Ab Initio Study of Molecular Properties and Decomposition Products of 1-AzidoalkynesA Challenge for Experimentalists

Abstract

In this publication, a characterization of different azidoalkyne compounds using high-level ab initio quantum chemical methods is presented. For this purpose, the molecular structures and the 13C NMR chemical shifts have been calculated at the MP2 and CCSD(T) level of theory and the influence of zero-point vibration as well as the solvent on the chemical shifts are discussed. Furthermore, a comparison of the energy barriers of the decomposition under N2 separation for a set of 1-azidoalkynes with different functional groups has been carried out. The molecular structures and properties of the resultant decomposition products have been investigated. It is remarkable that large deviations of the NMR chemical shifts of ethylthioethynyl azide occur in comparison to the experiment. These deviations are far outside of the error bars. Electron correlation effects are of high importance if an accurate description of the chemical shifts of 1-azidoalkynes shall be obtained. A comparison of the energy barriers of the decomposition under N2 separation of 1-azidoalkynes with different functional groups indicates that the stability of 1-azidoalkynes is not increased by typical donor or acceptor groups but rather by silyl or phenyl substituents. The molecular geometries of the decomposition products indicate that the equilibrium structures are of carbene character. Some of the results presented here are in contradiction to previous experimental publications and cast a new light on some open challenges for experimentalists.

Synthesis, Characterization and Reactions of (Azidoethynyl)trimethylsilane

“…The decomposition of ethynyl azide 6 is expected to lead to cyano(trimethylsilyl)carbene (8). The formation of this short-lived species is also supported by quantum chemical calculations that explained the generation of 8 from 6 through a synchronous reaction without a nitrene intermediate [17,21] (Figure 3). The activation barrier for this process was calculated to amount 20-22 kcal·mol −1 , and such a low value corresponds to the fact that 6 is a highly unstable compound.…”

“…The latter method was also utilized to analyze unstable 3 in the gas phase or in argon matrix, and to monitor the thermal or photochemical decay to generate cyanocarbene 4 [15,16]. In the case of the parent compound 3, previous ab initio studies predicted the highest barrier for the loss of dinitrogen, whereas ethynyl azides with a substituent at C-2 should more easily undergo the cleavage reaction [17]. Especially, 1-azidoethynes with a donor substituent Unfortunately, the parent compound 3 is a very explosive substance, and even recondensation of its solution under reduced pressure can lead to an explosion.…”

The Chemistry of Vinyl, Allenyl, and Ethynyl Azides

“…196 However, the known 197 trapping products of 306 including the cyclopropane framework cannot be detected when 304 was treated in the presence of cyclohexene or tolane. 39 Attempts to generate and to trap 1 -azido -2 -phenylethyne 157,193,196,198,199,201,202 process. 196a But alternative routes leading from 304 to 308 without the intermediary 1 -azidoalkyne 305 were also discussed.…”

“…196a But alternative routes leading from 304 to 308 without the intermediary 1 -azidoalkyne 305 were also discussed. 201 When the experiment starting with 310 was repeated quite recently, it has been shown that the real product 315 (10% yield), resulting via 306 , was erroneously taken for 312 . 196b Quite recently, it has been shown that irradiation of 304 in dichloromethane at − 70 ° C induced the complete transformation to the highly unstable heterocycle 307 generated in 90% yield and characterized by its NMR data.…”

“…The compound 312 seems to be a logical interception product of 305 and 311 , respectively, because the synthesis of similar sulfoximines from azides and dimethyl sulfoxide is a well -known Scheme 5. 201 It was shown that the parent compound as well as 305 and silyl -substituted ethynyl azides are relatively stable whereas azidoacetylenes with donor groups, especially such as amino or ethylsulfanyl, should split off dinitrogen very easily. 200 In the case of 311 , however, Auer and coworkers demonstrated in an ab initio study that the nitrene does not correspond to a local minimum of energy and that cleavage of dinitrogen from 305 should generate the carbene 306 .…”