Enol ethers hydroformylation

This method can also be applied to silyl enol ethers of homologous unsaturated ketones as well as of unsaturated aldehydes or esters [85-87]. While unmodified unsaturated esters give only the corresponding aldehydes without cyclization under tandem hydroformylation/aldol reaction conditions, the corresponding silylated ester enolates smoothly cyclize in a tandem hy-droformylation/ Mukaiyama aldol reaction (Scheme 32) [85-87]. 

Similarly, tandem hydroformylation/aldol sequences can be applied to the formation of bicyclic and spirocyclic compounds. Thus silyl enol ethers of 3-vinyl and 3-allyl cycloalkanones give ring anellated products (Scheme 33) [86,87]. 

Auto-tandem hydroformylation-cyclization, catalyzed by [RhCl(cod)]2, enables expansion of the organic skeleton of unsaturated silyl enol ethers (Scheme 10). Linear aldehydes generated in the hydroformylation step subsequently undergo Rh-catalyzed, intramolecular Mukaiyama aldol addition. Bicyclic ketones are also accessible from cyclic silyl enol ethers. 

Several interesting synthetic applications of hydroformylation are known. As one example, hydroformylation of enol ethers to afford -hydroxy aldehydes can be extended to catalytic aldol synthesis. The Rh-catalysed hydroformylation of the cyclic enol ether 42 affords the protected. vy -3,5-dihydroxyaldchydc 43 without forming the anti product 44. The regioselectivity of attack on the terminal carbon is also very high [28],... 

Hydrosilane HSiR.3 behaves similar to H2 toward transition metal complexes in some cases. When HSiR.3 is used instead of hydrogen in hydroformylation, two reactions are expected. One is a hydrocarbonylation-type reaction, by which formation of the silyl enol ethers 62 via the acylmetal intermediate 61, and the acylsilanes 64 via the acyl complex 63, are expected in practice both reactions are observed. The other possibility is silylformylation to form 65, which is unknown, even though silylformylation of alkynes is known. When Co2(CO)8 is used, the silyl enol ether of aldehyde 66 is obtained [36], However, the silyl enol ether 67 of acylsilane 68 is obtained when an Ir complex is used, and converted to the acylsilane 68 by hydrolysis [37],... 

Hydroformylation of the 1,2-bisilylalkene 2 is accompanied by Brooke rearrangement to provide a silyl enol ether (3). 

As the hydrosilation reaction is related to hydrogenation, it also bears a certain relationship to the hydroformylation (see Hydroformylation) reaction. The Co2(CO)g catalyzed process, which formylates an alkene in the presence of a hydrosilane instead of hydrogen, gives silyl enol ethers in excellent yields (equation 27). The hydrosilane/CO system can also react with cyclic ethers or aldehydes to give co- or... 

Silylcarbonylation is a known term describing formally the silicon version of hydroformylation (cf. Section 2.1.1) discovered by Collenille [147] and later developed by Murai and co-workers, in which hydrogen is replaced by trialkylsilane, eq. (10) [148]. The silylformylation of alkenes is catalyzed by Co2(CO)g [148], [RhCl(CO)2]2 [149], RhCl(PPh3)3 [150], Ru3(CO)i2, and HRujfCO),- [151] to give silyl enol ethers of the homologous aldehydes as the sole products. 

The reaction of olefins with hydrosilane and CO is a useful synthetic alternative to the hydroformylation of olefins , which instead of the corresponding aldehyde, gives silyl enol ethers bearing one more carbon atom than the starting olefins " ... 

With CO-H2 under pressure, the Rh complex mediates hydroformylation of alkenes and then cyclization involving remote silyl enol ethers. ... 

It catalyses the aminolysis of epoxides in an extraordinarily efficient manner in aprotic solvents (e.g. toluene, CH2CI2) with complete trans stereoselectivity and high regioselectivity [Chini et al. Tetrahedron Lett 35 433 1994], It also catalyses the trans addition of indole (at position 3) to epoxides (e.g. to phenoxymetltyloxirane) in >50% yields at 60° (42 hours) under pressure (10 Kbar) and was successfully applied for an enantioselective synthesis of (+)-diolmycin A2 [Kotsuki Tetrahedron Lett 37 3727 799(5]. Of the ten lanthanide triflates, Yb(OTf)3 gave the highest yields (> 90%, see above) of condensation products by catalytically activating formaldehyde, and a variety of aldehydes, in hydroformylations and aldol reactions, respectively, with trimethylsilyl enol-ethers in THF at room temperature. All the lanthanide triflates can be recovered from these reactions for re-use. [Kobayashi Hachiya J Org Chem 59 3590 1994.]... 

Dihydroxy aldehydes 54 are also stereoselectively prepared by hydroformylation of enol ether 52 (R = H, Me, Pr, BnOCCH ), R = Bu, Me). This suppose an alternative procedure to the aldol reaction, (Figure 17) [45,46],... 

Both hydroformylation and isomerization were foimd to occur from the same diastereoface of enol ether 55. The fact that isomerization must take place through a tertiary rhodium-alkyl which has a 1,3-diaxial interaction in chair conformation seems to indicate that the intermediate has a boat conformation. 

Hollmann andEilbracht investigated a hydroformylation-(Mukayama aldoliza-tion) reaction forthe cyclization of silyl enol ethers (Scheme 5.124) [5]. 

In order to overcome the tricky regioselectivity problem, hydroformylation of the corresponding enol ethers has been suggested (Scheme 6.67) [181]. These substrates are accessible via acetalization of a relevant propiophenone derivative, followed by elimination of 1 equiv ethanol. The hydroformylation reaction with a rhodium cluster was performed in a 100 g scale. The enolether reacts in the P position. A second elimination step afforded the a,p-unsaturated aldehyde, which was reduced with H2 to give selectively the desired 2-formyl compound. 

Recently, rhodium/poly(enolate-co-vinyl alcohol-co-vinyl acetate) catalysts have been developed for the biphasic hydroformylation of aliphatic alkenes and applied to the selective hydroformylation of functionalized alkenes [16], Although the conversions were low (< 25%), excellent selectivities for the hydroformylation of n-bu-tyl vinyl ether and methyl 3,3-dimethylpenten-4-onate can be achieved with such water-soluble polymer-anchored rhodium catalysts. For instance, the hydroformylation of methyl 3,3-dimethylpenten-4-onate gives only the linear aldehyde. 

Silanes normally reduce aldehydes or ketones under catalytic conditions to the silyl ethers. However, with certain catalysts such as nickel sulfide,Co2(CO)8, or (Ph3P)3RhCl, carbonyl compounds react with silanes to yield an equilibrium mixture of enol silyl ethers (Scheme 17). In a similar vein, the silyl-hydroformylation reaction of cycloalkenes with CO and silanes may be a practical way to prepare enol silyl ethers. An example is the preparation of compound (49). Catalytic 1,4-hydrosilation of a, -un-saturated ketones or aldehydes gives the corresponding enol silyl ethers. The reaction is similar to the reductive silylation referred to previously, but the reaction conditions are neutral and milder. The formation of the enol silyl ether (50) is outlined below. ... 

Aspects of the rhodium-catalysed hydroformylation of olefins have been reviewed. " Copper(ii) acetate catalyses the highly stereoselective solvolysis of -alkenylpentafluorosilicates to -alkenyl ethers under an atmosphere of air. Since the pentafluorosilicates can be obtained via hydrosilylation of acetylenes, the sequence represents a regio- and stereo-selective transformation of acetylenes into carbonyl precursors in moderate yield. The reaction of vinylmercurials and mercury carboxylates catalysed by palladium(ii) acetate provides a stereospecific route to enol carboxylates, which are valuable precursors of specific enolates. ... 

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