Propargylsilanes

Terminal alkynes can be converted readily into alkynylsilanes by reaction of the corresponding alkyne anion or its metalloid equivalent with a suitable chlorosilane (7). The reverse reaction, that of liberation of the alkyne, is quite facile, being effected by several reagent combinations, including hydroxide ion, methanolysis, fluoride anion, silver(i) followed by cyanide anion, and methyl lithium-lithium bromide (2). 

Terminal silylation of alkynes affords two main benefits the triple bond is protected against chemical attack, either for steric reasons or because the potentially acidic proton is masked, and, perhaps paradoxically, the bond is activated towards regioselective electrophilic attack under certain conditions. 

THF (500 ml) was cooled to 0°C, and saturated with dry ethyne. A solution of EtMgBr (0.8 mol) in THF (400 ml) was added in increments at such a rate that evolution of ethane had ceased before additional Grignard reagent was added. Ethyne was bubbled in throughout this step, and during the initial stages of the next. TMSC1 (0.7 mol) was added dropwise, and the solution was stirred at ambient temperature for 5 h. It was then poured into ice-water (500 ml), and the upper layer was separated. Preliminary distillation of this layer separated the more volatile product from the bulk of the THF. The 

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Propargylsilanes can also be employed in the Sakurai reaction. For example the enone 6, containing a propargylsilane side chain, undergoes an intramolecular Sakurai reaction, catalyzed by an acidic ion-exchange resin—e.g. Amberlyst-15—to give stereoselectively the bicyclic product 7 in good yield ... 

So far, only reactions in which the internal nucleophile is tethered to the nitrogen atom of the A -acyliminium ion have been discussed, however, cyclizations with nucleophiles attached to other positions are also possible. If the nucleophile is connected to the carbon atom adjacent to the carbonyl group, bridged azabicycloalkane derivatives are obtained in high yield by using the more reactive allyl- or propargylsilanes. 

Reactions of this type are mostly performed with internal nucleophiles attached to the carbon atom adjacent to the iminium nitrogen, thus leading to tropane-like azabicyclic systems. Both allyl- and propargylsilanes, activated and unactivated alkenes, and ketones have been successfully used as nucleophiles. The products 1 and 2 are also obtained via two consecutive C —C bond-forming reactions in a single operation. 

Internal propargylsilanes undergo hydroalumination much less successfully. However, hydroboration proceeds without difficulty ... 

Both terminal and internal propargylsilanes are versatile intermediates for the preparation of allylsilanes (Chapter 5). 

Monohydroalumination of terminal propargylsilanes (Chapter 7) with two equivalents of DIBAL proceeds in a stereoselective (9), though not regio-selective, manner to produce an approximately 1 1 mixture of the alkenyl-alanes (1) and (2). Protonolysis of the mixture yields the pure (Z)-allylsilanc... 

The reaction of propargyl chloride 83 and trichlorosilane 84 showed two different regioselectivities depending on the choice of transition metal catalysts [88]. Whereas the Sn2 substitution proceeded to give the propargylsilane 85 with 94% selectivity using a CuCl catalyst, the silylallene 86 was obtained via an SN2 pathway with >97% selectivity with 3mol% of Ni(PhCOCHCOPh)2 (Scheme 3.42). 

Scheme 4.52 Asymmetric synthesis of allenes via the allenyl- or propargylsilanes.

Table9.32 In situ additions of trichloallenyl/propargylsilane to aldehydes.

The ratios of nucleophilic substitution versus [3,3]-sigmatropic rearrangement for the collapse of allenyl(aryl)iodine(III), generated from the reaction of aryliodanes with propargylsilanes in the presence of Bp3.0Et2 in alcohols, have been determined. The mechanism proposed by the authors ° involves the generation of propargyl cations from the allenyliodine (III) via a unimolecular pathway. 

Enamine carbaldehyde 76 (Scheme 20) by sequential photochemical cycloaddition and iminium ion-propargylsilane cyclimtion furnishes allenes 77a-c in good yield and with high diastereoselectivity (92X2081). (Gas chromatographic fR values and thin-layer chromatographic Rp values have been reported.) Radical cyclization of ca-iodoalkyl isoquinolone 78a under... 

Polyene cyclization in terpene and steroid synthesis is critically dependent on the terminator in order to generate useful functionalities for further modification of the products. Allyl- and propargylsilanes have proven their value in facilitation of the cyclization and generation of an exocyclic methylene and allene, respectively. Thus, a concise approach to albicanyl acetate [126] and the rapid construction of a tetracyclic precursor of steroids [127] are sufficient to demonstrate the concept. Again, a comparison of the substrates with a silyl group with those having a simple alkyl moiety is very enlightening. 

The propargylsilane group can participate in intramolecular cyclization reactions to give a five-membered ring with an exocyclic allene group. An example is the cyclization of 2 to a mixture of 3 and 4 when treated with acetic acid. 1... 

A particularly interesting cyclization utilizes a propargylsilane group as a terminator for a biomimctic synthesis of a tetracyclic hydrocarbon (6) with a potential cortical side chain.5... 

Allenylation of acetals TiCl4 catalyzes a reaction of propargylsilanes with acetals to provide a-allenyl ethers in good yield. 

Propargylsilanes undergo addition to give a /3-silylcarbocation, which can then react further to give either addition or substitution, as shown in equation 101. 

Electrophilic substitution of propargylsilanes is an important route to allenes. Cycliza-tion gives allene-substituted cyclic compounds as shown in equations 103 and 104. 

Use of the allylsilanes as the alkene components in the Heck reaction is illustrated in the regioselective intramolecular cyclization of 120 (equation 98)180. In addition, a high enantioselectivity of 90 %ee is obtained when the chiral phosphine (S)-BINAP is employed. The propargylsilane analog undergoes a similar reaction181. 

Even if the SMS reaction typically involves allylsilanes, carbonyls and alcohols (or silyl ethers), some transformations can be considered as belonging to the same family. For example, in 2001, Yokozawa et al. described [43] a three-component reaction between aldehydes 6, alkoxysilanes 38 and propargylsilane 88 (instead of allylsilane). Tritylperchlorate was used as the catalyst and a-allenyl ethers 89 were... 

Alkenyl(phenyl)iodine(III) compounds can also serve as starting materials in rearrangements. Allenyl(aryl)iodine(III) compounds of type 86 can be synthesized from (diacetoxyiodo) derivatives 85 and propargylsilanes [145]. It depends on the leaving group ability of the aromatic substituent on iodine in 86 as to whether the reaction proceeds via nucleophilic substitution to compounds of type 87 or by an iodonio-Claisen rearrangement to compounds 88, Scheme 37 [146,147]. The easy access to propynyl compounds 87 has been shown [148] and solvent effects in these reactions have been investigated as well [149,150]. 

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