正方形平面イリジウムからβ-水素の除去のメカニズム (((I) アルコキシド複合体は,ラビル・ダティブ・リガンドを持つ
J Zhao1, H Hesslink, J F Hartwig
1Contribution from the Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8017, USA.
Journal of the American Chemical Society
|July 27, 2001
まとめ
この研究は,イリジウムアルコキソ複合体からのβ-水素の除去が,可逆的なプロセスであることを明らかにしています. この発見は,以前のモデルと対照的であり,有機金属反応経路に関する新しい洞察を提供します.
科学分野:
- 有機金属化学 有機金属化学
- 反応メカニズム 反応メカニズム
- 協調化化学について
背景:
- 正方形平面アルコキソ複合体, [Ir ((CO)) ((PPh3) 2 ((OR)) ]は,ユニークな安定性と反応性を示しています.
- ベータ-水素の除去を理解することは,触媒と合成化学にとって極めて重要です.
研究 の 目的:
- イリジウムアルコキソ複合体からβ-水素の除去のメカニズムを解明する.
- 反応経路におけるフォスフィン濃度と置換物の影響を調査する.
- 観察されたメカニズムをアルコキシドおよびアルキル除去の既存のモデルと比較する.
主な方法:
- 運動分析,同位体効果,およびラセミゼーション実験を含むメカニズム研究.
- トリフェニルフォスフィン (PPh3) の存在下でのイリジウムアルコキソ複合体の熱分解.
- フォスフィン濃度と溶媒を変化させ,反応運動を検出する.
主要な成果:
- ベータ-水素の除去は,ベータ-炭素の置換物によって速度定数が最小限に影響され,可逆性があることが判明しました.
- フォスフィン濃度に関する反応順序は多様で,複雑な運動性を示す.
- 1/k(obs) vs [PPh3]のグラフのy交差点で,一次運動同位体効果が観察されました.
結論:
- PPh3による関連的移位に続く可逆のβ-水素除去を含むメカニズムが,観測されたデータを説明しています.
- この経路は,提案された直接排除,プロティック,または二核メカニズムとは異なる.
- アルコキシドの除去は,アルキル複合体と同様の経路で進行し,以前の仮定に異議を唱えます.
関連する概念動画
02:54Lewis Structures of Molecular Compounds and Polyatomic Ions
To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:
02:06Newman Projections
Different notations are used to represent the three-dimensional structure of molecules on two-dimensional surfaces. One of the most commonly used representations is the dash-wedge formula. The dashed wedges, solid wedges, and the plane lines indicate the groups situated behind the plane, coming out of the plane, and in the plane, respectively.
The organic molecules rotate across the single bonds leading to numerous temporary three-dimensional structures of varying energy known as conformers.
The organic molecules rotate across the single bonds leading to numerous temporary three-dimensional structures of varying energy known as conformers.
03:08Hydroboration-Oxidation of Alkenes
In addition to the oxymercuration–demercuration method, which converts the alkenes to alcohols with Markovnikov orientation, a complementary hydroboration-oxidation method yields the anti-Markovnikov product. The hydroboration reaction, discovered in 1959 by H.C. Brown, involves the addition of a B–H bond of borane to an alkene giving an organoborane intermediate. The oxidation of this intermediate with basic hydrogen peroxide forms an alcohol.
02:04Oxidation of Alkenes: Anti Dihydroxylation with Peroxy Acids
Diols are compounds with two hydroxyl groups. In addition to syn dihydroxylation, diols can also be synthesized through the process of anti dihydroxylation. The process involves treating an alkene with a peroxycarboxylic acid to form an epoxide. Epoxides are highly strained three-membered rings with oxygen and two carbons occupying the corners of an equilateral triangle. This step is followed by ring-opening of the epoxide in the presence of an aqueous acid to give a trans diol.
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.
00:51Radical Formation: Elimination
Another method of radical formation is the elimination process. It is the opposite of the addition route and is driven by the instability of the radical. For example, as depicted in Figure 1, dibenzoyl peroxide yields a pair of unstable radicals upon homolysis. Given its instability, this radical spontaneously undergoes elimination via a C–C bond cleavage to form a relatively more stable phenyl radical. The mechanism involves cleavage of the bond between the α and β positions with respect to...