オーガニックゼオライトとして使用される自己組み立て可能なビスウレアマクロサイクルは,2-サイクロヘクセンオンの高度にステレオ選択的光二分化のために使用されます
Jun Yang1, Mahender B Dewal, Linda S Shimizu
1Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, USA.
Journal of the American Chemical Society
|June 22, 2006
まとめ
自ら組み立てられたビスウレアマクロサイクルは,2-サイクロヘクセノンの非常に選択的な2 + 2サイクロアディションを可能にし,ヘッド・トゥ・テール・フォトジマーを生成します. 再利用可能な触媒は,ゼオライト材料と同様に,効率的な製品抽出と回収を促進します.
科学分野:
- 有機化学 オーガニック・ケミストリー
- 超分子化学 超分子化学
- フォトケミストリー フォトケミストリー
背景:
- サイクル添加反応は有機合成において根本的な役割を果たします.
- フォトリアレンジメントは,サイクロアディション製品形成を複雑にすることができます.
- マクロサイクル化合物は,化学反応における触媒やテンプレートとして作用する.
研究 の 目的:
- 2-サイクロヘクセノンの2+2サイクル添加のための高度に選択的な方法を開発する.
- この反応を促すための自己組み立てビスウレアマクロサイクルの使用を調査する.
- 触媒システムの効率と再利用性を評価する.
主な方法:
- 超分子触媒として自己組み立てビスウレアマクロサイクルの利用.
- 光化学的条件下で,2-サイクロヘクセノンの2 + 2サイクル添加反応を実行する.
- 反応混合物の産物形成と選択性を分析する.
- 製品抽出と触媒回収の方法を調査する.
主要な成果:
- 2-サイクロヘクセノンの高度に選択的なヘッド・ツー・テール・フォトジマー形成を達成した.
- 高い反応変換が観察されました.
- 二次性写真再配置の発生率が低下していることが示されました.
- 製品を成功裏に抽出し,再利用のためにビスウレアマクロサイクル触媒を回収しました.
結論:
- 自ら組み立てられたビスウレアマクロサイクルは,2-サイクロヘクセノンの選択的光分解を効果的に触媒化する.
- このシステムは,高効率,選択性,および触媒の再利用性を提供します.
- このアプローチは,ゼオライトと同様の,再利用可能で回収可能な触媒システムを提供します.
関連する概念動画
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.
02:36Regioselectivity and Stereochemistry of Hydroboration
A significant aspect of hydroboration–oxidation is the regio- and stereochemical outcome of the reaction.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
01:26Thermal and Photochemical Electrocyclic Reactions: Overview
Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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
01:28Reduction of Benzene to Cyclohexane: Catalytic Hydrogenation
Unlike the easy catalytic hydrogenation of an alkene double bond, hydrogenation of a benzene double bond under similar reaction conditions does not take place easily. For example, in the reduction of stilbene, the benzene ring remains unaffected while the alkene bond gets reduced. Hydrogenation of an alkene double bond is exothermic and a favorable process. In contrast, to hydrogenate the first unsaturated bond of benzene, an energy input is needed; that is, the process is endothermic. This is...