Cyclopropane assistance in neopentyl solvolysis

Rhodes, Yorke E.; Takino, Toshio

doi:10.1021/ja00720a074

Cited by 20 publications

(5 citation statements)

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“…Subsequent capture of cationic intermediate 5 by water or possibly hydroxide ion would then afford 4 . The sterically demanding neopentyl system has been known for some time to undergo solvolysis with rearrangement similar to that observed here, but the leaving groups have typically been highly reactive sulfonates . We were therefore surprised to find that 1a, equipped with a formally dianionic leaving group with p K a LG = 1.8, also underwent this process.…”

Section: Discussionsupporting

confidence: 54%

Catalytic Proficiency: The Extreme Case of S–O Cleaving Sulfatases

2011

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As benchmarks for judging the catalytic power of sulfate monoesterases, we sought to determine the rates of spontaneous hydrolysis of unactivated alkyl sulfate monoesters by S-O bond cleavage. Neopentyl sulfate proved to be unsuitable for this purpose, since it was found to undergo hydrolysis by a C-O bond cleaving mechanism with rearrangement of its carbon skeleton. Instead, we examined the temperature dependence of the spontaneous hydrolyses of aryl sulfate monoesters, which proceed by S-O cleavage. Extrapolation of a Bronsted plot [log(k(25)(N)) = (-1.81 ± 0.09) pK(a)(LG) + (3.6 ± 0.7)] based on the rate constants at 25 °C for hydrolysis of a series of sulfate monoesters to a pK(a)(LG) value of 16.1, typical of an aliphatic alcohol, yields k(25)(N) = 3 × 10(-26) s(-1). Comparison of that value with established k(cat) values of bacterial sulfatases indicates that these enzymes produce rate enhancements (k(cat)/k(uncat)) of up to 2 × 10(26)-fold for the hydrolysis of sulfate monoesters. These rate enhancements surpass by several orders of magnitude the ~10(21)-fold rate enhancements that are generated by phosphohydrolases, the most powerful biological catalysts previously known. The hydrolytic rates of phosphate and sulfate monoesters are compared directly, and the misleading impression that the two classes of ester are of similar reactivity is dispelled.

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Section: Discussionsupporting

confidence: 54%

Catalytic Proficiency: The Extreme Case of S–O Cleaving Sulfatases

2011

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“…But the observed preferential cyclopropyl, rather than alkyl, migration is opposite to what is expected in view of the hybridization of the bonding orbitals of the migrating carbon in each case. 6 The migrating carbon in an alkyl group would use an sp3 hybridized orbital, whereas the migrating carbon of a cyclopropyl group would use an orbital that is of higher s character than the alkyl carbon orbital, making the electron pair more tightly bound and less able to stabilize the partially positive nitrogen to which it is becoming bonded in the transition state. Clearly there must be some effect in Schmidt reactions of alkyl cyclopropyl ketones (lOa-c) in 83% sulfuric acid that is overcoming this unfavorable hybridization of the cyclopropyl group and allowing (at high acid strength) cyclopropyl to migrate to a partially positive nitrogen to a greater extent than do alkyl groups.…”

Section: Resultsmentioning

confidence: 99%

Effects of acid strength on Schmidt reactions of alkyl cyclopropyl ketones and alkyl phenyl ketones

Fikes¹,

Shechter²

1979

J. Org. Chem.

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“…To begin with, phenyl assistance in neopentyl solvolyses is approximately five times faster than cyclopropyl neighboring group participation, at least under normal circumstances. 30 Secondly, molecular models disclose that a 1,2-phenyl shift leads to cation 27 (= 19) which is capable of appreciable stabilization (027 = 30°) while cyclopropyl migration would lead to cation 28 which is not (0g9 " 80°). The conclusion may be drawn, then, that cation 27 is more likely than ion 28.…”

Section: Discussionmentioning

confidence: 99%