Palladium‐Catalyzed β‐Selective Direct Arylation of Porphyrins

Abstract

Porphyrins are an important class of aromatic compounds that exhibit interesting optical and electrochemical properties and have attracted much attention as key components of functional materials and supramolecular architectures.[1] Creation of new porphyrins that feature intriguing properties is hence essential in developing the chemistry and applications of functional p-rich molecules. Peripheral functionalization of porphyrins can effectively modify the electronic and steric nature of the parent porphyrins, and is thus useful when investigating new porphyrins.[2] Among the possible functionalizations, arylation is promising since the introduced aryl groups can induce electronic perturbation through extension of p conjugation. Nucleophilic addition of aryllithium reagents to meso-unsubstituted porphyrins followed by oxidation can be employed for peripheral arylation reactions. [2c,d, 3] The nucleophilic arylation reaction inherently lacks functional-group compatibility because of the high reactivity of aryllithium reagents. Presently, the most reliable and chemoselective functionalization is based on palladiumcatalyzed cross-coupling reactions. [4,5] However, the crosscoupling strategy always requires preparation of halogenated [5,6] or metalated [7,8] porphyrins prior to arylation. More straightforward and efficient methods for peripheral arylation of porphyrins are thus needed.Transition-metal-catalyzed direct CÀH arylation of arenes with aryl halides has been rapidly emerging as a promising alternative to traditional cross-coupling arylations because preparations of aryl metal reagents can be omitted. [9][10][11] We envisaged that the direct arylation would be applicable to porphyrins. After extensive screening of porphyrin substrates and reaction conditions, we found that the pivalate-assisted conditions developed by Fagnou and co-workers, with some modifications, are effective for the synthesis of meso-free Ni II porphyrins. [9f, 10a] Treatment of the Ni II complex of 5,10,15-tris(3,5-di-tert-butylphenyl)porphyrin (1-Ni) with aryl halides in the presence of pivalic acid, [12] potassium carbonate, and catalytic amounts of palladium acetate and DavePhos in N,Ndimethylacetamide (DMA) at 100 8C resulted in diarylation with satisfactory efficiency (Table 1). To our surprise, the reaction proceeded exclusively at the b positions adjacent to the free meso position with excellent selectivity, and no mesoarylated products were detected. Although the sequential iridium-catalyzed borylation/palladium-catalyzed SuzukiMiyaura coupling had been the only efficient and practical method for b-selective arylation, [8] this sequence often suffers from the gradual protodeborylation of borylated porphyrins in protic media during purification and cross-coupling processes. Naturally, the present direct method is much more efficient and does not suffer from such demetalation. Moreover, this method is the first palladium-catalyzed reaction that selectively functionalizes the b positions of porphyrins.

Spectroelectrochemical Investigation of the One‐Electron Reduction of Nonplanar Nickel(II) Porphyrins

“…Therefore, an additional impact of the β‐tolyl groups aside from macrocycle distortion might arise from their electronic effects. In line with the calculations performed for 2 , Kawamata et al . found much lower dihedral angles for the β‐positions (48–54°) than for the meso ‐positions (86–89°) in the crystal structure of the structurally related nickel porphyrin [2,8,12,18‐tetrakis(3,5‐dimethylphenyl)‐5,15‐bis(3,5‐di‐ tert ‐butylphenyl)porphyrinato]nickel(II).…”

“…Compared with 1 , the first reduction potential of 2 is shifted to a more positive potential of −1.72 V versus Fc/Fc + , whereas 3 has the most negative reduction potential of −1.87 V versus Fc/Fc + . Compound 2 is substantially easier to reduce than 1 , which can be explained by the strong electronic effect of the β‐tolyl substituent, as reported in the literature . The electronic effect of these substituents proves that they are at least to some extent conjugated with the π system of the macrocycle.…”

“…The Soret band of 2 is centered at λ =426 nm despite the fact that the ruffling distortion of 2 is smaller than for 3 . Thus, an additional bathochromic shift of 0.05 eV is attributed to an electronic effect of the β‐arylation, which evidently extends the π system of the macrocycle.…”

“…found much lower dihedral angles for the β‐positions (48–54°) than for the meso ‐positions (86–89°) in the crystal structure of the structurally related nickel porphyrin [2,8,12,18‐tetrakis(3,5‐dimethylphenyl)‐5,15‐bis(3,5‐di‐ tert ‐butylphenyl)porphyrinato]nickel(II). Additionally, the bond length between the porphyrin core and peripheral aryl groups is shorter for the β‐positions (1.48 Å) than for the meso ‐positions (1.52 Å), which indicates conjugation …”

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

“…Recently, we reported that direct arylation reactions of nickel porphyrins 2 aNi provide β,β′‐diarylporphyrins 3 Ni . Such arylation reactions are not applicable to zinc or free‐base porphyrins 5a–c. The denickelation reaction of β,β′‐diphenylated and β,β′‐dinaphthylated porphyrins ( 3 aNi , 3 bNi , and 3 cNi ) resulted in yields comparable to conventional acid‐induced denickelations (Table 3).…”