Isolation, X-ray Structures, and Electronic Spectra of Reactive Intermediates in Friedel−Crafts Acylations

Abstract

[reaction: see text] Reactive intermediates in the Friedel-Crafts acylation of aromatic donors are scrutinized upon their successful isolation and X-ray crystallography at very low temperatures. Detailed analyses of the X-ray parameters for the [1:1] complexes of different aliphatic and aromatic-acid chlorides with the Lewis acids antimony pentafluoride and pentachloride, gallium trichloride, titanium and zirconium tetrachlorides provide unexpected insight into the activation mechanism for the formation of the critical acylium carbocations. Likewise, the X-ray-structure examinations of aliphatic and aromatic acylium electrophiles also isolated as crystalline salts point to the origins of their electrophilic reactivity. Although the Wheland intermediates (as acylium adducts to arene donors) could not be isolated in crystalline form owing to their exceedingly short lifetimes, transient (UV-vis) spectra of benzenium adducts of acylium carbocations with hexamethylbenzene can be measured and directly related to Wheland intermediates with other cationic electrophiles that have been structurally established via X-ray studies.

Catalyst Behavior in Metal-Catalyzed Carbonyl-Olefin Metathesis

“…Our observation of 21 is consistent with the solid state IR reported by the Susz lab . Further, Kochi and co-workers reported a crystal structure of an analogous GaCl 3 complex with 4-fluorobenzoyl chloride …”

Successive Protonation of an N‐Heterocyclic Imine Derived Carbonyl: Superelectrophilic Dication Versus Masked Acylium Ion

“…This results in a sequential increase in the C−O bond order. At the extreme, [ 5 ] + has a measured carbonyl stretching frequency of 2253 cm −1 , which is within the range for known acylium cations (≈2300 cm −1 ) and hints at C≡O triple bond character …”

“…Structurally, [ 5 ] + features a geometry which is linear about the central carbon [O1‐C1‐N1: 173.1(9)°] and bent at N1 [C1‐N1‐C2: 135.9(8)°]. The C1−O1 bond distance is significantly shortened in comparison to [ 3 ] + {1.160(7) vs. 1.216(2) Å}, and although it is similar to that found in carbon dioxide, for example (1.16 Å), it is somewhat longer than those previously reported for acylium ions (1.10–1.13 Å) . The N−C(O) distance is also markedly shortened compared to [ 3 ] + {1.219(8) vs. 1.326(2) Å}, while the N−C(N) bond is slightly elongated to 1.372(6) Å {cf.…”

“…1.13 Å for carbon monoxide) . More recently, Weiss and Kochi have characterized aryl‐substituted systems, [ArCO] + , but to date, heteroatom‐stabilized systems [XCO] + ( VI ), remain extremely rare ,. We postulated that such systems might be of interest not only from a fundamental perspective (i.e.…”

A reactivity-selectivity study of the Friedel-Crafts acetylation of 3,3′-dimethylbiphenyl and the oxidation of the acetyl derivatives

“…Although the reaction has been known for more than 130 years it still receives attention [6-16] the recent interest being focused on the use of ionic liquids as solvents [7], selectivity studies [6,9,11,13], the application of solid catalysts [16], and still, mechanism details [8,12,14,15]. Simple aromatic hydrocarbons and methyl substituted derivatives have featured as substrates in both earlier and more recent studies of the reaction including benzenes [6,7,17-19], biphenyls and fluorene [11,13,20], naphthalenes [21-23], anthracenes [14,24,25], phenanthrenes [12,26], pyrenes [27], and chrysenes [28] and as a part of our continuing interest in Friedel-Crafts acetylations [11,13,21,28-31], we now report details of the acetylation of 3,3′-dimethylbiphenyl (3,3′-dmbp).…”