Dante Macciantelli

Conformational studies by dynamic NMR—251: Stereomutations in trialkylamines

Abstract The trend toward steric acceleration for N-inversion and steric deceleration for C-N rotation has been verified in an homogeneous series of tertiary isopropylamines. Unambiguous evidence has been obtained that in a crowded amine (3-pentyl diisopropylamine) the barrier to rotation becomes higher than that to inversion. It has also been shown that this amine adopts an eclipsed, rather than a staggered dynamic conformation.

TRIS(TRIMETHYLSILYL)SILANE AS MEDIATOR IN THE RADICAL-BASED ALLYLATION REACTIONS OF ALLYL AND 2-FUNCTIONALIZED ALLYL PHENYL SULFONES

Radical allylations with 2-functionalized allyl phenyl sulfones have been performed by using tris(trimethylsilyl)silane as radical mediator. Yields varied from moderate to good depending on the nature of the starting materials. EPR studies on the alkyl radical adducts of allyl sulfones have been performed and conformational information has been obtained for the tert-butyl and tetrahydrofuranyl adducts of 2-carboxy allyl tris(trimethylsilyl)silane.

The electronic effect of positively charged substituents on an aromatic ring: a 13cmr and theoretical study

Abstract The effects of the substituents -N + Me 3 , -CH 2 N + Me 3 and -P + Me 3 on the relative 13 C chemical shifts at meta and para positions accord with the σ r ° values obtained from IR intensities provided the correlation equation of Syrova et al. is used. SCF-MO calculations using a minimal STO-3G basis set suggest that the -N + H 3 substituent acts as a weak π-electron donor and the -P + H 3 substituent as a π-electron acceptor in accord with the σ r ° constants for the methylated poles. In these calculations, the action of -PH 3 + as a π-electron acceptor can be rationalised in terms of an hyperconjugative effect involving the empty π-type group MO, that in -PH 3 + is located at a significantly lower energy than in -NH 3 + . This π-type group MO is formed by suitable combinations of the 3p AOs of P with the 1s AOs of the hydrogens and therefore there is no need to invoke 3d-orbitals participation in -PH 3 + to explain this differential behaviour.

The conformational problem of biphenyl in solution as investigated by the liquid crystal nuclear magnetic resonance spectrum of 3,3′,5,5′-tetrachlorobiphenyl

The interpretation of the 1H n.m.r. spectrum of a meta-substituted biphenyl (3,3′,5,5′-tetrachlorobiphenyl) dissolved in the nematic phase of a liquid crystalline solvent showed unambiguously that in solution it has neither a planar structure nor free rotation but exists in a twisted conformation. A fairly accurate measurement of the twisting angle could also be obtained (34° 20′), the experimental error being similar to that reported in the electron diffraction studies of the vapour phase. The effects of the substituents and of the nematic solvent upon the conformation are discussed and the conclusion is reached that they are rather small and acting in opposite directions. Biphenyl is thus expected to have a similar dihedral angle in solution.

Conformational studies by dynamic nmr. Part 18. Restricted rotation in nitrosobenzene and phenylhydrazine

The barriers to Ph-N rotation in the title compounds have been determined by C-13 NMR. The values are 7.6 and 6.9 kcal mol−1 respectively.

Reactivity of glucosyl radical in the presence of phenols

Abstract Glucosyl radicals from the photoreaction of α-bromo-2,3,4,6-tetra- O -acetylglucose ( ABG ) with hexabutylditin react with phenols. 4-H 3 COC 6 H 4 O• was identified by means of EPR spectroscopy in the case of 4-methoxyphenol, and the corresponding α - O -glucoside was isolated along with 1- and 2-deoxysugars and the dimers of glucosyl radical. The present results are consistent with the formation of α - O -glucosides observed in the electrochemical reaction of ABG and phenols, although in this case the dimers represent the main reaction products.

Synthesis and chemical behaviour of thioacylsilanes (silyl thioketones). Part 1. Oxidation to S-oxides, conversion into silylated thiiranes, silylated triarylethylenes, and α-silylated sulphides

Trimethylsilyl and triphenylsilylaryl thioketones have been synthesized by acid catalysed reaction of the corresponding ketones with hydrogen sulphide. Such silylated thioketones have been oxidized to give S-oxides and converted with diaryldiazomethanes into triaryl thiiranes; upon treatment with organolithium nucleophiles they undergo thiophilic addition. The ready desulphurization with triphenylphosphine of silylated thiirans to the corresponding silyl triaryl ethylenes is also reported.

Conformational studies by dynamic NMR. Part XV. The rotational barrier of N-methyl aniline☆

Abstract Low temperatures C-13 NMR spectra allowed the determination of the rotational barrier of N-methyl aniline in solution. The shifts of the anisochronous ortho and meta carbons were assigned by ab initio calculations.

On the Reactivity of Ferrocenoylsilanes

Reactions of ferrocenoylsilanes 1 with lithium aluminium hydride and organometallic reagents show unusual behaviour in that the ferrocenyl aldehydes 4 and ketones 5−9 are obtained instead of the expected secondary and tertiary silylated alcohols. This paper also reports on the synthesis and reactivity of the first planar chiral enantiomerically pure ferrocenoylsilane 1d that offers the possibility of synthesizing planar chiral enantiomerically pure 1,2-disubstituted aldehyde 4b and ketones 5b, 6b, 7b, 8 and 9. A mechanistic hypothesis is proposed for explaining the obtained results.

Configuration and conformation of acyl derivatives of hydroxylamine. Part 22. Hydroxamoyl chlorides. A revision

The analysis of the i.r. spectra of hydroxamoyl chlorides (Ia–d) under different conditions points to the Z-configuration in solution, i.e. the same configuration as established previously in the crystalline state by X-ray measurement. The previous arguments favouring the E-configuration are thus rebutted. This conclusion is supported by empirical correlations of i.r. frequencies, 1H and 13C n.m.r. chemical shifts, and by a revised analysis of dipole moments taking into account an aliphatic derivative and referring to the O-benzoyl derivatives (II) as model compounds. The original erroneous assignment was caused by the use of less efficient model compounds and by the low sensitivity of the dipole moments of (I) to the configuration.