Organozinc generation via the titanium-catalyzed activation of alkyl halides

Zinc-mediated carboxylations of allylic and propargylic halides in flow: synthesis of β-lactones via subsequent bromolactonization

Zinc-mediated carboxylation of allylic halides under flow conditions delivered β,γ-unsaturated carboxylic acids and subsequent bromolactonization provides a streamlined process for the synthesis of γ-bromo-β-lactones. The described process further demonstrates the utility of organozinc reagents prepared by passage of allylic halides through a metallic zinc column integrated into a flow process. Use of a tube-in-tube reactor for efficient CO₂ introduction led to improvements in conversion compared to a batch process and improved overall yields of β-lactones. The described flow process was also applied to propargylic bromides for the synthesis of allenic and propargylic acids.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Formyl MIDA Boronate: C1 Building Block Enables Straightforward Access to α-Functionalized Organoboron Derivatives

Formyl MIDA boronate has been known to be an elusive type of acylboronate that has not been obtained to date. In this work, an approach to the one-pot preparation and chemical transformations of formyl MIDA boronate were developed to provide new types of α-functionalized organoboron compounds. Among them are acylboronate reagents which present boron-substituted analogues of ynones and β-dicarbonyl compounds. The developed synthetic procedures, utilizing formyl MIDA boronate, are tolerant to diverse functional groups, making this reagent an advantageous C₁ building block for extending the scope of organoboron chemistry.

Trialkylborane-Mediated Multicomponent Reaction for the Diastereoselective Synthesis of Anti-δ,δ-Disubstituted Homoallylic Alcohols

The trialkylborane/O₂-mediated reaction of propargyl acetates having a tributylstannyl group at an alkyne terminus with aldehydes in a THF-H₂O solvent system gave anti-δ,δ-disubstituted homoallylic alcohols with good to high diastereoselectivity. Intriguingly, two alkyl groups derived from trialkylborane were embedded into the reaction product. The trialkylborane plays a key role not only as a radical initiator but also as a source of alkyl radicals.

Ti-Catalyzed Radical Alkylation of Secondary and Tertiary Alkyl Chlorides Using Michael Acceptors

Alkyl chlorides are common functional groups in synthetic organic chemistry. However, the engagement of unactivated alkyl chlorides, especially tertiary alkyl chlorides, in transition-metal-catalyzed C-C bond formation remains challenging. Herein, we describe the development of a TiIII-catalyzed radical addition of 2° and 3° alkyl chlorides to electron-deficient alkenes. Mechanistic data are consistent with inner-sphere activation of the C-Cl bond featuring TiIII-mediated Cl atom abstraction. Evidence suggests that the active TiIII catalyst is generated from the TiIV precursor in a Lewis-acid-assisted electron transfer process.

Iron Catalyzed Hydroboration of Aldehydes and Ketones

We report an operationally convenient room temperature hydroboration of aldehydes and ketones employing Fe(acac)₃ as precatalyst. The hydroboration of aldehydes and ketones proceeded efficiently at room temperature to yield, after work up, 1° and 2° alcohols; chemoselective hydroboration of aldehydes over ketones is attained under these conditions. We propose a σ-bond metathesis mechanism in which an Fe-H intermediate is postulated to be a key reactive species.

Navigating the Chiral Pool in the Total Synthesis of Complex Terpene Natural Products

The pool of abundant chiral terpene building blocks (i.e., "chiral pool terpenes") has long served as a starting point for the chemical synthesis of complex natural products, including many terpenes themselves. As inexpensive and versatile starting materials, such compounds continue to influence modern synthetic chemistry. This review highlights 21st century terpene total syntheses which themselves use small, terpene-derived materials as building blocks. An outlook to the future of research in this area is highlighted as well.

Highly α-regioselective neodymium-mediated allylation of diaryl ketones

The first utility of neodymium as a mediating-metal in the Barbier reaction of diaryl ketones with allyl halides is reported in this paper. This one-pot reaction was highly α-regioselective and was carried out under mild conditions.

Diastereo- and Enantioselective Iridium Catalyzed Carbonyl (α-Cyclopropyl)allylation via Transfer Hydrogenation

The first examples of diastereo- and enantioselective carbonyl α-(cyclopropyl)allylation are reported. Under the conditions of iridium catalyzed transfer hydrogenation using the chiral precatalyst (R)-Ir-I modified by SEGPHOS, carbonyl α-(cyclopropyl)allylation may be achieved with equal facility from alcohol or aldehyde oxidation levels. This methodology provides a conduit to hitherto inaccessible inaccessible enantiomerically enriched cyclopropane-containing architectures.

Titanium carbenoid-mediated cyclopropanation of allylic alcohols: selectivity and mechanism

A new method for the chemo- and stereoselective conversion of allylic alcohols into the corresponding cyclopropane derivatives has been developed. The cyclopropanation reaction was carried out with an unprecedented titanium carbenoid generated in situ from Nugent's reagent, manganese and methylene diiodide. The reaction involving the participation of an allylic hydroxyl group, proceeded with conservation of the alkene geometry and in a high diastereomeric excess. The scope, limitations and mechanism of this metal-catalysed reaction are discussed.

Design, synthesis, and evaluation of curcumin-derived arylheptanoids for glioblastoma and neuroblastoma cytotoxicity

Using an innovative approach toward multiple carbon-carbon bond-formations that relies on the multifaceted catalytic properties of titanocene complexes we constructed a series of C1-C7 analogs of curcumin for evaluation as brain and peripheral nervous system anti-cancer agents. C2-Arylated analogs proved efficacious against neuroblastoma (SK-N-SH & SK-N-FI) and glioblastoma multiforme (U87MG) cell lines. Similar inhibitory activity was also evident in p53 knockdown U87MG GBM cells. Furthermore, lead compounds showed limited growth inhibition in vitro against normal primary human CD34+hematopoietic progenitor cells. Taken together, the present findings indicate that these curcumin analogs are viable lead compounds for the development of new central and peripheral nervous system cancer chemotherapeutics with the potential for little effects on normal hematopoietic progenitor cells.

Zinc-mediated highly α-regioselective 1,4-addition of chalcones with prenyl bromide in THF

α-Regioselective addition to chalcones was realized by use of in situ generated prenylzinc reagents from zinc and prenyl bromide in the presence of SnCl4 in a 1,4-addition fashion. The approach uses the reagent combination of prenyl bromide, zinc, and SnCl4 in THF, all of which are inexpensive, readily available, and easily removable after the reaction.

Bifunctional titanocene catalysis in multicomponent couplings: a convergent assembly of β-alkynyl ketones

Herein is described a titanium-catalyzed three-component coupling to assemble β-alkynyl ketones in a single operation. Treatment of an aryl aldehyde with an acetylide and silyl enol ether in the presence of a bifunctional titanocene catalyst enables the highly convergent assembly of β-alkynyl ketones in good to excellent yields.

Cooperative titanocene and phosphine catalysis: accelerated C-X activation for the generation of reactive organometallics

The study presented herein describes a reductive transmetalation approach toward the generation of Grignard and organozinc reagents mediated by a titanocene catalyst. This method enables the metalation of functionalized substrates without loss of functional group compatibility. Allyl zinc reagents and allyl, vinyl, and alkyl Grignard reagents were generated in situ and used in the addition to carbonyl substrates to provide the corresponding carbinols in yields up to 99%. It was discovered that phosphine ligands effectively accelerate the reductive transmetalation event to enable the metalation of C-X bonds at temperatures as low as -40 °C. Performing the reactions in the presence of chiral diamines and amino alcohols led to the enantioselective allylation of aldehydes.

Titanocene-catalyzed multicomponent coupling approach to diarylethynyl methanes

A titanocene-catalyzed multicomponent coupling to provide diarylethynyl methanes is described. By combining the multifunctionality of Cp(2)TiCl(2) with the traceless dielectrophilicity of aryl aldehydes, all-carbon tertiary centers are obtained in 55-99% yield.

Aryl aldehydes as traceless dielectrophiles in bifunctional titanocene-catalyzed propargylic C-X activations

The titanocene-catalyzed construction of all-carbon substituted tertiary centers directly from aromatic aldehydes is described. The starting aldehyde behaves as a traceless functionality in the formation of multiple carbon-carbon bonds through consecutive carbon-heteroatom bond activations. The sequential addition of a metal acetylide and a second carbon nucleophile to the dielectrophilic aldehyde enables the construction of symmetrical and unsymmetrical 1,4-diynes in good yields.