Jacqueline Morgan

Reaction of arylboronic acids and their derivatives with lead tetra-acetate. The generation of aryl–lead triacetates, and meta- and para-phenylenebis(lead triacetate), in situ for electrophilic arylation

Arylboronic acids and some of their derivatives have been found to undergo a rapid boron–lead exchange with lead tetra-acetate. In the presence of a catalytic amount of mercury(II) acetate, the reaction produces mainly the aryl–lead triacetate, and it has been developed as a convenient method for in situ generation of these useful electrophilic C-arylating reagents. The reaction allows the generation for the first time of meta- and para-phenylenebis(lead triacetate), compounds which behave as meta- and para-phenylene dication equivalents.

Aryl fluoride syntheses involving reaction of aryllead triacetates with boron trifluoride-diethyl ether complex

Abstract Aryllead(IV) triacetates react at room temperature with BF 3 .Et 2 O to give the corresponding aryl fluoride in moderate to good yields; triarylboroxines, electron rich aryltrimethylsilanes and some arenes, which yield aryllead triacetates in acid catalysed reactions with lead tetraacetate, are converted directly into aryl fluorides when stirred with lead tetraacetate in BF 3 .Et 2 O. An investigation of the mechanism of the fluoro-deplumbation reaction indicates that it probably proceeds by acid catalysed heterolytic cleavage of the CPb bond to produce an aryl cation.

Mechanism of arylation of nucleophiles by aryllead triacetates. Part 2. Support for a ligand coupling process and X-ray molecular structure of (p-methoxyphenyl)-α-methylphenacyllead (IV) diacetate

In the presence of boron trifluoride–diethyl ether complex p-methoxyphenyllead triacetate 4 and the propiophenone silyl enol ether 6 were found to undergo a rapid reaction to give (p-methoxyphenyl)-α-methylphenacyllead diacetate 9 in high yield. Analogous products, 5, 10 and 11 respectively, were formed when the silyl enol ethers of acetophenone, butyrophenone and isobutyrophenone were treated under the same conditions with aryllead compound 4. The diorganolead diacetates 9, 10 and 11 were relatively unstable, giving a number of products when heated at 60 °C in chloroform. In each case there was significant elimination of Pb(OAc)2, with formation of the product of ligand coupling, the deoxybenzoin. α-Methylphenacyl(o-prop-2-enyloxy)phenyllead diacetate 21, which was produced to probe the mechanism of the coupling reaction, behaved similarly to compound 9, giving no dihydrobenzofuran derivatives. Therefore, it would appear that aryl free radicals are most probably not produced in these thermal reactions, and a ligand-coupling mechanism is proposed.

Reaction of organolead triacetates with 4-ethoxycarbonyl-2-methyloxazol-5-one. The synthesis of α-aryl and α-vinyl N-acetylglycine ethyl esters and their enzymic resolution

Abstract An efficient synthesis of the moisture-sensitive compound, 4-ethoxycarbonyl-2-methyloxazol-5-one, has been achieved. This compound undergoes high-yielding arylation and vinylation at the 4-position with organolead triacetates to give compounds which in water are converted to α-aryl and α-vinyl N-acetylglycine ethyl esters. These α-substituted glycine derivatives may be kinetically resolved in very good yield and high enantiomeric excess by enzymic hydrolysis of either the ester group or the amide function of the corresponding carboxylic acids.

Mechanism of arylation of nucleophiles by aryllead triacetates. Part 1. Exclusion of a pathway involving aryl free radicals

o-(Prop-2-enyloxy)phenyl lead triacetate 6, which was obtained by treatment of the corresponding boronic acid with lead tetraacetate, has been shown to react with iodide and azide ions, 2,4,6-trimethyl phenol, ethyl 2-oxocyclopentanecarboxylate, and the sodium salt of 2-nitropropane to give only those products which, in a formal sense, are derived by direct nucleophilic displacement of the Pb(OAc)3 group. The complete absence of 3-substituted dihydrobenzofurans among the products is strong evidence that aryl free radicals are not involved in the arylation reactions of aryllead triacetates.

Electrophilic metal-alkyl bond cleavage in tetraorganosilicon and tetraorganotin compounds by lead tetracarboxylates and aryllead tricarboxylates

Abstract Methyl-silicon bond cleavage of tetramethylsilane occurs with aryllead(IV) tricarboxylates in trifluoroacetic acid to produce aryl(methyl)lead(IV) bistrifluoroacetates in high yield. An extension of the study to tetraalkylstannanes has shown that alkyl-tin cleavage by lead(IV) carboxylates proceeds even in chloroform, while a detailed study of the reactions of lead tetraacetate with tetraalkylstannanes indicated the reaction to be an electrophilic cleavage rather than one involving an electron transfer mechanism.

α-Arylation of ketones by aryllead triacetates. Effect of methyland phenyl substitution at the α position

An examination of the α-arylation of a number of ketones and their enolate salts by p-methoxyphenyllead triacetate provides further evidence for a very marked selectivity in the arylation reaction. It is found that the reaction proceeds well at tertiary α-carbons and at secondary centres activated by the presence of a phenyl group, but fails where the secondary centre is unactivated and at primary α-carbons.

Reaction of organolead triacetates with4-ethoxycarbonyl-2-methyl-4,5-dihydro-1,3-oxazol-5-one. The synthesis ofα-aryl- and α-vinyl-N-acetylglycines andtheir ethyl esters and their enzymic resolution

4-Ethoxycarbonyl-2-methyl-4,5-dihydro-1,3-oxazol-5-one 7, which may be readily obtained from diethyl acetamidomalonate, undergoes high-yielding arylation and vinylation at the 4-position with organolead triacetates to give compounds which may be hydrolysed to give either the α-aryl- or α-vinyl-N-acetylglycine or the corresponding ethyl ester. The kinetic resolution of a number of these derivatives by enzymic hydrolysis of either the amide or ester function has been demonstrated.

Mechanism of arylation of nucleophiles by aryllead triacetates. Part 3. Concerning their reaction with phenols and X-ray molecular structure of p-methoxyphenyllead triacetate

(6-Methoxy-2,3,4-trimethylphenyl)phenyllead diacetate 17, the methyl ether of one of the two possible intermediates in the ortho phenylation of 3,4,5-trimethylphenol by phenyllead triacetate, is synthesised and shown to be unchanged in CHCl3–pyridine at 60 °C even after 6 days, thus excluding diaryllead diacetates as intermediates in this supposed ligand coupling process. A study of the arylation of a number of methyl substituted phenols by p-methoxyphenyllead triacetate shows that, in general, the rate of ortho arylation increases with methyl substitution; however, ortho methyl groups have a much greater effect than those in the meta and para positions. These results support the view that the arylation of phenols proceeds by an initial ligand exchange (or ligand exchange and pseudorotation) to produce an intermediate in which π-donation by the aryloxy ligand and electron release to the ortho position(s) are important factors. Phenols which are poor π-donors do not undergo the reaction.

Preparation of diorganolead dicarboxylates from aryllead triacetates: an investigation of ligand coupling in some diorganolead(IV) compounds

Aryllead triacetates have been found to react with arylboronic acids and vinylboronic acids to give diaryllead and aryl(vinyl)lead diacetates, respectively, in high yield. Symmetrical divinyllead diacetates are also readily accessed by reaction between vinylboronic acids and lead tetraacetate. It has been shown that diaryllead, aryl(vinyl)lead, and divinyllead diacetates produced in this way undergo a copper(I)-catalysed coupling to yield biaryls, vinylaromatics and buta-1,3-dienes, respectively, in high yield. With unsymmetrical diorganolead diacetates, it has been found that the coupling is not intramolecular, and thus the three possible products are produced.