Electrophilic Activation of Aliphatic C-H Bonds Mediated by Zirconium Hydride Entities and Applied to the Functionalization of the Porphyrinogen Periphery

A novel mode of electrophilic activation of aliphatic C-H bonds, assisted by zirconium(IV) and achieved by the use of an excess of MH [M = Li, Na, K], allows the functionalization of the periphery of meso-octaethylporphyrinogen. The reaction of [(eta(5)-eta(1)-eta(5)-eta(1)-Et(8)N(4))Zr(THF)] (1) with LiH (excess) and Nan (excess) led to the formation of [{eta(1)-eta(1)-eta(1)-eta(5)-Et(7)(CH2CH2)N-4}ZrH{Li(THF)}(2)] (2) and [{eta(1)-eta(1)-eta(1)-eta(5)-Et(7)(CH2CH2)N-4}ZrH(Na-(THF)(2)}(2)] (3), containing a Zr-C bond, derived from the metalation of one of the meso ethyl groups, and a triply bridged hydride ligand. The analogous potassium derivative, [{eta(1)-eta(1)-eta(1)-eta(5)-Et(7)(CH2CH2)N-4}ZrH{K(THF)(2)}(2) (4), has been obtained only from a metathesis reaction reacting 2 with KH at room temperature. The reaction of 1 with an excess of KH under drastic conditions gave a mixture of dimetalated forms derived from the metalation of two ethyl groups, they are [{eta(1)-eta(1)-eta(1)-eta(1)-Et(6)(CHCH3)(2)N-4}Zr{K(THF)(2)}(2)] (5) (75%) and [{eta(1)-eta(1)-eta(1)-eta(1)-Et(6)(CH2CH2)(2)N-4}Zr-{K(THF)(2)](2)] (6) (25%). The conversion of 4 into a mixture of 5 and 6 has been observed in the presence of an excess of KH under forcing conditions. Such a conversion gives some insight into the metalation mechanism. In particular, the transformation of 4 into 5 and 6 suggests a facile Zr-C and C-H sigma-bond metathesis. The insertion of Bu(1)NC into the Zr-C bond of 2 led to the formation of an eta(2)-iminoacyl, [{eta(1)-eta(1)-eta(1)-eta(5)-Et(7)(CH2CH2-eta(2)-C=NB1)} ZrH{Li(THF)}(2)] (7), which undergoes, in water, a hydrolytic cyclization to [Et(7)(CH2CH2COC4H4N)(C4H2NH)(3)] (9) via the attack of a carbenium eta(2)-iminoacyl on one of the pyrrolyl anions. The intermediacy of such a migrated carbenium eta(2)-iminoacyl has been observed during the controlled protolysis of 7 in aprotic solvents using PhNH(2).HCl, which led to the isolation of [eta(1)-eta(1)-eta(5)-eta(1)-Et(7)(CH2CH2C(NBu(1))C4H4N)(C4H2N)(3)ZrNHPh] (10). The spontaneous migration of a carbenium eta(2)-acyl from the metal to a pyrrolyl anion has been observed in the reaction of 2 with either carbon monoxide or [Mo(CO)(6)]. Both reactions led, via Intermediates very similar to 9 and 10, to the homologation of a pyrrole ring and the cleavage of the C-O bond. The resulting zirconyl compound [eta(1)-eta(1)-eta(5)- eta(1)-Et(7)(C4H2N)(3)(CH2CH2C5H2N)Zr=O-Li](2) (11) has been isolated as a dimer. The reaction of 2 with Bu(1)NC and CO emphasizes how the direct functionalization of an aliphatic chain in porphyrinogen chemistry can be achieved and used for synthetic purposes. Crystallographic details: The compound 2 is triclinic, space group P $($) over bar$$ 1, a 11.393(4) Angstrom, b = 20.135(5) Angstrom, c = 10.791(3) Angstrom, alpha = 103.34(2)degrees, beta = 117.88(2)degrees, gamma = 79.27(2)degrees, Z = 2, and R = 0.048. The mixture of 5 + 6 is monoclinic, space group C2/c, a = 14.090(1) Angstrom, b = 17.366(2) Angstrom, c = 21.365(3) Angstrom, alpha = gamma = 90 degrees, beta = 91.84(1)degrees, Z = 4, and R = 0.045. The compound 7 is monoclinic, space group P2(1)/n, a 12.152(2) Angstrom, b = 20.190(3) Angstrom, c = 20.039(3) Angstrom, alpha = gamma = 90 degrees, beta = 103.39(2)degrees, Z = 4, and R = 0.041. Compound 10 is monoclinic, space group P2(1)/c, a = 10.325(2) Angstrom, b = 19.824(3) Angstrom, c = 21.114(4) Angstrom, alpha = gamma = 90 degrees, beta = 102.72(2)degrees, Z = 4, and R = 0.060. Compound 11 is monoclinic, space group P2(1)/n, a = 14.774(4) Angstrom, b = 17.745(5)Angstrom, c = 15.771(4) Angstrom, alpha = gamma = 90 degrees, beta = 101.55(2)degrees, Z= 2, and R = 0.067.