Alkynyl iodonium salts with nucleophiles

In some instances the iodonium salt is not isolated but may react in situ with nucleophiles, for example upon hydrozirconation of alkynyl(phenyl)iodonium salts with Cp2Zr(H)Cl (Scheme 42) these salts were used for the stereoselective synthesis of some halogenated alkenes [128]. 

Recent progress on the use of hypervalent iodine reagents for the construction of carbon-het-eroatom (N, O, P, S, Se, Te, X) bonds is reviewed. Reactions of aryl-A3-iodanes with organic substrates are considered first and are loosely organized by functional group, separate sections being devoted to carbon-azide and carbon-fluorine bond formation. Arylations and alkenyla-tions of nucleophilic species with diaryliodonium and alkenyl(aryl)iodonium salts, and a variety of transformations of alkynyl(aryl)iodonium salts with heteroatom nucleophiles are then detailed. Finally, the use of sulfonyliminoiodanes as aziridination and amidation reagents, and reactions of iodonium enolates formally derived from monoketones are summarized. 

Appropriately substitued alkynyl iodonium salts afforded with nucleophiles cyclopentene derivatives. This annulation can be either [5 + 0], when all carbon atoms come from the alkyl chain of the alkynyl moiety, or a [2 + 3] process, in which three carbon atoms come from the nucleophile. Competition between [5 + 0] and [2 + 3] annulation may occur in some cases. 

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]. 

These highly reactive yet stable species are strong electrophiles of tetraphilic character, since nucleophiles may attack three different carbon atoms (a,/ ,a ) and iodine. In most reactions the first step is a Michael addition at fi-C with formation of an alkenyl zwitterionic intermediate (ylide) which normally eliminates iodoben-zene, generating an alkylidene carbene then, a 1,2-shift of the nucleophile ensues. The final result is its combination with the alkynyl moiety which behaves formally as an alkynyl cation. The initial adduct may react with an electrophile, notably a proton, in which case alkenyl iodonium salts are obtained also, cyclopentenes may be formed by intramolecular C-H 1,5-insertion from the alkylidenecarbenes ... 

The regiospecific coupling of triphenylphosphine with the alkynyl ligands of alkynyl(phenyl)iodonium ions in such photosubstitution reactions is remarkable. This may be contrasted with the less selective and somewhat unpredictable photolytic decomposition (i.e. high pressure mercury lamp, pyrex) of alkynyl(phenyl)iodonium salts in the absence of nucleophiles (e.g. see equation 70)89. 

Alkynyl(phenyl)iodonium salts react with certain carbon nucleophiles to yield products which are converted into carbenes by reductive elimination of iodobenzene. The carbenes... 

The first alkynyl(phenyl)iodonium salt, the rather unstable chloride (8), was reported in low yield, by the interaction of jS-phenylethynyl lithium (6) with (dichloroiodo)benzene (7) (equation 1). The Russian chemist Merkushev and his coworkers next reported the isolation of 9 in 64% yield (equation 2). However, this interesting but hygroscopic product was only sparsely characterized. It was not until the 1980s, and in particular the last half-dozen years, that these novel, uniquely functionalized acetylenes became readily available. Since then they have gained considerable importance and widespread use as synthons for the electrophilic acetylene species RC=C due to the versatility of their reactions with a wide variety of nucleophiles. This chapter will cover the preparation, characterization and chemistry of alkynyl(phenyl)iodonium salts (2) and related species with emphasis on our own recent contributions to this rapidly evolving area of acetylene chemistry. Two previous reviewsdescribe the early developments and chemistry in this new field. 

The vast majority of alkynyl(phenyl)iodonium salts are stable microcrystalline solids. Their exact stability depends upon both the counter-ion and the substituents on the alkyne. Stability decreases with the increasing nucleophilicity of the counter-ion. Hence, the most stable, and therefore widely used, counter-ions are CF3SO3 and BF4 with somewhat lower stability for ArS03 and MeS03 as well as CF3CO2, whereas halides as anions are rather unstable and decompose in a matter of hours or less. 

More complex nucleophiles can be reacted with alkynyl(phenyl)iodonium salts as well. For example, protected aminomalonates (50) and 18 give the corresponding alkynylmalonates 51 in 30-90% isolated yields (equation 19). Likewise, a variety of dicarbonyl enolates 52-54 react with 55 to give alkynyl products 56-58 (equations 20-22). These reactions may be looked upon as alkynylations or, in other words, the triple-bond analogs of the well-established alkylation reactions. Unfortunately, they only work with soft nucleophiles such as 52-54 (OTs, PhC02 PhsP, etc.). Nucleophiles such as RO, or simple enolates, do not seem to work. 

Alkynyl(phenyl)iodonium salts are also excellent substrates for interaction with organo-metalUc nucleophiles. Reaction of Vaska s complex (59) and its Rh-analog (60) with 18 results in cr-alkynyl complexes 61 and 62, respectively, in nearly quantitative yields (equations 23 and 24) . These reactions are oxidative additions to Ir and Rh, respectively, and represent one of the best ways to introduce a c-acetylide ligand into metal complexes. 

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]. 

The intramolecular variant of the alkylidene carbene cyclization is achieved by treating functionalized alkynyliodonium salts with a suitable nucleophile. These cyclizations are exemplified by the following works the preparation of various functionalized 2,5-dihydrofurans by treatment of 3-alkoxy-l-alkynyl-(phenyl)iodonium triflates with sodium benzenesulfinate [1002], employment of the alkylidene carbene cyclization in the total syntheses of natural products agelastatin A and agelastatin B [1003] and preparation of the tricyclic core of ( )-halichlorine through the use of an alkynyliodonium salt/alkylidenecarbene/1,5 C—H insertion sequence [1004]. In particular, Wardrop and Fritz have employed the sodium benzenesulfinate-induced cyclization of alkynyliodonium triflate 751 for the preparation of dihydrofuran 752 (Scheme 3.295), which is a key intermediate product in the total synthesis of ( )-magnofargesin [1002]. 

Various flve-membered heterocycles can be prepared by inter- or intramolecular addition/cyclizations of appropriate nucleophiles with alkynyliodonium salts via alkylidene carbene intermediates [856, 978, 979]. The intermolecular variant of this cyclization has been employed in the synthesis of 3-substituted-5,6-dihydroimidazo[2,l-( ]thiazoles [997], 2-substituted imidazo[l,2-a]pyrimidines [998] and 2-substituted-imidazo[l,2-fl]pyridines [999]. In a representative example, 2-substituted imidazo[l,2-fl]pyridines 744 were synthesized in good yield by cyclocondensation of 2-aminopyridine (742) with alkynyl(phenyl)iodonium tosylates 743 under mild conditions (Scheme 3.293) [999]. The mechanism of this cyclization involves... 

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