N-t-Butylketoketenimines

Recently Reported Crystalline Isothiazole Carbenes: Myth or Reality

“…In contrast to recent claims, [12] our calculations and experiments show that 1) isothiazole carbenes 2 S cannot be isolated or even observed at room temperature; 2) they isomerize into their 2-imino-2H-thiete isomers 3 S via a transition state TS located only about 1 kcal mol À1 higher in energy than carbenes 2 S; 3) in contrast to the original findings, no signals about 190 ppm were observed when monitoring by 13 C NMR spectroscopy the deprotonation of 2,4,5-triphenylisothiazolium perchlorate 1 S; 4) the formation of carbene dimers 5 S is doubtful, and in any case impossible starting from the free carbene 2 S, since the latter cannot be isolated; 5) for the same reasons, the "carbene-amine adducts" 6 S cannot be prepared from carbenes 2 S, but can be formed by nucleophilic addition to the cationic precursors 1.…”

“…However, despite their crystalline nature, single crystal diffraction studies were not performed. It was mentioned that the observed 13 C NMR signal for the carbene carbon atom of 2 S (d = 194.3-195.7 ppm) corresponds to the shifts observed for imidazol-2-ylidenes. This statement is correct; however, since the 13 C NMR signal for carbenes featuring a nitrogen and a carbon substituent (such as D-F) appears usually at much lower field, [7][8][9] these values were somewhat surprising.…”

“…This heterocycle [16] was isolated in 91 % yield and fully characterized including by a single-crystal X-ray diffraction study ( Figure 2). [17] As predicted by calculations, monitoring the reaction by 13 C NMR spectroscopy did not allow the observation of any intermediates; moreover no signal in the range reported for the carbene carbon of 2 S was observed.…”

“…[18] Despite their crystalline nature, the "dimers" have only been characterized by 1 H and 13 C NMR spectroscopy, as well as mass spectrometry. Assuming the presence of only one isomer, at least 15 13 C NMR signals are expected, all of them in the same range, which makes any attributions highly debatable. Since the "carbenes" 2 S do not exist, we have of course not been able to reproduce the reported results.…”

Stabile cyclische Carbene und verwandte Spezies jenseits der Diaminocarbene

“…Die bis dahin höchste Umsatzzahl betrug 14 000, erzielt mit einem Bis(9-cyclohexyl-9-phospha-9H-bicyclononan)-ruthenium-Komplex. [123] In sämtlichen bislang untersuchten Metathesereaktionen war der sterisch kleinste Katalysator der Serie, CAAC [125] und Isothiazolcarbenen [126] bekannt). Um einen solchen Prozess zu verhindern, müsste der kationische Teil Y des Ylids exocyclisch sein -wie in N-YHC 3 -, da die Bildung einer endocyclischen Dreifachbindung sehr unwahrscheinlich ist.…”