Aza-enamine—III

Brehme, R.; Nikolajewski, H. E.

doi:10.1016/s0040-4020(01)93798-6

Cited by 34 publications

(19 citation statements)

References 9 publications

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“…The strong electron-acceptor substituent R = NO 2 in hydrazones 19 increases the contribution of dipolar structure A and so favors umpolung, that is, the aza-enamine reactivity gives compounds 20ЈЈЈa-e (Scheme 4). The degree of the n-π interaction in dependence of the substituents NAlk 2 and R corresponding to canonical structures A and B parallels with the 1 H, [30] 13 C, [13,28] and 15 N [31] NMR shifts of the azomethine function and the λ max values [23,30] of the hydrazones 19.…”

Section: Reactions With Sulfonyl Isocyanatesmentioning

confidence: 59%

“…[18] Whereas benzaldehyde monophenylhydrazone 5 provided N-formyl product 10 [12] (Scheme 1), benzaldehyde N,N-dialkylhydrazones 15a-j with the pyrrolidino, the dimethylamino, and the dicyclohexylamino functionalities as the amino moiety were Ϫ according to the aza-enamine concept and in analogy to enamines [18,19] Ϫ attacked at the azomethine carbon, leading via triaza-pentadienium salts 16 and after hydrolysis to 2-phenylglyoxal hydrazones 17a-j [13,20,21] (Scheme 3).…”

Section: Formylations Of Aldehyde Nn-dialkylhydrazones With the Vilsmentioning

confidence: 99%

“…[22] In contrast, the pyrrolidino, piperidino, and morpholino hydrazones of the furan-2-carbaldehyde and of the aliphatic aldehydes did not furnish isolable compounds in the Vilsmeier reaction. [16,20] The benzaldehyde morpholino and piperidino hydrazones formed the corresponding phenylglyoxal hydrazones only in small amounts (1-25 %). Their reactions were not worked up, but the compounds were isolated and determined as phenylglyoxal bis (4-nitrophenylhydrazones).…”

Section: Formylations Of Aldehyde Nn-dialkylhydrazones With the Vilsmentioning

confidence: 99%

“…[15] It turned out that analogous reactions also take place with aldehyde hydrazones containing such tertiary electron-donating amino groups instead of the secondary amino ligands NHR [1] in 5. [13,16] Aldehyde N,N-dialkylhydrazones 13, [16,17] especially those with pyrrolidine as the amino group, react with electrophiles to compounds 14 at the azomethine carbon, which corresponds to the enamino β-carbon and thus can indeed be regarded as "aza-enamines". [13,14] Despite the higher electronegativity of the imine nitrogen relative to the α-carbon of enamines, these hydrazones have proved to be important reagents for umpolung, which are in contrast to basic anionic types 1, 2, 3, and 6 neutral equivalents of the acyl anion d 1 -synthon 4.…”

Section: Introductionmentioning

confidence: 99%

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Aldehyde N,N‐Dialkylhydrazones as Neutral Acyl Anion Equivalents: Umpolung of the Imine Reactivity

et al. 2007

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The isoelectronic replacement of the α‐carbon of enamines by nitrogen leads to the corresponding hydrazones, and thus, they can be termed “aza‐enamines”. Hence, aldehyde N,N‐dialkylhydrazones enable analogous electrophilic substitutions at the imine carbon that corresponds to the β‐carbon of enamines. In addition, after hydrolysis of the product hydrazones to the parent ketones or aldehydes, these reactions constitute an umpolung of the classical carbonyl reactivity and overall a nucleophilic acylation or formylation. This review covers the aza‐enamine chemistry from its very beginning in the late 1960s up to this year. In the first part, reactions of aromatic, aliphatic, and heterocyclic aldehyde N,N‐dialkylhydrazones with highly reactive substrates such as the Vilsmeier and Mannich reagents, sulfonyl isocyanates, perfluoroacetic anhydride, and inorganic electrophiles such as halogens and phosphorus tribromide are described. The hydrazones of α,β‐unsaturated aldehydes react as vinylogous aza‐enamines at the terminal carbon providing unsaturated aldehydes. As electron‐rich dienes and dienophiles, they also form Diels–Alder adducts and thus interesting N‐heterocycles. The second part covers carbon–carbon bond formations of the sterically less‐demanding formaldehyde N,N‐dialkylhydrazones with synthetically very useful electrophiles such as various Michael acceptors and carbonyl compounds. Formaldehyde SAMP‐hydrazone and related derivatives generally give excellent asymmetric inductions. Finally, first organocatalytic versions of the aza‐enamine chemistry are presented. In summary, the rich chemistry of aldehyde N,N‐dialkylhydrazones as neutral acyl anion, formyl anion, and cyanide equivalents is demonstrated. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

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Section: Reactions With Sulfonyl Isocyanatesmentioning

confidence: 59%

Section: Formylations Of Aldehyde Nn-dialkylhydrazones With the Vilsmentioning

confidence: 99%

Section: Formylations Of Aldehyde Nn-dialkylhydrazones With the Vilsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Aldehyde N,N‐Dialkylhydrazones as Neutral Acyl Anion Equivalents: Umpolung of the Imine Reactivity

et al. 2007

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show abstract

“…Hydrazones of aldehydes derived from the most active π-excessive five-membered heterocycles (pyrrole, furan, thiophene), however, enter into trifluoroacetylation, [6] aminomethylation, [7] and reactions with electron-deficient unsaturated compounds [8] preferably at the 5-position of the ring. We attribute a certain degree of regioselectivity to the fact that the highly electron-donating (dialkylhydrazono)methyl group is efficiently conjugated with the ring, thus activating it to a sufficient degree to enable phosphorylation [9] and trinitrophenylation [10] (i.e., electrophilic substitutions with low-reactivity electrophiles).…”

Section: Introductionmentioning

confidence: 99%