William D. Wulff

Addition of carbon nucleophiles to arene-chromium complexes

Abstract The electrophilic reactivity of arenes coordinated to the chromium tricarbonyl unit has been developed into several distinct methods for coupling carbon nucleophiles with aromatic rings. Addition of the nucleophile produces stable η 5 -cyclohexadienyl chromium complexes which can be oxidized to induce loss of the endo hydrogen and the metal, overall nucleophilic substitution for hydrogen. Alternatively, the intermediate can be protonated and the resulting cyclohexa-1,3-diene can be detached from the chromium, effecting nucleophilic addition with reduction of one double bond. If a halogen (F, Cl) is present as a ring substituent, and if the nucleophile can migrate about the arene ligand, then loss of halide can occur parallel with classical nucleophilic aromatic substitution for halogen in electron-deficient haloarenes. With substituted arenes, the regioselectivity of addition becomes important and is often very high. Particularly useful are strong resonance donor substituents (RO-, R 2 N-, F-) where selectivity for meta attack is high. Indole provides an excellent example of selective activation, as the six-membered ring complexes selectively and is then susceptible to nucleophilic substitution, predominantly at the 4 and 7 positions. Substitution for halogen is a somewhat limited process and depends upon the nature of the nucleophile. Very reactive nucleophiles add to unsubstituted positions and are often slow to isomerize to the ipso position from which loss of halide can occur.

Cycloadditions and annulations of transition metal carbene complexes

Abstract The synthetic aspects of several reactions from the multifaceted chemistry of Fischer carbene complexes are examined. Their benzannulation reactions with acetylenes are utilized in the synthesis of anthracyclinones via two approaches which differ by beginning at opposite ends of the molecule with either an aryl or an alkenyl substituted chromium carbene complex. The latter has been employed in a formal synthesis of daunomycinone. The Diels-Alder reactions of α,β-acetylenic chromium carbene complexes provide for a facile entry into substituted cyclohexenyl chromium carbene complexes that are subsequently employed in benzannulation reactions. These tandem cycloaddition/annulation reactions are incorporated into model studies for the synthesis of anthracyclinones and wentilactone A. Their potential is also demonstrated for coupling to yet a third reaction of organochromium compounds ; aromatic nucleophilic substitutions on arene chromium tricarbonyl complexes. The annulations of β,β-disubstituted alkenyl complexes provides for a regio- and stereoselective synthesis of 2,4-cyclohexadienones under neutral conditions at near ambient temperatures.

Solvent, chelation and concentration effects on the benzannulation reaction of chromium carbene complexes and acetylenes

The reactions of a number of chromium carbene complexes (CO)5CrC(OMe)R (R = Ph, o-OMePh, p-OMePh, o-O-tBuPh, 1-C6H9, 1-C5H7O) were examined with a variety of acetylenes (R′CCR2, R1, R2 = H, Me, Et, n-Pr, Ph, SiMe3) in solvents ranging from low to high coordinating ability. The high selectivity for the benzannulated product from the reactions of simple α,β-unsaturated complexes is not affected by changes in solvent or substituents on the acetylene. The reactions of aryl complexes with acetylenes are quite sensitive to the nature of the solvent and the acetylene. The highest selectivities and efficiencies for the benzannulated product from the reactions of aryl complexes are with solvents of low coordinating ability. Solvents with intermediate coordinating ability and small size give high selectivity for cyclobutenone formation for reactions with disubstituted acetylenes. Solvents with high coordinating ability give the least selective reactions and a considerable amount of indene products. An o-methoxy group on the aryl substituent of the carbene complex can chelate to the metal center during the course of the benzannulation reaction and, in acetonitrile, alter the product distribution in favor of the cyclobutenone product. An amino substituted complex, (CO)5 CrC(NMe2)Ph, was found to react with diethylacetylene in THF to give indene products. The reactions of aryl complexes with alkynes can be accelerated by ultraviolet irradiation which results in high selectivity for the benzannulated product at as low a temperature as −78°C, and high selectivity for indene products for those complexes having a chelating group (methoxy) on the aryl ring. Finally, the product distribution from the reaction of the o-methoxyphenyl complex with diethylacetylene was found to be dependent on alkyne concentration, a phenomenon not previously observed for the reactions of carbene complexes and acetylenes.

Metal–Carbene Cycloadditions

The chemistry of transition metal carbene complexes has been examined with an eye to applications in organic synthesis ever since their discovery by Fischer in 1964, and the growth in the number of useful applications has been exponential with time.1–3 There are two types of transition metal carbene complexes; those which have electrophilic carbene carbons and which are typified by the pentacarbonylchromium complex (1), and those which have nucleophilic carbene carbons and which are typified by the biscyclopentadienyltitanium complex (2). Complexes (1) and (2) are often referred to as ‘carbene’ and ‘alkylidene’ complexes, respectively. This review will be limited to the chemistry of electrophilic carbene complexes of the Fischer type. The chemistry of the nucleophilic alkylidene complexes will be covered in Chapter 9.3, this volume.4

Catalytic Asymmetric Synthesis of Trisubstituted Aziridines

A method is described which provides for the direct asymmetric catalytic synthesis of trisubstituted aziridines from imines and diazo compounds. While unactivated imines were not reactive to α-diazo carbonyl compounds in which the diazo carbon was disubstituted, N-Boc imines react with both α-diazo esters and α-diazo-N-acyloxazolidinones to give trisubstituted aziridines with excellent diastereo- and enantioselectivities.

Controlled diastereo- and enantioselection in a catalytic asymmetric aziridination.

Chiral polyborate based Brønsted acids prepared from the VANOL and VAPOL ligands are known to catalyze the reaction of diarylmethyl imines with diazoesters to give cis-aziridines. In the present work, this same catalyst is shown to catalyze the reaction of the same imines with diazoacetamides to give trans-aziridines with the same high asymmetric inductions as seen with cis-aziridines, enabling the development of an unprecedented universal catalytic asymmetric aziridination protocol. The substrate scope is broad and includes imines prepared from both electron-rich and electron-poor aromatic aldehydes and also from 1°, 2°, and 3° aliphatic aldehydes. The face selectivity of the addition to the imine was found to be independent of the diazo compounds. The (S)-VANOL or (S)-VAPOL derived catalyst will cause both diazoesters and diazoacetamides to add to the Si-face of the imine when cis-aziridines are formed and both to add to the Re-face of the imine when trans-aziridines are formed.

Synthesis of naphthoquinone antibiotics by intramolecular alkyne cycloaddition to carbene−chromium complexes

Abstract The reaction of carbene-chromium complexes with alkynes provides a direct route to naphthoquinone derivatives and is the key step in a new approach to the isochromanone antibiotics exemplified by deoxyfrenolicin (1) and nanaomycin A (2). While the regioselectivity of intermolecular addition of the appropriate unsymmetrical disubstituted alkyne is unfavourable, two successful approaches have been developed. Allylacetylene reacts with methoxy-(o-methoxyphenyl)methylidene-Cr(CO)5 with high regioselectivity. Bromination, lithiation, and reaction with acetaldehyde produced the desired precursor. Alkoxypalladation led to pyran ring formation and introduction of the acetate side chain. Following earlier procedures, nanaomycin A (2) was produced. A more convergent alternative involved intramolecular cycloaddition of an alkyne with the alkylidene-chromium unit. A series of model cyclizations established the feasibility and an alkyne with an ethylene glycol side chain was prepared. The ethylene glycol unit serves as the tether to hold the alkyne in place for cyclization and allows easy removal at a later stage. Deoxyfrenolicin (1) was produced in a highly convergent and efficient process.

Simultaneous Synthesis of Both Rings of Chromenes via a Benzannulation/o-Quinone Methide Formation/Electrocyclization Cascade

A new route to the chromene ring system has been developed which involves the reaction of an α,β-unsaturated Fischer carbene complex of chromium with a propargyl ether bearing an alkenyl group on the propargylic carbon. This transformation involves a cascade of reactions that begins with a benzannulation reaction and is followed by the formation of an o-quinone methide, and finally results in the emergence of a chromene upon an electrocyclization. This reaction was extended to provide access by employing an aryl carbene complex. This constitutes the first synthesis of chromenes in which both rings of the chromene system are generated in a single step and is highlighted in the synthesis of lapachenole and vitamin E.

Catalytic asymmetric aziridination with borate catalysts derived from VANOL and VAPOL ligands: Scope and mechanistic studies

An extended study of the scope and mechanism of the catalytic asymmetric aziridination of imines with ethyl diazoacetate mediated by catalysts prepared from the VANOL and VAPOL ligands and triphenylborate is described. Nonlinear studies with scalemic VANOL and VAPOL reveal an essentially linear relationship between the optical purity of the ligand and the product suggesting that the catalyst incorporates a single molecule of the ligand. Two species are present in the catalyst prepared from B(OPh)(3) and either VANOL or VAPOL as revealed by (1)H NMR studies. Mass spectral analysis of the catalyst mixture suggests that one of the species involves one ligand molecule and one boron atom (B1) and the other involves one ligand and two boron atoms (B2). The latter can be formulated as either a linear or cyclic pyroborate and the (11)B NMR spectrum is most consistent with the linear pyroborate structure. Several new protocols for catalyst preparation are developed which allow for the generation of mixtures of the B1 and B2 catalysts in ratios that range from 10:1 to 1:20. Studies with catalysts enriched in the B1 and B2 species reveal that the B2 catalyst is the active catalyst in the VAPOL catalyzed asymmetric aziridination reaction giving significantly higher asymmetric inductions and rates than the B1 catalyst. The difference is not as pronounced in the VANOL series. A series of 12 different imines were surveyed with the optimal catalyst preparation procedure with the finding that the asymmetric inductions are in the low to mid 90s for aromatic imines and in the mid 80s to low 90s for aliphatic imines for both VANOL and VAPOL catalysts. Nonetheless, the crystallinity of the N-benzhydryl aziridines is such that nearly all of the 12 aziridine products screened can be brought to >99 % ee with a single recrystallization.

Direct catalytic asymmetric aminoallylation of aldehydes: Synergism of chiral and nonchiral Brønsted acids

The development of a catalytic asymmetric method for the direct aminoallylation of aldehydes is described that gives high asymmetric inductions for a broad range of substrates including both aromatic and aliphatic aldehydes. This method allows for direct isolation of unprotected analytically pure homoallylic amines without chromatography. The unique catalyst system developed for this process involves the synergistic interaction between a chiral and a nonchiral Brønsted acid.