monoesters is proposed to proceed via a metaphosphate-like
transition state whilst the triesters must proceed through an
associative mechanism. The sulfate esters described here provide
an interesting contrast. The slope of the structure/reactivity
correlation for the sulfate monoester series, 24.7 6 1023, is
This work was supported by a University of Melbourne Early
Researcher Grant, the Australian Research Council (DP0449625)
and a Sir John and Lady Higgins Research Scholarship to ED.
Jeremy Paige is thanked for synthetic assistance.
21
equivalent to 435 kcal mol21
A
(calculated from blg = 21.2)12
˚
Notes and references
whereas Kirby calculated the corresponding slope in the phosphate
1023
{ Branda˜o and co-workers studied the structure of potassium 4-nitrophenyl
sulfate and 9 other structures taken from the Cambridge Crystallographic
Database (CCD) and identified a tentative correlation of increasing S–O
bond length and reactivity; however, this study was limited by the
uncertainty of assigning reliable pKa values to many of the leaving groups,
and the effects of libration on data collected at room temperature. A
personal communication with one of the authors of that paper (AJK)
revealed several errors in reporting their data apparently arising from a
vertical translation of data in Table 3 and from an incorrect pKa value for
4-nitrocatechol. These errors affect the slope of the correlations observed
but do not materially affect any conclusions presented.
§ Structures determined herein: potassium 4-methoxyphenylsulfate monoe-
ster 1 (2 independent molecules in unit cell), potassium 4-acetamidophe-
nylsulfate monoester 3, potassium 4-nitrophenylsulfate monoester 4,
potassium 2,2,2-trifluoroethylsulfate monoester 5, potassium methyl sulfate
6, 4-methoxyphenylsulfamate ester 2, 4-nitrophenylsulfamate ester 7,
3-nitrophenylsulfamate ester 8, 4-iodophenylsulfamate ester 9, 4-cyano-
phenylsulfamate ester 10, 4-chlorophenylsulfamate ester 11, 3-chlorophe-
nylsulfamate ester 12, phenylsulfamate ester 13, 3,4-dinitrophenylsulfamate
ester 14, ethylsulfamate ester 15, and 2,2,2-trifluoroethylsulfamate ester 16.
CCDC 285416–285431. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b513712h The low temperature X-ray
structure of potassium 4-nitrophenyl sulfate was reported by Sieroslawski
et al. whilst this work was in progress.20
monoester series to be 28
230 kcal mol21
6
,
equivalent to
21, indicating it is energetically more expensive
˚
A
to stretch an S–OR bond relative to a P–OR bond.
Williams18 has proposed that the degree of nucleophile
participation in sulfonyl (SO2) transfer mechanisms is inversely
related to the ability of the system to internally donate electron
density into sulfur, and Kirby17 has made similar comments for
phosphate mono- and triester solvolysis. Williams has studied
sulfuryl and phosphoryl transfer between nitrogen nucleophiles
and calculated rate constants where the entering and leaving group
are identical.19 The Brønsted relationships of these identity
reactions predict that at the transition state the bond orders of
the fissile bonds of sulfuryl transfer are greater than for phosphoryl
transfer and that a sulfuryl group surrenders less negative charge
than a phosphoryl group.19 The structural data reported here
provide strong evidence that internal electron donation into sulfur
during sulfuryl transfer is far less pronounced than the
corresponding electron donation into phosphorus during phos-
phoryl transfer. Internal electron donation to the central atom may
be effected by rearrangement of electron density at the nonbridging
oxygens, reflecting a shift from a polarized bonding interaction
with the central atom to a more covalent-type bonding interaction.
Thus, a more dramatic shift in X–Onb bond polarity is expected
along the phosphoryl transfer reaction coordinate (X = P) than
along the sulfuryl transfer (X = S) reaction coordinate.
" The same general trend is evident in the sulfamate ester data, however, a
quantitative relationship between sulfur geometry and S–Ob bond distance
or pKa(ROH) is ill-defined for the sulfamate series; this may be attributed
to geometric distortions around sulfur caused by random variations in the
intermolecular hydrogen bonding in the crystal network.
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This reasoning is consistent with the striking difference in the
magnitude of solvent effects observed for sulfuryl and phosphoryl
transfer, which both exhibit an acceleration of solvolysis rates in
solvents of reduced polarity. For phosphate monoesters the rate
acceleration is dramatic, being as high as 106 in DMSO/water. For
sulfate monoesters the effect is much more modest; under similar
conditions a 50-fold rate acceleration is observed.7 Solvents of
reduced polarity will afford greater transition state stabilization to
phosphoryl transfer relative to sulfuryl transfer if a more
significant shift in the X–Onb bond polarity occurs along the
reaction coordinate, resulting in greater acceleration of phosphoryl
transfer.
Structure/reactivity and structure/structure correlations of sul-
fate monoesters and sulfamate esters support a mechanism that
proceeds through significant bond lengthening of the scissile S–Ob
bond and a dissociative, sulfur trioxide-like transition state. The
similarity of the slope of the structure/reactivity plots suggests that
pH independent solvolysis of sulfate monoesters and neutral
sulfamate solvolysis proceed through qualitatively similar reaction
profiles. These correlations provide a powerful view of the
sensitivity of the ground state structures of these species to leaving
group ability and provide strong evidence for a sulfur trioxide-like
transition state in both reactions with minimal internal electron
donation from the non-bridging oxygens. This work suggests a
close mechanistic relationship of sulfamyl and sulfuryl group
transfer that is similar to but distinct from phosphoryl group
transfer.
19 A. Williams, J. Am. Chem. Soc., 1985, 107, 6335.
20 K. Sieroslawski, T. Popek and T. Lis, Acta Crystallogr., Sect. C, 2004,
60, M327.
316 | Chem. Commun., 2006, 314–316
This journal is ß The Royal Society of Chemistry 2006