ピロガロラーレンヘクサマーにおける分子封じ込めは,非均衡条件下で行われます
Jennifer C Chapin1, Miroslav Kvasnica, Byron W Purse
1Department of Chemistry and Biochemistry, University of Denver, 2199 S. University Blvd., Denver, Colorado 80208, USA.
Journal of the American Chemical Society
|September 4, 2012
まとめ
ピロガロール[4]アレンマクロサイクルは,溶媒のない条件下で,非常に安定した,非熱力学的封じ込め複合体を形成します. ゲストの罠の安定性は,ゲストと溶媒の分子特性に依存し,強力な運動トラップを作成します.
科学分野:
- 超分子化学 超分子化学
- マテリアルサイエンス 材料科学
- 化学物理 化学物理
背景:
- ピロガロール[4]アレンマクロサイクルは,オクタエドール対称性のヘクサマーに自己組み立てます.
- これらのヘクサマーは,小さな分子を閉じ込めることができる閉じられた空間を作り出します.
- 以前の封じ込め試験は,通常,溶媒の存在下で実施されていました.
研究 の 目的:
- ピロガロール[4]アレンヘクサマー内のゲストエンカプスレーションのための溶剤フリー条件を調査する.
- これらのカプセル化複合体の運動安定性を探求するために.
- 封じ込めの安定性に対するゲストと溶媒の分子特性の影響を理解する.
主な方法:
- 溶融のゲスト分子を用いた溶媒のない封装.
- 冷却中にヘクサマーカプセルを形成する.
- 構造的および運動的分析のための核磁共振 (NMR) スペクトロスコピー.
- ゲスト交換の障壁の決定 (ΔG‡).
主要な成果:
- 特殊な動力学的に安定したカプセル化複合体は,溶媒のない条件下で形成された.
- 運動安定性は,ゲストと溶媒のサイズ,形状,および硬さに大きく依存しています.
- CCl4 → pyrene. に対して32 ± 0.7 kcal mol−1の最大ゲスト交換バリアが観察されました.
- 形成された複合体は熱力学的に安定していないが,環境温度では持続している.
結論:
- 溶媒のない条件により,合成水素結合システムで前例のない運動トラップが作られます.
- 分子認識と構造的な互補性は,高い運動安定性を達成するために不可欠です.
- これらの持続的,非均衡複合体は,超分子化学のためのオープンシステムにおける潜在的な応用を提供します.
関連する概念動画
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.
01:26Thermal 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.
01:18Frost Circles for Different Conjugated Systems
The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.
01:18Aromatic Hydrocarbon Cations: Structural Overview
Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group with both...
Removing one hydrogen from the intervening CH2 group with both...
01:16Cycloaddition Reactions: MO Requirements for Thermal Activation
Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.
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