Diazo compounds structural properties

The properties of the diazirines and the analytical results showed that a new class of isomeric diazo compounds had been discovered. The three-membered ring structure (65), which is made probable by the synthetic methods, is confirmed by the reactions of the diazirines. 

The proof of the three-membered structure of the diazirines concludes the discussion on the three-membered ring structure of the aliphatic diazo compounds. The knowm linear aliphatic diazo compounds and the newly prepared cyclic diazo compounds (diazirines) are two independent classes of compounds completely different in their physical and chemical properties. An interconversion of the linear and cyclic diazo compounds has not so far been possible. 

Whereas the utility of these methods has been amply documented, they are limited in the structures they can provide because of their dependence on the diazoacetate functionality and its unique chemical properties. Transfer of a simple, unsubstituted methylene would allow access to a more general subset of chiral cyclopropanes. However, attempts to utilize simple diazo compounds, such as diazomethane, have never approached the high selectivities observed with the related diazoacetates (Scheme 3.2) [4]. Traditional strategies involving rhodium [3a,c], copper [ 3b, 5] and palladium have yet to provide a solution to this synthetic problem. The most promising results to date involve the use of zinc carbenoids albeit with selectivities less than those obtained using the diazoacetates. 

Bis(diazo)-l,2,4,5-cyclohexanetetraone (4.5) may be regarded as a derivative of a double 1,2-quinone diazide. Its X-ray analysis was reported by Ansell (1969). The synthesis, properties, and structure of this interesting compound will be discussed in the forthcoming book on aliphatic diazo compounds (Zollinger, 1995, Secs. 2.3 and 5.2). 

The diazo-compounds and corresponding aromatic carbenes that form the basis for our dissection of structure and reactivity are shown in Table 1. The carbenes in this group are carefully chosen so that the variation in structure is systematic the theory identifies the carbene bond angle and certain electronic factors as controlling chemical and physical properties, and as far as possible, these two features are varied independently of each other for these carbenes. Table 2 lists some other aromatic carbenes that have been studied. In general, the structures of these carbenes are not simply related to each other. Nevertheless, the principles uncovered by analysis of the compounds of Table 1 can be readily extended to those of Table 2. 

Abstraction, hydrogen atom, from O—H bonds, 9, 127 Acid-base behaviour macroeycles and other concave structures, 30, 63 Acid-base properties of electronically excited states of organic molecules, 12, 131 Acid solutions, strong, spectroscopic observation of alkylcarbonium ions in, 4, 305 Acids, reactions of aliphatic diazo compounds with, 5, 331 Acids, strong aqueous, protonation and solvation in, 13, 83 Acids and bases, oxygen and nitrogen in aqueous solution, mechanisms of proton transfer between, 22, 113... 

The synthesis and properties of diazo compounds have been covered in an excellent review by Regitz and Maas. This section does not attempt to duplicate that review. Rather, two key areas are covered the most practical methods for the preparation of diazo-containing compounds, and use of these intermediates in organic synthesis. To understand these areas, however, it is important first to appreciate the structure, bonding and reactivity of the diazo functional group. 

All these facts lead to the conclusion that the diazo compounds contain a pentavalent nitrogen atom, as it is only when nitrogen possesses this valence that its compounds show the properties of bases and salts. In order to emphasize this conclusion and thus indicate the analogy between the diazo and ammonium compounds, the word diazo has been replaced by the word diazonium in naming the compounds which have been described above. The compound of the formula C6H5N2CI is, thus, benzenediazonium chloride its structure is represented by the formula,—... 

It will be shown later that compounds of the structure C6H5.N=N.X exist these are called diazo compounds, and do not possess the salt-like properties of the diazonium compounds. 

The study of the salts of benzenediazo hydroxide has led to the discovery of facts which make it necessary to broaden still further the hypothesis of the structure of diazo compounds. When potassium benzenediazoate is heated with a strong solution of potassium hydroxide, it is converted into an isomeric salt, which is very stable and forms derivatives which differ widely in properties from the analogous derivatives of its isomer. The explanation which has been put forward of the isomerism in this case is based on stereochemical considerations. The arrangement of the atoms in the case of benzenediazo hydroxide is illustrated by the following formulas —... 

Hantzsch now" prepared isomeric diazocyanides (not cyanide and isocyanide) with the same structure C1C6H4 N2 CN, which Bamberger regarded as (A) and (B) compounds (see above). Hantzsch first considered that (B) is incompatible with the properties of diazo-compounds, which have different structures in the solid state and in solution but he then adopted (B) for salts in solution, calling the radical diazonium whilst the solids are yw-diazo-compounds and the diazo-cyanides are stereo-isomers ... 

We include here a short discussion of diazocyclopentadiene (4.21), its 2,3,4,5-tetra-cyano derivative (4.22), and the analogous heterocyclic compound 2-diazo-4,5-di-cyanoimidazole (4.23). Their synthesis and properties are discussed in Sections 2.6 and 12.2, where it is mentioned that they show in some respects the behavior of diazonium ions, i.e., of the mesomeric structure 4.21b (also analogously for 4.22 and 4.23). 

---