Iodonium chiral

Although formation of primary vinyl cation was disproved by the chirality probe approach, a vinyl cationic intermediate can be generated from a primary substrate via participation if a more stable cation could result. Unsymmetrically substituted 2,2-dialkylvinyl iodonium salt 24 gave mainly rearranged products on solvolysis.15 The products involve those of the 1,2-shift of either of the alkyl groups on the p position (Scheme 4). Those formed from migration of the alkyl... 

Under more basic conditions, a-elimination predominates and insertion of the carbene 40 to the solvent gives racemic 22. Non-basic and poorly nucleophilic conditions allow neighboring group participation to form the rearranged substitution product 23 with complete chirality transfer. The participation can be considered as an intramolecular nucleophilic substitution, and does occur only when it is preferable to the external reactions. Under slightly basic conditions with bases in HFIP, participation is allowed, and the weak base can react with the more electrophilic vinylic cation 21 (but not with the iodonium ion 19). A suitably controlled basicity can result in the formation of cycloalkyne 39, which is symmetrical and leads to racemization. These reactivities are illustrated in Scheme 6. 

Comparisons of product distributions in thermal and photochemical solvolyses show that the primary vinyl cation is not involved in thermolysis but is formed photochemically. The chirality probe approach using optically active 4-methylcyclohexylidenmethyl(aryl)iodonium tetrafluoroborate 19 was applied to the photosolvolysis.24 The rearranged product 4-methylcycloheptanone retained some optical activity, but the enantiomeric product in slight excess has a different structure depending on the iodoarene leaving group Arl of the substrate. The results indicate that the primary vinyl cation involved is not in a free, dissociated achiral form. 

Oenyuk, B., Whiteford, J.A. and Stang, P.J. (1996) Design and study of synthetic chiral nanoscopic assemblies. Preparation and characterization of optically active hybrid, iodonium-transition-metal and all-transition-metal macrocyclic molecular squares. J. Am. Chem. Soc., 118 (35), 8221-8230. 

Tetraphenylstannane was the reagent of choice for the conversion of two chiral precursors, i.e. 2-(diacetoxyiodo)- and 2,2 -bis(diacetoxyiodo)-l,r-binaph-thyls, into chiral iodonium salts (Scheme 35) [106]. 

A new type of iodonium salts constitute the conformationally rigid, tetranu-clear macrocyclic ring systems dubbed molecular boxes. The relatively simpler tetraaryltetraiodonium salts were obtained from 4,4 -bis(diacetoxyiodo)bi-phenyl and 4,4 -bis(trimethylsilyl)biphenyl [119]. The iodonium salt derived from 4-(4 -lithiophenyl)pyridine was made using the method of /J-(dichloroio-do)chloroethylene and it was used for the construction of hybrid iodonium-platinum (or palladium) cationic tetranuclear macrocyclic squares including some in which the ligand of the metal was a chiral biphosphine [120,121]. 

Chiral iodonium salts of the general type p-RC6H4C = CI+Ph X-, where R was S-2-methylbutyloxy or S-2-methylbutyloxycarbonyl and X was TsO or TfO, were prepared from silylated alkynes with either [hydroxy(tosyloxy)iodo]benzene or PhI(OTf)OI(OTf)Ph [138]. 

A similar procedure was employed in the asymmetric Heck-type coupling of iodonium salt 81 with 2,3-dihydrofuran (Scheme 38) [65]. When carried out in the presence of the chiral bidentate ligand (R)-BINAP, this reaction afforded optically active (up to 78% ee) coupling product 82 in moderate yield. 

Asymmetric phenylation of carbanions with chiral iodonium salts has recently been reported [70]. The chiral diiodonium salt 93 selectively reacts with potassium enolate of l-oxo-2-indancarboxylate 92 to give the a-phenylated indanone 94 with moderate enantioselectivity (Scheme 42). 

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]. 

The metal-catalyzed carbenoid decomposition of iodonium ylides can be applied in asymmetric reactions [149-152]. For example, the copper(II)-cat-alyzed intramolecular C-H insertion of phenyliodonium ylide 209 in the presence of several chiral ligands affords product 210 (Scheme 74) [151]. Enantiose-lectivities in this reaction vary in the range of 38-72% for different chiral... 

Several iodonium ylides, thermally or photochemically, transferred their carbene moiety to alkenes which were converted into cyclopropane derivatives. The thermal decomposition of ylides was usually catalysed by copper or rhodium salts and was most efficient in intramolecular cyclopropanation. Reactions of PhI=C(C02Me)2 with styrenes, allylbenzene and phenylacetylene have established the intermediacy of carbenes in the presence of a chiral catalyst, intramolecular cyclopropanation resulted in the preparation of a product in 67% enantiomeric excess [12]. 

Moreover, chiral jS.y-unsaturated acids bearing an oxygenated function at the a-position cyclize in a 5-endo mode under kinetic conditions leading to a single diastereomer. This behavior has been explained in terms of steric crowding of the intermediate iodonium ion50. 

A chiral bisphosphine such as 2,2 -h -(diphenylphosphino)-l,F-binaphthyl (BINAP) has been extensively used as a chiral chelator in asymmetric catalysis. When Stang et al. reacted the chiral metal complex 42 with 40, they synthesized a square box (Figure 25) and asymmetric induction was observed [79,82] with the formation of an excess of one of the preferred diastereoisomers as measured by NMR spectroscopy. The same reaction has been carried out with 42 and 6is-4-(4 -pyridyl)phenyl)iodonium triflate, but in this case the diaza ligands of the iodonium species possess rotational symmetry about their linkages. Consequently, the optical activity of the molecular squares obtained is due exclusively to the chiral transition metal auxiliary BINAP. 

When the reactive chiral building unit [M(/ -(+)-BINAP)(H20)][0Tf]2 with M = Pd (5.84) or M = Pt (5.85) was treated with the bridging ligand bis [4-(4 -pyridyl)phenyl) iodonium triflate (57), the related chiral hybrid supramolecular squares were formed (5.86-5.87) (hybrid = presence of iodonium and transition metal comers in the assembly). These hybrid molecular squares are optically active due to the chiral transition metal auxiliary (BINAP) in the assembly. Interestingly chiral induction was observed in these metallo-macrocycles since the preferred diastereomer in each preparation was formed in excess compared with the other isomers (Figure 5.34). ... 

It is worth mentioning that the reaction of 58 with the achiral diphosphine ligands [Pd (c -Et3P)2][OTf 2 and [Pt(cri-Et3P)2][OTf]2 under the above self-assembly conditions provided a diastereomeric mixture of products. In theory, six possible isomers could be formed (Eigure 5.36). In this example the chirality of the metallo-squares arises from the chiral conformation of the iodonium moiety in the assembly and the restricted rotation of the pyridine ligands around the M—N bond. 

The deprotonation of N—H bonds in diverse oxazoUdin-2-ones with KHMDS as the base followed by tbe treatment of tbe crade reaction mixtures with trimethylsilylethynyl iodonium triflate electrophiles afforded trimethylsilyl-terminated IV-ethynyl oxa-zolidinones in 50-60% yields (eq 74). Desilylation could be realized on the purified products or the crude reaction mixtures, and the alk)myl oxazolidinones were elaborated into novel stannyl enamines in the subsequent steps. In contrast, protocols employing -BuLi in toluene, or CS2CO3 in DMF gave yields lower than 20%. The procedure could be successfully applied to chiral oxazolidinones, since substitution at the C4 position of the oxazolidinones did not have a detrimental effect on reactivity. 

Aggarwal and Olofsson have developed a direct asymmetric a-arylation of prochiral ketones using chiral lithium amide bases and diaryliodonium salts [881]. In a representative example, the deprotonation of cyclohexanone derivative 684 using chiral Simpkins (/ ,/ )-base followed by reaction with the pyridyl iodonium salt gave the arylated product 685 in 94% ee (Scheme 3.275). This reaction has been employed in a short total synthesis of the alkaloid (-)-epibatidine [881]. 

Bis(methoxycarbonyl)(phenyliodinio)methanide (778), the most common iodonium ylide derived from malonate methyl ester, has found synthetic applications in the C—H insertion reactions [1044-1048] and the cyclopropanation of alkenes [1049-1055], including enantioselective cyclopropanations in the presence of chiral rhodium complexes [1056-1058], Representative examples of these reactions are shown in Scheme 3.306 and include the BFs-catalyzed bis(carbonyl)alkylation of 2-alkylthiophenes 777 [1045] and the optimized procedure for rhodium-catalyzed cyclopropanation of styrene 779 [1052]. 

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