カチオンのプラチナ (II) 複合体によるC[bond]H活性化:リガンド電子およびステリック効果
H Annita Zhong1, Jay A Labinger, John E Bercaw
1Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125, USA.
Journal of the American Chemical Society
|February 14, 2002
まとめ
プラチナ (((II) コンプレックスとビス (((アリル)) ディミネリガンドは,ベンゼン中のC-H結合を活性化し,フェニルプラチナカチオンを生成する. ステリックおよび電子的要因は,この触媒過程において,C-H活性化またはベンゼン調整が速度決定的かどうかに影響します.
科学分野:
- 有機金属化学 有機金属化学
- カタリシス カタリシス カタリシス
- C-H アクティベーション
背景:
- ビス・アリル・ダイミン結合メチル・プラチナ・II複合体は,触媒C−H活性化反応における重要な中間物質である.
- 芳香系におけるC−H結合の活性化を左右する要因を理解することは,新しい触媒変換の開発に極めて重要です.
研究 の 目的:
- カチオンのPt (II) 複合体によるベンゼンのC-H活性化のメカニズムを調査する.
- 反応経路と速度決定段階におけるリガンドの電子性およびステリック性能的影響を決定する.
主な方法:
- 様々なビス・アリル・ダイミン結合メチルPt・II複合体の合成.
- これらの複合体の反応は,ベンゼンおよび代替ベンゼンとトリフルオロエタノール.
- 反応産物の分析と,同位体効果を含む運動学的研究.
主要な成果:
- カチオンのPt (II) 複合体はベンゼンと反応して,Pt (IV) の中間体を通したフェニルPt (II) カチオンを形成する.
- アロマティックC-H活性化はベンジルC-H活性化よりも好ましいが,ステリックはこれを変化させることができる.
- 速度を決定するステップは,C-H活性化からベンゼン協調化に切り替わり,リガンドのステリックボールを増加させる.
- リンガンドの電子特性は主に基底状態の安定性と基質へのアクセスの容易さに影響し,C-H活性化ステップ自体に影響しません.
結論:
- ベンゼン置換のメカニズムは,溶媒によるアソシエーション経路を伴う.
- ビス・アリル・ダイミン・リガンドのステリックおよび電子特性により,Pt・II) コンプレックスによるC-H活性化の反応性とメカニズムが著しく調節される.
- 反応速度における観測された傾向は,主にC-H活性化段階ではなく,基質の調整に影響を与える基底状態効果に起因する.
関連する概念動画
02:48Ionic Bonding and Electron Transfer
Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence shell, and an anion (a negative ion) forms when a neutral atom gains one or more electrons in its valence shell. Compounds composed of ions are called ionic compounds (or salts), and their constituent ions are held together by ionic bonds: electrostatic forces of attraction between oppositely charged cations and anions.
02:46Covalent Bonding and Lewis Structures
Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual attraction of atoms for a “shared” pair of electrons.
01:17Thermal Electrocyclic Reactions: Stereochemistry
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
01:26Photochemical Electrocyclic Reactions: Stereochemistry
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Selection Rules: Photochemical Activation
00:54Radical Formation: Homolysis
A bond is formed between two atoms by sharing two electrons. When this bond is broken by supplying sufficient energy, either two electrons can be taken up by one atom forming ions by the cleavage called heterolysis, or the two electrons are shared by two atoms, with one each creating radicals by the cleavage called homolysis.
01:04¹³C NMR: ¹H–¹³C Decoupling
The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...