The microwave spectrum of 1-amino-1-ethenylcyclopropane has been investigated in the 13-40 GHz spectral region at about -45 °C. The prevailing (ap)-rotamer (sp with respect to the cyclopropyl and amino, ap with respect to cyclopropyl and the vinyl group) was assigned. This rotamer is virtually a hybrid of the most stable conformer of unsubstituted ethenylcyclopropane and unsubstituted cyclopropylamine, each in its preferred orientation; it has a symmetry plane (C s symmetry) with the CsN bond and the CdC double bond syn to each another. In this conformation, both of the hydrogen atoms of the amino group may be involved in weak intramolecular hydrogen bonds with the π electrons of the double bond. The dipole moment components and the total dipole moment are (in units of 10 -30 C m): µ a ) 2.00(1), µ b ) 0.89( 14), µ c ) 0.0 (for symmetry reasons), and µ tot ) 2.19(9). Four vibrationally excited states belonging to three different normal modes were assigned and their frequencies determined by relative intensity measurements. The microwave work has been assisted by quantum chemical computations at the elaborate MP2/cc-pVTZ, as well as at the B3LYP/6-31G* levels of theory.
The new 1,1-dimethylpropargylamine surrogates, 1-ethynylcyclopropylamine (3) and 1-ethynylcyclobutylamine (5), were prepared as hydrochlorides from cyclopropylacetylene and 6-chlorohex-1-yne in overall yields of 39 and 25%, respectively, on a scale of up to 300 mmol. The amine 3 was converted into the new ethynyl-extended 1-aminocyclopropanecarboxylic acid 4, and both the amine 3 as well as the amino acid 4 were made available as their N-Fmoc-protected derivatives.
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