Density functional theory study of mechanism of forming a spiro-Si-heterocyclic ring compound from Me₂Si⚌Si: and acetaldehyde

3.1 Reaction (1): channels of forming a four-membered Si-heterocyclic ring silylene (P1), methyl transfer products (P1.1, P1.3) and hydrogen transfer products (P1.2)

The geometrical parameters of intermediate (INT1), transition states (TS1, TS1.1, TS1.2, TS1.3) and products (P1, P1.1, P1.2, P1.3) appearing in reaction (1) between Me₂Si⚌Si: and acetaldehyde are given in Fig. 1. The energies are listed in Table 1, and the potential energy profile for the cycloaddition reaction is shown in Fig. 2. According to Fig. 2, it can be seen that reaction (1) consists of five steps: the first step is that the two reactants (R1, R2) form an intermediate (INT1), which is a barrier-free exothermic reaction of 67.7 kJ/mol; the second step is that the intermediate (INT1) isomerizes to a four-membered Si-heterocyclic ring silylene (P1) via the transition state (TS2) with an energy barrier of 24.5 kJ/mol; the third and fourth steps are that the four-membered Si-heterocyclic ring silylene (P1) undergoes Si—Si and C(3)—Si(2) methyl transfer via the transition states (TS1.1 and TS1.3) with energy barriers of 151.1 and 91.4 kJ/mol, respectively, resulting in the formation of products (P1.1 and P1.3). The fifth step is that the four-membered Si-heterocyclic ring silylene (P1) undergoes C(3)—Si(2) hydrogen transfer via the transition state (TS1.2) with energy barrier of 144.0 kJ/mol, resulting in the formation of product (P1.2). The reactions of P1 → P1.1, P1 → P1.2 and P1 → P1.3 are prohibited in thermodynamics, because the energies of P1.1, P1.2 and P1.3 are 55.5, 50.6 and 58.1 kJ/mol higher than that of P1, and the reaction (1) will be ended in product P1.

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Figure 1

Optimized B3LYP/6-31G^∗ geometrical parameters and the atomic numbering for the species in cycloaddition reaction (1). Bond lengths and bond angles are in angstrom and degree, respectively.

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Figure 2

The potential energy profile of the cycloaddition reactions between Me₂Si⚌Si: and MeHC⚌O with B3LYP/6-31G^∗.

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3.2 Reaction (2): channel of forming a spiro-Si-heterocyclic ring compound (P2)

In reaction (3), the four-membered Si-heterocyclic ring silylene (P1) further reacts with acetaldehyde to form a spiro-Si-heterocyclic ring compound (P2). The geometrical parameters of intermediate (INT2), transition state (TS2) and product (P2) appearing in reaction (2) are given in Fig. 3. The energies are listed in Table 1, and the potential energy profile for the cycloaddition reaction is shown in Fig. 2. According to Fig. 2, it can be seen that the process of the reaction (3) is as follows: on the basis of P1 formed in the reaction (1), the P1 further reacts with acetaldehyde (R2) to form an intermediate (INT2), which is also a barrier-free exothermic reaction of 38.3 kJ/mol; and then the intermediate (INT2) isomerizes to a spiro-Si-heterocyclic ring compound (P2) via the transition state (TS2) with an energy barrier of 28.0 kJ/mol.

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Figure 3

Optimized B3LYP/6-31G^∗ geometrical parameters of INT2, TS2, P2 and the atomic numbering for cycloaddition reaction (2). Bond lengths and bond angles are in angstrom and degree, respectively.

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3.3 Reaction (3): channels of forming four-membered Si-heterocyclic ring silylene (INT3) and

Its isomer (P3), H-transfer product (P3.1) and Me-transfer products (P3.2 and P3.3)

The geometrical parameters of the four-membered Si-heterocyclic ring silylene (INT3), transition states (TS3, TS3.1, TS3.2 and TS3.3) and products (P3, P3.1, P3.2 and P3.3) appearing in reaction (3) between Me₂Si⚌Si: and acetaldehyde are given in Fig. 4. The energies are listed in Table 1, and the potential energy profile for the cycloaddition reaction is shown in Fig. 2. According to Fig. 2, it can be seen that reaction (4) consists of five steps: the first step is that the two reactants (R1, R2) form a four-membered Si-heterocyclic ring silylene (INT3), which is a barrier-free exothermic reaction of 181.6 kJ/mol; the second step is that the INT3 isomerizes to a distorted four-membered ring product (P3) through the transition state (TS3) with energy barrier of 7.3 kJ/mol; the third step is that the four-membered Si-heterocyclic ring silylene (INT3) undergoes C(3)—Si(2) hydrogen transfer via the transition state TS3.1 with energy barrier of 10.5 kJ/mol, resulting in the formation of product (P3.1); the fourth and fifth steps are that the four-membered Si-heterocyclic ring silylene (INT3) undergoes Si—Si and C(3)—Si(2) methyl transfer via the transition states TS3.2 and TS3.3 with energy barriers of 53.1 and 67.2 kJ/mol, respectively, resulting in the formation of products (P3.2 and P3.3). The reaction of INT3 → P3.2 is prohibited in thermodynamics because the energy of P3.2 is 16.2 kJ/mol higher than that of INT3. The reaction of INT3 → P3 is the dominant reaction pathway the reaction (3) because the energy barrier of TS3 are 3.2 and 59.9 kJ/mol lower than TS3.1 and TS3.3. According to Figs. 1, 2 and 4 and statistical thermodynamics formula: $P_{\text{T}}(i)=\frac{e^{-\mathrm{\Delta }{G}_{\text{T}}(i)/\mathit{RT}}}{{\sum }_{\text{i}}{e}^{-\mathrm{\Delta }{G}_{\text{T}}(i)/\mathit{RT}}}$ and $\mathrm{\Delta }{G}_{\text{T}}(i)=-\mathit{RT}\ln K_{\text{i}}$ , it can be seen that INT1 and INT3 are isomerized, R1 + R2 → INT1 and R1 + R2 → INT3 are two parallel reactions, the equilibrium distributions of INT1 and INT3 are P_T(INT1) = K(INT1)/[K(INT1) + K(INT3)] ≈ 0.0, P_T(INT3) = K(INT3)/[K(INT1) + K(INT3)] ≈ 1.0, respectively. So, INT3 is the main distribution.

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Figure 4

Optimized B3LYP/6-31G^∗ geometrical parameters of INT3, TS3, P3, TS3.1, P3.1, TS3.2, P3.2, TS3.3, P3.3 and the atomic numbering for cycloaddition reaction (3). Bond lengths and bond angles are in angstrom and degree, respectively.

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3.4 Reaction (4): channel of forming a spiro-Si-heterocyclic ring compound (P4)

In reaction (4), the four-membered Si-heterocyclic ring silylene (INT3) further reacts with acetaldehyde (R2) to form a spiro-Si-heterocyclic ring compound (P4). The geometrical parameters of intermediate (INT5), transition state (TS4) and product (P4) appearing in reaction (4) are given in Fig. 5. The energies are listed in Table 1, and the potential energy profile for the cycloaddition reaction is shown in Fig. 2. According to Fig. 2, it can be seen that the process of the reaction (4) is as follows: on the basis of INT3 formed in the reaction (3), the INT3 further reacts with acetaldehyde (R2) to form an intermediate (INT4), which is also a barrier-free exothermic reaction of 70.9 kJ/mol; and then the intermediate (INT4) isomerizes to a spiro-Si-heterocyclic ring compound (P4) via the transition state (TS4) with an energy barrier of 34.1 kJ/mol. Compared reaction (4) with reaction (3), it is realizing that the two reactions compete mutually due to scrambling for INT3 together. In reaction (3), the energy barrier of INT3 → P3 is 7.3 kJ/mol. However, in reaction (4), INT3 + R2 → P4 can directly reduce the system energy of 70.9 kJ/mol. Therefore, reaction (4) should be the dominant reaction channel.

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Figure 5

Optimized B3LYP/6-31G^∗ geometrical parameters of INT4, TS4, P4 and the atomic numbering for cycloaddition reaction (5). Bond lengths and bond angles are in angstrom and degree, respectively.

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3.5 Theoretical analysis and explanation of the dominant reaction channel

According to the above analysis, reaction (4) should be the dominant reaction channel of the cycloaddition reaction between singlet Me₂Si⚌Si: and acetaldehyde. Namely:

In the reaction, the frontier molecular orbitals of R2 and INT3 are shown in Fig. 6. According Fig. 6, the frontier molecular orbitals of R2 and INT3 can be expressed in schematic diagram 7. The mechanism of the reaction could be explained with the molecular orbital diagram (Fig. 7) and Figs. 1, 4 and 5. According to Figs. 1 and 4, as Me₂Si⚌Si: initially interacts with acetaldehyde, the [2 + 2] cycloaddition of the bonding π-orbitals firstly results in a four-membered Si-heterocyclic ring silylene (INT3). Because INT3 is an active intermediate, so INT3 may further react with acetaldehyde (R2) to form a spiro-Si-heterocyclic ring compound (P4). The mechanism of the reaction can be explained with Figs. 5 and 7, when the INT3 interacts with acetaldehyde (R2), the 3p unoccupied orbital of the Si(2) atom in INT3 will insert the π orbital of acetaldehyde from the oxygen side, then the shift of π-electrons to the p unoccupied orbital forms a π → p donor–acceptor bond, leading to the formation of intermediate (INT4). As the reaction goes on, the ∠C(5)O(2)Si(2)(INT2: 136.9°, TS4: 98.5°, P4: 70.6°) decreases gradually, and the C(5)—O(2) bond(INT4: 1.253 Å, TS4: 1.273 Å, P4: 1.487 Å) gradually elongate, Finally, the Si(2) atom in INT4 hybridizes to an sp³ hybrid orbital after the transition state TS4, forming the stabler spiro-Si-heterocyclic ring compound (P4).

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Figure 6

The frontier molecular orbitals of R2 and INT3.

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Figure 7

A schematic interaction diagram for the frontier orbitals of INT3 and MeHC⚌O(R2).

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