Boron complexes 1,3-diketones

The reactivity profiles of the boronate complexes are also diverse.43 For example, the lithium methyl-trialkylboronates (75) are inert, but the more reactive copper(I) methyltrialkylboronates (76) afford conjugate adducts with acrylonitrile and ethyl acrylate (Scheme 16).44 In contrast, the lithium alkynylboronates (77) are alkylated by powerful acceptors, such as alkylideneacetoacetates, alkylidene-malonates and a-nitroethylene, to afford the intermediate vinylboranes (78) to (80), which on oxidation (peracids) or protonolysis yield the corresponding ketones or alkenes, respectively (Scheme 17).45a Similarly, titanium tetrachloride-catalyzed alkynylboronate (77) additions to methyl vinyl ketone afford 1,5-diketones (81).4Sb Mechanistically, the alkynylboronate additions proceed by initial 3-attack of the electrophile and simultaneous alkyl migration from boron to the a-carbon. 

A mixture of an acid anhydride and a ketone is saturated with boron trifluoride this is followed by treatment with aqueous sodium acetate. The quantity of boron trifluoride absorbed usually amounts to 100 mol per cent, (based on total mola of ketone and anhydride). Catalytic amounts of the reagent do not give satisfactory results. This is in line with the observation that the p diketone is produced in the reaction mixture as the boron difluoride complex, some of which have been isolated. A reasonable mechanism of the reaction postulates the conversion of the anhydride into a carbonium ion, such as (I) the ketone into an enol type of complex, such as (II) followed by condensation of (I) and (II) to yield the boron difluoride complex of the p diketone (III) ... 

The boron difluoride coordination complex is decomposed by heating under reflux with an aqueous solution of 2 mols of sodium acetate per mol of anhydride, whereupon the p diketone (acetylacetone) is liberated. 

MetalIa-/3-diketonate complexes, such as 1, are conveniently prepared by reacting acylmetal carbonyl complexes with strong bases that can also react as nucleophiles, such as organolithium, Grignard, or boron hydride reagents [Eq. (1)]. These reactions can be followed by IR spectroscopy. 

Metalla-j3-diketones or metalla-/3-diketonate anions react with trigonal boron compounds, BX2Y, in which X = Y = halogen or X = Cl and Y = Ph, to afford neutral (metalla-/3-diketonato)B(X) (Y) complexes. Complexes of this type are known with mangana-, rhena-, and ferra-/3-diketonato ligands. The earlier chemistry of these complexes has also been previously reviewed (J). 

In this procedure, the ketone is first converted to its enol acetate by reaction with acetic anhydride in the presence of a proton acid. Since this enol acetylation is performed under equilibrating conditions, the more stable enol acetate (usually the more highly substituted isomer) is produced. Acetylation of this enol acetate, catalyzed by the Lewis acid boron trifluoride, usually leads to the formation of the enol acetate of a /3-diketone which is cleaved by boron trifluoride to form acetyl fluoride and the borofluoride complex of the /3-diketone. Thus, this procedure offers a convenient and general synthetic route... 

Substitution of hydrogen by fluorine in compounds with acidic a-hydrogens can be achieved with xenon difluoride and some catalysts. Thus, room-temperature fluorination of 1,3-diketones, e g. 20, with xenon difluoride in the presence of the insoluble cross-linked polysty-rene-4-vinylpyridine, either complexed with boron trifluoridc or Nafion-H, as catalyst, gives mono and difluoro products.27-28... 

Bisphenylhydrazone complexes 50 and 52 of the corresponding polyfluorinated a-diketones with triethylamine or the triethylamine boron trifluoride complex give the pyrazoles 51143 and 53.119... 

The acylating reagent may be an acid chloride or an acid anhydride. Symmetrical ketones (—CH2 R = R2) yield only a single regioisomer. Thus acetone or cyclohexanone may be acylated with acetic anhydride in the presence of boron trifluoride-etherate to pentane-2,4-dione and 2-acetylcyclohexanone respectively (Expt 5.102). Both diketones are present in the reaction mixture as boron difluoride complexes [(10) and (11) respectively], from which they may be released by treatment with sodium acetate. Pentane-2,4-dione is appreciably water soluble and is isolated by means of its characteristic copper complex (12). 

Anthraquinonecarboxamide in its Sj state will photoadd two molecules of water. The photoreactions of diphenylhomobenzoquinones in the presence of amine donors have been studied. 1-Bromo substituted diphenylhomobenzoqui-none irradiated in the presence of triethylamine induces opening of the cyclopropane ring with formation of 2-diphenylmethyl-5-methyl-l,4-benzoquinone, and with dimethylaniline, a mixture of an aminated bicyclic dione and bis(p-dimethyl-aminophenyl)methane is produced. Irradiation of boron difluoride complexes derived from 1,3-diketones can lead to a number of different types of process, in particular exciplex formation and slow cycloaddition. Evidence has now been advanced to suggest that excitation of these complexes leads directly to an exciplex without the participation of an encounter complex, and that this solvolyses to free ions. 

The photostadDility of boron-containing orgamic complexes has been discussed, amd a study maule of the luminescent properties of p-diketonates of disubstituted boron amd of bis-p-diketonatoboron. A novel photochemical alkyl migration from B to C in the dialkyIboryl acetylacetonate complexes (1) ( R-cyclohexyl, Bu, isopinocaunpheyl, R -H R-cyclohexyl, Bu, R -Me ) has appeared. Crossover experiments show that the alkyl migration is essentially intraunolecular Silicon... 

The reagent, classified as a P-diketone dissolves, giving a yellow colour, in methanol, ethanol, acetone, and glacial acetic acid. In acid media, curcumin and boron form a violet-red 2 1 complex called rosocyanin. 

Even unsymmetrically substituted 1,3-diketones (338) are transformed into 1,3-dithiolanes (339) and (340) with boron trifluoride diethyl ether complex as a catalyst, as long as the reaction temperature does not exceed 15°C (Scheme 61) <88T2283>. 2-Oxo-aldehydes (341) react with ethane-... 

The photochemistry of p-diketones is dramatically altered if they are converted to their boron difluoride complexes (58). The reduction potential of the complex is lowered from that of the diketone or its enol so that their excited states can act as electron transfer sensitisers of alkene photochemistry or will form exciplexes with benzene derivatives, leading to the formation of products which are apparently produced by ortho addition to the arene... 

Complementary to the acylation of enolate anions is the acid-catalyzed acylation of the corresponding enols, where the regiochemistry of acylation can vary from that observed in base-catalyzed reactions. Although the reaction has been studied extensively in simple systems, it has not been widely used in the synthesis of complex molecules. The catalysts most frequently employed are boron trifluoride, aluminum chloride and some proton acids, and acid anhydrides are the most frequently used acylating agents. Reaction is thought to involve electrophilic attack on the enol of the ketone by a Lewis acid complex of the anhydride (Scheme 58). In the presence of a proton acid, the enol ester is probably the reactive nucleophile. In either case, the first formed 1,3-dicarbonyl compound is converted into its borofluoride complex, which may be decomposed to give the 3-diketone, sometimes isolated as its copper complex.164... 

Equally smooth is the reduction of aliphatic, alicyclic, and aromatic diketones to diols by the equivalent amount of complex boron hydrides. For instance, excellent yields of, mainly, m so-diols are obtained from acyclic 1,3-diols.389 The preparation of /ra .s-5-hydroxy-4a-methyl-zl1 8a-octalin-2-one from the corresponding dione by NaBN4390 is an example of selective reduction of one keto group of a dione ... 

The bonding of polypyrazolylborates to metal is determined primarily by the number of pyrazolyl groups attached to boron. Dipyrazolylborates are necessarily bidentate, and they form with divalent transition metal ions complexes, 1, that are similar to -diketonates but that are always monomeric for steric reasons. The major difference between dipyrazolylborates and -diketonates is that the R2B(pz)2M ring is not planar but is puckered in the boat form as in Structure 2. This results in asym-... 

The reaction of propargyl alcohols with dicobalt octacarbonyl to give the complex salts 148 (X = BF4 or PF6) and synthetic uses of the latter have been reviewed. The salts react with electron-rich aromatic compounds ArH, such as anisole, phenol or N,N-dimethylaniline, to yield substitution products 149 after oxidative demetallation with an iron(III) or cerium(I V) salt with j5-diketones or j -keto esters the corresponding propargyl-substituted compounds 150 are obtained k Acetone reacts in an analogous fashion to give 151. The action of the cobalt complexes 148 on allylsilanes 152 leads to enynes 153. Indole reacts with the complex 148 (R = H R = R = Me) in the presence of boron trifluoride etherate to give 154, which was converted into 155 by the action of iron(III) nitrate " ... 

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