Acid-Promoted sp³ C–H Bond Cleavage in a Series of (2-Allylphenoxo)ruthenium(II) Complexes. Mechanistic Insight into the Aryloxo–Acid Interaction and Bond Cleavage Reaction

Abstract

A series of RuCp[OC 6 H 3 (CH 2 CHCH 2 -2)(R)](PPh 3 ) n complexes (n = 2, R = H (1a); n = 1, R = 4-OMe (2b), 4-Me (2c), 4-Ph (2d), 4-Br (2e), 4-NO 2 (2f), 6-OMe (2g), 6-Me (2h), 6-Ph (2i)) have been prepared in 27−76% yields. These 2-allylaryloxo complexes 1a and 2b−f are in equilibrium between RuCp[OC 6 H 3 (CH 2 CHCH 2 -2)(R)-κ 1 O](PPh 3 ) 2 (1) and) is about the same for 1a and 2b−f (K 1 = 0.07−0.31 M). In contrast to the conventional aryloxo complexes of the late transition metals, treatment of 1a and 2a−g with weak Brϕnsted acids (HOR) gives a rapid equilibrium with 2·HOR. The association constant K 2 (K 2 = [2·HOR]/([2][HOR])) increases on decreasing the pK a value of the acid employed and on increasing the induction effect of substituents at the 4-position in the aryloxo group. These features suggest present association being regarded as a simple acid−base interaction. Interestingly, further association of 2·HOR with the second acid leads to the cleavage of the benzylic C−H bond, giving RuCp[C 3 H 4 {1-C 6 H 3 (OH-2)(R)}-η 3 C,C′,C″](PPh 3 ) (3). The thermodynamic and kinetic studies suggest formation of hydrogen bonds among two Brϕnsted acid molecules, lone-pair electrons in the aryloxo oxygen, and a benzylic methylene proton. Such association makes the Ru(II) center more electrophilic to attack the benzylic carbon to give 3.

Isomerization of Allylbenzenes

“…Hirano, Komiya, and co-workers recently investigated an interesting ruthenium system, in which they were able to demonstrate acid-promoted sp 3 C–H bond cleavages in a (2-allylphenoxo)­ruthenium­(II) complex . The complex 507 , readily synthesized from RuCpCl­(PPh 3 ) 2 and potassium 2-allylphenoxide, was studied by NMR spectroscopy with a series of Brønsted acids.…”

“…Hirano, Komiya, and co-workers recently investigated an interesting ruthenium system, in which they were able to demonstrate acid-promoted sp 3 C−H bond cleavages in a (2allylphenoxo)ruthenium(II) complex. 647 The complex 507, readily synthesized from RuCpCl(PPh 3 ) 2 and potassium 2allylphenoxide, was studied by NMR spectroscopy with a series of Brønsted acids. Of interest was that use of stronger acids, viz., CF 3 CO 2 H and HPF 6 , resulted in the formation of compound 508, as well as 2-allylphenol 158e and its isomerized isomer 159e, with the latter being observed in yields generally higher than 70%.…”