初等デウテリウム同位体に対する構造反応性効果は,環置換アルファメトキシチルネスのプロトネーションに対する効果である
Wing-Yin Tsang1, John P Richard
1Department of Chemistry, University of Buffalo, SUNY, Buffalo, New York 14260-3000, USA.
Journal of the American Chemical Society
|October 1, 2009
まとめ
この研究では,アルファ-メトキシスティレンのプロトネーションにおける一次運動同位体効果 (PIEs) を調査した. 最大のPIEは0の反応推進力の近くで観察され,これは水素移転の対称的な移行状態を示唆しています.
科学分野:
- 物理有機化学 物理有機化学
- 化学動力学 化学動力学
- イソトープ効果とは
背景:
- 主要運動同位体効果 (PIEs) は,反応機構と移行状態構造についての洞察を提供します.
- 陽子伝達反応は,有機化学と触媒学において根本的な役割を果たしています.
- PIEにおける反応の原動力の役割を理解することは,メカニズムの解明に不可欠です.
研究 の 目的:
- L ((+)) とカルボキシル酸によって触媒化された環置換アルファメトキシスティレンのプロトン化のためのPIEsを計算する.
- 移行状態の特徴を決定するために,反応の原動力とPIEsを相関させる.
- 水素転送における量子力学的トンネリングの可能性を調査する.
主な方法:
- プロトン核磁気共鳴 ((1) H NMR) を用いた原産物同位体効果 (PIEs) の定量化.
- 酸触媒による様々なプロトネーション反応における反応推進力 (DeltaG(o)) の測定.
- 活性化エネルギーの差異とPIEの限界値を決定するためにPIEsのArrhenius型プロットの分析.
主要な成果:
- 4-MeO-alpha-methoxystyreneを二酸化塩素酸 (DeltaG(o) = 1.0 kcal/mol) によってプロトン化すると,最大PIE 8.7 が観察されました.
- PIEは反応の原動力が増加または減少するにつれて減少し,最大値はDeltaG(o) = 0.
- 計算されたパラメータ [E (a)) H) - (E (a)) D) ] = -1.2 kcal/molと (A (h) /A (d)) = 1.0は,トンネル掘削の証拠なしに,エネルギー障壁の上での水素転送と一致しています.
結論:
- PIEのデータは,転送されたヒドロンが,酸ドナーと塩基受容体の間の距離がほぼ等しい移行状態を示唆しています.
- これらの発見は,ブロンステッドパラメータに基づいて製品のような移行状態を示す以前の研究と対照的です.
- 観測されたPIEは,量子力学的トンネリングではなく,バリアを越えた水素の移動を含む反応メカニズムをサポートしています.
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