Thermal and photochemical solvolysis of (E)- and (Z)-2-phenyl-1-propenyl(phenyl)iodonium tetrafluoroborate: benzenium and primary vinylic cation intermediates

Award Lecture Paper: Hypervalent Iodine-Mediated Oxidative Alkene Arylation: A Thorough Analysis

A hypervalent iodine-mediated, oxidative alkene arylation reaction has been developed. Both Koser’s reagent (PhI(OH)OTs) and (diacetoxyiodo)toluene (TolI(OAc)₂) were equally viable as oxidants, which reacted with ortho-vinylbiphenyl derivatives to produce tri-, tetra- and pentacyclic polycyclic aromatic hydrocarbons (PAHs) in yields up to 97%. Comparison of this stoichiometric reaction with a previously-reported catalytic process showed that these protocols were largely complementary, and that they likely operate via the same general mechanistic sequence involving vinyliodonium salts decomposing into vinylene phenonium ions. Various mechanistic control experiments were conducted, which ruled out epoxides as intermediates, and which showed that E- and Z- alkene geometry in 10-substituted ortho-vinylbiphenyls had no impact on the ensuing isomeric product distributions. These experiments strongly supported the formation of E-vinyliodonium ions as initial reaction intermediates, and while the occurrence of 1,2-phenyl shift products was a common phenomenon, we concluded that alkyl substitution on the ortho-vinylbiphenyl was a requirement for this alternate pathway to occur.

Palladium-Catalyzed Direct C-H Trifluoroethylation of Aromatic Amides

A simple and direct C-H trifluoroethylation of aromatic amides has been developed. The protocol is applicable to a variety of aromatic amides, including ones derived from amino acids. The developed method can be used for further modifications of peptides. Preliminary mechanistic studies have been done by isolating the reaction intermediate.

Highly regio- and stereoselective synthesis of (Z)-trisubstituted alkenes via propyne bromoboration and tandem Pd-catalyzed cross-coupling

Contrary to all previous reports, bromoboration of propyne with BBr(3) proceeds in >or=98% syn-selectivity to produce (Z)-2-bromo-1-propenyldibromoborane (1). Although 1 is readily prone to stereoisomerization, it can be converted to the pinacolboronate (2) of >or=98% isomeric purity by treatment with pinacol, which may then be subjected to Negishi coupling to give trisubstituted (Z)-alkenylpinacolboronates (3) containing various R groups in 73-90% yields. Iodinolysis of 3 affords alkenyl iodides (4) in 80-90% yields. All alkenes isolated and identified are >or=98% Z.

Regiospecific one-pot synthesis of diaryliodonium tetrafluoroborates from arylboronic acids and aryl iodides

Diaryliodonium salts have recently received considerable attention as mild arylation reagents in organic synthesis. This paper describes a regiospecific, sequential one-pot synthesis of symmetrical and unsymmetrical diaryliodonium tetrafluoroborates, which are the most popular salts in metal-catalyzed arylations. The protocol is fast and high-yielding and has a large substrate scope. Furthermore, the corresponding diaryliodonium triflates can conveniently be obtained via an in situ anion exchange.

Photochemical generation of six- and five-membered cyclic vinyl cations

The photochemical solvolyses of 4-tert-butylcyclohex-1-enyl(phenyl)iodonium tetrafluoroborate (1) and cyclopent-1-enyl(phenyl)iodonium tetrafluoroborate (2) in methanol yield vinylic ethers and vinylic cycloalkenyliodobenzenes and cycloalkenylbenzene, which are the trapping products of the geometrically destabilized C6-ring and C5-ring vinyl cation with the solvent and with the leaving group iodobenzene. Iodonium salt 2 also yields an allylic ether and allylic cyclopentenyliodobenzenes and cyclopentenylbenzene, which are the trapping products of the C5-ring allylic cation produced from the C5-ring vinyl cation by a hydride shift in a typical carbocationic rearrangement.

Photochemical generation of highly destabilized vinyl cations: the effects of alpha- and beta-trifluoromethyl versus alpha- and beta-methyl substituents

The photochemical reactions in methanol of the vinylic halides 1-4, halostyrenes with a methyl or a trifluoromethyl substituent at the alpha- or beta-position, have been investigated quantitatively. Next to E/Z isomerization, the reactions are formation of vinyl radicals, leading to reductive dehalogenation products, and formation of vinyl cations, leading to elimination, nucleophilic substitution, and rearrangement products. The vinyl cations are parts of tight ion pairs with halide as the counterion. The elimination products are the result of beta-proton loss from the primarily generated alpha-CH(3) and alpha-CF(3) vinyl cations, or from the alpha-CH(3) vinyl cation formed from the beta-CH(3) vinyl cation via a 1,2-phenyl shift. The beta-CF(3) vinyl cation reacts with methanol yielding nucleophilic substitution products, no migration of the phenyl ring producing the alpha-CF(3) vinyl cation occurs. The alpha-CF(3) vinyl cation, which is the most destabilized vinyl cation generated thus far, gives a 1,2-fluorine shift in competition with proton loss. The experimentally derived order of stabilization of the vinyl cations photogenerated in this study, alpha-CF(3) < beta-CF(3) < beta-CH(3) < alpha-CH(3), is corroborated by quantum chemical calculations, provided the effect of solvent is taken into account.

Catalytic Cyclization of o-Alkynylbenzaldehyde Acetals and Thioacetals. Unprecedented Activation of the Platinum Catalyst by Olefins. Scope and Mechanism of the Reaction

A general protocol for the synthesis of functionalized indenes from o-alkynylbenzaldehyde acetals and thioacetals has been elaborated. Acetals uniformly give cyclization products having the alkyl group from the starting acetylene migrated to the alpha-position, whereas the cyclization of the corresponding thioacetals proceeds without alkyl migration. Optimization of the catalytic system for the cyclization of o-alkynylbenzaldehyde acetals revealed an unknown activation effect: PtCl(2) was found to be a better catalyst for the cyclization of acetals in the presence of olefins than without. A similar catalytic system (PtCl(2)/benzoquinone) has been found to be appropriate for the cyclization of cyclic acetals, whereas the optimal catalyst for the reaction of thioacetals is PdI(2). NMR monitoring of two reactions, acetal 3a + Pd(CH(3)CN)Cl(2) in CD(3)CN and thioacetal 5j + PdI(2) in CD(2)Cl(2), revealed that in both reactions similar cationic species are formed at the early stage of the transformation. Computational data (B3LYP/SDD level of theory) suggest that the difference in the reaction pathways for acetals and thioacetals can be rationalized by taking into account the relative stabilities of the corresponding vinylpalladium intermediates (22 vs 20 and 19 vs 21), which suggests a reversible thermodynamically controlled alkyl migration in the intermediate vinylcationic species.

Photochemical generation of a primary vinyl cation from (E)-bromostyrene: mechanisms of formation and reaction

The photochemistry of (E)-bromostyrene was investigated to determine the nature of the product-forming intermediates and to clarify the mechanism of formation of vinylic cations and vinylic radicals. Both a cation- and a radical-derived product are formed, and the ionic origin of the former product is demonstrated by significant scrambling of the label, starting from specifically deuterated (E)-bromostyrene. MO calculations show that the isolated incipient primary vinyl cation is not a metastable species, but that specific interaction with a counterion in combination with a polar environment makes it metastable. The effects of variation of the wavelength of irradiation, solvent polarity, temperature, and isotopic substitution all agree with a mechanism of direct heterolytic C-Br bond cleavage producing an ion pair followed by formation of a radical pair via electron transfer. The vinylic cation is proposed to stem directly from the indirectly populated lowest excited singlet state of bromostyrene with an energy of activation of 6.7 kcal/mol. Branching between proton loss and electron transfer in the resulting ion pair determines the ratio of cation- to radical-derived product. The E/Z-isomerization occurs in a separate process and does not involve C-Br bond cleavage.

Polyvalent iodine in organic chemistry

Tricoordinate, I(III), and pentacoordinate, I(V), polyvalent iodine compounds have been known for over a century. In the last twenty years, new polyvalent iodine reagents have been introduced along with synthetic methodologies, based on these and derived reagents, that play an ever increasing role in contemporary organic chemistry. In this Perspective, an overview of these developments is provided with emphasis on the chemistry and uses of aryl-, alkenyl-, and alkynyliodonium salts in preparative and synthetic organic chemistry. It is hoped that this brief overview, along with recent more comprehensive reviews of the field, will stimulate further developments and applications of this useful class of compounds across a broad spectrum of organic chemistry.

Solvolysis of 4-methylcyclohexylidenemethyliodonium salt: chirality probe approach to a primary vinyl cation intermediate

Optically active 4-methylcyclohexylidenemethyl(aryl)iodonium tetrafluoroborate (1.BF(4)(-)) was prepared and its solvolysis was carried out at 60 degrees C in various solvents. The main product is optically active 4-methylcycloheptanone (or its enol derivative) in unbuffered solvents, accompanied by the iodoarene. The rearranged product always maintains the optical purity of the starting 1. Its stereochemistry conforms to a mechanism involving the rearrangement via the sigma-bond participation in departure of the nucleofuge, followed by trapping of the resulting chiral 5-methylcyclohept-1-enyl cation with a nucleophilic solvent. That is, the achiral, primary vinyl cation is not involved during the reaction. The unrearranged substitution product is also obtained in a minor fraction in unbuffered methanol, ethanol, and acetic acid, but not in trifluoroethanol or hexafluoro-2-propanol: the methoxy product from methanolysis is largely racemized, but the acetolysis product is obtained mainly via retention of configuration. Reactions of 1 with bromide, acetate, and trifluoroacetate in chloroform give unrearranged substitution products in different degrees of inversion. These unrearranged products are concluded to be formed via the direct nucleophilic substitutions. Added bases such as sodium acetate in methanol lead to the unrearranged methoxy products of complete racemization, which is ascribed to the alpha elimination (to give an alkylidenecarbene) followed by the solvent insertion.

Photosolvolysis of (E)-styryl(phenyl)iodonium tetrafluoroborate. Generation and reactivity of a primary vinyl Cation

The photochemistry of (E)-styryl(phenyl)iodonium tetrafluoroborate in methanol and 2,2,2-trifluoroethanol as well as in dichloromethane and toluene has been investigated. In all solvents the vinylic C [bond] I bond is more photoreactive than the aromatic C [bond] I bond. Homolysis as well as heterolysis of both bonds occurs, but the latter type of cleavage predominates. In alcoholic solvents, the incipient phenyl cation produces a nucleophilic substitution product. The primary styryl cation gives nucleophilic substitution, elimination, and rearrangement products. The dependence of the photoreaction on the nucleophilicity of the solvent indicates that in the presence of good nucleophiles a 10-I-3 compound is the reactive iodonium species. In this case the reaction proceeds via an S(N)i mechanism. In the absence of good nucleophiles an 8-I-2 species gives photoreaction via an S(N)1 mechanism. This is corroborated by the solvent dependence of the UV spectra, and the product composition upon photoreaction with bromide in varying concentration. Photoreaction of the iodonium salt in a chlorinated alkane yields (E)- and (Z)-beta-fluorostyrene in a Schiemann-type reaction. Reaction in toluene yields Friedel-Crafts products. The results of the photochemical reactions are compared to those of the thermal ones, and the implications of the differences are discussed.

Solvolysis of vinyl iodonium salts. New insights into vinyl cation intermediates

Solvolysis of some vinyl iodonium salts carrying an excellent leaving group is examined, focusing on whether or not a classical primary vinyl cation can be generated. Formation of the primary cation is avoided, when possible, by participation of the beta substituent in the heterolysis to form a vinylenebenzenium ion or a secondary vinyl cation. Definitive evidence against a primary vinyl cation is provided by a chirality probe approach in the solvolysis of 4-methylcyclohexylidenemethyl iodonium salt.