Allylsilanes coupling

Frey DA, Reddy SHK, Moeller KD (1999) Intramolecular anodic olefin coupling reactions the use of allylsilane coupling partners with allylic alkoxy groups. J Org Chem 64 2805-2813... 

Titanium(IV) is a powerful but selective Lewis acid which can promote the coupling of allylsilanes with carbonyl compounds and derivatives In the presence of titanium tetrachlonde, benzalacetone reacts with allyltnmethylsilane by 1,4-addition to give 4-PHENYL-6-HEPTEN-2-ONE. Similarly, the enol silyl ether of cyclopentanone is coupled with f-pentyl chloride using titanium tetrachlonde to give 2-(tert-PENTYL)CYCLOPENTANONE, an example of a-tert-alkylation of ketones. 

Optically active (Z)-l-substituted-2-alkenylsilanes are also available by asymmetric cross coupling, and similarly react with aldehydes in the presence of titanium(IV) chloride by an SE process in which the electrophile attacks the allylsilane double bond unit with respect to the leaving silyl group to form ( )-s)vr-products. However the enantiomeric excesses of these (Z)-allylsilanes tend to be lower than those of their ( )-isomers, and their reactions with aldehydes tend to be less stereoselective with more of the (E)-anti products being obtained74. 

Arenediazonium salts also couple in acetonitrile with another group of activated ethene derivatives, the allylsilanes (Mayr and Grimm, 1992). At low temperatures the... 

Carbon-Chiral Allylsilanes by Asymmetric Grignard Cross-Coupling... 

An allylsilane-generating CM using catalyst C between the sensitive /J-lactone 319 and allyltrimethylsilane served to introduce the allylsilane moiety in intermediate 320 as an inconsequential mixture (ca. 3 1) of (EIZ)-isomers in 80% yield. Cyclization of /J-lactone 320 with TiCl4 smoothly delivered cyclopentane 321 with inversion at the /J-carbon. Acid 321 was converted to key aldehyde 322 in three steps. The convergent fragment coupling was performed by a uniquely... 

Allylsilane additions were used in a formal synthesis of roflamycoin [51] (Eq. 24). A one-pot, three-component sequential coupling of bis-allylsilane 138 with 4-acetoxy-l,3-dioxanes 137 and 139 provided the C11-C22 polyol chain (140) in moderate yield. 

Tetraalkylstannanes with double bonds at allylic or more remote positions undergo cross-coupling with allylsilanes to yield dienes, as illustrated in reaction 47299. 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

The alkoxycarbenium ions generated by the cation pool method react with various carbon nucleophiles such as substituted allylsilanes and enol silyl ethers to give the corresponding coupling products in good yields. It should be noted that the reactions of alkoxycarbenium ion pools with such nucleophiles are much faster than the Lewis acid promoted reactions of acetals with similar nucleophiles. A higher concentration of the cationic species in the cation pool method seems to be responsible. 

The intramolecular coupling of enolethers with enolethers, styrenes, alkyl-substituted olefins, allylsilanes, and vinylsilanes was systematically studied by Moeller [69]. Many of these coupling reactions turned out to be compatible with the smooth formation of quaternary carbon atoms (Eq. 11) [70], which were formed diastere-oselectively and led to fused bicyclic ring skeletons having a ds-stereochemistry [71]. The cyclization is compatible with acid-sensitive functional groups as the allylic alkoxy group. Moeller has demonstrated in some cases that these reactions can be run without loss of selectivity and yield in a simple beaker with either a carbon rod or reticulated carbon as anode without potential control and a 6-V lantern battery as power supply [71]. 

Fig. 43 Stereoselective intramolecular coupling of enolether with allylsilane [201, 202].

The most direct route to the 1,4-dicarbonyl equivalent required for the aldol condensation would be to couple the enol ether of an aldehyde with the enol ether of a ketone. However, this sequence proved impractical due to the hydrolytic instability of the ketone enol ether. Even after an extensive effort, the substrate for the electrolysis reaction could not be reproducibly prepared in high yield. These problems were readily avoided with the use of an allylsilane based... 

Further investigations are needed to establish, whether this approach is really useful to obtain chiral allylsilanes 2, which are synthetically quite interesting intermediates. They are available otherwise only by asymmetric cross-coupling of silyl alkyl Grignard reagents with bromoethylenes in the presence of a chiral ferrocenylphosphine-palladium catalyst54. 

The cross-coupling route to allylsilanes is effective with either aromatic or aliphatic a-silylated Grignard reagents16, and palladium catalysts are more reactive and stereoselective than the corresponding nickel complexes. Unsubstituted or i+monosubstituted alkenyl bromides work well but the Z-substituted bromides give lower yields and an inferior enantiomeric excess. The enantiomeric excess increases quite markedly with decreasing temperature, and optimum results are obtained at 0 C or below. 

Allyl- and vinylsilane chemistry was one of the first areas of reagent synthesis impacted by CM methodology. Allylsilanes are commonly employed in nucleophilic additions to carbonyl compounds, epoxides, and Michael acceptors (the Sakurai reaction) vinylsilanes are useful reagents for palladium-coupling reactions. As the ubiquitous application of CM to this substrate class has recently been described in several excellent reviews, this topic will not be discussed in detail, with the exception of the use of silane moieties to direct CM stereoselectivity (previously discussed in Section 11.06.3.2). 

IJ-Enynes.1 (Propargyl)dicobalt hexacarbonyl cations (8, 148-149) couple with allylsilanes to give complexes of 1,5-enynes, generally in satisfactory yields. A typical reaction is formulated in equation (I). 

As shown in Scheme 78, with or without zinc chloride, benzylic Grignard reagents couple with vinyl bromide to form allylbenzene derivatives in high ee (188). This method has been used to prepare optically active allylsilanes in up to 95% ee. Vinylic bromides with E configurations lead to the E allylsilanes with high ee, and Z bromides lead to Z allylsilanes with lower ee. 

The asymmetric cross-coupling was successfully applied to the synthesis of optically active allylsilanes [27,28] (Scheme 8F.9). The reaction of a-(trimethylsilyl)benzylmagnesium bro-... 

The coupling reaction of allylsilane with the w-thiomethoxyacetal is catalyzed by TMSOTf51. TiCLj-mediated reaction of a-bromoallylsilane 7 with 1,1-diethoxyethane leads to homoallylic ether 12 stereoselectively in excellent yield (equation 9)40. Under similar reaction conditions, double substitution of allylsilane to diketals 13 affords 14 in high diastereoselectivity (equation 10)52. 

Carbamate and amide groups have been found to be stable under these coupling conditions73. In the presence of TiCLt or SnCLt, chiral a-keto amides 36 react with allyl-silane to produce, after hydrolysis, optically active tertiary alcohols 37 with extremely high optical selectivity (equation 23)74. The addition reaction appears to occur from the Si face of the carbonyl group. In a similar manner, a high degree of stereoselectivity is obtained from the reactions of A-Boc-a-amino aldehydes 38 with 2-substituted allylsilanes (equation 24)75. 

The reaction of (Z)-/J-methylcrotylsilane with 2-benzyloxypropanal in the presence of chelative SnCL gives the awfi-homoallylic alcohol 40 diastereoselectively (equation 26)78. The use of chelative TiCLt versus nonchelative BF3 OEt2 Lewis acids also gives different stereoselectivity in the coupling of allylsilane with the aldehydes 41 (equation 27)79. 

The TiCU-induced three-component coupling reaction of an a-haloacylsilane, allylsilane and another carbonyl compound gives 48 in good yield. A silyl enol ether intermediate is suggested (equation 31)82. The reaction of a cyclopropyl ketone with allylsilane yields a mixture of skeletal rearranged products83. 

Treatment of 7 with acid chloride at — 60 °C in the presence of TiCLt gives the corresponding coupling product 89 stereoselectively (equation 67)40. The reaction of aryl chloroformate with allylsilane in the presence of aluminum trichloride gives aryl 4-alkenoates 90 in excellent yield (equation 68)137. 

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