Article Cite This: J. Org. Chem. 2018, 83, 1804−1809
pubs.acs.org/joc
Computational Study of the Isomerization Reactions of Borirane Małgorzata Krasowska† and Holger F. Bettinger* Institut für Organische Chemie, Auf der Morgenstelle 18, 72076 Tübingen, Germany S Supporting Information *
ABSTRACT: Borirane is isoelectronic to the cyclopropyl cation, but is stable toward electrocyclic ring opening to 2-bora-propa-1,3-diyl A, the boron analogue of the allyl cation. A computational investigation using density functional theory (B3LYP) in combination with highly correlated electronic structure theory methods of the coupled-cluster [CCSD(T)] and multireference configuration interaction (MRCI) type in conjunction with basis sets of up to quadruple-ζ quality reveal that formation of A is endothermic by roughly 15 kcal mol−1 and that A collapses almost without barrier (0.2 kcal mol−1) to borirane. The vinylborane isomer B is more stable than borirane and its formation is associated with a barrier of 36−38 kcal mol−1. Methyl methylideneborane E, only slightly less stable than B, can only be accessed by pathways involving barriers of at least 60 kcal mol−1.
■
corresponding boratiranes.6−9 Even earlier, bicyclic boriranes were obtained by Klusik and Berndt.10−12 Uncoordinated boryl substituted boriranes undergo various ring opening reactions at elevated temperatures according to Berndt et al.13 Either breaking of the C−C bond or the C−B bond opposite or adjacent to the boryl substituent can occur. What is remarkable is that the preferred mode of ring opening depends very strongly on the electronic and steric effects of the boryl substituent connected to the three-membered ring.13 Although the mechanisms for ring opening have not been elucidated, the intermediacy of a diboryl carbene14 was suggested in one of the reactions.13 A closely related mechanism was proposed based on theory and confirmed experimentally to be operative in the ring opening of cyclopropane.15,16 Boriranes were invoked as reactive intermediates,17,18 but remained rather scarce boron containing heterocycles,13,19−21 until more recently significant advances were made. Wang et al. synthesized derivatives of boranorcaradienes photochemically that are stabilized by intramolecular interaction with nitrogencontaining aromatic bases.22−30 Braunschweig et al.31,32 as well as Curran et al.33 obtained boriranes that are stabilized by coordination to N-heterocyclic carbenes. Khafizova et al. reported that the cycloboration of terminal olefins using BCl3·SMe2, BF3·THF in the presence of magnesium, and Cp2TiCl2 catalyst gives 1-halo-2-alkyl-boriranes as adducts with the Lewis base as synthetic intermediates.34 Most remarkably, catalyzed cycloboration of terminal olefins with dichlorophenylborane yields 1-phenyl-2-alkyl-boriranes that could be isolated in substance and characterized by NMR techniques according to Khafizova et al.35 Early36,37 theoretical investigations have established that among C2H5B isomers vinylborane is more stable than borirane,38,39 and this was confirmed by more sophisticated
INTRODUCTION The highly strained cyclopropyl cation is known to easily undergo disrotatory ring opening.1−4 Somewhat surprisingly, the isoelectronic borirane (Chart 1) is quite resistant toward ring opening. Chart 1. Borirane and Cyclopropyl Cation
Denmark et al.5 demonstrated this in an elegant stereochemical experiment. Trapping of (2S,3S)-1,2,3-triphenylborirane (1), generated in situ from the (S)-nicotine adduct (2), by pyridine is faster than ring opening to the 2-bora-propa-1,3-diyl (2-boraallyl) derivative 3 even at temperatures as high as 150 °C (Scheme 1). Denmark et al.5 obtained the nicotine adduct from a di-πmethane rearrangement as the first example of a Lewis base adduct of a simple borirane. They followed earlier work of Schuster and co-workers who observed that the photochemical di-π-methane rearrangement of tetraarylborates yields the Scheme 1. Stereochemical Test for Generation and Valence Isomerization of Borirane 1 According To Denmark et al.5
Received: October 26, 2017 Published: January 31, 2018 © 2018 American Chemical Society
1804
DOI: 10.1021/acs.joc.7b02715 J. Org. Chem. 2018, 83, 1804−1809
Article
The Journal of Organic Chemistry
(MRCI),53,54 and included the effects of unlinked quadruple excitations via Davidson’s correction (MRCI+Q).55,56 Unless noted otherwise, the MRCI computations are based on complete active space self-consistent field (CASSCF) reference wave functions that were obtained by taking the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) and two electrons into account, i.e., CASSCF(2,2). Only for 2-bora-prop-1,3-diyl (compound A) a larger active space consisting of the three π orbitals and two electrons, i.e., CASSCF(2,3), was taken into account. The structure of A was also computed at the complete active space second order perturbation theory level (CASPT2) as described by Werner57 using an active space that consisted of three π orbitals and two electrons and the cc-pVTZ basis set. Geometry optimization at the CASPT2/ccpVTZ level used analytic gradients,58 while harmonic vibrational frequencies were subsequently computed by finite differences of analytic gradients. All electron-correlated computations employed the frozen core approximation and were performed using the Molpro program.59,60 Spherical harmonic basis functions were used throughout this study.
computational analyses.40 Ring opening of borirane to 2borapropane-1,3-diyl was identified to be endergonic, in contrast to the strongly exothermic formation of the allyl cation from cyclopropyl cation.13,38,39 The diradical 2borapropane-1,3-diyl has more recently received some interest in the context of nonlinear optical properties. 41 The interconversion of C2H5B isomers, involving borirane, has been studied by Taylor et al. in 1986 at the MP4/6-31G**// HF/3-21G level of theory.39 The authors state that pathways were “somewhat troublesome to determine and only limited success was achieved.”39 This is the only computational study of reaction pathways for ring opening of borirane that we are aware of. The recent advances in the chemistry of Lewis base adducts of boriranes as well as free boriranes call for a modern computational treatment of the potential energy surface (PES) for interconversion of borirane into its isomers. This provides an understanding of the kinetic stability of the borirane ring and allows judging the suitability of intramolecular transformations for synthetic purposes.
■
■
RESULTS AND DISCUSSION The investigated isomers and transition states connecting them are summarized in Scheme 2, while the computed structures are displayed in Figure 1 and their energies are given in Table 1. An energy profile of the potential energy surface is shown in Figure 2. Electrocyclic Ring Opening to 2-Bora-propa-1,3-diyl A. The electrocyclic ring opening of borirane is expected to yield 2-borapropane-1,3-diyl A. Early computations at the HF level found this compound to be a saddle point,39 while the more recent study by Kubota et al.41 appears not to have investigated this aspect. We find A to be a minimum at the UB3LYP/6-311+G**, as well as at the CASPT2/cc-pVTZ levels of theory. Using the UB3LYP geometry, the electronic structure of A was investigated with the multireference configuration interaction (MRCI) method based on a complete active space involving two electrons and three orbitals (HOMO, LUMO, LUMO+1). The weight of the reference configuration is only 0.758 while that of the doubly excited configuration (HOMO,HOMO → LUMO,LUMO) is 0.125. This indicates significant diradical character for A (0.25, by taking twice the weight of the doubly excited configuration), somewhat less than obtained previously (0.37) by projected spin-unrestricted HF.41 A hallmark of compounds with pronounced diradical character is a small vertical singlet−triplet energy difference. At the UB3LYP/6-311+G** level used for optimization of the singlet geometry, this splitting amounts to 21.3 kcal mol−1. To check the reliability of the symmetry broken treatment, this energy difference was computed at a number of levels of theory and with correlation consistent basis sets up to quadruple-ζ quality (Table 2). The HF method, as expected, underestimates the stability of the singlet state significantly. This is qualitatively corrected by taking into account the doubly excited configuration already at the CASSCF level. At the MRCI level including multireference Davidson correction, the energy difference is 24.4 kcal mol−1. Despite the large weight of the doubly excited configuration that causes a large D1 diagnostic value of 0.085 in the coupled cluster calculation (the T1 diagnostic is only 0.022), CCSD(T) and MRCI+Q agree to within 0.4 kcal mol−1. Increase of the basis set to cc-pVQZ has no effect on the energy difference, suggesting that this value is already converged at the triple-ζ level. The isomer A is 15−18 kcal mol−1 higher in energy than borirane according to our most sophisticated computations.
COMPUTATIONAL METHODS
Geometry optimizations were performed with the B3LYP42,43 functional as implemented44 in Gaussian 0945 in conjunction with the 6-311+G** basis set.46 The stability of the RKS solution was probed and a UKS treatment was chosen if a lower energy solution was obtained. This happened to be the case for 2-bora-propa-1,3-diyl (A), TS8 (Abis), TS1, and 1-bora-3-propylidene F (Scheme 2, Figure
Scheme 2. Minima and Interconnecting Transition States on the C2H5B Potential Energy Surface Investigated in the Present Study
1). Harmonic vibrational frequencies were computed to identify minima and transition states and to obtain zero point vibrational energies (ZPVE) that were used for correcting classical energy barriers. Starting from selected transition states, intrinsic reaction coordinates (IRC) were computed in order to identify the minima that are interconnected.47,48 The B3LYP/6-311+G** geometries were used for further refinement of energies by single point energy computations using Dunning’s49 correlation consistent basis sets of triple-ζ and quadruple-ζ quality, cc-pVTZ and cc-pVQZ, in conjunction with highly correlated single reference and multireference theories. These include coupled-cluster theory with single, double, and a perturbative estimate of triple excitations, CCSD(T).50,51 The partially spin projected algorithm was used for open-shell species [RHF-RCCSD(T)].51,52 In addition, we performed internally contracted multiconfiguration-reference configuration interaction computations 1805
DOI: 10.1021/acs.joc.7b02715 J. Org. Chem. 2018, 83, 1804−1809
Article
The Journal of Organic Chemistry
Figure 1. Structures of minima (borirane, A−F) and transition states (TS1 to TS10) computed at the B3LYP/6-311+G** level of theory. Important bond lengths and angles are given in Å and degrees. The boron atom is depicted in gray scale.
The transition state for disrotatory ring opening TS1 was located at the UB3LYP/6-311+G** level of theory. It is of Cs symmetry and has an imaginary vibrational frequency of i225 cm−1. The CBC bond angle is 108.8° indicative of a late transition state. The transition state is only 0.4 kcal mol−1 higher in energy than A, indicating that this isomer corresponds to an extremely shallow minimum on the potential energy surface. To evaluate the reliability of the UB3LYP treatment, we have also performed multireference computations (Table 3). We note that all the methods agree that the barrier for disrotatory electrocyclic ring closure of A is very small. Both MRCI+Q and CCSD(T) arrive at a barrier of 0.2 kcal mol−1 showing that the UB3LYP method is sufficiently accurate for describing this reaction. Inclusion of zero-point vibrational energy computed at UB3LYP/6-311+G** hardly has an effect on the barrier height (
