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. 2023 Aug 8;8(33):30410–30420. doi: 10.1021/acsomega.3c03560

Quantum Chemical Study of the Cycloaddition Reaction of Tropone with 1,1-Diethoxyethene Catalyzed by B(C6F5)3 or BPh3

Ken Sakata 1,*, Sarina Suzuki 1, Tsubasa Sugimoto 1, Takeshi Yoshikawa 1
PMCID: PMC10448487  PMID: 37636958

Abstract

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Cycloaddition reaction of tropone with 1,1-diethoxyethene catalyzed by Lewis acid (LA), B(C6F5)3 or BPh3, was examined by using ωB97X-D-level density functional theory (DFT) calculations. In the absence of LA, the reaction proceeds in a stepwise fashion to form two chemical bonds, first between the C2 atom in tropone and the C2 atom in ethene and then between the C5 atom in the former and the C1 atom in the latter. When B(C6F5)3 is attached to the O atom in tropone, the C5 atom in tropone is attacked preferentially by the C1 atom in ethene in the second stage. The attack of the O atom in tropone is shown to be less likely; thus, the [4 + 2] addition is favored in the B(C6F5)3-catalyzed reaction. In contrast, the attack of the O atom in the BPh3-attached tropone to the C1 atom in ethene is preferred over the attack of the C5 atom, indicating that the [8 + 2] cycloaddition instead of the [4 + 2] cycloaddition proceeds in the BPh3-catalyzed reaction. Whether the C1 atom in ethene is attacked by C5 or by O in the second bond formation step is shown in this study to be governed mainly by the nucleophilicity of σ-lone pair electrons of the carbonyl O atom of tropone in the presence of LA. These results are consistent with the experiments reported by Li and Yamamoto.

Introduction

Tropone and its derivatives have attracted the attention of both synthetic and theoretical chemists for more than half a century.1−5 A variety of higher-order cycloaddition reactions utilizing its nonbenzenoid aromaticity, such as [6 + 3], [6 + 4], [8 + 2], and [8 + 3] additions, have been reported.6−10 In contrast, the Diels–Alder [4 + 2] cycloaddition reaction is relatively limited because of the electron-deficient nature of tropones, although the reaction can directly provide the bicyclo[3.2.2] structures found in natural products or bioactive compounds.11−16 Nozoe et al. examined the reaction of tropolones with maleic anhydride under reflux conditions and obtained [4 + 2] adducts.11 Takeshita et al. reported the Diels–Alder reactions of tropones under high-pressure conditions.12 Thus, harsh conditions are required for the Diels–Alder reaction of tropones.

One method for overcoming the low reactivity of tropones toward the [4 + 2] cycloaddition reaction is to increase the nucleophilicity for use as dienes in the normal-electron-demand Diels–Alder reactions. Jørgensen et al. reported Brønsted base-catalyzed asymmetric [4 + 2] cycloaddition reactions of tropolones.17 Okamura et al. examined the Diels–Alder reaction between α-tropolone and electron-deficient dienophiles prompted by Et3N or silica gel.18 More recently, Wu et al. proposed a theoretical carbonyl umpolung strategy for activating tropone.19

Another method is the use of Lewis acid (LA) to activate tropones as dienes in the inverse-electron-demand Diels–Alder (IEDDA) reaction with electron-rich dienophiles. Li and Yamamoto reported the IEDDA reaction of tropone with electron-rich dienophiles.20 Reaction of tropone (1) with 1,1-diethoxyethene (2) catalyzed by B(C6F5)3 gave the [4 + 2] cycloadduct, 9,9-diethoxybicyclo[3.2.2]nona-3,6-dien-2-one (3), while the reaction catalyzed by other LAs such as BPh3, Me2AlCl, BF3·OEt2, and TiCl4 provided 2,2-diethoxy-3,3a-dihydro-2H-cyclohepta[b]furan (4), which corresponds to the [8 + 2] cycloadduct. Both 3 and 4 were obtained when Me3Al, Et2Zn, and Ti(OiPr)4 were used as the LA catalyst. They further applied the bicyclo[3.2.2] compounds obtained by the B(C6F5)3-catalyzed reaction to the formal synthesis of platencin.21

The difference in catalysis toward the IEDDA reaction among those LAs is of great interest to us. Domingo and Pérez recently examined the reactions of tropone with cyclic ketene acetal in the presence of B(C6F5)3 or BF3 by using molecular electron density theory, and concluded that a series of weak attractive/repulsive interactions control the selectivity in giving [4 + 2] or [8 + 2] cycloadduct.22 In this study, we focused on the difference in catalytic roles for the cycloaddition reactions between B(C6F5)3 and BPh3, both of which have the same molecular structural frameworks, and examined the precise reaction mechanisms for the reaction of 1 with 2 catalyzed by B(C6F5)3 or BPh3, as shown in Scheme 1, by using ωB97X-D-level density functional theory (DFT) calculations.

Scheme 1. Reaction of 1 with 2 Catalyzed by a Lewis Acid (LA).

Scheme 1

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Results and Discussion

Reaction without LA

We first examine the cycloaddition between 1 and 2 in the absence of LA. Stepwise pathways were found both for [4 + 2] and for [8 + 2] cycloadditions (Figure 1).23 In the [4 + 2] addition pathway, nucleophilic attack of the C2 atom in 2 by the C2 atom in 1 gives an intermediate complex INTA4 through the transition state TS1A4. In INTA4, the distances between C2 in 1 and C2 in 2 and between C5 in 1 and C1 in 2 are 1.59 and 3.38 Å, respectively (Figure S2). The stability analysis for the solution of the closed-shell Kohn–Sham equations showed that the obtained Kohn–Sham wave function of INTA4 is stable. The bond formation between C5 in 1 and C1 in 2 proceeds through the second transition state TS2A4 to afford the [4 + 2] adduct 3. The Gibbs free energies of TS1A4, INTA4, and TS2A4, relative to the initial state (1 + 2), ΔG298K, are located relatively high, at 25.1, 18.9, and 22.1 kcal/mol, respectively (the activation energies, ΔG298K‡, of TS1A4 and TS2A4 are 21.3 and 3.2 kcal/mol, respectively24). On the other hand, the transition state TS1A8, in which the direction of the C=C bond in dienophile is different from that in TS1A4, leads to the intermediate complex INTA8. The complex INTA8 provides the [8 + 2] product 4 through the transition state for the second bond formation between the O atom in 1 and the C1 atom in 2, TS2A8. The ΔG298K of TS1A8, 27.5 kcal/mol (ΔG298K‡ = 22.9 kcal/mol), is higher than that of TS1A4, while the ΔG298K of TS2A8, 20.3 kcal/mol (ΔG298K‡ = 1.1 kcal/mol), is slightly lower than that of TS2A4 (ΔG298K = 22.1 kcal/mol; ΔG298K‡ = 3.2 kcal/mol). In addition, the calculation revealed that the intermediate complex in the [4 + 2] addition pathway, INTA4, is transformed into INTA8 in the [8 + 2] addition pathway via rotation around the bond formed first between the C2 atom in 1 and the C2 atom in 2 through another transition state, TS3A4. One notes that the relative free energy of TS3A4 is not high, 22.5 kcal/mol (ΔG298K‡ = 3.4 kcal/mol), compared with those of other transition states.

Figure 1.

Figure 1

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Gibbs free energy diagrams for the reaction of 1 with 2 without LA (kcal/mol). The [4 + 2] and [8 + 2] addition pathways are represented in red and blue, respectively.

Thus, the first bond formation through TS1A4 is preferred over TS1A8. The resulting INTA4 leads, however, to TS2A4 and also to TS3A4 because these transition state structures have almost the same relative free energies (22.1 and 22.5 kcal/mol). This indicates that both 3 and 4 are formed in the addition of 1 and 2 in the absence of a catalyst (Scheme 2 and Figure S3). Experimentally, Takeshita et al. examined the cycloaddition reaction of tropone with 1,1-diethoxyethane under thermal conditions and observed [8 + 2], [4 + 2], and [6 + 2] cycloadducts.12c We could locate the transition state structure for bond formation between the C7 atom in 1 and the C1 atom in 2 to afford the [6 + 2] adduct 5, TS4A4 (Figure S2). The ΔG298K of TS4A4, 25.1 kcal/mol, is seen, however, to be higher than those of TS2A4 and TS3A4 to give 3 and 4, respectively. We also examined transition state structures with other conformations of diethoxy moieties for the first bond formation (Figure S4), and TS1A4 has the lowest free energy among the obtained structures. Moreover, similar stepwise pathways were obtained for TS1B4 and TS1B8 (Figure S5).

Scheme 2. Pathways for the Reaction of 1 with 2 without LA.

Scheme 2

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B(C6F5)3-Catalyzed Reaction

It was shown above to be difficult to obtain selectively the [4 + 2] or [8 + 2] cycloadduct in the reaction between 1 and 2 without any catalyst. The formation of two bonds should take place in a stepwise manner to avoid the strong overlap repulsion that would intervene in the cycloadditions.25 We examine next the pathways for the reaction between tropone attached to B(C6F5)3 (1···B(C6F5)3), which is lower in Gibbs free energy by 13.4 kcal/mol than (1 + B(C6F5)3), and 2. As in the case of noncatalyzed reaction system, stepwise pathways were found both for the [4 + 2] and for the [8 + 2] cycloadditions (Figures 2 and S7). In the [4 + 2] addition pathway, the first bond formation through the transition state, TS1FA4, is followed by the second bond formation through the intermediate complex INT1FA4 and the transition state TS2FA4 (Figure 3).26 The ΔG298K values of TS1FA4, INT1FA4, and TS2FA4, are 13.7, 0.6, and 11.3 kcal/mol, respectively, much lower than those in the noncatalyzed reaction pathway (ΔG298K‡ of TS1FA4 and TS2FA4 are 11.5 and 10.7 kcal/mol, respectively). In the [8 + 2] addition pathway, on the other hand, the first bond formation via the transition state TS1FA8 (ΔG298K = 17.8 kcal/mol; ΔG298K‡ = 16.7 kcal/mol) gives an intermediate complex INT1FA8 (ΔG298K = −0.7 kcal/mol). This complex INT1FA8, in which the dienophile moiety is located above the tropone plane, is then transformed into the other complex INT2FA8 (ΔG298K = 4.0 kcal/mol), in which the dienophile moiety lies on the side of tropone (Figures S7 and S8). The complex INT2FA8 is further transformed into the other conformation complex INT5FA8 (INT2FA8 → TS3FA8 (ΔG298K = 5.8 kcal/mol) → INT3FA8 (ΔG298K = 2.2 kcal/mol) → TS4FA8 (ΔG298K = 6.3 kcal/mol) → INT4FA8 (ΔG298K = 4.7 kcal/mol) → TS5FA8 (ΔG298K = 6.0 kcal/mol) → INT5FA8 (ΔG298K = 5.5 kcal/mol; Figures S7 and S8)), and then the complex INT5FA8 provides INT6FA8 via the transition state, TS6FA8, for the bond formation between the O atom in tropone and the C1 atom in ethene (Figure 3 for the structure of TS6FA8). The LA, B(C6F5)3, is now dissociated from the O atom in tropone through TS7FA8 to give finally the [8 + 2] cycloadduct 4···B(C6F5)3 (Figure S8). The distance between the boron atom in B(C6F5)3 and the oxygen atom in tropone at INT5FA8, TS6FA8, INT6FA8, TS7FA8, and 4···B(C6F5)3 is 1.51, 1.67, 1.80, 1.89, and 4.18 Å, respectively. The ΔG298K of TS6FA8 and TS7FA8 is 16.9 and 16.3 kcal/mol, much higher than that of TS2FA4 for the second bond formation in the [4 + 2] addition pathway (ΔG298K‡ of TS6FA8 and TS7FA8 is 11.4 and 1.6 kcal/mol, respectively). The intermediate complex in the [4 + 2] addition pathway, INT1FA4, is transformed into the intermediate complex in the [8 + 2] pathway, INT1FA8 (INT1FA4 (ΔG298K = 0.6 kcal/mol) → TS3FA4 (ΔG298K = 2.0 kcal/mol) → INT2FA4 (ΔG298K = −0.4 kcal/mol) → TS4FA4 (ΔG298K = 6.4 kcal/mol) → INT3FA4 (ΔG298K = −0.5 kcal/mol) → TS5FA4 (ΔG298K = 1.7 kcal/mol) → INT4FA4 (ΔG298K = −1.0 kcal/mol) → TS6FA4 (ΔG298K = 0.5 kcal/mol) → INT5FA4 (ΔG298K = −0.7 kcal/mol) → TS7FA4 (ΔG298K = 0.9 kcal/mol) → INT1FA8 (ΔG298K = −0.7 kcal/mol); Figures 2, S7, and S8). One sees in Figure 2 shows that the [4 + 2] addition pathway should take place preferentially in the B(C6F5)3-catalyzed reaction (Scheme 3).

Figure 2.

Figure 2

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Gibbs free energy diagrams for the reaction of 1 with 2 catalyzed by B(C6F5)3 (kcal/mol). The [4 + 2] and [8 + 2] addition pathways are represented in red and blue, respectively.

Figure 3.

Figure 3

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Structures of TS1FA4, TS2FA4, TS1FA8, and TS6FA8. Distances are shown in angstrom.

Scheme 3. Pathways for the Reaction of 1 with 2 Catalyzed by B(C6F5)3.

Scheme 3

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BPh3-Catalyzed Reaction

The stepwise pathways for the reaction between tropone attached to BPh3 (1···BPh3), which is lower in Gibbs free energy by 1.2 kcal/mol than (1 + BPh3), and 2 were examined next (Figures 4, 5, S9, and S10). For the [4 + 2] addition pathway, the first bond formation through the transition state, TS1HD4, is followed by the second bond formation through intermediate complex INT1HD4 and transition state TS2HD4 (Figure 5 for the structures of TS1HD4 and TS2HD4). The ΔG298K of TS1HD4, INT1HD4, and TS2HD4 is 17.5, 3.2, and 13.4 kcal/mol, respectively, which are slightly higher than the corresponding values in the B(C6F5)3-catalyzed pathway (ΔG298K‡ of TS1HD4 and TS2HD4 is 15.1 and 10.2 kcal/mol, respectively).27 In the [8 + 2] addition pathway, on the other hand, the ΔG298K of the transition state for the first bond formation TS1HD8 and of the resulting intermediate complex INT1HD8 is 21.1 and 5.4 kcal/mol, respectively, being higher than those of TS1HD4 and INT1HD4 (ΔG298K‡ of TS1HD8 is 18.9 kcal/mol). The migration of BPh3 via TS2HD8 (ΔG298K = 7.3 kcal/mol) and the rotation of a phenyl ring in BPh3 via TS3HD8 (ΔG298K = 6.5 kcal/mol) provide complex INT3HD8 (ΔG298K = 5.1 kcal/mol; Figure S9). Further migration of both BPh3 and the dienophile moiety gives complex INT4HD8 (ΔG298K = 0.4 kcal/mol) via TS4HD8 (ΔG298K = 10.6 kcal/mol). Finally, bond formation between the O atom in 1 and the C1 atom in 2, accompanied by a simultaneous bond breaking between the O atom in 1 and the B atom in BPh3, gives the [8 + 2] cycloadduct 4···BPh3 via the transition state TS5HD8 (ΔG298K = 10.3 kcal/mol and ΔG298K‡ = 9.9 kcal/mol; Figure 5 for the structure of TS5HD8). The pathway from the complex INT1HD8 to 4···BPh3 is lower in free energy than TS2HD4 in the [4 + 2] pathway (ΔG298K = 13.4 kcal/mol), although TS1HD8 in the [8 + 2] pathway is higher in free energy than TS1HD4 in the [4 + 2] pathway.

Figure 4.

Figure 4

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Gibbs free energy diagrams for the reaction of 1 with 2 catalyzed by BPh3 (kcal/mol). The [4 + 2] and [8 + 2] addition pathways are represented in red and blue, respectively.

Figure 5.

Figure 5

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Structures of TS1HD4, TS2HD4, TS1HD8, TS5HD8, and TS3HD4. Distances are shown in angstrom.

The intermediate complex in the [4 + 2] addition pathway, INT1HD4, is transformed easily into the complex in the [8 + 2] pathway, INT1HD8, via TS3HD4 (Figure 5 for the structure of TS3HD4). The ΔG298K of TS3HD4 is 11.3 kcal/mol, which is lower than that of TS2HD4 (ΔG298K = 13.4 kcal/mol; Figure 4). These results indicate that the [8 + 2] cycloadduct is produced preferentially through the transformation of INT1HD4 to INT1HD8 via TS3HD4 (Scheme 4).

Scheme 4. Pathways for the Reaction of 1 with 2 Catalyzed by BPh3.

Scheme 4

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Reactivity and Selectivity

We investigated orbital interactions for the noncatalyzed [4 + 2] cycloaddition by using the interaction frontier orbitals (IFOs)28,25 calculated at the ωB97X/6-311G**(6d)//ωB97X-D(IEFPCM)/6-311G**(5d) level of theory. Electron delocalization from ethene to tropone at TS1A4 is represented by a pair of orbitals (ϕ1′; ψ1′) (Figure S11a). The orbital ϕ1′ consists of the unoccupied Kohn–Sham orbitals of the tropone fragment, showing a large amplitude on the C2 atom. The orbital ψ1′ is the π bonding orbital localized on the C=C bond of ethene, given by a linear combination of the occupied Kohn–Sham orbitals of the ethene fragment. The orbitals ϕ1′ and ψ1′ are located at +0.07 and −7.88 eV in energy, respectively. Electron delocalization from tropone to ethene is governed by a pair of orbitals (ϕ2′; ψ2′) (Figure S11a). The orbital ϕ2′ is localized at the pπ orbital at the C2 atom, which is composed of the occupied canonical orbitals of tropone. The orbital ψ2′ is the π antibonding orbital localized on the C=C bond, given by a combination of the unoccupied orbitals of ethene. The orbitals ϕ2′ and ψ2′ are located at −9.65 and +7.89 eV, respectively. The energy gap between ϕ2′ and ψ2′ is much larger than that between ϕ1′ and ψ1′. That is, electron delocalization from ethene to tropone, inverse-electron demand, is essential for the first bond formation step of the [4 + 2] cycloaddition.29

At TS2A4 for the transition state structure in the second bond formation step giving the [4 + 2] adduct, electron delocalization from ethene to tropone is represented by a pair of orbitals (ϕ3′; ψ3′), as illustrated in Figure S11b. The orbitals, ϕ3′ and ψ3′ are localized around the bond formed through TS1A4, showing that electron delocalization remains of importance in strengthening the first bond between the C2 atom in tropone and the C2 atom in ethene. A pair of orbitals (ϕ4′; ψ4′), which shows the electron delocalization from tropone to ethene, come to have large amplitudes in between the C5 atom in tropone and the C1 atom in ethene. The energy gap between ϕ4′ and ψ4′ has been reduced compared with that between ϕ2′ and ψ2′ at TS1A4 (the orbitals ϕ4′ and ψ4′ are located at −8.89 and +2.34 eV in energy, respectively). Thus, the latter electron delocalization plays an important role in the formation of a bond between the C5 atom in tropone and the C1 atom in ethene.

Based on the results for the noncatalyzed cycloaddition, we next examined the first bond formation step in the [4 + 2] addition pathway in the presence of an LA. At TS1FA4, two pairs of interacting orbitals similar in shape to those in TS1A4 were obtained (Figures S12 and S13), indicating that the reaction mechanism is essentially unchanged by the attachment of B(C6F5)3. However, the electron-accepting orbital ϕ1″ is lowered by 2.00 eV compared to the orbital ϕ1′ in TS1A4. The electron-accepting ability of the C2 atom in tropone is seen to be strengthened significantly by the attachment of B(C6F5)3.30 Meanwhile, the electron-donating orbital ψ1″ of ethene is slightly elevated by 0.23 eV in TS1FA4 relative to the orbital ψ1′ in TS1A4. In response to the enhancement of the acidic character of the C2 atom, the energy gap between the electron-donating and electron-accepting levels is significantly reduced in TS1FA4, compared with that in TS1A4, to facilitate electron delocalization. The first role of the LA is the strengthening of electrophilicity of the C2 atom, consistent with findings reported by Domingo and Pérez.22 At TS1HD4, the energy level of the electron-accepting orbital in tropone ϕ1″ (Figure S12) is 1.45 eV lower than the orbital ϕ1′ in TS1A4, but 0.55 eV higher than the orbital ϕ1″ in TS1FA4. This signifies that the electron-accepting ability of the C2 atom in the BPh3-attached tropone is weaker than that in the B(C6F5)3-attached tropone but is significantly stronger than that in tropone without any LA. This difference in the strength of the electron-accepting ability of C2 is reasoned in terms of the decrease in electron population and polarization of charges induced by the LA on the tropone framework. The natural population atomic charge of C2 and the sum of the atomic charge in tropone at TS1HD4 are shown to be −0.165 and −0.064, and −0.129 and −0.018 at TS1FA4.31,32 They were −0.182 and −0.466, respectively, in the noncatalyzed case.

For the second bond formation step in the [4 + 2] addition pathway in the presence of the LA, electron delocalization from tropone to ethene is represented by a pair of orbitals, (ϕ4″; ψ4″) at TS2FA4 and TS2HD4 (Figure S14). The electron-donating orbitals, ϕ4″ in B(C6F5)3- and BPh3-attached tropone, are −10.98 and −10.24 eV, respectively, showing that the nucleophilicity of the pπ-orbital in B(C6F5)3-attached tropone is lower than that of BPh3-attached tropone.

Next, we examine the orbital interactions at the transition state, TS6FA8 or TS5HD8, for bond formation between the O atom in B(C6F5)3- or BPh3-attached tropone and the C1 atom in ethene in the [8 + 2] addition pathway. Electron delocalization from tropone to ethene at TS6FA8 is represented by a pair of orbitals (ϕ5″; ψ5″), as illustrated in Figure 6a. The electron-donating orbital ϕ5″ is localized in the σ-lone pair orbital at the O atom, while the electron-accepting orbital ψ5″ is the π-antibonding orbital localized on the C=C bond in ethene. At TS5HD8, a pair of orbitals very similar in shape was obtained (Figure 6b). The energy of the electron-donating orbital ϕ5″, −11.14 eV, is higher than that in the B(C6F5)3 attached tropone, −12.32 eV, indicating that the electron-donating ability of the σ-lone pair electrons at the O atom is considerably stronger in the BPh3-attached tropone.

Figure 6.

Figure 6

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Pair of interacting orbitals (ϕ5″; ψ5″) for (a) TS6FA8 and (b) TS5HD8 calculated at the ωB97X-D/6-311G**(6d)//ωB97X-D(IEFPCM)/6-311G** level of theory. The orbitals ϕ5″ for TS6FA8, ψ5″ for TS6FA8, ϕ5″ for TS5HD8, and ψ5″ for TS5HD8 are located at −12.32, +2.05, −11.14, and +1.87 eV in energy, respectively.

In summary, the attachment of the LA, B(C6F5)3 or BPh3, to tropone strengthens the electrophilicity of the pπ-orbital, having a large amplitude on the C2 atom in tropone. The electrophilicity of the C2 atom in the B(C6F5)3-attached tropone is seen to be higher, as is expected from the stronger Lewis acidity of B(C6F5)3 than that of BPh3.33,34 Thus, the first bond formation between the C2 atom in the B(C6F5)3-attached tropone and the C2 atom in ethene is much easier in this step. In contrast, the LA weakens the electron-donating character of both the pπ-orbital in the tropone ring and the also σ-lone pair orbital on the O atom in tropone. In particular, the nucleophilicity of the σ-lone pair electrons on the O atom, which plays the dominant role in the second bond formation in the [8 + 2] cycloaddition, is weakened markedly by the attachment of the LA. Here, the effect of LA is seen to be more significant in the B(C6F5)3-attached tropone.35 Thus, the [8 + 2] addition is suppressed and the [4 + 2] addition proceeds in the case of B(C6F5)3, whereas the [8 + 2] addition is preferred in the case of BPh3.

Conclusions

We examined the cycloaddition reaction of tropone with 1,1-diethoxyethene catalyzed by an LA, B(C6F5)3 or BPh3, by using ωB97X-D-level DFT calculations. In the absence of a LA, we identified two stepwise pathways, the [4 + 2] and [8 + 2] additions. The first bond formation between C2 in tropone and C2 in 1,1-diethoxyethene prefers the [4 + 2] pathway, but the bond formation in the second stage between the C5/O in tropone and C1 in the ethene prefers the [8 + 2] process. In addition, the calculations revealed a path to transfer from the [4 + 2] pathway to the [8 + 2] pathway. Thus, both the [4 + 2] and [8 + 2] adducts were formed under a thermal condition. The LA-catalyzed additions take place in two stages, as is the case in the noncatalyzed cycloadditions. In the B(C6F5)3-catalyzed reaction, the attack of the C1 atom in ethene by the C5 atom in tropone is preferred to the attack by the O atom in tropone in the second stage. On the contrary, the attack of the C1 atom in ethene by the O atom in tropone is preferred in the BPh3-catalyzed reaction, indicating that the formation of the [8 + 2] adduct is favored in this case. Whether C1 of the dienophile is attacked by C5 or by O of tropone in the second bond formation step is related to the nucleophilicity of the σ-lone pair electrons at the LA-attached O atom in tropone. These results are consistent with the experimental results reported by Li and Yamamoto.

Computational Details

DFT calculations were carried out with the Gaussian0936 program package. Geometry optimization and analytical vibrational frequency analysis were performed by the restricted Kohn–Sham DFT by using the long-range corrected hybrid functionals with dispersion corrections (ωB97X-D).37,38 A larger grid (superfinegrid) was used in the numerical integration.36 Pople’s 6-311G** basis set was used for the Gaussian basis functions (5d polarization functions).39 The polarizable continuum model with integral equation formalism (IEFPCM)40 was used for the solvent effects of dichloromethane. To confirm that the obtained transition state structures connect the reactant and product structures, IRC calculations41 or structural optimizations from the initial structures which were displaced along the imaginary frequency mode of the transition states were performed. To explore the conformers of 2, the conformer search program CONFLEX42 was initially used and the structures with lower energies were then optimized by using DFT calculations.

Acknowledgments

K.S. thanks Emeritus Professor Hiroshi Fujimoto of Kyoto University for his valuable discussion and comments. This work was supported by the JSPS KAKENHI, Grant Numbers 20K05532, 20H02733, and 20H05671, Japan. Some of the calculations were performed using the Research Center for Computational Science, Okazaki, Japan (Project: 23-IMS-C047). T.Y. and K.S. are grateful to the Center for generous permission to use its computing facilities.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c03560.

  • Tables listing energies and geometries and figures containing optimized structures (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao3c03560_si_001.pdf (1.6MB, pdf)

References

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