C O M M U N I C A T I O N S
Table 1. Nucleophilic Addition of Aniline to 1-3, 5, and 6a
Table 2. APT Charges for Model η3-Benzyl and Allyl Complexes
a
b
c
8
9
0.113
0.343
0.063
-0.193
0.113
0.122
complexes [(PH3)2Pd(η3-allyl)]+ (8) and [(PH3)2Pd(η3-benzyl)]+ (9)
were conducted. The coordination geometries of the organic ligands
were similar to those of experimental structures. To gain insight into
the relative electrophilicities of these two compounds, APT charge
calculations were performed. These calculations showed that the de-
gree of positive charge at the site of nucleophilic attack (carbon a,
Table 2) correlates with the observed rates of nucleophilic addition.14
In conclusion, studies of the effect of the electronic and steric
properties of organometallic electrophiles on stoichiometric C(sp3)-N
bond formation revealed that addition to these η3-benzyl complexes
is faster than to the corresponding η3-allyl complexes. This result
implies that it should be possible to develop a body of transforma-
tions occurring by attack on η3-benzyl and phenethyl ligands.3,15
a Reaction conditions: 0.022 M Pd complex, 1.08 M aniline, 0.22 M
PhCCPh, 9.0 mM 1,3,5-trimethylbenzene (internal standard) in 0.2 mL of
CD2Cl2 and 0.6 mL of THF-d8 at 60 °C. b Results in parentheses were
obtained from reactions in 0.8 mL of DMSO-d6.
In addition to revealing the fast rate of reaction of η3-benzyl
complexes, the data in Table 1 show the affect of steric and
electronic components on the rate. The influence of steric interac-
tions at the site of reaction is revealed by entries 1 and 2. The
naphthylethyl complex 6 reacted about 4 times slower than the
naphthylmethyl complex 5. The influence of steric interactions
ancillary to the site of attack is revealed by comparing entries 4
and 5 and was the opposite of the effect of steric interactions at
the point of reaction. The two methyl groups on the dimethylallyl
ligand of complex 2 increased the rate of reaction; complex 2
reacted roughly 4 times faster than the parent allyl complex 1.
The effect of the extended aromatic system on the rate is revealed
by comparing reactions of the naphthylmethyl and benzyl complexes.
This extension of the aromatic system accelerated reaction of the
η3-R-arylethyl ligand by more than an order of magnitude. We sug-
gest that this rate is faster because the η2-arene complex formed ini-
tially from addition to the η3-R-naphthylalkyl complex is more stable
than that formed after addition to the η3-R-phenylalkyl complexes.
To compare the rate of reaction of nucleophiles with these
π-ligands with the rate of reaction with a σ-bound alkyl ligand, we
conducted the reaction of aniline with [((R)-BINAP)Pd(CH3)-
(pyridine)]OTf. No reaction occurred, even at high concentrations,
elevated temperatures, and extended reaction times. This result is
consistent with the scarcity of alkyl complexes that undergo
nucleophilic attack or C-N bond-forming reductive elimination.12
To determine if these results were specific to reactions conducted
in this solvent system, we conducted the reaction of aniline with
1-3, 5, and 6 in the more polar DMSO-d6 (Table 1). Reactions of
the R-arylalkyl complexes 3, 5, and 6 were faster in DMSO-d8 than
in THF-d8/CD2Cl2 by a factor of 1.2-2.8; reactions of the allyl com-
plex 1 were faster in DMSO by a factor of 8.8, while reactions of the
dimethylallyl complex 2 were faster by a factor of about 80.13 Thus,
the relative order of reactivity of the benzyl and allyl com-
plexes was retained, with the exception of complex 2. Reaction of
this complex did not occur with a clear first-order decay in DMSO,
and future studies will be directed at understanding the factors that
control the rate and regiochemistry of the reactions of this complex.
Computational studies were performed on model η3-benzyl and
η3-allyl complexes to probe the origin of the difference in rate of reac-
tion of these two classes of complexes. DFT calculations of the model
Acknowledgment. We thank the NIH (GM-55382) for support
of this work.
Supporting Information Available: Spectroscopic and analytical
data of new compounds and experimental and computational proce-
dures. This material is available free of charge via the Internet at http://
pubs.acs.org.
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