Hypervalent iodine-guided electrophilic substitution (HIGES) was described previously for the para-selective benzylation of aryl-λ 3 -iodane diacetates. One drawback of the method was the synthesis and isolation of hypervalent iodine starting materials. An improvement is reported herein in which the benzylation product can be afforded from an aryl iodide via an in situ oxidation. The metal-like properties of hypervalent iodine have been demonstrated in the transmetallation of metal-The recent and rapid development of hypervalent iodineguided electrophilic substitutions (Scheme 1) began from a series of papers by Khatri and Zhu in which they rediscovered and evaluated of the reductive iodonio-Claisen rearrangement (RICR). [1][2][3] RICR-type reactions have undergone review by Shafir and also in a section of a review on C-C bond forming reactions by Hyatt et al. in 2019. [4-6] While allylic and benzylic groups often have similarities, substituting the allyl metalloid with a benzyl metalloid in the RICR would seem to theoretical fail due to their different π-systems and the theorized Claisen-type rearrangement. [7][8][9][10][11][12] However, when the allyl metalloid was replaced by a benzyl metalloid under the RICR reaction conditions, different regioselectivity was discovered; a para-selective substitution of the aryl iodine as opposed the RICR's ortho selectivity. [13,14] The C-C bond forming methodology described herein hints at, and provides increasing evidence for, a new class of reactions involving HIGES. Elegant synthetic pathways incorporating HIGES have resulted in tetraasubstituted benzenes via four C-C bond forming reactions from iodobenzene, [13] the total synthesis of clopidogrel, [3] the total synthesis of broussin, [2] the synthesis of various heterocycles, [1] and the selective bromination of arenes. [15] DFT-calculations investigating the mechanistic aspects of RICR propargylation found an acid-activated hypervalent iodine intermediates was required. [16,17] Calculations have also shown [a]
A method of increasing the reactivity of phenyliodine(III) diacetate (PIDA) involves the addition of a Lewis acid. The acid-activation of PIDA was investigated toward the goal of elucidating relevant intermediates that participate in other mechanisms such as the transmetallation of hypervalent iodine compounds with metalloids and diaryliodane synthesis. Our computational analysis focused on the reaction coordinate of PIDA reacting with TMSOTf. Acetate-triflate substitutions were found to be thermodynamically preferred over PIDA, and the possibility of a four-coordinate iodine intermediate is evaluated. Other structural and electronic considerations involving trans-influence, σ-hole, and the 3-center-4-electron (3c-4e) bond are presented.
Hypervalent iodine (HVI) reagents are employed in organic synthesis as versatile, proficient, and environmentally friendly reagents. Despite the utility of such reagents, the application of HVI reagents, especially phenyliodonium diacetate (PIDA), has been limited due to its poor solubility in a myriad of solvents. The aggregated and polymeric structures of many HVI reagents account for their poor solubility, thus limiting the reactivity and use of HVI reagents in reactions in non-polar solvents. The research presented herein outlines ligand exchange reactions of universal carboxylic acids promoted by phenyliodonium diacetate (PIDA) reagents, in which the acetate moiety of PIDA is modified, ultimately enhancing the solubility and reactivity of HVI reagents.
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