Studies in claisen rearrangement - novel thermal and mercuric trifluroacetate induced transformations of 2,4-di(N-aryl)amino-1,3,5-triazin-6yl-prop-2-y
01 Jan 1985-Tetrahedron (Pergamon)-Vol. 41, Iss: 1, pp 111-120
TL;DR: The thermal rearrangement of 2,4-di(N-aryl)amino-1,3,5-triazin-6yl-prop-2-ynyl ethers 1 yield a mixture of 6-methyleneimidazo(1,2-a)-1, 3, 5-triazine-4-one 6 and 6-methylimidazo (1, 2-a) 1, 3.5-trifluroacetate 4-one 7 at room temperature, whereas under the influence of mercuric trif
Abstract: The thermal rearrangement of 2,4-di(N-aryl)amino-1,3,5-triazin-6yl-prop-2-ynyl ethers 1 yield a mixture of 6-methyleneimidazo(1,2-a)-1,3,5-triazine-4-one 6 and 6-methylimidazo(1,2-a)-1,3,5-triazine-4-one 7 , whereas under the influence of mercuric trifluroacetate the ethers 1 yield only 6 , at room temperature. Mechanisms invoking [3,3] sigmatropic rearrangement of ethers 1 were proposed to account for the product formation.
TL;DR: The Claisen rearrangement has been widely used in the synthesis of natural heterocyclic products as discussed by the authors, and there are now several alkaloid syntheses in which the Claisen re-arrangement features as a key step.
Abstract: Publisher Summary The Claisen rearrangement of heteroaromatic allyl ethers shows many similarities to the rearrangement of simple benzenoid aromatic allyl ethers. Mechanistically, the reactions are for the most part genuine [3,3]-sigmatropic rearrangements, although the presence of nitrogen in the aromatic ring does allow alternative pathways to operate, and hence more than one product may be formed. However, these alternative products are usually easily rationalized on mechanistic grounds. One other difference between the Claisen rearrangement of heteroaromatic ally1 ethers (or thioethers) and their benzene counterparts is that the rearrangement product is often more stable in the one form than as the hydroxy (or mercapto) heteroaromatic compound. The Claisen rearrangement has been widely used in the synthesis of natural heterocyclic products. Although much of this work has been in the coumarin series, there are now several alkaloid syntheses in which the Claisen rearrangement features as a key step. Indications are that the Claisen rearrangement continues to hold an important role in organic chemistry.
TL;DR: A new method for the synthesis of dihydroimidazo[1,2-a][1,3,5]triazin-4(6H)-ones via copper(I)-catalyzed hydroamination was developed and for the first time, iodoalkynes were shown to participate in the copper( I)-catalystzed intramolecular hydroamination reaction with exclusive formation of E-isomers.
Abstract: A new method for the synthesis of dihydroimidazo[1,2-a][1,3,5]triazin-4(6H)-ones via copper(I)-catalyzed hydroamination was developed. In addition, for the first time, iodoalkynes were shown to participate in the copper(I)-catalyzed intramolecular hydroamination reaction with exclusive formation of E-isomers.
TL;DR: The mechanism of the catalysis of the reversible (propargyl ester)/(allenyl esters) rearrangement (10 ⇄ 11) by silver(I) ions was investigated, using optically active and diastereoisomeric esters as well as 14C- and 18O-labelling as discussed by the authors.
Abstract: 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. In order to work with crystalline materials, mainly p-nitrobenzoates (10, 11: R4 = pO2NC6H4) 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: 1 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. ), 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. 2 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). 3 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. 4 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). 5 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. ). 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.   –). 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.
TL;DR: It is known that propargyl-phenylethers rearrange at about 200° to 2 H-chromenes [1, 4] as mentioned in this paper, and this rearrangement occurs in benzene or chloroform at lower temperatures (20-80°) in the presence of silver-tetrafluoroborate (or-trifluoracetate).
Abstract: It is known that propargyl-phenylethers rearrange at about 200° to 2 H-chromenes [1–4]. It is shown that this rearrangement occurs in benzene or chloroform at lower temperatures (20–80°) in the presence of silver-tetrafluoroborate (or-trifluoracetate). The ethers examined are presented in Scheme 1. Thus in chloroform at 61° in the presence of AgBF4, phenyl-propargylether (3) yields 2 H-chromene (13). With 0.78 molar equivalents AgBF4 in benzene at 80° the same ether 3 yields a 3:1 mixture of 2-methyl-cumaron (14) and 2 H-chromene (13). From 1′-methylpropargyl-phenylether (4) and 2′-butinyl-3,5-dimethylphenylether (5) under similar conditions the corresponding chromenes 16 and 17 resp. are obtained. Rearrangement of propargyl- and 2′-butinyl-1-methyl-2-naphthylether (6 and 7 resp.) in benzene at 80° in the presence of AgBF4 gives the corresponding allenyl-naphthalenones 18 and 19 resp. Treatment of propargyl- and 2′-butinyl-mesityl-ether (8 and 9 resp.), and propargyl- and l′-methylpropargyl-2,6-dimethyl-phenylether (10 and 11 resp.) in benzene at 80° with AgRF, yields as the only product the corresponding 3-allenyl-phenols 21, 22,24 and 25 (Scheme 3). It is shown that in the presence of μ-dichlor-dirhodiuni (1)-tetracarbonyl in benzene a t 80° the ether 4 rearranges to 2-methyl-2H-chromene (16). However with this catalyst the predominant reaction is a cleavage to phenol. No reaction was observed when ethers 3 and 12, (Scheme 7 ) were treated with the tris-(trimethylsily1)-ester of vanadic acid in benzene a t 80° (see also ). By analogy with the known mechanism for thc silver catalysis of the reversible propargylesterl/allenylester rearrangement [S], the silver (1)ion is assumed to form a pre-equilibrium π-complex with the C, C-triplebond of the substrate. This complex then undergoes a [3s, 3s]-sigmatropic rearrangement (Scheme 2). In the case of the others 6, 7 and 12 the resulting allenyldienones were isolated. The 2,G-dimethyl substituted ethers 8, 9, 10 and 11 resp. first give the usual allenyl- dienones (Scheme 3). These then undergo a novel silver catalysed dienon-phenol-rearrangement (Sclzenzu4) to give the 3-allenylphenols 21, 22, 24 and 25. Thc others 3, 4 and 5 with free ortho positions presumably rearrange first to the non-isolated 2-allenyl-phenols 15, 42 and 43 resp.(Scheme 7). These then rearrange, either thermally or by silver (1)ion catalysis to the 2H-chromenes 13,16 and 17 resp. The rate of the rearrangement of 2-allenylphenol (15) to 13 at room temperature in benzene or chloroform is approximately doubled when silver ions are present as catalyst.