Silylformylation 1-alkynes

The development of the first alkyne silylformylation reaction was reported in 1989 by Matsuda [27]. Alkynes were treated with Me2PhSiH and Et3N with 1 mol% Rh4(CO)i2 under CO pressure to produce yS-silyl-a,/ -unsaturated aldehydes (Scheme 5.20). A second report from Ojima detailed the development of rhodium-cobalt mixed metal clusters as effective catalysts for alkyne silylformylation [28]. Shortly thereafter, Doyle reported that rhodium(II) perfluorobutyrate was a highly efficient and selective catalyst for alkyne silylformylation under remarkably mild reaction conditions (0°C, 1 atm CO) [29]. In all these reports, terminal alkynes react regiospedfically with attachment of the silane to the unsubstituted end of the alkyne. The reaction is often (but not always) stereospecific, producing the cis-product preferentially. 

When this reaction is carried out under 1 atm of nitrogen or GO atmosphere, a cyclopentane 276 is formed selectively in a minute at 25 °G (Scheme 13, mode 2). Although the Pauson-Khand reaction of 1,6-enyne 273 (Scheme 13, mode 3) gives 21H, this transformation is completely suppressed under the conditions of mode 1. Even simple alkyne silylformylation product 277 is not detected at all. This contrasts sharply to the silylformylation of l-penten-4-yne 48 carried out under similar conditions (Equation (12)). These results can be explained by a pathway similar to the reaction of 1,6-diynes (i) stepwise insertion of the acetylenic and olefmic moieties into the Rh-Si bond in this order, and (ii) subsequent interaction of GO and Mc2PhSiH with the resultant intermediate to give 275. The... 

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... 

Silylformylation, defined as the addition of RsSi- and -CHO across various types of bonds using a silane R3SiH, CO, and a transition metal catalyst, was discovered by Murai and co-workers, who developed the Co2(CO)8-catalyzed silylformylation of aldehydes, epoxides, and cyclic ethers [26]. More recently, as described in detail in Section 5.3.1, below, alkynes and alkenes have been successfully developed as silylformylation substrates. These reactions represent a powerful variation on hydroformylation, in that a C-Si bond is produced instead of a C-H bond. Given that C-Si groups are subject to, among other reactions, oxidation to C-OH groups, silylformylation could represent an oxidative carbonylation of the type described in Scheme 5.1. 

Alkynes are viable substrates for the tandem intramolecular silylformylation/allylsilyla-tion reaction. The 1,5-diastereoselechvity is reversed from that observed with alkene substrates, as the 1,5-anti product predominates (Scheme 5.24) [38]. Synthetic flexibility was demonstrated in that either of two carbon-silicon cleavage procedures may be employed. Protodesilylahon with tetra-n-butylammonium fluoride followed by acetylation (determi-... 

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. ... 

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)). ... 

Rhodium-catalyzed silylformylation proceeds smoothly in branched 1-alkynes at 25 °C as shown in Table 3. The stereochemistry at the chiral carbon involved in alkynes is retained intact under the silylformylation conditions. (A)-28, (rhodium particles co-condensed with mesitylene. 3-Trimethylsilyl-l-propyne 40 reacts similarly to give 41 (Equation (7)). " / //-Butylacetylene does not work as a substrate for the silylformylation because of the bulky tert-huty group on the i/>-carbon. In contrast to /< r/-butylacetylene, trimethylsilylacetylene 42 gives 43 in a fair yield (Equation (8)). ... 

Presence of a certain type of functional groups in alkynes does not interfere with silylformylation. For example, the chlorine atom in 5-chloro-l-pentyne 44 and the cyano group in 5-cyano-l-pentyne 46 do not affect the silylformylation to give 45 and 47 respectively (Equations (10) and (11)). 4-Bromo-l-butyne and 6-bromo-l-hexyne also give the corresponding silylformylation products." ... 

Table 3 Z-Selective silylformylation of branched 1 -alkynes with Me2PhSIH ...

Conditions Rh particle/mesitylene (0.1-1 mol%), CO (10-50atm), toluene, 25 °C, 24-48 h. Reprinted with permission from Wiley-VCH Verlag GmbH Co KG by Aronica, L. A. Terreni, S. Coporusso, A. M. Salvadori, P. Silylformylation of chiral 1-alkynes, catalysed by solvated rhodium atoms. European Journal of Organic Chemistry, 2001,22, 4321-4329. 

Ethene and 1-hexene do not react with hydrosilane under GO pressure in the presence of an Rh catalyst. Any product is obtained under the conditions similar to the reaction of 1-alkynes. Thus, the alkynyl moiety in pent-l-ene-4-yne 48, 4-oxahept-l-en-6-yne 50, and 4-oxa-2-methylhept-l-en-6-yne 52 is solely silylformylated to give 49, 51, and 53, respectively (Equations (12)-(14)). 

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... 

In a manner similar to silylformylation, germylformylation of 1-alkynes catalyzed by 12 proceeds with Bu aGeH under CO pressure (20 atm). However, it is difficult to completely suppress hydrogermylation of 1-alkynes as the side-reaction. ... 

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. 

In contrast to the results of Equation (25), Scheme 4, and Scheme 5, four rhodium complexes, Rh4(CO)i2, 7a, 7b, and 11a, uniformly interact with a stoichiometric mixture of Me2PhSiH and phenylacetylene 13 to give 14a within 1 h under GO pressure. The respective starting complex remains intact after completion of silylformylation (Scheme 8). The results in Equation (26) and the behavior of 7b in Scheme 8 suggest strongly that the alkyne moiety in 7 is not incorporated directly as a product of silylformylation. 

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. 

Accordingly, two possible cycles A and B are proposed for the silylformylation of 1-alkynes (Scheme 9), both starting with oxidative addition of hydrosilane to give an Rh-Si species, 113. This step is presumably reversible and triggers the degradation of cluster frameworks. In cycle A, 113 is allowed to react with acetylene to form vinyl-Rh... 

In cycle B, complex 118 is derived from 113 by oxidative addition of R3SiH and then interacts directly with alkyne to form 119 and then 120. Insertion of GO into the Rh-vinyl carbon bond in 120 leads to 117. The rest of the cycle is shared with A. It is difficult to conclude unequivocally which of these cycles dominates silylformylation, because such observable steps as formation of 11 and silyl exchange discussed in Scheme 5 require far longer time than that sufficient for silylformylation. 

In addition to alkynes, aldehydes can undergo silylformylation (Equation (27)). Although this reaction pattern was previously carried out with cobalt catalysts, the most important merit in the use of [Rh(cod)Gl]2 or [Rh(GO)2Gl]2 is that the reaction proceeds under far milder conditions. Since such mild conditions make it possible to discriminate starting aldehydes 121 from resultant sterically demanding a-silyloxyaldehydes 122, adjustment of molar ratios of the starting substrates is unnecessary for isolation of product aldehydes. ... 

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