Insertion Reactions of Six-Membered Cyclopalladated N,N′,N″-Triarylguanidine, [Pd{κ2(C,N)‑C6H3Me-3(NHC(NHAr)(NAr))-2}(μ-Br)]2 (Ar = 2‑MeC6H4) with PhCCC(O)OR (R = Me and Et): A Gateway to Second Orthopalladation through Novel Rearrangements

The reaction of [Pd­{κ2(C,N)-C6H3Me-3­(NHC­(NHAr)­(NAr))-2}­(μ-Br)]2 (Ar = 2-MeC6H4; 1) with 4 equiv of PhCCC­(O)­OMe in CH2Cl2 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)­C6H3Me-3­(NC­(NHAr)2)-2}­Br] (Ar = 2-MeC6H4; 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-MeC6H4; R = Me (5a/5b), Et (6a/6b)) in 94 and 93% yields, respectively. Palladacycles 3a/3b and 4a/4b, upon reaction with AgOTf in CH2Cl2/Me2C­(O) (1/1, v/v) mixture at ambient temperature for 15 min, afforded [Pd­{κ3(N,C,O)-OC­(OR)­C5Ph3(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/Me2C­(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)­C5Ph2(C­(O)­OMe)2C­(C­(O)­OMe)­C6H3Me-3­(NC­(NHAr)2)-2}­(NCMe)] (Ar = 2-MeC6H4; 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.