Enolates rhodium

In the reductive aldol condensation of an ,/J-unsaturated ester and an aldehyde shown in Eq. 291, the initial step is believed to be the addition of an in situ formed rhodium hydride to the a,/Tunsaturated ester, followed by reaction of the resulting rhodium enolate with the aldehyde.470 The reaction has been carried out both inter-470 and intramolecularly471,472 as well as in an asymmetric fashion (Eq. 291). 

This catalytic reaction was believed to proceed analogously to those with phenylboronic acids (Scheme 49) 137 137a Transmetallation of the arylstannane with the cationic rhodium complex generated the rhodium aryl species a and trimethyltin tetrafluoroborate. Conjugate addition generated rhodium enolate b, which subsequently reacted with... 

Krische and co-workers have revealed efficient reductive generation of rhodium enolates under hydrogenation conditions.403 40311-403 Both inter- and intramolecular reductive aldol reactions proceed smoothly and stereoselec-tively, although intermolecular reactions generally show low diastereoselectivity (Equations (52) and (53)). 

Analysis of the Mukaiyama-type aldol coupling (Eq. 2) and the well-known hydrosilyla-tion of a,/l-unsaturated carbonyl compounds 11 in the presence of a rhodium catalyst, indicate that both can be explained by the intervention of the rhodium enolate 13. This line of reasoning provided the impetus to develop a new crossed aldol coupling using a hydrosilane, an a,yS-unsaturated ketone 11, and an aldehyde to form 15 (Scheme 6.4). 

Rhodium-catalyzed addition of boronic acids to enone moiety 89 led to a rhodium-enolate 90 which can be trapped by addition to the adjacent carbonyl function giving functionalized cyclopentanes or cyclohexanes 91. An important feature of this methodology is that this process allows the creation of three contiguous stereocenters with a high level of stereoselectivity. An asymmetric version of this reaction has also been realized with a chiral ligand (BINAP) giving excellent enantiomeric excesses (77 to 95%) (Scheme 34). 

There are several relevant aspects of rhodium enolate chemistry. Rh(I) catalyzes the isomerization of allylic aUtoxides to enolates. We welcome this reaction done in a direct thermochemical context analogous to the related isomerizations of allyl halides , ethers and allyl amines . From the enthalpies of formation of allyl alcohol and propanal (—185.6 and —124.5 kJmol ), and their respective gas phase deprotonation enthalpies (1564 and 1530 kJ mol 252,253 jj. jjg concluded that the rearrangement of the allyloxide to propen-1-olate is exothermic by 95 kJmoR. ... 

The aldol reactions of rhodium enolates have limited synthetic utility when employed stoichiometri-cally. 0-Bound rhodium enolates of ketones, e.g. (36), have been prepared in high yield by reaction of preformed potassium enolates with carbonyldi(trimethylphosphine)ihodium(I) chloride or fluoride. However, the basic nature of these particular enolates restricts their aldol reactions to nonenolizable aldehydes. 

If rhodium enolates are used in a catalytic cycle they can promote aldol reactions under reasonably mild conditions. For example, the aldol reactions of trimethylsilyl enol ethers and ketene silyl acetals (37) with aldehydes can be catalyzed by various rhodium(I) complexes, under essentially neutral conditions, to give p-trimethylsiloxy ketones and esters (38 equation 14 and Table 6). The study of Matsuda and coworkers suggests that use of the rhodium complex Rlu(CO)i2 (39 at 2 mol %) in benzene at 100 C gives best results for the formation of adduct (38 Table 6, entries 1-7). There is negligible diastereoselectivity in most cases. Various cationic ihodium complexes such as (40) also catalyze the reaction. Reetz and Vougioukas have found that this aldol reaction proceeds well with the more reactive ketene silyl acetals, (37) for R = OMe or OEt, in CH2CI2 at room temperature (Table 6, entries 8-13). The intermediacy of an ti -O-bound rhodium enolate, such as (41), in the catalytic cycle is like-... 

Crossed aldol reactions of enol silyl ethers with aldehydes have been successfuly performed with the aid of catalytic amounts of the rhodium complex ((COD)Rh(DPPB)]+X (X = PFs or CIO4) or Rh4(CO)i2. Although the intermediacy of rhodium enolate has been suggested for these reactions, the fact that the same rhodium catalysts can promote the condensations of acetals as well (Scheme 38) tends to indicate that the reactive species may not be a metal enolate. 

On the basis of this strategy, a conceptually related desymmetrization reaction of enone-diones was introduced by the same group [26aj. An initial enone carbometalation furnished a rhodium enolate, which efficiently differentiated between the two carbonyl groups of the neighboring dione. Compound 55 as a representative example could be converted into the aldol product 57 in 87% yield and excellent selectivities. Only a single diastereomer out of 16 potential stereoisomers was formed selectively (Scheme 8.16). 

The authors proposed a Zimmerman-Traxler type transition state 56 bearing a Z-enolate, which was based on the fact that mainly T) -haptomeric rhodium enolates exist after conjugate additions. Additionally, this concept could be extended to parallel kinetic resolution reactions with racemic unsymmetrically substituted diones. 

Because water is excluded in the handling and use of arylzinc reagents, the possibility of trapping the resulting rhodium enolate with different electrophiles arises. Similar yields and enantioselectivities were obtained when this protocol... 

Most of the currently applied protocols for rhodium-catalyzed conjugate addition chemistry involve the use of aqueous solvent systems which ensure catalytic turnover by protonation of the intermediate rhodium enolate. Consequently, tandem reaction sequences with electrophiles other than a proton are troublesome. In early investigations, Hayashi reported a rhodium/BINAP-catalyzed conjugate addition-aldol reaction under anhydrous conditions by use of 9-aryl-9-borabicyclo[3.3.1]no nanes (9-Ar-9-BBN) as aryl sources [117]. The reaction between tert-butyl vinyl ketone (145) with 9-(4-fluorophenyl)-9-BBN (146) and propionaldehyde (147) led to the formation of a syn/anti-mixiuve of 148 in a 0.8 to 1 ratio (Scheme 8.39). 

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