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The utility of Sato's titanium-mediated reduction of alkynes towards the synthesis of all cis-poly(phenylenevinylene)s (PPVs) is demonstrated by the syn-selective reduction of a variety of model diynes as well as a tetrayne. This technique was then applied to the reduction of a poly(phenyleneethynylene) (PPE) to provide the corresponding all-cis PPV polymer.
Poly(phenylene vinylene)s (PPVs) are among the most actively studied conjugated polymers.iii Although many techniques to synthesize high molecular weight PPVs exist, they are largely limited to the synthesis of predominantly trans-PPVs.iii Recent work by Katayama and Ozawa has, for the first time, provided access to all cis-PPVs by way of a stereospecific Suzuki-Miyaura cross-coupling polymerization of 1,4-bis((Z)-2-bromovinyl)benzenes with aryl-bis-boronic acids.iv We have been interested in an alternative approach, where rather than build a PPV with a pre-ordained stereochemistry, a post-polymerization syn-selective reduction on a poly(phenylene ethynylene) (PPE) is employed. This scheme has the advantage that high molecular-weight PPEs can be synthesized using either Pd-catalysis or alkyne metathesis.v This route could also potentially allow for the access to an additional array of PPVs that are uniquely accessible from PPEs. The transformation of the triple bonds in PPEs and other acetylene building blocks to alkenesvi has considerable potential.
Although there are many means by which to reduce alkynes to disubstituted alkenes,vii we judged the titanium-mediated reduction developed by Sato to be the most promising.viii This transformation is stoichiometric in both titanium and magnesium, but the reduction is quantitative and completely cis-selective for a wide variety of alkyne systems. Additionally, the titanium and magnesium oxide byproducts can be easily removed with an aqueous workup, thus minimizing the amount of impurities in the polymer product.ix
In contrast to Sato's work, diethyl ether was not a suitable solvent for these substrates. However, with toluene as the solvent, the desired cis-olefins are obtained in excellent yields and selectivities (entries 1 and 2, Table 1). The geometry of the olefins were assigned by the coupling constants of the vinyl protons. The reaction is tolerant of a variety of substitution patterns, most notably the ortho-bromo groups of 7 (entry 5) and the meta-alkyne isomer (9, entry 6). The tetrayne, 11, was also successfully reduced (entry 7), although isolation difficulties resulted in a slightly diminished yield.
The low temperature of the reaction made application to polymer systems challenging, as many PPEs are insoluble in toluene at −78 °C.x However, the reaction appears viable for systems that are soluble at low temperatures. Using the standard conditions, polymer 15 was cleanly reduced to PPV 16 (Scheme 1, Figure 1).xi,xii The isolated polymers behaves similarly to the materials described in earlier work describing all cis-PPVs,3 and these materials undergo an irreversible red-shift in absorbance when exposed to UV-light (Figure 2).
A means to effectively expand the scope of this reaction in the synthesis of polymers having all-cis PPV-linkages is to convert p-bromo-functionalized diyne systems such as 9 to the corresponding cis-diene and then perform a Sonogashira polymerization with a diyne to make an all-cis PPV/PPE co-polymer (Scheme 2). Polymer 17 shows a similar cis-trans isomerization under irradiation as 16 (Figure 3). All-cis PPV/PPE co-polymers should have greater availability and versatility compared to the all-cis PPV accessed via the titanium mediated reduction of PPEs.
This work represents the first and only example of converting a PPE to an all cis-PPV system. Although limited in polymer scope, this method does appear complimentary to existing cis-PPV syntheses, which required lengthy monomer synthesis and did not provide an example of a PPV possessing substitution on both phenyl subunits.3 Additionally this technique provides access to potentially useful all-cis monomers for use in polymer synthesis.
This work was supported by the US Army Medical Research (W81XWH-07-1-0649) and the Army Research Office's IED Stand-Off Detection Research Program (W911NF-07-1-0654). C.G.E. is grateful for a postdoctoral fellowship from the National Institutes of Health.