- 1,4-Bis-Dipp/Mes-1,2,4-Triazolylidenes: Carbene Catalysts That Efficiently Overcome Steric Hindrance in the Redox Esterification of α- And β-Substituted α,β-Enals
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As reported by Scheidt and Bode in 2005, sterically nonencumbered α,β-enals are readily converted to saturated esters in the presence of alcohols and N-heterocyclic carbene catalysts, e.g., benzimidazolylidenes or triazolylidenes. However, substituents at the α- or β-position of the α,β-enal substrate are typically not tolerated, thus severely limiting the substrate spectrum. On the basis of our earlier mechanistic studies, a set of N-Mes- or N-Dipp-substituted 1,2,4-triazolium salts were synthesized and evaluated as (pre)catalysts in the redox esterification of various α- or β-substituted enals. In particular the 1,4-bis-Mes/Dipp-1,2,4-triazolylidenes overcome the above limitations and efficiently catalyze the redox esterification of a whole series of α/β-substituted enals hitherto not amenable to NHC-catalyzed transformations. The synthetic value of 1,4-bis-Mes/Dipp-1,2,4-triazolylidenes is further demonstrated by the one-step bicyclization of 10-oxocitral to (racemic) nepetalactone in diastereomerically pure form.
- Yatham, Veera Reddy,Harnying, Wacharee,Kootz, Darius,Neud?rfl, J?rg-M.,Schl?rer, Nils E.,Berkessel, Albrecht
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supporting information
p. 2670 - 2677
(2016/03/12)
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- Regioselectivity of the ortho- and para-semidine, and diphenyline rearrangements
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The regioselectivity of the o-semidine, p-semidine, and diphenyline rearrangements of unsymmetrical N,N′-diarylhydrazines was studied experimentally. The results indicate that their electron-rich nitrogen atom is first protonated and then the electron-poor non-protonated nitrogen atom undergoes an N[1,3]-sigmatropic shift to the ortho-position of the electron-rich aryl rings, generating key intermediates. The intermediates can undergo (1) a direct proton transfer to give o-semidines, (2) a second N[1,3]-shift of the electron-poor nitrogen atom and then proton transfer to furnish p-semidines, and (3) a [3,3]-sigmatropic shift and subsequent proton transfer to yield diphenylines. It is the first N[1,3]-sigmatropic shift step that plays an important role in controlling the regioselectivity in the three rearrangements, further determining the structures of o-semidines, p-semidines, and diphenylines. The current results provide new insights into the o/p-semidine and diphenyline rearrangements and useful information for controlling and predicting the structures of the rearrangement products.
- Yang, Zhanhui,Hou, Shili,He, Wei,Cheng, Baoxiang,Jiao, Peng,Xu, Jiaxi
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p. 2186 - 2195
(2016/04/09)
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- α-Aroyloxyaldehydes: Scope and limitations as alternatives to α-haloaldehydes for NHC-catalysed redox transformations
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α-Aroyloxyaldehydes are readily prepared bench stable synthetic intermediates. Their ability to act as α-haloaldehyde surrogates for NHC-promoted redox esterifications and in [4+2] cycloadditions is described.
- Ling, Kenneth B.,Smith, Andrew D.
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supporting information; experimental part
p. 373 - 375
(2011/02/24)
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- ENANTIOSELECTVE REACTIONS CATALYZED BY CHIRAL TRIAZOLIUM SALTS
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This invention provides a convenient method for converting imines and other electrophiles into heterocyclic ring systems. The process does not require the use of metallic reagents, and is catalyzed by an organic heterocyclic carbene catalyst. Accordingly,
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Page/Page column 54-55
(2010/11/28)
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