Fischer Alkoxy Carbene Complexes Research Articles

ChemInform Abstract: Recent Advances in Selective Organic Synthesis Mediated by Transition Metal Complexes

This short review covers some useful synthetic transformations mediated by group 6 Fischer alkoxy carbene complexes. Alkenyl carbene complexes derived from (±)-8-phenylmenthol are suitable Michael acceptors towards organolithium reagents -alkyllithium and lithium enolate derivatives-yielding ultimately b-alkyl substituted aldehydes with 80±98% enantiomeric excess. The reaction of (menthyloxy)aryl carbene complexes with sBuLi and tBuLi resulted in clean dearomatization leading to 1,4-cyclohexadiene derivatives. If alkynyl lithium, instead of alkyllithium, compounds are reacted with aryl carbene complexes different products -furans, propargyl ethers, alkoxyindenes are selectively produced. Enamines derived from (S)-methoxymethylpyrrolidine and both aldehydes and ketones react with alkenyl carbenes to afford with very high selectivity [3‡2] cycloadducts which are elaborated into useful, enantiopure 3,4-disubstituted cyclopentanones and 2,3-disubstituted 2-cyclopentenones, respectively. The [4‡3] carbocyclization occurs from alkenyl carbene complexes and 2-aminobutadiene derivatives, while the analogous heterocyclization can be accomplished from various types of azabutadienes and both alkenyl and alkynyl derivatives. Ef®cient structural modi®cations of terpenes can be done using boroxycarbene complexes via C±H insertion reactions. Finally, representative examples of cyclopropanation of electronically neutral alkenes with Fischer carbene complexes are provided.

Oxidative Demetalation of Fischer Alkoxy Carbene Complexes with Stoichiometric Pyridine N‐Oxide and NaBH4‐Promoted Demetalation of Fischer Iminocarbene Complexes with Sulfur and Selenium.

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Oxidative demetalation of Fischer alkoxy carbene complexes with stoichiometric pyridine N-oxide and NaBH 4-promoted demetalation of Fischer iminocarbene complexes with sulfur and selenium

Oxidation of Fischer alkoxy carbene complexes were systematically investigated with stoichiometric pyridine N-oxide (PNO) under mild conditions, forming ester products in good to excellent yields from the corresponding monocarbene complexes. Fischer alkoxy biscarbene complexes efficiently underwent stepwise oxidative demetalation under controlled conditions, resulting in ester-monocarbene and diester products, respectively. This oxidation protocol has demonstrated a generally efficient method to oxidize Fischer alkoxy carbene complexes under mild conditions, providing a new route to novel monocarbene complexes from Fischer biscarbene complexes. In the presence of NaBH 4, reactions of Fischer iminocarbene complexes with elemental sulfur or selenium in ethanol at ambient temperature regioselectively afforded thione or selone complexes by insertion of a sulfur or selenium atom into the M C bonds in Fischer carbene complexes, and metal-free selone was also obtained. The molecular structures of the iminocarbene complexes and selone derivatives were confirmed by X-ray crystallographic study. The NaBH 4-promoted demetalation protocol suggests a potential new route to demetalate Fischer aminocarbene complexes.

Fluoride‐Promoted Oxidation of Fischer Alkoxy Carbene Complexes: Stoichiometric and Catalytic Conditions.

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Fluoride-Promoted Oxidation of Fischer Alkoxy Carbene Complexes: Stoichiometric and Catalytic Conditions

The utility of fluoride anion as promoter of the oxidation of Fischer carbene complexes is presented. Two different and complementary methods that allow the fast and convenient preparation of carbene-derived esters in good yields have been developed using stoichiometric or catalytic quantities of fluoride ion.

Synthesis of new chiral alkenyl Fischer carbene complexes

New chiral group 6 Fischer alkoxycarbene complexes 1 and 2 have been synthesized by condensation of enantiopure aldehydes 6 and 7, respectively, with methylcarbene complexes 5. The corresponding aminocarbene complexes 3 and 4 were accessible from 1 and 2 by methoxypyrrolidine exchange. The chromium carbene 1b underwent the benzannulation reaction with alkynes affording chiral substituted phenols 11.

Recent advances in selective organic synthesis mediated by transition metal complexes

This short review covers some useful synthetic transformations mediated by group 6 Fischer alkoxy carbene complexes. Alkenyl carbene complexes derived from (±)-8-phenylmenthol are suitable Michael acceptors towards organolithium reagents -alkyllithium and lithium enolate derivatives-yielding ultimately b-alkyl substituted aldehydes with 80±98% enantiomeric excess. The reaction of (menthyloxy)aryl carbene complexes with sBuLi and tBuLi resulted in clean dearomatization leading to 1,4-cyclohexadiene derivatives. If alkynyl lithium, instead of alkyllithium, compounds are reacted with aryl carbene complexes different products -furans, propargyl ethers, alkoxyindenes are selectively produced. Enamines derived from (S)-methoxymethylpyrrolidine and both aldehydes and ketones react with alkenyl carbenes to afford with very high selectivity [3‡2] cycloadducts which are elaborated into useful, enantiopure 3,4-disubstituted cyclopentanones and 2,3-disubstituted 2-cyclopentenones, respectively. The [4‡3] carbocyclization occurs from alkenyl carbene complexes and 2-aminobutadiene derivatives, while the analogous heterocyclization can be accomplished from various types of azabutadienes and both alkenyl and alkynyl derivatives. Ef®cient structural modi®cations of terpenes can be done using boroxycarbene complexes via C±H insertion reactions. Finally, representative examples of cyclopropanation of electronically neutral alkenes with Fischer carbene complexes are provided.