溶解反応におけるイオン化段階の直接観察:カルボカシオンの電友性対電流性
Heike F Schaller1, Alexander A Tishkov, Xinliang Feng
1Department Chemie und Biochemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13 (Haus F), 81377 München, Germany.
Journal of the American Chemical Society
|February 20, 2008
まとめ
高度安定したベンジジドリリウムイオンは,カルボキシラートイオンと反応する. イオン化率と反応性は相関関係がないため,S (((N)) 1反応における変数内在の障壁と非カルボケーションのような移行状態を示す.
科学分野:
- 有機化学 オーガニック・ケミストリー
- 反応の動力学
背景:
- アミノ置換されたベンジジドリリウムイオン (Ar2CH+) は,高度に安定したカルボケーションである.
- それらの反応の動力学と熱力学を理解することは,反応機構の研究にとって極めて重要です.
研究 の 目的:
- ベンジヒドリウムイオンとカーボキシラートイオン間の反応の速度と均衡を決定する.
- イオン化率 (電流性) と反応性 (電友性) の関係を調査する.
- S(N) 1反応における移行状態構造と溶媒効果を分析する.
主な方法:
- アセトンとアセトニトリルにおける反応速度と均衡の光度測定.
- 結合エステルからベンジヒドリリウムイオンの再生によるS(N) 1反応の第一段階の直接測定.
- イオン化のための自由エネルギープロファイルの構築.
主要な成果:
- ベンジヒドリルカルボキシラートはベンジヒドリリウムイオンを再生し,最初のステップでS(N) 1を直接測定することができます.
- 電気流動性の順序は,ベンジヒドリリウムイオンの電友性順序と相関しない.
- 類似の速さで生成されるイオンに対して,2桁以上の反応性差が観察されました.
- 変数の固有の障壁は,レート・バランス関係の崩壊を説明する.
- 移行状態はカルボケーション型ではなく,ウィンシュタイン-グランワルド方程式における小さな溶媒効果 (m = 0.35-0.55) によって支えられています.
結論:
- ベンジヒドリルカルボキシラートのイオン化には,完全にはカルボケーション型ではない移行状態が伴う.
- 小さな溶媒の効果は,必ずしもS(N) 2のイオン化メカニズムを示さない.
- 本質的なバリアは,ベンジヒドリリウムイオンの反応性において重要な役割を果たします.
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