Base catalysed rearrangements involving ylide intermediates. Part 14. Rearrangements of 4-oxidoanilinium ylides and 3-oxidoanilinium betaines

Abstract

The N-allyl-4-oxidoanilinium ylides (1 la+) undergo a [3,2] sigmatropic rearrangement at 0 "C to give initially the products (9a-c), which undergo a further [3,3] rearrangement to give the isolated reaction products (8a-c). The N-(3,3-dimethylallyl)-4-oxidoanilinium ylide (1 1 d) reacts at 0 'C by the alternative pathway involving homolysis and radical pair recombination to give a mixture of the products (8d) and (1 3d). The ylides (1 5a-c) rearrange by a similar radical pair mechanism. The N-pentadienyl-4-oxidoanilinium ylides derived from the salts (24a and b) also rearrange at 0 'C by a sequence of [3,2] and [3,3] sigmatropic rearrangements to give in each case the product (26b). The N-allyl-3-oxidoanilinium betaines (29a and b) undergo [3,3] rearrangements on heating at 40 "C to give mixtures of the products (32) and (33) whereas the N-pentadienyl-3-oxidoanilinium ylide (30) gives only the product (35) of a [5,5] rearrangement followed by aromatisation.

Base catalysed rearrangements involving ylide intermediates. Part 15. The mechanism of the Stevens [1,2] rearrangement

“…The important experimental facts, established prior to the investigation reported in this paper, are (i) the rearrangement is intramolecular,' (ii) for a chiral migrating group R (for example R = CHMePh) the product is formed with almost complete retention of the configuration of R,8-10 and (iii) in suitable cases chirality can be transferred from nitrogen to carbon. Two principal mechanisms can be envisaged for the reaction: (a) a concerted [1,2] sigmatropic rearrangement,12 with retention of configuration of the migrating group and simultaneous bond breaking and making, or (b) a process in which bond breaking precedes bond making which must, therefore, involve initial homolysis to give a radical pair 9*13*14 or heterolysis to give an ion pair 15*16 followed by rapid pair recombination. The first possibility, a concerted mechanism, requires a process with suprafacial use of the n-bonding between atoms X and Y in the transition state and would be symmetry forbidden by the Woodward-Hoffmann rules.…”

“…We chose as reaction probes (a) the structures of reaction products additional to the rearrangement product (5), (b) the intramolecularity of the rearrangement, and (c) the stereoselectivity of the rearrangement of a chiral ylide (4b) -(5b). The investigation was also conducted in the light of a number of reports 9 9 1 4 * 2 0 * 2 1 of the observation of CIDNP during Stevens rearrangements and other related [1,2] anionic rearrangements. A further method of investigation using ambident migrating groups has been described in preliminary…”

“…75-82 "C, which could be stored at 0 "C for one year without noticeable decomposition. Thermal rearrangement of the ylide (4a) in chloroform at 60 "C gave the [1,2] rearrangement product (5a) as the major product together with 1,2-diphenylethane, the diastereoisomeric diamines (6), and N,N-dimethylbenzylamine. A similar set of products was obtained under a variety of reaction conditions in yields that depended upon the conditions used (see Table 6, Experimental section) and these products accounted for 1 9 5 % of the total reaction product.…”

“…The reaction was therefore investigated by treating the ammohiurn salt (3b) with base. In methanol at 55 "C, using sodium methoxide as the base, the salt (3b) gave the [1,2] rearrangement product (5b) as the major product as a mixture of both diastereoisomers (1 : 1 ratio), together with small quantities of both diastereoisomers of 2,3-diphenylbutane (1 : 1 ratio), and diastereoisomers A and B of the diamine (6) (3 : 1 ratio). The rearrangement of the salt (3b) in water at 0 "C, using sodium hydroxide as the base, gave the [1,2] rearrangement product (5b) as a mixture of diastereoisomers (5 : 4 ratio) and small quantities of 1-phenylethanol and both diastereoisomers of the acyloin (8) (5 : 3 ratio).…”

“…In methanol at 55 "C, using sodium methoxide as the base, the salt (3b) gave the [1,2] rearrangement product (5b) as the major product as a mixture of both diastereoisomers (1 : 1 ratio), together with small quantities of both diastereoisomers of 2,3-diphenylbutane (1 : 1 ratio), and diastereoisomers A and B of the diamine (6) (3 : 1 ratio). The rearrangement of the salt (3b) in water at 0 "C, using sodium hydroxide as the base, gave the [1,2] rearrangement product (5b) as a mixture of diastereoisomers (5 : 4 ratio) and small quantities of 1-phenylethanol and both diastereoisomers of the acyloin (8) (5 : 3 ratio). The origin of (8) was not clear from this work, but subsequent work 23a showed that hydroxyketones may be formed during Stekens rearrangements in aqueous solution by a competing [1,3] rearrangement of the ylide (4b) to give a betaine (9).…”