Oligomerization is kinetically favored in RCM reactions catalyzed by RuCl2(PCy3)(IMes)(CHPh), for a range of unhindered α,ω-dienes leading to large or medium-sized rings, even at dilutions designed to minimize intermolecular reaction. Reversible metathesis (i.e., ethenolysis) is inhibited by rapid volatilization of ethylene. At appropriately high dilutions, however, the RCM products are efficiently liberated by backbiting.

Ruthenium alkylidene complexes containing one aryloxide "pseudohalide" ligand catalyze ring-closing metathesis of diene and ene-yne substrates with exceptionally high efficiency. Chromatographic removal of Ru residues is unexpectedly facile, offering a convenient means of isolating pure organic products in high yields.

The intramolecular α-arylation of aldehydes has been accomplished using singly occupied molecular orbital (SOMO) catalysis. Selective oxidation of chiral enamines (formed by the condensation of an aldehyde and a secondary amine catalyst) leads to the formation of a 3π-electron radical species. These chiral SOMO-activated radical cations undergo enantioselective reaction with an array of pendent electron-rich aromatics and heterocycles thus efficiently providing cyclic α-aryl aldehyde products (10 examples: ≥70% yield and ≥90% ee). In accordance with our radical mechanism, when there is a choice between arylation at the ortho or para position of anisole substrates, we find that arylation proceeds selectively at the ortho position.

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The combination of photoredox catalysis and enamine catalysis has enabled the development of an enantioselective α-cyanoalkylation of aldehydes. This synergistic catalysis protocol allows for the coupling of two highly versatile yet orthogonal functionalities, allowing rapid diversification of the oxonitrile products to a wide array of medicinally relevant derivatives and heterocycles. This methodology has also been applied to the total synthesis of the lignan natural product (−)-bursehernin.Keywords photoredox catalysis; organocatalysis; enantioselective; alkylation; aldehyde; nitrile; total synthesisThe enantioselective α-alkylation of carbonyl compounds with sp 3 -hybridized halidebearing electrophiles has long been considered an elusive goal for practitioners of asymmetric catalysis. [1] Indeed, the most commonly employed strategy to achieve the stereoselective construction of α-alkyl carbonyls involves the coupling of auxiliary-based metal enolates with halo or tosyloxy alkanes. [2], [3] A critical issue for the development of catalytic variants of this venerable reaction has been the insufficient electrophilicity of alkyl halides towards silyl or alkyl enol ether π-nucleophiles (enolate equivalents that are broadly employed in asymmetric catalysis). This limitation has man-dated the use of lithium-, sodium-, or cesium-derived enolates for auxiliary controlled carbonyl α-functionalization at higher carbonyl oxidation states. Recently, however, the application of secondary amine organocatalysts has overcome several of these constraints via the direct use of aldehydes or ketones in a variety of chiral enamine α-functionalization reactions. [4] As one example, our laboratory disclosed the synergistic merger of enamine catalysis with visible-light

The first highly enantioselective α-fluorination of ketones using organocatalysis has been accomplished. The long-standing problem of enantioselective ketone α-fluorination via enamine activation has been overcome via high-throughput evaluation of a new library of amine catalysts. The optimal system, a primary amine functionalized Cinchona alkaloid, allows the direct and asymmetric α-fluorination of a variety of carbo- and heterocyclic substrates. Furthermore, this protocol also provides diastereo-, regio- and chemoselective catalyst control in fluorinations involving complex carbonyl systems.

An efficient route towards biologically relevant pentose derivatives is described. The de novo synthetic strategy features an enantioselective α-oxidation reaction enabled by a chiral amine in conjunction with copper(II) catalysis. A subsequent Mukaiyama aldol coupling allows for the incorporation of a wide array of modular two-carbon fragments. Lactone intermediates accessed via this route provide a useful platform for elaboration, as demonstrated by the preparation of a variety of C-nucleosides and fluorinated pentoses. Finally, this work has facilitated expedient syntheses of pharmaceutically active compounds currently in clinical use.

Reaction of RuHCl(PPh3)3 4 with 3‐chloro‐3‐methyl‐1‐butyne effects transformation into RuCl2(PPh3)2(CHCHCMe2) 1c. Starting 4 is available commercially, or via quantitative reaction of RuCl2(PPh3)3 with one equivalent of alkali phenoxides or isopropoxides in refluxing benzene‐2‐propanol. Phosphane exchange between 1c and PCy3 or 1,3‐(CH2PCy2)2C6H4 is rapid at RT, affording RuCl2(PCy3)2(CHCHCMe2) 1b or the novel alkylidene complex RuCl2[1,3‐(CH2PCy2)2C6H4](CHCHCMe2) 7. Much slower exchange occurred on use of RuCl2(PCy3)2(CHPh) (1a) as precursor. Complex 1c is stable indefinitely (months) in the solid state at RT under N2, but dimerizes slowly in solution to give RuCl(PPh3)2(μ‐Cl)3Ru(PPh3)2(CHCHCMe2) 6a. 2,7‐Dimethyl‐octa‐2,4,6‐triene, the formal product of carbene coupling, is observed by 1H NMR. Dimerization does not compete with phosphane exchange. A side‐product arising from use of excess 3‐chloro‐3‐methyl‐1‐butyne in the synthesis of 1c was identified as Ru(IV) carbyne complex RuCl3(PPh3)2(≡CCHCMe2) 5, the structure of which was confirmed by X‐ray crystallography.