Norbornene 5-alkyl derivatives

To improve the processability of PI and to alleviate or eliminate volatiles, work began in the late 1960 s under NASA Lewis sponsorship (23), to end-cap oligomers with reactive groups. The first report involved the use of 3,6-endomethylene-l,2,3,6-tetrahy-drophthalic anhydride (5-norbornene-2,3-dicarbo ylic anhydride, nadic anhydride) and alkyl derivatives thereof (e.g. citraconic anhydride) and 1,2,3,4-tetrahydrophthalic anhydride (4-cyclohexene-1, 2-dicarboxylic anhydride) as the reactive end group on imide oligomers (23-25). This initial effort led to the development of P13N [with T for polyimide, 13 for Mn of 1300g/mole and N for nadic end cap] whose structure is shown below. 

Similarly, using macromonomers to which the dendrimer had already been appended, Holerca and Percec found a dependence of the polymer s shape on that of the dendrimer. Homopolymers were prepared from norbornene dendrimer derivatives where the dendrimer branch consisted of an aryl group with two or three ether chains extending outward. When both ether linkages were located in the meta positions, a cylindrical polymer structure was obtained. However, the addition of the third ether at the para position resulted in spheres or cylinders dependent on the number of monomers incorporated. The rationale for the differing outcomes was attributed to the greater planarity of the disubstituted dendrimer as opposed to the trisubstituted wherein the alkyl chains... 

Alkylation of norbornene with acrylic acid derivatives occurs with ruthenium catalysts like [RuCl2(C6H6)]2/Zn in protic solvent.31 (,E)-o -2-norbonylacrylates are obtained with high regio- and stereoselectivity in good yields. 

A few examples are known using homogeneous transition-metal-catalyzed additions. Rhodium(III) and iridium(III) salts catalyze the addition of dialkylamines to ethylene.302 These complexes are believed to activate the alkene, thus promoting hydroamination. A cationic iridium(I) complex, in turn, catalyzes the addition of aniline to norbornene through the activation of the H—N bond.303 For the sake of comparison it is of interest to note that dimethylamino derivatives of Nb, Ta, and Zr can be used to promote the reaction of dialkylamines with terminal alkenes.304 In this case, however, C-alkylation instead of /V-alkylation occurs. 

A wide range of organic azides such as alkyl,80 alkenyl,79,100 aryl, including dinitrophenyl and picryl,101-103 heterocyclic,28,91 trimethylsilyl,79,104 acyl,9 phosphoryl,105 sulfonyl,106 alkoxycarbonyl,9 and oxime azides107 has been found to add to norbornene,25,79,90,104,105,107-121 its derivatives,104 106,1 13,120,122-128 or to related bridged bicyclic... 

Likewise, Lautens and coworkers have demonstrated the strength of the concept of sequential alkylation-alkenylation by application to the synthesis of fused aromatic rings. Hence, in the presence of 10% of Pd acetate, 20% of tri-2-furylphosphane, two equivalents of norbornene, and two equivalents of cesium carbonate, bromoenoates and related derivatives, ortho-substituted aryl iodides react in boiling acetonitrile in the sense of a Pd-catalyzed ortho-alkylation-intramolecular Heck reaction to furnish fused aromatic carbocy-cles 47-53 in moderate to excellent yields (Scheme 17) [74]. 

Chromans and isochromans can be obtained by the reductive Pd-catalysed Heck cyclisation of 4-(2-bromophenoxy)-2-methylbut-l-ene and 3-(2-bromobenzyloxy)-2-methylpropene respectively <07TL2307>. Bromopropyl derivatives of 3,5-dihydroxy-iodobenzene and of 3,5-dihydroxymethyliodobenzene undergo sequential intramolecular alkylations and an intermolecular Heck coupling on treatment with an acrylate under microwave irradiation and Pd-catalysis mediated by norbornene. The products are 5-alkenyl derivatives of pyrano[3,2-g]benzo-2/7-1 -pyrans and pyrano[4,3-/]bcnzo-l77-2-pyrans (Scheme 13) <07JOC775>. 

Our group investigated the influence of the spacer length of 2,3-disubsti-tuted norbornene derivatives with cyanobiphenyl-mesogenic units [42] and with methoxybiphenyl-mesogenic units [43] on the mesophases. Exo,endo-his-[(4 -cyanobiphenyl-4-yl)oxy-n-alkyl]norborn-5-ene-2,3-dicarboxylates with different alkyl spacer lengths (IV-n, n=2-12) were synthesized in a straightforward manner. 

The intermediacy of palladacycles in the reaction of bromobenzene with norbornene in the presence of Pd(PPh3)4 as catalyst and KOAc as a base in anisole as solvent was initially suggested by the isolation, among other products, of two compounds (Eq. 9), the former resulting from norbornene insertion into an alkylpalladium bond, the latter clearly deriving from palladium migration from the alkyl to the aryl site and double norbornene insertion. In both cases the termination step involved p,y-C-C bond cleavage followed by P-H elimination. The stereochemistry of the norbornane unit invariably was exo [19]. 

The reason why certain bromides are more active than iodides in the attack to palladium(II) is probably connected to steric effects. We ascertained that at room temperature the order of reactivity of aryl iodides and bromides with palladium(O) and paUadiiun(II) is the same (I>Br), so the difference is hkely to lie in the easier accessibility of the reaction center of the palladacycle to suitably activated bromo derivatives than to o-alkyl-substituted aryl iodides, which are considerably bulkier. We ascertained that several groups are compatible as shown in Table 4, referring to the reaction of o-substituted aryl iodides, substituted aryl bromides, norbornene, methyl acrylate, K2CO3, and Pd(OAc)2 in the molar ratio 50 50 50 80 120 1. 

Apart from standard subsequent transformations, it was later discovered by Catellani et al. that the C(sp )—Pd bond in norbornene-derived paUadium(II) alkyls activates proximal C(sp )—H bonds, thereby rendering this chemistry a general methodology [87]. 

Numerous new examples of cyclopentenone synthesis from acetylenehexacarbonyl-dicobalt complexes and norbornene derivatives have been disclosed there is evidence of steric control, and the bulky trimethylsilyl group can be employed as a removable direction-determining group to allow synthesis of 3- instead of 2-sub-stituted cyclopentenones. Reaction of (731) with sodamide in toluene gave the new octahydro-2,5-methanoazulene system (732 R = CONHj) successive hydrolysis, reduction, and esterification converted (732 R = CONHj) into (732 R = CHjOTs) which, on solvolysis, gave the homoprotoadamantane derivative (733 X = H, R R = CH2) in high yield.The carboxylic acid (732 R = COjH) underwent spontaneous Friedel-Crafts intramolecular acylation on conversion into its acid chloride to give (733 XX = O R R = CHj) and (733 XX = O, R = Cl, R = Me). Reaction of methyl a-bromocrotonate (mixture of E- and Z-isomers) with the enolates of cyclohex-2-enone affords a mixture of stereoisomeric tricyclo-[3,2,l,0 ]octan-6-ones (734 R —R are variously H or alkyl) in moderate yield. The reaction involves double Michael addition and subsequent substitution. 

FIGURE 16.10 Basic concept of Promerus product lines based on copolymers of norbornene derivatives. Different substituents are used to tailor specific polymer properties (R i = alkyl group, oxygen-containing group R2 = trialkylsiloxy group R3 = photosensitive ester group). 

The complex catalytic cycle commences with oxidative palladium insertion on the aryl iodide bond. The following carbopalladation with norbornene creates a bulky intermediate that is not prone to p-hydride elimination and allows C-H activation of the ortho position of the aryl moiety. Subsequent oxidative addition of the alkyl iodide allows cross-coupling and elimination of the norbornene by retro-carbopalladation. The remaining intermediate 146 then undergoes aMizoroki-Heck reaction with the acrylate, which completes the catalytic process and furnishes the desired cinnamate 148 derivative. 

A great number of norbornene-like monomers [e.g., m = 1-3, R] and R2 = alkyl and aryl groups, Eqs. (103) and (104)] with or without substituents have been employed in polymerization reactions induced by Ziegler-Natta and ROMP catalysts derived from ruthenium, osmium, iridium, palladium, platinum, molybdenum, and tungsten halides or vanadium and zirconium halides or acetylacetonate associated with organometallic compounds [162, 163]. Both addition and ring-opened polymers have been obtained by this way depending on the catalyst employed [Eqs. (103) and (104)]. 

Appear polymers have a similar composition and consist of >90 wt.% of an alkyl norbornene and <10 wt.% of an oxygen-containing norbornene derivative, which is used to increase chain-chain interactions and, thereby, the overall polymer properties. Applications for these polymers are flat panel displays and optical wave guides, both of which are accessible due to the high optical transmission and low birefringence of the polymers, combined with their excellent moisture resistance and ability to be used at high temperature. 

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