Reactivity of β-stannylketones. Elimination vs. substitution

Article abstract of DOI:10.3390/50300388

In the present work we report the results obtained in the reaction of β-stannylketones (I) with t-BuONa in dimethylsulfoxide (DMSO) and acetonitrile (ACN) as solvents. The reaction mechanisms probably involved are proposed.

Full text of DOI:10.3390/50300388

388
Molecules 2000, 5
Reactivity of β-Stannylketones. Elimination vs. Substitution
Ana Paula Murray and Alicia B. Chopa*
Instituto de Investigaciones en Química Orgánica, Universidad Nacional del Sur, Avda. Alem 1253
(8000) Bahía Blanca, Argentina
Tel/Fax: 54 291 4595187, E-mail: achopa@criba.edu.ar
*Author to whom correspondence should be addressed.
Abstract: In the present work we report the results obtained in the reaction of β-
stannylketones (I) with t-BuONa in dimethylsulfoxide (DMSO) and acetonitrile (ACN) as
solvents. The reaction mechanisms probably involved are proposed.
Introduction
In the reaction of β-functionalized organotin compounds (I) with t-BuONa in t-BuOH there is a
competition between an elimination reaction [(E1cB)R], leading to olefins with high diastereoselectiv-
ity, and a nucleofilic substitution reaction yielding the reduction product (II) [1]. Now we report on the
reactions of β-stannylketones (I) with t-BuONa in DMSO and ACN in order to compare the reactivity
in these solvents.
R1
Ph
H
R1
Z
tBuONa
Z
R3-nX
nSn
+
Ph
Z
DMSO
R1
II
Ph
I
or ACN
Z / E
Z = COPh, CN ; R1 = Me, Ph ; R = Me, Ph ; X = Cl, Br ; n = 1, 3
Experimental
Anhydrous solvents and sublimated t-BuONa were used. The β-stannylketones were synthesized in
our laboratory [2-3]. The reaction mixtures were analysed by CGL. The reaction conditions are de-
tailed in the Table.

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Molecules 2000, 5
Results and Discussion
The results summarised in the Table show that these reactions lead to higher yields in shorter reac-
tion times, depending on the substrates. Thus, while β-stannylketones carrying electron-withdrawing
groups attached to tin lead to the elimination product in high yield, β-trialkylstannylketones give
mainly the reduction product.
Table. Reactions of R3-nXnSn-CH(Ph)-CH(R1)-COPh with tBuONaa.
R1
R3-nXnSn
N°
DMSO
ACN
% Elim (Z/E) % Elim (Z/E)
1
2
3
4
5
6
7
8
9
Me
Me
Me
Me
Me
Me
Ph
Ph3Sn
Ph3Sn
58 (10/90)b
86 (19/81)b
84 (10/90)
39 (1/99)c
85 (0/100)c
83 (0/100)
37 (30/70)d
85 (0/100)
80 (100/0)
76 (16/84)c
58 (17/83)c
69 (90/10)
Ph2BrSn
Me3Sn
Cl3Sne
Cl3Snf
Me3Sne
Me3Snf
Cl3Sng
d
-
92 (0/100)
81 (100/0)
d
-
d
Ph
-
Ph
98 (89/11)
aSubstrate/base ratio 1/1.1 Similar results are obtained from both
erythro and threo isomers; bStarting substrate is recovered like
c
a diastereoisomeric mixture; No isomerization of the starting
d
substrate was observed; High yield of the reduction product;
eErythro isomer; fThreo isomer; gErythro/threo 13/87.
The stereochemical results show that, in most cases, these reactions are stereoconvergent. The same
ratio Z/E is obtained independently of the configuration of the starting substrate. (Table, entries 1-4,7
and 8). Taking into account the stereochemistry observed we are able to say that, probably, the elimi-
nation reaction goes through an (E1cB)R mechanism in DMSO and through an (E1cB)I in ACN.
On the other hand, β-trichlorostannylketones (Table, entries 5, 6 and 9) give high yields of olefins
through a stereospecific elimination reaction. Two possible mechanisms could be proposed to explain
these results: a concerted E2, or an (E1cB)I in which, because of electronic interactions between the
trichlorostannyl group and the oxygen atom, the intermediate carbanions are not interconvertibles.
Acknowledgements: This work was partially supported by CIC, CONICET and UNS (Argentina). The
authors thank Dr. M. González Sierra (IQUIOS, Argentina) for the NMR spectra.

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Molecules 2000, 5
References and Notes
1. Murray, A.P. Thesis, 1999.
2. Chopa, A.B.; Murray, A.P. Diastereoselective synthesis of β-trichlorostannyl and β-trimethyl-
stannylketones. Main Group Metal Chem. 1998, 21, 347.
3. Chopa, A.B.; Murray, A.P.; Lockhart, M.T. Evidence of single electron transfer in the dias-
tereoselective synthesis of β-stannylketones. J. Organomet. Chem. 1999, 585, 35.