Mechanisms of the Thermal Cyclotrimerizations of Fluoro- and Chloroacetylenes: Density Functional Theory Investigation and Intermediate Trapping Experiments

Abstract

Theoretical studies of the mechanisms of the thermal cyclotrimerization of fluoro- and chloroacetylenes, which were reported by Viehe and Ballester, respectively, were conducted with the aid of density functional theory calculations of the (U)B3LYP functional, indicating that the thermal cyclotrimerizations of fluoro- and chloroacetylenes involve tandem processes of regioselectively stepwise [2+2] and stepwise [4+2] cycloadditions. These tandem processes generate 1,2,6-trihalo-Dewar benzenes and 1,2,4-trihalo-Dewar benzenes, which then isomerize to the corresponding benzenes when heated. The rate-determining step of the cyclotrimerizations of haloacetylenes is the dimerization step involving open-shell singlet diradical transition states and intermediates. The substituent effects in the thermal cyclotrimerization of haloacetylenes have been rationalized using frontier molecular orbital theory. The higher reactivity of fluoroacetylenes compared to that of chloroacetylenes is due to the fact that fluoroacetylenes have lower singlet-triplet gaps than chloroacetylenes and more easily undergo dimerization and cyclotrimerization. In this report, additional experiments were performed to verify the theoretical prediction about the cyclotrimerization of chloroacetylene and to trap the proposed 1,4-dichlorocyclobutadiene intermediate. Experiments revealed that the thermal reaction of phenylchloroacetylene at 110 °C gave 1,2,3-triphenyltrichlorobenzene and 1,2,4-triphenyltrichlorobenzene together with a tetramer, cis-1,2,5,6-tetrachloro-3,4,7,8-tetraphenyltricyclo[4.2.0.0(2,5)]octa-3,7-diene. The proposed 1,4-diphenyldichlorocyclobutadiene intermediate in the thermal cyclotrimerization of phenylchloroacetylene was successfully trapped using dienophiles of maleic anhydride and dimethyl acetylenedicarboxylate.

Discerning the thermal cyclotrimerizations of fluoro- and chloroacetylenes through ELF, NBO descriptors and QTAIM analysis: pseudodiradical character

“…In Figure 1 , we can see the alkyne cyclotrimerization reported by Yao et al. [ 34 ].

Figure 1 Mechanism of the cyclotrimerization of fluoroacetylene reported by Yao et al.

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“…(b) Thermodynamic parameters: ΔG, ΔH, ΔE 0 (kcal/mol) profiles along the IRC, reported by Yao et al. using DFT calculations [ 34 ]. Note: Yao et al [ 34 ].…”

Cyclic Compounds Incorporating Two or Four Alkyne Units in Close Proximity – Theory and Experiments

“…Calculations for 80b (Y = F) and 80c (Y = Cl) were also performed by UB3LYP/6–31G*. These results show that the 1,1′ dimerization is energetically favored by more than 5 kcal mol –1 over to the corresponding 2,2′ dimerizations . The results summarized in Figure suggested that 80b (Y = F) and 80d (Y = OMe) should be good candidates for trapping experiments.…”

A Structurally Characterized Fluoroalkyne

“…Those that have been reported are often highly reactive and, in marked contrast to fluoroalkanes, fluoroalkenes and fluoroaromatic compounds, fluoroalkynes readily undergo oligomerization reactions. For example, FC≡CF is unstable at temperatures above −196 °C and t BuC≡CF undergoes cyclotrimerization below 0 °C . Fluoroalkynes having perfluoroalkyl groups, however, appear to have greater stability .…”