Triflates, vinyl reaction with aldehydes

Vinylchromium compounds (12,137).6 These compounds can be obtained by reaction of vinyl triflates with CrCl2 catalyzed by NiCl2 in DMF. They undergo selective Grignard-type addition to aldehydes. Some commercial sources of CrCl2 do not require a catalyst, presumably because they contain a metal contaminant. 

Vinyl triflates (10, 123).1 The most satisfactory conditions for preparation of vinyl triflates from carbonyl compounds using triflic anhydride and catalyzed by the pyridine 1 or the polymer-bound reagent 2 are discussed by Wright and Pulley. In the case of aldehydes, the reaction is best carried out in CHC13 or (CH2C1)2 at 50-70°, and results in a gem-bistriflate, which decomposes thermally to the vinyl triflate. These conditions, when applied to a ketone, particularly aldol-prone ones, are usually not useful. The preferred solvent is nonpolar (CClt) with a reaction temperature of about 50°. In addition, the Tf20 should be freshly distilled from P205. 

Lithiation of compound 560 with s-BuLi-TMEDA in THF at —78 °C following an inverse addition protocol provided the anion 561. It reacts with primary alkyl iodides and triflates, silyl chlorides, diphenyl disulfide, epoxides, aldehydes, ketones, imines, acyl chlorides, isocyanates and sulfonyl fluorides to yield the expected compounds 562 (Scheme 152). The transmetallation of compound 561 with ZnBr2 allowed the palladium-catalyzed cross-coupling reaction with aryl and vinyl bromides837. When the reaction was quenched with 1,2-dibromotetrafluoroethane, the corresponding bromide 562 (X = Br) is obtained838. 

Cyclic ketals of acetophenone can be prepared directly under basic conditions by the reaction of aryl triflates, bromides or iodides with hydroxyatkyl vinyl ethers in the presence of a catalytic amount of palladium(II) acetate and 1,3-bis(diphenylphosphino)propane (DPPP).84 The example illustrated in Scheme 2.39 is noteworthy since a protected methyl ketone was appended to an arene in the presence of a reactive aldehyde functionality. 

Blaser and Spencer used aroyl halides in place of aryl halides, with aroyl chlorides being of specific interest as ubiquitous, relatively cheap compounds ( Blaser reaction ) [24], This latter reaction is normally conducted in aromatic solvents phosphines are not used here as catalyst ligands since they fully inhibit the reaction. In the same way, benzoic acid anhydrides can be used as the aryl source in combination with PdCl2 and catalytic amounts of NaBr [79]. In this reaction, one of the arenes is used in the coupling reaction by elimination of CO, whereas the other benzoate serves as the base. The benzoic acid thus formed can easily be recycled into the anhydride. The use of aryl and vinyl triflates according to Cacchi [25] and Stille [26] extends the scope of the Heck coupling to carbonyl compounds phenol derivatives act via triflate functionalization as synthetic equivalents of the aryl halides. The arylation of cyclic alkenes [27], electron-rich vinyl ethers [28], and allylic alcohols [29] is accessible through Heck reactions. Allylic alcohols yield C-C-saturated carbonyl compounds (aldehydes) for mechanistic reasons (y9-H elimination), as exemplified in eq. (6). 

In Section 12.7.B, the Stille coupling reaction reacted a vinyl tin compound with a vinyl triflate, in the presence of palladium(O). Tetravalent tin complexes add to aldehydes and ketones, in the presence of a Lewis acid. Allyltin complexes are, by far, the most widely used of these compounds.297 a typical example is taken from the work of Keck, in which a chiral aldehyde (455) was treated with allyltributyltin, in the presence of various Lewis acids. S As shown in Table 12.19, a mixture of syn (456) and anti (457) products was obtained. The ratio of 456/457 was dependent on the structure of the R group in 455, the solvent and the Lewis acid.The anti product (457) was obtained by using the tert-butyldimethylsilyloxy derivative (sec. 7.3.A.i) of 455 with 2 equivalents of boron trifluoride in dichloromethane. The syn product is obtained preferentially when the benzyloxy derivative of 455 is used with titanium tetrachloride in dichloromethane.298... 

The mechanism involves the oxidative addition of the vinyl or aromatic triflate or halide to give an organopalladium intermediate. TransmetaUation with the organostannane forms another organopalladium intermediate with two Pd—C a bonds. A reductive elimination step releases the product and thereby regenerates the palladium(0) catalyst. Vinyl triflates can be made from enolizable aldehydes or ketones and aryl triflates from phenols, but the reaction also works with vinyl and aryl halides. 

The protected indanone 109 was obtained by the Pd-catalyzed one-pot reaction of the triflate of salicylic aldehyde 108 with 2-hydroxy ethyl vinyl ether and addition of AcOH [65], In this case, selective a-arylation is followed by annulation promoted by Pd and AcOH to give 109. 

Acylsilanes. The reaction of anion (1) with alkyl halides generates a functionalized dithiane which, when hydrolyzed under mild conditions, provides the corresponding acylsilane (eq 16). More highly substituted acylsilanes have been accessed by initial formation of the dithiane from the aldehyde, followed by deprotonation and silylation with TMSCl (eq 17). Acylsilanes have been used as sterically hindered aldehyde equivalents forre-giocontrol in addition reactions (eq 18) and also as precursors for silyl-substituted vinyl triflates (eq 19). ... 

In subsequent studies, methyl vinyl ketone (2.0 mmole) was chosen as the dienophile so as to determine the combined effect of the ionic liquid (2 mL) and the Lewis acids (0.2 and 0.5 wt%) upon the yield and selectivity. Without the Lewis acid catalyst, this system demonstrated a 52% conversion of the cyclopentadiene (2.2 mmol) in 1 h with the endojexo selectivity being 85/15. The cerium triflate-catalyzed reaction was quantitative in 5 min and the endo. exo selectivity was very good for this experiment as well (94 6, endo. exo). Also with the scandium or yttrium salts tested, reactions came to completion in a short time with high stereo-selection. Cerium, scandium and yttrium triflates are strong Lewis acids known to be quite effective catalysts in the cycloadditions of cyclopentadiene with acyclic aldehydes, ketones, quinones and cycloalkenones. These compounds are expected to act as strong Lewis acids because of their hard character and the electron-withdrawing triflate group. On the other hand, reaction times of 1 hour were required for... 

Later, the dimethylphenylsilyl adduct 75 d was not only converted to the vinyl-ketene acetal 81 by a Wittig-Horner reaction via the ester 80, but also by reduction of 80 to the aldehyde 84 followed by silylation of the corresponding anion to the silylenol ether 85. Vinylcyclohexenes without terminal substitution (e. g., 83) was prepared via an alcohol derived from 80 or 84 [73,74]. The least substituted diene 83 was alternatively prepared from the triflate 82 in a Stille coupling with trialkylvinyl stannane [75] (Scheme 24), a reaction also used by Toshima etal.[76]. 

Kobayashi et al. found that lanthanide triflates were excellent catalysts for activation of C-N double bonds —activation by other Lewis acids required more than stoichiometric amounts of the acids. Examples were aza Diels-Alder reactions, the Man-nich-type reaction of A-(a-aminoalkyl)benzotriazoles with silyl enol ethers, the 1,3-dipolar cycloaddition of nitrones to alkenes, the 1,2-cycloaddition of diazoesters to imines, and the nucleophilic addition reactions to imines [24], These reactions are efficiently catalyzed by Yb(OTf)3. The arylimines reacted with Danishefsky s diene to give the dihydropyridones (Eq. 14) [25,26], The arylimines acted as the azadienes when reacted with cyclopentadiene, vinyl ethers or vinyl thioethers, providing the tet-rahydroquinolines (Eq. 15). Silyl enol ethers derived from esters, ketones, and thio-esters reacted with N-(a-aminoalkyl)benzotriazoles to give the /5-amino carbonyl compounds (Eq. 16) [27]. The diastereoselectivity was independent of the geometry of the silyl enol ethers, and favored the anti products. Nitrones, prepared in situ from aldehydes and N-substituted hydroxylamines, added to alkenes to afford isoxazoli-dines (Eq. 17) [28]. Addition of diazoesters to imines afforded CK-aziridines as the major products (Eq. 18) [29]. In all the reactions the imines could be generated in situ and the three-component coupling reactions proceeded smoothly in one pot. 

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