A.A. Dabbawala et al. / Catalysis Communications 12 (2011) 403–407
407
acceptor ability than phosphine and favored branched aldehydes,
particularly P(ONp)3 gave branched aldehydes selectivity comparable
to PNp3. To confirm further whether 1-hexene first isomerized to 2- and
3-hexenes and then hydroformylation occurs to yield branched
aldehydes in the case of P(ONp)3 also, a few set of experiments were
performed. In the first set of experiment, effect of pressure was studied
(entry 7), at 2.0 MPa syngas pressure, 71% isomerized hexene was
observed with only 29% aldehyde chemoselectivity. In the second ex-
periment, effect of ligand/Rhratio wasstudied (entry8), at3.0 ligand/Rh
ratio (syngas pressure 4.0 MPa) and 79% selectivity to branched
aldehyde with 92% aldehyde chemoselectivity was observed. The
results indicated that 1-hexene first isomerized to 2- and 3-hexenes
and then yielded branched aldehydes on hydroformylation. The ligand
free system (unmodified Rh catalyst) also yielded 70% branched
aldehydes with 44% aldehyde chemoselectivity. From the above
data it can be concluded that ligands PNp3 and P(ONp)3 resulted in
best (78–79%) branched aldehydes selectivity (Scheme 3).
The PNp3 gave considerably high branched aldehydes selectivity
thanPPh3. However, aldehyde chemoselectivitywaslow ascompared to
PPh3 at 110 °C and 4.0 MPa syngas pressure (Table 3, entries 2 and 9). As
reported the appropriate combination of two different monodentate
phosphine ligands can greatly influence the regioselectivity as com-
pared to pure ligand [24,25]. Bulky monodentate phosphite ligands are
known to show high activity toward the hydroformylation of olefins
[26,27]. Moreover phosphites as auxiliary ligands have been employed
in several reports [28–30]. Here, we utilized the appropriate ratio of
monodentate bulky phosphine and phosphite ligand and studied their
influence on regioselectivity. The combination of bulky phosphine and
bulky phosphite ligands led to improve the chemo as well as
regioselectivity to branched aldehydes (Table 3, entries 11 and 12).
The chemoselectivity to aldehyde was increased (71 to 96%) while
branched aldehyde selectivity decreased (78 to 72%) using P(OPh)3 as
auxiliary ligand (Table 3, entry 10). Interestingly, bulky monodentate
phosphite P(ONp)3, as auxiliary ligand increased the aldehyde chemo-
(71 to 98%) as well as branched aldehyde (78 to 82%) selectivity
(Table 3, entry 12). On the other hand, the individual ligands PNp3 and P
(ONp)3 resulted in ~78% branched aldehydes selectivity with 71% and
92% aldehyde chemoselctivity respectively (Table 3, entries 8 and 9).
From the above results, it is evident that proper combination of steric
and electronic effect of ligands led to increased overall selectivity (regio
as well as chemo selectivity). The addition of P(OPh)3 as auxiliary ligand
(entry 10) increased only aldehyde chemoselectivty while use of bulky
monodentate phosphite P(ONp)3 as auxiliary ligand (entry 12)
increased the aldehyde chemo- (98%) as well as branched aldehydes
(82%) selectivity. These results clearly demonstrated the potential of
ligands combination for obtaining high branched aldehydes with high
aldehyde chemoselectivity compared to the individual ligands as well as
the conventional Rh/PPh3 system.
complexes modified with PNp3 and P(ONp)3 were found to be an
effective catalytic system to produce branched aldehydes in the
hydroformylation of 1-hexene. The ligands PNp3 and P(ONp)3 having
more steric nature than PPh3 favored formation of branched
aldehydes (110 °C and 4.0 MPa syngas pressure) ensuing in up to
78–79% branched aldehydes. The aldehyde selectivity was 71% in the
case of PNp3. The chemo– as well as regioselectivity were remarkably
increased by the addition of P(ONp)3 as auxiliary ligand in Rh/PNp3
catalyzed hydroformylation of 1-hexene. Such permutation repre-
sents a potential catalytic system without synthesizing complicated
ligands with an increased formation of branched aldehydes in case of
linear alkene like 1-hexene.
Acknowledgements
Authors are thankful to analytical division of the institute for
assistance with analyses and CSIR for financial support under CSIR
Network Project. Authors are also thankful to Dr E. Suresh for single
crystal X-ray data. AAD thanks CSIR, New Delhi, for the award of
Senior Research Fellowship.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
doi:10.1016/j.catcom.2010.10.026.
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4. Conclusions
The hydroformylation of 1-hexene catalyzed by rhodium complex
modified by PPh3 favors generally linear aldehyde. The rhodium