Synthesis of a Sterically Demanding Dispiropiperidine and Its Application in Monoamidodialkyl Zincate Complexes

The new sterically
hindered piperidine analog, dispiro­[cyclohexane-2,2′-piperidine-6′,2″-cyclohexane]
(CPC­(H), 2), and its N-methylated derivative
CPC­(Me) (3) were synthesized from commercially available
starting materials in short steps. The N-lithiated
amide LiCPC (4) was also isolated from 2 as a cyclictrimer in single crystals and showed slightly larger
steric hindrance than that of lithium 2,2,6,6-tetramethylpiperidide
(LiTMP) in the competitive methylation reaction with methyl trifluoromethanesulfonate.
In addition, the heterobimetallic heteroleptic zincate complexes [Li­(μ-NR2)­(μ-Et)­Zn­(Et)] (NR2 = CPC, 5, and NR2 = TMP, 6) were obtained as THF-
and TMEDA-coordinated monomer 5·(THF)2, 6·(THF)2, 5·TMEDA, and 6·TMEDA (THF =
tetrahydrofuran, TMEDA = N,N,N′,N′-tetramethylethylenediamine).
These molecular structures bearing different amido ligands in single
crystals showed little structural differences from crystallographic
studies. Diffusion-ordered spectroscopy (DOSY) revealed that the solution
structures of the zincate complexes 5·(THF)2 and 6·(THF)2 only differ in the number of coordination THF molecules. In
the deprotonation reactions with tert-butyl 3-bromobenzoate,
the zincate complexes containing the CPC ligand [Li­(μ-CPC)­(μ-R)­Zn­(R)]
(R = Et (5), tBu) showed moderately
improved regioselectivity for the 6 position in comparison to those
containing the TMP ligand [Li­(μ-TMP)­(μ-R)­Zn­(R)] (R = Et
(6), tBu).