Chemistry Reference
In-Depth Information
For both classes of four-membered chelate forming ligands (Scheme 8.6,
2
and
3
),
the phenyl-substituted systems do not show essentially catalytic activity in the copo-
lymerization of ethene in the presence of propene using palladium acetate and
B(C
6
F
5
)
3
as the promoter and methylene chloride as the solvent. However, increas-
ing the size of the R substituent leads to very efficient catalysts (turnover frequency
up to
2
10
4
mol (mol h)
-1
at 70
C). The incorporation of propene under the ex-
perimental conditions is more efficient for the methane-diphosphine
2
[40].
For the diphosphines that form a five-membered chelate ring (Scheme 8.6,
4
-
7
), increasing the rigidity of the backbone leads to an increase in the activity (e.g.,
one order of magnitude from
4
to
7
) [11]. The best turnover frequency reported at
85
C is close to 0.6
10
4
mol (mol h)
-1
[41].
Several dppp-like ligands (which form a six-membered chelate ring), bearing
different substituents on the carbon backbone (Scheme 8.6,
8
-
14
), were also used
as palladium trifluoroacetate complexes [42] or bis-chelate complexes with various
dinitrogen ligands [43]. Under identical reaction conditions (85
C), the trifluoro-
acetate complexes showed turnover frequencies in the range 1.5
10
4
-0.7
10
4
,
the highest activity corresponding to the
u
-ligand
14
. The
l
-ligand
13
was found to
be 1.4
less active than its
u
-counterpart. The difference in reactivity was attrib-
uted to a subtle balance of steric and electronic effects (see also the copolymeriza-
tion of propene). The catalytic activity of the bis-chelate complexes was somewhat
higher. Various alkyl homologues of dppp were also used. The catalytic activity of
the corresponding cationic bis(acetonitrile)complexes was somewhat lower than
that of the parent dppp
8
[44]. Furthermore, supporting the dppp system on
polysiloxane matrices leads to a loss of catalytic activity [45]. Hemilabile dppp
homologues, with the general formula CH
2
{CH
2
P(CH
2
CH
2
OR)
2
}
2
, on the other
hand, showed comparable catalytic activity depending on the substitution [46].
Activation of [Pd(dppp)(O
2
CCH
3
)
2
] with alumoxanes [47, 48] and B(C
6
F
5
)
3
[49] has
been reported. Cationic palladacycles with the general formula [Pd(P-P){
o
-C
6
H
4
(CH
2
NR
2
)}][X] (where P-P is dppp
8
or
9
) were found to be very active catalyst
precursors (turnover number
6
10
5
mol mol
-1
) [50].
The catalytic activity of dppb
15
containing catalysts for ethene was reported to
be about 2.6 times lower than that of the analogous dppp
8
catalysts [7]. Again,
the use of more rigid systems (Scheme 8.6,
16
and
17
) [51, 52] was reported to be
beneficial. Ligand
17
was about four times more active than the corresponding
1,3-bis(di-
o
-methoxyphenylphosphino)propane [52]. The catalytic activity of the
other C
4
-bridged diphosphines
18
-
22
(Scheme 8.6) is lower. Ligand
18
has the
highest activity (turnover frequency
0.6
10
3
mol (mol h)
-1
); the unusual selec-
tivity of
21
, as well as of the
trans
-homologue of
22
, is remarkable, causing the
preferential formation of methyl propanoate under the same reaction conditions
[53, 54]. Palladium acetate complexes of the tetramethylphosphole ligands (
23
-
26
)
in the presence of methanesulfonic acid have also been tested. Whereas
23
was
inactive towards copolymerization,
25
showed turnover frequencies close to
0.8
10
3
mol (mol h)
-1
at 90
C [55].
Catalyst precursors modified with other types of ligands, such as bis(
N
-methyl-
imidazole)
27
[56], N-heterocyclic carbene chelates
28
[57], and calix[6]arene-derived
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