Cyclic (alkyl) (amino) carbenes

from Wikipedia, the free encyclopedia
The structure of a CAAC
A CAAC - The carbene functionality is indicated by the two dots that indicate the unpaired electrons that are able to bind to a number of vacant coordination sites. The R substituents illustrate the potential for the derivatization of CAACs.

In cyclic (alkyl) (amino) carbenes (CAACs) is carbenes , which in a cyclic structure of only one nitrogen atom are adjacent. They are related to the N -heterocyclic carbenes (NHCs), which have two adjacent nitrogen atoms. In contrast to these, however, CAACs are stronger π acceptors and σ donor ligands .

CAACs play a role primarily as ligands in organometallic compounds .

presentation

The representation of many CAACs follows the route originally described by Guy Bertrand and his colleagues. This begins with the condensation of 2,6-diisopropylaniline with isobutanal . The deprotonation of the resulting aldimine with lithium diisopropylamide (LDA) leads to an azaallyl anion , which leads to a ring opening of 1,2-epoxy-2-methylpropane. The resulting lithium alcoholate is then reacted with trifluoroacetic anhydride , which leads to the aldiminium salt. In a final step, this is deprotonated by means of LDA, whereby the carbene is formed as a colorless solid.

A schematic diagram representing the synthesis of a CAAC
CAAC synthesis according to Guy Bertrand et al.

Reactivity

In 2007, Guy Bertrand's group was able to split nucleophilic hydrogen using several CAACs , which is not possible with NHCs. This is attributed to an energetically somewhat higher highest occupied molecular orbital (HOMO) and a smaller singlet-triplet distance . In this reaction, this leads to a significantly lower activation energy of around 100 kJ / mol compared to NHCs (around 150 kJ / mol).

Individual evidence

  1. ↑ a b c Vincent Lavallo, Yves Canac, Carsten Presang, Bruno Donnadieu, Guy Bertrand: Stable Cyclic (Alkyl) (Amino) Carbenes as Rigid or Flexible, Bulky, Electron-Rich Ligands for Transition-Metal Catalysts: A Quaternary Carbon Atom Makes the difference . In: Angewandte Chemie International Edition . tape 44 , no. 35 , September 5, 2005, pp. 5705-5709 , doi : 10.1002 / anie.200501841 , PMID 16059961 , PMC 2427276 (free full text).
  2. ↑ Research Highlights. Retrieved November 8, 2019 .
  3. ↑ Erik A. Romero, Rodolphe Jazzar, Guy Bertrand: (CAAC) CuX-catalyzed hydroboration of terminal alkynes with pinacolborane directed by the X-ligand . In: Journal of Organometallic Chemistry . tape 829 , February 2017, p. 11–13 , doi : 10.1016 / j.jorganchem.2016.09.025 .
  4. ↑ Eder Tomás-Mendivil, Max M. Hansmann, Cory M. Weinstein, Rodolphe Jazzar, Mohand Melaimi: Bicyclic (alkyl) (amino) carbenes (BICAACs): Stable Carbenes More Ambiphilic than CAACs . In: Journal of the American Chemical Society . tape 139 , no. 23 , June 14, 2017, p. 7753-7756 , doi : 10.1021 / jacs.7b04640 .
  5. ↑ a b c G. D. Frey, V. Lavallo, B. Donnadieu, WW Schoeller, G. Bertrand: Facile Splitting of Hydrogen and Ammonia by Nucleophilic Activation at a Single Carbon Center . In: Science . tape 316 , no. 5823 , April 20, 2007, p. 439–441 , doi : 10.1126 / science.1141474 .