Hydroxyalkylierung, Acylierung, Formylierung und Carboxylierung von 2‐Nitro‐ und 2‐Chlor‐1‐(trimethylsilyl)benzol

Abstract

Die präparative Anwendung der basekatalysierten Carbodesilylierung von Aryltrimethylsilanen wird an den Umsetzungen von 2‐Nitro‐ (1a) und 2‐Chlor‐1‐(trimethylsilyl)benzol (1b) mit Aldehyden, Ketonen, Carbonsäurefluoriden bzw. ‐anhydriden, Dimethylformamid und Kohlendioxid beschrieben. In guten Ausbeuten werden dabei die entsprechenden [(Trimethylsiloxy)alkyl]benzole 3 und 8, (Hydroxyalkyl)benzole 4, 6 und 9, die Benzophenone 12, die Benzaldehyde 14 sowie die Benzoesäuren 17 erhalten. Die neue Methode ist eine nützliche Alternative zur elektrophilen Substitution sowie der Verwendung metallorganischer Verbindungen bei der Darstellung mehrfach substituierter Aromaten.

CO2 Conversion into Esters by Fluoride‐Mediated Carboxylation of Organosilanes and Halide Derivatives

“…The group of Kondo developed the carboxylation of alkynylsilanes (Scheme d) . Early on, Effenberger and Spiegler reported the carboxylation of electron deficient arylsilanes, bearing ortho ‐NO 2 and ortho ‐Cl groups, using KF or CsF under high pressures of CO 2 (50 atm), presumably to avoid the formation of the free aryl anion (Scheme e) . These few examples highlight the small scope of reactive organosilanes that is limited to the g eneration of stabilized carbanions having a corresponding p K a lower than 35.…”

“…Overall, this mechanism requires a low energy demand of 28.6 kcal mol −1 , and it highlights the unique role of CO 2 in this transformation that acts both as a reactant and catalyst able to facilitate the C−Si bond cleavage. In fact, whereas the carboxylation of electron deficient arylsilanes is possible, the method reported herein uses reversible CO 2 coordination as a means of traceless heteroarene activation. The driving force for the fluoride‐mediated carboxylation of 2 a 2 derives from the stabilization of anion C that is reflected in the decreased p K a of the pyridine–CO 2 adduct at the 2‐position (30.2 versus 43.4 in pyridine, Scheme ).…”

“…We found that introducing an electronwithdrawing group (EWG), namely CO 2 ,s ignificantly facilitates the CÀSi bond cleavage and the release of as tabilized anion (C)b ya na ccessible TS 5 at 28.6 kcal mol À1 . C readily undergoes carboxylation at the carbanionic position to yield D,p rior to ad ecarboxylation of the NÀCO 2 À moiety to 2-PyCO 2 À À .O verall, this mechanism requires al ow energy demando f2 8.6 kcal mol À1 ,a nd it highlightst he unique role of CO 2 in this transformation that acts both as ar eactant and catalyst able to facilitate the CÀSi bond cleavage.I nfact, whereas the carboxylation of electron deficient arylsilanes is possible, [12] the methodr eportedh erein uses reversible CO 2 coordination as am eans of tracelessh eteroarene activation.T he drivingf orce for the fluoride-mediated carboxylationo f2a 2 derives from the stabilization of anion C that is reflected in the decreased pK a of the pyridine-CO 2 adduct at the 2-position (30.2 versus 43.4 in pyridine, Scheme 3). In contrast, ac arbanion at the 3-position of the pyridine-CO 2 adduct is characterized by pK a of 32.0 and its formation from 2l is 3.9 kcal.…”

“…[11] Early on, Effenberger and Spiegler reported the carboxylation of electron deficient arylsilanes, bearing ortho-NO 2 and ortho-Cl groups, using KF or CsF under high pressures of CO 2 (50 atm), presumably to avoid the formation of the free aryl anion (Scheme 1e). [12] These few examples highlight the small scope of reactive organosilanes that is limitedt ot he g eneration of stabilized carbanions having ac orresponding pK a lowert han 35. Additionally,t he esterification step must be carried out in as econd step to prevent ad irectq uenching of the carbonn ucleophile by the electrophile (RCHN 2 or RX) in the formation of the ester product.…”

2‐Arylsilacyclobutane as a Latent Carbanion Reacting with CO2

“…[1] Most methods employed for generating carbanions include a) metallation of organic (pseudo)halides with metallic agents like magnesium-(0) metal and n-butyllithium, and b) deprotonation of an acidic hydrogen by abase.Amilder pathway is available with organosilicon compounds which are activated by af luoride anion. [2] Formation of as trong Si À Fb ond provides ad riving force for the generation of carbanionic species.Acarbanionic species is also assumed in the carboxylation reaction of organic iodides with carbon dioxide using as trained disilane and cesium fluoride as the activating agents. [3] Thes train energy inherent in three-and four-membered ring structures often brings forth unique reactivities that are not available with the corresponding acyclic structures.T his also applies to organosilicon compounds.…”