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The Mechanism of the Cycloaddition Reaction of 1,3-Dipole Molecules with Acetylene: an Investigation with the Unified Reaction Valley Approach

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The Mechanism of the Cycloaddition Reaction of 1,3-Dipole Molecules with Acetylene: an Investigation with the Unified Reaction Valley Approach Theor Chem Acc (2014) 133:1423 DOI 10.1007/s00214-013-1423-z REGULAR ARTICLE The mechanism of the cycloaddition reaction of 1,3-dipole molecules with acetylene: an investigation with the unified reaction valley approach Marek Freindorf • Thomas Sexton • Elfi Kraka • Dieter Cremer Received: 10 September 2013 / Accepted: 12 November 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract The unified reaction valley approach (URVA) between vibrational modes lead to an unusual energy is used in connection with a dual-level approach to describe exchange between just those bending modes that facilitate the mechanism of ten different cycloadditions of 1,3- the formation of radicaloid centers. The relative magnitude dipoles XYZ (diazonium betaines, nitrilium betaines, azo- of the reaction barriers and reaction energies is rationalized methines, and nitryl hydride) to acetylene utilizing density by determining reactant properties, which are responsible functional theory for the URVA calculations and for the mutual polarization of the reactants and the stability CCSD(T)-F12/aug-cc-pVTZ for the determination of the of the bonds to be broken or formed. reaction energetics. The URVA results reveal that the mechanism of the 1,3-dipolar cycloadditions is determined Keywords Unified reaction valley approach Á early in the van der Waals range where the mutual orien- 1,3-Dipolar cycloadditions Reaction mechanism tation of the reactants (resulting from the shape of the Mutual polarization EnergyÁ transfer and dissipationÁ Á enveloping exchange repulsion spheres, electrostatic attraction, and dispersion forces) decides on charge trans- fer, charge polarization, the formation of radicaloid cen- 1 Introduction ters, and the asynchronicity of bond formation. All cycloadditions investigated are driven by charge transfer to 1,3-Dipolar cycloadditions are pericyclic reactions leading the acetylene LUMO irrespective of the electrophilic/ to five-membered rings with heteroatoms [1–5]. The nucleophilic character of the 1,3-dipole. However, an reactions involve a 1,3-dipole molecule and a reaction insufficient charge transfer typical of an electrophilic 1,3- partner with a multiple bond as a dipolarophile. The dipole leads to a higher barrier. Energy transfer and energy reaction mechanism is in most cases concerted and stereo- dissipation as a result of curvature and Coriolis couplings and regiospecific with regard to both 1,3-dipole and di- polarophile. The interest in these cycloadditions results from the fact that a large variety of 1,3-dipole molecules Dedicated to Professor Thom Dunning and published as part of the special collection of articles celebrating his career upon his XYZ rapidly reacts in an exothermic fashion to the desired retirement. cycloaddition product yielding a wealth of heterocycles. A suitable combination of groups X, Y, and Z containing C, Electronic supplementary material The online version of this N, or O leads to 18 different 1,3-dipole molecules, 12 of article (doi:10.1007/s00214-013-1423-z) contains supplementary material, which is available to authorized users. which are N-centered and 6 are O-centered. The XYZ framework of a 1,3-dipole molecule can contain 16 valence M. Freindorf T. Sexton E. Kraka (&) D. Cremer (&) Á Á Á electrons as in the case of the diazonium betaines 1, 2, and Computational and Theoretical Chemistry Group (CATCO), 3 or the nitrilium betaines 4, 5, and 6 of Fig. 1. Alterna- Department of Chemistry, Southern Methodist University, 3215 Daniel Ave, Dallas, TX 75275-0314, USA tively, XYZ may have a total of 18 valence electrons as in e-mail: [email protected] the case of the azomethines 7, 8, and 9 of Fig. 1. D. Cremer 1,3-Dipole molecules are better described by resonance e-mail: [email protected] structures rather than simple Lewis structural formulas 123 1423 Page 2 of 18 Theor Chem Acc (2014) 133:1423 Fig. 1 1,3-Dipolar cycloaddition systems 1–9 investigated in this work. The name of the 1,3-dipole and the five-membered ring formed is given with standard single, double, or triple bonds. The propar- and identifying 1,3-dipolar cycloadditions as concerted, gyl-allenyl-type resonance structures (only the former is symmetry-allowed pericyclic p4s ? p2s reactions involv- shown in Fig. 1) are typical of the 16 valence electrons ing 4 p-electrons of the 1,3-dipole and 2 p-electrons of the systems 1–6, and the allyl-type resonance structures (only double or triple bond of the dipolarophile leading in this one shown) are typical of the N-centered 18 valence way to a Hu¨ckel-aromatic, stabilized transition state (TS) electron systems 7–9. Although the electronic structures of according to the Evans–Dewar–Zimmerman rules [7]. The the 1,3-dipole molecules are different, the mechanism of state- and orbital-symmetry analysis can explain the rela- their cycloadditions to dipolarophiles is similar as reflected tively low barriers of 1,3-dipolar cycloadditions and their by their exothermicity, similar reaction barriers, and a stereochemistry. Detailed trends in reaction rates and similar stereochemistry [1–5]. This has been explained to changes in the stereochemistry have been explained, often some part by applying the Woodward–Hoffmann rules [6] with success, on the basis of frontier orbital theory [8]. An 123 Theor Chem Acc (2014) 133:1423 Page 3 of 18 1423 even more detailed insight has been provided by a large of elementary electronic events that take place in each number of quantum chemical investigations [9–21] where phase of the reaction starting from the first long-range especially the thorough and extensive work of Houk et al. interactions between the reactants in the entrance channel [22–37] has to be mentioned. and terminating with the finalization of the products in the 1,3-Dipolar cycloadditions are versatile tools for the exit channel. synthesis of heterocyclic molecules [1–5]. Applications We will show that the fate of the reaction is determined range from asymmetric organic synthesis [38, 39], catalysis early on in the van der Waals region of the entrance channel. [40–43], materials science [44], and drug design [45] to Furthermore, we will reveal that all 1,3-dipolar cycloadditions chemical biology [46, 47]. The mechanism of the 1,3- investigated in this work pass through the same reaction dipolar cycloaddition reactions had been controversially phases; however, there are distinct differences between dif- debated starting in the late 1960s when Huisgen [48] ferent classes of cycloadditions. Also, it will become apparent postulated a concerted mechanism and Firestone [49] that there is a difference between reactions with radicaloid and advocated a stepwise biradical mechanism. Numerous biradicaloid character and that the dipolarophile rather than quantum chemical investigations have been performed the 1,3-dipole initiates bond formation. since then to shed light on the mechanism of 1,3-dipolar The current investigation is based on a detailed analysis cycloadditions [33, 50, 51], to provide accurate energetic of the reaction path and the reaction valley, which will lead data [29, 34–36] and kinetic data [24], and to investigate to a better understanding of the mechanism than it is pos- the regioselectivity of these reactions [9, 11, 17, 28]. Cal- sible with a conventional mechanistic analysis based on the culations could solve the Huisgen–Firestone controversy; features of the stationary points along the reaction path. the concerted mechanism is favored for the reactions of the Our focus will be on well-defined transient points along the unsubstituted 1,3-dipoles with ethylene and acetylene [21, reaction path corresponding to bond breaking and bond 26, 27], whereas the stepwise mechanism becomes likely formation, rehybridization, and (bi)radicaloid formation. when the 1,3-dipoles and dipolarophiles are substituted by The results of this work are presented in the following way. radical-stabilizing groups [25, 31, 52]. In Sect. 2, the computational methods used in this work are Still, open mechanistic questions concern the driving described. The results of the quantum chemical investigation force of the cyclization and the electronic factor(s) that of ten 1,3-dipolar cycloadditions are presented and discussed determine(s) the barrier height. Currently, three different in Sect. 3. Conclusions will be drawn in the final section. rate-determining factors are discussed in the literature: (1) the distortion energy of dipole and dipolarophile as defined by Houk et al. [22, 23, 30, 32]; (2) reaction acceleration by 2 Computational methods an excitation of the bending vibrations of the 1,3-dipole [13, 14, 26, 27]; and (3) the biradical character of the 1,3- The current investigation is based on URVA [53–57], the dipole molecule [10, 12]. reaction path Hamiltonian of Miller, Handy, and Adams Most of the previous work on the 1,3-dipolar cycload- [58], and the local vibrational mode description of Konkoli ditions has focused on finding a rationale for the relatively and Cremer [59–62] recently theoretically justified by Zou low reaction barriers because they predominantly deter- and Cremer [63, 64]. URVA has been repeatedly described mine the reaction rate and the yield of a given product. A in previous publications [55, 65–69] as well as several general assumption in this connection is that the similarity review articles [53, 54, 56, 57]. Therefore, we refrain from of the activation energies indicates a similarity of the a detailed account of the theory of URVA and the use of reaction mechanism. local vibrational modes and instead outline the methods In this work, we will show that comparable reaction used in a more qualitative fashion. barriers do not necessarily imply a similarity in the reaction Each 1,3-dipolar cycloaddition system is investigated mechanism. Conversely, two reactions may have a similar with a dual-level approach: (1) A high level such as mechanism although they have different barriers.
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