Functionalized Triarylcarbenium Ions as Catalysts in Mukaiyama Aldol Addition: Effects of Counter Ions and Silyl Groups on the Intervention of Silyl Catalysis

Abstract

Stereochemical studies have indicated that functionalized Tr + SbCl 6 -may serve as efficient aldol reaction catalysts by judicious choice of a silyl component. The aldol addition between TMS ketene acetal derived from γ-butyrolactone and benzaldehyde provided the silyl aldolates with high syn diastereoselectivities.Triphenylcarbenium ions (trityl salts) constitute a unique mode of carbon-centered Lewis acids for the Mukaiyama-type aldol addition. 1 So far, the precise nature of the catalytic species in this system remains elusive in view of several documented facile silyl-mediated pathways. 2 Recently, a cogent mechanistic study by Bosnich strongly suggests that Me 3 Si-X species is the real catalyst in the trityl perchlorate and triflatemediated reactions. 3 Their results disagree with our previous mechanistic findings by the employment of trityl ions based on a dibenzosuberane scaffold. 4 One reasonable hypothesis for this discrepancy is that our trityl ions are more catalytically active due to the peculiar confinements of two of the three aryl groups in a sevenmembered ring. An alternative explanation is that the operation of silyl catalysis requires a matched use of silyl group and counter ion (e.g., TMS and triflate). To support the conjectures, we have conducted a more thorough stereochemical study of this catalyzed process with functionalized triarylcarbenium ions.To gain insights into the effects of substituents and counter ions on the reactivity of trityl ions and extent of silyl catalysis, triarylcarbinols with 5 different aryl appendages of varying electronic and/or steric demands were synthesized by treatment of dibenzosuberone with the respective aryllithium. The trityl alcohols were all delivered in good yields ranging from 71 to 91% even in the 2,6-disubstituted cases (1d-f), Scheme 1. 5 Their structural identities (1d-f) were further proven by X-ray crystallographic analysis. Scheme 1We have recently developed a novel way of accessing carbenium ions with functionalized aryl groups, which involves Meerwein saltpromoted ionization of the corresponding trityl methyl ethers 2. The requisite ethers can be produced in good to excellent yield (70-97%) by the standard Williamson etherification procedure (NaH/CH 3 I). Three commercial Meerwein salts (Et 3 O + BF 4 -, Et 3 O + PF 6 -, and Me 3 O + SbCl 6 -) have been examined to transform the respective trityl methyl ethers into the corresponding triarylcarbenium ions. The efficiency of this process was assessed by the recovery yield of trityl alcohols on column chromatography and on HPLC analysis after buffered, aqueous quenching of the resulting trityl salts. In marked contrast to triethyloxonium salts, clean delivery of all trityl hexachloroantimonates except 3d was achieved by the employment of Me 3 O + SbC1 6 -. The optimal procedure involved treatment of the individual trityl methyl ether with Me 3 O + SbCl 6 -(1.0 equiv.) in a mixed solvent of EtNO 2 /CH 2 Cl 2 (1/3). The reaction proceeded with full strength in less than 4 hours. In all cases but...

Triarylcarbenium Chlorides as Catalysts in Allylation Reaction: A Unique Type of Reaction with Negligible Intervention of Silyl Catalysis

“…Recently, we were engaged in clarifying this issue by directly observing the catalytic behavior of chiral triarylcarbenium ions in asymmetric Mukaiyama aldol additions and have identified the possible cause of competing silyl catalysis as the direct attack of a nucleophile (e.g., silyl ketene acetal) to the carbenium center of the catalyst with concomitant release of silyl-X species . An intrinsically viable solution to this dilemma is by stereoelectronic modification of the trityl ions to increase their reactivity and compatibility and, in the meantime, to judiciously choose a counterion for the minimization of silyl catalysis . On the basis of these two considerations, we sought the possible utility of trityl-type chlorides in catalytic reactions in view of their well-documented partially ionic characters and the extremely weak Lewis acidic character of TMS-Cl in most solvent systems .…”

“…To find out the best combination of a diarylmethyl template and a pendant aryl group to reach a maximal reactivity of the corresponding trityl-type chloride, we have prepared various trityl-type alcohols by independent treatment of xanthone, fluorenone, benzophenone, and dibenzosuberone with five different aryllithiums of varying electron demands. , The resulting alcohols can be readily converted to the corresponding chlorides by treatment with SOCl 2 (5 equiv) in anhydrous CCl 4 at 0 °C or at ambient temperature, Scheme . In the cases with the parent (Ar = C 6 H 5 ) and electron-releasing (Ar = 4- t -BuC 6 H 4 , 4-MeOC 6 H 4 ) aryl appendages, the trityl chlorides are stable enough to allow for full spectroscopic characterizations .…”

Unexpected behaviour of tosylated and acetylated imidazolinium salts

“…Nlithiotrifluoromethanesulfonimide (1.22 g, 3.6 mmol) were added and the mixture was stirred for 3.5 h at r.t. After adding water (8 mL) stirring was continuing for another hour, the phases were separated by using a pipette and the organic layer was washed with water (3 x 2 mL), dried (Na 2 SO 4 ) and evaporated in vacuum. Drying under high vacuum yielded the title compound 14c as a white solid (1.18 13 C NMR (50 MHz) d 213.9 (q, C=O), 164.9 (q, NCN), 133.0 (+, Ar), 128.9 (+, Ar), 121.5 (q, Ar), 58.2 (q, C(1)), 51.9 (−), 48.2 (q, C(7)), 47.9 (−), 42.0 (−, C(4)), 41.9 (+, C(3)), 35.5 (+, NMe), 26.2 (−, C(6)), 24.9 (−, C(5)), 19…”

Trityl Cation-Catalyzed Hosomi-Sakurai Reaction of Allylsilane with β,γ-Unsaturated α-Ketoester to Form γ,γ-Disubstituted α-Ketoesters

“…Trityl tetrakis (pentafluorophenyl) borate [(Ph 3 C)[BPh( F ) 4 ]] [ 8 ] is well-known for providing stable and easily available carbocations. Since the pioneering work of Mukaiyama and co-workers [ 9 ], the use of trityl cations as Lewis acid catalysts has been explored in Mukaiyama aldol reactions [ 10 , 11 , 12 , 13 , 14 , 15 , 16 ], Hosomi-Sakurai allylations [ 17 , 18 ], Michael additions [ 19 ], Diels–Alder reactions [ 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ], halogenations [ 32 ], epoxide rearrangements [ 27 ], ene reactions [ 33 , 34 ] and other transformations [ 35 , 36 , 37 , 38 ]. In these transformations, trityl cations display much higher catalytic reactivity than traditional metal-based Lewis acids.…”