The dimeric η3-allylpalladium chloride complexes
formed from various cycloalkenes (C7−C13) and some methyl- and tert-butyl-substituted
cycloalkenes have been characterized by
1H and 13C NMR spectroscopy and in selected
cases by X-ray crystallography. The formation
of syn and anti isomers in the larger ring
systems is demonstrated, and complexes with a
rearranged ring system are formed from tert-butylcyclodecene
and tert-butylcyclododecene.
The sesquiterpene, carophyllene, is shown to form an
η3-allyl exocyclic complex, exclusively
from the (E)-double bond. Cis and trans isomers, with
respect to allyl group orientation in
these halo-bridged dimers, have been identified by low-temperature
(190K) NMR spectroscopy, and their interconversion (ΔG
⧧ ≈ 11
kcal/mol) is considered to involve a “cubic”
η3-allylpalladium chloride tetramer.
Cyclo-1,2-dienes (C&13) are converted into the synthetically promising 2,3-bis(trimethylsilyl)and 2,3-bis-(trimethylstanny1)cycloalk-1-enes by disilylation with
Molecular mechanics calculations (MM3-92) suggest that a slightly distorted diaxial chair conformation of trans-1,2-bis(trimethylsilyl)cyclohexane is at least 1.26 kcal mol Ϫ1 (1 cal = 4.184 J) lower in steric energy than the diequatorial form, and this is supported by NOED spectra, magnitudes of vic-1 H-1 H spin coupling constants, and low temperature (190 K) 13 C NMR spectroscopic measurements, which place the free energy difference between 1.5 and 1.7 kcal mol Ϫ1 .
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