Direct cyanation of heteroaromatic compounds mediated by hypervalent iodine(III) reagents: In situ generation of PhI(III)-CN species and their cyano transfer.

Hypervalent iodine(III) reagents mediate the direct cyanating reaction of a wide range of electron-rich heteroaromatic compounds such as pyrroles 1, thiophenes 3, and indoles 5 under mild conditions (ambient temperature), without the need for any prefunctionalization. Commercially available trimethylsilylcyanide is usable as a stable and effective cyanide source, and the reaction proceeds in a homogeneous system. The N-substituent of pyrroles is crucial to avoid the undesired oxidative bipyrrole coupling process, and thus a cyano group was introduced selectively at the 2-position of N-tosylpyrroles 1 in good yields using the combination of phenyliodine bis(trifluoroacetate) (PIFA), TMSCN, and BF3.Et2O at room temperature. In the reaction mechanism, cation radical intermediates of heteroaromatic compounds are involved as a result of single electron oxidation, and the key to successful transformations seems to depend on the oxidation potential of the substrates used. Thus, the reaction was also successfully extended to other heteroaromatic compounds having oxidation potentials similar to that of N-tosylpyrroles such as thiophenes 3 and indoles 5. However, regioisomeric mixtures of the products derived from the reaction at the 2- and 3-positions were obtained in the case of N-tosylindole 5a. Further investigation performed in our laboratory provided insights into the real active iodine(III) species during the reaction; the reaction is induced by an active hypervalent iodine(III) species having a cyano ligand in situ generated by ligand exchange reaction at the iodine(III) center between trifluoroacetoxy group in PIFA and TMSCN, and effective cyanide introduction into heteroaromatic compounds is achieved by means of the high cyano transfer ability of the hypervalent iodine(III)-cyano intermediates. In fact, the reaction of N-tosylpyrrole 1a with a hypervalent iodine(III)-cyano compound (e.g., (dicyano)iodobenzene 8), in the absence of TMSCN, took place to afford the 2-cyanated product 2a in good yield, and an effective preparation of the intermediates is of importance for successful transformation. 1,3,5,7-Tetrakis[4-{bis(trifluoroacetoxy)-iodo}phenyl]adamantane 12, a recyclable hypervalent iodine(III) reagent, was also comparable in the cyanating reactions as a valuable alternative to PIFA, affording a high yield of the heteroaromatic cyanide by facilitating isolation of the cyanated products with a simple workup. Accordingly, after preparing the active hypervalent iodine(III)-CN species by premixing of a recyclable reagent 12, TMSCN, and BF3.Et2O for 30 min in dichloromethane, reaction of a variety of pyrroles 1 and thiophenes 3 provided the desired cyanated products 2 and 4 in high yields. The iodine compound 13, recovered by filtration after replacement of the reaction solvent to MeOH, could be reused without any loss of activity (the oxidant 12 can be obtained nearly quantitatively by reoxidation of 13 using m-CPBA).

The synthesis of head-to-tail (H-T) dimers of 3-substituted thiophenes by the hypervalent iodine(III)-induced oxidative biaryl coupling reaction.

The head-to-tail (H-T) dimers could be obtained selectively by the oxidative coupling reaction of 3-substituted thiophenes using a combination of hypervalent iodine(III) reagents and trimethylsilyl trifluoromethanesulfonate.

Novel and direct oxidative cyanation reactions of heteroaromatic compounds mediated by a hypervalent iodine(III) reagent.

The hypervalent iodine(III) reagent phenyliodine bis(trifluoroacetate) (PIFA) mediates the selective cyanation reactions of a wide range of electron-rich heteroaromatic compounds such as pyrroles, thiophenes, and indoles under mild conditions. These reactions proceed via a cation radical intermediate, and the key for the successful transformation presumably depends on the oxidation-reduction potential of the substrates used. The synthetic utility has been demonstrated through the conversion of these biologically important pyrroles 2f and 2g. [reaction: see text]

A novel and direct synthesis of alkylated 2,2'-bithiophene derivatives using a combination of hypervalent iodine(III) reagent and BF3.Et2O.

A novel nonmetallic oxidative coupling of alkylthiophene derivatives leading to the corresponding 2,2'-bithiophene derivatives using a combination of a hypervalent iodine(III) reagent, phenyliodine bis(trifluoroacetate) (PIFA), and BF3.Et2O was developed.

The first total synthesis of discorhabdin A.

The first stereoselective total synthesis of a potent antitumor alkaloid, discorhabdin A (1), which is a unique sulfur-containing pyrroloiminoquinone alkaloid, is described. The key step in the stereocontrolled total synthesis of 1 involves both a diastereoselective oxidative spirocyclization using a hypervalent iodine(III) reagent and an efficient construction of the labile and highly strained N,S-acetal skeleton. These methodologies provide a breakthrough in the total syntheses of these promising new antitumor agents, discorhabdins and their analogues, which should serve as valuable probes for structure-activity studies.