Norbornene insertion mechanism

Catellani and Lautens have independently reported unique palladium/ norbornene-catalyzed reactions of aryl halides, which mechanistically involve a reversible alkene insertion/p-carbon elimination process [11]. For example, iodobenzene reacted with 1-iodobutane and methyl acrylate to form the multiply-alkylated benzene 29 (Scheme 7.9) [12]. The following mechanism is proposed oxidative addition of phenyl iodide onto palladium generates phenylpalladium(ll) iodide. A double bond of norbornene inserts into the C-Pd bond to form an alkylpalladium species, which cleaves a C-H bond nearby to form the palladacycle 25. -Butyl iodide then reacts with 25 to form the Pd(IV) intermediate 26, which undergoes reductive elimination. Repetition of the cyclometalation/alkylation process leads to the formation of 27. Then, P-carbon elimination affords the arylpalladium species 28 together with norbornene. Subsequently, a Heck-type reaction takes place with methyl acrylate, giving rise to 29. 

Allyl methylcarbonate reacts with norbornene following a ruthenium-catalyzed carbonylative cyclization under carbon monoxide pressure to give cyclopentenone derivatives 12 (Scheme 4).32 Catalyst loading, amine and CO pressure have been optimized to give the cyclopentenone compound in 80% yield and a total control of the stereoselectivity (exo 100%). Aromatic or bidentate amines inhibit the reaction certainly by a too strong interaction with ruthenium. A plausible mechanism is proposed. Stereoselective CM-carboruthenation of norbornene with allyl-ruthenium complex 13 followed by carbon monoxide insertion generates an acylruthenium intermediate 15. Intramolecular carboruthenation and /3-hydride elimination of 16 afford the -olefin 17. Isomerization of the double bond under experimental conditions allows formation of the cyclopentenone derivative 12. 

The reaction of Co(tc-Cp)(CO)2 and of [Co(7t-Cp)NO]2 with nitric oxide in the presence of norbornene has been reported. In both cases the species shown in Figure 14 may be isolated in high yield.125 The mechanism of these three component syntheses could well be related to that of the NO insertion reactions (Scheme 2) in that here NO insertion might occur into the metal-carbon tt-bond of a cobalt-norbornene intermediate. 

The first successful catalytic animation of an olefin by transition-metal-catalysed N—H activation was reported for an Ir(I) catalyst and the substrates aniline and norbornene 365498. The reaction involves initial N—FI oxidative addition and olefin insertion 365 - 366, followed by C—FI reductive elimination, yielding the animation product 367. Labelling studies indicated an overall. vyw-addition of N—FI across the exo-face of the norbornene double bond498. In a related study, the animation of non-activated olefins was catalysed by lithium amides and rhodium complexes499. The results suggest different mechanisms, probably with /5-arninoethyl-metal species as intermediates. 

These reactions are considered to involve insertion of the unsaturated compounds to arylpalladium species followed by the formation of palladacycle intermediates. Oxidative addition of another halide molecule to them leads to the products. In the reaction with norbornene [105 -108] and diphenylacety-lene [109],the corresponding 3 1 and4 1 products and 3 1 product,respectively, are also formed under somewhat different conditions. The mechanisms to account for the formation of these unusual products involving multiple C-H cleavage steps have been proposed. It is noted that, in contrast to Eq. (49), treatment of aryl bromides with aliphatic internal alkynes gives allene derivatives (Eq.50) [110]. 

The reaction of allyl carbonates with 2-norbornene under 3 atm of CO catalyzed by [RuCl2(CO)3]2 gives cyclopentenones. A reaction mechanism involving successive insertion of 2-norbornene and CO into a Jt-allyl-ruthenium bond is proposed (Eq. 11.39) [83], the details of which discussed in Chapter 5. 

The stereospecific formation of syn-deutero-(indolyl)norbornane from C2-D-indole and norbornene suggests the hydroheteroarylation occurs via a mechanism involving olefin insertion into an C—Ir or Ir—H bond. The relatively small kinetic isotope effect (KIE = 1.4) could result from reversible addition of the indole C—H bond to the Ir center prior to the turnover-limiting step (Scheme 5.63). NMR monitoring indicates that C—H bond activa-... 

SCHEME 16.2 Stereochemistry during the polymerization of a prochiral cycloolefin such as norbornene nsing Cs- (A) and C2-symmetiic (B) catalysts. Independent of the relative topicities of the single insertion events (contrasted by pathways a and b), apphcation of the mechanisms known from a-olefin polymerization predict an erythrodisyndiotactic polymer to be formed by C -symmetric catalysts and an erythrodiisotactic polymer to be formed by C2-symmetric catalysts. 

The mechanism by which ethylene/norbornene copolymerization proceeds depends on the exact nature of the metallocene or CGC catalyst employed. The insertion rate depends not only on the last inserted monomer unit, but can also be influenced by the second last inserted monomer unit. The copolymerization parameters described above (Markov 1 model) are derived from the following insertion events ... 

Cl-symmetric catalysts such as 9 and 10 bear a Cp ring substituent pointing in the same direction as one of the chlorine atoms on the Zr center. In the active catalyst species, the chlorine is replaced by the growing polymer chain or a n-complex-bonded olefin. The coordination site on the same side as the Cp substituent (site B) is more sterically hindered than the other coordination site (site A). After insertion of a monomer coordinating at site B, the polymer chain is walked back to less sterically hindered site A by the next monomer insertion (alternating/chain migratory mechanism). But in some cases, the polymer chain can back skip to site A before the next monomer insertion (site epimerization). Norbornene, as a bulky olefin, can be rr-bonded only at site (see Chapter 2 for a further discussion of the alternating and site epimerization mechanisms with catalyst 9). 

It was noted that norbomene-ethylene copolymers, obtained in the presence of the nickel ylide catalysts, can contain comonomer units in nearly equimolar proportions. The mechanism of the formation of these copolymers can be viewed as follows. The norbornene molecule coordinated with the Ni atom cannot insert itself into the Ni-phenyl or Ni-norbomyl bond for steric reasons, while the insertion of ethylene into these bonds takes place readily. This leads to copolymers in which comonomer units alternate with a high degree of regularity. 

The highly controlled living copolymerization mechanism for E-N copolymerization with a PI catalyst/MAO is the result of the stabilization of an ethene-last-inserted species toward chain transfers. The coordination of highly nucleophilic and sterically encumbered norbornene to the ethene-last-inserted active species probably stabilizes the active species toward chain transfer and catalyst decay. Such a coordination would reduce the electrophilicity of the active site and, in addition, provide steric hindrance around the active site, which probably reduces all the possible chain transfers (e.g., hydrogen transfer to a reacting monomer, chain transfer to alkyl A1 species). Thus, the achievement of the controlled living copolymerization probably results from the stabilization of a Ti-E species toward chain transfers and its smooth change to a Ti-N species stable toward chain transfers. 

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