Alkynes, activation silylformylation

The Pauson-Khand Reaction and Silylformylation. Perhaps one of the earhest and most widely studied bimetallic alkyne activation processes is the Pauson-Khand Reaction (PKR), which involves the [2-1-2-1-1] cycloaddition of an alkyne, alkene and CO to yield a cyclopentenone product. The key intermediate in both the PKR and the related silylformylation reaction is a (p—r r )-bonded intermediate of structure E (Fig. 2). Extensive DFT modelling of the catalytic cycles has provided an excellent insight into the electronic changes that occur within the bimetallic unit during the reaction. 

Silylformylation of 1-alkynes is not affected by the presence of small amounts (2 to 3 molar equiv.) of water or methanol in the reaction mixture. The corresponding products are formed in yields similar to the reaction without these additives.This information implies that the silylformylation of alkyne is not affected by the presence of a protic functional group, such as hydroxy and amino groups in alkyne. In fact, 2-propyn-l-ol 54 smoothly gives 55, in which the hydroxy group remains intact (Equation (15)). (Z)-Selectivity in 55 is drastically improved by the reaction in the absence of EtsN or by use of Rh2(pfb)4 as the catalyst. The Rh complex is known to work as an active catalyst for alcoholysis of hydrosilane." " In particular, since Rh2(pfb)4 shows high activity for alcoholysis of EtsSiH in the absence of it is quite notable that the product derived from silane alcoholysis is not detected at... 

All the starting compounds in Scheme 8 have a sufficient potential for both catalytic and stoichiometric silylformylation, when Me2PhSiH and 1-alkyne are present in a reaction vessel at the same time. Stable mononuclear complex, RhH(GO)(PPh3)3, is far inferior in catalyst efficiency at 25 °G, though the efficiency is improved under practical operation at 100 °C (Table 6). Though 7 and 11 are derived from Rh4(GO)i2 under controlled conditions and work as an active catalyst of silylformylation, their position in the catalytic cycle is still a precursor of truly active species, because it takes a far longer induction period for activation than that for silylformylation. 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

Tandem intramolecular silylformylation-allylation reaction of diallylhydrosilyl ethers derived from homoallyl alcohols is convenient for rapid, stereoselective synthesis of 1,3,5-triols convertible to more oxygen-functionalized compounds (Scheme 12).142,142a,142b 143 [ he second uncatalyzed allylation step would be facilitated by the formation of a strained silacycle intermediate, which has enough Lewis acidity to activate the formyl group. A similar tandem reaction via alkyne silylformylation has been reported.144... 

With excellent chemo- and regioselectivity, alkynes and especially terminal alkynes 49 undergo silylformylations (Scheme 7) [21]. Instead of hydrogen, phenyldimethylsilane is used, which adds, after activation of the Si-H bond by the metal catalyst, exclusively via the silicon-metal and not via the hydrogen-metal bond onto the al-... 

Reactions of alkynes with hydrosilanes under CO (1-30 atm) catalyzed by Rh4(CO),2 or Rh2Co2(CO),2 give /3-formylvinylsilanes in high to excellent yields . This reaction, silylformylation, is dso a silylcarbonylation, but it proceeds via a different mechanism from the Co2(CO)8-catalyzed silylcarbonylation. In fact, Co2(CO)g is inactive for silylformylation. The reaction catalyzed by Co2Rh2(CO),2 is proposed to involve the Co-Rh mixed metal dinuclear complexes as active catalyst species. ... 

---