Recently, as part of an NMR investigation of heterocyclic
R-aminoorganometallics, we examined the carbon-13 NMR
spectra of N-methyl-2-tributylstannylpiperidine (1) at low
temperature. We were surprised to find evidence of two
nearly equally populated conformers, suggesting that there
is a stereoelectronic effect that influences the solution
structure of R-aminoorganostannaes.
for N-methyl-2-tributylstannylpiperidine, as shown in Scheme
1, resulting from ring inversion (RI, vertical arrows) and
Scheme 1a
Partial 13C spectra (C-2, C-6, and the N-methyl region) of
N-methyl-2-tributylstannylpiperidine and the same region
(now including C-4) of cis-N-methyl-4-tert-butyl-2-tributyl-
stannylpiperidine, 2, at various temperatures, are shown in
Figure 1. The absence of any dynamic phenomena in the
a NI ) nitrogen inversion; RI ) ring inversion. N&RI )
simultaneous nitrogen and ring inversion.
nitrogen inversion (NI, horizontal arrows). From the Meeq-
Sneq conformer, RI gives the high-energy MeaxSnax conformer
and then NI affords the MeeqSnax conformer. The latter is
more easily accessed via the MeaxSneq conformer by a
sequential NI-RI sequence from the MeeqSneq conformer.
It has also been suggested that these individual motions could
be coupled such that conversion of the MeeqSneq to the Meeq-
Snax conformer takes place directly (N&RI).18 The energetic
preference for the N-methyl of N-methylpiperidine for the
equatorial orientation is 2.41 kcal/mol (chloroform solu-
tion),19 so the two Meax conformers are unlikely contributors,
leaving the MeeqSneq and MeeqSnax as the two species
observed by NMR.
Figure 1. Partial 13C NMR spectra (100 MHz, THF-d8) of
N-methyl-2-tributylstannylpiperidine and cis-N-methyl-4-tert-butyl-
2-tributylstannylpiperidine.
The free energy difference for a methyl in the 2-position
of N-methylpiperidine is 1.7 kcal/mol,20 the same as its A
value in methylcyclohexane. The A values for several
trialkylstannyl groups (Me3Sn, i-Pr3Sn, Me2PhSn) range from
1 to 1.1 kcal/mol in substituted cyclohexanes,21 and those
for tributylstannyl should be similar. Since the free energy
differences for a methyl in cyclohexane and N-methyl-
piperidine are the same, it is reasonable to assume similar
energy differences for stannyl groups in cyclohexanes and
N-methylpiperidine. Therefore, one might expect ∆G° )
1.0-1.1 kcal/mol for the MeeqSneq a MeeqSnax equilibrium
at low temperature (the Me3Sn value was determined at low
temperature). This would correspond to an equilibrium
constant of ∼1/14, reflecting an 7:93 mixture. The ratio of
peaks in the low-temperature NMR of 1 is 55:45 favoring
the diequatorial isomer (vide infra), corresponding to ∆G°
) 73 cal/mol at -70 °C. To compensate for the expected
spectra of 2 fix the dynamic phenomena of 1 as ring
inversions. The coalescence temperature for the N-methyl
signal of 1 is near -30° and corresponds to an energy of
activation of approximately 11 kcal/mol. In comparison, the
barrier to ring inversion for N-methylpiperidine (in methanol)
is 14.4 kcal/mol.17
Only two species are observed in the DNMR spectra at
slow exchange. There are four possible chair conformations
(9) Pearson, W. H.; Lindbeck, A. C. J. Am. Chem. Soc. 1991, 113, 8546.
(10) Burchat, A. F.; Chong, J. M.; Park, S. B. Tetrahedron Lett. 1993,
34, 51.
(11) Gawley, R. E.; Zhang, Q. J. Am. Chem. Soc. 1993, 115, 7515.
(12) Gawley, R. E.; Zhang, Q. Tetrahedron 1994, 50, 6077.
(13) Gawley, R. E.; Zhang, Q.; Campagna, S. J. Am. Chem. Soc. 1995,
117, 11817.
(14) Gawley, R. E.; Campagna, S. In ECHET96. Electronic Conference
on Heterocyclic Chemistry; Rzepa, H., Snyder, J., Eds.; Royal Society of
Chemistry: London, 1997 (CD-ROM).
(15) Tsunoda, T.; Fujiwara, K.; Yamamoto, Y.; Itoˆ, S. Tetrahedron Lett.
1991, 32, 1975.
(16) Gawley, R. E.; Evanseck, J. D.; Pearson, W. H.; Stevens, E. P. In
ECHET96. Electronic Conference on Heterocyclic Chemistry; Rzepa, H.,
Snyder, J., Eds.; Royal Society of Chemistry: London, 1997 (CD-ROM).
(17) Lambert, J. B.; Keske, R. G.; Carhart, R. E.; Jovanovich, A. P. J.
Am. Chem. Soc. 1967, 89, 3761.
(18) Delpuech, J.-J. In Cyclic Organonitrogen Stereodynamics; Lambert,
J. B., Taieuchi, Y., Eds.; VCH: New York, 1992; p 169.
(19) Crowley, P. J.; Robinson, M. J. T.; Ward, M. G. Tetrahedron 1977,
33, 915.
(20) Eliel, E. L.; Kandasamy, D.; Yen, C.; Hargrave, K. D. J. Am. Chem.
Soc. 1980, 102, 3698.
(21) Kitching, W.; Olszowy, H. A.; Harvey, K. J. Org. Chem. 1982, 47,
1893.
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Org. Lett., Vol. 1, No. 4, 1999