Diaza [1,4] Wittig-type rearrangement of N-allylic-N-Boc-hydrazines into γ-amino-N-Boc-enamines

Basicity-Tuned Reactivity: diaza-[1,2]-Wittig versus diaza-[1,3]-Wittig Rearrangements of 3,4-Dihydro-2H-1,2,3-benzothiadiazine 1,1-Dioxides

The base-induced (t-BuOK) rearrangement reactions of 3,4-dihydro-2H-1,2,3-benzothiadiazine 1,1-dioxides result in a ring opening along the N–N bond, followed by ring closure with the formation of new C–N bonds. The position of the newly formed C–N bond can selectively be tuned by the amount of the base, providing access to new, pharmacologically interesting ring systems with high yield. While with 2 equiv of t-BuOK 1,2-benzisothiazoles can be obtained in a diaza-[1,2]-Wittig reaction, with 6 equiv of the base 1,2-benzothiazine 1,1-dioxides can be prepared in most cases as the main product, in a diaza-[1,3]-Wittig reaction. DFT calculations and detailed NMR studies clarified the mechanism, with a mono- or dianionic key intermediate, depending on the amount of the reactant base. Also, the role of an enamide intermediate formed during the workup of the highly basic (6 equiv of base) reaction was clarified. The substrate scope of the reaction was also explored in detail.

Mechanistic investigation in the [1,4] and [1,2] Wittig rearrangement reactions: a DFT study

The mechanistic pathways for the [1,4] and [1,2] Wittig rearrangements of 2-silyl-6-aryl-5,6-dihydro-(2H)-pyrans have been studied at the M06-2X/6-31+G(d,p), 6-311++G(d,p) and cc-pVTZ level of theory. The crucial C-O bond cleavage step in the mechanism has been analysed initially, using two model reactions covering aliphatic as well as cyclic allylic ethers. The barriers for the one-step as well as two-step pathways have been calculated and the mechanisms for both the [1,4] and [1,2] Wittig rearrangement reactions are predicted to occur through a two-step mode. An energetic analysis of the reaction pathways reveals that the [1,4]-rearrangement has a lower barrier than the [1,2]-Wittig rearrangement. The C-O cleavage transition state was found to have the highest barrier and is thus the rate determining transition state for all of the studied molecules. This is in agreement with the previously published experimental studies. The role of the allylic trimethylsilane group in the stabilization of the intermediate anions of the Wittig reactants has also been investigated while comparing it with the phenyl and allylic t-butyl groups through Natural Bond Orbital (NBO) calculations.

Synthesis and Magnetic Properties of Stable Radical Derivatives Carrying a Phenylacetylene Unit

A nitronyl nitroxide derivative, 2-phenylethynyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-oxide (1), and two verdazyl derivatives carrying a phenylacetylene unit, 1,5-diphenyl-3-phenylethynyl-6-oxo-1,2,4,5-tetrazin-2-yl (2) and 1,5-diisopropyl-3-phenylethynyl-6-oxo-1,2,4,5-tetrazin-2-yl (3), were synthesized and their packing structures were studied by X-ray crystallographic analysis and magnetically characterized in the solid state. While 1 and 3 had an isolated doublet spin state, 2 formed an antiferromagnetically coupled pair (2J/k_B = −118 K). Density functional theory (DFT) calculations reveal that the spin density polarized in the phenyl group decreases as the dihedral angle between the phenyl ring and radical plane increases.