The electronic structures and dissociation energies of diazocyclopropane (1), diazo methane (2), 2 diazopropane (3), and diazocyclobutane (4) were calculated at the density functional B3LYP and the ab initio MP2 levels using the 6 31G(d) basis set and at the G2(MP2,SVP)//B3LYP/6 31G(d) level. Distinctive features of diazocyclopropane 1 are the low energy of dissociation with loss of the nitrogen molecule; ∆E = 18.7 kcal mol -1 , B3LYP; 9.2 kcal mol -1 , G2 at 0 K) and a nonplanar structure, in which the C=N bond forms an angle of 115.7° with the plane of the cyclopropane ring. The behavior of molecules 1 and 2 in the 1,3 dipolar cycloaddition to ethylene (5), acrylonitrile (6), and methyl acrylate (7) was stud ied. The reactions of 1 with 6 and 7 have very low activation barriers (∆E a = 4.7 and 4.4 kcal mol -1 , respectively; at the B3LYP level). For these reactions, the G2 method gives even smaller activation parameters (1.8 and 0.3 kcal mol -1 , respectively). The results of our calculations provide a good explanation for high reactivity of diazocyclopropane 1.The characteristic feature of diazo compounds is their ability to react as 1,3 dipoles in [3+2] cycloaddition re actions with unsaturated compounds. Such reactions are well studied both experimentally and theoretically. 1 The reaction mechanisms of cycloaddition 2-4 and the regio selectivity 5,6 of the process were studied in depth by mod ern quantum chemical methods. We found that diazo cyclopropane (1) is distinguished from a whole series of aliphatic diazo compounds by high reactivity. 7Diazocyclopropane (1) is very unstable and remains undetectable by spectroscopic methods. However, when generated in situ in the presence of appropriate trapping agents, compound 1 can give the corresponding trapping products in preparative yields. These reactions afford pyrazolines containing the spiro fused cyclopropane frag ment (Scheme 1).
Scheme 1 B is a baseDiazocyclopropane 1 is most efficiently trapped by substrates in which the C=C bond is elongated due to the presence of a strained hydrocarbon moiety 8 or is conju gated with electron withdrawing groups. 9 Attempts to pre pare addition products of compound 1 to substrates con taining an inactivated double bond (for example, to alk 1 enes) failed. In this case, reactions involving the ini tial dediazotization of diazocyclopropane 1 play the main role. 10 The hypothesis of a nonplanar structure of diazo cyclopropane 1, which has been put forward more than 40 years ago, 10 is a possible key to understanding the unusual behavior of this compound. The nonplanar struc ture of 1 is also supported by the results of quantum chemi cal calculations at the RHF/6 31G(d) level of theory. 11 However, the geometric structure of diazocyclopropane has not been described in the latter study.To elucidate the geometric features and the electronic structure of molecule 1 and explain its chemical proper ties, we carried out quantum chemical calculations by the B3LYP, MP2, and G2 methods.
ExperimentalQuantum chemical calculations of ...