Alkenes diazo

FIGURE 22 5 The diazo mum ion generated by treatment of a primary al kylamine with nitrous acid loses nitrogen to give a car bocation The isolated prod ucts are derived from the carbocation and include in this example alkenes (by loss of a proton) and an al cohol (nucleophilic capture by water)... 

Diazo compounds react with alkenes to afford A -pyrazolines, which in turn izomerize to A -pyrazolines if there is a hydrogen atom a to the N=N bond (Scheme 54). In those cases where two possible ways of isomerization exist, the more acidic hydrogen migrates preferentially. The alkene configuration is conserved on the A -pyrazoline (stereospecificity) but the regioselectivity depends on the substituents of both the alkene and the diazo compound. 

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. 

In the case of the reaction between 2-diazopropane and diphenyldiacetylene, the reverse (as compared with other diynes) orientation of addition of the first molecule of the diazo compound with a predominant formation of 4-phenylethynylpyrazole is observed. Therefore, it is noteworthy that whereas the regioselectivity of the addition of diazoalkanes to alkenes is well studied audits products have, as a rule, the structure been predicted with respect to electron effects, the problem of orientation... 

The most comprehensive modern works on the subject are the relevant volumes of Patai s series The Chemistry of Functional Groups, namely the two volumes on diazonium and diazo groups (Patai, 1978), the two volumes on hydrazo, azo, and azoxy groups (Patai, 1975) and the two Supplement C volumes on triple-bonded groups (Patai and Rappoport, 1983). Supplement C contains chapters on arene- and alkene-diazonium ions and on dediazoniation reactions. 

In this chapter we will discuss only the dediazoniation of arenediazonium ions (Group a). The dediazoniation of alkene- and alkanediazonium ions and of diazoalkanes (Groups b, c, and d) will be treated in the second book on diazo chemistry (Zollinger, 1995, Chs. 7-9). 

With regard to the mechanism of these Pd°-catalyzed reactions, little is known in addition to what is shown in Scheme 10-62. In our opinion, the much higher yields with diazonium tetrafluoroborates compared with the chlorides and bromides, and the low yields and diazo tar formation in the one-pot method using arylamines and tert-butyl nitrites (Kikukawa et al., 1981 a) indicate a heterolytic mechanism for reactions under optimal conditions. The arylpalladium compound is probably a tetra-fluoroborate salt of the cation Ar-Pd+, which dissociates into Ar+ +Pd° before or after addition to the alkene. An aryldiazenido complex of Pd(PPh3)3 (10.25) was obtained together with its dediazoniation product, the corresponding arylpalladium complex 10.26, in the reaction of Scheme 10-64 by Yamashita et al. (1980). Aryldiazenido complexes with compounds of transition metals other than Pd are discussed in the context of metal complexes with diazo compounds (Zollinger, 1995, Sec. 10.1). 

Other interesting three-component cycloadditions are the following Sulfur dioxide and diazo compounds lead to episulfones (equation 75)436—in a special case to 4,5-dihydrothiepine S,S-dioxides437 sulfur dioxide, ketene, and arylimine lead to thiazole derivatives438 (equation 76) sulfur dioxide, quinone, and alkenes lead to benzoxathiane derivatives439 (equation 77). 

Since their first introduction by Brunner and McKervey as chiral catalysts for the asymmetric cyclopropanation of alkenes with diazo compounds, chiral dirhodium tetra(A-arylsulfonylprolinates) complexes have been widely used by Davies,in particular, in the context of these reactions. Therefore, the use of... 

---