones catalyzed by the oxime-palladacycle in biosolvent 2-MeTHF

Oxime-palladacycle catalyzed aerobic cross-coupling reaction of arylboronic acids and acyl chlorides to yield aryl ketones was developed. The reaction proceeds efficiently in 2-MeTHF, which is derived from corncobs, sugar cane, and fulfils the 3rd, 5th and 7th principles of Green Chemistry.

Weakly nucleophilic potassium aryltrifluoroborates in palladium-catalyzed Suzuki-Miyaura reactions: relative reactivity of K[4-RC6F4BF3] and the role of silver-assistance in acceleration of transmetallation

Small differences in the reactivity of weakly nucleophilic potassium aryltrifluoroborates are revealed in the silver-assisted Pd-catalyzed cross-coupling of K[4-RC₆F₄BF₃] (R = H, Bu, MeO, EtO, PrO, iPrO, BuO, t-BuO, CH₂=CHCH₂O, PhCH₂O, PhCH₂CH₂O, PhO, F, pyrazol-1-yl, pyrrol-1-yl, and indol-1-yl) with ArX (4-BrC₆H₄CH₃, 4-IC₆H₄F and 3-IC₆H₄F). An assumed role of silver(I) compounds Ag_mY (Y = O, NO₃, SO₄, BF₄, F) consists in polarization of the Pd–X bond in neutral complex ArPdL_nX with the generation of the related transition state or formation of [ArPdL_n][XAg_mY] with a highly electrophilic cation and subsequent transmetallation with the weakly nucleophilic borate. Efficiency of Ag_mY as a polarizing agent decreases in order Ag₂O > AgNO₃ ≈ Ag₂SO₄ > Ag[BF₄] > AgF. No clear correlation between the reactivity of K[4-RC₆F₄BF₃] and substituent electron parameters, σ_I and σ_R°, of the aryl group 4-RC₆F₄ was found.

Stereospecific couplings of secondary and tertiary boronic esters

This Minireview highlights advances in the Suzuki-Miyaura cross-coupling of secondary boron reagents for the creation of C-C bonds with control of stereochemistry. It also includes non-transition-metal coupling of secondary and tertiary boronic esters to electron-rich aromatics.

Enantioselective homocrotylboration of aliphatic aldehydes

A practical route to optically pure syn-homocrotylation reagents is described, including highly diastereo- and enantioselective preparation of numerous syn-homocrotyl products, as well as several matched mismatched pairs. NMR experiments suggest that the active homocrotylating species is a cyclopropylcarbinyldichloroborane generated by chloride exchange from the PhBCl(2) activator. Computational studies support the intermediacy of chloroboranes and suggest that homoallyl/homocrotyl transfers occur through Zimmerman-Traxler transition states.

New preparation and synthetic reactions of 3,3,3-trifluoropropynyllithium, -borate and -stannane: facile synthesis of trifluoromethylated allenes, arylacetylenes and enynes

Background: Since trifluoromethyl-containing compounds have found diverse applications in the fields of pharmaceuticals and agrochemicals, facile and selective synthetic methods for trifluoromethyl-substituted compounds are an essential tool for advanced medicinal chemistry. Now that diverse synthetic transformations of carbon–carbon triple bonds are available, 3,3,3-trifluoropropynyl-substituted compounds serve as versatile building blocks for target molecules containing trifluoromethyl groups. Thus, novel synthetic methods and reagents for incorporation of a 3,3,3-trifluoropropynyl group into an organic compound have been the major concern in exploration and modification of fluorine-based biologically active substances. Results & discussion: We report a preparative method of 3,3,3-trifluoropropynyllithium and its reaction with electrophiles including aldehydes, ketones, trimethoxyborane, chlorosilanes and chlorostannanes in detail. Palladium-catalyzed synthetic reactions of the carbonyl adducts and the trifluoromethyl-containing metalloid reagents are also demonstrated, which provide simple synthetic methods of trifluoromethylated allenes, arylacetylenes and enynes. The synthetic transformations presented here expand the repertoire of trifluoromethylated compounds available for medicinal scientists and contribute to the advance of future medicinal chemistry.

A general solution for unstable boronic acids: slow-release cross-coupling from air-stable MIDA boronates

Many boronic acids, including 2-heterocyclic, vinyl, and cyclopropyl derivatives, are inherently unstable, which can limit their benchtop storage and/or efficient cross-coupling. We herein report the first general solution to this problem: in situ slow release of unstable boronic acids from the corresponding air-stable MIDA boronates. This remarkably general approach has transformed all three classes of these unstable boronic acids into shelf-stable and highly effective building blocks for cross-coupling with a wide range of aryl and heteroaryl chlorides.

Enantioselective reactions of donor/acceptor carbenoids derived from alpha-aryl-alpha-diazoketones

The reaction of a variety of alpha-aryl-alpha-diazo ketones with activated olefins, catalyzed by the adamantyl glycine-derived dirhodium complex Rh(2)(S-PTAD)(4), generates cyclopropyl ketones with high diastereoselectivity (up to >95:5 dr) and enantioselectivity (up to 98% ee). Intermolecular C-H functionalization of 1,4-cyclohexadiene by means of carbenoid-induced C-H insertion was also possible with this type of carbenoid.

A concise and diversity-oriented approach to the synthesis of SAG derivatives

An efficient and rapid solution-phase combinatorial synthesis of the SAG library was developed. The salient features for this library synthesis is the application of carbothioamide-derived palladacycle-catalyzed Suzuki coupling reactions for the parallel synthesis of a series of pyridine-based biaryl aldehydes under aerobic conditions and a direct N-alkylation of carbamates using NaH as base in DMF in the presence of catalytic amount of water. The resultant library has been submitted to biological screening to evaluate their potential role in the regulation of Hedgehog pathway.

Acyl palladium species in synthesis: single-step synthesis of alpha,beta-unsaturated ketones from acid chlorides

Conditions are reported for the facile, one-pot synthesis of alpha,beta-unsaturated ketones via the palladium-catalysed cross-coupling of acyl chlorides with hydrozirconated acetylenes, and its use in the 2-step synthesis of D-5-O-benzyl deoxyxylulose.

A new ketone synthesis by palladium-catalyzed cross-coupling reactions of esters with organoboron compounds

[reaction: see text] The palladium-catalyzed coupling reaction of 2-pyridyl esters with organoboron compounds is described. The reaction is compatible with various functional groups and proceeds under mild reaction conditions. The coordination of the nitrogen atom to Pd is a key step for efficient reaction.

Preparation and Suzuki−Miyaura Coupling Reactions of Tetrahydropyridine-2-boronic Acid Pinacol Esters

[reaction: see text] A study on the conversion of lactam-derived vinyl triflates and phosphates into the corresponding vinyl boronates was carried out. While delta-valerolactam-derived vinyl triflates were successfully converted into 1,4,5,6-tetrahydropyridine-2-boronic acid pinacol ester derivatives by Pd-catalyzed coupling reaction with both bis(pinacolato)diboron and pinacolborane, pyrrolidinone and epsilon-caprolactam derivatives either did not react or were readily reduced. The delta-valerolactam-derived vinyl boronates are thermally stable compounds that efficiently coupled, under Pd catalysis, with structurally diverse aryl and heteroaryl bromides and triflates, vinyl iodides and bromides, and aromatic acid chlorides, to give the corresponding 2-substituted piperidines in good to excellent yields. The number of electrophiles that can virtually be coupled with these new boronic esters makes them very useful reagents for the synthesis of N-heterocyclic compounds.

A unique catalyst effects the rapid room-temperature cross-coupling of organozinc reagents with carboxylic acid fluorides, chlorides, anhydrides, and thioesters

We have identified a catalyst capable of effecting the rapid, room-temperature cross-coupling between acid fluorides and diorganozinc reagents. The use of acid fluorides as electrophilic partners allows this reaction to tolerate epimerizable functionality as well as beta leaving groups. The same catalyst is also capable of cross-coupling acid chlorides, acyl cyanides, anhydrides, thioesters, and pyridyl esters. Noteworthy is the ability of the reaction to successfully transfer both groups from the organometalic fragment.