This web page continues the theme of conductive polymers discussed in another report , Modified Polyaniline and other Conductive Polymers . We will explore the application of the Wittig olefination reaction to the preparation of a hypothetical aromatic hydrocarbon material , polynapthalene ,
which would probably be a semiconducting polymer . I will also outline the synthesis of a related material , poly(bis-1,4-dimethylamino)napthalene , which when doped with one-electron oxidizers might have an exceptional ambient electrical conductivity . It has a structure similar to the sort of polymer proposed in the early 1960s by W.A. Little (the references are below , at the end of the page) as an ambient superconductor .
The Wittig reaction is invaluable in the synthesis of a wide range of functionalized olefins . It and its related olefination reaction , the Horner-Wadsworth-Emmons phosphonate modification , have found many applications in organic synthesis . The original Wittig reaction is useful in cases where halides with unactivated carbon-halogen bonds are used to form the phosphonium salt intermediate . The phosphonate modification is usually employed in preparations where the carbon-halogen bond of the halide is more labile , as in those with an activating group (electron-withdrawing , such as COOR , CN , phenyl , etc.) alpha to it .
The stereochemistry of the two olefination reactions is rather different , too . Generally , the Wittig procedure using triphenylphosphine , non-activated halides , nonpolar solvents , no soluble salts in the reaction mix , and lower temperatures provides olefin products with a pronounced cis (Z) stereochemistry . The phosphonate method , with phosphite esters , activated halides , polar aprotic solvents (especially 1,2-dimethoxyethane , DME) , a soluble lithium salt in the reaction mixture , and higher - i.e. ambient - temperatures yields olefin products with a very high , often exclusive , trans (E) selectivity . I must admit that when I was studying the literature concerning conditions influencing the stereochemistry of the two reactions , I became somewhat confused by the apparently conflicting findings in this area . I will cite the pertinent references below , at the end of the page , and let the reader decide for him/herself .
In the web page mentioned above concerning polyaniline and other conductive polymers , we saw how important the olefination stereochemistry was when the phosphonate method was the key step in the proposed synthesis , for example , of the hypothetical material poly(p-quino)phenylene . It was essential to have an all-trans (all-E) geometry for the polymer , otherwise its growth could be slowed or terminated by cis-geometry kinking of the chains . Therefore , the phosphonate procedure , with the important modifying features noted above , was recommended for the synthesis of this intriguing polymer . In the present case of polynapthalene , exactly the reverse will be true ; we will require as high a degree of all-cis (all-Z) selectivity as possible in the chain-creating olefination . The "classic" Wittig reaction , using all the known cis-promoting conditions , will thus be required in the synthesis scheme I'll describe for polynapthalene .
The synthesis route to polynapthalene is actually fairly simple and straightforward , requiring the preparation of only two intermediates before the final polymer-creating step :
Methylolation is a very general type of chemical reaction in which two substrates , A-H and B-H , combine with the reactive electrophile formaldehyde , such that A-H and B-H are linked together by a methylene bridge and a water molecule is expelled :
AH + HCHO + HB ----------------> ACH2B + H2O
The Mannich reaction is an especially valuable type of methylolation , in which carbon-carbon bonds are formed :
However , methylolation has quite a broad scope , and the two substrates A-H and B-H can be selected from a wide range of organic molecules , provided they have reasonably labile hydrogens . The following are two examples illustrating the utility of methylolation , the latter example especially relevant to the proposed synthesis route to polynapthalene :
This second example is reminiscent of the methylolation of phenol , in the formation of phenol-formaldehyde polymer , the famous Bakelite thermoset resin . The related polymers , urea-formaldehyde and melamine-formaldehyde , are formed similarly from methylolation of the substrates , urea and melamine respectively . In the hydroquinone example above , the added dimethyamine serves as the B-H component , and so a small molecule product is formed rather than a polymer .
In our case , the A-H substrate is hydroquinone and the B-H component is triphenylphosphonium chloride , formed in advance by neutralization of the mildly basic triphenylphosphine , Ph3P , with hydrogen chloride (or concentrated hydrochloric acid) :
(1) Ph3P + HCl (aq) ----------------> Ph3PH+ Cl- ;
(2) Hydroquinone H + HCHO + H P+Ph3 Cl- ---------------->
(at C2 on the ring) HydroquinoneCH2P+Ph3 Cl- + H2O ;
(3) Repeat with a second methylolation at C5 on the hydroquinone ring .
In general , one equivalent of hydroquinone reacts with two equivalents of an electrophile , first at C2 (by default) , then at C5 on its ring . For example , dialkylation of hydroquinone in the Friedel-Crafts reaction produces 2,5-dialkylhydroquinones , such as the commercial antioxidant 2,5-di-tert-butylhydroquinone . And of course there is the example of the bis-methylolation of hydroquinone sketched above .
Hydroquinone and 1,4-benzoquinone are a closely related redox couple in electrochemistry , the latter compound being a moderately strong oxidizing agent . It can be readily reduced , for example with hydrogen over a platinum catalyst , and with metal hydrides such as lithium aluminum hydride . Conversely , hydroquinones can be easily oxidized by several reagents to the corresponding benzoquinones . One of the neatest such oxidizing systems for hydroquinones is oxygen gas itself , catalyzed by copper(II) cation . The hydroquinone and copper(II) salt , in this case CuCl2 , are dissolved in methanol and oxygen gas is bubbled into the solution . The oxidation is somewhat exothermic , and the reaction vessel should be cooled . A good example of this oxidation technique is shown in the following sketch of the preparation of 2,5-bis(dimethylamino)-1,4-benzoquinone :
In this reaction , the first dimethylamine molecule adds a,b to one of the benzoquinone double bonds . The ene-dione immediately tautomerizes to the hydroquinone , which is oxidized back to the quinone (2-dimethylamino-1,4-benzoquinone) . This intermediate accepts a second dimethylamine molecule at its C5 , to form 2,5-bis-dimethylaminohydroquinone . The oxygen and Cu(II) then oxidize this second intermediate to the end-product , 2,5-bis(dimethylamino)-1,4-benzoquinone . Thus , two successive oxidations have been neatly done by the oxygen /copper(II) couple in this system .
The third step , in which polynapthalene is hopefully formed , is a "classical" Wittig reaction carried out on the 2,5-bis-methylenetriphenylphosphonium chloride salt of 1,4-benzoquinone . I have indicated the use of a very nonpolar solvent , hexane , with n-butyl-lithium as the base to abstract two equivalents of HCl from the salt and so form the bis-ylid . The by-product lithium chloride is insoluble in this medium , and remains suspended in it . The all-cis (all-Z) olefin must be produced , as a trans (E) geometry will inhibit the formation of polynapthalene :
I don't want to minimize the difficulty of this polymerization by any means . The condensation of bis-ylids with difunctional carbonyl compounds has been extensively studied , and yields of the cyclized olefins are often disappointingly low . The synthesis of 1,2,5,6-dibenzocyclooctatetriene is typical of such Wittig cyclizations :
In examining the Griffin-Peters synthesis above , I must say that I would have used somewhat different reaction conditions ; for example , with n-butyl-lithium in hexane or sodium hydride in benzene , but certainly not with the very polar DMF and ethanol .
Another problem , probably unique to benzoquinone and its derivatives , is that they are oxidizers that react with nucleophiles with great avidity in an a,b manner across their double bonds , rather than at the carbonyls . Not inconceivably , therefore , the two ylid molecules could condense in an a,b manner to yield only the undesirable small molecule bis-hydroquinone derivative :
Note , however , that the a,b-unsaturated aldehyde cinnamaldehyde has been successfully reacted at its carbonyl group with a phosphonate ylid , to provide a good yield of the all-trans (all-E) 1,4-diphenyl-1,3-butadiene . Fieser , in his excellent organic chemistry laboratory text , describes this experiment for students :
The success of the Wittig and phosphonate olefinations can be attributed mostly to the great strength of the phosphorus-oxygen bond . P-O bond formation is favoured in the reactions because of the complimentary polarities of the carbonyl and ylid bonds :
The phosphorus atom in the ylid is "magnetically attracted" to the carbonyl oxygen and will tend to bond preferentially to it first , thereby minimizing the competing a,b addition of the ylid carbon to the quinone's double bond .
The proposed synthesis of dimethylamino-substituted polynapthalene follows much the same route as its parent material . An additional step is required to introduce the two dimethylamino groups into the benzoquinone monomer . The Baltzly-Lorz oxidative procedure , sketched above , might be used to accomplish this , substituting CuCl2 for the copper(II) acetate salt they used :
My interest in this polymer lies in the possibility that its electrical conductivity might be substantially enhanced by treating it with a one-electron oxidizer , in effect converting it into a Wurster Blue . The original Wurster Blue was first prepared by reacting N,N,N',N'-tetramethyl-p-phenylenediamine with a one-electron oxidizer , which removes an electron from one of the amine nitrogens . The resulting singlet electron on the aminium nitrogen is delocalized by resonance over the entire molecule :
Treatment of poly(bis-1,4-dimethylamino)napthalene with a suitable one-electron oxidizer should therefore convert it into a giant-sized Wurster Blue compound . As is well-known , the doping of olefin polymers such as polyacetylene with a one-electron oxidizer for example , a halogen such as bromine or iodine can dramatically enhance their ambient electrical conductivities . Since the halogens might react with the aromatic rings , they are probably undesirable as doping oxidizers in this present case . Two other types of one-electron oxidizers might be examined here : antimony pentafluoride and the nitrosonium salts .
Antimony pentafluoride is a mild oxidizer (E = 0.8 V , approx.) , but nevertheless has achieved spectacular success in activating graphite electrically . When intercalated into graphite up to 75% by weight , SbF5 causes the ambient electrical conductivity of the composite to rise from around 25,000 ohm-1-cm-1 (undoped) to a million ohm-1-cm-1 . This highly conductive graphite , first prepared at the University of Sherbrooke in 1973 , still holds the world record for the material with the highest electrical conductivity at room temperature [silver , the most conductive metallurgical metal , is far behind at 629,000 ohm-1-cm-1] . The stoichiometry for the formation of the Wurster Blue from the aromatic amine is :
2 [ polymer = N : ] + 3 SbF5 ------------------> 2 [ polymer = N .+ ] SbF6- + SbF3
The nitrosonium cation is a far more powerful oxidizer than antimony pentafluoride :
NO+ + e- --------------> NO (g) ; E0red = 1.45 V .
Several nitrosonium salts are commercially available (Aldrich , Alfa-Aesar) . They are crystalline , stable compounds with inert , non-nucleophilic spectator anions such as perchlorate , tetrafluoroborate , and hexafluorophosphate . Treatment of a suspension of the polymer in a polar solvent such as acetonitrile with a nitrosonium salt should yield the doped material with Wurster Blue properties . In fact , the nitrosonium cation is probably a strong enough one-electron oxidizer to form extensively delocalized radical carbocations in polynapthalene :
The Wurster Blue salt of poly(bis-1,4-dimethylamino)napthalene offers another alluring possibility , that of synthesizing an ambient superconductor . As mentioned near the beginning of this web page , W.A. Little made the controversial suggestion back in the early 1960s that superconductors with transition temperatures well above room temperature might be fabricated from organic olefin polymers (like polyacetylene) with side-groups in which there is a significant resonance of singlet electrons . Little's proposal was based on classical (BCS) superconductor theory , in which the electrical conductors in superconductors , the Cooper pairs , are form by the interaction of singlet conduction electrons with vibrating atomic kernels ("phonons") in the crystal lattice . In Little's picture of an organic polymeric superconductor , the conduction electrons moving along the spine of the polymer would be "excitonically pumped" by the resonating electrons in the side chains , thus causing them to form Cooper pairs at and above room temperature :
Little's theory has never been verified or disproved since he first brought it forward , but it has been remarkably influential to successive generations of scientists (for example , the synthetic metals such as KCP aroused considerable interest at the time of their re-discovery and intensified study in the late 1960s and early 1970s) . Since poly(bis-1,4-dimethylamino)napthalene's Wurster Blue salt has the sort of structure envisaged by Little for his room temperature superconductor , its synthesis and characterization would have added importance and relevance in the study of conductive polymers .
The only reference to "polynapthalenes" I can find in the literature is in U.S. Patent no. 6828406 , Method of producing organic semiconductors having high charge carrier mobility through pi-conjugated crosslinking groups (R. Haasmann et al.) , at http://www.freepatentsonline.com/6828406.html . These polymers seem to be composed of phenylene or napthylene units linked together by alkynyl bridges , which is a quite different structure than that of the "polynapthalenes" proposed in this report .
For general references concerning conductive polymers , see in the References section of the web page , Modified Polyaniline and Other Conductive Polymers .
W.A. Little : W.A. Little , Possibility of Synthesizing an Organic Superconductor, Phys. Rev. 134 (6A) , pp. A1416-A1424 (1964) ; ibid. , Superconductivity at Room Temperature, Scientific American 212 (2) , pp. 21-27 (February 1965) ; ibid. , The Exciton Mechanism in Superconductivity, pp. 17-26 in W.A. Little (ed.) , Proceedings of the International Conference on Organic Superconductors , J. Polymer Sci. , Part C , Polymer Symposia 29 , Interscience , New York , 1970 ; p. 26 . I proposed a different system in an earlier study , based on a hypothetical molecular metal , as an intriguing possible candidate for ambient superconductiviity .
Wittig : A. Maercker , The Wittig Reaction, Organic Reactions , Vol. 14 , Ch. 3 , pp. 270-490 , A.C. Cope (ed.) , John Wiley , New York , 1965 ; S. Trippett , The Wittig Reaction, Quart. Rev. 17 (4) , pp. 406-440 (1963) ; in Wikipedia : Wittig reaction, at http://en.wikipedia.org/wiki/Wittig_reaction . For the phosphonate modification , see the article , Horner-Wadsworth-Emmons reaction, at http://en.wikipedia.org/wiki/Horner-Wadsworth-Emmons_reaction . An overview of the synthetic utility of the "classic" Wittig reaction is provided by H.B. Hopps and J.H. Biel , The Wittig Reaction, Aldrichimica Acta 2 (2) , pp. 3-6 (1969) . This article can be downloaded for free from the Internet (PDF , 2760 KB) . NB : the entire Issue no. 2 must be downloaded to obtain the article , hence the rather large file size .
literature : H.O. House , Modern Synthetic Reactions , 2nd ed. , W.A. Benjamin , Menlo Park , CA , 1972 , pp. 701-708 ; K.P.C. Vollhardt , Bis-Wittig Reactions in the Synthesis of Nonbenzenoid Aromatic Ring Systems, Synthesis 1975 (12) , pp. 765-780 : Non-stabilized ylids (alkylidenetriphenylphosphoranes) generally give a predominately cis-alkenic linkage in the absence of added salts ........The presence of lithium salts or the use of protic media leads to an increase in the production of trans-alkene (p. 766) . Optimization of reaction conditions for the phosphonate modification : S.K. Thompson and C.H. Heathcock , Effect of Cation , Temperature , and Solvent on the Stereoselectivity of the Horner-Emmons Reaction of Trimethyl Phosphonoacetate with Aldehydes, J. Org. Chem. 55 (10) , pp. 3386-3388 (1990) .
Methylolation : H.E. Zaugg and W.B. Martin , a-Amidoalkylations at Carbon, Organic Reactions , Vol. 14 , Ch. 2 , pp. 52-269 , A.C. Cope (ed.) , John Wiley , New York , 1965 ; example from p. 111 .
Mannich : F.F. Blicke , The Mannich Reaction, Organic Reactions , Vol. 1 , Ch. 10 , pp. 303-341 , R. Adams (ed.) , John Wiley , New York , 1942 .
Friedel-Crafts : C.C. Price , The Alkylation of Aromatic Compounds by the Friedel-Crafts Method, Organic Reactions , Vol. 3 , Ch. 1 , pp. 1-82 , R. Adams (ed.) , John Wiley , New York , 1946 ; Table XI , p. 71 ; G.A. Olah and D. Meidar , Friedel-Crafts Reactions, Kirk-Othmer Encyclopedia of Chemical Technology , Vol. 11 , pp. 251-268 , M. Grayson and D. Eckroth (eds.) , John Wiley , New York , 1980 ; C.A. Thomas , Anhydrous Aluminum Chloride in Organic Chemistry , Reinhold Publishing , New York , 1941 ; J. Varagnat , Hydroquinone , Resorcinol , and Catechol, Kirk-Othmer Encyclopedia of Chemical Technology , Vol. 13 , pp. 39-69 (1981) ; p. 60 ; Wikipedia : Friedel-Crafts reaction at http://en.wikipedia.org/wiki/Friedel-Crafts_reaction .
studied : see the review in Synthesis by Vollhardt , cited above .
graphite : J.-M. Lalancette and J. Lafontaine , Intercalation of Antimony Pentafluoride in the Lattice of Graphite, J.C.S. Chem. Comm. 1973 , p. 815 ; F.L. Vogel , The Electrical Conductivity of Graphite Intercalated with Superacid Fluorides : Experiments with Antimony Pentafluoride, J. Mater. Sci. 12 (5) , pp. 982-986 (1977) .
nitrosonium cation : W.J. Plieth , Nitrogen, Ch. 5 , pp. 321-479 in Encyclopedia of Electrochemistry of the Elements , Vol. 8 , A.J. Bard (ed.) , Marcel Dekker , New York , 1978 ; p. 325 .
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