Yutaro Yamamoto

Demetalation of Metal Porphyrins via Magnesium Porphyrins by Reaction with Grignard Reagents

Many naturally occurring porphyrins play pivotal roles in biological processes such as oxygen transport, oxygen storage, and photosynthesis. In attempts to artificially mimic these biological processes, a wide variety of porphyrin derivatives have been synthesized and studied. Amongst metalated porphyrins, nickel porphyrins are the most widely employed building blocks in the construction of porphyrinbased architectures as diamagnetic nickel porphyrins can be readily characterized by NMR spectroscopy. Additionally, nickel porphyrins are chemically robust towards numerous harsh reaction conditions. However, photoexcited states of nickel porphyrins decay to the ground state within a few hundred picoseconds. Therefore, artificial constructs that incorporate nickel porphyrins are inconvenient for the study of photophysical properties. Conventionally, denickelation of the compound of interest is necessary before assessing its photophysical properties. Denickelation often requires strongly acidic conditions. Typically, concentrated H2SO4 in trifluoroacetic acid is employed, which can often lead to undesired decomposition of substrates. Much milder protocols for achieving denickelation are highly sought after. During our studies on the functionalization of nickel porphyrins, 5] we have serendipitously discovered that nickel porphyrins are smoothly converted to the corresponding magnesium porphyrins upon treatment with 4-methylphenylmagnesium bromide (Scheme 1, bottom). An interesting observation worth noting is that no reaction occurred when phenylmagnesium bromide was employed (Scheme 1, middle). In contrast, meso-arylation proceeds with more nucleophilic aryllithium reagents (Scheme 1, top). The denickelation/magnesiation reaction was applicable to a wide variety of nickel porphyrins and the resulting acid-sensitive magnesium porphyrins were readily transformed to the corresponding free-base porphyrins via hydrolysis upon treatment with aqueous 1m HCl. Overall, nickel porphyrins are efficiently converted to free-base porphyrins in a twostep methodology. Treatment of {5,10,15,20-tetrakis ACHTUNGTRENNUNG[3,5-di(tert-butylphenyl)]porphyrinato}nickel (1 Ni) with 4-methylphenylmagnesium bromide in toluene at 25 8C for 4 h provided the corresponding magnesium porphyrin 1 Mg in 96 % yield (Table 1, entry 1). Further purification by recrystallization from CH2Cl2/methanol afforded 1 Mg in 76 % yield. Concurrently, 4,4’-dimethylbiphenyl was also isolated, which indicates that the displaced Ni was reduced to Ni by the Grignard reagent. Note that addition of one equivalent of PPh3 completely suppressed the reaction. In an attempt to assess the scope of the magnesiation reaction, reaction conditions were screened. The magnesiation reaction also proceeded smoothly in THF, albeit in slightly lower yield (Table 1, entry 2). Porphyrins 1 Ni and 1 Mg did not survive the addition of the Grignard reagent in dioxane. We assume the decomposition of 1 Ni and 1 Mg in dioxane was facilitated by the formation of a highly nucleophilic diACHTUNGTRENNUNGarylmagnesium species generated by the Schlenk equilibrium (entry 3). No magnesium porphyrin 1 Mg was observed in the reactions performed in either ether or dichloromethane (entries 4 and 5). Interestingly, the reaction was specific to 4-methylphenylmagnesium bromide. Only trace amounts of 1 Mg were obtained in the reactions with 3and 2-methylphenylmagnesi[a] Dr. K. Murakami, Y. Yamamoto, Prof. Dr. H. Yorimitsu, Prof. Dr. A. Osuka Department of Chemistry, Graduate School of Science Kyoto University Kitashirakawa, Sakyo-ku Kyoto 606-8502 (Japan) Fax: (+ 81) 75-753-3970 E-mail : yori@kuchem.kyoto-u.ac.jp osuka@kuchem.kyoto-u.ac.jp [b] Prof. Dr. H. Yorimitsu ACT-C (Japan) Science and Technology Agency (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201301146. Scheme 1. Comparison of reactions of nickel porphyrin with arylmetal reagents.

Activator-free palladium-catalyzed silylation of aryl chlorides with silylsilatranes.

The palladium-catalyzed silylation of aryl chlorides with silylsilatranes proceeds under activator-free conditions; hence, wide functional group compatibility is displayed and boryl and siloxy groups are able to survive. Experimental and computational studies revealed that smooth transmetalation from the silylsilatrane to the arylpalladium chloride is facilitated by strong interaction between the Lewis acidic silicon and the chloride.

Base-Free Palladium-Catalyzed Borylation of Aryl Chlorides with Diborons

The base‐free palladium‐catalyzed borylation of aryl chlorides with diborons was achieved. The base‐free conditions offered acceptable functional group compatibility. Based on experimental and computational studies, it was shown that smooth boryl transfer from the diborons to the arylpalladium chloride was promoted by strong interaction between the Lewis acidic boron and the chlorine atom on palladium.

Palladium-Catalyzed Tetraarylation of 5,15-Dialkylporphyrins with Aryl Bromides

Nickel complexes of 5,15-dialkylporphyrins are subjected to palladium-catalyzed direct arylation under the modified Fagnou conditions. The arylation takes place still exclusively at the four less hindered ! positions although the meso-nonyl, hexyl, and propyl groups are considered to impose less steric hindrance than the meso-3,5-di-tert-butylphenyl group in the previous report. INTRODUCTION Due to the important roles that porphyrins adopt in a variety of pivotal biological processes, chemists have devoted much time to the design and synthesis of new artificial porphyrins that can be utilized in advanced functional materials. Peripheral functionalizations of a porphyrin core is an effective strategy that allows for the systematic construction of porphyrin-based architechtures. Palladium-catalyzed cross-coupling reactions can be used to successfully introduce a direct carbon–carbon bond at the periphery of a porphyrin. However, these reactions generally take place in moderate yield. Unlike benzene-based building blocks, which are cheap and readily available, prefunctionalized porphyrins such as bromoporphyrins and borylporphyrins are far more precious. It is therefore essential that extensive efforts be made to develop much more efficient, scalable, and reliable synthetic methodologies for achieving these highly desirable molecules. A recent dramatic growth in reports of transition-metal-catalyzed direct C–H arylations has changed the landscape of biaryl synthesis. Direct arylation does not require either an aryl metal reagent or an aryl halide and therefore represents the ideal arylation. We have been interested in the modification of functional aromatics by direct arylation and we have recently developed conditions for palladiumThis paper is dedicated to Professor Victor Snieckus on the occasion of his 77th birthday. HETEROCYCLES, Vol. 88, No. 1, 2014 223