Table 6 Rates and ionic product distributions for negative ion reactions with lactonesa
∆Hacid
ADO
Ion (AϪ)
(HA)b
P/torr
Neutral reagent
Ionic productsc
Rxn path
Rated
Rate e
Eff.f
OHϪ
391
380
371
357
0.5
0.5
0.5
0.5
0.8
1.0
0.5
0.5
0.5
0.5
β-Propiolactone
β-Propiolactone
β-Propiolactone
β-Propiolactone
(M–H)Ϫ
PT
PT
PT
Adduct
Adduct
Adduct
PT
PT
PT
45.0 3.0
30.0 2.0
41.5
33.1
39.7
28.9
28.9
28.9
1.00
0.91
CH3OϪ
FϪ
(M–H)Ϫ
(M–H)Ϫ
CH3SϪ
CH3SϪ(β-lactone)
CH3SϪ(β-lactone)
CH3SϪ(β-lactone)
(M–H)Ϫ
7.9 0.6
8.2 0.4
8.2 0.6
12.3 0.3
0.27
0.28
0.28
OHϪ
391
380
373
356
γ-Butyrolactone
γ-Butyrolactone
γ-Butyrolactone
γ-Butyrolactone
CH3OϪ
(M–H)Ϫ
Ϫ
NCCH2
O2NCH2
(M–H)Ϫ
Ϫ
No observed reaction
a Rates for the reactions with γ-butyrolactone, δ-valerolactone and ε-capriolactone were tried with little or no success due to vapor pressure problems
(see text). b Values from the NIST website (http://webbook.nist.gov). c %Yields are 100%, extrapolated for a zero flow rate of neutral reagent in STP
cm3 sϪ1
.
d Rates in units of (10Ϫ10 cm3 moleculeϪ1 sϪ1) and the reported errors are one standard deviation of at least triplicate measurements. e Rates in
units of (10Ϫ10 cm3 moleculeϪ1
s
Ϫ1) and the polarizability for β-propiolactone is approximated using group additivity. f Efficiency = (experimental
rate/ADO rate), and limited to a value of 1.
esters, and the reactivity with the ethyl esters is dominated by
chemistry at the alkyl group. This change is due to the avail-
ability of the E2 pathway for alkyl reactions with the ethyl
esters.5,24,25
project, in particular. Bob was gifted as a scientist, a teacher,
and a friend. Bob made us all better because he was there to
help and to guide. We will miss him. Goodbye, Bob.
A comparison of formyl and acetyl esters demonstrates that
the formyl esters react more readily with less basic ions, but
the rates of reaction of formyl esters with more basic ions are
slower than the acetyl esters. The main reason for the increased
reaction rate of basic nucleophiles with the acetyl esters is the
References
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availability of the proton transfer pathway. The acidity (∆Hacid
)
of the formyl esters is approximately 391 kcal molϪ1, while the
acidity of the α-hydrogen in the acetyl esters is approximately
374 kcal molϪ1. Other reaction pathways do not compete effect-
ively when proton transfer is favorable (i.e. exothermic).
The Riveros pathway dominates the reactivity of methyl
formate. The BAC2 pathway is observed to only a very small
extent with methyl formate as a substrate. FϪ ion demonstrates
some interesting reactivity with the esters considered here.
Computational (B3LYP/6-31ϩG(d)) results demonstrate that
the BAL2 pathway (Fig. 5a) has the lowest activation barrier;
however, the experimental results at 298 K (Table 5) show a
slight preference for the Riveros pathway. The calculated
Riveros pathway has a slightly endothermic (ϩ0.3 kcal molϪ1
)
barrier, and variable temperature experimental studies demon-
strate an increase in the Riveros product as the temperature is
increased from 298 to 448 K.
Moreover, from a mechanistic sense, the Riveros product can
be generated through two different routes: a FϪ-assisted C–O
bond cleavage (Fig. 5b) or through nucleophilic attack at the
carbonyl carbon (BAC2, Fig. 5c). In the BAC2 pathway, the
alkoxide leaving group abstracts a proton from the formyl or
acetyl group prior to departing from the complex.
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Further examples of nucleophilic reactivity with carbonyl
groups will be presented in due course from both experimental
and computational approaches.
Acknowledgements
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This research was supported by the National Science Found-
ation (CHE-9733457). We also acknowledge the donors of the
Petroleum Research Fund, administered by the American
Chemical Society, for partial support of this research. We also
acknowledge support by the Ohio Supercomputer Center where
some of these calculations were performed. In addition, we are
grateful to the reviewers for many helpful comments.
We are particularly pleased to dedicate this paper to Bob
Squires, who was an inspiration and a role model for us. Bob
never hesitated to offer his advice, his energy, and his creativity
for our flowing afterglow development and for this carbonyl
2406
J. Chem. Soc., Perkin Trans. 2, 1999, 2397–2407