Acetophenone, enol silyl ether

Another important variant of the preceding approach is the cycloaddition reaction between monocarbonyl iodonium salt 47 and an alkene to give dihydrofuran 48 (88TL3703 89JOC2605). The iodonium salt 47 is generated by the oxidation of acetophenone silyl enol ether (46) with iodosobenzene in the presence of fluoboric acid. 

ACETONE TRIMETHYLS ILVL ENOL ETHER SILANE, (ISOPROPE NYLOXY )TR IMETHYL SILANE, TRIMETHYL[(1-METHYLETHENYL)0XY]- (1833-53-0), 65, 1 Acetonitrile, purification, 66, 101 Acetophenone Ethanone, 1-phenyl- (98-86-2), 65, 6, 119 Acetophenone silyl enol ether Silane, trimethyl[(1-phenylvinyl)oxy] Silane, tririethyl[(l-phenylethenyl )oxy]- (13735-81-4), 65, 12 4-ACET OXYAZET ID IN-2-ONE 2-AZET IDINONE, 4-HYDROXY-ACETATE (ESTER) 2-AZET ID1N0NE, 4-(ACETYL0XY)- (2 8562 - 53-0), 65, 135 Acetylene Ethyne (74-86-2), 65, 61... 

Acetophenone Ethanone, 1-phenyl- (98-86-2), 65, 6, 119 Acetophenone silyl enol ether Silane, trimethyl[(1-phenylvinyl)oxy]- ... 

Nicholas reaction of acetophenone silyl enol ether with the bis-acetal-alkynylcobalt complex 18 using an excess of Lewis acid affords a furyl-a-pyrone in quantitative yield (Equation 37) <2003EJ01652>. 

A remote functionalization sequence which employs a tandem retro-[l, 4] Brook rearrangement to transfer a silyl moiety of an acetophenone silyl enol ether to the ortho position of the aromatic ring, has been developed see Scheme 11. The enolate (35) derived from this process has been used in cross-aldol condensations to afford ketones (36). 

More recently, further developments have shown that the reaction outlined in Scheme 4.33 can also proceed for other alkenes, such as silyl-enol ethers of acetophenone [48 b], which gives the endo diastereomer in up to 99% ee. It was also shown that / -ethyl-/ -methyl-substituted acyl phosphonate also can undergo a dia-stereo- and enantioselective cycloaddition reaction with ethyl vinyl ether catalyzed by the chiral Ph-BOX-copper(ll) catalyst. The preparative use of the cycloaddition reaction was demonstrated by performing reactions on the gram scale and showing that no special measures are required for the reaction and that the dihydro-pyrans can be obtained in high yield and with very high diastereo- and enantioselective excess. 

OS 75] ]R 4b] ]P 55] For the reaction of 4-bromobenzaldehyde with the silyl enol ether of acetophenone, 100% conversion with respect to the silyl enol ether was achieved in 20 min for a given set of electrical fields (375, 409, 381 and 0 V cm ) [15]. The corresponding batch synthesis time was about 1 day. 

With the use of NMR spectroscopy Giering and coworkers studied the reaction of acetophenone and HSiBu3 in more detail. The catalyst was [(1,5-cod)RhCl]2 and R-BINAP [36], They noted that the silyl enol ether by-product was formed mainly at the beginning of the reaction and thus this must form via an independent pathway. The common intermediate for silyl ether product and... 

In order to obtain further insight into the mechanism of the Mannich-type reaction, sulfone IP and silyl enol ether derived from acetophenone were reacted in the presence HOTf or TMSOTf, which could be produced in the reaction medium when using Bi(0Tf)3-4H20 as catalyst. It appeared that these two compounds efficiently catalyze the Mannich-type reaction (Table 7, entries 2 and 3). The reaction does not occur in the presence of 2,6-di-/<7V-buty I-4-methyl-pyridine [DTBMP] (1.0 equiv. of lp, 1.3 equiv. of silyl enol ether, 0.5 mol% of Bi(0Tf)34H20, 1.5 mol% of 2,6-di-/c/V-buty l-4-methy I-pyridine, 22 °C, 20 h, 99% recovery of lp), which indicates that triflic acid is involved in the mechanism (Table 7, entry 4). 

A photochemical reaction of the silyl enol ether of acetophenone and benzaldehyde provided the 2,3-diphenyl-3-trimethylsilyloxyoxetane (13) with excellent regioselectivity (> 95 5) and diaster-eoselectivity (> 95 5) (91TL7037). In this example, the diastereoselection was explained by anti-approach of the two phenyl groups during the carbon-carbon bond forming step from the diradical intermediate (Scheme 7). 

The reaction requires the formation of enolates that by themselves are one of the most profound spedes enabling C—C bond formation [15]. Reducing processing time is a driver for microchannel processing of aldol readions [15] that can be accomplished using reactive readants such as silyl enol ethers. For example, the reaction between 4-bromobenzaldehyde and the silyl enol ether of acetophenone was performed in a microreactor [15]. 

Silyl enol ethers are quite reactive towards IOB-boron trifluoride (or tetrafluoroboric acid) and can be considered as valuable starting materials for several reactions of synthetic importance. Of special interest is their use for carbon-carbon bond formation 1,4-diketones and unsaturated ketones are the products of such reactions further, they can be transformed to oc-hydroxy, methoxy or trifyloxy ketones. With tetrafluoroboric acid IOB forms a yellow solution containing the highly electrophilic Phi+ OH BF4 , stable up to 0°C. This species reacts readily with silyl ethers of several ketones, notably acetophenones, at —78°C, forming an unstable iodonium ion (ArCOCH2I+ Ph) which with another silyl ether affords 1,4-diketones. 

An interesting reaction between the bis phenyl iodonium triflate of acetylene and the silyl enol ether of acetophenone afforded an allene (PhCOCH=C=C=CHCOPh, 84%) [6], Also, alkynyl iodonium tosylates and carbon monoxide in methanol or ethanol, with palladium catalysis, furnished alkyne carboxylates [53]. 

It has already been noted that the enolates of unactivated monocarbonyl compounds do not undergo alkynylation with alkynyliodonium salts3. It is therefore particularly intriguing that [bis(phenyliodonium)]ethyne ditriflate reacts with the silyl enol ether (SEE) of acetophenone to give an allenic diketone (equation 151)41. Except for the SEE of cyclohexanone, which gives a black tar with the bisiodonium compound41, similar studies of other SEEs have not been reported. 

Complex 1 also catalyzes the regioselective radical addition of perhalogeno-ethanes to silyl enol ethers. The primary addition-desilylation products undergo the facile /1-elimination of a chloride to afford a,/3-unsaturated ketones [26, 27]. For example, CF2C1CC13 adds to the trimethylsilyl enol ether of acetophenone to yield /i-chloro-/i-(chlorodifluoromethyl)-a,/ -acetophenone in 80% yield (Eq. 9). 

Bis(pentafluorophenyl) tin dibromide effects the Mukaiyama aldol reaction of ketene silyl acetal with ketones, but promotes no reaction with acetals under the same conditions. On the other hand, reaction of silyl enol ether derived from acetophenone leads to the opposite outcome, giving acetal aldolate exclusively. This protocol can be applied to a bifunctional substrate (Equation (105)). Keto acetal is exposed to a mixture of different types of enol silyl ethers, in which each nucleophile reacts chemoselectively to give a sole product.271... 

The silyl enol ether may be obtained from the Fluka Chemical Corp., 255 Oser Avenue, Hauppauge, NY 11788. Alternatively, it may be prepared by the following modification of the procedure of Walshe and co-workers.2 The Walshe procedure is followed exactly with 36 g (0.30 mol) of acetophenone, 41.4 g (0.41 mol) of tri ethyl amine, 43.2 g (0.40 mol) of chlorotri-methylsilane, 60 g (0.40 mol) of sodium iodide, and 350 nt of acetonitrile. After extraction, the organic layer is dried over potassium carbonate and then concentrated with a rotary evaporator under reduced pressure. The crude product is a mixture of 97% of the desired silyl enol ether and 3% of acetophenone, as shown by gas chromatography. The crude product is distilled in a Claisen flask at a pressure of about 40 mm. After a small forerun (ca. 3... 

The silicon-carbon double-bonded intermediates generated photo-chemically from a-alkenyldisilane derivatives react with both enolizable and nonenolizable ketones to give olefins (98). For instance, the photolysis of a-styrylpentamethyldisilane (49) in the presence of one molar equivalent of acetone gives l-trimethylsilyl-2-phenyl-3-methyl-2-butene in 13% yield as a single product. No silyl enol ether to be expected from the reaction of the intermediate with the enol form of acetone is observed. Similar irradiation of 49 with acetophenone affords (E)- and (Z)-l-trimeth-... 

Cationic Pd complexes can be applied to the asymmetric aldol reaction. Shibasaki and coworkers reported that (/ )-BINAP PdCP, generated from a 1 1 mixture of (i )-BINAP PdCl2 and AgOTf in wet DMF, is an effective chiral catalyst for asymmetric aldol addition of silyl enol ethers to aldehydes [63]. For instance, treatment of trimethylsi-lyl enol ether of acetophenone 49 with benzaldehyde under the influence of 5 mol % of this catalyst affords the trimethylsilyl ether of aldol adduct 113 (87 % yield, 71 % ee) and desilylated product 114 (9 % yield, 73 % ee) as shown in Sch. 31. They later prepared chiral palladium diaquo complexes 115 and 116 from (7 )-BINAP PdCl2 and (i )-p-Tol-BINAP PdCl2, respectively, by reaction with 2 equiv. AgBF4 in wet acetone [64]. These complexes are tolerant of air and moisture, and afford similar reactivity and enantioselec-tivity in the aldol condensation of 49 and benzaldehyde. Sodeoka and coworkers have recently developed enantioselective Mannich-type reactions of silyl enol ethers with imi-nes catalyzed by binuclear -hydroxo palladium(II) complexes 117 and 118 derived from the diaquo complexes 115 and 116 [65]. These reactions are believed to proceed via a chiral palladium(fl) enolate. 

If the 1,5-dicarbonyl compound is required, then an aqueous work-up with either acid or base cleaves the silicon-oxygen bond in the product but the value of silyl enol ethers is that they can undergo synthetically useful reactions other than just hydrolysis. Addition of the silyl enol ether derived from acetophenone (PhCOMe) to a disubstituted enone promoted by titanium tetrachloride is very rapid and gives the diketone product in good yield even though a quaternary carbon atom is created in the conjugate addition. This is a typical example of this very powerful class of conjugate addition reactions. 

Trost and coworkers developed a chiral zinc phenoxide for the asymmetric aldol reaction of acetophenone or hydroxyacetophenone with aldehydes (equations 62 and 63) . This method does not involve the prior activation of the carbonyls to silyl enol ethers as in the Mukaiyama aldol reactions. Shibasaki and coworkers employed titanium phenoxide derived from a phenoxy sugar for the asymmetric cyanosilylation of ketones (equation 64). 2-Hydroxy-2 -amino-l,l -binaphthyl was employed in the asymmetric carbonyl addition of diethylzinc , and a 2 -mercapto derivative in the asymmetric reduction of ketones and carbonyl allylation using allyltin ° . ... 

In recent years, catalytic asymmetric Mukaiyama aldol reactions have emerged as one of the most important C—C bond-forming reactions [35]. Among the various types of chiral Lewis acid catalysts used for the Mukaiyama aldol reactions, chirally modified boron derived from N-sulfonyl-fS)-tryptophan was effective for the reaction between aldehyde and silyl enol ether [36, 37]. By using polymer-supported N-sulfonyl-fS)-tryptophan synthesized by polymerization of the chiral monomer, the polymeric version of Yamamoto s oxazaborohdinone catalyst was prepared by treatment with 3,5-bis(trifluoromethyl)phenyl boron dichloride ]38]. The polymeric chiral Lewis acid catalyst 55 worked well in the asymmetric aldol reaction of benzaldehyde with silyl enol ether derived from acetophenone to give [i-hydroxyketone with up to 95% ee, as shown in Scheme 3.16. In addition to the Mukaiyama aldol reaction, a Mannich-type reaction and an allylation reaction of imine 58 were also asymmetrically catalyzed by the same polymeric catalyst ]38]. 

This ligand coupling method was quite efficient for acetophenone and rerf-butylketone derivatives. However, coupling of the TMS ether of cyclohexanone (121) failed. The oxidative coupling of the TMS ether of cyclohexanone (121) to give 2,2 -bicyclohexanone was successful only when this silyl enol ether was treated with the iodosobenzene-tetrafluoroborate complex.230,231... 

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