Study of the Origin of the Hindered Rotation of an Aryl Ring in Chromium Aminocarbene Complexes Bearing an Aromatic Ring Attached to the Carbene Carbon Atom
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- 作者：Tomá ;
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- 刊名：Organometallics
- 出版年：2006
- 出版时间：November 6, 2006
- 年：2006
- 卷：25
- 期：23
- 页码：5540 - 5548
- 全文大小：177K
- 年卷期：v.25,no.23(November 6, 2006)
- ISSN：1520-6041
文摘
The substituted chromium aminocarbene complexes cis-tetracarbonyl[(
2-N-allyl-N-allylamino)(4-X-phenyl)carbene]chromium(0) (X = MeO, Me, H, Cl, CO2Me, CF3) were prepared by the reaction of thecorresponding N,N-diallylamides with Na2Cr(CO)5 and Me3SiCl. Using dynamic NMR, the rotationalbarrier of the aryl ring in these complexes was established. The obtained values are higher for the complexesbearing electron acceptors compared to those bearing donor substituents and correspond with the Hammettequation. This indicates that the restricted rotation of aryl ring in the chromium aminocarbene complexesis a balance between steric and conjugation effects; while the former increases the barrier, the latterdecreases it by stabilizing the transition state of the rotation. This finding is compatible with the fact thatin the related cis-tetracarbonyl[(N-alkyl-2-N-allylamino)(phenyl)carbene]chromium(0) complexes (alkyl= Me, Et, i-Pr) the rotational barrier grows steadily with the growing steric demand of the substituent.The introduction of the methyl group to the ortho position of the aryl ring resulted in an increase of therotational barrier above 76 kJ mol-1 even for the N-Me derivative.
2-N-allyl-N-allylamino)(4-X-phenyl)carbene]chromium(0) (X = MeO, Me, H, Cl, CO2Me, CF3) were prepared by the reaction of thecorresponding N,N-diallylamides with Na2Cr(CO)5 and Me3SiCl. Using dynamic NMR, the rotationalbarrier of the aryl ring in these complexes was established. The obtained values are higher for the complexesbearing electron acceptors compared to those bearing donor substituents and correspond with the Hammettequation. This indicates that the restricted rotation of aryl ring in the chromium aminocarbene complexesis a balance between steric and conjugation effects; while the former increases the barrier, the latterdecreases it by stabilizing the transition state of the rotation. This finding is compatible with the fact thatin the related cis-tetracarbonyl[(N-alkyl-2-N-allylamino)(phenyl)carbene]chromium(0) complexes (alkyl= Me, Et, i-Pr) the rotational barrier grows steadily with the growing steric demand of the substituent.The introduction of the methyl group to the ortho position of the aryl ring resulted in an increase of therotational barrier above 76 kJ mol-1 even for the N-Me derivative.