Die Harnstoff‐Renaissance

Abstract

Im letzten Jahrzehnt hat die Verwendung von Harnstoffderivaten als wertvolle Reagentien, Katalysatoren und Struktureinheiten in der organischen Chemie rasch zugenommen. Sie finden nun Verwendung als Wasserstoffbrückendonoren in Organokatalysatoren und Anionentransportern, als wichtige Struktureinheit in der supramolekularen Chemie, für die Steuerung von Lithiierungen, als Aminierungssubstrate, für die Vermittlung von Metallierungen und als Substrate für neuartige Umlagerungen. In diesem Aufsatz wird die bemerkenswert rasche aktuelle Entwicklung der Chemie des Harnstoffs hervorgehoben, der für viele Jahre als unreaktiv, schwer zu handhaben und vergleichsweise wertlos betrachtet wurde.

A Bifunctional Tungstate Catalyst for Chemical Fixation of CO2 at Atmospheric Pressure

“…[7] Notably, the A-catalyzed reaction proceeded efficiently with CO 2 at atmospheric pressure (0.1 MPa) to give 2 a in 91 % yield. Reactions hardly proceeded in the absence of A or in the presence of the other POMs such as TBA 2 , and is in accord with the computational results. The reaction rate (10 mm min À1 ) of A was 48-fold faster compared with those (< 0.01-0.21 mm min À1 ) of typical inorganic and organic strong bases such as Cs 2 CO 3 , TBAOH, 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU), and 1,1,3,3-tetramethylguanidine (TMG).…”

“…Catalytic CO 2 fixation at atmospheric pressure has been limited to reactive substrates (strained cyclic molecules, unsaturated compounds, etc. [2] Although various base catalysts such as CsOH, Cs 2 CO 3 , and ionic liquids have been used for the synthesis of urea derivatives with CO 2 , these systems have disadvantages: 1) high CO 2 pressures (2.5-8.0 MPa) and reaction temperatures (423-453 K), 2) narrow applicability to substrates, and 3) need of dehydrating agents or additives (see Table S1 in the Supporting Information). [1d] Catalytic CÀN and CÀO bond-formation processes with CO 2 are important in both industry and academia because they offer economical and environmental advantages such as high atom efficiency and water is the only by-product.…”

“…[1a,d-f] Urea derivatives, made from CO 2 , are important end products or intermediates for pharmaceuticals, agricultural pesticides, antioxidants in gasoline, dyes, and resin precursors. [2] Although various base catalysts such as CsOH, Cs 2 CO 3 , and ionic liquids have been used for the synthesis of urea derivatives with CO 2 , these systems have disadvantages: 1) high CO 2 pressures (2.5-8.0 MPa) and reaction temperatures (423-453 K), 2) narrow applicability to substrates, and 3) need of dehydrating agents or additives (see Table S1 in the Supporting Information). [3] Recently, we have developed a series of polyoxometalates (POMs) as catalysts for various functional-group transformations.…”

Acyclic and Cyclic Ureas (Update 2013)

“…[25] Although the reaction proceeds in principle without a catalyst, the influence of heteroleptic calcium amides [{Ar 1 NC(Me)CHC(Me)NAr 1 }Ca(NR 1 2 )(THF); Ar 1 = 2,6-iPr 2 C 6 H 3 ;R 1 =TMS, Ph] and homoleptic heavier alkaline earth amides [M{N(TMS) 2 } 2 ; M = Ca, Sr, Ba] on the course of the reaction has been investigated. [26] Reaction of the calcium amide 1 with 1-adamantyl isocyanate in benzene-d 6 at room temperature affords insertion product 2 (Scheme 2), the structure of which has been confirmed by X-ray crystallography. Subsequent reaction of 2 with diphenylamine regenerates the catalyst and liberates the hydroamination product 1-(1-adamantyl)-3,3-diphenylurea.…”

“…These methods can benefit from the ability of urea groups to direct metalation of aromatic substituents at the ortho position [74] or to transfer an aryl group onto an appended lithiated allyl residue. [75] Most of this work has been reviewed; [6] however, since it does not primarily address the synthesis or chemical transformation of the urea moiety itself, it will not be considered further here. 18 Alkylation of the NH groups of ureas can usually be achieved in a simple fashion by deprotonation followed by treatment with an alkyl halide.…”