The direct vinylation of aryl halides leading to styrene derivatives and of vinyl halides leading to conjugated dienes represent important transformations in organic synthesis. The cross-coupling of the commercially available vinyltrimethoxysilane or vinyltriethoxysilane with aryl bromides and chlorides brings about the vinylation leading to the corresponding styrene in good yields. Best results are obtained when microwave stimulation is applied (Eq. 39).43,44 The reaction also works well with vinyl bromides. (Eq. 40).
The readily available vinylsilane, tetravinyltetramethylcyclotetrasiloxane, (D4V) can be used to vinylate aryl and vinyl iodides and bromides in good yields (Eqs. 41 and 42).46,47
4-Silyl homoallylic alcohols, prepared by lithiation of allyldimethyl(isopropoxy)silane and reaction of the resulting lithium reagent with aldehydes can then be cross-coupled with aryl or vinyl iodides (Eqs. 43, 44).48
The bis(silyl)ethylene, 1, was employed for the stereoselective introduction of an aryl group via the more reactive diethoxymethylsilyl moiety followed by electrophilic desilylation of the phenyldimethylsilyl or trimethylsilyl group (Eq. 45). This leads to α,ß-unsaturated ketones (Eq. 46).48
Alkynyldimethylsilanols react with aryl iodides to give arylacetylenes in good yields (Eqs. 47, 48).49,50 Aryl bromides work as well, but aryl chlorides are poorer substrates under these conditions (Eq. 49).51 The difference in the reactivity between the sp-carbon and a hydrogen and the sp-carbon and a trimethylsilyl group was shown in the synthesis of 1,4-bis(3-bromophenylethynyl)benzene (Eq. 50).32
Alkynyltrimethylsilanes can be cross-coupled with propargyl chlorides as shown in Eqs. 51, 52.52,53 A similar reaction occurs with propargyl alcohols (Eq. 53).54 ln one case an aldehyde is converted to the di-alkynyl derivative via the propargyl alcohol intermediate formed by the addition of the first alkynyl group to the aldehyde (Eq. 54).55
The Cu(I)-catalyzed homocoupling of alkynylsilanes can produce 1,3-butadiynes (Eq. 55) The yields are good to excellent.55
The formation of hydroxymethyl enynes is accomplished by the addition of bis(trimethylsilyl)acetylene to a propargyl alcohol (Eq. 56). These can be converted via a cyclization, cross-coupling sequence to 2-aryl-dihydrofuran derivatives (Eq. 57).56
The silicon-Sonogashira cross-coupling occurs over that of the silicon-based vinylation reaction as shown in Eq. 58.57
Non-basic, Cu(I)-catalyzed cross-coupling of alkynylsilanes with aryl iodides have been reported and provides a route to unsymmetrical diaryl acetylenes in good to excellent yields (Eq. 59).58 A typical problem with these is the homo-coupling to 1,3-dialkynes (Glaser-type product). The use of bis(trimethylsilyl)acetylene allows the generation of unsymmetrical diaryl acetylenes in a one-pot procedure.
The use of a 1-trimethylsilylalkyne rather than the terminal acetylene permits the three-component coupling of an internal acetylene, an aryl iodide and the alkynyltrimethylsilane (Eq. 60).60 Use of the terminal acetylene gives a high yield of the direct Sonogashira coupling product without incorporation of the internal acetylene.
Trimethylsilylbutadiyne was subjected to a zinc-mediated cross-coupling with 2-bromoiodoethylene and the product of this similarly cross-coupled with a 1-octenylzinc reagent to form a conjugated diyne-diene. This was in turn used in a trimethylsilylethynyl cross-coupling with (E)-1-bromopropargyl alcohol (Eqs. 61, 62).60