Cyclopropanation, alkenes

Because in metathesis reactions with most catalyst systems a selectivity of nearly 100% is found, a carbene mechanism seems less likely. Banks and Bailey ( ) reported the formation of small quantities of C3-C6-alkenes, cyclopropane, and methylcyclopropane when ethene was passed over Mo(CO)6-A1203, which suggests reactions involving carbene complexes. However, similar results have not been reported elsewhere most probably the products found by Banks and Bailey were formed by side reactions, typical for their particular catalyst system. 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

The first examples of alkene cyclopropanation reactions with alkynylcarbene complexes were reported by Barluenga et al. in 2002 [15]. These intermolecular... 

To simplify the catalytic system further, Kodadek and Woo investigated the activity of [Fe(F2o-TPP)Cl] for alkene cyclopropanation with EDA in the absence of cobaltocene. These workers proposed that electron-deficient porphyrin would render the Fe(III) porphyrin more easily reduced by EDA. Indeed, [Fe(F2o-TPP)Cl] efficiently catalyzes alkene cyclopropanation with EDA with high catalyst turnover... 

Aryldiazomethane can also be used for iron porphyrin-catalyzed alkene cyclopropanation [55]. For example, the treatment of p-tolyldiazomethane with styrene in the presence of [Fe(TTP)] afforded the corresponding arylcyclopropapane in 79% yield with a high transicis ratio of 14 1 (eq. 1 in Scheme 11). Interestingly, when bulkier mesityldiazomethane was used as carbene source, ds-selectivity was observed (cisitrans = 2.0 1). Additionally, mesityldiazomethane was found to react with frans-p-styrene, the latter was found not to react with EDA or trimethyl-silyldiazomethane under the similar reaction conditions, to give l-mesityl-2-methyl-3-phenylcyclopropane in 35% yield. Trimethylsilyldiazomethane is also an active carbene source for [Fe(TTP)]-catalyzed cyclopropanation of styrene, affording l-phenyl-2-trimethylsilylcyclopropane in 89% yield with transicis ratio of 10 1 (eq. 2 in Scheme 11). 

The complex [Fe(D4-TmAP)Cl] with Halterman s porphyrin ligand can effect asymmetric alkene cyclopropanation with diazoacetate in high product yield and high stereoselectivity [57]. The reaction occurs smoothly at room temperature without the need for addition of CoCp2, affording the cyclopropyl esters... 

Fournier and Charette proposed a new gem-dizinc carbenoid, IZnCHIZnI 279, for alkene cyclopropanation.389 They reported that EtZnI reacted with CHC13 to form unstable 279, which was capable of reacting with the Unprotected allylic alcohols 280a-c. The final step of the reaction sequence was quenching the Zn-containing intermediate 281a-c with an electrophile (Scheme 147). 

The popularity of Cu(acac)2, where acac = acetylacetonato, as a precatalyst in alkene cyclopropanation using diazoesters has led to the investigation of chiral 1,3-dicarbonyls as a source of asymmetric induction in this process. Mathn et al. (26) report a selective cyclopropanation of styrene with a dimedone-derived diazocarbonyl in the presence of a camphor-derived diketone, Eq. 12. The reaction is con-... 

Non-heteroatom-substituted vinylcarbene complexes are readily available from alkynes and Fischer-type carbene complexes. These intermediates can undergo the inter- or intramolecular cyclopropanation reactions of non-activated alkenes. Cyclopropanation of 1,3-butadienes with these intermediates also leads to the formation of cycloheptadienes (Entry 4, Table 2.24). 

These carbene (or alkylidene) complexes are used as either stoichiometric reagents or catalysts for various transformations which are different from those of free carbenes. Reactions involving the carbene complexes of W, Mo, Cr, Re, Ru, Rh, Pd, Ti and Zr are known. Carbene complexes undergo the following transformations (i) alkene metathesis (ii) alkene cyclopropanation (iii) carbonyl alkenation (iv) insertion to C—H, N—H and O—H bonds (v) ylide formation and (vi) dimerization. Their chemoselectivity depends mainly on the metal species and ligands, as discussed in the following sections. 

Heteroatom-containing ene analogues react with imine or carbene equivalents to give heterocycles in reactions similar to [2-1-1] carbene/alkene cyclopropanation. Alkylidenoaminoboranes R2N=B=CR 2 react slowly with various azides with loss of N2 to form azaboriridines 36 in poor yield. An example appears in Equation (21) (R = PhCH2 or Ph) <1985AGE416, 1989CB595>. 

---