Alkenyliodonium salts

Evidence for a Michael addition of a nucleophile to alkenyl(phenyl)iodonium salts at the Cp atom has now been reported for the first time. Nucleophilic vinylic substitutions of (Z)-(/3-bromoalkenyl)iodonium tetrafiuoroborates (161) and its (Z)-(/3-chloroalkenyl) analogue with sodium benzenesulfinate in THE afforded stereoselectively (Z)-l,2-bis(benzenesulfonyl)alkene (163) with retention of configuration. Intermediate formation of (Z)-[/3-(benzenesulfonyl)alkenyl]iodonium salt (162) in these reactions was established by NMR experiments in CDCI3. The formation of (Z)-(162) involves a hitherto unobserved Michael addition of benzenesulfinate anion to the alkenyliodonium salts at the Cp atom, followed by halogen extrusion. ... 

Several important improvements of this methodology have been reported since 1994. A broad range of organotins were used in the reactions with aryl-, heteroaryl-, or alkenyliodonium salts in aqueous organic solvents. This reaction (Equation (2)) takes place at room temperature in the presence of phosphine-free palladium... 

It has been shown that selective a-vinylation of enolate anions derived from 1,3-dicarbonyl compounds can be achieved by reaction with 4-/-butyl-1 -cyclohexenyl-(aryl)iodonium and 1-cyclopentenyl(aryl)iodonium tehafluoroborates without competing a-arylation, provided that the alkenyliodonium salt used bears a / -mcthoxyphcnyl, rather than phenyl, group.24... 

The newest and perhaps best method of general application in which the double bond already exists is the reaction of alkenylboronic acids or their esters with (diacetoxyiodo)benzene in presence of BF3 Et20. Since the precursors are readily available from alkynes in a well defined stereochemistry, the alkenyliodonium salts may be obtained in pure E- or Z-form without difficulty and in high yield (Scheme 40) [122]. 

Alkenyl(phenyl)iodonium salts have attracted a significant interest recently as stable and readily available powerful alkenylating reagents. Several convenient, general procedures for the stereoselective synthesis of alkenyliodonium salts from silylated or stannylated alkenes and the appropriate hypervalent iodine reagents are known [5]. The chemistry of alkenyliodonium salts has been extensively covered in several recent reviews [42 - 45]. 

The alkylidenecarbenes generated via base-induced a-elimination can also be trapped by cycloaddition with external alkenes. For example, the treatment of alkenyliodonium salt 55 with a strong base in the presence of excess styrene gives methylenecyclopropane 56 in good yield (Scheme 26) [47]. 

The selectivity of the alkenylation reactions and the yields of products can be dramatically improved by carrying out the reaction of alkenyliodonium salts with carbon nucleophiles in the presence of transition metal compounds in stoichiometric or catalytic amounts. Thus, the reactions of bicycloalkenyldiiodo-nium salts 62 with cyanide anion or with alkynyllithium in the absence of transition metals are non selective and lead to a wide spectrum of products, while the same reactions in the presence of the equimolecular amount of copper(I) cyanide afford the respective products of vinylic nucleophilic substitution in good yields (Scheme 29) [52,53]. 

Copper(I) mediated reactions of alkenyliodonium salts with nucleophiles proceed with complete retention of configuration, which is probably explained by the mechanism involving oxidative addition of cuprates, ligand coupling at the iodine center of 67, and then ligand coupling at the copper(III) of 68 (Scheme 31) [42,54]. 

Alkenyliodonium salts can serve as highly reactive substrates in various transition metal-catalyzed reactions. In the presence of a copper(I) catalyst iodo-... 

Alkenyliodonium salts can be used as highly reactive substrates for Heck-type olefination and similar palladium-catalyzed cross-coupling reactions [63 -65]. In a recent example, a series of dienes 80 were stereo- and regioselectively prepared by a palladium-catalyzed Heck-type reaction of alkenyliodonium salts 79 with a,/3-unsaturated carbonyl compounds (Scheme 37) [64]. 

Organotin compounds can be effectively used as substrates in the palladium-catalyzed cross-coupling reactions of alkenyliodonium salts [66,67]. For example, the reaction of alkenyliodonium triflate 84 with 5-stannylated uracil 83 proceeds smoothly to provide cross-coupled products 87 in moderate yield. The same products 87 are obtained in almost quantitative yield in the palladium-catalyzed cross-coupling of uracil iodonium triflate 85 and vinyltin 86 (Scheme 39) [67]. 

The palladium-catalyzed carbonylation reaction of alkenyliodonium salts in the presence of alcohols proceeds at room temperature under one atmosphere of carbon monoxide to afford esters 89 in good yields (Scheme 40) [68]. 

Similar to alkenyliodonium salts (see Sect. 6.3), aryliodonium salts are highly reactive substrates in Heck-type olefination and other palladium-catalyzed coupling reactions. Aryliodonium salts can serve as very efficient reagents in the palladium-catalyzed arylations of acrylic acid 101 [75], organotin compounds 102 [76], sodium tetraphenylborate 103 [77], and copper acetylide 104 [78] (Scheme 45). 

Kang, S.-K., Yoon, S.-K., Kim, Y.-M. Copper-Catalyzed Coupling Reaction of Terminal Alkynes with Aryl- and Alkenyliodonium Salts. Org. 

The outcome of the reaction of alkenyliodonium salts with halide salts depends on the substitution pattern of the iodonium substrate. Thus, treatment of ( )-alkenyliodonium salts with tetra(n-butyl)ammonium halides (X = Cl, Br or I) afforded exclusively the corresponding (Z)-alkenyl halides, the product of inversion of configuration.The mechanism of these reactions with configuration inversion still remains a matter of debate, an SN2-type transition being postulated. When the same reaction was performed with a combination of cuprous halides and potassium halides at room temperature in the dark, complete retention of stereochemistry was observed. 

The effects of the /3-aIkyl groups of the 1-alkenyliodonium salts 11 and 20-22 on the product ratios and rate constants in their reaction with chloride ion are... 

A stereochemical investigation using substrates of opposite configuration (e.g., Z-11 next to -11) is desirable to establish that the substitution reaction is stereospecifically inversion. However, Z-11 does not undergo nucleophilic substitution with halides but yields quantitatively the elimination product, 19. (Z)-l-Alkenyliodonium salts are known to be too labile to be isolated due to their facile a/if/-/3-elimination (Scheme 17). ... 

Other nucleophiles, which give substitution with inversion in their reaction with ( )-l-alkenyliodonium salts, include dialkyl sulfide and selenide," phosphorose-lenoates, and sulfonates (Scheme 18). Amides also react with E)- -alkenyliodonium salts and give a Z enol formate (Scheme 19). " The mechanism of these reactions, involving very weakly basic or non-basic nucleophiles, may well be in-plane SnVo- attack. Moderately basic nucleophiles such as amines and carboxylates, even dichloroacetate, give -elimination. 

The intramolecular Friedel-Crafts-type reaction of alkenyliodonium salts 23 proceeds efficiently at 40-60°C in various solvents including methanol, acetonitrile, and chloroform (Scheme 21). The Z isomer of 23c does not undergo ring closure, thus the reaction does not involve the formation of a vinyl cation. The mechanism of the reaction may well be in-plane SnVo- attack by the intramolecular aromatic nucleophile. 

Thus far, we have discussed typical vinylic Sn2 and SnI reactions. The former involve reactions of good nucleophiles with simple 1-alkenyliodonium salts (S Ycr) and unactivated j8-halo derivatives (SnVtt), while the latter reactions occur under poor nucleophilic conditions (solvolysis) if the substrates can give stabilized vinylic cations of secondary or bridged structure. In this section, reactions which have characteristics of both categories are presented. 

Defunctionalization of heteroalkenes. The C-X bonds of alkenyliodonium salts, l,2-bis(phenyltelluro)alkenes, and ketene bis(methylthio)acetals, which carry an electron-deficient substituent, are selectively replaced. Only one group of the chalcogenides is affected, and the ketene dithioacetals undergo reductive cleavage. However, the alkenylcopper intermediates can be acylated. ... 

Replacement of an unsaturated phenyliodo group. Formation of unsaturated sulfones by a tandem Michael addition-elimination is a highly efficient process that allows the synthesis of (Z)-l,2-bis(benzenesulfonyl)alkenes from (Z)-jS-(benzenesul-fonyl)alkenyliodonium salts. In S-ketoethynyl(phenyl)iodonium salts the electron-withdrawing power of the ketone group is weaker, the Michael addition is followed by carbene formation. Cyclopentenones are formed. A seemingly direct substitution of alkynyl(phenyl)iodonium salts gives alkynyl sulfones efficiently. ... 

Asymmetric Heck reaction. The chiral product from an intramolecular process can he trapped. Thus, the method has been applied to a synthesis of (-)-A -capnellene and halenaquinone. Hypervalent alkenyliodonium salts can be used instead of triflates. 

Alkene derivatives such as alkenylboronic acids and alkenylzir-conanes reacht with PhI(OAc)2 to furnish alkenyliodonium salts (eq 46). These transformations proceed with retention of olefin configuration. Similarly, alkenylboron species add to PhI(OAc)2 in the presence of Nal to give vinyl acetate products (eq 47). In these examples, ( )-alkenylboronates give stereochemically pure (Z)-configured enol acetates. ... 

Several examples of alkenyliodonium salts have been known for more than a hundred years, but it was only in the 1990s that these compounds become readily available and found some synthetic application. The chemistry of alkenyl(aryl)iodonium salts has been covered in several reviews by Ochiai [431], Okuyama [432-434] and Zefirov and coauthors [435]. 

Preparation First representatives of alkenyliodonium salts, dichlorovinyl(phenyl)iodonium species, were reported by Thiele and Haakh in the early 1900s [436]. The first general synthetic approach to alkenyl(phenyl)iodonium salts was developed by Ochiai in the mid-1980s [437,438], This method is based on the reaction of silylated alkenes 299 with iodosylbenzene in the presence of Lewis acids, leading to the stereoselective formation of various alkenyliodonium tetrafluoroborates 300 in good yield (Scheme 2.85). 

Structural Studies Examples of alkenyliodonium salts whose X-ray crystal structures have been reported in literature include the following ( )-((3-trifluoromethanesulfonyloxyalkenyl)phenyliodonium triflate 336 [465], 2,3-bis(iodonium)norbornadiene 332 [460], 4-fert-butylcyclohexenyl(phenyl)iodonium... 

Figure 2.12 Examples of alkenyliodonium salts analyzed by single-crystal X-ray diffraction.

Steroidal dienes 36 react with (difluoroiodo)arenes 37 to afford fluorinated product 38 with a high degree of regioselectivity and stereoselectivity (Scheme 3.14) [14]. (Difluoroiodo)arenes react with terminal alkynes with stereo- and regioselective formation of synthetically useful ( )-2-fluoro-l-alkenyliodonium salts [36-39]. A convenient procedure for the preparation of various ( )-2-fluoroalkenyliodonium fluorides 39 is based on the addition of difluoroiodotoluene to terminal acetylenes (Scheme 3.15) [37-39]. Products 39 can be... 

Alkenyliodonium salts have been used as highly reactive reagents for Heck-type olefination [39, 971], Sonogashira-type coupling with alkynes [965, 972] and similar other palladium-catalyzed cross-coupling reactions [966, 973, 974]. In a specific example, (Z)-p-fiuoro-a,p-unsaturated esters 721 were stereoselec-tively synthesized from (Z)-2-fluoro-l-alkenyliodonium salts 720 by the Pd-catalyzed methoxycarbonylation reaction (Scheme 3.288) [974]. This reaction proceeds at room temperature and is compatible with various functional groups on the substrate. 

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