Silylformylation, of alkynes

Silylformylation and hydroformylation reactions figured prominently in Ojima s approach to isoretronecanol and trachelanthamidine [40]. Thus, silylformylation of alkyne 70 proceeded smoothly to produce aldehyde 71 (Scheme 5.26). Reduction and protodesilylation provided allylic alcohol 72, which was protected to give 73. Hydroformylation in the presence of HC(OEt)3 led to a 2 1 mixture of 74 and 75. Deprotection and amide and amidal reduction then provided the target compounds. 

In sharp contrast to silylformylation of alkynes, specific combination [Rh(GOD)Gl]2/THF of catalyst precursor and solvent is quite important for selective silylformylation of aldehydes. Go-use of A-methylpyrazole is crucial to perform silylformylation of epoxides ( [Rh(GO)2Gl]/GH2Cl2 " and oxetanes [Rh(GO)2Gl]2/toluene. ... 

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... 

Choice of THF as the solvent, Me2PhSiH as the hydrosilane, and operation under anhydrous conditions are critical for selective silylformylation of 121 to give 122, whereas these regulations are not necessary in the silylformylation of alkynes. Similar silylformylation is applied to 2-(iV-methylpyrrolidyl)aldehyde (60%), 2-furylaldehyde (90%), 2-thiophenecarboxaldehyde (72%), 2,6-dimethyl-5-heptenal (60%), butanal (60%), 2-methylpropanal (75%), ferrocenecar-boxaldehyde (88%), and phenylacetaldehyde (80%) ... 

Independent discovery of the silylformylation of alkynes was reported by the Matsuda and Ojima groups. The general reaction involves addition of both CO and tertiary hydrosilane to an alkyne to yield silyl alkenals, catalyzed by rhodium or rhodium-cobalt mixed metal clusters [Eq. (46)]. 

Ojima and co-workers have also extensively studied the catalytic silylformylation of alkynes, employing a number of rhodium and rhodium-cobalt mixed metal complexes.122 The reaction of 1-hexyne with hydrosilanes under either ambient pressure or 10 atm of CO is catalyzed by a variety of metal complexes to yield (Z)-l-silyl-2-formyl-l-hexene, 1, and/or ( )-l-silyl-l-hexene, 2 [Eq. (48)].122a>c... 

A surprising variation on the silylformylation reaction has been reported by Zhou and Alper. A zwitterionic rhodium(I) complex, (l,5-COD)Rh+(Tj6-PhBPh3), catalyzes the silylformylation of alkynes under normal reaction conditions. However, if H2 is added to the system, the reaction may proceed to yield silylalkenals of a different structure. Interestingly, although the H2 must play a key role in the reaction, it is not incorporated in the product. At this time, the mechanistic role of the hydrogen remains unclear. The authors term this reaction a silylhydroformylation [Eq. (51)].126... 

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. 

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],... 

When hydrosilane is used instead of hydrogen in the Rh-catalysed carbonylation of alkynes, the silylformylation of alkynes occurs to give the / -silyl-a,/ -unsaturated aldehydes 71 [38,39]. Oxidative addition of silane to Rh generates the silylrhodium hydride 69. It should be noticed that insertion of alkyne to the Rh-SiR.3 bond, but not... 

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. 

While platinum and rhodium are predominantly used as efficient catalysts in the hydrosilylation and cobalt group complexes are used in the reactions of silicon compounds with carbon monooxide, in the last couple of years the chemistry of ruthenium complexes has progressed significantly and plays a crucial role in catalysis of these types of processes (e.g., dehydrogenative silylation, hydrosilylation and silylformylation of alkynes, carbonylation and carbocyclisation of silicon substrates). 

Several examples of stereoselective silylformylation of alkynes in the presence of rhodium(I) catalysts have been reported80-83. 

Co2Rh2(CO)i2] and [Co3Rh(CO)i2] catalyze the hydrosilation of isoprene, cyclohexanone, cyclohexenone, and 1-alkynes, the regioseleetive inter- or intramolecular (Eq. 5) silylformylation of alkynes and 1-alkynals, and the silylcarbo-cyclization of enynes, diynes, and alkynals. These synthetically important reactions have been reviewed recently. 

Highly functionalized olefins are easily accessible via silyformylation of alkynes. When coupled to other reaction systems, silylformylation reactions become even more powerful. Eilbracht et al. demonstrated that stabilized phosphorous ylides 74 could be trapped by P-silylated a-P-unsaturated aldehydes formed via silylformylation of alkynes in a one-pot synthesis. The tandem reaction proceeds with high yields for both alipathic and aromatic terminal alkynes. Protected propargyl alcohols also showed good reactivity however, propargyl amines react with low selectivity. The reaction shown below is the best-afforded result for this process. ... 

The silylformylation of alkynes involves the coupling of an alkyne with a silane and carbon monoxide to yield p-silylvinyl aldehydes, which are versatile building... 

Alkynes can also undergoes consecntive process under silylformylation conditions. Thus, rhodium catalyzed silylformylation of alkyne 217 in the presence of primary or secondary amines leads directly to the azadiene 221 by silylformylation and enamine formation [ 139]. These azadienes undergoes Diels-Alder reaction with dimethyl acetylendicarboxilate to give dihydropyridiaes 222. 

Silylformylation of alkynals with Ph3SiH proceeds in a 5yn-selective fashion upon treatment with Rh(acac)(CO)2 under... 

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