Structure benzoin

The triazole 76, which is more accurately portrayed as the nucleophilic carbene structure 76a, acts as a formyl anion equivalent by reaction with alkyl halides and subsequent reductive cleavage to give aldehydes as shown (75TL1889). The benzoin reaction may be considered as resulting in the net addition of a benzoyl anion to a benzaldehyde, and the chiral triazolium salt 77 has been reported to be an efficient asymmetric catalyst for this, giving the products (/ )-ArCH(OH)COAr, in up to 86% e.e. (96HCA1217). In the closely related intramolecular Stetter reaction e.e.s of up to 74% were obtained (96HCA1899). 

Infrared spectral data have been advanced to support the oxo formulation for 96 (R —propyl). Structure 96 (R= phenyl) has also been proposed on the basis of the fact that benzoin does not condense with urea, and this conclusion is probably correct, although the argument is not convincing. The oxo forms of benzimidazolone, 97 106,10- related pyrazinoimidazolone 98 have been shown... 

Acyloins (a-hydroxy ketones) are formed enzymatically by a mechanism similar to the classical benzoin condensation. The enzymes that can catalyze reactions of this type arc thiamine dependent. In this sense, the cofactor thiamine pyrophosphate may be regarded as a natural- equivalent of the cyanide catalyst needed for the umpolung step in benzoin condensations. Thus, a suitable carbonyl compound (a -synthon) reacts with thiamine pyrophosphate to form an enzyme-substrate complex that subsequently cleaves to the corresponding a-carbanion (d1-synthon). The latter adds to a carbonyl group resulting in an a-hydroxy ketone after elimination of thiamine pyrophosphate. Stereoselectivity of the addition step (i.e., addition to the Stand Re-face of the carbonyl group, respectively) is achieved by adjustment of a preferred active center conformation. A detailed discussion of the mechanisms involved in thiamine-dependent enzymes, as well as a comparison of the structural similarities, is found in references 1 -4. 

The acyl phosphonates, acyl phosphine oxides and related compounds (e.g. 81. 82) absorb strongly in the near UV (350-400 nm) and generally decompose by rescission in a manner analogous to the benzoin derivatives.381"285 Quantum yields vary from 0.3 to 1.0 depending on structure. The phosphinyl radicals are highly reactive towards unsaturated substrates and appear to have a high specificity for addition v.v abstraction (see 3.4.3.2). 

These conclusions were supported by the results obtained in a study of the reactions of various types of acetylenes with TTN (94). Hydration of the C=C bond was found to occur to a very minor extent, if at all, with almost all of the compounds studied, and the nature of the products formed was dependent on the structure of the acetylene and the solvent employed. Oxidation of diarylacetylenes with two equivalents of TTN in either aqueous acidic glyme or methanol as solvent resulted in smooth high yield conversion into the corresponding benzils (Scheme 23). The mechanism of this oxidation in aqueous medium most probably involves oxythallation of the acetylene, ketonization of the initially formed adduct (XXXV) to give the monoalkylthallium(III) derivative (XXXVI), and conversion of this intermediate into a benzoin (XXXVII) by a Type 1 process. Oxidation of (XXXVII) to the benzil (XXXVIII) by the second equivalent of reagent would then proceed in exactly the same manner as described for the oxidation of chalcones, deoxybenzoins, and benzoins to benzils by TTN. The mechanism of oxidation in methanol solution is somewhat more complex and has not yet been fully elucidated. 

Attack by eCN is slow (rate-limiting), while proton transfer from HCN or a protic solvent, e.g. HzO, is rapid. The effect of the structure of the carbonyl compound on the position of equilibrium in cyanohydrin formation has already been referred to (p. 206) it is a preparative proposition with aldehydes, and with simple aliphatic and cyclic ketones, but is poor for ArCOR, and does not take place at all with ArCOAr. With ArCHO the benzoin reaction (p. 231) may compete with cyanohydrin formation with C=C—C=0, 1,4-addition may compete (cf. p. 200). 

The isopavine bases, ( )-amurensinine (25) (81,135), ( )-0-meth-ylthalisopavine (26) (136), ( )-reframidine (27) (77), and ( )-reffamine (28) (77) were synthesized by the above-mentioned classical route, where deoxy-benzoins were utilized as starting materials. In some cases, some modification to the method has been introduced, particularly involving the formation of the requisite benzylaminoacetals (77,110). The synthesis of ( )-thalisopavine (30) was undertaken along parallel lines to confirm the structure of the naturally occurring base (53). Moreover, both ( )-reframoline (29) and its positional isomer ( )- 146 were synthesized in an attempt to establish the position of the phenolic hydroxyl (110). The synthesis of ( )-reframine (28) from the properly substituted deoxybenzoin 94 has been outlined in Scheme 16 as a typical example (77). 

Although benzil was the first a-dicarbonyl compound to be investigated,1-4 its photoreactions have not been as thoroughly studied as those of biacetyl. By 1916 it had been established that a precipitate was formed when a solution of benzil in ether,1,3 ethanol,2 aldehydes,3 or alkyl substituted benzenes3 was exposed to sunlight. The thermal and photochemical instability of the precipitate caused some confusion about its structure until Cohen demonstrated that it was pinacol 10 which decomposed to benzil and benzoin on heating.4... 

For related reasons, and because their excited-state energies are lower than for dialkyl ketones, diaryl ketones and simple alkyl aryl ketones do not fragment on irradiation in solution, even at higher temperatures. This leads to a photostability that is one factor contributing to the successful employment of ketones such as benzophe-none tPh-CO) or acetophenone (PhCOMe) as triplet sensitizers. a-Cleavage for ketones in solution at room temperature is promoted if structural factors cause the bond adjacent to the carbonyl group to be somewhat weaker than normal. Hence t-alkyl ketones give decar-borylation products readily (4.5), as do benzyl ketones (4.6 and benzoin derivatives (4.7). 

Benzoic acid [65-85-0], C6H5COOH, the simplest member of the aromatic carboxylic acid family, was first described in 1618 by a French physician, but it was not until 1832 that its structure was determined by Wnfiler and Liebig. In the nineteenth century benzoic acid was used extensively as a medicinal substance and was prepared from gum benzoin. Benzoic acid was first produced synthetically by the hydrolysis of benzotrichloride. Various other processes such as the nitric acid oxidation of toluene were used until the 1930s when the decarboxylation of phthalic acid became the dominant commercial process. During World War II in Germany the batchwise liquid-phase air oxidation of toluene became an important process. 

The thiazolium salt 3-benzyl-5-(2-hydroxyethyl)-4-methyl-l,3-thiazolium chloride is an excellent catalyst for the addition of unsaturated aliphatic aldehydes to vinylketones (79CB84). The presence of a base such as sodium acetate or triethylamine is required, for the thiazolium salt must first be transformed into the ylide structure (615), which then exerts a catalytic effect resembling that of cyanide ion in the benzoin condensation (Scheme 137). Yields of 1,4-diketones (616) produced in this process were generally good. The use of thiazolium salts for other related reactions has been reviewed (76AG(E)639). 

Benzamido-cinnamic acid, 20, 38, 353 Benzofuran polymerization, 181 Benzoin condensation, 326 Benzomorphans, 37 Benzycinchoninium bromide, 334 Benzycinchoninium chloride, 334, 338 Bifiinctional catalysts, 328 Bifiinctional ketones, enantioselectivity, 66 BINAP allylation, 194 allylic alcohols, 46 axial chirality, 18 complex catalysts, 47 cyclic substrates, 115, 117 double hydrogenation, 72 Heck reaction, 191 hydrogen incorporation, 51 hydrogen shift, 100 hydrogenation, 18, 28, 57, 309 hydrosilylation, 126 inclusion complexes, oxides, 97 ligands, 19, 105 molecular structure, 50, 115 mono- and bis-complexes, 106 NMR spectra, 105 olefin isomerization, 96... 

The apparent fickleness of the acyl-pyrroles and -indoles in their reaction with carbanions to form new C—C bonds arises from the contribution made by the zwitterionic structure, e.g. (410b), to the resonance hybrid and the choice of the reaction conditions is critical for a successful nucleophilic reaction. Thus, formyl-pyrroles and -indoles do not normally undergo the Cannizzaro reaction nor do they form stable cyanohydrins or undergo benzoin-type reactions. However, surprisingly, 2-formylpyrrole reacts with arylaldehydes in the presence of potassium cyanide to yield (428), which is easily oxidized to (429) (B-77MI30505). It is noteworthy that the presence of an ester substituent adjacent to the formyl group modifies the mesomeric interaction to such an extent to allow the formation of (430) in low yield, as a result of an initial benzoin-type self-condensation (Scheme 76) (68BSF637). 

Importantly, TV-carbamoyl derivatives of primary amines obtained from photolabile benzoins exist in varying proportions as cyclic hydroxyoxazolidinone tautomers. Therefore, the preparation of TV-carbamoyl derivatives of benzoins is applicable only for secondary amines and TV-alkylated amino acids. 244 At basic pH unsymmetrical benzoins, such as 3,5-dimethoxybenzoin, their mixed carbonate and carbamate derivatives, tend to equilibrate to the isomeric forms. Nevertheless, in TFA and aqueous solutions at pH 8 the structural integrity is fully maintained. 244 Preparation of 3,5-dimethoxybenzoin-derived carbamates of secondary amines and amino acids can be mediated by either CDI/methyl triflate in ni-tromethane 246 or 4-nitrophenyl chloroformate/DMAP in dry THF. 244 ... 

FIGURE 16 The chemical structures of some drugs resolved on different CSPs based on a- and /1-cyclodextrin derivatives (Fig. 15) Troger s base (I), tra .s-2,.1-diphenyloxirane (II), l-(9-anthryl)-2,2,2-trifluoroethanol (III), 1,2,2,2-tetraphenylethanol (IV), 2,2 -dihy-droxy-6,6-dimethylbiphenyl (V), 2-phenylcyclohexanone (VI), flavanone (VII), benzoin (VIII), and tnms -cyclopropanedicarboxylic acid anilide (IX). (From Ref. 49.)... 

In the thiazolium cation the proton in the 2-position is acidic and its removal gives rise to the ylide/carbene 227. This nucleophilic carbene 227 can add, e.g., to an aldehyde to produce the cationic primary addition product 228. The latter, again via C-deprotonation, affords the enamine-like structure 229. Nucleophilic addition of 229 to either an aldehyde or a Michael-acceptor affords compound(s) 230. The catalytic cycle is completed by deprotonation and elimination of the carbene 227. Strictly speaking, the thiazolium salts (and the 1,2,4-triazolium salts discussed below) are thus not the actual catalysts but pre-catalysts that provide the catalytically active nucleophilic carbenes under the reaction conditions used. This mechanism of action of thiamine was first formulated by Breslow [234] and applies to the benzoin and Stetter-reactions catalyzed by thiazolium salts [235-237] and to those... 

The structures of all three oxidation states in the tungsten bdt series have been reported. The W(IV) and W(V) states are similar to the Mo analogs [178], The bis(oxido) W(VI) complex is a distorted octahedron. The W—S bonds tram to the oxido ligand are lengthened with W—S distances cis to the oxido at 2.425(4) A and tram to the oxido at 2.597(4) A, respectively. The W(VI) complex reacts with benzoin to give benzil (Eq. 12), but only reacts slowly with PPh3 [183],... 

The triazol-5-ylidene 12 was found to be a powerful catalyst for the conversion of formaldehyde to glycolaldehyde in a formoin reaction [25.] The concept of triazolium salt catalysis appeared to show promise, and consequently our research group undertook the synthesis of a variety of chiral triazolium salts for the asymmetric benzoin reaction [26]. However, the ce-values and catalytic activities shifted widely with slight structural changes in the substitution pattern of the triazolium system. The most active catalyst 15 (Fig. 9.4) afforded benzoin (6, Ar = Ph) in its (R -configuration with 75% ee and a satisfactory yield of 66%. 

This ligand, which has a bicyclic structure strongly related to Leeper s and Rovis ligands (vide supra) [20-25], was found to be a very efficient organocatalyst in the asymmetric benzoin condensation (ee s up to 99%, see Scheme 5, Eq. 1). 

The stereochemical outcome of the reaction above is explained in terms of a structurally rigid 1,3-bridged six-membered chairlike transition state (Scheme 3.2i). Transition state B would be disfavored relative to transition state A because the phenyl group of benzoin is engaged in nonbonded interactions with the methyl and the hydrogen of the crotylsilane. 

A comparison of the H-bonding motif between supraminols 28 30, 29 30, and 29 32 suggests that the structure of the partner diol (e.g. cyclic diol versus acyclic diol) is an important factor in the control of the efficiency of the full or partial coordination in these supramolecular structures. Further insight into this aspect was obtained by the X-ray crystal analysis of heterochiral and homochiral complexes 29 33 and 29 34, formed between the diamine (R,R)-29 and (S, -hydro-benzoin (33) and (i ,7 )-hydrobenzoin (34), respectively [51]. After crystallization from benzene, the heterochiral complex 29 33 showed a higher melting... 

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