om960790e_si_001.pdf (635.36 kB)
Insertions of Allenes into Palladium−Carbon Bonds of Complexes Containing Bidentate Nitrogen Ligands. Structural and Mechanistic Studies
journal contribution
posted on 1997-02-18, 00:00 authored by Johannes G. P. Delis, Johannes H. Groen, Kees Vrieze, Piet W. N. M. van Leeuwen, Nora Veldman, Anthony L. SpekThe insertion reactions of the allenes propadiene and
1,2-heptadiene in the Pd−C bond of
complexes (N⌒N)Pd(R)X (N⌒N = 8-PQ, p-An-BIAN,
i-Pr-DAB, p-An-DAB, i-Pr-PyCa; R
=
Me, C(O)Me, C(O)Ph, C(O)i-Pr; X = Cl, Br)
have been investigated. An X-ray crystal
structure determination of
(8-PQ)Pd{(1−3-η)-2-methylpropenyl}Cl exhibited
the unexpected
monodentate coordination of the nitrogen ligand. The monodentate
coordination in apolar
solvents and bidentate coordination in polar solvents was demonstrated
by means of NOE
NMR experiments. Kinetic measurements revealed that the reactions
are first order in the
palladium concentration and occur via an allene concentration
independent and dependent
pathway. Reactions of complexes containing flexible bidentate
nitrogen ligands were retarded
by additional free bidentate nitrogen ligand indicating that initial
dissociation of a nitrogen
donor is an important step in the reaction. We have strong
indications that the migration
of the R group to the precoordinated allene is the rate-determining
step. Instead of mass-law retardation by excess X- (X = Cl-,
Br-), an enhancement of the reaction has
been
observed in case of the complexes (8-PQ)Pd(Me)Cl,
(8-PQ)Pd(Me)Br, and
(i-Pr-DAB)Pd(C(O)Me)Cl. Flexible bidentate nitrogen ligands greatly enhance the
reaction, owing to the easy
formation of an accessible site on the metal center. The insertion
of allenes into the Pd−C
bonds of complexes containing rigid bidentate nitrogen ligands probably
proceeds via initial
allene association followed by either halide or nitrogen dissociation
and subsequent migration
of the R group to the precoordinated allene.