ボリレンがエチン,エチン,メタンに対して反応する
Małgorzata Krasowska1, Holger F Bettinger
1Institut für Organische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany.
Journal of the American Chemical Society
|October 6, 2012
まとめ
この研究では,ボリレン反応性を計算的に調査し,BHが最も反応性があり,電好性であり,BFが最も反応性なく,最も核好性であることを発見しました. これらの発見は,ボリレン添加反応の洞察を提供します.
科学分野:
- コンピューティング・ケミストリー
- 量子化学とは,量子化学である.
- 有機金属化学 有機金属化学
背景:
- ボリレン,カルベンのシリコンアナログは,反応性中間物質である.
- ボリレンの電子構造と反応性を理解することは,合成アプリケーションにとって極めて重要です.
研究 の 目的:
- 置換ボリレンの電子構造と幾何学的構造を研究する.
- ボリレンの炭化水素に対する反応性を調べる.
- ボリレン添加および挿入反応のメカニズムとエネルギーの解明.
主な方法:
- ハイブリッド密度関数理論 (B3LYP) とモラー・プレセットの混乱理論 (MP2) です.
- クープレッド・クラスター計算 [CCSD(T) ].
- 6-311+G**,cc-pVTZ,およびaug-cc-pVTZのベースセットを使用しています.
主要な成果:
- BHは,最も電友性および反応性の高いボリレンであり,エネルギー障壁がないまたは低い熱外加反応を示します.
- BFは最も核愛性の高いボリレンであり,反応障壁が高く,エクソ熱性が低い.
- ボリレンと不飽和炭化水素 (エチネ,エチネ) の反応とメタンへのC-H結合の挿入が研究されました.
結論:
- ボリレン反応性は,置換剤 (R群) によって強く影響されます.
- BHは最も高い反応性を示し,BFは最も低い反応性を示しています.
- 計算による洞察は,ボリレン反応機構と潜在的な応用に関する理解を導く.
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.
02:38Electrophilic Addition to Alkynes: Halogenation
Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
01:17Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene
The electrophilic addition of hydrogen halides such as HBr to alkenes and nonconjugated dienes gives a single product as per Markovnikov’s rule.
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.
02:47Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation
Introduction
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
02:42Acidity of 1-Alkynes
The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.