1.3- dipolar cycloaddition of diazo compounds

These results can be interpreted in terms of competition between recombination of the diradical intermediate and conformational equilibration, which would destroy the stereochemical relationships present in the azo compound. The main synthetic application of azo compound decomposition is in the synthesis of cyclopropanes and other strained-ring systems. Some of the required azo compounds can be made by 1,3-dipolar cycloadditions of diazo compounds (see Section 6.2). 

The 1,3-dipolar cycloaddition of diazo compounds with alkynes represents a standard method for the preparation of pyrazoles. The initially formed... 

Another limitation of the 1,3-dipolar cycloaddition of diazo compounds and selenoketones is that it is reversible. Consequently, mixtures of aikenes may be obtained as illustrated in Equation (22) <2004EJ0820>. 

This reaction has general applications in organic synthesis, especially for the 1,3-Dipolar Cycloaddition of diazo compounds. 

Pyrazoles from the 1,3-dipolar cycloaddition of diazo compounds and alkynes. 

Diazo transfer from p-toluenesulfonyl azide onto acylthioacetamide (256) takes place in EtOH to give 90% of 4-acyl-5-phenylamino-l,2,3-thiadiazole (258).1 The diazo intermediate (257) could not be isolated, since the cycloaddition is evidently faster than the diazo transfer reaction.1 Thiadiazoles are also prepared by 1,3-dipolar cycloaddition of diazo carbonyl compounds to isothiocyanates in moderate yields.1... 

The synthesis of 1,2,3-selenadiazole derivatives has been reported. The reaction of aroyl chlorides such as 102 with potassium isoselenocyanate and ethyl diazoacetate yielded 5-(aroylimino)-2,5-dihydro-l, 2,3-selenadiazole-4-carboxylate esters such as 104. A reaction mechanism via the initial formation of the corresponding aroyl isoselenocyanate 103 followed by a 1,3-dipolar cycloaddition of the diazo compound with the C=Se bond is proposed <00HCA539>. 

Diazo compounds have also been used as precursors in the preparation of pyrazoles and indazoles. The copper-promoted cycloaddition reaction of lithium acetylides 18 with diazocarbonyl compounds 19 provided a direct and efficient approach to the synthesis of pyrazoles 20 <07AG(I)3242>. A facile, efficient, and general method for the synthesis of 1-arylated indazoles 22 and A-unsubstituted indazoles 23 by the 1,3-dipolar cycloaddition of benzynes, generated from 21, with diazomethane derivatives has been reported <07AG(I)3323>. Reaction of diazo(trimethylsilyl)methylmagnesium bromide with aldehydes or ketones gave 2-diazo-2-(trimethylsilyl)ethanols, which were applied to the synthesis of di- and trisubstituted pyrazoles via [3+2] cycloaddition reaction with ethyl propiolate or dimethyl acetylenedicarboxylate <07S3371>. 

Diazonium intermediates have also been employed in the synthesis of pyrazoles. A convenient one-pot procedure for the preparation of 3-phenyl- or 3-pyridylpyrazoles 27 from the 1,3-dipolar cycloadditions of phenylacetylene or 3-(pyridyl)acetylene with diazo compounds 26 generated in situ from aldehydes 25 has been reported <03JOC5381>. Cyclization of ortho-(arylethynyl)benzene diazonium salts 28 having substituents at the para-position of the aryl ring furnished indazoles 29 <03TL5453>. 

As pointed out previously, seleniranes thermally extrude selenium to yield the corresponding alkenes <1996CHEC-11(1)259>. Thus, thermolysis of the 1,3,4-selenadiazoline 37, as shown in Equation (6), results in the loss of N2 to form the presumed selenirane intermediate. This intermediate loses selenium to form an alkene . The 1,3,4-selenadiazoline is prepared but usually not isolated by the 1,3-dipolar cycloaddition of a diazo compound... 

Density functional theory (DFT) calculations at the B3LYP/6-31H-G"" level were carried out on the 1,3-dipolar cycloadditions of various heterophospholes, including 1,3-azaphosphole, with diazo compounds across the P=N bond <2003JP0504>. In most cases, the dominant frontier orbital interaction is between HOMO(diazo) with LUMO(heterophosphole) however, 1,3-azaphosphole has a HOMO of high energy and for it, HOMO(heterophosphole)-LUMO(diazo) is also important (HOMO = highest occupied molecular orbital LUMO = lowest unoccupied molecular orbital). 

As mentioned already in Section 2.6, it is somewhat arbitrary to discuss diazo transfer reactions to alkenes in isolation from those to activated methylene compounds. The most important activation in methylene compounds is that of a neighboring carbonyl group and, as a consequence, the active methylene compound is in equilibrium with the corresponding enol, i.e., with an alkene as established by the systematic work of Huisgen (review Huisgen, 1984), typical diazo transfers involve 1,3-dipolar cycloaddition of a 1,3-dipole (azides) to a multiple-bond system, the dienophile (see Chapt. 6). In diazo transfer, this dienophile is an alkene or an alkyne, and the primary product is a A -l,2,3-triazoline or a A -l,2,3-triazole,... 

---