Silylformylation internal alkynes

Internal alkynes are particularly suitable for silylformylation, and the conditions similar to the reaction of 1-alkynes are applicable. 2-Butyne 100 and diphenylacetylene 2 give corresponding 3-silylalkenals 101 and 102, respectively. 

A hydrosilyloxy moiety also works as a directing group for the intramolecular silylformylation regardless of terminal and internal alkynes (Table 9)7 ... 

A second example of intramolecular silylformylation involves reactions of terminal and internal alkynes linked to a dimethylsiloxy group, as depicted in Eq. (50).125... 

Although the regioselectivity for the silylformylation of 1-alkynes is excellent, that of internal alkynes is low except for 2-alkynoates (vide supra). Also, in the reactions of 1-alkynes, the silyl group is always delivered to the terminal position and the formyl to the C-2 position, thus it is impossible to synthesize 3-silyl-2-alkenals, which requires opposite regioselectivity. Intramolecular directed reactions can circumvent these limitations and expand the scope of the silylformylation of alkynes. 

Internal acetylenes, Ni-mediated reactions, 10, 546 Internal alkenes, ethylene co-polymers, 4, 1145 Internal alkynes in alder-ene reaction, 10, 567 intermolecular hydrosilylation with ruthenium, 10, 802 with yttrium, 10, 801 silylboration, 9, 163 silylformylation, 11, 483... 

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. 

Silylformylation of alkynes.1 0-Silyl-a,0-enals can be prepared by reaction of dimethylphenylsilane and carbon monoxide with alkynes catalyzed by Rh4(CO)12. Yields are improved if triethylamine is also present. Regioselectivity in the reaction with internal alkynes is controlled by steric factors. 

The silylformylation of 1-alkynes has been studied extensively affording (Z)-l-silyl-2-formyl-l-alkenes high regioselectivity. However, internal alkynes have not demonstrated high regioselectivity. Biffis et al. have reported using a cationic dirhodium(II) complex to achieve improved chemoselectivity for the silylformylation of internal alkynes.In addition, intramolecular silylformylation reactions were developed to aid in the regioselectivity of silylformylation of internal alkynes. ... 

Zwitterionic rhodium(I) complex, Rh+(COD)(/ -C6H5BPh3), is also found to be an efficient catalyst for the silylformylation of 1-alkynes at 40 °C and 40 atm of CO in CH2CI2 (equation 125) although no reaction occurs with internal alkynes. However, silylhydroformylation takes place when the reaction is carried out under hydroformylation conditions, i.e. in the presence of CO and H2 (CO/H2 = 1/1), to give ( )-2-silylmethyl-2-alkenals (319) in 54-92% isolated yields (equation 128). The intermediacy of jr- lenyl-Rh species is proposed to account for the formation of 319 1. When 4-acetoxy-l-butyne and 4-(p-tosyloxy)-l-butyne are used as the substrates, saturated silylhydroformylation products are obtained. ... 

The most notable point of this reaction is that the internal sp-c xhon is selectively carbonylated to form (Z)-14a predominantly, although the ZjE ratio is likely to depend on reaction temperature, time, and catalyst precursor. It is revealed that the stereochemistry of the transition metal-catalyzed addition to alkynes is intrinsically cis. Isomerization from (Z)-14a to ( )-14a proceeds as an independent event from silylformylation. This feature sharply contrasts to the results observed in hydrosilylation of 13 with Me2PhSiH (Equation (3)). ... 

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