Insertion reactions of six-membered cyclopalladated N, N′, N″ -Triarylguanidine, [Pd{κ2(C, N)-C6H 3Me-3(NHC(NHAr)(=NAr))-2}(μ-Br)]2 (Ar = 2-MeC 6H4) with PhC≡C - C(O)OR (R = Me and Et): A gateway to second orthopalladation through novel rearrangements

Article abstract of DOI:10.1021/om4010719

The reaction of [Pd{κ²(C,N)-C₆H ₃Me-3(NHC(NHAr)(=NAr))-2}(μ-Br)]₂ (Ar = 2-MeC ₆H₄; 1) with 4 equiv of PhC≡C - C(O)OMe in CH ₂Cl₂ afforded [Pd{κ²(C,N)-C(Ph)=C(C(O) OMe)C(Ph)=C(C(O)OMe)C₆H₃Me-3(N=C(NHAr)₂)-2}Br] (Ar = 2-MeC₆H₄; 2) in 70% yield, and the aforementioned reaction carried out with 10 equiv of PhC≡C - C(O)OR (R = Me, and Et) afforded an admixture of two regioisomers of [Pd{κ³(N,C,O)-O= C(OR)C₅Ph₃(C(O)OR)C(C(O)OR)C₆H ₃Me-3(N=C(NHAr)₂)-2}Br] (Ar = 2-MeC₆H ₄; R = Me (3a/3b), Et (4a/4b)) in 80 and 87% yields, respectively. In one attempt, the minor regioisomer, 4b, was isolated from the mixture in 6% yield by fractional crystallization. Palladacycles 3a/3b and 4a/4b, upon stirring in CH₂Cl₂/MeCN (1/1, v/v) mixture at ambient condition for 5 days, afforded [Pd{(η³-allyl, κ¹N)-C₅(C(O)OR)₂Ph₃C(C(O)OR) C₆H₃Me-3(N=C(NHAr)₂)-2}Br] (Ar = 2-MeC ₆H₄; R = Me (5a/5b), Et (6a/6b)) in 94 and 93% yields, respectively. Palladacycles 3a/3b and 4a/4b, upon reaction with AgOTf in CH ₂Cl₂/Me₂C(O) (1/1, v/v) mixture at ambient temperature for 15 min, afforded [Pd{κ³(N,C,O)-O=C(OR)C ₅Ph₃(C(O)OR)C(C(O)OR)C₆H₃Me-3(N= C(NHAr)₂)-2}(OTf)] (Ar = 2-MeC₆H₄; R = Me (7a/7b), Et (8a/8b)) in 79 and 77% yields, respectively. Palladacycles 7a/7b and 8a/8b, upon reflux in PhCl separately for 6 h, or palladacycles 5a/5b and 6a/6b, upon treatment with AgOTf in CH₂Cl₂/Me ₂C(O) (7/3, v/v) mixture for 15 min, afforded [Pd{(η²- Ph)C₅Ph₂(C(O)OR)₂κ²(C,N)- C(C(O)OR)C₆H₃Me-3(N=C(NHAr)₂)-2}(OTf)] (Ar = 2-MeC₆H₄; R = Me (9a/9b), Et (10a/10b)) in ≥87% yields. Palladacycles 9a/9b, upon stirring in MeCN in the presence of excess NaOAc followed by crystallization of the reaction mixture in the same solvent, afforded [Pd{κ³(N,C,C)-(C₆H₄)C ₅Ph₂(C(O)OMe)₂C(C(O)OMe)C₆H ₃Me-3(N=C(NHAr)₂)-2}(NCMe)] (Ar = 2-MeC₆H ₄; 11a/11b) in 82% yield. The new palladacycles were characterized by analytical, IR, and NMR (¹H and ¹³C) spectroscopic techniques, and the molecular structures of 2, 3a, 4a, 4b, 5a, 6a, 7a, 9a, 10a, and 11a-d₃ were determined by single crystal X-ray diffraction. The frameworks in the aforementioned palladacycles, except that present in 2, are unprecedented. Plausible pathways for the formation of new palladacycles and the influence of the guanidine unit in 1, substituents in alkynes, reaction conditions, and electrophilicity of the bromide and the triflate upon the frameworks of the insertion products have been discussed.