Molecular Simulation of Potential Energies, Steric Changesand Substituent Effects in Urethane Formation Reactionsfrom Isocyanates

Abstract

ウレタン樹脂の製造と物性に関する基本的情報である，イソシアナートのウレタン化反応性と触媒作用，生成するカルバミン酸とそのエステルの脱炭酸と安定性および副生物の挙動について，計算化学を用いて検 討した．その際各節に示す基本的な実験結果につき，反応過程のエネルギーと立体化学変化をMOPAC-PM6 法 でシミュレーションして解析した．置換フェニルイソシアナートとアルコールとのウレタン化反応性が，フロンティア軌道HOMOとLUMOのエネルギーを利用する置換基分子のMullikenの電気陰性度値((IP + EA)/2)，と大きい相関係数で表されることを示す．そのウレタン化反応は，1：2モル比の6員環錯体を経て，遷移状態の活性化エネルギー(Ea)は4∼13 kcal mol⁻¹ と検証された．メチルアミンとのウレア化は反応性が高いが，それは会合体を形成し易く，Ea が小さいためと判断された．遷移状態構造の結果からは，3分子間の水素結合とその役割が理解される．第3級アミンの触媒作用は，上記1：2 錯体中のプロトンの捕捉促進とNCOへの受け渡しで，Eaを低下させると解析された．イソシアナートと水の反応で生成するカルバミン酸の不安定で発泡の事実等に関しては，分解の素過程の Ea が11 kcal mol⁻¹と低く，アミンと CO₂ に分解すると検証された．

Figures

Figure 2.

 PM6 simulation of reaction between PhNCO and two moles of MeOH.

Figure 1.

 PM6 simulation process of the reaction (PhNCO + 2MeOH), and the stable complex (B), transition state (C), Ea and product (D) energies, relatives (Parenthesis: HOF (kcal mol⁻¹)). Hb: Hydrogen bond energy, In: Interaction energy.

Figure 3.

 Transition state structures of the reaction (HNCO + 2MeOH: Entry 4) by PM6 (and by ab initio of reference [5]) in Table 4.

Figure 4.

 Interaction between PhNCO and Et₃N (A1), and the catalytic reaction with 2MeOH for the urethane (B).

Figure 5.

 Energy profile of catalytic urethane formation from PhNCO and 2MeOH by Et₃N. Energies (HOF: kcal mol⁻¹).

Figure 6.

 IRC data of catalytic urethane formation via the complex (Bcom) from PhNCO and 2MeOH by Et₃N.

Figure 7.

 IRC data on the reaction of phenyl-carbamic acid with 2H₂O.

Figure 8.

 IRC data on the contact of methl phenyl-carbamate with 2H₂O.

Tables

Table 1.  Relationship between Frontier orbital parameters and urethanization rates of substituted phenyl isocyanates

a) Reference [2]: Part 1, p.143 Table VII and Figure 2 (28 °C).

b) Reference [12]: p. 47 (Mulliken' Electronegativity).

Table 2.  Frontier orbital parameters of R-Y-H, the addition reactions with PhN=C=O and the relative rates

a) Reference [15]. b) Atom. C: Net Atomic Charge of the Y atom in RYH by PM6. c) Reference [1b]. d) Reference [2],

p. 140; Activation energy (Ea, kcal mol⁻¹).

Table 3.  Simulation results of addition reactions of PhNCO with H₂O, MeOH and MeNH₂

Table 4.  Simulation result of addition reactions of X-NCO with methanol

a,c. Reference [2]: kcal mol⁻¹. b. Interaction energy, kcalmol⁻¹. d. Relative Ea. e. Reference [5], ab initio calc. data.

Table 5.  Comparison of the TS structures (a- f in Figure 3) by ab initio^a), PM6 and PM6-D3H4^b) methods

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