Substituent‐induced chemical shifts (SCS) by the phenyl group in sterically congested styrene derivatives

Pivaloin (2), prepared from ethyl pivalate (1) or from dimethyl oxalate (3), reacts with tert-butyllithium by reduction (45% of 10) and addition (45 % of 9), whereas the corresponding Barbier variant takes a different course. Productive transformations of the a,a,P-tri-tert-butyl glycol 9 lead to a,p,p-tritert-butylethanone (17, overall 4 steps), or a,a,P-tri-tert-butyl-ethene (20, 3 or 4 steps), or a,p,P-tri-tert-butyl-P-hydroxyethanone (19, 3 steps, also by Grignard addition to bipivaloyl 4). Steric congestion is assessed by searches for restricted internal rotation.. The alkene 20 and its epoxide 27 are studied with respect to NMR assignments and chemical degradation.We describe short routes to several a,a,P-tri-tert-butyl compounds which are either new or have been previously prepared only by long or inefficient procedures. Some of their properties are reported to show how intramolecular mobility and chemical reactivity may depend on the overcrowding caused by three tert-butyl ("tBu") groups. A. Preparation and teut-Butylation of PivaloinPivaloin (2) is the common source of all material described in this work but is commercially no longer avail-abler2]. For its preparation from ethyl pivalate (1) and sodium metal, the leading references"] are rather laconic and do not mention the considerable amount (ca. 2.50/) of yellow bipivaloyl (4) generated in the process. We preferred to run this acyloin condensation on large scales in to provide for an easier separation of 4r41 and purification of 2 that yielded at least 52% of colourless pivaloin, with the 'H NMR doublet splitting typical"] of the very clean material. Diketone 4 was certainly formed by a protolytic disproportionation of its radical anion that would have either been present as one of the primary products or perhaps resulted from rapid oxidation[6] of the expectedr71 enediolate 5. Trapping of 5 with chlorotrimethylsilane did not appear yield. 4 Na i 25% o be helpful in view of a much lower reported[7]> 52%

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Sterically Congested Molecules, 17. – Novel Syntheses of α,α,β‐Tri‐tert‐butyl Compounds

Böhrer

Knorr

Böhrer

et al. 1997

Liebigs Ann./Recl.

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Substituent‐induced chemical shifts along CC double bonds

Knorr¹,

Roman²,

Freudenreich³

et al. 1993

Magnetic Reson in Chemistry

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'H and 13C NMR signals were assigned and CH coupling constants ('J, 'J, '4 determined for a series of a-monoand a,a-disubstituted (1,1,3,3-tetramethyl-2-indanylidene)methanes with the following a-substituents: (mesityl),B, n-propyl, phenyl, rerr-butyl-C(=NH), cyano, (rert-butyl),C(OH), pivaloyl, H,N-CO, PhNH-CO, carboxy, nitro, acetoxy, Me,SiO, Me,Si, PhS, PhSMe+, PhSO, PhSO, , bromo and trimethylstannyl. The ' J couplings with the oleíinic proton span the range 124.3-193.7 Hz. Substituent-induced chemicai shifts (SCS) of most of the nuclei with respect to the a-unsubstituted olefin obey simple additivity in the a,a-disubstituted compounds and are very similar to the SCS values along the C=N double bond in the isoelectronic (1,1,3,3-tetramethyl-2-indanylidene)amines within the error limits. The exceptions concern nuclei in the immediate vicinity of the perturbing substituent. A dominant mechanistic contrihution of electric field effects appears likely for the more distant aromatic part of the indanylidene moiety. The chemical shifts of two (2,2,5,5-tetramethylcyclopentyiidene)methanes are shown to be compatible with the SCS parameters from the indanylidene series.

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¹⁷O and¹³C NMR spectra of stable simple enols

Frey¹,

Eventova²,

Rappoport³

et al. 1995

J. Chem. Soc., Perkin Trans. 2

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Substituent‐induced chemical shifts (SCS) by the phenyl group in sterically congested styrene derivatives

Cited by 3 publications

References 21 publications

Sterically Congested Molecules, 17. – Novel Syntheses of α,α,β‐Tri‐tert‐butyl Compounds

Sterically Congested Molecules, 17. – Novel Syntheses of α,α,β‐Tri‐tert‐butyl Compounds

Substituent‐induced chemical shifts along CC double bonds

¹⁷O and¹³C NMR spectra of stable simple enols

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Substituent‐induced chemical shifts (SCS) by the phenyl group in sterically congested styrene derivatives

Cited by 3 publications

References 21 publications

Sterically Congested Molecules, 17. – Novel Syntheses of α,α,β‐Tri‐tert‐butyl Compounds

Sterically Congested Molecules, 17. – Novel Syntheses of α,α,β‐Tri‐tert‐butyl Compounds

Substituent‐induced chemical shifts along CC double bonds

17O and13C NMR spectra of stable simple enols

Contact Info

Product

Resources

About

¹⁷O and¹³C NMR spectra of stable simple enols