Reactivity Studies of Rhodium(III) Porphyrins with Methanol in Alkaline Media

Abstract

Rh(ttp)Cl (1a) (ttp = 5,10,15,20-tetrakistolylporphyrinato dianion) was found to react with methanol at a high temperature of 150 °C in the presence of inorganic bases to give a high yield of Rh(ttp)CH 3 (2a), up to 87%. Rh(ttp)H ( 1d) is suggested to be the key intermediate for the carbonoxygen bond cleavage.

Alkylation of Rhodium Porphyrin Complexes with Primary Alcohols under Basic Conditions

“…It has been reported that Rh III (ttp)Cl readily reacts with K 2 CO 3 to give Rh III (ttp)­CO 3 K via ligand substitution in MeOH, and we proposed that it is the same case in benzonitrile solvent, given its polar nature. Rh III (ttp)­CO 3 K might then react with PhCH 2 OH to give Rh III (ttp)­OCH 2 Ph.…”

“…Initially, we employed the previously reported conditions and examined the C–O cleavage of benzyl alcohol ( 2a ) with Rh III (ttp)Cl ( 1 ). To our delight, Rh III (ttp)Cl ( 1 ) reacted with PhCH 2 OH ( 2a ) in the presence of 10 equiv of K 2 CO 3 under solventless conditions at 150 °C to afford the C–O cleavage product Rh III (ttp)­CH 2 Ph ( 3a ) in 50% yield in 1 day (eq ).…”

“…Under the optimized reaction conditions, methanol ( 2j ) without β-hydrogens reacted with Rh III (ttp)­Cl ( 1 ) to afford the desired product Rh III (ttp)­Me ( 3j ) in 69% yield after 24 h (Table , entry 1). This yield is similar to that with the previously reported solventless conditions . To our delight, the C–O cleavage of ethanol ( 2k ) took place to give a 37% yield of Rh III (ttp)­Et ( 3k ) in 24 h (Table , entry 2).…”

“…On the basis of the previous studies on C–O cleavage of MeOH and current experimental findings, Scheme illustrates the proposed mechanism of C–O bond cleavage in primary alcohols using benzyl alcohol as an example. Initially, K 2 CO 3 deprotonates benzyl alcohol to generate a small amount of PhCH 2 O – anion, which reacts with Rh III (ttp)Cl via ligand substitution to give the Rh III (ttp)­OCH 2 Ph intermediate.…”

“…Alternatively, the C–O bond is relatively strong with BDE values of 92.0 kcal/mol in Me–OH and 79.9 kcal/mol in PhCH 2 –OH . Our group has reported the C–O bond cleavage of MeOH with Rh III (ttp)Cl under basic conditions to give Rh III (ttp)­Me . Rh III (ttp)H is proposed as the active intermediate and cleaves the C–O bond in MeOH via σ-bond metathesis to yield Rh III (ttp)­Me and H 2 O.…”