Bielsa et al.
of the reaction. We can speak of “almost” equivalent
positions, since in the two cases the activation of a C(sp2)
- H bond is involved and the formation of a five-membered
palladacycle is achieved. The difference between the two
activations lies, obviously, in the nature of the substituents,
since the Ph ring at the phosphonium group is strongly
deactivated with respect to that of the benzyl unit due to the
presence of a formal positive charge at the P atom. In
addition, the location of the PdN double bond with respect
to the metallacycle is also determinant, since the endo
arrangement seems to be more stable than that of the exo
arrangement (Figure 1).9 We have observed for most systems
that the endo arrangement is kinetically and thermodynami-
cally favored, except for Ph2MePdNCH2Ph (1g), in which
a regioselective exo- or endo-metalation is obtained as a
function of the reaction temperature. All these facts have
been adequately explained through molecular modeling of
the complete systems, without simplifications, and study of
the reaction mechanism using theoretical methods.
Figure 1. Orientations of the palladation and substituents changed through
this work.
prone to be activated in similar structural environments. A
typical case is the palladation of benzyl-benzylidene-
amines [C6H4C(H)dNCH2C6H5]. These substrates react with
Pd(II) giving endo metallacyclesswith an endocyclic CdN
double bondswhile exo metalation is obtained only if the
endo form is disfavored by steric or electronic reasons.6 This
fact limits further reactivity of the starting substrate since
only endo functionalization could be obtained.
Following our research work on C-H bond activations
promoted by Pd(II) on iminophosphoranes,7 a subject cur-
rently attracting the interest of chemists,8 we present here a
study of the palladation of several benzyl derivatives (Figures
1 and 2) in which a tailored change of substituents has been
performed (Figure 1). These substrates display two almost
equivalent metalation positions, one of them at one Ph ring
of the PPhR2 unit and another one at the aryl ring of the
benzyl unit, which can behave as competitive activation sites,
one of our main purposes being the study of the orientation
Results and Discussion
1. Orientation of the Palladation Position on Benzyl
Iminophosphoranes. Compound 1a has been prepared from
the corresponding azide10a using the Staudinger method.11
The reaction of 1a with Pd(OAc)2 (OAc ) acetate, 1:1 molar
ratio, Scheme 1) in refluxing toluene for 30 min, followed
by solvent evaporation, dissolution of the residue in MeOH,
and treatment of the solution with excess LiCl, gives the
dimer [Pd(µ-Cl){C,N-C6H4(PPh2dNCH2Ph)-2}]2 (3a). Com-
pound 3a is obtained as a mixture of the cis and trans
geometric isomers (1:1.5 molar ratio), although an unam-
biguous assignment of each structure could not be done.
Compound 3a can also be obtained in less drastic reaction
conditions (CH2Cl2, reflux, 1 h; CH2Cl2, room temperature
(r.t.), 5 h), although in these cases the yields are substantially
lower. The preparation of the acetate dimer [Pd(µ-OAc)-
{C,N-C6H4(PPh2dNCH2Ph)-2}]2 (2a) can be done by direct
reaction of Pd(OAc)2 with 1a, as we have just described,
but treating the residue with Et2O after toluene evaporation.
However, the product 2a thus obtained contains many
impurities, which were very difficult to remove. The reaction
of 3a with AgOAc (1:2 molar ratio) in CH2Cl2 affords pure
2a, the latter method being the best synthetic alternative (see
the Supporting Information (SI)).
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The IR spectra of 2a and 3a clearly show that the
iminophosphorane is N-bonded, since the νPN stretch appears
at lower energies (1257 cm-1, 3a; 1278 cm-1, 2a) than those
in the free ligand 1a (1303 cm-1). This shift to low energies,
compared with the corresponding free ligands, is a general
feature of all complexes reported here. The 31P{1H} NMR
spectrum of 3a shows two peaks at 52.29 and 53.73 ppm.
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10134 Inorganic Chemistry, Vol. 46, No. 24, 2007