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 degrees 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.

Barriers to rotation about the Ccarbene−N bonds in diaminocarbene (H2N)2C 1a, bis(dimethylamino)carbene ((CH3)2N)2C 1b, the related formamidinium ions (H2N)2CH+ 2a and ((CH3)2N)2CH+ 2b, and the Li+ complexes (H2N)2CLi+ 3a and ((CH3)2N)2CLi+ 3b have been calculated using density-functional theory in order to study the extent of π-bond stabilization of the carbene center. Experimental barriers from DNMR are reported for 1b and 2b and compared with those for bis(diisopropylamino)carbene 1c and the N,N,N‘,N‘-tetraisopropylformamidinium ion 2c; rotational barriers computed for 1b and 2b including thermal corrections compare well with experiment. The dimerization of 1a and 1b have been studied with (full) geometry optimization up to the levels QCISD(T)/cc-pVDZ//MP2/cc-pVDZ and B3LYP/cc-pVDZ//B3LYP/cc-pVDZ, respectively. The minimum-energy path for the dimerization of 1a has been computed using the BPW91/cc-pVDZ method. It is shown that the transition state geometries for the dimerizations of 1a and 1b have C 2 and C 1 symmetry, respectively, the latter being strongly polarized. The possible involvement of catalysis by protons and lithium ions in the dimerization processes is discussed. Calculations of the proton affinities of 1a, 1b, and some related species are reported. 13C NMR shielding constant calculations on a series of diaminocarbenes have been performed using the gauge-including atomic orbitals (GIAO) method. The variation in the extremely downfield-shifted 13C NMR signal of the carbene carbon in 1a, 1b, and related species is reproduced reasonably well by GIAO calculations, the latter being 2−8 ppm more upfield than the experimentally observed signals. It is shown that the paramagnetic contributions to the shielding tensor at the carbene nucleus play an important role in the chemical shift changes upon substitution in the RXC(NR2) species.

Bisher wurde kein eindeutiger Nachweis für eine einfache, nicht katalysierte Dimerisierung von Diaminocarbenen gefunden. Es ist somit an der Zeit zu hinterfragen, welche Faktoren die Thermodynamik dieser Reaktion steuern und welche Mechanismen für die beobachteten Dimerisierungen verantwortlich sind. In Übereinstimmung mit qualitativen experimentellen Beobachtungen ergab die Berechnung, dass die Dimerisierung von einfachen Fünf‐ und Sechsringdiaminocarbenen um 100 kJ mol−1 weniger begünstigt ist als die der acyclischen Pendants. Dieser große Unterschied kann halbquantitativ durch die Änderung von Bindungs‐ und Torsionswinkeln um die Carbenzentren herum erklärt werden. Carbene wie (Et2N)2C sind, in Einklang mit den berechneten Energiebarrieren, in THF bei 25 °C kinetisch stabil, aber sie dimerisieren schnell in Gegenwart des entsprechenden Formamidiniumions. Dieser protonenkatalysierte Prozess ist wahrscheinlich der häufigste Mechanismus der Dimerbildung. Er führt zur Bildung von C‐protonierten Dimeren, was in geeigneten Fällen beobachtet werden kann. Es wird auch die Möglichkeit der durch Alkalimetalle geförderten Dimerisierung diskutiert; hierfür werden einige Belege vorgestellt.

Palladium acetate in a mixture of TBAB and water can be used as an effective catalyst for the Suzuki coupling of deactivated aryl chloride substrates.

The synthesis of a range of chiral palladium bis(phosphite) pincer complexes has been achieved via C-H activation of the parent ligands and one of the complexes formed shows good activity in the catalytic allylation of aldehydes.