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TetrahedronLettersNo. 36, pp 3315 - 3318.
OPergamon Ress Ltd. 1978. Printedin Great Britain.
INTRINSIC FRAGMENTATION MODES OF PRIMARY ALKOXIDES
D. A. Evans* and D. J. Baillargeon
Contribution No. 5808 from the Laboratories of Chemistry
California Institute of Technology, Pasadena, California 91125
In the previous communication we have provided thermochemical estimates on
(eq. 1). 1 The
purpose of this communication is to compare the energetics of gas phase homolysis
the perturbation of negatively charged oxygen on bond homolysis
(eq. 1) with that of heterolysis
(eq. Z), and to correlate these results with
analogous processes in solution where metal counterions are known to play an
important role in the mode of alkoxide fragmentation.
DW
-OCH2-R
-
-OCH2-R
-
-O-t&*
+
lR
(1)
+
-R
(2)
AH;
O=CH,
Alkoxide fragmentation processes, both in solution 2-7 and in the gas phase, 3
have been reported by several investigators.
From these studies the fragmentation
mode (homolysis vs. heterolysis) appears to be defined not only by substrate
alkoxide but also by counterion and solvent.
In a classic study Cram demonstrated
the importance of alkali metal counterions in dictating the course of alkoxide
fragmentation via heterolytic or homolytic modes.
(M = K, Na, Li), predominant heterolysis
In the case of substrate 1
(eq. 2) was observed for M = K while
homolysis (eq. 1) was preferred for M = Li.'
Ph CH3
MO-k-f!
Ph
bh a,,2
I
3315
tvl
khet ’ khom
K
2.2
Na
1.6
Li
0.05
No. 36
331.6
In conjunction
ments,
we
of
metal
of
reaction
have
counterion
Calculation
limited
for
the
of
these
highly
bond
dissociation
(eq.
2)
can
employing
of
in
effects
modes,
can
be
the
primary
data
generalized
Analogous
the
enthalpy
for
the
thermochemical
(EA)
to
the
cycle
and bond
and
the
include
ionic
(AH;).
due
present
for
In
estimating
process
in
strengths.
the
fragmentation
fragmentation
illustrTt;d
to
time.
the
procedure
absence
enthalpies
alkoxides
to
the
the
phase
at
ionic
rearrange-
in
gas
(DH”)
for
thermochemical
affinities
alkoxides
radical
possible
alkoxides.
from
molecular
of
by estimating
only
approach
electron
Table
is
relevant
energies,
known
summarized
reactivity
fragmentation
this
derived
on alkoxide-promoted
intrinsic
energies
substituted
be
studies
the
two
however,
more
our
and solvent
availability
principle,
of
with
determined
Scheme
1 by
The results
’
are
I.
I
Scheme
AH;
-0-CH,-R
0=CH2
-
+
+e-
-R
AH;
/
I
t
AH;
*O-CH,-R
= EA(.0CH2--R)
AH;
In the
solvent,
absence
by
34,
heterolysis
in
dependent
cleavage
the
free
homolytic
Due to
conclusions.
of
metal
for
At
metal
the
of
oxygen
in
the
DH” and
should
are
of
of
radical
increase,
curtailed
coupling
process
alkoxides
lend
is
not
study,
alkoxides.
to
5
to
homolysis
the
above
on the
However,
as
than
Our therm0
out.
support
increasingly
bond
concluded
rather
available
increments,
pairing
reported
solution
they
and AH; (MOCH2-R) .
by differing
ion
by Cram’s
and magnesium
data
counterion
in
ruled
bond
have
heterolytic
be
over
for
on the
products,
via
and
favored
and Mclver’
couldn’t
primary
but
is
observation
Arnett
thermochemical
by
counterion
The preference
proceeding
latter
metal
tri-tert-butylcarbinol
M+, on DH” (MOCH2-R)
As suggested
many lithium
salt
AH: for
time
both
EA1.R)
3-butenoxide
with Cram’s
5
Recently,
was probably
present
cations,
counterions.
preferred
absence
of
and
.R
-
respectively.
agrees
potassium
although
DH” and AH: values
properties
the
ethoxide
solution.
decomposition
estimates
effect
1 in
the
influence
28 kcal/mol
alkoxides
of
cleavage,
chemical
moderating
+
DH(*OCH2-RI
+
methoxide,
and
of
DMSO).
alkoxide
the
of
17,
fragmentation
(25”C,
that
of
heterolysis
homolysis
O=CH2
-
the
both
donor
electronegative
may well
be
3317
No. 36
Table
Calculated
I.
Fragmentation
Primary
Alkoxides,
R
42
CH3
68
51
CH,CH=CH,
58
30
values
2 (Scheme
2 have
been
In the
radical
Wittig
in
II).
to
the
bond
rearrangement
pair
Scheme
the
homolysis
covalent
is
such
Wittig
as well
phase,
298OK.
those
discussed
this
carbon
subject
to
12*13
Thus,
metal
rearrangement
issues.
in
charged
alkyls
as
as heterolytic
mechanistic
intermediates
fragmentation.
while
well-known
examined
rearrangement
alkoxide
b
Gas
kcal/mol.
fragmentations
Homolytic
l2
intensively
carbon-oxygen
found
alkoxide
resemblance
ethers
in
reported
I.
Mechanistically,
for
AH;
DH”(&H2-R?
76
‘Ref.
evidence
for
H
‘All
a striking
Energies
a,b
-OCH2-R
donor
effects
dissociated
which
similar
metal
Finally,
have
process.
substituent
counterion
modes
studies
sigmatropic
the
II
M+
C-O-R,
II
e-
-
/
R2
L
RI
\
/
R2
C=O
+
transfer
M-R3
RI
==s
provided
12
that
undergo
radical
I
MO-C-R,
R2
of
facilitates
to
alkyls
+ ‘R3
RI\..-
bear
metallated
dissociation
Recent
[1,2]
do not.
of
above
and/or
NO. 36
3318
charged intermediates accessible from metallated ethers or alkoxides are simply
related by an electron transfer process.
This corresponds to the single electron
transfer (SET) mechanism proposed by Ashby for the addition of organometallics to
14
carbonyl compounds.
Accordingly, those factors identified by Ashby which
favor the SET mechanism in organometallic-carbonyl
addition should be relevant
to defining the course of alkoxide fragmentation.
Acknowledgement.
_1_-.,___5____q-.* Support from the National Science Foundation is gratefully
acknowledged.
REFERENCES
1.
D. A. Evans and D. J. Baillargeon, Tetrahedron Lett., 0000 (1978) and
references cited therein.
2.
H. D. Zook, J. March, and D. F. Smith, J. Am. Chem. Sot., 53, 1617 (1959).
3.
E. M. Arnett, L. E. Small, R. T. McIver, Jr., and J. S. Miller, J. Org. Chem.,
43, 815 (1978) and references cited therein.
4.
R: A. Benkeser M. P. Siklosi, and E. C. Mozdzen, J. Am. Chem. Sot., __100,
2134 (1978) ani references cited therein.
5.
D. J. Cram, A. Langemann, W. Lwowski, and K. R. Kopecky, ibid., __,
81 5760
(1959).
N. Hirota and S. I. Weissman, ibid., SC, 2538 (1964).
6.
7.
G. 0. Schenck, G. Matthias, M. Pape, M. Cziesla, and G. von Bt'nau, Liebigs
Ann. Chem., __I
719, 80 (1968).
8.
Pertinent EA values in kcal/mol are as follow: EA(.H) = 18.9 (ref. 9);
= 12.7 (ref. 11).
EA(.CH3) = 1.8 (ref. 10); EA(.CH2CH=CH2)
9.
D. R. Stull and H. Prophet, Ed., Natl. Stand. Ref. Data Ser., Natl. Bur.
Stand ., _3_7(1971).
G. B. Ellison, P. C. Engelking, and W. C. Lineberger, J. Am. Chem. Sot.,
100, 2556 (1978).
10.
11.
12.
13.
14.
A.
---H. Zimmerman and J. I. Brauman, ibid., 9,9,, 3565 (1977).
For recent discussions of the different possible mechanisms see: u. Schollkopf, Angew. Chem., Int. Ed. Engl., 2, 763 (1970); J. F. Garst and C. D.
Smith, J. Am. Chem. Sot., __
98, 1526 (1976).
H. F. Ebel, V. Db'rr, and B. 0. Wagner, Angew. Chem., Int. Ed. Engl., 2, 163
(1970).
I. G. Lopp, J. D. Buhler, and E. C. Ashby, J. Am. Chem. SOC.,
zz,
4966
(1975) and references cited therein.
(Receivedin USA 15 June 1978;received
inUK for publication4 July 1978)