2,4-Pentanediones benzylation

Using different mono- and diketones in acetic acid (at room temperature) afforded the following products from benzophenone, 2,2-diphenyl-2//-imid-azo[4,5-/]quinoline from dibenzylketone, the 2-benzyl-imidazo[4,5-/]quino-line and from 2,4-pentanedione, 2-methyl-imidazo[4,5-/]quinoline. Cyclohexanone under reflux gave 2-n-pentyl-, whereas at room temperature it afforded the. s pira[cyclohexane-l,2 ]-(2//)-imidazo[4,5-/]quinoline 108 (R R =(CFl2)5) (86UC527). 

Benzyl [15N]-3,4,5-trimethylpyrrol-2-carboxylate, 94, has been obtained77,79 in 38% yield in the reaction of [15N]-sodium nitrite with benzyl acetoacetate in AcOH at 10-5 °C, during 18 h, followed by addition of 3-methyl-2,4-pentanedione, AcONa, powdered zinc in AcOH, heating the suspension at 60 °C during 1 h, pouring the suspension over ice-water (5°C, 18 h) and recrystallization (Me0H-H20). 

This first example of a Bi(OTf)3-catalyzed Friedel-Crafts alkylation originated in the following procedures, including benzylations of 2,4-pentanediones or hydroarylation and hydroalkylation reactions. A related procedure was simultaneously developed by Bonrath et al. [39]. The authors utilized Bi(OTf)3 in the synthesis of (all-rac)-a-tocopherol (Vitamin E) [39], Besides rare earth metal triflates, such as Ga(OTf)3, Hf(OTf)3, Sc(OTf)3 and Gd(OTf)3, Bi(OTf)3 was shown to be the most efficient catalyst for the Friedel-Crafts-type reaction between trimethylhydroquinone acetate 10b and isophytols 11a, b. With only 0.02 mol% Bi(OTf)3 (substrate to catalyst ratio 5,000 1) the desired a-tocopherols 12a and 12b were isolated in excellent yields (Scheme 10). 

Beside the Friedel-Crafts-type alkylation of arenes, the direct functionalization of 2,4-pentanediones is of great interest in Lewis acid catalysis. Although Pd-catalyzed Tsuji-Trost type allylations of 1,3-diketones are known, direct benzylation procedures catalyzed by Lewis acids are less explored [40-43]. Based on the previously described Friedel-Crafts alkylation of arenes and heteroarenes, the Rueping group developed a Bi(OTf)3-catalyzed benzylation of 2,4-pentanediones. Alcohols such as benzyl, allyl or cinnamyl alcohols were used as the electrophilic component to yield important 2-alkylated 1,3-dicarbonyl compounds. Initially, different Bi(III) salts were screened. In contrast... 

From a mechanistic point of view, it can be envisaged that this reaction proceeds via the desired benzylated pentanedione intermediate 14f. The subsequent intramolecular Friedel-Crafts alkylation of the electron-rich arene results in the quaternary benzyl alcohol II, which readily eliminates water to give the highly substituted indene 16 (Scheme 14). 

In addition to 1-phenylethanol and benzyl alcohol, cinnamyl alcohol has also been utilized as alkylating reagent. In contrast to the Bi(OTf)3-catalyzed Friedel-Crafts alkylation of benzyl alcohols, in which the corresponding acetate was shown to be more reactive than the free benzyl alcohol (Scheme 8), in this case the alkylation with cinnamyl alcohol or the corresponding acetate provided almost similar results. With 5 mol% BifOTfh. the desired allylated 2,4-pentanediones were isolated in good yields (Scheme 15). 

The concept of a diastereoselective Friedel-Crafts alkylation of a-chiral benzyl alcohols was first examined by Bach and coworkers [62, 63]. The initial protocol required stoichiometric amounts of strong Brpnsted acids like HBF4 and was followed by a more valuable methodology in which catalytic amounts of AuC L were employed for the diastereoselective functionalization of chiral benzyl alcohols [64], Beside arenes, allyl silanes, 2,4-pentanediones and silyl enol ethers have been used as nucleophiles. Depending on the diastereodiscriminating group and on the catalyst (Brpnsted or Lewis acid), the authors observed either the syn or the anti diastereoisomer as the major product. 

Moreover, 3-benzoyl-2,4-pentanedione (215) is obtained when benzoylchloride is employed to the acetylacetone derivate i94143) whereas the reaction with benzaldehyde results a mixture of 3-benzylidene-2,4-pentanedione (213) and 3-[a-hydro-xy-benzyl]-2,4-pentanedione (214)143 ... 

Allyl halides have been reduced with electrogenerated tris(bipyridine)cobalt(I) to afford 1,5-hexadiene [369,370]. Some of the earliest work with cobalt(I) salen involved its use for the catalytic reduction of bromoethane [371], bromobenzene [371], and /er/-butyl bromide and chloride [372]. More recently. Fry and coworkers examined the cobalt(I) salen-cata-lyzed reductions of benzal chloride [373-375] and of benzotrichloride [376], and the catalytic reductions of 1-bromobutane [377,378], 1-iodobutane [378], 1,2-dibromobutane [378], benzyl and 4-(trifluoromethyl)benzyl chlorides [379], iodoethane [380], diphenyl disulfide [381], 1,8-diiodooctane [382], and 3-chloro-2,4-pentanedione [383] have been investigated. 

When l-phenyl-2,4-pentanedione is doubly deprotonated with potassium amide in ammonia, the ensuing aldol reaction with benzophenone gives the expected aldol (19).55 However, if sodium amide is employed to generate the dianion, the reaction product with benzophenone is the dihydro-7-pyrone (20 equation 36).56 This result suggests the intermediacy in the latter reaction of a dianion involving the benzylic position. Further study of this interesting reaction is warranted. 

The 7-benzyl derivative of XII-53 (W-55) prepared from the sodium salt of 5-phenyl-l, 3-pentanedione, potassium amide, and methyl benzoate, was not isolated but was converted to S-benzyl-6-phenyl4-pyridone with ammonia in ethanol. " ... 

Mg2,8Al-IJX) catalyzes the direct C-alkylation of 2,4-pentanedione with alkyl halides (560). For example, C-monoethylation of 2,4-pentanedione with ethyl iodide occurs at 100% selectivity and 87% conversion. Similarly, Choud-hary et al. used MgGa-LDHs that were treated with gaseous HCl as catalysts for benzylation of benzene and toluene with benzyl chloride at high conversion (561a,561b). 

With the combination of dimethyl sulfoxide as solvent and NaH as base, di-alkylated derivatives in yields superior to those previously found can be obtained from malononitrile and 2,4-pentanedione with methyl iodide, butyl bromide, or benzyl chloride. Reaction conditions are mild and reaction times short.— E A soln. of malononitrile in dimethyl sulfoxide added during 15 min. to a stirred slurry of NaH in the same solvent, stirring continued 15 min. with occasional cooling, then benzyl chloride added during 20 min. with occasional cooling, and stirred 8.5 hrs. at room temp. dibenzylmalonitrile. Y 64.5-75.9%. F. e. s. J. J. Bloomfield, J. Org. Ghem. 26, 4112 (1961). 

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