Steric interplay between alkyl groups bonded to planar frameworks

Berg, Ulf; Liljefors, Tommy; Roussel, Christian; Sandstroem, J.

doi:10.1021/ar00111a003

Cited by 77 publications

(38 citation statements)

References 54 publications

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“…With regard to conformational effects in the sidechains themselves, it may well be that proton loss from methyl changes the conformation from one that is gear-clashed methyl-methyl (16,17) to one that is gear-meshed (or gearlocked) methyl-methylene. The possibility that this effect is favourable, together with a consideration of polarizability effects and hybridization changes at nitrogen, will be addressed in a subsequent publication.…”

Section: Resultsmentioning

confidence: 99%

Synthesis, base strength, and prototropic behaviour of a number of alkyllumazines

Stewart¹,

Gumbley²

1985

Can. J. Chem.

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A number of lumazines and 5-deazalumazines containing a methyl group at C-7 have been prepared, their pKBH+ values determined, and measurements made of the rates at which the hydrogen atoms of their 7-methyl groups undergo isotopic exchange in aqueous sulfuric acid. The presence of an alkyl group at N-8 in the protonated forms of these compounds activates the neighbouring methyl group at C-7; the effect is considerably larger than that previously observed for a methyl group at C-6, which is the other neighbouring position. The comparison of methyl and hydrogen at N-8 can be made only for the acid-catalyzed reaction because the structures of the neutral compounds, which take part in the base-catalyzed reaction, are not analogous.Ross STEWART et S. J. GUMBLEY. Can. J. Chem. 63, 3290 (1985) On a prkpark un certain nombre de lumazines et de dCaza-5 lumazines contenant un groupement mkthyle en position C-7; de plus, on a determink les valeurs de leurs pKBH+ et, opkrant dans de I'acide sulfurique aqueux, on a mesure les vitesses d'kchange isotopique des atomes d'hydrogenes de leurs groupements mkthyles en C-7. La prksence d'un groupement alkyle dans la position N-8 des formes protonkes de ces composCs active le groupement mCthyle voisin en C-7; cet effet est beaucoup plus important que celui observk antkrieurement pour un groupement mCthyle en C-6, I'autre position adjacente. On ne peut faire une cornparaison entre les effets des groupements mkthyles et ceux des atomes d'hydrogenes en N-8 que pour les rkactions acido-catalysCes puisque les structures des composks neutres, qui prennent part 3 la rkaction catalyske par les bases, ne sont pas semblables.[Traduit par le journal]

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Section: Resultsmentioning

confidence: 99%

Synthesis, base strength, and prototropic behaviour of a number of alkyllumazines

Stewart¹,

Gumbley²

1985

Can. J. Chem.

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“…Apparently, this involves a so-called gear-mesh mechanism. [19] This mechanism has been properly analyzed for aminoboranes solely in the case of {η 5 -[1-(diisopropylamino)borole]}metal complexes. [20] In the case of compound 4, it is noteworthy that the barrier to rotation is rather high, when compared with the aforementioned complexes (for the borole-Cr(CO) 3 complex: ΔG # coal = 42 Ϯ2 kJ mol -1 [20] ) as shown by the evaluated [21] energy of activation for 4 (ΔG # coal = 52 Ϯ2 kJ mol -1 ).…”

Section: Exchange Reactions Of the Selenasilanes 1 And 2 With Boron Hmentioning

confidence: 99%

1,3,2‐Diselenaborolanes with an Annelated Dicarba‐closo‐dodecaborane(12) Unit: Synthesis, Molecular Structure and Reactivity

2011

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Keywords: Main group elements / Carboranes / Boron / Selenium / Heterocycles / Density functional calculations / Multinuclear NMR spectroscopy Exchange reactions of 2,2-dimethyl-1,3-diselena-2-silacyclopentane and 2,2,3,3-tetramethyl-1,4-diselena-2,3-disilacyclohexane, as the 4,5-and 5,6-[1,2-dicarba-closo-dodecaborano(12)] derivatives, respectively, with boron trihalides (BCl 3 , BBr 3 , BI 3 ) and dichlorides (PhBCl 2 , iPr 2 NBCl 2 ) afford the corresponding 1,3,2-diselenaborolanes in essentially quantitative yield. The products were characterized in solution by multinuclear magnetic resonance spectroscopy ( 1 H,

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“…It is known that o-xylene prefers the gear-clashed conformation 24 rather than the other gear-clashed form 25, or the gear-meshed form 26 (16,17).…”

Section: Gear Effectsmentioning

confidence: 99%

“…Roussel and co-workers (17,18) have recently examined the interplay between alkyl groups attached to aromatic rings and have rationalized reactivity differences in a number of systems on the basis that adjacent methyl and similar alkyl groups prefer to avoid the interactions shown in 25 and 26. Moreover, Schug and Viers (19) have calculated the energies of the rotational conformers of 2,3-lutidine and found that the gear-clashed rotamer 27 is more stable than the two gear-meshed rotamers 28 and 29 by 0.36 and 0.59 kcal mol-', respectively; it is more stable than the gear-clashed rotamer 30, which has a frontal interaction of hydrogens, by 1 .…”

Section: Gear Effectsmentioning

confidence: 99%

Steric activation in prototropic reactions of pyrazine derivatives. I. Polar and conformational effects

Lee¹,

Stewart²

1986

Can. J. Chem.

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The rates of proton loss from methyl groups in a number of pyrazine derivatives have been measured in D2O by monitoring the replacement of protium by deuterium at the acidic sites of the substrates; they include the reaction of four methylpyrazines with NaOD/D2O and nine alkylpyrazinium ions with D2O/DCl. The reactive site, which is a methyl group conjugated to the aza or azonium group of the substrate, is in all cases activated by an adjacent alkyl group, with the greatest effect, a factor of about 30, occurring when a proton on nitrogen is replaced by a methyl group. The phenomenon is analysed in terms of polar effects, ring strain and inter-alkyl interactions (including gear effects), and hybridization changes at nitrogen.

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Steric interplay between alkyl groups bonded to planar frameworks

Cited by 77 publications

References 54 publications

Synthesis, base strength, and prototropic behaviour of a number of alkyllumazines

Synthesis, base strength, and prototropic behaviour of a number of alkyllumazines

1,3,2‐Diselenaborolanes with an Annelated Dicarba‐closo‐dodecaborane(12) Unit: Synthesis, Molecular Structure and Reactivity

Steric activation in prototropic reactions of pyrazine derivatives. I. Polar and conformational effects

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