P-Alkoxide eliminations

In a reaction similar to the P-alkoxide elimination reactions seen with zir-conocenes, catalytic Rh(OH)(cod)2 and 2 eq. of arylboronic acids gave cyclic products 165 from enynes 166 (Scheme 35) [100]. In this reaction, transmet-allation of Rh - OR with B - Ph gave Rh - Ph species 167, which inserted into the alkyne, cyclized to 168, and finally underwent P-alkoxide elimination to provide Rh-OCHa. This reaction is limited to the formation of five-membered rings, but it can also undergo cascade type reactions of enediynes to give multicyclic products [100]. 

Transfer hydrogenation of aldehydes with isopropanol without addition of external base has been achieved using the electronically and coordinatively unsaturated Os complex 43 as catalyst. High turnover frequencies have been observed with aldehyde substrates, however the catalyst was very poor for the hydrogenation of ketones. The stoichiometric conversion of 43 to the spectroscopically identifiable in solution ketone complex 45, via the non-isolable complex 44 (Scheme 2.4), provides evidence for two steps of the operating mechanism (alkoxide exchange, p-hydride elimination to form ketone hydride complex) of the transfer hydrogenation reaction [43]. 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

The hydrido(ethoxo) complex carrying an electron-donating q -CsMes (= Cp ) ligand, [Cp IrH(OEt)(PPh3)] (4), was prepared by a metathesis reaction between [Cp Ir Cl2(PR3)] (3) and NaOEt followed by P-H elimination from the intermediate diethox-ide complex (Eq. 6.4) [7]. Several other iridium alkoxide analogs [Cp IrH(OR)... 

The empirical observation that (—)-sparteine 55 is necessary for catalysis implicates a base-promoted pathway in the mechanism. In the first step, a palladium alk-oxide is formed after alcohol binding, followed by p-hydride elimination of the alkoxide to yield a ketone product. On the basis of a kinetic study of the enantio-selective oxidation of 1-phenylethanol, it was revealed that (—)-sparteine plays a dual role in the oxidative kinetic resolution of alcohols, as a ligand on palladium and an exogeneous base " ... 

The molecular derivatives of platinum group metals are usually rather well soluble in organic solvents and volatile in vacuum. At normal pressure they demonstrate very low thermal stability and easily decompose producing fine metal powders. This decomposition occurs more easily for the derivatives of branched radicals as it is based on a P-hydrogen elimination process. An important feature of the chemical behavior of these alkoxide complexes is their rather high stability to hydrolysis. Some derivatives can even form outer sphere hydrates when reacted with water in organic solvents. This stability to hydrolysis can at least partially be due to the kinetic inertness of the complexes of this group. 

Here the Pd(0) complex reacts with diallyl carbonate to form a n-allyl palladium alkoxide that ligates to 16. The resulting intermediate then undergoes P-hydride elimination to produce the lactone and propene (Scheme 6.10). 

Fig. 7. TPD traces from alkyl iodides adsorbed on oxygen- (left) and hydroxide- (right) precovered Ni(lOO) surfaces. The bottom traces correspond to the formation of acetaldehyde from ethyl iodide, while those on top display the desorption of acetone from 2-propyl iodide conversion. The enhancing power of OH surface groups towards partial oxidation pathways is indicated by two observations from these data (1) the yield for acetone increases to the point of resembling that seen with 2-propanol and (2) some acetaldehyde is detected as well. It is at the present time unclear if the OH groups favor the formation of alkoxide intermediates or the subsequent P-hydride elimination step.

As shown in Scheme 3, classic approaches to p-hydroxy ketones are often ineffective for the formation of hydroxypropionates. Standard aldol reactions are complicated by the presence of the P-alkoxide of 4. Enolization of 4 typically results in facile P-elimination, thus preventing aldol addition and formation of hydroxypropionate 7. This approach was later successfully applied to the myriaporone problem by Loh through the use of boron enolates. Likewise, relatively few allylation reactions are appropriate for hydroxypropionate synthesis as the required organometallic species 8 would also be prone to elimination. 

The reaction sequence includes (1) an oxidative addition of C-X bond of the aromatic substrate Ar-X to a Pd° center, (2) a substitution of OR for X in the LnPd (Ar)X intermediate, and (3) a reductive elimination of the C-O bond from the Pd center. The ability of palladium(ll) alkoxides bearing p-hydrogen atoms to undergo p-hydride elimination imposes some limitations on the type of alkoxide groups that can be involved in these C-O coupling reactions [2]. The C(sp )-0 reductive elimination reactions from Pd and Pt centers have also been studied computationally [4]. The reactions were suggested to proceed via a concerted three-center mechanism. 

On the other hand, the arylative ring opening takes place in the presence of a Pd(0) catalyst, an aryl halide, and a base (Scheme 3, reaction b) [32-35]. Oxidative addition of aryl halides toward Pd(0) gives ArPdX species, which can readily interact with the alcohols affording arylpalladium alkoxide intermediates. Then, p-carbon elimination and subsequent reductive elimination occur to give y-arylated ketones and regenarate Pd(0) species. An example is shown in Eq. 15. 

Here the R1CH2OH alcohol adds to the Ru-O bond allowing the formatiOTi of Ru-alkoxide, which undergoes p-hydride elimination to produce the RjCHO aldehyde. The inability of the aldehyde to bind at the metal center, owing to the strong effect of the trans-NBC hgand, is credited for selective imine formation. 

P-Eliminations (Equation 10.1) are the most common type of elimination reaction from transition metal complexes, and p-hydrogen eliminations from metal-alkyl complexes are the most common type of p-elimination reactions. p-Hydrogen elimination from aUcoxo and amido complexes has also been observed in a few cases. p-Alkyl elimination, p-aryl elimination, p-aUcoxide elimination, and p-chloride elimination have also been observed and have been studied carefully because of their importance as side reactions in catalytic chemistry. Although P-hydrogen elimination from metal-alkyl complexes occurs almost exclusively by migratory de-insertion pathways, p-hydrogen ehmmation from alkoxides has been shown to occur by several different pathways. 

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