Nature and composition of Taft-Hancock steric constants

Fujita, Toshio; Takayama, Chiyozo; Nakajima, Minoru

doi:10.1021/jo00949a001

Cited by 92 publications

(67 citation statements)

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“…Possibly, transition states energy difference is determined by steric interaction of the bulky methyl group with chlorine [29]. As presented in Fig.…”

Section: Regioselective Thiiranium Intermediate Ringopening Processesmentioning

confidence: 99%

Theoretical studies on the methylsulfenyl chloride addition to propene

Vektarienė

Vektaris

2010

Heteroatom Chemistry

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“…Possibly, transition states energy difference is determined by steric interaction of the bulky methyl group with chlorine [29]. As presented in Fig.…”

Section: Regioselective Thiiranium Intermediate Ringopening Processesmentioning

confidence: 99%

Theoretical studies on the methylsulfenyl chloride addition to propene

Vektarienė

Vektaris

2010

Heteroatom Chemistry

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“…It has to be mentioned that the larger the group R i , the smaller r i , thus r 1 corresponds to the small size group, r 2 to the medium size group, and r 3 to the large size group. [33]. f ) At 293 K. It was assumed that the difference of temperature was not significant.…”

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confidence: 99%

Re‐formation Reaction of Cyclic Nitroxide‐Based Alkoxyamines: Steric and Polar/Stabilization Effects

Fischer

Marque²,

Nesvadba

2006

Helvetica Chimica Acta

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Dedicated to Professor Hanns Fischer for his invaluable contribution in the fields of nitroxide-mediated polymerization and radical chemistryIn nitroxide-mediated radical polymerization, the polymerization times decrease with the increasing re-formation rate constant of the CÀON bond (! alkoxyamine) between the growing polymer chain and the nitroxide radical. The factors influencing the re-formation rate constant are of considerable interest, but up to now, the polar/stabilization effects have not been addressed thoroughly. The combination of new data with previously reported data now showed that the re-formation rate constant k c increases with the increasing polar character of the substituents attached to the nitroxide moiety. The polar/stabilization effects are weaker for the re-formation than for the homolysis of the CÀON bond, and may be mainly attributed to the relocation of the odd electron onto the O-atom of the NÀO moiety, i.e., the stabilization of the nitroxide moiety. Hence, it is possible to predict the values of k c by combining both the polar/stabilization (s I ) and steric effects (E , nitroxide-mediated radical polymerization (NMP) has often been used for the synthesis of living polymers and copolymers with narrow polydispersities and controlled molecular masses and architecture [4 -6]. The mechanism of NMP outlined in Scheme 1 involves the cleavage of capped chain molecules (dormant species) into propagating alkyl and persistent nitroxide radicals (k d ). The propagating radicals grow by monomer addition (k p ) and form longer dormant chains by cross-coupling with the nitroxide radicals (k c ). Simultaneously, the usual self-termination (k t ) of the propagating radicals into unreactive polymer products takes place. This removes the propagating radicals and increases the concentration of nitroxide radicals over time; the cross-coupling reaction (k c ), therefore, dominates over self-termination [7] [8]. For an extended time range, a quasi-equilibrium of the reversible cleavage exists which is characterized by weakly time-dependent radical concentrations and a large excess of the persistent radicals over the propagating radicals.For the well-controlled and living radical polymerization of a given monomer with propagation and termination rate constants k p and k t , the rate constants of the reversible cleavage k d and k c must fall into proper ranges [7 -9]. In general, one aims at rela-

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“…The values of s U , E s (2) , k d (2) , E s (3) , and k d (3) are gathered in Table 2. All the individual s U (R i ) [20] and r(R i ) [21]d,e values needed to estimate s U and E s are stored in Table 3. [21,23] To highlight the leveled steric effect, k d (2) and k d (3) were estimated and then compared to the experimental values for alkoxyamines 1-4.…”

Section: Resultsmentioning

confidence: 99%

“…All the individual s U (R i ) [20] and r(R i ) [21]d,e values needed to estimate s U and E s are stored in Table 3. [21,23] To highlight the leveled steric effect, k d (2) and k d (3) were estimated and then compared to the experimental values for alkoxyamines 1-4. The predicted rate constants k d (2) are 850, 530, 16, and 530 times larger than the experimental ones for 1, 2, 3 0 , and 4, respectively.…”

Section: Resultsmentioning

confidence: 99%

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Leveled Steric Effect in Alkoxyamines of SG1‐Type

Acerbis¹,

Beaudoin²,

Gigmès³

et al. 2004

Macro Chemistry & Physics

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Summary: During nitroxide mediated polymerization (NMP) the polymerization time decreases with the increasing rate constant of the cleavage of the NOC bond of the dormant alkoxyamines. Hence, the knowledge of the factors influencing this cleavage is of considerable interest. Recently, we demonstrated that the influence of the nitroxyl moieties of 24 alkoxyamines on the values of the rate constant for the homolysis of the CON bond of the alkoxyamine (kd) was accounted for by a biparametric equation log kd = −3.07 × σU − 0.88 × Es − 5.88, with σU for the universal electrical effect and Es for the steric demand of the substituents bound to the nitroxyl function. During the course of the work, it was assumed for a number of molecules that the steric effect was leveled. We present herein three new SG1‐based alkoxyamines with sufficiently different steric strains to confirm the leveled steric effect.The fit of the rate constant for the homolysis of the CON bond of the alkoxyamines (kds) with the equation log kd = −0.37 × σU − 0.88 × Es when a leveled steric effect is assumed.imageThe fit of the rate constant for the homolysis of the CON bond of the alkoxyamines (kds) with the equation log kd = −0.37 × σU − 0.88 × Es when a leveled steric effect is assumed.

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Nature and composition of Taft-Hancock steric constants

Abstract: The Taft steric, E,, and the Hancock corrected steric, Ea°, constants of a set of various alkyl-and heteroatomsubstituted groups were analyzed as to whether they are separable into components. The analyses were carried (1) R.

Cited by 92 publications

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Theoretical studies on the methylsulfenyl chloride addition to propene

Theoretical studies on the methylsulfenyl chloride addition to propene

Re‐formation Reaction of Cyclic Nitroxide‐Based Alkoxyamines: Steric and Polar/Stabilization Effects

Leveled Steric Effect in Alkoxyamines of SG1‐Type

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