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476
Organometallics 2001, 20, 1476-1478
Nu cleop h ilicity vs Ba sicity in th e Rea ction of Sod iu m
ter t-Bu toxid e w ith â-Sta n n yl Keton es
Alicia B. Chopa* and Ana P. Murray
INIQO, Departamento de Qu ı´ mica e Ingenier ı´ a Qu ı´ mica, Universidad Nacional del Sur,
Avenida Alem 1253, 8000 Bah ´ı a Blanca, Argentina
Received November 29, 2000
Summary: The reaction of 3-stannyl-1,2,3-triphenyl- and
-stannyl-1,3-diphenyl-2-methylpropanones with sodium
tert-butoxide in either t-BuOH or dimethyl sulfoxide
DMSO) as solvent leads to elimination and/ or substitu-
as an electrofugal sense, leading to elimination or
substitution products, respectively.
Thus, the reaction of erythro-1,3-diphenyl-2-methyl-
3
(
3
-(triphenylstannyl)propanone (1) with sodium tert-
tion products. The composition of the product mixtures
depends essentially on the nature of the ligands attached
to the tin atom, on the solvent, and also on the nature of
the substituent on C-2. Thus, â-(triphenyl), â-(bromo-
diphenyl), and â-(trichlorostannyl) ketones undergo an
exclusive elimination reaction leading to an unsaturated
ketone in good to high yields (72%-96%) both in t-BuOH
and in DMSO. In the latter the reactions lead to higher
yields in shorter times. On the other hand, â-(trimeth-
ylstannyl) ketones lead to mixtures of olefins and sub-
stitution products in t-BuOH and exclusively to substi-
tution products in DMSO (96%-98%). Stereochemical
results suggest that the elimination reactions proceed
through an (E1cB)R mechanism.
butoxide in t-BuOH led (3 h) to a mixture of 1,3-
diphenyl-2-methylpropenone (2) (41%; Z/E 8/92) and the
isomerized starting substrate (erythro/threo 30/70). The
olefin yield increased to 72% after a 24 h reaction time
(Table 1, entries 1 and 2).
On the other hand, under similar conditions, erythro-
1
,3-diphenyl-2-methyl-3-(trimethylstannyl)propanone (3)
led (3 h) to a mixture of 2 (34%; Z/E 30/70) and 1,3-
3
diphenyl-2-methylpropanone (4) (57%), together with
a little amount of isomerized starting adduct (erythro/
threo 15/85) (Table 1, entry 3).
Attempts to increase the olefin yield by increasing
either the temperature or the ratio base/substrate (3/1)
led to mixtures of undesirable secondary products
starting from 1 and to an increase in the amount of 4
(85%) starting from 3 (Table 1, entry 4).
In tr od u ction
In previous investigations1 we have found that a
Similar reactions were carried out with erythro-1,3-
diph en yl-2-m et h yl-3-(br om odiph en ylst a n n yl)pr o-
series of threo-â-(triorganostannyl)nitriles led to Z/E
mixtures of R-methylene nitriles in different yields upon
treatment with lithium diisopropylamide (LDA) in tet-
rahydrofurane (THF) or sodium tert-butoxide in tert-
butyl alcohol (t-BuOH) under the appropriate condi-
tions.
4
panone (5) and with erythro-1,3-diphenyl-2-methyl-3-
5
(
trichlorostannyl)propanone (6). These substrates led
to higher yields of elimination product in shorter times.
Thus, ketones 5 (3 h, 30 °C) and 6 (2 h, 30 °C) led to 2
in 88% (Z/E 4/96) and 93% (Z/E 6/94) yield, respectively
As far as we know, in the chemical literature there
are few references to elimination reactions of organotin
compounds via carbanions, and these are restricted to
(Table 1, entries 5 and 6). It should be noted that no
substitution product was detected.
2
The substantial amounts of 4 found in the reaction
of 3 could be explained as the result of a nucleophilic
â-(trichlorostannyl) ketones and aldehydes.
Therefore, we considered that it would be of interest
to study the reactivity of â-stannyl ketones under base
conditions. Now, we wish to report the results obtained
in the reaction of 3-stannyl-1,2,3-triphenyl- and 3-stan-
nyl-1,3-diphenyl-2-methylpropanones with sodium tert-
butoxide in both t-BuOH and dimethyl sulfoxide (DMSO)
as solvents.
6
attack of the tert-butoxide anion at the tin atom. On
the other hand, the formation of 2 is probably due to
an elimination reaction where the tert-butoxide anion
acts as a base abstracting the proton from C-2 and the
R3Sn anion acts as a nucleofuge. To obtain direct
evidence for the existence of organotin anions, we
carried out the reaction between 3 and sodium tert-
butoxide in t-BuOH in the presence of an equimolecular
amount of allyl bromide. Triphenylallyltin was formed
in 37% yield.
Resu lts a n d Discu ssion
The reactions were carried out with R,â-disubstituted-
â-stannyl ketones because we could ascertain at the
same time the course and stereochemistry of the reac-
tion by using individual diastereomers as substrates and
examining isomeric compositions of the products.
The results indicate that in the studied reactions the
stannyl group can depart in a nucleofugal sense as well
All these observations suggest that the tert-butoxide
anion could react with these substrates as a base by
(3) Quantified by GC by comparing with an authentic sample.
(4) Synthesized by electrophilic cleavage of one phenyl group in 1
by bromine. Chopa, A. B.; Koll, L. C.; Podest a´ J . C.; Mitchell, T. N. J .
Organomet.Chem. 1989, 376, 283.
(
5) Synthesized by hydrotrichlorostannation of the appropriate
*
To whom correspondence should be addressed. E-mail: abchopa@
uns.edu.ar. Fax: 54-291-4595187.
1) Podest a´ , J . C.; Chopa, A. B.; Ayala, A. D.; Koll, L. C. J . Chem.
Soc., Perkin Trans. 2 1993, 2323
2) Nakahira, H.; Ryu, I.; Ikebe, M.; Oku, Y.; Ogawa, A.; Kambe,
N.; Sonoda N.; Murai, S. J . Org. Chem. 1992, 57, 17
olefin. Chopa A. B.; Murray, A. P. Main Group Metal Chem. 1998, 21,
347.
(6) There are many examples in the literature about the susceptibil-
ity of organostannanes to nucleophilic attack at the tin atom. Farah,
D.; Karol T.; Kuivila, H. Organometallics, 1985, 4, 662, and references
therein.
(
(
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0.1021/om001022x CCC: $20.00 © 2001 American Chemical Society
Publication on Web 03/07/2001