Silylformylations Me2PhSiH

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. 

In sharp contrast to the unique pattern for the incorporation of carbon monoxide into the 1,6-diyne 63, aldehyde 77 was obtained as the sole product in the rhodium-catalyzed reaction of 1,6-enyne 76 with a molar equivalent of Me2PhSiH under CO (Scheme 6.15, mode 1) [22]. This result can be explained by the stepwise insertion of the acetylenic and vinylic moieties into the Rh-Si bond, the formyl group being generated by the reductive elimination to afford 77. The fact that a formyl group can be introduced to the ole-finic moiety of 76 under mild conditions should be stressed, since enoxysilanes are isolated in the rhodium-catalyzed silylformylation of simple alkenes under forcing conditions. The 1,6-enyne 76 is used as a typical model for Pauson-Khand reactions (Scheme 6.15, mode 2) [23], whereas formation of the corresponding product was completely suppressed in the presence of a hydrosilane. The selective formation of 79 in the absence of CO (Scheme 6.15, mode 3) supports the stepwise insertion of the acetylenic and olefmic moieties in the same molecules into the Rh-Si bond. 

Phenylacetylene 13 readily reacts with Me2PhSiH under CO pressure to give 3-dimethylphenylsilyl-2-phenyl-propenal 14a in the presence of a catalytic amount of Rh4(CO)i2 (Equation (2)). When 10-20 atm of CO pressure and 0.1-0.2 mol% of Rh4(GO)i2 are applied, the reaction is completed within 2 h at 100 °G. Silylformylation proceeds smoothly even at room temperature, though the reaction becomes slower (600-700 turnover frequency at room temperature after 17 h). The reaction does proceed in the absence of Et3N, but its presence improves yield and (Z)-selectivity of product 14a. ... 

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)). ... 

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

On the other hand, alkenal 14a is selectively formed with recovery of Rh4(GO)i2 under CO pressure (20 atm) in a stoichiometric reaction mole ratio = Rh4(GO)i2 13 Me2PhSiH = 1 4 4 as well as a catalytic reaction. When 13 and Me2PhSiH are mixed at once in a GDGI3 solution of Rh4(GO)i2 under CO atmosphere, 14a is smoothly formed as a major product with concomitant formation of small amounts of 15 and Me2PhSiOH. In the case that 13 and Me2PhSiH are added separately, it is critical to add 13 to a solution of Me2PhSiH and Rh4(CO)i2 for the production of 14a. Reverse addition results in hydrosilylation of 13 only. Similar results are observed in the silylformylation catalyzed by Rh2(pfb)4. ... 

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. 

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%) ... 

It is worth noting that the reaction of 284 with Me2PhSiH gives 285 as a sole cyclic compound a simple silylformylation product is formed in less than 3% yield (Equation (49)). The outcome is rationalized by intervention of 286 in which intramolecular coordination of an olefinic moiety dominates the orientation of silylrhodation... 

The outcome of the reaction is affected by hydrosilane, CO pressure, and choice of catalyst. The hydrosilane substituents influence the selectivity of the reaction, i.e., relative ratio of silylformylation to hydrosilylation. For example, silanes with phenyl substituents, such as Me2PhSiH, Ph2MeSiH, and Ph3SiH, lead exclusively to silylformylation products, but at least 80% of the product derived from trimethoxysilane is due to hydrosilylation. Trialkylsilanes generally give 40 60 mixtures of hydrosilylation and silylformylation products. The carbon monoxide pressure under which the reaction... 

---