Cyclization by Catalytic Ruthenium Carbene Insertion into C sp 3H Bonds

Hydrogenative Cycloisomerization and Sigmatropic Rearrangement Reactions of Cationic Ruthenium Carbenes Formed by Catalytic Alkyne gem-Hydrogenation

gem‐Hydrogenation of propargyl alcohol derivatives with [Cp^XRu(MeCN)₃]PF₆ (Cp^X=substituted cyclopentadienyl) as catalysts affords cationic pianostool ruthenium carbene complexes which are so electrophilic that they attack a tethered olefin to furnish cyclopentene products; cyclopropanation or metathesis do not compete with this novel transformation. If the transient carbenes carry appropriate propargylic substituents, however, they engage in ([2,3]‐sigmatropic) rearrangements to give enol esters (carbonates, carbamates, sulfonates) or alkenyl halides. Both pathways are unprecedented in the vast hydrogenation literature. The proposed mechanistic scenarios are in line with labeling experiments and spectroscopic data; most notably, PHIP NMR spectroscopy (PHIP=parahydrogen induced polarization) provides compelling evidence that the reactions are indeed triggered by highly unorthodox gem‐hydrogenation events.

Cp*RuCl-Vinyl Carbenes: Two Faces and the Bifunctional Role in Catalytic Processes

Ruthenium vinyl carbenes derived from Cp/Cp*RuCl-based complexes (Cp=cyclopentadiene, Cp*=1,2,3,4,5-pentamethylcyclopentadiene) have been routinely invoked as key intermediates in tandem reactions involving a carbene/alkyne metathesis (CAM). A priori, these intermediates resemble the Grubbs-type family of catalysts, but they exhibit a completely different reactivity pattern that few, if any, other catalytic system can reproduce so far. The reactivity of these species with α-unsubstituted and α-substituted alkynals showcases the peculiarities of these intermediates. Although Z-vinyl dihydrooxazines are preferentially obtained with the former, Z-vinyl epoxypyrrolidines are obtained with the latter. A combination of spectroscopic and computational data now prove that a η³ -coordination mode of the ruthenium vinyl carbene and the presence of a Lewis basic chloride ligand give rise to two markedly different stereoelectronic faces, which are responsible for the unconventional reactivity of these species.

A Divergent Synthetic Route to the Vallesamidine and Schizozygine Alkaloids: Total Synthesis of (+)-Vallesamidine and (+)-14,15-Dehydrostrempeliopine

The total synthesis of representative members of the schizozygine alkaloids, (+)-vallesamidine and (+)-14,15-dehydrostrempeliopine, were completed from a late-stage divergent intermediate. The synthesis took advantage of efficient nitro-group reactions with the A/B/C ring skeleton constructed concisely on a gram scale through an asymmetric Michael addition, nitro-Mannich/lactamisation, Tsuji-Trost allylation, and intramolecular C-N coupling reaction. Other key features of the synthesis are a novel [1,4] hydride transfer/Mannich-type cyclisation to build ring E and a diastereoselective ring-closing metathesis reaction to construct ring D. This approach gave access to a late-stage C14,C15 alkene divergent intermediate that could be simply transformed into (+)-vallesamidine, (+)-14,15-dehydrostrempeliopine, and potentially other schizozygine alkaloids and unnatural derivatives.

Alkyl Ethers as Traceless Hydride Donors in Brønsted Acid Catalyzed Intramolecular Hydrogen Atom Transfer

A new protocol for the deoxygenation of alcohols and the hydrogenation of alkenes under Brønsted acid catalysis has been developed. The method is based on the use of either a benzyl or isopropyl ether as a traceless hydrogen-atom donor, and involves an intramolecular hydride transfer as a key step, which is achieved in a regio- and stereoselective manner.

Nucleophilic addition of amines to ruthenium carbenes: ortho-(alkynyloxy)benzylamine cyclizations towards 1,3-benzoxazines

A new ruthenium-catalyzed cyclization of ortho-(alkynyloxy)benzylamines to dihydro-1,3-benzoxazines is reported. The cyclization is thought to take place via the vinyl ruthenium carbene intermediates which are easily formed from [Cp*RuCl(cod)] and N2 CHSiMe3 . The mild reaction conditions and the efficiency of the procedure allow the easy preparation of a broad range of new 2-vinyl-2-substituted 1,3-benzoxazine derivatives. Rearrangement of an internal C(sp) in the starting material into a tetrasubstituted C(sp(3) ) atom in the final 1,3-benzoxazine is highly remarkable.