Facile Aerobic Alkylation of Rhodium Porphyrins with Alkyl Halides

Abstract

Alkylation of rhodium porphyrins was achieved in moderate to high yields in the presence of air and water. With this facile alkylation method, various alkyl Rh III (por) species, including those with tertiary alkyl, were synthesized. Mechanistic investigations suggest a parallel S N 2 via [Rh I (ttp)] − with halogen atom transfer pathway via [Rh II (ttp)] • . ■ INTRODUCTIONRhodium porphyrin alkyls ( Figure 1) play an important role in catalysis. They are precursors of [Rh II (por)] • (por = porphyrinato dianion) metalloradicals for carbon−hydrogen bond 1 and carbon−carbon single bond activation. 1d,2 Activation of these strong bonds has gained much current interest for more efficient utilization of hydrocarbons. 3 To the best of our knowledge, only primary and secondary alkyl Rh III (por) species have been reported so far. Tertiary alkyl Rh III (por) species remain unknown, which may be of great interest in investigating its role in catalysis. Hence, developing robust methods to access alkyl Rh III (por) species, especially tertiary alkyl Rh III (por) species, is important.The synthesis of rhodium porphyrin alkyls was first reported by Ogoshi in 1972 by the reaction between Rh III (oep)Cl (oep =2,3,7,8,12,13,17,18-octaethylporphyrinato dianion) and MeLi (Scheme 1a). 4 A more versatile method involves the reduction of Rh III (por)Cl by NaBH 4 to generate [Rh I (por)] − . [Rh I (por)] − then undergoes nucleophilic substitution with alkyl halides to give the corresponding rhodium porphyrin alkyls (Scheme 1b). 5 In 1986, Kadish and co-workers reported the Rh−C bond formation by the electrochemically generated monomeric species [Rh II (tpp)] • (tpp = 5,10,15,20-tetraphenylporphyrinato dianion) (Scheme 1c). 6 In 1990, Wayland discovered the formation of Rh−C bond by carbon−hydrogen bond activation of CH 4 and toluene to give Rh(por)Me and Rh(por)Bn (Scheme 1d). 1a,b The carbon−oxygen bond cleavage of methanol by Rh III (ttp)Cl (ttp =5,10,15,20-tetratolylporphyrinato dianion) in alkaline media was discovered to give a high yield of Rh III (ttp)Me and was reported by Chan in 2009 (Scheme 1e). 7 In 2010, Chan and co-workers found the formation of the Rh−C bonds by carbon−nitrogen bond activation (CNA) of amines (Scheme 1f). 8 This CNA method, further developed by the Dong group, tolerates air and water by utilizing ammonium salts as the alkylating agents (Scheme 1g). An S N 2-like reaction pathway involving a [Rh I (por)] − intermediate was proposed. 9 These reported methods are unlikely to prepare a tertiary alkyl Rh III (por) species due to the steric hindrance. Herein, we disclose a facile Figure 1. Structure of rhodium porphyrin alkyls. Scheme 1. Synthesis of Rhodium Porphyrin AlkylsArticle pubs.acs.org/Organometallics

User-friendly aerobic reductive alkylation of iridium(iii) porphyrin chloride with potassium hydroxide: scope and mechanism

“…Based on our previously reported mechanism on the base-promoted cleavage of aryl halides with Ir(ttp)(CO)Cl (1) 16,17 and recently reported base-promoted alkylation of rhodium porphyrins, 25 1). [Ir I (ttp)] − is likely more air compatible since it is a weak base.…”

“…It is in contrast with the occurrence of some extent of halogen atom transfer for the rhodium porphyrin analogues with a weaker Rh-Rh bond (∼16 kcal mol −1 ). 25,29,30 For 3°alkyl halides, it undergoes base-promoted elimination by hydrodehalogenation to give an alkene like that of t BuCl. For 1-bromoadamantane, this elimination gives a bridge-headed olefin which is highly unfavorable.…”

Acylation of Rhodium(III) Porphyrin Complexes with Carboxylic Acids: Scope and Mechanism

“…Previous studies by our group and others have shown that rhodium­(III) porphyrin complexes, Rh III (por)­X (por = porphyrinato dianions; X = H, alkyl, acyl, halides, Figure ), can mediate various stoichiometric bond activations of different organic functional groups. Reported examples include (i) carbon–carbon bond activation (CCA) with hydrocarbons, − ketones, − ethers, , amides, esters, nitriles, , and nitroxides; , (ii) carbon–hydrogen bond activation (CHA) with alkanes, − toluenes, ,, alcohols, aldehydes, , and ketones; (iii) carbon-halogen bond activation (CXA) with alkyl halides and aryl halides; (iv) carbon–oxygen bond activation (COA) with alcohols , and ethers; (v) carbon–nitrogen bond activation (CNA) with amines; and (vi) silicon–hydrogen bond activation with silanes . However, to the best of our knowledge, the reactivity of Rh III (por)­X toward carboxylic acids has not been reported.…”

Alkylation of Rhodium Porphyrin Complexes with Primary Alcohols under Basic Conditions

“…Our group has communicated the alkylation of Rh III (ttp)Cl with alkyl halides under basic conditions. 7 Both the Rh II (ttp) metalloradical and Rh I (ttp) − anion generated in situ from Rh III (ttp)Cl are responsible for cleaving the alkyl R−X bond (X = Cl, Br, I) by halogen atom abstraction and nucleophilic substitution, respectively (Scheme 1c). This alkylation protocol gives access to Rh III (ttp)R with 1°, 2°, and even 3°alkyl ligands in air.…”

“…This air-tolerant transformation also takes advantage of the nucleophilicity of the Rh I (tpp) − anion generated in situ under basic conditions. Our group has communicated the alkylation of Rh III (ttp)Cl with alkyl halides under basic conditions . Both the Rh II (ttp) metalloradical and Rh I (ttp) − anion generated in situ from Rh III (ttp)Cl are responsible for cleaving the alkyl R–X bond (X = Cl, Br, I) by halogen atom abstraction and nucleophilic substitution, respectively (Scheme c).…”