Phosphine-induced reductions

For Zr( j -Cp)2(HXR), generation of Zr(Cp- )2 by phosphine-induced reductive elimination followed by treatment with n-butyl chloride gives the desired oxidative addition product, which is stable ... 

The nucleophilic displacement reactions with azide, primary amines, thiols and carboxylatc salts arc reported to be highly efficient giving high (>95%) yields of the displacement product (Table 9.25). The latter two reactions are carried out in the presence of a base (DBU, DABCO). Radical-induced reduction with tin hydrides is quantitative. The displacement reaction with phenolates,61j phosphines,6M and potassium phthalimide608 gives elimination of HBr as a side reaction. 

Along similar lines, Schwartz and Gell later reported that tertiary phosphines would also induce reductive elimination in bis(i7-cyclopenta-dienyl) (cyclohexylmethyl) (hydrido)zirconium resulting in high yields of zirconocene bis(phosphine) complexes (53-55). Carbon monoxide was found to readily react with a benzene solution of Cp2Zr(PMePh2)2... 

Alkylnickel amido complexes ligated by bipyridine have been prepared that undergo reductive elimination of V-alkyl amines (Equation (54)).207,208 Unlike the phosphine-ligated palladium arylamides, these complexes underwent reductive elimination only after oxidation to nickel(III). Thermally induced reductive elimination of alkylamines from phosphine-ligated nickel complexes appears to occur after consumption of phosphine by arylazides 209... 

More recently, reductive elimination of aryl ethers has been reported from complexes that lack the activating substituent on the palladium-bound aryl group (Equation (55)). These complexes contain sterically hindered phosphine ligands, and these results demonstrate how steric effects of the dative ligand can overcome the electronic constraints of the reaction.112,113 Reductive elimination of oxygen heterocycles upon oxidation of nickel oxametallacycles has also been reported, but yields of the organic product were lower than they were for oxidatively induced reductive eliminations of alkylamines from nickel(II) mentioned above 215-217... 

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). 

The important oxidative addition reaction was found in 1972 by Osborn and coworkers to proceed according to an ATC mechanism in the case of the addition of certain halides to Ir. For instance, irradiation at 436 nm was proposed to induce reduction of EtI from the photoexcited state of Ir, generating Et and Ir a process accelerated by addition of an electron-rich phosphine which produces a better re-ductant (eq. (6) and Scheme 10) [49]. 

The mechanism of ligand-induced reductive eliminations involving the addition of CO or tertiary phosphines to (>j -Cp)2Zr(R)(H) has been studied ... 

In the case of L = CO, CO insertion into the Zr-R bond competes with reduction however, for R = C-C6H11CH2, the reduction just shown [reaction (h)] is the predominant reaction . If L is a tertiary phosphine, reaction (h) does not work well if L is too bulky. Further, pure a donors such as THF and NEts will not effect reduction presumably, L must be able to stabilize the Zr(II) oxidation state . Pyridine-induced reductive elimination has also been utilized as a route to the preparation of aryloxide Ti(II) and Ti(III) complexes from Ti(IV) precursors . 

The activation energy for reductive elimination from [NiR2(bipy)] decreases on coordination of an electron-deficient alkene , and second-order kinetics is observed in the phosphine-induced elimination from the same complex, indicating that the rate-determining step is formation of a five-coordinate intermediate . In general, reductive elimination from cis-dialkylnickel(II) complexes may be promoted by addition of CO, phosphines, or alkenes, and it proceeds by an associative mechanism, whereas the corresponding trans complexes are more resistant to elimination Carbon monoxide induced reductive elimination may also produce ketones ... 

Radical cyclization. Phosphinic acid has been used to induce reductive cyclization of haloalkenes. 

An X-ray analysis of the air-sensitive crystal of the bis( 1-adamantyl isocyanide)pal-ladacycle complex 3b revealed a square planar structure with two coordinated isocyanide figands on the palladium atom. An addition of phosphine Ugands to a solution of 3 induced reductive elimination of the Si—Si bond to give 1,2-disilacyclopentane 2. Thus, tert-alkyl isocyanide ligand plays a critical role to stabilize bis(organosilyl)palladium(ll) complexes. 

The efficiency of the catalytic reaction is typically highly dependent on the employed palladium precursor and the ligand. In most cases the precursor needs to be transformed into the active palladium(O) species prior to the initial oxidative addition and therefore lies off the catalytic cycle. This can be accomplished in a number of ways including ligand exchange, phosphine or base induced reduction, reductive elimination pathways etc. generating an unsaturated low valent palla-dium(O) complex (Scheme 1.1) [5-7]. 

One exception to the use of primary phosphines is in the reported syntheses of the catASium M dass of ligands 20 [46-49]. In one report, reaction of the cyclic sulfate with P(TMS)3 yields the TMS-protected secondary phospholane, which could then be reacted with the appropriate 1,2-dichloro spedes [46]. An alternative procedure to the same intermediate involves preparation of 1-phenyl-phospholane via the bismesylate, subsequent lithium-induced P-Ph deavage, and quenching with TMSC1 [49]. The ligand based on 2,3-dichloromaleic anhydride (20a originally referred to as MalPHOS [46]) has been shown to be effective for the chiral reduction of a- and /1-deliydroarnino acid derivatives and itaconate derivatives. 

In addition to direct DuPhos and BPE analogues, several other ligands containing five-membered phosphacycles have been reported (Fig. 24.6). As early as 1991, non-C2-symme trie phospholane-containing phosphines 37-39 were reported by Brunner and Limmer [7]. These were prepared by base-induced addition of the secondary phospholane to the appropriate diphenylphosphino-substi-tuted olefin. As for the symmetrical 3,4-disubstituted bisphospholanes, enantios-electivities for the Rh-catalyzed reduction of a-acetamidocinnamate were poor. 

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