Dipolar Cycloaddition of Fulminic Acid to Ethyne

Given the concerted, almost synchronous nature of this gas-phase reaction it might seem reasonable to suppose that the electronic mechanism would resemble those for the Diels-Alder and cyclohexadiene ring-opening reactions, described above. However, our spin-coupled calculations along the IRC reveal a somewhat different picture [3]. 

Analysis of the total spin function reveals that the spins associated with the pairs (v /2,V /4), (v3,V6) and (v / V /6) remain essentially singlet coupled throughout the course of the reaction, with no evidence for any aromatic structure along the IRC. As such, the spin-coupled description corresponds to a mechanism that involves the simultaneous relocation of three orbital pairs, as might be represented by the following simplistic scheme  

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The combination of modem valence bond theory, in its spin-coupled (SC) form, and intrinsic reaction coordinate calculations utilizing a complete-active-space self-consistent field (CASSCF) wavefunction, is demonstrated to provide quantitative and yet very easy-to-visualize models for the electronic mechanisms of three gas-phase six-electron pericyclic reactions, namely the Diels-Alder reaction between butadiene and ethene, the 1,3-dipolar cycloaddition of fulminic acid to ethyne, and the disrotatory electrocyclic ringopening of cyclohexadiene. 

The article is organized as follows. In the next Section we present a brief outline of the theoretical background for the present work. Section 3 contains summaries of the SC models for the electronic mechanisms of the gas-phase Diels-Alder reaction between butadiene andethene [11] and the 1,3-dipolar cycloaddition of fulminic acid to ethyne [12]. In Section 4 we provide, for the first time, a description of the SC model for the electronic mechanism of the gas-phase disrotatory electrocyclic ring-opening of cyclohexadiene. Conclusions and final comments are presented in Section 5. 

We now turn to the gas-phase 1,3-dipolar cycloaddition of fulminic acid to ethyne. The concerted, almost synchronous nature of this reaction might create the impression that the electronic mechanism of this process should be very similar to that of the Diels-Alder reaction. Such an expectation is reinforced by frontier orbital theory, which treats both reactions in very much the same way (see Ref. 32). The only significant differences are related to the fact that the lowest unoccupied MO (LUMO) for a linear 1,3-dipole... 

We believe that reactions that follow a scheme with fill-arrows, such as the gas-phase 1,3-dipolar cycloaddition of fulminic acid to ethyne, are most likely to remain nonaromatic along the whole reaction coordinate. 

At the RHF level of theory, which uses a wavefunction that is relatively straightforward to interpret, the subtle differences between the half- and full-arrow reaction schemes would remain well-hidden within the doubly-occupied, usually delocalized orbitals. While it can be argued that the application of an orbital localization procedure could produce a semblance of the SC description for the 1,3-dipolar cycloaddition of fulminic acid to ethyne, the double-occupancy restriction makes it impossible to obtain the analogue of a half-arrow SC mechanism using an RHF wavefunction. 

Figure 2. SC orbitals for the gas-phase 1,3-dipolar cycloaddition of fulminic acid to ethyne along the CASSCF(6,6) IRC at IRC -1.2 amu bohr (leftmost column), the IS (IRC = 0) and IRC = +1.2 amu bohr (rightmost column). The plot details are as for Fig. I except that the isovalue surfaces correspond to V / = 0.1.

Figure 5. Symmetry-unique spin-coupled orbitals for the 1,3-dipolar cycloaddition of fulminic acid to ethyne.

The SC descriptions of the electronic mechanisms of the three six-electron pericycUc gas-phase reactions discussed in this paper (namely, the Diels-Alder reaction between butadiene and ethene [11], the 1,3-dipolar cycloaddition of fulminic acid to ethyne [12], and the disrotatory electrocychc ring-opening of cyclohexadiene) take the theory much beyond the HMO and RHF levels employed in the formulation of the most popular MO-based treatments ofpericyclic reactions, including the Woodward-Hoffmann rules [1,2], Fukui s frontier orbital theoiy [3] and the Dewar-Zimmerman model [4-6]. The SC wavefunction maintains near-CASSCF quality throughout the range of reaction coordinate studied for each reaction but, in contrast to its CASSCF counterpart, it is veiy much easier to interpret and to visualize directly. 

Microwave irradiation accelerates the rate of Mg(II)-catalysed 1,3-dipolar cycloaddition between mesitonitrile oxide and jS-hydroxy-2-methylene esters, but has little effect on the diastereoisomeric excess. FMO interactions and regiochemical drift due to steric effects have been used to determine the regiochemistry of 3 + 2-cycloadditions of nitrile oxides with a,/3-unsaturated amides. The gas-phase 1,3-dipolar cycloaddition of fulminic acid to ethyne has been investigated using valence-bond theory in the spin-coupled form and using intrinsic reaction... 

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