Silyl-hydroformylation

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. ... 

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]. 

Our study on the synthesis, structure and catalytic properties of rhodium and iridium dimeric and monomeric siloxide complexes has indicated that these complexes can be very useful as catalysts and precursors of catalysts of various reactions involving olefins, in particular hydrosilylation [9], silylative couphng [10], silyl carbonylation [11] and hydroformylation [12]. Especially, rhodium siloxide complexes appeared to be much more effective than the respective chloro complexes in the hydrosilylation of various olefins such as 1-hexene [9a], (poly)vinylsiloxanes [9b] and allyl alkyl ethers [9c]. 

Metal chemical shifts have not found extensive use in relation to structural problems in catalysis. This is partially due to the relatively poor sensitivity of many (but not all) spin 1=1/2 metals. The most interesting exception concerns Pt, which is 33.7% abundant and possesses a relatively large magnetic moment. Platinum chemistry often serves as a model for the catalytically more useful palladium. Additionally, Pt NMR, has been used in connection with the hydrosilyla-tion and hydroformylation reactions. In the former area, Roy and Taylor [82] have prepared the catalysts Pt(SiCl2Me)2(l,5-COD) and [Pt()i-Cl)(SiCl2Me)(q -l,5-COD)]2 and used Pt methods (plus Si and NMR) to characterize these and related compounds. These represent the first stable alkene platinum silyl complexes and their reactions are thought to support the often-cited Chalk-Harrod hydrosilylation mechanism. 

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. 

Use of a hydrosilane instead of molecular hydrogen in combination with a transition metal has opened the door leading to hydrosilylation and dehydrogenative silylation of unsaturated bonds. Thus, replacement of hydrogen by a hydrosilane is a reasonable strategy to improve serious issues in hydroformylation of alkenes. Along this line, some types of silylcarbonylation were extensively studied by Murai and his co-workers. However, the silicon moiety always attaches to the oxygen atom of incorporated GO molecule (Scheme 1). 

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],... 

Regioselective hydroformylation. The rhodium-catalyzed hydroformylation of (Z)-f-butyldiphenylsilylalkenes is an excellent route to p-silyl aldehydes (equation I). A bulky silyl group is essential for this regiocontrol. 

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... 

Our investigation was aimed at studying the effects of different substituent patterns in the silyl fragment, variation of the chain length of the alkenyl group, the reaction temperature and the concentration of additional PPhs on the yield and regioselectivity of the hydroformylation. The results obtained for the hydroformylation of different types of allylsilanes are listed in Table 1. 

Reactions based on syngas, in analogy to hydroformylation, have been performed as well. These include aminomethylation [85], amidocarbonylation (see Section 2.1.2.4), homologation of acids [86] and alcohols, (cf. Section 3.2.7) [87] or silyl-formylation (cf. Section 2.6) [88]. All these reactions are far beyond the scope of this chapter and are not discussed further here. 

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 was successfully extended to the hydroformylation of propargyl-type alcohols [154] and propargylamine [155], the silylative cyclocarbonylation of alkynes [156], silylcarbocyclization of alkenynes and diynes [157-160], and other transformations of C=C bonds in the presence of HSiRa and CO (e. g., [161]). A generalized catalytic cycle for the silylformylation of 1-alkynes catalyzed by rhodium-cobalt clusters is illustrated in Scheme 6. 

Rhodium cationic and zwitteiionic complexes proved to be superior catalysts for the hydroformylation of vinylsilanes, producing either a- or ff-silyl aldehydes depending on the reaction conditions [162], On the other hand, carbonylation of vinylsilanes in the reaction related to hydrocarboxylation and hydroesterification afforded P- and a-silyl esters in high yields (eq. (14) [163]). 

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 " ... 

A similar technique was used to immobilize rhodium with silyl-ether phosphine ligands (2) on silica for the hydroformylation of 1-hexene [16-18]. This method... 

---