Amines, activation oxidative addition

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

The mechanism involves a Pd(0) monocoordinate complex as the active species that undergoes oxidative addition to the aryl halide [141]. Thereafter, coordination of the amine to the palladium centre and deprotonation by the external base results in halide abstraction. After reductive elimination, the coupling product is obtained and the catalytic active species regenerated (Scheme 6.45). 

Although the oxidative addition of the N-H bond of NH3 and amines to transition metal complexes had been known for some time [140], it was only in the late 1980s that Milstein et al. succeeded in designing a homogeneously catalyzed hydroamina-tion reaction involving such an activation process (Eq. 4.27) [141]. 

Allyl carbonate esters are also useful hydroxy-protecting groups and are introduced using allyl chloroformate. A number of Pd-based catalysts for allylic deprotection have been developed.209 They are based on a catalytic cycle in which Pd° reacts by oxidative addition and activates the allylic bond to nucleophilic substitution. Various nucleophiles are effective, including dimedone,210 pentane-2,4-dione,211 and amines.212... 

The elimination of HC1 was proposed to occur also during the H2 activation with the [Pd(PNP)Cl]Cl complexes (PNP = bis-2-(diphenylphosphino)ethyl benzy-lamine, bis-2-(diphenylphosphino)ethyl amine or tris-2-(diphenylphosphino)ethyl amine) [24, 25]. Based on the findings of 31P 1H - and 1H-NMR investigations, the hydride [HPd(PNP)]Cl was detected under H2 atmosphere. The alternative mechanism which involves the oxidative addition of H2 with formation of a Pd(IV)-dihydride intermediate, appeared less likely on the basis of thermodynamic considerations. 

The fact that complex 38 does not react further - that is, it does not oxidatively add the N—H bond - is due to the comparatively low electron density present on the Ir center. However, in the presence of more electron-rich phosphines an adduct similar to 38 may be observed in situ by NMR (see Section 6.5.3 see also below), but then readily activates N—H or C—H bonds. Amine coordination to an electron-rich Ir(I) center further augments its electron density and thus its propensity to oxidative addition reactions. Not only accessible N—H bonds are therefore readily activated but also C—H bonds [32] (cf. cyclo-metallations in Equation 6.14 and Scheme 6.10 below). This latter activation is a possible side reaction and mode of catalyst deactivation in OHA reactions that follow the CMM mechanism. Phosphine-free cationic Ir(I)-amine complexes were also shown to be quite reactive towards C—H bonds [30aj. The stable Ir-ammonia complex 39, which was isolated and structurally characterized by Hartwig and coworkers (Figure 6.7) [33], is accessible either by thermally induced reductive elimination of the corresponding Ir(III)-amido-hydrido precursor or by an acid-base reaction between the 14-electron Ir(I) intermediate 53 and ammonia (see Scheme 6.9). 

Notably, NH3 is indispensable for the catalytic phenol synthesis. In the absence of NH3, neither benzene combustion nor phenol formation occurred on the Re-CVD/HZSM-5 catalyst (Table 10.6). Other amine compounds such as pyridine and isopropylamine did not promote the catalytic reaction at aU, which indicates that the role of NH3 in the catalysis is not due to its basic function. Fe/ZSM-5 has been reported to be active and selective for phenol synthesis from benzene using N2O as an oxidant [90, 91], but N2O did not act as an active oxidant on the Re-CVD/ HZSM-5 catalyst Furthermore, no positive effects were observed by the addition of both N2O and H2O. Notably, the NH3-pretreated Re-CVD/HZSM-5 catalyst selectively converted benzene into phenol with O2 in the absence of NH3, as discussed below. 

The liquid polymer is converted to the rubbery state by reagents that react with mercaptan (-SH) and side groups of the polymer segments by oxidation, addition or condensation to effect sulfide (-S-S-) bond formation. The oxidation reactions are exothermic and accelerated by an alkaline environment. The most commonly employed oxidizing agents which are suitable for curing liquid polymers are cobalt or manganese or lead octoate, p-quinonedioxime and di- or tri-nitrobenzene. Epoxy resin also reacts with liquid polysulfide polymers by addition in the presence of an aliphatic or aromatic amine and polyamide activator as shown in Equation 5.8 ... 

Asymmetric hydrometallation of ketones and imines with H-M (M = Si, B, Al) catalyzed by chiral transition-metal complexes followed by hydrolysis provides an effective route to optically active alcohols and amines, respectively. Asymmetric addition of metal hydrides to olefins provides an alternative and attractive route to optically active alcohols or halides via subsequent oxidation of the resulting metal-carbon bonds (Scheme 2.1). 

Vinyloxiranes are used for facile 7i-allyl complex formation [14], The -allylic ferralactone complex 41 was prepared by oxidative addition of Fe2(CO)9 to the functionalized vinyloxirane 40 and CO insertion. Treatment of the ferralactone complex 41 with optically active a-methylbenzylamine (42) in the presence of ZnCl2 gave the 7r-allylic ferralactam complex 45 via 44. In this case, as shown by 43, the amine attacks the terminal carbon of the allylic system and then the lactone carbonyl. Then, elimination of OH group generates the 7r-allylic ferralactam complex 45. Finally the /1-lactam 46 was obtained in 64% yield by oxidative decomplexation with Ce(TV) salt. The <5-lactam 47 was a minor product (24%). The precursor of the thienamycin 48 was prepared from 46 [15,16]. This mechanistic explanation is supported by the formation of both 7r-allyllactone and lactam complexes (49 and 51) from the allylic amino alcohol 50 [17]. 

A plausible mechanism for the one-pot synthesis ofcarbazoles is shown in Scheme 5. It consists of two interlinked catalytic cycles. In the first cycle a classical Buchwald-Hartwig amination reaction occurs to generate an intermediate 5 which then enters the second cycle by oxidative addition to Pd(0). The resulting Pd(II) complex then undergoes intramolecular C-H activation to give a six-membered palladacycle which subsequently yields the carbazole by reductive elimination. 

The 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) radical was first prepared in 1960 by Lebedev and Kazarnovskii by oxidation of its piperidine precursor. TEMPO is a highly persistent radical, resistant to air and moisture, which is stabilized primarily by the steric hindrance of the NO-bond. Paramagnetic TEMPO radicals can be used as powerful spin probes for investigating the structure and dynamics of biopolymers such as proteins, DNA, and synthetic polymers by ESR spectroscopy [7]. A versatile redox chemistry has been reported for TEMPO radicals. The radical species can be transformed by two-electron reduction into the respective hydroxyl-amine or by two-electron oxidation into the oxoammonium salt [8]. One-electron oxidations involving oxoammonium salts have also been postulated [9]. The TEMPO radical is usually employed under phase-transfer conditions with, e.g., sodium hypochlorite as activating oxidant in the aqueous phase. In oxidations of primary alcohols carboxylic acids are often formed by over-oxidation, in addition to the de-... 

The unique transformation of formamides to ureas was reported by Watanabe and coworkers [85]. In place of carbon monoxide, formamide derivatives are used as a carbonyl source. The reaction of formanilide with aniline was conducted in the presence of a catalytic amount of RuCl2(PPh3)3 in refluxing mesitylene, leading to N,AT-diphenylurea in 92% yield (Eq. 56) [85]. They proposed that the catalysis starts with the oxidative addition of the formyl C-H bond to the active ruthenium center. In the case of the reaction of formamide, HCONH2, with amines, two molecules of the amine react with the amide to afford the symmetrically substituted ureas in good yields. This reaction evolves one molecule of NH3 and one molecule of H2. 

Among the carbonylative cycloaddition reactions, the Pauson-Khand (P-K) reaction, in which an alkyne, an alkene, and carbon monoxide are condensed in a formal [2+2+1] cycloaddition to form cyclopentenones, has attracted considerable attention [3]. Significant progress in this reaction has been made in this decade. In the past, a stoichiometric amount of Co2(CO)8 was used as the source of CO. Various additive promoters, such as amines, amine N-oxides, phosphanes, ethers, and sulfides, have been developed thus far for a stoichiometric P-K reaction to proceed under milder reaction conditions. Other transition-metal carbonyl complexes, such as Fe(CO)4(acetone), W(CO)5(tetrahydrofuran), W(CO)5F, Cp2Mo2(CO)4, where Cp is cyclopentadienyl, and Mo(CO)6, are also used as the source of CO in place of Co2(CO)8. There has been significant interest in developing catalytic variants of the P-K reaction. Rautenstrauch et al. [4] reported the first catalytic P-K reaction in which alkenes are limited to reactive alkenes, such as ethylene and norbornene. Since 1994 when Jeong et al. [5] reported the first catalytic intramolecular P-K reaction, most attention has been focused on the modification of the cobalt catalytic system [3]. Recently, other transition-metal complexes, such as Ti [6], Rh [7], and Ir complexes [8], have been found to be active for intramolecular P-K reactions. 

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