Mechanistic studies on the role of cationic palladium hydride initiators in addition polymerization of exo-and endo-functionalized norbornenes

Abstract

Simple routes to [Pd(P-i-Pr(_3))(_2)(O(_2)CCH(_3))(NCCH(_3))][(B(C6F5)4] (Pdl206) have been developed. Reactions of trans-[(i-Pr(_3)P)(_2)Pd(CO(_2)CCH(_3))(_2)] with Li(Et(_2_O)(_2.5)[B(C(_6)F(_5))(_4)] or [HN(Ph)(Me)(_2)]- [B(C(_6)F(_5))(_4)] in acetonitrile led to carboxylate abstraction and formation of Pdl206 in excellent yield. Reaction of trans-[(i-Pr(_3)P)(_2)Pd(0(_2)CCH(_3))(_2)] with K[B(C(_6)F(_5))(_4)] in acetonitrile led to carboxylate abstraction and formation of Pdl206 in only 40% yield but the addition of 1 equivalent of LiOTf pushed the reaction to completion. The thermolysis behaviour of Pdl206 and [Pd(P-i-Pr(_3))(_2)(K(^2)-OAc)][B(C(_6)F(_5))(_4)] (Pdll65) in THF-d(_8) and CDCI(_3), is shown to be complex by (^1)H and (^31)P NMR spectroscopy. The reactions reveal a number of new thermodynamically unstable intermediates that lead to the thermodynamically stable [Pd(P-i-Pr(_3))(_2)(NCCH(_3))(H)][B(C(_6)F(_5))(_4)] (Pdll48) in approximately 40-45% yield and a number of other closely realated cationic palladium hydride complexes. The thermolysis behaviour of Pdl206 and Pdll65 depends heavily on the reaction solvent and temperature, with coordinating THF-d(_8) giving a faster reaction rate than non-coordinating CDCI(_3). (^1)H and (^31)P VT- NMR, (^31)P-(^1)H and (^31)P-(^31)P 2D-NMR, between 25 and -60 C, using CDCI(_3) as solvent, allowed the characterisation of key reaction intermediates and products including cis/trans-[Pd(P-i-Pr(_3))(P(i-Pr(_2))(K(^2)C(CH(_3))(_2)))(NCCH(_3))][B(C(_6)F(_5))(_4)] (Pdll46M), [Pd(P-i-Pr(_3))((i-Pr(_2))P-C(CH(_3))=CH(_2))(NCCH(_3))(H)][B(C(_6)F(_5))(_4)] (Pdll46H)and [Pd(P-i-Pr(_3))((i-Pr(_2))P-C(CH(_3))=CH(_2))(_2)(H)][B(C(_6)F(_5))(_4)] (Pdl273H). The cyclometallated cis/trans- Pdll46M is formed via the reversible carboxylate abstraction reaction from Pdll65. The Pdll46H is identified using (^31)P NMR spectroscopy at -60 C (202.3 MHz, CDCI(_3)): δ (ppm) = cis-Pdll46M, 92.6%; 50.4 (d, non-metalated P, J = 31.56), 41.7 (d, metalated P,J= 31.56): trans-Pdll46M, 7.4%; 38.2 (d, non-metalated P, J = 236.30), 14.8 (d, metalated P, J = 236.30). The cyclometallated phosphine ligand in Pdll46M undergoes a rapid hydride abstraction reaction, above 0 C, resulting in Pdll46H that now has two different trans-phosphine ligands. The Pdll46H is identified using (^31)P NMR spectroscopy (121.4 MHz, CDCI(_3)): δ (ppm) = 52.2 (AB-system, J = 322.0 Hz). Pdll46H is identified using (^1)H NMR spectroscopy (200.0 MHz, CDCI(_3)): δ (ppm) = 5.86 (d, IH, (^3)J(_ph) = 34.8, vinylic H trans to P), 5.67 (d, IH, (^3)J(_ph) =16.1, vinylic H cis to P), -15.25 (t, IH, J = 7.16, Pd-H). The Pdll46H is highly thermodynamically unstable above 0 C, resulting in rapid phosphine ligand exchange to give Pdll48 and a range of other mixed phosphine ligand cationic palladium hydride complexes. The tris-phosphine complex Pdl273H was identified using (^31)P NMR spectroscopy (121.4 MHz, CDCI(_3)): δ (ppm) = 69.8 (dd, J = 110.4 & 29.3, ((i-Pr(_2))P-C(CH(_3))=CH(_2)), 60.0 (dd, J= 32.5 & 29.3, ((i-Pr(_2))P-C(CH(_3))=CH(_2)), 56.9 (dd, J= 110.4 & 32.5, P-i-Pr(_3)).The thermolytic transformation of Pdl206 into cationic palladium hydrides is non-quantitative since a decarboxylation pathway leads to the formation of carbon dioxide and [Pd(P-i-Pr(_3))(_2)(NCCH(_3))(CH(_3))][B(C(_6)F(_5))(_4)] (Pdll62). Pdll62 readily undergoes decomposition to Pd metal under thermolysis conditions. The complicated thermolysis mechanism can be obviated and Pdll48 generated in >90% yield from Pdl206 via the addition of 1 equivalent of Et(_3)SiH in CDCI(_3). This methodology may be useful for the conversion of any general complex, [Pd(PR(_3))(_2)(OAc)(NCCH(_3))][FABA], into [Pd(PR(_3))(_2)(H)(NCCH(_3))][FABA], for any trialkylphosphine. The Pdll48 polymerizes endo/exo-5-decyl-2-norbomene (k = 4.54 x 10(^-3) s(^-1)) approximately four times faster than Pdl206 (k = 1.28 x 10(^-3) s(^-1)) in toluenee-d(_s), at 70 C. The GPC data shows Pdll48 generates a lower molecular weight polymer (M(_w), = 33,000 versus 60,000), sharper PDl (M(_w)/M(_n) = 1.87 versus 2.87), and higher yield than Pdl206 under the same conditions. (^1)H NMR spectroscopy is demonstrated as an analytical tool for probing the initiation and propagation steps of the stable cationic palladium hydride initiator, [Pd(PCy(_3))(_2)(NCCH(_3))(H)]- [B(C(_6)F(_5))(_4)] (Pdl388), with a range of technologically important functionalised norbornenes. The addition of CuCl causes instantaneous initiation and an increase in the polymerisation rate of norbornene monomers by a factor of x3. An increase in polymerisation solvent polarity causes an increase in polymerisation rate. enc/o-Substituted norbornenes are polymerised more slowly than their exo-analogues. The rate of polymerisation is independent of the steric size of the exo-functionality, however, for the endo-regioisomer, functional groups with larger steric size polymerise more slowly. The electronic nature of the functionality plays the major role in lowering polymerisation activity; with electron withdrawing functionalities causing reduced polymerisation activity. The introduction of methylene spacers between norbomene and its functionality reduces the electronic effect of the functionality and increases the monomers polymerisation activity. The polymerisation activities of endo- and exo-functionalised norbomene monomers are greatly affected by the penultimate group effect during co- polymerisation reactions. The new single regioisomer monomers: exo-methoxy-2-norbomene (exo-NB-OMe), exo-5- acetate-2-norbomene (exo-NB-OAc), exo-5-benzyloxy-2-norbomene (exo-NB-OBz) and endo-5- acetate-2-norbomene (endo-NB-OAc) have been synthesised from exo- or endo-5-hydroxy-2- norbomene (NB-OH) and their polymerisation activity investigated. exo-NB-OMe showed very high polymerisation activity. Surprisingly, exo-NB-OAc showed lower polymerisation activity than endo-NB-OAc. The new single isomer acetate functionalised tetracyclododecenes: exo-TD- OAc or endo-TD-OAc, have been synthesised from the Diels-Alder cycloaddition reaction of cyclopentadiene with the corresponding exo- or endo-NB-OAc monomers. In both cases cycloaddition occurred selectively at the norbornene exo face. A single crystal of exo-TD-OAc was obtained from hexanes at -10 C over 4 weeks, and an X-ray structural diagram produced. Steric effects cause both TD-OAc isomers to show low polymerisation activity. The single regioisomer monomers: exo-5-methyl methoxy-2-norbornene (exo-NB-CH(_2)OMe), exo-5-methyl acetate-2-norbomene (exo-NB-CH(_2)OA(_c)), endo-5-methyl methoxy-2-norbomene (endo-NB-CH(_2)0Me) and endo-5-methyI acetate-2-norbomene (endo-NB-CH(_2)0Ac) have been synthesised from exo- and endo-5-carboxylic acid-2-norbomene (NB-CO(_2)H) and their polymerisation activity investigated. Single crystals of exo-NB-C0(_2)H and endo-NB-C0(_2)H were obtained via slow sublimation at 20 C and X-ray structural diagrams produced. The new single regioisomer monomers: endo-5-methyl-exo-5-trimethylsilyl carboxylate-2- norbomene (NB(endo-Me)(exo-C0(_2)TMS)), endo-5-methyl-exo-5-methyl methoxy-2-norbomene (NB(endo-Me)(exo-CH(_2)0Me)) and endo-5-methyl-exo-5-methyl trimethylsilyl carboxylate-2- norbomene (NB(endo-Me)(exo-CH(_2)C0(_2)TMS)) have been synthesised from endo-5-methyl-exo- 5-carboxylic acid-2-norbomene (NB(endo-Me)(exo-C02U)) and their polymerisation activity investigated. A Single crystal of NB(endo-Me)(exo-C0(_2)H) was obtained via slow recrystallisation from ethylacetate at 20 C and a X-ray structural diagram produced. NB(endo-Me)(exo-CH(_2)C0(_2)TMS) was produced using standard chain extension techniques. The (^13)C NMR spectroscopic shifts were employed in an attempt to rationalise the effects of introducing methylene spacers between norbornene and its functional group. The key bicyclic carbons within the monomer structures displayed no regular or consistent trends in (^13)C NMR signal shifts, upon the methylene spacing of the –CO(_2)TMS, -OMe, -NHSO(_2)CF(_3) and –Oac functionalities. Theoretical calculations are performed on isolated molecules of endo- and exo-NB-(CH(_2))(_n)C0(_2)H, where n = 0, 1 and 2, using the Gaussian 03 program, to rationalise the effects of introducing methylene spacers between norbornene and its functional group. The relationships between polymerisation activity and bond lengths, bond angles and Mulliken atomic charges are examined. Although the relationships shown for the exo-regioisomers differ from those occurring in the analogous endo-regioisomers, the main structural changes for both regioisomers involve the C3, C4, C5 and perhaps C6, atoms, which is not surprising given that these carbon atoms are nearest the electron withdrawing substituent. The structural changes occurring could not be used to explain the monomers changes in polymerisation activity.