Room Temperature Decarboxylative and Oxidative [2+2+2] Annulation of Benzoic Acids with Alkynes Catalyzed by an Electron‐Deficient Rhodium(III) Complex

Abstract

It has been established that an electron-deficient (η -cyclopentadienyl)rhodium(III) [Cp Rh ] complex is capable of catalyzing the decarboxylative and oxidative [2+2+2] annulation of benzoic acids with alkynes to produce substituted naphthalenes at room temperature. The appropriate choice of the additive and the solvent is crucial for this transformation. This catalyst system allowed use of oxygen as a terminal oxidant and broadened the substrate scope including both aromatic and aliphatic alkynes. In this catalysis, the electron deficient nature of the Cp Rh catalyst would cause the strong rhodium-π interaction, which accelerates the decarboxylation as well as the C-H bond cleavage.

Rhodium‐Catalyzed [2+1+2+1] Cycloaddition of Benzoic Acids with Diynes through Decarboxylation and C≡C Triple Bond Cleavage

“…However, these reactions required high temperatures (xylene, reflux to 160 °C) and stoichiometric amounts of metal oxidants. Very recently, our research group has reported that the use of an electron‐deficient Cp‐rhodium(III) complex (Cp E Rh III ) significantly mildened the reaction conditions (at RT–80 °C under air or O 2 ) . The highly Lewis acidic nature of the Cp E Rh III complex may accelerate the decarboxylation step as well as the C−H bond cleavage step as a result of the strong π‐rhodium interaction in their transition states.…”

“…Very recently,our research group has reportedt hat the use of an electron-deficient Cp-rhodium(III) complex (Cp E Rh III )s ignificantly mildenedt he reactionc onditions (at RT-80 8Cu nder air or O 2 ). [16] The highly Lewis acidic nature of the Cp E Rh III complex may accelerate the decarboxylation step as well as the CÀH bond cleavage step [17] as ar esult of the strong p-rhodium interaction in their transition states. Here, we have established that the Cp E Rh III complex catalyzes the oxidative and decarboxylative [2+ +1+ +2+ +1] cycloaddition of benzoica cids with acyclic diynes (Scheme 1b,r ight).…”

“…As mentioned above, benzoica cid (1a)r eactedw ith two internal monoynes in the presence of the Cp E Rh III complex to give oxidative and decarboxylative[ 2 + +2+ +2] cycloaddition products C in good yields without forming [2+ +1+ +2+ +1] cycloaddition products D (Scheme2a). [16] On the contrary,t he reaction of 1a with 1,6-diyne 2a instead of two monoynes afforded [2+ +1+ +2+ +1] cycloaddition product 3aa as am ajor product along with as mall amount of [2+ +2+ +2] cycloaddition product 4aa (Scheme 2b).…”

“…As with 1,6-diyne 2a,1 ,7-diyne 2j reacted with as eries of substitutedb enzoic acids 1b-f to furnish the corresponding [2+ +1+ +2+ +1] cycloaddition products 3bj-fj. Based on the previous reports concerning the oxidative and decarboxylative[ 2 + +2+ +2] cycloaddition of benzoica cids with two alkynes, [14][15][16] and the [2+ +1+ +2+ +1] cycloaddition of three al-Scheme3.Substrate scope. [Cp E RhCl 2 ] 2 (0.0050 mmol), AgOAc (0.020 mmol),Cu(OAc) 2 ·H 2 O(0.010 mmol), 1 (0.100 mmol), 2 (0.100 mmol), and (CH 2 Cl) 2 (2.0 mL) wereu sed.…”

“…Subsequently,i nsertion of one alkyne unit of diyne 2 affords regioisomeric seven-membered rhodacycle G or G'.D ecarboxylation from G furnishes five-membered rhodacycle H,and intramolecular alkyne insertion and reductive eliminationg enerate [2+ +2+ +2] cycloaddition product 4 and rhodium(I) complex I. [14][15][16] Alternatively,d ecarboxylationf rom G' affords H',i n which intramolecular alkyne insertion cannotp roceed. Thus, reductivee limination furnishes cyclobutadiene-rhodium(I) complex J,and the subsequento xidativea ddition of tautomeric cyclobutadiene J' (dearomatized form) to rhodium(I)g enerates rhodacycle K.S ubsequent alkyne insertion and reductive elimination generate [2+ +1+ +2+ +1] cycloaddition product 3 and rhodium(I) complex I.F inally,o xidation of I by the copper(II) co-catalyst regenerates rhodium(III) complex E.T his mechanism is consistent with the observed completer egioselectivity (absenceo fr egioisomers 3' in Scheme3)i nt he reactions of substituted benzoica cids.…”