Chemistry Reference
In-Depth Information
O
O
PS-TBD
(30 mol%)
CD
3
Me
CDCl
3
, 50 °C, 64 h
MeO
MeO
86%
Scheme 6.24
Effective H-D exchange reaction in CDCl
3
It was found that the use of a triflate salt of PS-PAPT (0.1 equiv.) in conjunction with
sodium hydride (2.5 equiv.) could be a remarkably effective method for achieving
dehydrohalogenation of RX or debromination of vicinal dibromides to the corresponding
olefins [84] (Scheme 6.23). In this case, deprotonation of a triflate salt of PS-PAPT with
sodium hydride may have generated the actual strong base species, since sodium hydride
itself did not work well or at all under the same reaction conditions.
Recently, PS-TBD was found to be basic enough to cause hydrogen/deuterium (H/D)
exchange reactions with acidic substrates such as methyl ketones and terminal alkynes in
CDCl
3
as a deuterium source as well as a solvent [85] (Scheme 6.24). An advantage of this
method is that the conditions are compatible with other sensitive functional groups and
aqueous work-up can be avoided, providing a convenient technique for incorporating
deuterium into the acidic substrates.
It has also been reported that the use of PS-TBD as a sequestering agent in periodinate mediated
oxidations was convenient for removing by-products and excess starting reagents [86].
For other works on the use of poly(aminophosphazene) catalysts or mesoporous silica-
supported TBD reagents, only references are shown for convenience [87,88].
6.6 Concluding Remarks
The research area of organic synthesis using polymer-supported superbase reagents or
catalysts has grown rapidly after reports of combinatorial chemistry appeared in the
literature. Accordingly, this method provides a new frontier in the rapid production of a
large number of chemical libraries that consist of structurally diverse molecules, and
particularly with the use of automation or parallel flow systems in multi-step sequence.
Furthermore, thismethodmay be very useful in synthetic chemistry because of its significant
ability to cleanly separate undesired by-products, unreacted starting materials and excess
reagents from the desired products.
Despite these fascinating properties, there have beenvery few studies on the development
of asymmetric organobase catalysts [31,39,89], compared with the dramatic progress in
polymer-supported chiral lithium amide based asymmetric transformations [90]. It can be
expected that new effective polymer-supported chiral superbase reagents will be discovered
in the near future.
References
1. Shuttleworth, S.J., Allin, S.M. and Sharma, P.K. (1997) Functionalised polymers: recent
developments and new applications in synthetic organic chemistry. Synthesis, 1217-1239.
