Acetylenes trimethylsilyl iodide reactions

Addition of trimethylsilyl cyanide to carbon-carbon triple bonds proceeds effectively in the presence of palladium catalyst to give cyano-substituted alkenylsilanes in good yields. Reaction of trimethylsilyl iodide with alkynes under the coexistence of acetylenic tin compounds gives three-component coupling products effectively. 

Potassium or lithium derivatives of ethyl acetate, dimethyl acetamide, acetonitrile, acetophenone, pinacolone and (trimethylsilyl)acetylene are known to undergo conjugate addition to 3-(t-butyldimethylsiloxy)-l-cyclohexenyl f-butyl sulfone 328. The resulting a-sulfonyl carbanions 329 can be trapped stereospecifically by electrophiles such as water and methyl iodide . When the nucleophile was an sp -hybridized primary anion (Nu = CH2Y), the resulting product was mainly 330, while in the reaction with (trimethylsilyl)acetylide anion the main product was 331. 

Nitration of acetylenes with nitryl iodide followed by elimination of HI gives nitroacetylenes, but nitroacetylenes are too thermally unstable to be useful for organic synthesis.77 Recently, nitro-trimethylsilyl-acetylenes are prepared as stable nitroacetylenes by the reaction of bis(trimethylsily)acetylene with nitronium tetrafluoroborate (Eq. 2.39).78... 

Efficiency of the deprotection and coupling reactions are critical to the success of any iterative solid-phase synthesis. Shown in Scheme 1 is a triad of reactions for phenylacetylene oligomer synthesis trimethylsilyl deprotection,28 29 triazene unmasking of an iodobenzene,30 and the Sonogashira coupling of a terminal acetylene with an aryl iodide.31-33 Representative procedures for each step in this sequence are included at the end of this chapter. 

Scheme 5.34 shows the transformations used for the synthesis of dendritic wedges (147) that are functionalized with aryl iodide and trimethylsilyl protected arylalkyne moieties at the focal region. Reaction of 3,5-dibromobenzotriazene (148) with one equivalent of isopropoxy-protected acetylene 149, followed by treatment with base, gave the 3-bromo-5-ethynylaryltriazene (150). Higher selectivity of Pd-catalyzed alkynylation of aryl iodides (e.g., 146) afforded the difunctionalized wedge (151). 

Additionally, acetylene itself is a useful two-carbon building block but is not very convenient to handle as it is an explosive gas. Trimethylsilyl acetylene is a distillable liquid that is a convenient substitute for acetylene in reactions involving the lithium derivative as it has only one acidic proton. The synthesis of this alkynyl ketone is an example. Deprotonation with butyl lithium provides the alkynyl lithium that reacted with the alkyl chloride in the presence of iodide as nucleophilic catalyst (see Chapter 17). Removal of the trimethylsilyl group with potassium carbonate in methanol allowed further reaction on the other end of the alkyne. 

A mixture consisting of the Step 1 product (2.1 g), trimethylsilyl acetylene (0.83 g), triphenyl-phosphine (24 mg), bis(triphenylphosphine)palladium(II) dichloride (12 mg), and copper(I) iodide (4 mg) dissolved in 20 ml of triethylamine was refluxed for 10 hours under a nitrogen atmosphere. After cooling, water was added, and the reaction solution was extracted with EtOAc, washed with saturated brine, and concentrated. The residue was purified by column chromatography, and 1.5 g of product was isolated. 

Selective phosphonate ester dealkylation. Alkyl phosphonate esters are selectively and nearly quantitatively cleaved by bromotrimethylsilane in the presence of alkyl carboxylate esters, carbamates, acetylenes, ketones, and halides. Alkyl iodides do not exchange under the reaction conditions. The resulting bis(trimethylsilyl) phosphonates are hydrolyzed in acetone by a small excess of water. 

Among the applications of allyl silanes reported this year their reaction with phenylselenyl chloride and subsequent oxidation of the resulting allyl selenides to allylic alcoholshas been used in the sequence outlined in Scheme IS. 3-Bromoallyltrimethylsilane therefore behaves as a hydroxypropenyl synthon. Nitroalkenes react with allylsilanes to give, after hydrolysis, y,S-unsaturated ketones, and a synthesis of substituted cyclopentenones based on this procedure has been described.Allyl trimethylsilyl ethers undergo a palladium-promoted coupling with aryl iodides to give /3-aryl-a,/3-unsaturated ketones, and palladium also catalyses the reaction of various aryl bromides with trimethylsilyl acetylene to produce ethynylated aromatics. 

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