The first synthesis of cyclopropanone acetals from the reaction of Fischer carbene complexes with ketene acetals

The reaction of iso-propoxy stabilized Fischer carbene complexes with ketene acetals gives moderate to excellent yields of cyclopropanone acetals when carried out under a carbon monoxide atmosphere. This is in contrast to the known reaction of methoxy substituted complexes which give cyclic ortho esters under the same conditions. A mechanism is proposed which involves a branch point between the two products as the zwitterionic intermediate resulting from nucleophilic addition of the ketene acetal to the carbene carbon. A 1,3-migration of the methoxyl group to the cationic center leads to the ortho ester and a ring closure by backside attack leads to the cyclopropanone acetal. A double-labeling experiment shows that the 1,3-migration occurs by an intramolecular process that is proposed to involve a bridging oxonium ion. The effect of the isopropoxy group is thus interpreted to be to sterically hinder the formation of a bridged oxonium ion.     

Thermal and photochemical reactions of Fischer carbene complexes with trialkylsilyl-substituted alkynes

Thermal reaction of Fischer carbene complexes with triisopropylsilyl (TIPS) substituted alkynes in benzene afforded TIPS-substituted vinylketenes or 2-TIPS-substituted cyclobutenones as major products while photochemical reaction of Fischer carbene complexes with trimethylsilyl (TMS) substituted alkynes in acetonitrile afforded 3-TMS-substituted cyclobutenones.     

Unsaturated carbene and allenylidene ruthenium complexes from alkynes

The author's studies aimed at activation of terminal alkynes by metal complexes and reactivity patterns and selective preparations of unsaturated carbene, allenylidene, and cumulenylidene derivatives of (arene)ruthenium complexes are reviewed.The review is based on the lecture given at Workshop «Modern Problems of Organometallic Chemistry» (May 1994).Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 827–838, May, 1995.     

Electrochemical approach to Fischer carbene complexes

Although the chemistry of Fischer carbene complexes is recently a growing direction of research, a review summarizing their electrochemical investigations is still missing. In the present, text evaluation and comparison of experimental data allow insight on how the structural changes of Fischer carbene complexes influence their redox properties. It is evident that the respective reduction and oxidation potentials depend on multiple parameters. To shed light on this problem, the available data were systematized in several series, where effects of individual structural changes were evaluated as shifts of oxidation and reduction potentials with respect to a reference compound. The changes in electron delocalization caused by different substitutions were rationalized using quantum chemical calculations.     

Stereoselective [3+2+2] cycloaddition utilizing optically active binuclear Fischer carbene complexes with alkynes

The reaction of optically active α,β-unsaturated binuclear Fischer carbene complexes with alkynes gave planar chiral cycloheptatriene chromium complexes via [3+2+2] cycloaddition with high diastereoselectivity.     

Diastereoselective cyclopropanation of ketone enols with Fischer carbene complexes

Treatment of aryl/heteroaryl methoxycarbene complexes of chromium with -substituted ketone lithium enolates between -78 degrees C and room temperature resulted in the diastereoselective synthesis of 1,2,2,3-tetrasubstituted cyclopropanols. An exception has been observed in the reaction with cyclohexanone lithium enolate that yielded a bicyclic 2-buten-4-olide. 1,2-Dimethoxycyclopropanes and 1-methoxy-2-trimethylsilyloxycyclopropanes were isolated by quenching the reactions with MeOTf or Me3SiCl, respectively. This novel cyclopropanation process involves formation of lithium (3-oxoalkyl)pentacarbonylchromate intermediates which on warming undergo intramolecular addition to the carbonyl group. This cyclization is equivalent to the cyclopropanation in smooth conditions of electron-rich alkenes with Fischer carbene complexes.     

Synthesis of indenes from alkynes and phenyl amino chromium carbene complexes

Reactions of phenyl morpholino or pyrrolidino chromium carbene complexes with alkynes in DMF at 120–125°C resulted in exclusive formation of the indene derivatives.     

Hydrosilylation of alkynes catalyzed by ruthenium carbene complexes

Hydrosilylation of terminal alkynes with a variety of silanes catalyzed by Cl₂(PCy₃)₂RuCHPh (1) affords mainly the Z-isomer via trans addition in excellent yields. The presence of a hydroxyl group in close proximity to the triple bond was observed to exert a strong directing effect, resulting in the highly selective formation of the α-isomer. Intramolecular hydrosilylation of a homopropargylic silyl ether was demonstrated to give the cis addition product.     

Fischer type carbene ligands in dinuclear complexes

A large number of iron and ruthenium dinuclear complexes containing heteroatom substituted carbene ligands have been obtained by two different synthetic routes. The first method consists in reacting heteroatom substituted -carbyne cationic complexes with CN^– ion. The second involves the displacement of the SMe₂ molecule in the sulfonium [Fe₂{-C(CN)(SMe₂)}(-CO)(CO)₂CP₂]SO₃CF₃ by appropriate nucleophilesX ^– (X=OR, SR, NR₂, PR₂). Spectroscopic (IR, NMR) and structural investigations together with reactivity studies on these complexes have greatly contributed to better understanding the factors which favor bridging vs. terminal coordination of heteroatom substituted carbene ligands.     

Synthetic approach to substituted cyclopropanes based on the coupling reaction of lithiated chloroalkyloxazolines with Fischer carbene complexes

Regioselective addition of lithiated oxazoline 2a, easily available from 2-(1-chloroethyl)-4,4-dimethyl-2-oxazoline 1a (LDA, THF, -98 degrees C), to alpha,beta-unsaturated Fischer carbene complexes 3 afforded cyclopropylcarbene complexes 4 as sole diastereoisomers. Exposure of carbene complexes 4a-c (M = Cr) to air and sunlight gave cyclopropane carboxylate derivatives 5a-c. A plausible mechanistic explanation is proposed. Moreover, when lithiated oxazoline 2b was generated from 1b in the presence of the carbene complex 3a,b, the oxazolinylcyclopropane carboxylates 6a,b formed as a 1:1 mixture of diastereoisomers. Chiral lithiated oxazoline 2c added regioselectively and diastereoselectively to chromium complexes 3a,b and to tungsten complexes 3d,e, leading, after oxidation of the metal fragment, to esters 7a,b with good diastereoselectivity (dr = 4:1). The reaction of lithiated oxazoline 2d with chromium complex 3b and tungsten complex 3e proceeded less diastereoselectively, furnishing, in both cases, after oxidation, the ester 7c as a 3:2 diastereoselective mixture.     

Novel thermal reaction of Fischer carbene complexes with imines: synthesis of beta-methoxy allylic amine derivatives

[formula: see text] Alkyl-substituted chromium Fischer carbene complexes were found to react with various N-tosylimines to give beta-methoxy allylic amine derivatives under thermal conditions. This result is essentially distinct from the photochemical outcome in which beta-lactam derivatives are obtained.     

Synthesis of naphthalenes through three-component coupling of alkynes, Fischer carbene complexes, and benzaldehyde hydrazones via isoindole intermediates

The synthesis of naphthalene derivatives through three-component coupling of 2-alkynylbenzaldehyde hydrazones with carbene complexes and electron-deficient alkynes has been examined. The reaction involves formation of an isoindole derivative, followed by intramolecular Diels-Alder reaction, followed by nitrene extrusion. The reaction was highly regioselective using unsymmetrical alkynes.     

Synthesis of pyrroles through coupling of enyne-hydrazones with Fischer carbene complexes

[reaction: see text] The coupling of enyne-imines with Fischer carbene complexes leads to the formation of alkenylpyrrole derivatives. Maximum yields of pyrrole adducts were obtained using N,N-dimethylhydrazones. A mechanism involving alkyne insertion followed by nucleophilic attack of the imine nitrogen at the intermediate alkenylcarbene complex was proposed.     

Transmetalation reactions from Fischer carbene complexes to late transition metals: a DFT study

Transmetalation reactions from chromium(0) Fischer carbene complexes to late-transition-metal complexes (palladium(0), copper(I), and rhodium(I)) have been studied computationally by density functional theory. The computational data were compared with the available experimental data. In this study, the different reaction pathways involving the different metal atoms have been compared with each other in terms of their activation barriers and reaction energies. Although the reaction profiles for the transmetalation reactions to palladium and copper are quite similar, the computed energy values indicate that the process involving palladium as catalyst is more favorable than that involving copper. In contrast to these transformations, which occur via triangular heterobimetallic species, the transmetalation reaction to rhodium leads to a new heterobimetallic species in which a carbonyl ligand is also transferred from the Fischer carbene to the rhodium catalyst. Moreover, the structure and bonding situation of the so far elusive heterobimetallic complexes are briefly discussed.     

Rhodium(I) catalyzed four-component reaction of Fischer alkenyl carbene complexes and 1,1-diphenylallene

Alkyl substituted chromium Fischer carbene complexes react with 1,1-diphenylallene in the presence of rhodium(I) catalysts (10 mol%) to yield highly substituted dienyl indenone derivatives. In this process a catalytic chromium(0)-rhodium(I) exchange occurs, four new C-C bonds are created, and four-components (two allenes, the carbene ligand and one CO ligand) are joined in a chemo- and regioselective manner.     

Synthesis of nitrogen-containing compounds using chromium Fischer carbene complexes

Synthetic utility of the Fischer-type carbene complexes of chromium for the preparation of nitrogen-containing compounds is demonstrated. Alkoxy carbene complexes reacted with imines to give (optically active) 3-pyrrolines and beta-methoxy allylic amine derivatives in good yields. The amino carbene complexes reacted with alpha,beta-unsaturated aldehydes to give substituted pyrroles in good yields.     

Contingency and serendipity in the reactions of Fischer carbene complexes with conjugated triynes

The first examples of reactions of Fischer carbene complexes with triynes are reported. The regioselectivity of the reaction of the two different alkyne functions in the symmetrical triyne depends on the nature of the substituent of the triyne. Bis-silyl-substituted triynes react at the central alkyne unit, whereas bis-aryl- and bis-alkyl-substituted triynes react at the end alkyne unit. The reaction of a Fischer carbene complex with a phenyl substituent also reacts with a bis-silyl-substituted triyne at the central alkyne unit but gives a furan product rather than the normal phenol product. It was also demonstrated that all three of the alkyne units in conjugated triynes could react in turn with a Fischer carbene complex to give access to trisquinones.