Aromatic Substitution of Olefins. XXI. Reaction of o-Alkylstyrenes with Benzene in the Presence of Palladium(II) Acetate

Abstract

In aromatic substitution of olefins, steric factor of o-alkyl substituents of styrene was investigated. The formation of arylated products was found to be decreased by introduction of alkyl groups to ortho position of styrene.

Intermolecular Dehydrogenative Heck Reactions

“…Given the relative reactivity of benzene and monosubstituted benzenes toward styrene in the presence of Pd(OAc) 2 , Fujiwara et al suggested on November 13, 1975, that “the reaction involves an electrophilic attack of Pd(II) on the aromatic ring to form an aromatic palladium σ complex” . This agreed with the Heck-type mechanism proposed by Yamamura, one year before (manuscripts submitted on October 4, and November 13, 1974) , to schematize the DHR of olefins with benzene, this proposal being retained in a subsequent report published in 1978 …”

“…Under these conditions, β-acetoxystyrene was obtained as a side-product (eq ). With alkylbenzenes, the DHR efficiency decreased with the steric hindrance as shown in eqs −4; − this was also exemplified from competitive reactions of benzene and monosubstituted benzenes toward styrene (eq ). , These oxidative couplings have also been mediated with PdCl 2 and an excess of NaOAc or KOAc, but in lower yields; metathesis leading to coordination of the acetate anion to palladium was thus suspected. ,, Use of palladium powder with 2 equiv of silver acetate afforded also stilbene from the reaction of styrene with benzene, but in no more than 21% yield …”

“…The efficiency of the oxidative coupling and the regioselectivity depend not only on the arylating agent (eqs −), but also on the substitution of the styrene-type compound as exemplified in eqs and . ,− It is worth mentioning that the same compounds, that is, triphenylethylene in one case (eq ; 28% and 72% yields) and 1,3,5-trimethyl-2-styrylbenzene in another case (eqs 4 and ; 9% and 55% yields), are synthesized in yields depending highly on the starting units. …”