Boron difluoride

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. 

Recently, novel polymethine carbonyl-dyes based on coumarin moiety and their boron difluoride complexes 9a-d and lOa-d [34—36] were evaluated as fluorescent dyes for the detection of native proteins using bovine serum albumin (BSA) as a model protein, and as probes for the nonspecific detection of proteins using a BSA/ sodium dodecyl sulfate (SDS) mixture [37]. Optical properties of these compounds in the absence and presence of BSA, as well as in SDS and BSA/SDS mixture, were measured in Tris-HCl buffer (pH 8.0) (Table 1). 

Studies of fluorescence properties of the dye pair (i.e., boron difluoride complexes dye 9a-d and the nonsubstituted one lOa-d) in the BSA/SDS mixture revealed that coumarins 9b and 10c showed two excitation and emission bands, while other heterocycles showed single bands in the corresponding spectra. For all studied compounds, excitation and emission maxima occurred in the range 405-603 nm and 467-680 nm, respectively. 

This rearrangement has been generalized to also include mangana- and rhena-/3-diketonato boron difluoride complexes [Eq. (8)] (45). A diagnostic indication of the formation of allyl products 36 is the observation of the... 

Boron trifluoride-mediated acylation of pyrrolidine-2,4-dione 68 led to the isolation of the neutral boron difluoride complex 2, which was sufficiently stable to be purified by column chromatography (Equation 18). This compound has been characterized crystallographically as noted above <1990J(P1)1959>. 

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). 

Chow, Y.L., Cheng, X., Wang, S.S., and Wu, S.P. (1997) Further examples of photocycloadditions of 1,3-diketonato-boron difluorides to olefins a description of the reaction pattern and mechanism. Canadian Journal of Chemistry, 75, 720-726. 

In order to illustrate the Molecular symmetry Group Theory, let us consider the methyl boron difluoride molecule CH3 — BF2), which contains a nearly free rotating methyl group. We shall see next that the torsional levels of this molecule can be classified according to the irreducible representations of the symmetry point group Csv, although this molecule does not possess, in a random configuration, any symmetry at all. 

Fig. 2. The twelve isoenergetic configurations and non-rigid operations corresponding to the internal rotation in methyl boron difluoride.

In sections 2 and 5., we have already considered linear molecules boron difluoride methyl and distorded methylhalide. Let us consider next a more symmetric linear molecule bearing two equivalent rotors the ethane molecule. 

Rapid saturation of the reaction mixture with BF3 leads to the formation of the product derived from the kinetic enol in the presence of a protic acid, the product formed from the thermodynamic enol can be obtained exclusively. It is assumed that in the first case the nucleophile is the boron complex of the enol, whereas an enol ester is involved in the latter case (Eq. 69). In both sequences the final product is the boron difluoride complex. 

Open-chain Systems - Chow and his coworkers have carried out further studies on the cycloaddition reactions of dibenzoyl(methanato)boron difluoride (1) to alkenes and dienes. This present work has examined the mechanistic details of the process. Previously they have reported the addition of the same reactant to enol ethers. The influence of the chiral centre in the alkene (2) on the cycloaddition of a,P-unsaturated nitriles such as acrylonitrile has been evaluated. The products obtained from these (2+2)-photoaddition reactions are the azabicyclooctanes (3)." ... 

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 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... 

In a series of papers on the synthesis of benzopyrone derivatives, benzyl protection-deprotection of phenolic hydroxy groups has been a standard protocol. It should be noted that in this series the yields of the debenzylated products are poor unless the boron difluoride complex is preformed. The example (eq 5) shown is part of a fulvic acid synthesis. ... 

Dicarbonyl compounds can be specifically reduced to a/ -unsaturated ketones via the corresponding boron difluoride complexes, as exemplified in Scheme 89. 

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