Ambient temperature activation of haloporphyrinic-enediynes: electronic contributions to bergman cycloaromatization

We have synthesized the nickel(II) 2,3-bis(haloethynyl)-5,10,15,20-tetraphenylporphyrins with -Br (2a) or -I (2b) at the alkyne termini position from the corresponding 2,3-diethynyl analogue (1). The cross coupling of nickel(II) 2,3-dibromo-5,10,15,20-tetraphenylporphyrin with trimethyl(trimethylstannanylethynyl)silane in the presence of a Pd0 catalyst and subsequent deprotection with base under aqueous conditions yields the nickel(II) 2,3-diethynyl-5,10,15,20-tetraphenylporphyrin (1). Subsequent reaction of 1 with N-bromo- or N-iodosuccinimide in dry acetone in the presence of AgNO3 yields 2,3-bis(haloethynyl)-5,10,15,20-tetraphenylporphyrins in 70% (2a) and 68% (2b) yields. The X-ray crystal structures of 2a,b show that the porphyrin backbone deviates significantly from planarity due to a Ni(II)-induced mixture of the classic ruffle and saddle distortions. Thermolysis of 2a at 190 degrees C for 6 h in chlorobenzene and 30-fold 1,4-cyclohexadiene (CHD) generates the Bergman cyclized nickel(II) dibromopicenoporphyrin product (3) in 65% yield, which derives from diradical addition across the adjacent meso-phenyl substituents. Similarly, nickel(II) 2,3-bis(iodoethynyl)-5,10,15,20-tetraphenylporphyrin, 2b, cyclizes at 190 degrees C in chlorobenzene/CHD via high-temperature substitution of iodine by hydrogen (from CHD) or chlorine (from solvent) to afford a mixture of 4 (15%) and 5 (45%). Remarkably, ambient temperature incubation of 2a in MeOH/CHCl3 (1:3, 22 h) or chlorobenzene/CHD (3:1, 24 h) leads to generation of 3 in 15% and 22% isolated yields, respectively. Addition of 1.2 equiv of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in CHCl3/MeOH dramatically accelerates the rate of reaction, producing 3 in 30% yield within 0.5 h. The origin of the ambient temperature activation of 2a derives from the ability of electron-withdrawing functionalities at the alkyne termini to decrease the activation barrier to the Bergman product.