Alkynes alkynylsilanes

Alkynes, alkynylsilanes and alkynylstannanes constitute the usual precursors for these compounds. A simplified procedure has been developed involving readily available starting materials and resulting in very good yields. 

Terminal alkynes can be converted readily into alkynylsilanes by reaction of the corresponding alkyne anion or its metalloid equivalent with a suitable chlorosilane (/). 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). 

To a stirred solution of the alkynylsilane (20 mmol) and triethylbenzyl-ammonium chloride (0.7 mmol) in MeCN (15 ml) cooled to 0°C was added aqueous sodium hydroxide (15 ml, 12m). After 5-10min, the mixture was diluted with ether and extracted with ether/dichloromethane. Drying, concentration and suitable purification gave the free alkyne (80-90%). 

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

Nolan and co-workers reported on the coupling of arylbromides with TMS acetylenes making use of the imidazolium salt 34 in combination with Pd(OAc)2 and Cul. Slightly lower yields were obtained in the absence of the copper salt. Ag20 and Agl, instead of copper salts, have also been used with Pd(PPh3)4 for the coupling of aryl iodides with bis(TMS)alkynes and the coupling of vinyltriflates with a variety of alkynylsilanes, respectively. 

Although the cyclization of allenylzincs to alkynylsilanes proceeded quite efficiently, it was demonstrated that a metallated alkyne was an even much higher reactive enophilic partner. Treatment of the diyne 360 with s-BuLi in ether at room temperature resulted in... 

Dehydrogenative silylation has also been observed for terminal alkyne substrates. Doyle and co-workers reported in 1991 that a small amount (6%) of alkynylsilane was observed in the product mixture that results from reaction of phenylacetylene and Et3SiH catalyzed by Rh2(pfb)4.41 The remaining components of the product mixture resulted from hydrosilylation. Crabtree and co-workers have found that in reaction of terminal alkynes with various tertiary silanes, dehydrogenative silylation can become the predominant route, depending on reaction conditions [Eq. (7)].42... 

Overall electrophilic addition to alkynes is rare, although bromination of alkynylsilanes has been described and takes place in the trans sense (equation 60)132. 

TMS group is used for protection of terminal alkynes. However, alkynylsilanes themselves can be used for the coupling with aryl and alkenyl triflates using Pd-CuCl as a catalyst [74], Thus the internal alkyne 160 is prepared by stepwise reactions of two different triflates 157 and 159 with trimethylsilylacetylene (134) via 158. 

Conversion of a terminal alkyne to its alkynylsilane prevents loss of the relatively acidic terminal hydrogen (pKa of ethyne c. 25) during later synthetic steps. For example, the terminal hydrogen of propyne was masked whilst its propargylic anion was used in a synthesis of Cecropia juvenile hormone, a chemical which plays ail important role in insect development (Figure Si5.2). 

Reactions of internal alkynes may be performed in the presence of terminal alkynes if the terminal alkyne is first converted to an alkynylsilane. In the example depicted in Figure Si5.3, hydrogenation of a silylated diyne using the Lindlar catalyst partially reduces the non-silylated alkyne and leaves the sterically hindered alkynylsilane intact. 

Negishi182 generalised this cyclisation to show that trialkylsilylalkynes will trap alkyl- (403, 404), vinyl- (405, 406), allenyl- (407) and aryllithiums (408) intramolecularly. Piers183 has cyclised a cyclopropyllithium onto an alkynylsilane (409 - phenyl and alkyl substituted alkynes performed less well), and Coldham61 found that the oc-amino organolithium cyclises onto a silyl alkyne (410) though in low yield. 

Alkynylsilanes can be converted into alkynylcoppers by treatment with Cu salts in an aprotic polar solvent such as l,3-dimethyl-2-imidazolidinone (DMI).62 The Cu-promoted coupling reactions via the Si-Cu transmetallation of alkynylsilanes are valuable for the syntheses of alkynyl ketones, 1,3-diynes, 1,3-enynes, and 1-aryl-l-alkynes from acid chlorides,62 alkynyl chlorides,63 alkenyl iodides,64 and aryl iodides,64 respectively (Equation (12)). In addition, symmetric 1,3-diynes can be prepared by the CuCl-mediated homocoupling of alkynylsilanes.65 653... 

Allylzinc bromide can undergo regioselectivc addition to l-(trimethylsilyl)-l-alkynes to give adducts in which ZnBr and SiCCH,), are attached to the same carbon atom. In the case of reactive alkynylsilanes, the reaction proceeds by cii-addition. However, if the reaction requires higher temperatures, products resulting from a net trans-addition are obtained (equation II). ... 

If alkyldichloroboranes are specifically required, dichloroborane-dimethyl sulfide is the reagent of choice. It is more stable and more convenient than the dichloroborane-diethyl ether complex, but its hydroborating properties are very similar." Dichloroborane complexes ethers even more strongly than monochloroborane, and its reactions with alkenes in this solvent are slow and lead to mixtures. Therefore, it is generally used in pentane and trichloroborane is added to liberate uncomplexed dichloroborane. Under these conditions it readily gives alkyldichloroboranes on reaction with alkenes or alkenyldichloro-boranes on reaction with alkynes. " " The latter reaction has been applied to alkynylsilanes (equation 44)." ... 

Despite this gamut of competing reactions, reaction conditions can usually be found for preparing either the syn or the anti adduct of many terminal alkynes with high regio- and stereo-selectivity (Table 2). Failing this the 1-alkynylsilanes can be employed as masked forms of 1-alkynes. In hydroalumination reactions, such silanes have little or no tendency to undergo the side reactions (equations 19-24) to... 

Oxasilacyclopentenes have been synthesized either by silacyclopropenation of alkynes utilizing Ag3P04 as catalyst <2004JA9522>, or by reaction of ketones and alkynylsilanes in the presence of a catalytic amount of a nucleophilic initiator (Equation 48) <2005OL4995>. 

Most importantly, even asymmetric induction with a maximum of 64 % d.e. is achievable by starting with optically active 8-phenylmenthyl acrylate" . Recently the Binger group has also reported the cycloadditions to alkynylsilanes forming 4-methylene-1-cyclopentenes (equations 194 and 195)" However, in many examples regiochemical problems concerning methylenecyclopropane as well as alkyne substituents might limit practical application. 

Alkynylsilanes have been used as terminators in allylic alcohol and ketene thioacetal initiated cycliza-tions. In the late 1970s Heathcock and Johnson independently established the utility of alkynylsilanes in cyclization reactions. The example reported by Heathcock illustrated the electronic effect of silicon in the formic acid promoted transformation of the alkynylsilane (76) to the bicyclic ketone (78) in 76% yield. In contrast cyclization of the analogous alkyne (77) produced the bicyclo[2.2.2]octene (79), as illustrated in Scheme 38. 

Alkynylsilanes are readily accessible by metalation of terminal alkynes, followed by addition of the appropriate trialkylchlorosilane. 

Among the many available procedures for preparing alkenylsilanes are hydrosilylation of alkynes and partial reduction of alkynylsilanes. Hydrosilylation of 1-alkynes with triethylsilane in the presence of catalytic chloroplatinic acid results in regioselective... 

Sonogashira coupling. Stereodefmed enynes including those bearing an alkoxy substituent (i.e., enol and ynol ethers) are readily prepared from alkynes on coupling with alkenyl iodides. Alkynylsilanes can be used instead of 1-alkynes in the coupling with alkenyl and aryl triflates. ... 

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