Iodonium-substituted alkynes

Alkynyl(phenyl)iodonium salts can be used for the preparation of substituted alkynes by the reaction with carbon nucleophiles. The parent ethynyliodonium tetrafluoroborate 124 reacts with various enolates of /J-dicarbonyl compounds 123 to give the respective alkynylated products 125 in a high yield (Scheme 51) [109]. The anion of nitrocyclohexane can also be ethynylated under these conditions. A similar alkynylation of 2-methyl-1,3-cyclopentanedione by ethynyliodonium salt 124 was applied in the key step of the synthesis of chiral methylene lactones [110]. 

Likewise, alkynyliodonium tosylates can be coupled with dialkyl- and diphenyl cuprates 136 to afford the respective alkyl- and phenyl-substituted alkynes 137 (Scheme 56) [114]. An interesting example of this reaction involves the coupling of (trimethylsilyl)ethynyl iodonium triflate with cubyl cuprate generated in situ from iodocubane 138 [116]. 

Lithium diphenylcuprates and lithium dialkylcuprates reacted similarly with alkynylphenyl iodonium tosylates to afford phenyl- and alkyl-substituted alkynes respectively... 

Alkynyl(phenyl)iodonium salts have found synthetic application for the preparation of various substituted alkynes by the reaction with appropriate nucleophiles, such as enolate anions [980,981], selenide and telluride anions [982-984], dialkylphosphonate anions [985], benzotriazolate anion [986], imidazolate anion [987], N-functionalized amide anions [988-990] and transition metal complexes [991-993]. Scheme 3.291 shows several representative reactions the preparation of Ai-alkynyl carbamates 733 by alkynylation of carbamates 732 using alkynyliodonium triflates 731 [989], synthesis of ynamides 735 by the alkyny-lation/desilylation of tosylanilides 734 using trimethylsilylethynyl(phenyl)iodonium triflate [990] and the preparation of Ir(III) a-acetylide complex 737 by the alkynylation of Vaska s complex 736 [991]. 

Hydrozirconation of terminal alkynes R-C=CH (R= aryl, alkyl) with 1 affords terminally ( )-Zr-substituted alkenes with high efficiency and excellent stereochemical and regiochemical control (>98%). These alkenylzirconocene complexes are of particular interest for synthetic use [136, 143, 144]. Moreover, beside the electropositive halogen sources [145] and heteroatom electrophiles [3] used in the pioneering studies to directly cleave the Zr-C bond, ( )-vinyl-Zr complexes were recently transformed into a number of other trans-functionalized alkenes such as ( )-vinyl-sul-fides[146], vinylic selenol esters [147], vinyl-sulfones [148], vinyl-iodonium [149], vinyl-(R0)2P(0) [150], and vinilic tellurides [143]. 

Sn2 mechanism (Scheme 47) [138,139]. Concerted,in-plane nucleophilic substitutions of this type are facilitated by the exceptional nucleofugality of iodoben-zene, and appear to be operative with weakly basic nucleophiles. Similar reactions of ( )-/ phenylvinyl(phenyl)iodonium tetrafluoroborate are slower, less stereoselective, and more prone to competitive alkyne formation. 

The exceptional nucleofugality of the phenyliodonio group has been determined in an alkenyl salt and it is about 106 times greater than that of triflate [30]. This remarkable property makes alkenyl iodonium salts excellent vinyl cation equivalents in nucleophilic substitutions. The chemistry of alkenyl iodonium salts is dominated by the transfer of their aliphatic moiety to a variety of nucleophiles other important reactions involve Michael-type addition and alkylidenecarbene generation, along with elimination to alkynes which is actually an undesirable side-reaction. 

The photochemical hydration of one of the triple bonds of naphthyl-1,3-butadiynes is reported to yield naphthyl substituted ynones if the reaction is carried out in methanol rather than water then the primary products include (240) which result from addition of solvent or photoreduction. These compounds undergo secondary photochemistry and are found to photocyclise to the phenanthrenes (241). A similar reaction occurs for phenyl substituted 1,3-butadiynes. Photolysis of the phenyl iodonium ylid of dibenzoylmethane, i.e. (242), with terminal alkynes yields (243) which photocyclises in a similar manner to (240) to yield benzoyl-naphthols (244). °... 

Likewise, interaction of terminal alkynes with 20 in refluxing acetonitrile in the presence of toluenesulfonic acid gives the intramolecular heterocyclic alkynyliodonium salts 21 [Eq. (11)] [33]. A wide range of the alkyl- and phenyl-substituted congeners 21 of these intramolecular analogs of the acyclic iodonium tosylates, 9, are obtained in 26-70% isolated yields. 

Rh(II)-catalyzed reaction of iodonium yhdes with conjugated compounds leading to efficient synthesis of dihydrofurans, oxazoles and dihydrooxepines [ 1068] synthesis of various heterocycles by Rh(II)-catalyzed reactions of iodonium ylides with vinyl ethers, carbon disulfide, alkynes and nitriles [1055] Rh(II)-catalyzed reaction of iodonium ylides with electron-deficient and conjugated alkynes leading to substituted furans... 

In-plane, backside, vinylic nucleophilic substitution of an appropriate halide leaving group can also be effected. Thus, the ( )-P-alkylvinyl(phenyl)iodonium salt, which can be prepared from the corresponding trimethylsilyl alkene and iodoso-benzene, undergoes both Sn2 substitution to produce the inverted chloroalkene and elimination to the corresponding alkyne (Scheme 7.11). 

The copper-catalyzed cascade annulation of nitriles, diaryliodonium salts, and alkynes was recently described by Chen and coworkers. N-Arylation of the nitrile resulted in an V-arylnitrilium intermediate, which was trapped by the aUcyne and subsequently underwent electrophilic annulation to yield substituted quinolines (Scheme 6d) [107]. The concept was later varied to reach polycyclic quinolines, quinazolines, quinazolinimine, and acridine scaffolds [108—110], A similar cascade reaction delivered iminobenzoxazines by N-aiylation of ort/io-cyanoanilides followed by C-O cyclization [111], and the reactimi of nitriles with [1,1-biphenyl]-derived iodonium salts resulted in phenanthiidines [112]. The corresponding C-arylation cascade reactions are discussed in Sect. 4.3. 

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