Recent Progress in Synthetic Applications of Hypervalent Iodine(III) Reagents

Hypervalent iodine(III) compounds have found wide application in modern organic chemistry as environmentally friendly reagents and catalysts. Hypervalent iodine reagents are commonly used in synthetically important halogenations, oxidations, aminations, heterocyclizations, and various oxidative functionalizations of organic substrates. Iodonium salts are important arylating reagents, while iodonium ylides and imides are excellent carbene and nitrene precursors. Various derivatives of benziodoxoles, such as azidobenziodoxoles, trifluoromethylbenziodoxoles, alkynylbenziodoxoles, and alkenylbenziodoxoles have found wide application as group transfer reagents in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Development of hypervalent iodine catalytic systems and discovery of highly enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important recent achievement in the field of hypervalent iodine chemistry. Chemical transformations promoted by hypervalent iodine in many cases are unique and cannot be performed by using any other common, non-iodine-based reagent. This review covers literature published mainly in the last 7-8 years, between 2016 and 2024.

Difluorocarbene Enables Access to 2,2-Difluorohydrobenzofurans and 2-Fluorobenzofurans from ortho-Vinylphenols

2-Fluorobenzofurans are the backbone structures of many drug molecules and have many potential therapeutic bioactivities. Despite the potential applications in medicinal chemistry, practical and efficient synthetic methods for the construction of 2-fluorobenzofuran are very limited. Herein, we report an efficient and general method for the construction of 2-fluorobenzofurans. Contrary to the previous functionalizations of the existing backbone of benzofuran, our strategy directly constructs benzofuran scaffolds alongside the incorporation of fluorine atom on C2 position in a formal [4 + 1] cyclization from readily accessible ortho-vinylphenols and difluorocarbene. In our strategy, ClCF2H decomposes into difluorocarbene in the presence of base, which is further captured by the oxygen anion from the hydroxy group in ortho-hydroxychalcones; subsequent intramolecular Michael addition to the α, β-unsaturated system leads to 2,2-difluorohydrobenzofurans, and further fluorine elimination renders 2-fluorobenzofurans by forming one C-O bond and one C-C double bond. Of note, various complex 2,2-difluorohydrobenzofurans and 2-fluorobenzofurans could be readily accessed through our protocol via the late-stage elaborations.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Revisiting Aromatic Claisen Rearrangement Using Unstable Aryl Sulfonium/Iodonium Species: The Strategy of Breaking Up the Whole into Parts

ConspectusSince Ludwig Claisen's discovery of the sigmatropic rearrangement of allyl aryl ethers in 1912, aromatic Claisen rearrangement has continuously attracted the attention of both experimental and theoretical chemists. Over more than a century of growth, this protocol has proven to be a practical and powerful synthetic tool in many aspects. However, the reaction scope has long been limited to aryl ethers and their S or N analogs until the serendipitous discovery of aromatic iodonium-Claisen rearrangement by Oh et al. in 1988 and the development of aromatic sulfonium-Claisen rearrangement by Kita et al. in 2004. Unlike traditional Claisen rearrangements, these hypervalent-bonding-based Claisen-type rearrangements can be performed by simply mixing electrophilically activated aryl sulfoxides/iodanes with certain nucleophiles to directly deliver rearrangement products. In addition to the simple operation, remarkable features, such as readily available substrates, valuable products and intriguing rearrangement patterns, have led to a dramatic resurgence of this rearrangement chemistry.In this Account, we summarize our recent works on developing new aromatic rearrangement modes using sulfonium/iodonium reagents. Interestingly, the program started with an accidental discovery that aryl sulfoxides could be coupled with alkyl nitriles in the presence of Tf₂O and base. Mechanistic studies reveal that the reaction proceeds in three major steps, including the Tf₂O-triggered assembly of both coupling partners, base-promoted deprotonation of in situ-generated aryl sulfonium-imine species leading to a key rearrangement precursor called aryl sulfonium-ketenimine species, and subsequent facile and rapid [3,3]-rearrangement. On the basis of the mechanistic underpinning, we divided the one-step operation into two steps called the "assembly/deprotonation" protocol for constructing unstable rearrangement precursors. Most notably, the switch from the commonly used one-step to mechanism-based multiple-step manipulation, which can be termed "breaking up the whole into parts", not only enables the independent control of each step of the reaction, thus significantly expanding the accessible synthetic scope, but also raises opportunities for developing new rearrangement patterns. For example, the "assembly/deprotonation" protocol has also been applied to the development of [5,5]-rearrangement of aryl sulfoxides and the asymmetric rearrangement of aryl iodanes, thus enabling the unprecedented regio- and stereocontrol of the rearrangement process. Furthermore, the "breaking up the whole into parts" thinking triggered us to merge the Morita-Baylis-Hillman (MBH) reaction into the rearrangement process to accomplish Z-selective MBH-type [3,3]-rearrangement of α,β-unsaturated nitriles and E-selective MBH-type [3,3]-rearrangement of α,β-unsaturated 2-oxazolines, which expands the scope of rearrangement partners to include α,β-unsaturated carbonyls. In addition, the impressive rapidity of the rearrangement process found in our initial discovery has also been recognized as a congestion-acceleration effect, which was further utilized to forge the rapid ortho-cyanoalkylative rearrangement of aryl iodanes, and thus leading to the first dearomatization of aryl iodanes. We anticipate that our protocols and ideas behind the methods will be complementary to the traditional thinking of the aromatic Claisen rearrangement.

Transition-metal-free nucleophilic 211At-astatination of spirocyclic aryliodonium ylides

The transition-metal-free 211At-astatination of spirocyclic aryliodonium ylides via a nucleophilic aromatic substitution reaction is described. This method enables the preparation of 211At-radiolabeled compounds derived from multi-functionalized molecules and heteroarenes in good to excellent radiochemical yields.

Morita-Baylis-Hillman-Type [3,3]-Rearrangement: Switching from Z- to E-Selective α-Arylation by New Rearrangement Partners

α-aryl α,β-unsaturated carbonyls represent an important class of derivatizable synthetic intermediates, however, the synthesis of such compounds still remains a challenge. Recently, we showcased a novel Z-selective α-arylation of α,β-unsaturated nitriles with aryl sulfoxides via [3,3]-rearrangement involving an Morita-Baylis-Hillman (MBH) process. Herein, we demonstrate the feasibility of reversing the stereoselectivity of such MBH-type [3,3]-rearrangement by switching to a new pair of rearrangement partners consisting of aryl iodanes and α,β-unsaturated oxazolines. As a result, the two protocols complement each other in approaching E- or Z-α-aryl α,β-unsaturated carbonyl derivatives. Mechanistic studies reveal a possible reaction pathway and provide an explanation for the opposite stereoselectivities.

Chemoselective Cleavage of Si-C(sp3) Bonds in Unactivated Tetraalkylsilanes Using Iodine Tris(trifluoroacetate)

Organosilanes are synthetically useful reagents and precursors in organic chemistry. However, the typical inertness of unactivated Si-C(sp³) bonds under conventional reaction conditions has hampered the application of simple tetraalkylsilanes in organic synthesis. Herein we report the chemoselective cleavage of Si-C(sp³) bonds of unactivated tetraalkylsilanes using iodine tris(trifluoroacetate). The reaction proceeds smoothly under mild conditions (-50 °C to room temperature) and tolerates various polar functional groups, thus enabling subsequent Tamao-Fleming oxidation to provide the corresponding alcohols. NMR experiments and density functional theory calculations on the reaction indicate that the transfer of alkyl groups from Si to the I(III) center and the formation of the Si-O bond proceed concertedly to afford an alkyl-λ³-iodane and silyl trifluoroacetate. The developed method enables the use of unactivated tetraalkylsilanes as highly stable synthetic precursors.