Rhodium catalysts configuration

The most common oxidation states, corresponding electronic configurations, and coordination geometries of iridium are +1 (t5 ) usually square plane although some five-coordinate complexes are known, and +3 (t7 ) and +4 (t5 ), both octahedral. Compounds ia every oxidation state between —1 and +6 (<5 ) are known. Iridium compounds are used primarily to model more active rhodium catalysts. 

Cyclopropanation of C=C bonds by carbenoids derived from diazoesters usually occurs stereospeciflcally with respect to the configuration of the olefin. This has been confirmed for cyclopropanation with copper 2S,S7,60 85), palladium 86), and rhodium catalysts S9,87>. However, cyclopropanation of c -D2-styrene with ethyl diazoacetate in the presence of a (l,2-dioximato)cobalt(II) complex occurs with considerable geometrical isomerization88). Furthermore, CuCl-catalyzed cyclopropanation of cis-2-butene with co-diazoacetophenone gives a mixture of the cis- and trans-1,2-dimethylcyclopropanes 89). 

Rhodium catalysts have also been used with increasing frequency for the allylic etherification of aliphatic alcohols. The chiral 7r-allylrhodium complexes generated from asymmetric ring-opening (ARO) reactions have been shown to react with both aromatic and aliphatic alcohols (Equation (46)).185-188 Mechanistic studies have shown that the reaction proceeds by an oxidative addition of Rh(i) into the oxabicyclic alkene system with retention of configuration, as directed by coordination of the oxygen atom, and subsequent SN2 addition of the oxygen nucleophile. 

In entries 10-13 (Table 21.8) of trisubstituted alkenes, very high diastereo-selectivity is realized by the use of a cationic rhodium catalyst under high hydrogen pressure, and the 1,3-syn- or 1,3-anti-configuration naturally corresponds to the ( )- or (Z)-geometry of the trisubstituted olefin unit [48, 49]. The facial selectivity is rationalized to be controlled by the A(l,3)-allylic strain at the intermediary complex stage (Scheme 21.2) [48]. 

The complexes may be applied to some very demanding and complex transformations as illustrated by a key step in the synthesis of Bafilomycin A1 (Scheme 9), in which a neutral rhodium catalyst was employed69. In contrast, a cationic iridium-based complex was the catalyst of choice in an exacting selective homoallylic alcohol reduction to achieve a vital inversion of configuration in the synthesis of Brevetoxin B70. Functionalized alkenes may also be effectively reduced, significantly trialkylstannanes which... 

Automobile and Hydrocarbon Emissions. The oxidation of carbon monoxide and hydrocarbons is catalyzed by platinum/palladium/rhodium on alumina. If catalyst poisons such as lead and phosphorus are not present, the major problems become initiation of oxidation at low temperature, thermal stability at high temperature, resistance to thermal schock, and a high external surface area catalyst configuration. 

Values in the parentheses are the optical yields on using polymer-supported rhodium catalyst. R or 5 in parentheses stands for the configuration of the product. 

Unsaturated amines, such as 5-amino-l -pentenes, upon hydrocarboxylation give lactams (2-piperidones)34,33. With appropriate substituents, mixtures of diastereomers are formed in up to 87% yield with ratios dependent on the phosphanes used for modification of the rhodium catalyst Rh,(OAc)4. The relative configurations introduced with the substrates are retained. Although chiral phosphanes have been used, no asymmetric induction is reported33. 

The rhodium catalyst also efficiently promotes the cyclopropenation of alkyncs with high diastereoselectivities or with enantioselectivities up to 98 %. Compared with the related cyclo-propanations this process seems to be more dependent on the size and the configuration of the alkyl group of the diazoester114. If the 5-catalyst is employed the configuration at the cyclo-propene is predominantly S, w hile the 7 -catalyst induces the opposite configuration. 

In addition to rhodium phosphane complexes, ruthenium phosphane complexes have also been successfully applied as catalysis for enantioseleetive hydrogenation of 2-acylamino-2-alkenoic acids and esters1 71,72b 3, enol acetates 18 (R = i-Pr E = COOEt X = OCOCH3 98% ee with BINAP)137, and itaeonic acid138. The absolute configuration of the products from the ruthenium-catalyzed reactions shown below is opposite to that obtained with the corresponding rhodium catalysts. 

Diazo-l-oxoalkenes with Z-configuration on treatment with rhodium catalysts yield furans (Equation (7)). This reaction can be envisaged as an electrocyclization reaction of the corresponding carbene <83JOC38i3, 90TL6835> (see also Section 2.07.2.2.3). TTiis route is also useful in the benzo[6)-and benzo[c]furan series (see Sections 2.07.3 and 2.07.4). 

In order to explain the marked effect of silane structure on the stereochemical outcome, it seems reasonable, in the absence of contrary evidence, to assume that diastereomeric a-siloxyalkyl-rhodium intermediates are formed in the rate determining step, where a predominant configuration and the extent of enantiomeric excess of the product would have already been determined. It is, therefore, evident that the steric demands of not only the chiral phosphine ligand but also the substituents on the silicon bound to the rhodium catalyst exhibit a remarkable effect on the selection of enantiotopic faces of a prochiral ketone. An extreme example is given by the reversal of preferred configuration in the reactions of pivalophenone with dimethylphenylsilane (VIII) on the one hand and with trimethylsilane (DC) on the other. 

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