Vittorio Rosnati

The reactions of phenolic reagents with α-bromo Michael acceptors in the K2CO3-acetone system. A stereospecific AdSNE process.

Abstract α-bromo Michael acceptors undergo ipso-substitution by phenol or benzenethiol in the K 2 CO 3 -acetone system, the reaction originating the (Z) isomers, via a stereospecific Ad SN E process.

Single electron transfers in zinc-promoted reactions. The mechanisms of the clemmensen reduction and related reactions

Abstract A parallel study of the Clemmensen reaction of benzophenone, 1 , benzhydryl chloride, 2 , and dichlorodiphenylmethane, 3 , suggests new hypotheses about the mechanism of this reduction of diarylketones. Two main pathways have been recognized, both involving ionic and non-ionic reactions. The devised mechanisms provide a tool for interpreting literature data not understood hitherto, concerning Clemmensen-type as well as other zinc-promoted reactions.

Ring opening and ring-chain tautomerism in 2-carbalkoxy- and 2-acyl-2,3-dihydro-1,4-benzodioxins

Abstract The behaviour of benzodioxin esters 1 a-c and ketones 2 a-c under the action of K2CO3 in acetone and of NaH in DMSO has been studied; the experiments were also carried out in the presence of scavengers of phenolic intermediates or products. Under the action of potassium acetonate the esters did not undergo ring opening and ring-chain tautomerism was absent; however, in the presence of 1-chloro-2-(diethylamino)ethane partial ring cleavage occurred, while the tautomerism was evidenced. The esters were easily converted into mixtures of the corresponding lactones 6 a-c and acids 7 a-c by the action of NaH in DMSO. Ketones 2 a and 2 b were stable in the K2CO3/acetone system, but ring-chain tautomerism was shown to be at work in the case of 2 b and 2 c, while the latter also underwent ring cleavage. In the same system, but in the presence of an alkylating agent, the three ketones easily underwent ring opening.

Zinc-promoted reactions. 8. The effect of ring strain in the reduction of 1,2-dibenzoylcycloalkanes

Abstract Ring cleavage was the main route in the Zn reduction of 1 in neat AcOH, while selective carbonyl reduction predominated in the presence of LiCl. The less strained 2 underwent only carbonyl reduction with Zn/AcOH. The Clemmensen reduction of both 1 and 2 resulted mainly in acyclic products. The unstrained 3 was fairly resistant towards reduction, and did not undergo ring cleavage.

Zinc-promoted reactions. The processes of carboncarbon bond formation and cleavage interpreted according to a general mechanism

Abstract Several zinc-promoted reactions of 1,2-, 1,3-, and 1,4-diketones are interpreted according to a general mechanism, involving ionic and nonionic pathways. The mechanism accounts for the preservation of the carbonyl function and the observed carboncarbon bond formation and cleavage.

Nucleophilic substitution of α-halo-ketones. XXIII. Acetolysis of 1-chloro-3-phenoxy-1-phenylthio-2-propanones. An intramolecular trans-acetylation involving phenoxide anion as the leaving group

Abstract The acetolyses of α-chloro-ketones1a-c,2a-c,9a and11a have been investigated parallely. Several aspects of the mechanisms involved in chlorine normal and cine substitution have been elucidated. Intramolecular trans-acetylation, ultimately leading to fragmentation of acetoxy-ketones3b,4a and4c, have been postulated to account for the formation of thiol ester6, aldehyde5a and ketone5c, respectively.

IONIC REACTIONS AND SINGLE ELECTRON TRANSFERS IN THE REDUCTION OF ARYL CARBONYL COMPOUNDS IN THE PRESENCE OF CHLOROTRIMETHYLSILANE

Chlorotrimethylsilane, when used as the solvent, promotes the Zn reduction of aryl carbonyl compounds (simple ketones, esters, as well as α-halo ketones and esters) by reacting as a Lewis acid, its coordination to the carbonyl ultimately resulting in the adduct formation. Ionic reactions, leading to carbenium ions of different types, precede the true reductive process involving single electron transfers from the metal to such electron deficient species. Cyclopropanation was either absent, or a very minor event, when the substrates under investigation were reduced in the presence of a carbene scavenger.

Zinc-promoted reactions. Part 5. The behaviour of alkyl substituted 1,3-diketones

Abstract The zinc-promoted reaction of 2,4-pentanedione and related β-dicarbonyl substrates have been investigated under a variety of conditions. The results were explained according to a general mechanism, involving ionic and nonionic pathways.

Neighboring group participation in the acetolysis of 1,1,1-triaryl-3-diazo-2-propanones. An unprecedented 1,3 shift of an aryl group via a five-membered transition state.

Abstract The acetolysis of diazo ketones 1a,b,c, leads to the corresponding indanones 3a,b,c, and to the rearranged acetates 4a,b,c. The formation of the acetates 4 can be explained in terms of a mechanism involving the same transition state responsible for the ring closure to 3.

Zinc-promoted reactions. Part 11. 1 Ionic reactions and single electron transfers in the Zn/TMSCI reduction of benzaldehyde

The Zn/TMSCI reduction of benzaldehyde in the presence of styrene follows different pathways, depending on the nature of the solvent: 1,2diphenylcyclopropane was formed only in pure TMSCI and in non-polar solvents, while l,2-diphenyl-l,2-trimethylsilyloxye thane, was always the main product in polar solvents. Evidence was obtained indicating that in all cases TMSCI initially reacts as a Lewis acid, its coordination to the carbonyl ultimately resulting in the adduct formation. The true reductive process is then preceded by simple ionic reactions leading to carbenium ions PhCHCI and PhCHOSiMe3, the precursors of the products originating in non-polar and in polar solvents, respectively. Introduction Chlorotrimethylsilane (TMSCI) has been used successfully as a coreagent in several Zn reductions of carbonyl compounds. Thus, for instance, the Zn/TMSCI reduction of alicyclic ketones has been reported to afford the corresponding monomeric alkenes, while 3-ketosteroids and related substrates gave dimeric ketones. Selective desulfurization was obtained in the Zn/TMSCI reduction of a-phenylthioketones, while in the case of sterically hindered a-haloketones reductive dehalogenation resulted in the formation of enol trimethylsilylethers. More interestingly, a practical cyclopropanation method was achieved when the Zn/TMSCI reduction of aromatic aldehydes and α,βunsaturated ketones was performed in the presence of a carbene scavenger under properly selected experimental conditions. In spite of the interest of the above results, the role of TMSCI in such reductions has not been elucidated as yet. We now report the results obtained when benzaldehyde, 1, was reduced under standard conditions in anhydrous solvents, in the presence of styrene as a carbene scavenger and TMSCI rigorously free from HCl. Since 1 is not affected by Zn in the absence of a suitable catalyst, our working hypothesis was that TMSCI might react as a Lewis acid, its coordination to the carbonyl eventually leading to the adduct formation. The following literature data supported our views: a) the Zn reduction of 1 in dry EtjO in the presence of BF3 has been reported to generate phenylcarbene (or a carbenoid species), which could be trapped by an alkene; b) aldehydes are known to react with Me3Sil and TMSCN to give the corresponding carbonyl adduct. Results and Discussion When TMSCI was used as the solvent in the reduction of 1, 1,2diphenylcyclopropane, Ζ was obtained (two steroisomers) in 82% yield, along with a mixture of l-chioro-3-phenylindane, 3, and 1-phenylindene, 4. When the reduction was performed in a series of solvents of different polarity, using a TMSCI/substrate 3:1 molar ratio, the results reported in Table 1 were obtained, As shown, considerable amounts of 2 were formed only in C6H6 and Et20 solution while the reduction in polar solvents always afforded l,2-diphenyl-l,2trimethylsilyloxyethane, 5, as the main product. Also, diphenylacetaldehyde, 6, and