Communication
[
8]
1
2
1
33 h was observed. Similarly, heating 3 in C D6 for up to
PtÀMe complex 2. However, it could also be an indication of
reduced catalyst lifetime of 3 compared to 1.
6
00 h at 1008C produced a mixture of isomers in a ratio of
.23:1 favoring the isomer with the Pt–phenyl trans to the pyr-
Remarkably, increasing the propylene concentration in-
creased the TON to a level that was comparable to that ob-
served when precatalyst 1 was used in place of precatalyst 3.
As shown in Table 1, entries 3–5, increasing the propylene con-
centration resulted in higher TON for the hydroarylation reac-
tion with precatalyst 3. The reactions shown for entries 3–5
idyl group.
The four complexes (1–4) were tested as pre-catalysts for hy-
droarylation with propylene, 1-hexene, and neohexene (3,3-di-
methyl-1-butene) at 1008C. Reactions were monitored in situ
1
by H NMR spectroscopy with yields and TON determined after
1
either 120 or 240 h by using gas chromatography (Table 1).
were monitored by H NMR spectroscopy, and the similar initial
slopes in the plots of TON versus time suggests that the in-
creased TON with increased olefin concentration results in
large part from greater catalyst longevity. As shown in
Figure 4, the TON begins to plateau at longer reaction times
with higher olefin concentration.
[
a,b]
Table 1. Hydroarylation TON and ratio of products.
[
c]
[d,e]
Entry
Pre-catalyst
Olefin
M/olefin
TON
A/B/C
1
2
3
4
5
6
7
8
9
1
2
3
3
3
4
1
3
3
3
3
1
3
3
propylene
propylene
propylene
propylene
propylene
propylene
1-hexene
1-hexene
1-hexene
1-hexene
1-hexene
neohexene
neohexene
neohexene
0.54
0.54
0.26
0.54
0.80
0.54
0.51
0.51
17.9 (0.9)
10.4 (0.7)
9.3 (0.8)
10.7 (0.9)
15.5 (0.3)
2.7 (0.1)
11.8 (1.1)
8.4 (0.2)
11.7 (0.8)
9.0 (1.5)
10.5 (0.6)
2.5 (0.2)
3.1 (0.1)
11.8 (0.2)
13:84:3
48:52:0
47:53:0
48:52:0
49:51:0
[
g]
3:17:80
16:83:1
57:43:0
57:43:0
59:41:0
58:42:0
15:48:37
85:5:10
90:9:1
[
f]
0.51
1.6
1
0
[
f]
1
1
1.6
1
1
1
2
3
4
0.51
0.51
2.6
[a] Experimental conditions: 0.1–1.0 mmol olefin, 1.3 mol% catalyst,
excess C , 1008C, 120 h, unless otherwise noted. [b] Experiments were
6
H
6
conducted 2–4 times and averaged. [c] Standard deviation noted in pa-
rentheses. [d] C=product of branched b-hydride elimination. [e] Standard
deviations for these values were all less than 1 and averaged 0.4. [f] Reac-
tion time of 240 h. [g] A large amount of disubstituted product (D) was
observed. A ratio of A/B/C/D 1:10:45:44 was calculated. TON is for mono-
substituted product.
Figure 4. Plot of TON for the total products generated by using 1 at differ-
ent concentrations of propylene. Experimental conditions: 0.5–2.6m olefin,
1.3 mol% catalyst, excess C
6
H
6
, 1008C. ~ =0.80 m propylene;
&
=0.54 m pro-
pylene; ^ =0.26 m propylene.
As can be seen by comparing entries 1, 2, and 4 in Table 1,
the removal of the methyl groups from the pyrrolide portion
of the ligand led to a striking change in the anti-Markovnikov
When 1-hexene was used as the substrate for the hydroaryl-
ation reaction, a comparable product ratio (16:83 for A/B,
entry 7) to that obtained with propylene was observed with
complex 1 as the precatalyst. In contrast, with catalyst 3
(entry 8), the anti-Markovnikov selectivity is enhanced to the
extent that it becomes the favored product with an A/B ratio
of 57:43. With 1-hexene, increasing the olefin concentration
past 0.5m had little effect on the TON. For example, when
a threefold increase to 1.6m was employed with complex 3 as
the precatalyst, a similar TON was measured (entries 8 and 11).
When the significantly bulkier olefin neohexene was used as
the substrate, selectivity for the anti-Markovnikov product
drastically increased to a ratio of almost 85:5 for A/B by using
3 (entry 13). An additional product, the branched b-hydride
elimination product (C), which in this case was a-tert-butylstyr-
ene, was also observed comprising 10% of the hydroarylated
product. Unfortunately, while selectivity for the anti-Markovni-
kov product was high, the TON was quite limited. However, in-
creasing the olefin concentration by fivefold (from 0.5m to
2.6m, entries 13 and 14) led to a fourfold increase in TON. The
b-hydride elimination pathway was also inhibited under these
conditions and the A/B selectivity was measured at 90:9.
(
A) to Markovnikov (B) selectivity for the hydroarylation of pro-
pylene. With the original complex 1, the Markovnikov product
was strongly favored (A/B ratio of 15:85). Utilizing complexes 2
or 3 under the same reaction conditions, the products A and B
were formed in a 48:52 ratio, representing a significant in-
crease in the anti-Markovnikov selectivity. Monitoring the hy-
1
droarylation reactions for n-propylbenzene by H NMR spec-
troscopy revealed that the selectivity (A/B ratio) was conserved
over time when complexes 1, 2, or 3 were used as precata-
[
14]
lysts.
Although removing the methyl group on the pyrrolide ring
led to a significant increase in the selectivity for the anti-Mar-
kovnikov product, it also resulted in approximately 40% lower
TON (entries 1 and 4). This difference may be related to the de-
creased electron density at platinum leading to a lower pro-
pensity for oxidative addition of the arene CÀH bond. This
would be consistent with the catalyst synthesis results, in
which the phenyl complex 1 was formed directly from the re-
Me2
action of py pyrÀH with [PtMe (SMe )] in benzene, but the
2
2 2
corresponding reaction with pypyrÀH primarily gave only the
Chem. Eur. J. 2014, 20, 1 – 6
3
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&
&
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