Nucleophile alkynyl ions

Wipf has shown that 4,4-disubstituted cyclohexanones undergo nucleophilic attack where the facial selectivity is determined by dipolar control. Thus, compounds of the type 23 underwent nucleophilic attack anti to the electronegative substituent at C(4), whereas the fluorinated analogue, 24, underwent attack syn to the oxygen, in accordance with the inversion of the dipole moment. They found that the logarithm of the experimentally observed facial selectivity for nucleophilic attack was correlated linearly (R = 0.998) with the calculated dipole moments. The facial selectivities were also shown to depend upon the nature of the nucleophile, hydride ions and alkynyl carbanions being essentially unselective. 

In most cases, it has been proposed that the cationic intermediate resulting from electrophilic attack by the mercuric ion is, in fact, a mercurinium species 109 in which the mercury cation is unsymmetrically coordinated to the two alkynyl carbon atoms. Hence, these reactions typically afford Markovnikov products. In the presence of excess Hg(OAc)2, nucleophilic attack of the mercurinium intermediate is intermolecular and occurs in an anti-fashion leading to the (ft)-isomer. When no excess of Hg(OAc)2 is present, nucleophilic attack of the mercurinium intermediate is intramolecular and occurs in a ry -fashion leading to the (Z)-isomer.1... 

Rueping has recently reported an interesting alknylation reaction of a-imino esters employing both phosphoric acid Ip and AgOAc as orthogonal cocatalysts [35]. As seen in the catalytic cycle in Scheme 5.21, generation of chiral iminium ion pair I nucleophilic and alkynyl-silver species II proceeds simultaneously. Subsequent nucelophilic addition completes both parallel cycles [36]. 

Alkynyliodonium ions, 1 and 2, are hypervalent iodine species in which one or two alkynyl ligands are bound to a positively charged iodine(III) atom. They are sensitive to nucleophiles, especially at the /1-carbon atom(s) of the alkynyl ligand(s), and for that reason, the isolation of stable alkynyliodonium salts generally requires the incorporation of nucleofugic anions. A list of known alkynyliodonium compounds (i.e. as of 4/1/94), containing 134 iodonium salts derived from 103 iodonium ions, and references (5-45) to their preparation and characterization are presented in Table 1. Among these compounds, alkynyl(phenyl)iodonium sulfonates and tetrafluoroborates are the most common, while alkynyl(alkyl)iodonium salts of any kind are unknown. 

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. 

Many asymmetric C-C bond formation processes are based upon the addition of nucleophiles to the prochiral faces of double bonds to create the chiral carbon center. Since the oxidative alkynylation is believed to occur through the addition of the terminal alkyne to an in situ generated iminium ion, Li and co-workers examined the potential for the development of an asymmetric variant to the oxidative alkynylation process (Scheme 16) [33, 34],... 

An indolizine may also be synthesized from a 2-aIlylpyridine by heating the compound at 120°C [3753]. A good yield of a tricyclic indolizine is obtained by oxidative cyclization of the alkyne (42.4) at about 5°C with MCPBA the first-formed iV-oxide undergoes a rearrangement of the alkynyl chain. Addition of a nucleophile towards the end of this one-pot synthesis leads to a peri-fused quinoline in good yield [3535]. A(-Benzyl-)V-propynylanilines undergo oxidative cyclization on treatment with a perbenzoic acid and a nucleophile (such as thiophenol or cyanide ion) under mild conditions [2784]. 

The reaction of alkoxide ions with alkynyliodonium salts is unproductive, leading to only decomposition products rather than the desired alkoxyacetylenes. Similarly, reaction of R3SiO does not lead to any siloxyalkynes. In contrast the softer sulfonate, carboxylate, and phosphate nucleophiles all readily react with alkynyliodonium salts leading to the corresponding alkynyl sulfonate, carboxylate and phosphate esters [4]. 

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. 

The great majority of known2 reactions of acetylenes are with electrophiles either via the acetylide ions RC=C", or by way of electrophilic addition reactions to the triple bond. Nucleophilic acetylenic substitutions (Sat-A) are generally unfavorable2. Alkynyl(phenyl)iodonium species may serve as synthons for the electrophilic alkynyl... 

Gagosz et al. reported Au(l) catalyst 147-catalyzed alkylation of alkynyl ethers which produced cyclohexane 146 as major product (Scheme 54) [127]. Theoretically, the electrophilic activation of the alkyne 145 by Au(l) initiates a [1,5]-hydride shift to furnish oxocarbenium ion 1, interaction of which with the pendant nucleophilic vinyl-gold moiety affords cyclopropenium intermediate II. Carboca-tion IV, which would finally collapse into cyclohexene 146 after elimination of the gold(I) catalyst might be generated via a [1,2]-alkyl shift on Au-carbene intermediate III. 

The Stephenson s work undoubtedly stimulates a rapid development of visible-light-driven C(sp )-H bond functionalization of tertiary amines. Soon afterward, numerous nucleophiles were employed under the principle of iminium ions chemistry by visible-light photoredox catalysis (Scheme 3.4). The oxidative Mannich reaction of Af-Aryl tetrahydroisoquinoUnes with ketones [17] and enol silanes [18] were developed by Rueping and Xia, respectively. As a follow-up work. Rueping et al. [19] reported an efiicient alkynylation of a-C(sp )-H bond... 

NIS-promoted iodocarbocychzation reaction of various functionalized 1,5-enynes occurs via a 5-endo diastereoselective process to form iodo-functionalized cyclopentenes (Scheme 6.22). Initially, the iodonium ion activates the alkynyl functionality through jt-coordination. Upon nucleophilic attack of the alkenyl moiety in an anti fashion, an iodocyclization reaction occurs and a carbocation is formed. Further, proton abstraction by the succinimide anion affords the iodo-functionalized cyclopentenes [26]. 

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