Polymer Wittig

Polymer Wittig reagents are useful for the conversion of an aldehyde or ketone to an alkene according to the sequence in Eq. 9-70 [Ford, 1986a,b],... 

For the purpose of our discussion, a polycrown is here defined as a polymer system arising by polymerization of a crown monomer unit. Extensive work has been done in this field by Kopolow, Hogen Esch and Smid and the examples presented here are taken from a paper by all three of these workers. A typical preparation of vinylbenzo-15-crown-5 is accomplished according to the scheme shown as Eq. (6.25). One might also have utilized a formylation/Wittig sequence on benzo-15-crown-5 to accomplish the same end. 

Scheme 4 Microwave-assisted Wittig reactions using polymer-supported triphenylphosphine in methanol...

The Wittig reaction has been carried out with polymer-supported ylids (see p. 509). It has also been done on silica gel. " ... 

However, thermolysis of the phosphonium salts (X=+PPh3) leads directly to the indolic products without need of acid catalyst or PPh3, and thus may not proceed via a normal Wittig pathway. Alternatively, Hughes has effected a solid-phase version of this reaction employing a polymer-hound phosphonium salt and potassium tert-butoxide as base <96TL7595>. In this case, the phosphine oxide by-product remains bound to the polymer resin. 

Wittig-Type and Wittig-Horner-Type Polycondensations Polymers... 

Very recently a novel one-pot three-step Wittig reaction using microwave irradiation and polymer-supported triphenylphosphine has been reported [57]. By use of... 

Scheme 12.15 Microwave-mediated one-pot, three-component Wittig olefinations utilizing polymer-supported triphenylphosphine.

In the effort to make pure blue-emitting materials Shim and coworkers [146] synthesized a series of PPV-based copolymers containing carbazole (polymers 95 and 96) and fluorene (polymers 97 and 98) units via Wittig polycondensation. The use of trimethylsilyl substituents, instead of alkoxy groups, eliminates the electron donor influence of the latter and leads to chain distortion that bathochromically shifts the emission (Amax = 480 nm for 95 and 495 nm for 97). In addition, a very high PLQY was found for these polymers in the solid state (64 and 81%, respectively). Single-layer PLEDs fabricated with 95 and 97 (ITO/polymer/Al) showed EL efficiencies of 13 and 32 times higher than MEH-PPV, respectively (see also Ref. [147] for synthesis and PLED studies of polymers 99 and 100) (Chart 2.20). 

Kim et al. have introduced silicon atoms in PPV block copolymers to confine the conjugation length and achieve blue EL materials. Copolymers 188-190 [215] and 191 [216] have been synthesized by Wittig-Horner and Knoevenagel condensation, respectively. The emission band in this series can be tuned between 410 and 520 nm, and ITO/polymer/Al PLEDs with turn-on voltages 7 V have been reported (Chart 2.41). 

Although of little general use under normal Wittig conditions, tetra-n-butyl-ammonium iodide aids the reaction between polymer-bound benzylphosphonium salts and aromatic aldehydes with yields generally >90% [17]. 

Wittig reaction with polymer-bound phosphonium salts... 

Scheme 2.2 Polymer-supported triphenylphosphine equivalents for use in Wittig reactions.

For the synthesis of alkenes, the Wittig and Horner-Wadsworth-Emmons reactions have become important tools. Triphenylphosphine that is used in the Wittig reaction can be immobilized either on the polymer or can be used in solution for solid-phase chemistry (Scheme 3.18). The Horner-Wadsworth-Emmons reaction for example is used for the synthesis of aldehyde building blocks [261]. 

Ford, W. T., Wittig Reactions on Polymer Supports, Chap. 8 in Polymeric Reagents and Catalysts, W. T. Ford, ed., Am. Chem. Soc. Symp. Ser., Vol. 308, American Chemical Society, Washington, DC, 1986b. 

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