P. Pongrácz et al. / Journal of Organometallic Chemistry 695 (2010) 2381e2384
2383
CHO
Ph2P
Ph2P
CHO
Pt(II) cat, 1-5
+
CO/H2
8
6
7
RO2C
CO2R
RO2C
Scheme 1.
1: R = Et
2: R = i-Pr
3: R = t-Bu
4: R = Me
5: R = t-Bu
In addition to the branched and linear aldehyde products 7 and
8, respectively, some hydrogenation product (ethylbenzene, 9) was
also formed. Although the formation of aldehydes was highly fav-
oured under all conditions, the presence of 9 was not negligible
even under optimal conditions (vide infra).
Fig. 1. The P-alkene ligands (1e5) used in this study.
spectra thereof (Table 1). In these spectra, the coupling constants
are diagnostic, showing values of 2600e2700 Hz or 3100e3200 Hz
for the trans and cis complexes, respectively. The 31P NMR spectra of
the platinum complexes provide a sensitive probe for the structures
of complexes, even in complicated mixtures. The magnitude of the
1J(195Pt, 31P) coupling constants is strongly dependent on whether
the P atom has another P atom or a chlorine atom as ligand in
the position trans to it on the Pt. It is known from the literature that
the trans PeCl arrangement features 1J(195Pt, 31P) coupling
constants typically larger than 3500 Hz, while trans-PeP arrange-
ment displays typical platinumephosphorus coupling constants of
2500e3000 Hz [15].
It is worth mentioning that although the ligands used in the
present study can be considered as ortho-substituted PPh3
analogues, the 1J(195Pt,31P) coupling constants for 4b and 5b are
significantly smaller than that observed in the corresponding cis-
PtCl2(PPh3)2 (3678 Hz,14.7 ppm) [16]. Interestingly, the 1J(195Pt,31P)
coupling constants for 1ae5a are almost the same as that measured
in case of the trans-PtCl2(PPh3)2 (2605 Hz, 22.0 ppm) complex [17].
Accordingly, on the basis of NMR investigations, a weaker interac-
tion between our P-ligands and the platinum(II) centre can be
envisaged for the cis complexes than in case of the triar-
ylphosphines investigated earlier [16,17]. Indeed, similarly weak
interactions can be invoked on the basis of the 1J(195Pt,31P) coupling
constants for the trans complexes. This phenomenon is probably
due to the small effect of the ortho-substituents that would be
minimised between the phosphine ligands occupying trans posi-
tions on the metal centre.
The hydroformylation activity of the in situ-generated plati-
numetin(II)chloride catalysts, obtained from PtCl2L2 (L ¼ 1e5)
precursors, is moderate. The conversions obtained in 24 hours are
strongly dependent on the structure of the ligand. The highest
catalytic activity was obtained from the two diester derivatives 1
and 2, resulting in 66 and 88% conversion (Table 1, entries 1 and 2).
Lower conversions were obtained with the two monoester-type (4,
5) and the di-tert-butyl ester (3) ligands (entries 3, 5 and 7). The
activity of these catalysts is lower than those of the corresponding
rhodium catalysts and most of the platinumediphosphineetin(II)
chloride systems tested to date [8,11]. Nevertheless, the novel
ligand-containing catalysts are of interest from several theoretical
points of view.
The regioselectivity towards branched aldehyde varies between
50 and 55%. The introduction of an
a-unsaturated mono/diester
moiety into ortho-positions of one of the phenyl rings of PPh3,
yields catalysts that afford slightly higher branched selectivities
than is obtained with PPh3. (The application of the obvious analo-
gous catalytic precursor, cis-PtCl2(PPh3)2 resulted in 45% regiose-
lectivity [12]. It is worth mentioning that similar results have been
obtained by using the corresponding PtePPh3eSnCl2 ‘in situ’
system for the hydroformylation of styrene [18].) It is worth noting
that higher conversions and practically identical chemo-
selectivities, accompanied by a decrease in the regioselectivity,
were observed for reaction times as long as 72 h (entries 4, 6 and 8).
The fact that both the cis-PtCl2(PPh3)2 precursor, reported
previously, and the trans-PtCl2L2 precursors in this study gave
similar regioselectivities, i.e. almost equimolar formation of 7 and 8,
Interestingly, in our earlier work [14] using these P-alkene
ligands with Pd, each was shown to bind in a bidentate (P-binding
and alkene-binding to the metal) fashion to the Pd centre. This was
manifested by significant upfield shifts of the resonances charac-
teristic of the alkene hydrogen atom(s) in the 1H NMR spectra of the
Pd-complexes when compared to the free ligands. For example, free
ligand 4 gives resonances at 8.44 ppm and 6.27 ppm for the alkene
protons, while the Pd complex thereof gave resonances at 5.95 ppm
and 4.44 ppm, respectively. In general in the present instance, it
appears as if the P-alkene ligands prefer a monodentate binding
fashion to the Pt centre, as evidenced by the low field resonances
for the alkene protons that are reflective of a free alkene moiety in
the ligand. In only one case is this not so, namely for complex 4b,
where the alkene signals for the bound ligand shows similar upfield
shifts as those observed for the Pd complex, namely showing
resonances at 6.29 ppm and 4.58 ppm, respectively, indicating
a bidentate binding mode to the Pt ion.
Table 2
Hydroformylation of styrene with platinum complexes containing 2-diphenyl-
phosphinobenzaldehyde-derived P-alkene phosphines (1e5).a
Entry
Catalyst
Conv. [%]
Rcb [%]
Rbrc [%]
1
2
3
4
5
6
7
8
1a þ 2SnCl2
2a þ 2SnCl2
3a þ 2SnCl2
3a þ 2SnCl2
4a þ 2SnCl2
4a þ 2SnCl2
5a þ 2SnCl2
5a þ 2SnCl2
66
88
43
70
41
53
36
48
46
35
58
45
14
87
81
82
84
85
82
86
84
79
79
80
81
77
50
54
55
52
55
44
54
44
45
48
56e
50
50
d
d
d
e
e
e
e
e
9
PtCl2(PhCN)2 þ 1 þ SnCl2
PtCl2(PhCN)2 þ 2 þ SnCl2
PtCl2(PhCN)2 þ 3 þ SnCl2
PtCl2(PhCN)2 þ 4 þ SnCl2
PtCl2(PhCN)2 þ 5 þ SnCl2
10
11
12
13
a
Reaction conditions (unless otherwise stated): Pt/styrene ¼ 1:100; 0.05 mmol
Pt-complex precursor, 0.1 mmol SnCl2; p(CO) ¼ p(H2) ¼ 40 bar; T ¼ 100 ꢀC; t ¼ 24 h;
solvent: toluene.
3.2. Hydroformylation reactions
b
Rc ¼ chemoselectivity towards aldehydes [(moles of 7 þ moles of 8)/(moles of
7 þ moles of 8 þ moles of 9) ꢁ 100].
Pre-formed complexes PtCl2(L)2 (where L ¼ 1e5, Fig. 1) were
used as catalysts for the hydroformylation of styrene (Scheme 1) in
the presence of 2 equivalents of tin(II) chloride per Pt under ‘oxo-
conditions’ (p(CO) ¼ p(H2) ¼ 40 bar, 100 ꢀC, as described in Table 2).
c
Rbr ¼ regioselectivity towards branched aldehyde regioisomer [moles of 7/
(moles of 7 þ moles of 8) ꢁ 100].
d
Reaction time: 72 h.
Pt/ligand (1e5)/SnCl2 ¼ 1/0.55/1.5 was used.
e