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Stable Organic Lewis Acids.

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Topotactic reactions occur in all processes involving
participation of solids; they occur much more frequently
than has hitherto been assumed and must always be
taken into consideration in investigations on the interelation of structure, appearance and chemical reactivity. Thus,
topotaxy plays an important role in numerous branches of
applied chemistry such as sintering processes and corrosion
in aqueous solutions or in hot gases. The interesting biological
process of bone formation also involves topotactic reactions.
Lecture a t Bonn, February 4, 1969 [VB 201 IE]
German version: Angew. Chem. 81,470 (1969)
[*] Prof. Dr. H. R. Oswald
Anorganisch-chemisches Institut der Universitat
CH-8001 Zurich, Ramisstrasse 76 (Switzerland)
Stable Organic Lewis Acids
groups presumably lead t o coplanarity of only one COOR’
group and t o decoupling of the other. Accordingly, the K
band of (la)(v’ = 3 1 600 em-1) lies bathochromic to that of
(2a) (v’ = 36000 cm--I), which actually corresponds more to
the K band of methyl cinnamate (v‘ = 36500 cm-1).
The structure of the anion A 0 has been established by synthesis and by UV- and N M R spectroscopy.
The stability of the Lewis acids under discussion is limited by
hydrolytic cleavage of the C=-Cdouble bond (retro Knoevenagel reaction). This reaction rollows a complicated multistep
mechanism according to an overall first order rate law; the
rate constant of the overall reaction is dependent on pH and
has a maximum when p H FZ p K i . In the p K i determination
the retro Knoevenagel reaction is taken into consideration by
extrapolation t o time t =. 0.
The Lewis acid character of the olefins L is occasionally
adversely affected by acid H atoms or by acidification of H
atoms in the R1 and Rz groups.
By 0. E. Polansky[*l
Olefins of structure L (in which XI and X2 are strongly electron accepting groups that are fully conjugated with the C = C
double bond and R1 and Rz are groups that have little or n o
acceptor or donor properties) usually behave as electrically
neutral, reversibly titratable organic Lewis acids. They
combine with bases via the @-carbon atom with polarization
of the C = C double bond, the negative charge being distributed over the site of the XI and X2 groups so that only a relatively slight partial charge results on the @-carbonatom in
the anion AO.
(3)
The synthesis of the above Lewis acids can be achieved either
by direct condensation in a water extractor (in the case where
Y = 0) or by reaction of the corresponding N-alkylamine
(Y = NRI) in a glass autoclave.
R:,C=Y
R2
A0
L
The apparent pKL values of the Lewis acids L can be determined, inter alia, by potentiometric titration with the base
conjugated t o the solvent
PKL
=
PKL+ PKHB
(~KL
= true p K of Lewis acid, KHB = ionic product of the
solvent).
The substituents R affect the p K i of the Lewis acids to a
greater extent than they do the p K values of the corresponding
carboxylic- or ammonium acids; increasing acceptor strength
of the substituent XI and X2 lowers the p K i value.
The relatively low pKL values of ( 1 ) compared with those
of ( 2 ) are due to the positive entropy of reaction in the reaction of bases with (I). i n (1) the two C O O groups are coplanar; in (2) repulsive forces between the two COOR’
+
H,C
7’A
-HY
”( 7’
k2
The synthesis is limited: 1. Aldehydes of the constitution
RCH2CHO have hitherto a1ways afforded resinous products; 2. strong Lewis acids (such as (31) obviously immediately react with XlCH2X2 t o form Michael-addition
products ( 4 ) .
In accord with their Lewis acid character the olefins L catalyze the etherification of alcohols by diazo compounds as well
as the rearrangement of epoxides to carbonyl compounds.
They react with diazoalkanes at low temperature to give,
among other things, thermally labile pyrazol-1-ines. At
room temperature, on the other hand, the reaction is usually
characterized by spontaneous evolution of N z ; depending on
the substitution the reaction products that can be isolated are,
cyclopropanes, dihydrofurans, alkylation- or rearrangement
products.
Lecture at Basel on January 23, 1969 [VB 200 1El
German version: Angew. Chem. 81, 469 (1969)
[*I
Angew. Chem. internal. Edit. i Vol. 8 (1969) 1 No. 6
(4)
Prof. Dr. 0. E. Polansky
Institut fur Theoretische Chemie der Universitat
A-1090 Wien, Wahringer Strasse 38 (Austria) and
Max-Planck-Institut fur Kohlenforschung,
Abt. Strahlenchernie
433 MulheimiRuhr, Stiftstrasse 34 -36 (Germany)
46 1
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