Revealing the Activity of π‐Acid Catalysts using a 7‐Alkynyl Cycloheptatriene

Abstract

A compound that isomerizes into distinct products depending on the particular Lewis acid or Brønsted acid catalyst used is disclosed. One product can only be obtained with the softest π-acids, such as Au, Pt, Ga, or In complexes. Another is formed only with harder π-acids incorporating Ag or Cu salts. The formation of the third category requires even harder π-acids or protons. This simple benchmark reaction allows for prediction of the category of transformations that can be catalyzed by a new complex. It also informs whether protons have been unintentionally generated in the reaction mixture.

Exploring the Versatility of 7‐Alkynylcycloheptatriene Scaffolds Under π‐Acid Catalysis

“…This transformation gives an easy access to functionalized alkenes or alkanes . As previously described by Kitagawa and more recently by us,[11a] 7‐alkynylcycloheptatrienes can rearrange into phenylallenes. Therefore, in the presence of a suitable catalyst, the pendant aryl moiety could then promote the hydroarylation of the allene moiety.…”

“…Under hard Lewis acidic conditions, the allene can be trapped by a pendant aryl group to give a hydroarylation product (Equ. 4) …”

“…In order to predict the general π‐Lewis acidity pattern of a catalyst, we designed substrate 1a , containing a 7‐alkynylcycloheptatriene moiety tethered to a phenyl group (Scheme ). [11a] This compound is readily synthesized in two steps from dimethyl propargyl malonate after benzylation and functionalization by tropylium tetrafluoroborate. Treated with soft π‐Lewis acids [Au(I), Pt(II), Hg(II), Ga(III), In(III), In(I), …], it leads to the regioisomeric indenes 2a and 3a , leaving the Ph group untouched.…”

Exploring the Limits of π‐Acid Catalysis Using Strongly Electrophilic Main Group Metal Complexes: The Case of Zinc and Aluminium

“…In DCE, as olvent that can give rise to superacids, such as HSbF 6 ,i nt he presence of AgSbF 6 or SbF 5 , [8,9 ] or by using HNTf 2 ,t he conversion of 4 into the tricyclic compound 5 is observed with time, whichc orresponds to the hydroarylation of the styrene moiety through ab enzylic carbocation. [5] One interest of this test is to identify new soft p-acids, as we did in the gallium(I) and indium(I) series. [5,10] Another interesti s to show the efficiency of an anion exchange.…”

“…[2a, 3c] The classification of Lewis acids between the softest ones,whichwill be good for mediating enynecycloisomerizations, and harder ones,w hichw ill be more potent for CÀC p bond functionalization through classical carbocations, can be easily made by using the cycloheptatriene test that we developed. [5] As described by Echavarren et al,7 -alkynylcycloheptatrienes are treated like enynesb yg old(I)c omplexes (Scheme 1, reaction2). [6] Likewise, we reported that the reaction of compound 1 with catalysts of the gold, platinum, mercury,g allium and indium seriesl eads to its skeletal reorganization into the regioisomeric indenes 2 and 3 in 1,2-dichloroethane (DCE) or toluene (Scheme 2).…”