Alkenes from diazo compounds

DCA reactions are an important means of synthesis of a wide variety of heterocyclic molecules, some of which are useful intermediates in multistage syntheses. Pyrazolines, which are formed from alkenes and diazo compounds, for example, can be pyrolyzed or photolyzed to give cyclopropanes. 

B. From Diazo Compounds and Alkenes Bearing Suitable Leaving Groups... 

Optically active metal complexes have been recognized as excellent catalysts for the enantioselective cyclopropanation of carbenes with alkenes. Normally, diazo compounds react under metal catalysts in the dark to afford carbenoid complexes as key intermediates. Katsuki et al. have reported the ds-selective and enantioselective cyclopropanation of styrene with a-diazoacetate in the presence of optically active (R,R)-(NO + )(salen)ruthenium complex 80, supported under illumination (440 nm light or an incandescent bulb) [59]. The irradiation causes dissociation of the apical ligand ON + in 80, and thus avoids the splitting of nitrogen from the a-diazoacetate. 

Episulfides, generated from diazo compounds and a thioketone, can also undergo desulfurization with phosphines to give alkenes (see section 4.3.7). 

It has also proved possible to make alkenes in a one-pot process from diazo compounds N2CR2, where R is an electron-withdrawing group, and aldehydes or ketones by heating them in the presence of tributylstibine no base was required Reaction was assumed to proceed through a stibonium ylide as intermediate, although it proved impossible to isolate this ylide, which probably reacted further too rapidly to allow this. 

There are three main approaches for the synthesis of thiiranes the first is C—S bond formation (Scheme 64), the second is C—C bond formation (Scheme 65), and the third is based on the formation of two bonds (Scheme 66), which includes the reaction of elemental sulfur with alkenes and the reaction of thiocarbonyl compounds or elemental sulfur with carbenes derived from diazo compounds. These methods are described in detail in CHEC-I, and references until 1983 are cited therein <84CHEC-i(7)i7i>. A survey of reports, published mainly after 1983, of the formation of thiiranes, thiirane 5-oxide, and thiirane S,S-dioxides is given in the following sections. 

A ruthenium carbene complex in the presence of a chiral ligand is capable of catalyzing the formation of optically active cyclopropane derivatives from alkenes and diazo compounds in high enantiomeric excess [177]. A mixture of [RuCl2(/ -cymene)] in the presence of pybox-(5,5)-/ catalyzes the asymmetric cyclopropanation of styrene (eq (48)). The key intermediate is proposed to be a dichloro(pybox)ruthenium carbene complex. 

The cyclopropanation of alkenes via carbene transfer from diazo compounds can be achieved by a variety of transition metals. The most popular catalysts for this transformation are based on copper(I) and rhodium(II) complexes, however, paUadium(II) has been shown, as demonstrated first by Kirmse and Kapps, "- to be effective and even superior for certain substrates and reagents. " ... 

Ruthenium carbenoid species, formed from diazo compounds, can react with alkenes to produce cyclopropane derivatives. For example, the CpRuCl... 

Pyrazoles are formed when the diazo compounds react with alkynes or with functionalized alkenes, viz. the enols of /3-diketones. Pyrazolenines (353 Section 4.04.2.2.1) are isolated from disubstituted diazomethanes. Many pyrazoles, difficult to obtain by other methods, have been prepared by this procedure, for example 3-cyanopyrazole (616) is obtained from cyanoacetylene and diazomethane (7iJCS(C)2i47), 3,4,5-tris(trifiuoromethyl)pyrazole (617) from trifluorodiazoethane and hexafluoro-2-butyne (8lAHC(28)l), and 4-phenyl-3-triflylpyrazole (618 R =H) from phenyltriflylacetylene and diazomethane (82MI40402). An excess of diazomethane causes iV-methylation of the pyrazole (618 R = H) and the two isomers (618 R = Me) and (619) are formed in a ratio of 1 1. 

Figure 7.29. (Top) Molecular representations based on X-ray structural data of the diazo compound 88N2 and the alkene product 89Z (the migrating hydrogen is shown in black in both reactant and product). (Bottom) Schematic reaction path showing the minimal structural changes in the transition from the diazo compound to the product, via the probable transition structure 88TS.

An important competing process with significant practical consequences is the catalytic dimerization of diazoacetate to form maleate and fumarate esters. Most catalysts suffer from this side reaction, leading to the use of the alkene as solvent in order to accelerate the productive pathway and the slow addition of diazo compound in order to minimize dimerization. Since this problem is generally shared across most catalyst architectures, it will be mentioned in discussions of individual asymmetric catalyst systems only in those instances where these precautions prove to be unnecessary. 

There are no mechanistic details known from intermediates of copper, like we have seen in the studies on metathesis, where both metal alkylidene complexes and metallacyclobutanes that are active catalysts have been isolated and characterised. The copper catalyst must fulfil two roles, first it must decompose the diazo compound in the carbene and dinitrogen and secondly it must transfer the carbene fragment to an alkene. Copper carbene species, if involved, must be rather unstable, but yet in view of the enantioselective effect of the ligands on copper, clearly the carbene fragment must be coordinated to copper. It is generally believed that the copper carbene complex is rather a copper carbenoid complex, as the highly reactive species has reactivities very similar to free carbenes. It has not the character of a metal-alkylidene complex that we have encountered on the left-hand-side of the periodic table in metathesis (Chapter 16). Carbene-copper species have been observed in situ (in a neutral copper species containing an iminophosphanamide as the anion), but they are still very rare [9],... 

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