Barriers to internal rotation in 1,3,5-trineopentylbenzenes. VI. Correlation between barriers to internal rotation (.DELTA.G.dag.) and substituent size

Abstract. This paper presents a critical discussion of the perceived similarity between alkyl groups and mechanical gears. Specifically, we examine the notion that the availability of "free spaces" between the ligands of the central atom in an alkyl group leads to a reduction in the steric requirements of the group and, hence, to an effect on the rate of conformational interconversion. It is concluded that there is no compelling experimental evidence to support the operation of this type of dynamic gear effect. STATIC AND DYNAMIC GEAR EFFECTSModels of alkyl groups or, in general, of n-fold chemical rotors bear a manifest resemblance to toothed gears. If the ligands bound to an alkyl group center (e.g. the hydrogen atoms in CH3 or the methyl groups in t-Bu) are likened to the teeth of a gear, a formal analogy can be drawn between the rotors and their mechanical counterparts. Under conditions of intramolecular crowding, arrangements of neighboring alkyl groups can then be compared to meshed gears, as is strikingly obvious merely by inspection of molecular models.' The meshing of alkyl groups in the ground state is one kind of gear effecr'" for which we propose the collective term static gear effect"The gear analogy has also been widely applied to the dynamics of conformational interconversions. We propose the term dynamic gear effect to describe the special effect on the rate or mechanism of a process which may be attributed to the intermeshing of a chemical rotor with a neighboring group. Based on this classical mechanical model, a variety of claims and suggestions have been advanced that the availability of "free spaces" between the ligands of the central atom in an alkyl group leads to a reduction in the effective size of the group and hence to an effect on rates of conformational interconversions. In this paper we present a critical discussion of some of these proposed dynamic gear effects following the classical approach, i.e. ignoring quantum mechanical phenomena (e.g. tunneling)," In order to place what is to follow into proper perspective, we devote the next section to a more detailed discussion of the gearing analogy. THE NATURE OF DYNAMIC GEARINGThe central analogy, that between chemical and mechanical gears, is often invoked in context with the coupled motion of rotors: the torsion of "gear-meshed" chemical rotors plainly evokes an image of rotating cogwheels. This analogy rests on the formal similarity between meshed alkyl groups and external gears." In these chemical and mechanical systems, a disrotatory motion of the component parts would appear to be an obligatory feature." In actuality, profound dissimilarities exist between chemical and mechanical gear systems. One vital difference is that mechanical gears are designed to move with uniform angular velocity; this cannot be the case for chemical rotors. 11 Net disrotatory motion of chemical rotors by no means implies that the motions along the pathway connecting the initial and final states resemble those of mechanical cogwheels. Rotations of chemica...

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“…3 in Ref. 16. Chemistry 20 1980 effects and thus introduces an element of ambiguity in the interpretation of rate effects.…”

Section: Discussionmentioning

confidence: 99%

Is the Effective Size of an Alkyl Group a Gauge of Dynamic Gearing?

Hounshell

Iroff

Iverson

et al. 1980

Israel Journal of Chemistry

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“…I3C and 2H relaxation times suggest a AG* of 18-23 kJ/mol (5). For 2-bromo-3,5-di-tert-butylneopentylbenzene (7) only a linebroadening is observed in the 'H nmr spectrum, attributed to a rate process characterized by an upper limit to AG* of 23 kJ/mol for rotation past the ortho C-H bond.…”

Section: Introductionmentioning

confidence: 93%

“…The present nmr measurements are meant to probe further the idea (1)(2)(3) that hyperconjugation of the carbon-metal bond with the aromatic .rr electron system determines the conformational preference about the C,2-C, 3 bond in benzyltrimethyl X (X = Si, Ge, Sn, Pb). Such an origin for its conformational behaviour was not suggested for benzyltrimethylmethane (neopentylbenzene), where steric interactions presumably are dominant (4)(5)(6)(7).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

An NMR study of steric and hyperconjugative barriers in benzyl X(CH₃)₃, X = C, Si, Ge, Sn, Pb

Schaefer¹,

Penner²,

Takeuchi³

et al. 1989

Can. J. Chem.

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The 1H nuclear magnetic resonance spectra and their analyses are reported for benzyltrimethyl X (X = C, Si, Ge, Sn, Pb) and for the 2,6-dichloro derivatives where X = Si, Ge, Sn, Pb. The steric barrier to rotation about the Csp2—Csp3bond for X = C, based on the long-range proton–proton coupling constant over six bonds, 6J(H,CH2), must be greater than 20 kJ/mol. Unexpectedly, 6J(H,CH2) implies that the barriers for X = Si, Ge, Sn, Pb have relatively minor steric components. If these barriers are dominated by hyperconjugative interactions of the C—X bond with the aromatic π system, then they are probably mainly twofold in nature. Furthermore, their magnitude is likely to be near 9.5 kJ/mol for X = Si, Ge, Sn and may be as high as 12 kJ/mol for X = Pb. Keywords: NMR, benzyl X(CH3)3, X = C, Si, Ge, Sn, Pb; NMR, conformations; NMR, hyperconjugation.

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“…Although the values for AG* from Table 4 are not all at the same temperature, the values of AS* are small so a,a,a1,a',2,4,5,6-octachloro-m-xylene (OCMX), AG* = 14.9 -1-0.5 kcal mol-' in toluene-d, (4) and an explanation has been advanced suggesting that buttressing may increase the ground state energy more than the transition state energy (5). If each of the dichloromethyl groups of a,a,af,cr',a",a",2,4,6-nonachloromesitylene (NCM) were treated as an independent rigid rotor flanked by a single chlorine on each side, the value of AG* should be comparable to that for 2,4,6-PCT; however, the consideration of the 'effective' size (24) for the dichloromethyl groups suggests that the AG* value be comparable to OCMX. The experimental value for NCM, AG* = 16.1 -1-0.06 kcal mol-' in toluene-d, (6) indicates that these trends are not reliably predicted using buttressing effects.…”

Section: Comparison Of Activation Parameters For Relatedmentioning

confidence: 99%

The proton magnetic resonance determination of the hindered rotation in α,α,α′,α′,3,4,6-heptachloro-o-xylene

Hutton¹,

Hiebert²,

Mark³

1978

Can. J. Chem.

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, and VICTOR MARK. Can. J. Chem. 56.1261Chem. 56. (1978.A high resolution proton magnetic resonance investigation of the hindered rotation of the dichloromethyl groups as a function of temperature in a,a,a1,a',3,4,6-heptachloro-o-xylene (HCOX) has been conducted using dynamic nuclear magnetic resonance line shape analysis, supplemented with a homonuclear double resonance saturation transfer technique. The rate process in HCOX involves a coupled rotation of the two substituted methyl groups and the nonmutual exchange of the three pairs of protons between two equally populated conformers. The Arrhenius and transition state parameters were determined from the rate constants obtained over a temperature range of -5°C to 93°C for a 5 mol% solution of HCOX in methyl- On a effectut, en faisant appel a une analyse de la forme des raies en resonance magnttique nucltaire dynamique complCmentte par une technique de transfert de saturation de double rtsonance homonucltaire, une etude par resonance rnagnttique nucltaire du proton a haute rtsolution de la rotation empechte des groupes dichlororntthyles en fonction de la temperature dans des a,a,a',a',3,4 (7) have previously studied the hindered rotation in a series of o-bis(dich1oromethyl) benzenes, including HCOX in tetradecane, using the variation of the line width of the coalescing resonances as a function of temperature (7). Because the results for H C O X were somewhat ambiguous using this technique the activation parameters were not

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Barriers to internal rotation in 1,3,5-trineopentylbenzenes. VI. Correlation between barriers to internal rotation (.DELTA.G.dag.) and substituent size

Cited by 38 publications

References 6 publications

Is the Effective Size of an Alkyl Group a Gauge of Dynamic Gearing?

Is the Effective Size of an Alkyl Group a Gauge of Dynamic Gearing?

An NMR study of steric and hyperconjugative barriers in benzyl X(CH₃)₃, X = C, Si, Ge, Sn, Pb

The proton magnetic resonance determination of the hindered rotation in α,α,α′,α′,3,4,6-heptachloro-o-xylene

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Barriers to internal rotation in 1,3,5-trineopentylbenzenes. VI. Correlation between barriers to internal rotation (.DELTA.G.dag.) and substituent size

Cited by 38 publications

References 6 publications

Is the Effective Size of an Alkyl Group a Gauge of Dynamic Gearing?

Is the Effective Size of an Alkyl Group a Gauge of Dynamic Gearing?

An NMR study of steric and hyperconjugative barriers in benzyl X(CH3)3, X = C, Si, Ge, Sn, Pb

The proton magnetic resonance determination of the hindered rotation in α,α,α′,α′,3,4,6-heptachloro-o-xylene

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An NMR study of steric and hyperconjugative barriers in benzyl X(CH₃)₃, X = C, Si, Ge, Sn, Pb