Norbornene catalysts, palladium complexes

As a consequence of the living nature of the copolymerization wifh fhis catalyst, palladium-capped block copolymers of norbornene and ethene as well as of norbornene, ethene, and styrene were synthesized [61]. Higher activities (up to ten times higher) were observed for a series of oxazohne-phosphine complexes (e.g., 31). Several complexes, modified with bisphosphine monooxide and monosulfide ligands (Scheme 8.8, 32 and 33), were also used as catalysts precursors. The best reported turnover frequency is 0.6x10 mol (molh) at 80°C [62, 63]. A slightly lower activity was observed for fhe ketophosphine containing catalyst precursor 34 [64]. The activity of catalyst precursors 35, 36, and 37 is even lower [65]. 

The first addition polymerization of norbornene catalyzed by a late transition metal compound dates back to work of Schultz in 1966 using PdCb as a catalyst. In the late 1970s, palladium complexes of the type L2PdCl2 (L = CeHsCN, Ph3P) were reported to promote the addition polymerization of norbornene,and based on the work of Sen and Lai, " the dicationic system [(CH3CN)4Pd] " [BF4]J was found to be a very active catalyst. 

Asymmetric hydrosilylation of alkenes, catalysed by ferrocenylphosphine-palladium complexes, can be utilized in a synthesis of optically active alcohols from alkenes (Scheme 1). The initial optically active silane adducts are converted to alkyl pentafluorosilicates (1) which are then cleaved oxidatively, with retention of configuration, using peracid (c/. 3,132). Complex (2) was the best of several hydrosilylation catalysts examined using this, alcohols with enantiomeric purities of approx. 50% were obtained from norbornene and styrene as the alkenes. 

Oxatrimethylenemethanepalladium complexes can also be generated by oxidative addition of palladium(O) to 5-methylene-l,3-dioxolan-2-ones and subsequent decarboxylation. Again, reaction with norbornene, norbornadiene and dicyclopentadiene yields polycyclic cyclopropyl ketones in medium to high yield (Table 19). In this case, tetrakis(triphenylphosphane)pal-ladium(O) was the best catalyst found, whereas tris(dibenzylideneacetone)palladium(0)-chloro-form/triphenylphosphane (see above) and bis(cycloocta-l,5-diene)nickel/triphenylphosphane (used in stoichiometric amounts) proved less efficient. 

The following reactions of norbornene and other nonfunctionalized alkenes with substituted methylenecyclopropanes illustrate these points. (1-Methylethylidene)- and (diphenyl-methylene)cyclopropane (1 R = Me, Ph) give rise to the same type of cycloadducts in the presence of either nickel(O) or palladium(O) catalysts. Even at temperatures as low as 40 "C with bis(> -cycloocta-l,5-diene)nickel(0) as catalyst, 2 may be isolated in 70% yield. The reaction can be extended to vinylbenzene and ethene at temperatures of between 40 and 60 °C, where it may be advantageous to use (cyclododeca-l,3,5-triene)nickel(0) ° as a source of the catalytically active nickel(O) species instead of bis(j7" -cycloocta-l,5-diene)nickel(0). ° This is because ligand dissociation from the former complex is more facile, especially at relatively low temperatures. 

Thus, until the last decade, three families of catalysts have been reported to catalyze the addition, or vinyl-type homopolymerization of norbornene resulting in poly(2,3-bicyclo[2.2.1]hept-2-ene). These three catalyst types are the classical TiC -based Ziegler systems (type 1), the zirconocene/aluminoxane systems (type 2) and certain electrophilic palladium(II) complexes (type 3). 

When phosphane-free palladium catalysts, such as bis(dibenzylideneacetone)palladium, bis(jy4-cycloocta-l,5-diene)palladium, tris(norbornene)palladium or a catalyst generated in situ from bis(acetylacetonato)palladium and ethoxydiethylaluminum, are used with 3,3-dimethylcyclopropene, dimer 17 is obtained as the major product (76% when R1 = R2 = Me), along with three tetrameric products in a combined yield of 12.3%. Complexes such as bis(7r-allyl)pal-ladium and bis(acetonitrile)dichloropalladium also act as catalysts but mainly lead to higher oligomers.42 Other 3,3-dialkylcyclopropenes react in the same manner.36 ... 

In a more detailed study, the structure of the catalyst precursor was determined and found to be Pd(Diop),32. Other L2Pd and L2Ni complexes [L = Diop, BPPM, BINAP, etc.] were prepared [e.g., by in situ reduction of Pd(Il)Cl,L with sodium borohydride or as isolated palladium(O) complexes] and used as catalysts for the asymmetric addition of hydrogen cyanide to norbornene. norbornadiene, benzonorbornadiene, and cyclopentadiene dimer. In the presence of excess ( + )-Diop and L,Pd, norbornene gives 91 -95% of exo-2-cyanonorbornane with 24% cc of the ( + )-(15.25,4/ )-isomer. Similarly, use of the ( —)-Diop complex leads to the (-)-(l/ ,2f ,4S)-isomer with 24% ee (95% yield). Lower reaction temperatures, instead of the 120 "C used above, give better ee values (80 =C 32% ee with 94% yield 35 °C 35 % ee with 6% yield)32. 

In light of the initial contribution by Chiusoli et al., it is somewhat surprising that efficient procedures for the palladium-catalyzed hydroarylation of norbornene (144) were reported almost a decade later by Cacchi et al. [3] and, shortly after, by Larock et al. (4) (144 155 Scheme 7.36). These protocols resemble that of Chiusoli et al. (86), as well as those for the conjugate hydroarylation of a, 3-unsaturated carbonyl compounds (see Chapter 8) [88]. In recent years, catalysts such as pal-ladacycles [89-91] and palladium-carbene complexes [92] were also shown to catalyze the hydroarylation of 144. 

The reaction of norbornene with iodobenzene or bromobenzene in the presence of [Pd(PPh3)4] as the catalyst leads to pentacycle 83 (Scheme 11.29) [6, 31, 90-94], This process is known as the Catellani reaction. The parent transformation involves an insertion of the phenylpalladium(II) complex into the double bond of norbornene to give (Ti -phenyl)norbornyl palladium(II) complex 84, which then undergoes intramolecular paUadation to form 85. Further reaction of this paUadacycle with iodobenzene leads to 83. Biphenyl, tetracycle 86, and more complex derivatives 87 and 88, have also been isolated in this reaction [90, 93c, 94], Benzocyclobutenes... 

Single site catalysts, such as metallocene compounds, CGCs, and nickel or palladium diimine complexes, used in combination with MAO or borate cocatalysts, are highly active for the homopolymerization of norbornene and its copolymerization with ethylene. The structure of the norbornene homo- and copolymers can be widely influenced by the symmetry and structure of the ligands on the transition metal complexes. 

Formyl pyrrole 86 was arylated with a series of electron-deficient aryl chlorides in the presence of a palladium—Ai-heterocyclic carbene complex. The catalyst complexes were found to he air stable and delivered the pyrrole products in moderate to good yields with loadings as low as 1% (13BJOC303). Jiao and Bach reported the direct C—H alkylation of electron-deficient pyrroles (87) with a palladium catalyst and norbornene yields were very good and the alkyl partner was tolerant of a wide variety of substituents including esters, acetals, olefins, and nitriles (13AG(I)6080). 

Narbonne et al. developed a one-pot catalytic method for the synthesis of dibenzo[c,e]azepines and their imine analogues in 2014. ° The reaction was catalyzed by a joint palladium-norbornene organometallic catalyst. The complete diastereoselectivity observed originates from a chelated Pd(iv) complex via atroposelective atyl-atyl coupling. The desired dibenzo[c,e]aze-pines were isolated in good yields (Scheme 4.23). 

Nickel-0- and palladium-O-complexes are very active catalysts for the polymerization of norbornene and also for cyclopentene [552-554], Nickel catalysts produce soluble polymers with a molecular weight of over one million while polymers obtained with palladium or metallocene complexes are insoluble. The soluble polymers have an atactic structure. The microstructure of the polynorbornene depends on the catalyst used and is isotactic by synthesis with chiral metallocenes. 

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