Aryl silyl enol ethers

The introduction of a t-butyl group a - to a carbonyl group has been achieved by Friedel-Crafts alkylation of the appropriate trimethylsilyl enol ether. In the presence of a nickel(ii) catalyst, u/c-bromotrimethylsilyloxyalkenes couple with Grignard reagents to produce alkylated and arylated silyl enol ethers, which, after hydrolysis, furnish the corresponding a-alkylated or a-arylated ketones (Scheme 36). ... 

Enolizable compounds can be used for Meerwein reactions provided that the keto-enol equilibrium is not too far on the side of the ketone for example, P-dicar-bonyl compounds such as acetylacetone are suitable (Citterio and Ferrario, 1983). The arylation of enol esters or ethers (10.12) affords a convenient route for arylating aldehydes and ketones at the a-carbon atom (Scheme 10-48). Silyl enol ethers [10.12, R = Si(CH3)3] can be used instead of enol ethers (Sakakura et al., 1985). The reaction is carried out in pyridine. 

Diazonium salts react with oximes to give aryl oximes, which are easily hydrolyzed to aldehydes (R = H) or ketones." A copper sulfate-sodium sulfite catalyst is essential. In most cases higher yields (40-60%) are obtained when the reaction is used for aldehydes than for ketones. In another method for achieving the conversion ArNj —> ArCOR, diazonium salts are treated with R4Sn and CO with palladium acetate as catalyst. In a different kind of reaction, silyl enol ethers of aryl ketones, Ar C(OSiMe3)=CHR, react with sohd diazonium fluoroborates, ArNj BF4, to give ketones, ArCHRCOAr. " This is, in effect, an arylation of the aryl ketone. 

Conditions for arylation of enolate equivalents have also been developed. In the presence of ZnF2, silyl enol ethers, silyl ketene acetals, and similar compounds react. For example, the TMS derivatives of /V-acyl oxazolidinones can be arylated. 

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

On the other hand, the use of [Rh(CO)2Cl]2 as a catalyst results in ring opening of the siloxycyclopropanes 13 to the silyl enol ethers 14 with high stereoselectivity [10]. The 2-siloxyrhodacyclobutane 15a is proposed to undergo j8-elimination to give jr-allylrhodium 16a followed by reductive elimination to the silyl enol ether 14a. 1-Trimethylsiloxybicyclo[n.l.0]alkanes serve as / -metallo-carbonyl compounds via desilylation with a variety of transition metals [11]. The palladium-catalyzed reaction of the siloxycyclopropanes 17 under carbon monoxide in chloroform provides a route to the 4-keto pimelates 18. In the presence of aryl triflates, the 1,4-dicarbonyl compounds 19 are... 

Different nucleophiles such as methanol, allylsilanes, silyl enol ethers, trimethylsilyl-cyanide, and arenes can be used in this process [62]. When the sulfide itself contains an unsaturated or aromatic fragment and the process is carried out in the absence of a nucleophile, an intramolecular anodic sub-stitution/cyclization might occur [61-63]. Methyl esters of 2-benzothiazolyl-2-alkyl or aryl-acetic acid, oxidized in MeOH/Et4 NCIO4 or H2SO4 in the presence of CUCI2, form 2,2-dimethoxy products (Eq. 7) [64]. 

MeOC6H4, respectively. The titanium enolates were converted into silyl enol ethers 54 by treatment with chlorotrimethylsilane and lithium isopropoxide. Additionally, cyclic enones lb and Ic, and linear enones Id and le, are also good substrates for the asymmetric conjugate addition of phenyltitanium triisopropoxide, giving the corresponding arylation products with over 97% enantioselectivity. 

Tin enolates of ketones can be generated by the reaction of the enol acetate 733 with tributyltin methoxide[601] and they react with alkenyl halides via transmetallation to give 734. This reaction offers a useful method for the introduction of an aryl or alkenyl group at the o-carbon of ketones[602]. Tin enolates are also generated by the reaction of silyl enol ethers with tributyltin fluoride and used for coupling with halides[603]. 

Stephen L. Buchwald at MIT has reported (Org. Lett. 2004, 6, 4809) a complementary approach. Enantioselective conjugate reduction of the inexpensive 3-methylcyclopentenone 4 led to the silyl enol ether 5, Pd-mediated coupling of which with the aryl halide gave the product 6. 3-Methylcyclohexenone gave the analogous product in 84% . 

Keywords Catalyst, Alkylation, Allylation, Arylation, Mannich reaction, Carbon-nitrogen double bond, Imine, Nitrone, Aldimine, Organozinc reagents, Silyl ketene acetal, Silyl enol ether, Amine, (3-Amino acid... 

Silyl enol ethers with stereogenic silicon atoms bearing chiral alkoxy groups on silicon, as in 193, induce modest stereoselectivity in peracid epoxidation of the enol double bond306. Aryl n participation has been observed in the epoxidation of the bicylooctene 194307. 

Arylation of silyl enol ethers.1 Silyl enol ethers of methyl n- or sec-alkyl ketones undergo arylation when treated with an aryl bromide and tri-n-butyltin fluoride and a... 

The relatively unstable / -oxoalkyl(aryl)iodonium salts 46 can be generated by a low temperature reaction of silyl enol ethers with reagent 45 (Scheme 21) [40]. 

Diaryliodonium fluorides can be used as efficient reagents for the arylation of silyl enol ethers [71, 72]. The reaction of silyl enol ethers 95 with o-nitro-phenyliodonium fluoride 96 results in a regiospecific arylation yielding arylke-tones 97 in good yields (Scheme 43) [72]. This arylation has been applied in a new, regiocontrolled synthesis of carbocycle-fused indoles [72]. 

The use of aryl-A3-iodanes for C-heteroatom bond formation at the a-carbon atoms of ketones and / -dicarbonyl compounds, and related transformations of silyl enol ethers and silyl ketene acetals, has been exhaustively summarized in recent reviews (Scheme 27) [5,8]. Reactions of this type are especially useful for the introduction of oxygen ligands (e. g., L2 = OH, OR, OCOR, 0S02R, OPO(OR)2), and have been extensively utilized for the synthesis of a-sulfonyl-oxy ketones and a-hydroxy dimethyl ketals. 

However, all attempts to activate the a-position of the cyclohexanone ring in order to facilitate a subsequent diazotization, which was to be followed by rhodium carbenoid-mediated aryl C-H insertion onto C-4 [21], were unsatisfactory [22], One of the approaches was based on the generation of the silyl enol ether 35, but attempts to achieve its a-acylation led only to the formation of Paal-Knorr-type cyclization products 36. Chemoselective formylation of 34 to 37 was possible by reaction with ethyl formate in the presence of a large excess of sodium ethoxide, but in situ oxidation of the desired compound 37 to 38, which was the major isolated product, made the reaction impractical (Scheme 6). 

Pandey, G., Karthikeyan, M., and Mumgan, A. (1998) New intramolecular a-arylation strategy of ketones by the reaction of silyl enol ethers to photosensitized electron transfer generated arene radical cations construction of benzannulated and benzospiroannulated compounds. Journal of Organic Chemistry, 63, 2867-2872. 

The photo-SNl reaction with particular classes of alkenes - namely enamines, silyl enol ethers and silyl ketene acetals - affords a smooth synthesis of a-aryl ketones,... 

A versatile strategy for efficient intramolecular oc-arylation of ketones was achieved by the reaction of silyle enol ethers with PET-generated arene radical cations. This strategy involved one-electron transfer from the excited methoxy-substituted arenes to ground-state DCN [42]. Pandey et al. reported the construction of five- to eight-membered benzannulated as well as benzospiroannulated compounds using this approach (Sch. 20) [42a]. The course of the reaction can be controlled via the silyl enol ether obtained... 

The reactions of silenes with aldehydes and ketones is another area whose synthetic aspects have been particularly well-studied4,6 7 10 12. The favoured reaction pathways for reaction are generally ene-addition (in the case of enolizable ketones and aldehydes) to yield silyl enol ethers and [2 + 2]-cycloaddition to yield 1,2-siloxetanes (equation 44), but other products can also arise in special cases. For example, the reaction of aryldisilane-derived (l-sila)hexatrienes (e.g. 21a-c) with acetone yields mixtures of 1,2-siloxetanes (51a-c) and ene-adducts (52a-c) in which the carbonyl compound rather than the silene has played the role of the enophile (equation 45)47,50 52 98 99. Also, [4 + 2]-cycloadducts are frequently obtained from reaction of silenes with a,/i-unsaturated- or aryl ketones, where the silene acts as a dienophile in a formal Diels-Alder reaction6 29,100-102. 

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