Abstract

A comparison of externally coordinated arylallyllithiums with internally coordinated arylallyllithiums using a combination of X-ray and NMR studies shows that 2-bis(2-methoxyethyl)aminomethyl-1-phenylallyllithium 7, and 2-bis(2-methoxyethyl)aminomethyl-1,3-diphenylallyllithi um 8, are indeed fully internally coordinated with all phenyls endo, and lithium is close to one terminal allyl carbon. By contrast, among the externally coordinated analogs 1-phenylallyl-lithium.(HMPT)4 5, and 1-phenylallyllithium.(THF) 6, both phenyls are exo and the proximity of lithium to anion was only detected in compound 6. Lithium-7 NMR clearly identifies the Li(HMPT)4+ complex in 5, whereas a 7Li{1H} HOESY experiment reveals 7Li to be coordinated to THF in 6 and close to the PhC terminal allyl carbon. Carbon-13 NMR shows that all of the above compounds are fully delocalized despite differences among the sites of lithium and their separations from the anions. Changes in the 13C NMR line shapes show that the diphenyl compound 8 undergoes a very fast 1,3-lithium sigmatropic shift, and all of the phenyls in the above compounds undergo fast rotation around their phenyl Cipso-Callyl bonds. Barriers to rotation, DeltaH from NMR line shapes for 5, 6, 7, and 8 respectively, are 19.8, 14.6, 10.2, and 8.9 kcal x mol-1. The decrease in barriers is clearly correlated with a decrease in separation of lithium from an allyl terminal carbon and implies that especially among the latter three, 6, 7, and 8, lithium is involved in the mechanism for phenyl rotation. This question is discussed.

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call

Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.