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When and How Do Diaminocarbenes Dimerize?
Author(s) -
Alder Roger W.,
Blake Michael E.,
Chaker Leila,
Harvey Jeremy N.,
Paolini François,
Schütz Jan
Publication year - 2004
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.200400654
Subject(s) - dimer , carbene , protonation , chemistry , photochemistry , computational chemistry , chemical physics , crystallography , catalysis , ion , organic chemistry
Abstract No example of a simple uncatalyzed dimerization of a diaminocarbene has been clearly established, so it is timely to ask what factors control the thermodynamics of this reaction, and what mechanisms are responsible for the observed dimerizations? In agreement with qualitative experimental observations, the dimerizations of simple five‐ and six‐membered‐ring diaminocarbenes are calculated to be 100 kJ mol −1 less favorable than those of acyclic counterparts. This large difference is semiquantitatively accounted for by bond and torsional angle changes around the carbene centers. Carbenes such as (Et 2 N) 2 C are kinetically stable in THF at 25 °C in agreement with calculated energy barriers, but they rapidly dimerize in the presence of the corresponding formamidinium ion. This proton‐catalyzed process is probably the most common mechanism for dimer formation, and involves formation of C‐protonated dimers, which can be observed in suitable cases. The possibility of alkali‐metal‐promoted dimerization is raised, and circumstantial evidence for this is presented.