Geltungsbereich und Mechamismus der durch Silberionen katalysierten Propargylester‐Allenylester‐Umlagerung nach Saucy und Marbet
Authors: Schlossarczyk Horst , Sieber Willi , Hesse Manfred , Hansen Hans‐Jürgen , Schmid Hans 
Published: December 2004

The mechanism of the catalysis of the reversible (propargyl ester)/(allenyl ester) rearrangement (10 ⇄ 11) by silver(I) ions was investigated, using optically active and diastereoisomeric esters as well as 14C‐ and 18O‐labelling. magnified image

In order to work with crystalline materials, mainly p‐nitrobenzoates (10, 11: R4 = pO2NC6H4) were used. In some cases the rearrangement 10 ⇄ 11 was studied using acetates (R4 = CH3). The alkyl substituents R1, R2, R3, were widely varied (cf. Tables 1, 2). The solvents in which the rearrangements were performed were in most cases dry chlorobenzene and 96% aqueous dioxane. Silver tetrafluoroborate, the benzene complex of the latter, and silver trifluoroacetate (in chlorobenzene) as well as silver nitrate (in aqueous dioxane) served as catalysts. The amounts of the silver catalysts used varied between 0,5 and 10 mol‐%; reaction temperatures applied were in the range 35–95°, The results obtained are as follows:

The rate‐determining step of the (propargyl ester)/(allenyl ester) rearrangement (10 ⇄ 11) occurs in a silver(I) complex with the substrates (10, 11), which is formed in a pre‐equilibrium. This follows from kinetic experiments (cf. Fig. 6, 7, 8, 10) and the fact that the rate of rearrangement (of 10a) is strongly decreased when cyclohexene is added (cf. Fig. 9). In solvents which are known to form complexes with silver(I) ions the rate of rearrangement (of 10a)is much slower than in solvents with similar dielectric constants but with small capacity for complex formation with silver(I) ions (cf. Table 4). Taking into account what is known about silver(I)‐alkene and ‐alkyne complexes (cf. [18]), it is obvious that the (propargyl ester)/(allenyl ester) rearrangement occurs in a π‐complex of the silver(I) ion with the triple bond in the propargyl ester or one of the two C,C double bonds in the allenyl ester, respectively.

The shift of the carboxyl moiety in the reversible rearrangement 10 ⇄ 11 occurs intramolecularly. p‐Nitrobenzoic acid‐[carboxyl‐14C] is not incorporated during the rearrangement, neither in the reactant 10 nor in the product 11 and vice versa. A crossing experiment gave no mixed products (cf. Scheme 2, p. 882).

An internal ion pair can be excluded for the rearrangement 10 ⇄ 11 because the 18O‐carbonyl label in the reactant is found exclusively in the alkoxy part of the product (cf. Scheme 3, p. 886, and Table 9). Thus, the rearrangement 10 ⇄ 11 occurs with inversion of the carboxyl moiety.

The rearrangement of optically active propargyl esters (10g, 10i) leads to completely racemic allenyl esters (11g, 11i). However, rearrangement of erythro‐ and threo‐10j‐[carbonyl‐18O] (Scheme 3) shows that the stereospecifically formed allenyl esters erythro‐ and threo‐11j‐[18O]‐epimerize rapidly in the presence of silver(I) ions. This epimerization is twice and forty times, respectively, as fast as the rearrangement of the corresponding propargyl esters (cf. Fig. 1–5). During epimerization or racemization the 18O‐label is not randomized (cf. also Scheme 4, p. 898).

The equilibrium of the rearrangement 10 ⇄ 11 depends on the bulkiness of the substituents R1, R2, R3 and of the carboxyl moiety (cf. Table 2).

Taking into account these facts (points 1–5), the reversible (propargyl ester)/(allenyl ester) rearrangement promoted by silver(I) ions can be described as a [3s, 3s]‐sigmatropic reaction occurring in a silver(I)‐π‐complex with the C,C triple bond in 10 and a C,C double bond in 11. It is suggested that complex formation in 10 and 11 occurs with the π‐bond which is not involved in the quasicyclic (containing six orbitals and six electrons) transition state of the rearrangement (Fig. 11). Thus, the rearrangement is of a type which has recently been called a charge‐induced sigmatropic reaction (cf. [26]). Therefore, in our case, the catalysis by silver(I) ions is of a different type from that of transformations of strained cyclic molecules promoted by silver(I) ions (cf. [14] [16] [27]–[31]).

Side reactions. Whereas the rearrangement of propargyl esters 10 in presence of silver tetra‐ fluoroborate in chlorobenzene or silver nitrate in aqueous dioxane leads to the corresponding allenyl esters 11, the rearrangement of 10 with silver trifluoroacetate, especially in the presence of trifluoroacetic acid, results in the formation of the dienol esters 12 and 13, which clearly are derived from 11 (see Scheme 1, p. 881). As shown by the rearrangement of 11 in the presence of p‐nitrobenzoic acid‐[carboxyl‐14C], 12 and 13 arise in part from a not isolated di‐p‐nitrobenzoate (cf. Scheme 6, p. 905), since radioactivity is found in 12 and 13.