Alkenes diazo compounds

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... 

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... 

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.

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. 

Additions to the double bond of phospha-alkenes continue to be explored. The reactions of (177) with aliphatic diazo compounds... 

The second intermediate s identity has been debated since the mid-1980s. In 1984, Liu and Tomioka suggested that it was a carbene-alkenc complex (CAC).17 Similar complexes had been previously postulated to rationalize the negative activation energies observed in certain carbene-alkene addition reactions.11,30 A second intermediate is not limited to the CAC, however. In fact any other intermediate, in addition to the carbene, will satisfy the kinetic observations i.e., that a correlation of addn/rearr vs. [alkene] is curved, whereas the double reciprocal plot is linear.31 Proposed second intermediates include the CAC,17 an excited carbene,31 a diazo compound,23 or an excited diazirine.22,26 We will consider the last three proposals collectively below as rearrangements in the excited state (RIES). 

When other acceptor systems such as tetracyanoethylene, ethyl propiolate, dibenzoylacetylene, or dimethyl azodicarboxylate were reacted with 41, no additional products were formed. Accordingly, the scope of the reaction for the nucleophilic addition of 41 to electron-poor alkenes, alkynes, and diazo compounds is quite narrow. 

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],... 

Photolysis ofbenzylchlorodiazirine (3) in the presence of tetramethylethylene (TME) is known to produce ( )- and (Z)-/l-chlorostyrene (4) and the cyclopropane (5). Plots of [5]/[4] vs [TME] are curved, consistent with the existence of two pathways for the formation of the alkenes (4). Benzylchlorocarbene (BnClC ) was generated by laser flash photolysis of the phenanthrene (6) in the presence of TME. In this case, plots of [5]/[4] vs [TME] are linear, mling out the possibility that the second pathway to the alkenes (4) involves reaction of a carbene-alkene complex. Time-resolved IR spectroscopy revealed that diazirine (3) rearranges to the corresponding diazo compound, but this process is too inefficient to account for the curvatures. It is proposed that the second pathway to alkene formation involves the excited state of the diazirine. 

Other novel diazo compounds that have been subjected to 1,3-dipolar cycloaddition with activated alkenes, and that give unusually functionalized pyrazolines (Scheme 8.7), include l-diazo-3-trimethylsilylpropan-2-one (20) (49), 2-diazo-methyl-4(57/)-furanones (21) (50), methyl 2-diazo-5-methylanilino-5-oxopentano-ate (22) (51), 2-(acylamino)-2-diazoacetates (23) (51), ethyl 2-diazo-4,4,4-trichloro-3-(ethoxycarbonylamino)butyrate (24) (52), and diazopropyne (53). 

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