John C. W. Lohrenz

[Lithium tert‐butylperoxide]12: Crystal Structure of an Aggregated Oxenoid

The X-ray crystal structure of the dodecameric lithium tert-butylperoxide [2]12 is the first of an alkali or alkaline earth peroxide. It shows the lithium ion bridging the two oxygen atoms of the peroxide unit and a slight lenghtening of the O-O bond, in agreement with quantum-chemical calculations. A calculation for the model reaction of MeLi with LiOOH to give MeOLi and LiOH reveals the importance of Li bridging the O-O bond in the transition state of this reaction, as similarly discussed for many oxidation reactions of (transition-) metal peroxides. Preliminary theoretical studies of the O-O bond length (and thus of the oxenoid character) as a function of the aggregation of 2 disclose that increasing aggregation leads to stabilization of the charge at the anionic oxygen atom and thus to a reduction of the O-O bond length (oxenoid character). Related considerations of the effect of aggregation should also be valid for other lithium (organometallic) compounds and their structure and reactivity as well as other properties.

Lithio-diazomethane and lithio-(trimethylsilyl)diazomethane : theoretical and experimental studies of their structures, reactions and reaction products

Abstract The first report of a metalated diazomethane by Eugen Muller and his coworkers appeared in 1933. Then, in a thirty-six years ongoing story, 1–6 this group revealed several highly interesting but also puzzling details about the chemistry of metalated diazomethane and metalated substituted diazomethanes: 1. Metalation of diazomethane 1a (the parent compound) to give the corresponding Li (or Na) species followed by protonation led to “isodiazomethane” which finally turned out to be N -isocyano amine 1c . Ab initio calculations of the structures and energies of the three non-cyclic CH 2 N 2 isomers 1a-c and of the four non-cyclic lithiated diazomethane isomers 2 -Li(I)- 2 -Li-(IV), as well as their dimers, provide an understanding of this reaction sequence. 2. Metalation of mono-substituted diazomethanes RCHN 2 , R Ph, Me, Me 3 Si, e.g. by methyllithium, followed by protonation, resulted in a completely different observation, namely the formation of 4,5-bis-R-substituted 1,2,3-thiazoles (e.g. 10 ) and methylamine 11 . In the crystals of lithiated trimethylsilyl-diazomethane 13 -Li, which allowed for the first time to determine the crystal structure of a lithiated (substituted) diazomethane, is also present the corresponding Li-triazole 14 -Li ([6 13 -Li · 2 14 -Li · 6 Et 2 O · hexane]). Model calculations traced the pathway of the triazole formation via a nitrenoid (e.g. 9 -Li) which is also responsible for the formation of methylamine 11 . The completely different chemistry of diazomethane 1a and mono-substituted diazomethanes in the deprotonation-protonation sequence is due to the different structures of the corresponding Li(Na) species and their different reactivities (selectivities).

Crystal Structure of [2-ZnCl-benzoxazole⋅2 THF]2: The Remarkable Difference between 2-ZnHal- and 2-Li-oxazoles

The strong influence of the metal (M) on the equilibrium 1-M⇌2-M (shown below) is clearly evident: it is far towards the 2-M side with M = Li, whereas it is on the 1-M side for M = ZnCl. The first crystal structure of a 2-metalated oxazole, [3-ZnCl⋅2 THF]2 (below right), has been determined.