Carbon nucleophile addition to sp2-unsaturated Fischer carbene complexes

Fischer Carbene Complexes: A Glance at Two Decades of Research on Higher-Order Cycloaddition Reactions

The structure of Fischer carbene complexes (FCCs) is electron deficient. If bearing an α,β-unsaturated system, it can generate a wide variety of compounds by undergoing many different transformations, including higher-order cycloadditions. The latter are described as pericyclic reactions in which more than six electrons participate. These reactions have been employed in various areas of organic synthesis, resulting in highly selective compounds with a broad range of scaffolds. The first studies on higher-order cycloadditions involving FCCs frequently yielded competing byproducts. Many groups have attempted to increase selectivity by exploring distinct reaction conditions, reagents and co-catalysts (e. g., metal-mediated cycloadditions). The present review is the first to focus exclusively on using higher-order cycloadditions involving FCCs to synthesize carbocycles and heterocycles. Based on two decades of reports, an analysis is made of the main aspects of the mechanisms proposed for higher-order cycloadditions and the structural diversity obtained by the substituent effect.

Selective synthesis of trisubstituted pyrroles through the reactions of alkynyl Fischer carbene complexes with oxazolones

The unexpected reactivity of fluorinated oxazolones with alkynyl Fischer carbenes is reported. The loss of the –CF₃ and a cascade process involving nucleophilic additions were observed.

On the mechanism of imine elimination from Fischer tungsten carbene complexes

(Aminoferrocenyl)(ferrocenyl)carbene(pentacarbonyl)tungsten(0) (CO)5W=C(NHFc)Fc (W(CO) 5 ( E -2)) is synthesized by nucleophilic substitution of the ethoxy group of (CO)5W=C(OEt)Fc (M(CO) 5 (1 (Et) )) by ferrocenyl amide Fc-NH(-) (Fc = ferrocenyl). W(CO) 5 ( E -2) thermally and photochemically eliminates bulky E-1,2-diferrocenylimine ( E -3) via a formal 1,2-H shift from the N to the carbene C atom. Kinetic and mechanistic studies to the formation of imine E -3 are performed by NMR, IR and UV-vis spectroscopy and liquid injection field desorption ionization (LIFDI) mass spectrometry as well as by trapping experiments for low-coordinate tungsten complexes with triphenylphosphane. W(CO) 5 ( E -2) decays thermally in a first-order rate-law with a Gibbs free energy of activation of ΔG (‡) 298K = 112 kJ mol(-1). Three proposed mechanistic pathways are taken into account and supported by detailed (time-dependent) densitiy functional theory [(TD)-DFT] calculations. The preferred pathway is initiated by an irreversible CO dissociation, followed by an oxidative addition/pseudorotation/reductive elimination pathway with short-lived, elusive seven-coordinate hydrido tungsten(II) intermediates cis (N,H)-W(CO) 4 (H)( Z -15) and cis (C,H)-W(CO) 4 (H)( Z -15).

Mechanism of the aminolysis of Fischer alkoxy and thiocarbene complexes: a DFT study

B3LYP calculations have been carried out to study the reaction mechanism of the aminolysis of Fischer carbene complexes of the type (CO)(5)Cr=C(XMe)R (X = O and S; R = Me and Ph). We have explored different possible reaction mechanisms either through neutral or zwitterionic intermediates as well as a general base catalysis assisted by an ammonia molecule. Our results show that the most favorable pathway for the aminolysis of Fischer carbene complexes is through a stepwise reaction via a zwitterionic intermediate generated by the initial nucleophilic attack. We have found that the ammonia-catalyzed mechanism entails a significantly lower barrier for the rate-determining step than the uncatalyzed one. At lower pressure gas-phase conditions, the rate-determining step corresponds to the concerted proton transfer and MeXH elimination. Thiocarbene complexes show a higher energy barrier for this rate-determining step due to the lower basicity of the MeS(-) substituent. At higher pressure or in solution, the rate-determining step corresponds to the initial nucleophilic attack. Our results indicate that the transition state of the nucleophilic attack is more advanced and has a higher barrier for alkoxycarbene than thiocarbene complexes due to the stronger pi-donor character of the alkoxy group that reduces the electrophilicity of the attacked carbene atom making the nucleophilic attack more difficult.

Diastereoselective Synthesis of Three-, Five-, Six-, and Seven-Membered Rings from Fischer Carbene Complexes and 4-Unsubstituted 1-Amino-1,3-Dienes

Fischer carbene complexes react with 4-unsubstituted 1-amino-1,3-dienes to give different carbocyclization products depending on the nature of the carbene complex and on the substitution pattern of the aminodiene. Thus, the reaction of arylcarbene chromium complexes and 1-aminodienes diastereoselectively affords cyclopropane derivatives by means of a formal [2+1] carbocyclization reaction. In particular, pentacarbonyl[(2-furyl)(methoxy)methylene]chromium complex furnishes formal [4+1] carbocyclization products. Starting from beta-substituted alkenylcarbene complexes, formal [4+1] reactions occur and cyclopentenamine derivatives are diastereoselectively formed. However, when the alpha,beta-disubstituted alkenylcarbene complex pentacarbonyl[(5,6-dihydro-2H-pyran-2-yl)(methoxy)methylene]tungsten is used, the outcome of the reaction depends on the substitution on the carbon atom at the 3-position of the aminodiene, generating the [3+2] or [4+3]-cyclization products if the substituent is or is not a hydrogen atom, respectively. Finally, when the reaction is performed with alkynylcarbene complexes, benzaldehyde derivatives are obtained if the triple-bond substituent is a phenyl group or indene derivatives if the group is an alkenyl moiety.

Diastereoselective multicomponent cyclizations of Fischer carbene complexes, lithium enolates, and allylmagnesium bromide leading to highly substituted five- and six-membered carbocycles

The one-pot sequential reaction of a chromium alkoxycarbene complex, a ketone or ester lithium enolate, and allylmagnesium bromide enabled the selective synthesis of novel diastereomerically pure pentasubstituted cyclopentanols or tetrasubstituted 1,4-cyclohexanediols, depending on the degree of substitution at the Cbeta position of the enolate anion. A few exceptions have been encountered in which tetrasubstituted cyclopentanols or pentasubstituted 1,4-cyclohexanediols were selectively formed. The use of 2-iodoethoxycarbene complexes gave access to 1,2,4-cyclohexanetriols. These multicomponent-coupling reactions involved the formation of lithium alkylpentacarbonylchromates as key intermediates, which further evolved through intramolecular processes, such as insertion of an alkene, CO insertion or addition to a carbonyl group, and, moreover, could be trapped in intermolecular reactions with different electrophiles and styrene. The substitution pattern of the alkylchromate carbon chain has been proposed to control the nature of the annulation process.

Synthesis of Isochromene Esters Utilizing 1,6-Addition of Nucleophiles to Benzopyranylidenetungsten(0) Complexes

[reaction: see text] A concise method for the preparation of isochromene carboxylates has been developed by the regioselective 1,6-addition of various nucleophiles such as Grignard reagents, alkoxide, and cyanide onto benzopyranylidenetungsten(0) complexes, followed by iodine oxidation of the addition intermediates.