Alkenes cyclopropanations, dicarbonyl

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-... 

The reductive cyclization of readily available enol phosphates of 1,3-dicarbonyl compounds bearing pendant olefinic units has been explored [66,67]. The chemistry is exceptionally interesting, and provides a unique route to structures possessing a cyclopropyl unit which is suitable for structural elaboration. The reaction occurs in a manner wherein the phosphate-bearing carbon behaves like a carbene that adds to the pendant alkene to form a cyclopropane. While this provides a useful way of viewing the transformation, mechanistic studies indicate that a carbene is not an actual intermediate. Examples are portrayed in Table 11. 

Motherwell and Roberts dicarbonyl coupling reaction19 can be improved by substituting l,2-bis(chlorodimethylsilyl)ethane for TMSC1. The intermediate organozinc carbenoids can be trapped with alkenes to produce cyclopropanes, as exemplified in the intramolecular case in equation 3. 

Dicarbonyl(ri5-cyclopentadienyl)iron-alky] complexes represent useful precursors for iron-carbene complexes [47]. For example, iron-carbene complexes are intermediates in the acid-promoted reaction of Fp-alkyl ether derivatives with alkenes to provide cyclopropanes via a [2 + l]-cycloaddition (Scheme 1.16). 

Alkenylboron compounds cyclopropanations, 9, 181 haloetherification, 9, 182 hydrogenation and epoxidation, 9, 182 metal-catalyzed reactions, 9, 183 metallic reagent additions, 9, 182 via radical addition reactions, 9, 183 5-Alkenylboron compounds, cross-coupling reactions, 9, 208 Alkenyl complexes with cobalt, 7, 51 with copper, 2, 160, 2, 174 with Cp Re(CO) (alkene)3 , 5, 915-916 with dicarbonyl(cyclopentadienyl)hydridoirons, 6, 175 with gold, 2, 255... 

In intermolecular cyclopropanations [100], it was found better to use a-bromoesters and amides as ylide precursors and a,/ -unsaturated ketones and esters as electron-deficient alkenes - rather than using a-haloketones as the ylide precursor. (For experimental details see Chapter 14.11.4). The reaction gives access to a range of 1,2-dicarbonyl-substituted cyclopropanes (see Fig. 10.5). The al-kene could have an aryl-, alkyl- or indole-substituted ketone, and a-substitution was also tolerated. Notably, Weinreb amides could be used as the ylide precursor and the product subsequently transformed into a diketocyclopropane. Both enan-... 

The carbenoid reaction between a-diazo ketones and simple alkenes or styrenes leads to acylcyclopropanes. (For the enantioselective cyclopropanation of styrene with 2-diazo-5,5-dimethylcyclohexane-l,3-dione, see Section 1.2.1.2.4.2.6.3.2.). With ketene acetals, 2,3-dihyd-rofurans are obtained. In contrast, l-acyl-2-oxycyclopropanes or 2-oxy-2,3-dihydrofurans can be formed in reactions with enol ethers and enol acetates the result depends strongly on the substitution pattern of both reaction partners.Whereas simple diazo ketones usually lead to cyclopropanes (Table 15), 3-diazo-2-oxopropanoates and 2-diazo-l,3-dicarbonyl compounds, such as 2-diazoacetoacetates, 3-diazopentane-2,4-dione, and 2-diazo-5,5-dimethylcy-clohexane-1,3-dione, yield 2,3-dihydrofurans and occasionally acyclic structural isomers thereof when reacted with these electron-rich oxy-substituted alkenes. 

---