Organometallic catalyzed reactions

The opposite of entry 2 is the phase combination of entry 5, a fully organic reaction in the presence of an aqueous phase which contains the catalyst. This type of reaction system is the most often used for the technical realization of aqueous-phase organometallic-catalyzed reactions, for instance in the oligomerization of ethylene using the SHOP process (cf. Section 7.1) or in the Ruhrchemie/Rhone-Poulenc process (cf. Section 6.1.1) of propene hydroformylation (see also Section 4.2.2). 

Tab. 2 Important examples of polar-phase organometallic-catalyzed reactions.

Furthermore, a vast number of organometallic catalyzed reactions can be performed in a biphasic manner thus proving that also uncommon reactions may be worth to be investigated in liquid/liquid systems. For instance, Braddock describes the atom economic nitration of aromatics in a two-phase process [192], Nitration of aromatics leads usually to excessive acid waste streams and the classical Lewis acid catalysts such as boron trifluoride are destroyed in the aqueous quench after the reaction thus making any recycle impossible. In the method of Braddock the ytterbium triflate catalyst is solved in the aqueous phase and can be recycled by a simple evaporative process. Monflier and Mortreux [193] investigated the nickel catalyzed isomerization of olefins, for instance allylbenzene, in a two phase system yielding good yields of cis- and trans-methylstyrene. 

In conclusion, it can be shown that asymmetric catalysis, and particularly asymmetric hydrogenation, occurs in a two-phase system, allowing the very easy recycling of the catalyst without loss of enantioselectivity. Different techniques have been used in order to solubilize the catalyst and the products in the aqueous phase. It is obvious that in the future other asymmetric organometallic-catalyzed reactions in water or in a two-phase system will appear in the literature. 

OrganometaUic complexes are usually assumed to be rather unstable, for instance with respect to high temperatures. It is an interesting fact that organometallic-catalyzed reactions are successfully carried out in SCW. The advantage of organo-metallic reactions with respect to their technical application can be the facile separation of products upon cooling. 

Homogeneous Catalysis in the Aqueous Phase as a Special Unit Operation 115 Table 1 Examples of organometallic-catalyzed reactions. 

Having demonstrated the potential of artificial metalloenzymes for the reduction of V-protected dehydroaminoacids, we turned our attention towards organometallic-catalyzed reactions where the enantiodiscrimination step occurs without coordination of one of the reactants to the metal centre. We anticipated that incorporation of the metal complex within a protein enviromnent may steer the enantioselection without requiring transient coordination to the metal. In this context, we selected the palladium-catalyzed asymmetric allylic alkylation, the ruthenium-catalyzed transfer hydrogenation as well as the vanadyl-catalyzed sulfoxidation reaction. Indeed, these reactions are believed to proceed without prior coordination of the soft nucleophile, the prochiral ketone or the prochiral sulfide respectively. Figure 13.5. 

Acyl halides react with organometallic reagents without catalysts, but sometimes the Pd-catalyzed reactions give higher yields and selectivity than the Lincatalyzed reactions. Acyl halides react with Pd(0) to form the acylpalladium complexes 846, which undergo facile transmetallation. 

Benzodiazepines as antianxiety agents, 1, 170 as anticonvulsants, 1, 166 organometallic complexes, 7, 604 as sedatives, 1, 166 IH- 1,2-Benzodiazepines conversion to 3H-1,2-benzodiazepines, 7, 604 synthesis, 7, 597, 598, 604 3H-1,2-Benzodiazepines acid-catalyzed reactions, 7, 601 nucleophilic reactions, 7, 604 oxidation, 7, 603 synthesis, 7, 596 thermal reactions, 7, 600 5H-1,2-Benzodiazepines photochemical reactions, 7, 599 synthesis, 7, 603... 

In summary, palladium-mediated reactions, especially cross-coupling reactions have found many applications in quinoline synthesis. It is noteworthy that due to the a and S activation for the C(2) and C(4) positions, even 2-chloro- and 4-chloro-quinolines are viable substrates for palladium-catalyzed reactions under standard conditions. With the advent of the palladium chemistry and more commercially available organometallic substrates, more palladium-mediated quinoline syntheses are to be added to the repertoire of quinoline chemistry. 

As described in Section 2.3.2, vinylaziridines are versatile intermediates for the stereoselective synthesis of (E)-alkene dipeptide isosteres. One of the simplest methods for the synthesis of alkene isosteres such as 242 and 243 via aziridine derivatives of type 240 and 241 (Scheme 2.59) involves the use of chiral anti- and syn-amino alcohols 238 and 239, synthesizable in turn from various chiral amino aldehydes 237. However, when a chiral N-protected amino aldehyde derived from a natural ot-amino acid is treated with an organometallic reagent such as vinylmag-nesium bromide, a mixture of anti- and syn-amino alcohols 238 and 239 is always obtained. Highly stereoselective syntheses of either anti- or syn-amino alcohols 238 or 239, and hence 2,3-trans- or 2,3-as-3-alkyl-2-vinylaziridines 240 or 241, from readily available amino aldehydes 237 had thus hitherto been difficult. Ibuka and coworkers overcame this difficulty by developing an extremely useful epimerization of vinylaziridines. Palladium(0)-catalyzed reactions of 2,3-trons-2-vinylaziri-dines 240 afforded the thermodynamically more stable 2,3-cis isomers 241 predominantly over 240 (241 240 >94 6) through 7i-allylpalladium intermediates, in accordance with ab initio calculations [29]. This epimerization allowed a highly stereoselective synthesis of (E) -alkene dipeptide isosteres 243 with the desired L,L-... 

Scheme 6/4.12. Four component Ni°-catalyzed reaction of alkynes, organometallics, enones and TMSCI.

Development of new methodologies for formation of carbon-carbon bonds has been one of the major tasks in organic chemistry. Obviously, organometallic compounds, particularly zinc derivatives, have found great use in such reactions. During the past several years, there have been several significant reports of nickel- and palladium-catalyzed reactions of dialkylzincs and alkylzinc halides with alkyl halides of diverse structure. A detailed account of most of these studies can be found in a recent review by Knochel et al,246... 

The first examples of the use of palladium as a catalyst for carbon-carbon coupling reactions were reported almost thirty years ago [14], and over recent decades a massive effort has been devoted to the extension of the scope of palladium-catalyzed reactions. Organic and organometallic chemists have received extensive input from palladium-coordination chemistry in the task of understanding the mechanisms behind these efficient synthetic procedures [14]. 

Heating by means of single-mode microwave irradiation enables readily adjustable and controlled bulk heating which can be performed safely and with very low energy consumption. The synthetic chemist of today can take advantage of the unique carbon-carbon bond formation reactions afforded by organometallic chemistry and make the reaction happen in seconds or minutes, an important feat, because many transition-metal-catalyzed reactions are time-consuming. 

In addition to aryl halides and triflates, organometallic reagents can be utilized for the catalytic arylation reaction. The rhodium-catalyzed arylation of arylpyridines proceeds with the use of tetraarylstannanes (Equation (67)).83 The ruthenium-catalyzed reaction of aromatic ketones with arylboronates affords the ortho-arylated aromatic ketones (Equation (68)).84... 

The Molecular and Reaction Engineering Basis of Organometallic-Catalyzed Hydrogenations using Compressed and scC02... 

While the transmetalation step is often the rate-determining step for Pd-catalyzed reactions with organometallics, the oxidative addition step is often the rate-determining step in the Heck reactions, although olefin insertion can be rate-limiting in some cases — this is why the Heck reactions of tri- and tetra-substituted olefins sometimes proceed slower than those of di-substituted and terminal olefins. 

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