Evidence of single electron transfer in the diastereoselective synthesis of β-stannylketones

Article abstract of DOI:10.1016/S0022-328X(99)00186-2

The reaction of trimethyl- and triphenylstannylpotassium with mono and disubstituted enones (1-5) in acetonitrile as solvent was studied. In a few seconds and under mild conditions these reactions lead in nearly quantitative yields either to a mixture of diastereomers or to a pure diastereomer of β-stannylketones 6-16. The reactions with triphenylstannylpotassium gave higher yields than trimethylstannylpotassium. The partial or total inhibition of the reactions by addition of a free radical scavenger (galvinoxyl) or a radical-anion scavenger (p-dinitrobenzene) leads us to believe that these reactions could follow a two-stage reaction mechanism involving an initial electron transfer step. Our results indicate that these reactions are stereoselective but certainly not stereospecific. Full ¹H- and ¹³C-NMR data of the new β-stannylketones are given.

Full text of DOI:10.1016/S0022-328X(99)00186-2

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 585 (1999) 35–42
Evidence of single electron transfer in the diastereoselective
synthesis of b-stannylketones
A.B. Chopa *, A.P. Murray, M.T. Lockhart
Instituto de In6estigaciones en Qu´ımica Orga´nica, Uni6ersidad Nacional del Sur, A6enida Alem 1253, 8000 Bah´ıa Blanca, Argentina
Received 9 November 1998
Abstract
The reaction of trimethyl- and triphenylstannylpotassium with mono and disubstituted enones (1–5) in acetonitrile as solvent
was studied. In a few seconds and under mild conditions these reactions lead in nearly quantitative yields either to a mixture of
diastereomers or to a pure diastereomer of b-stannylketones 6–16. The reactions with triphenylstannylpotassium gave higher
yields than trimethylstannylpotassium. The partial or total inhibition of the reactions by addition of a free radical scavenger
(galvinoxyl) or a radical–anion scavenger (p-dinitrobenzene) leads us to believe that these reactions could follow a two-stage
reaction mechanism involving an initial electron transfer step. Our results indicate that these reactions are stereoselective but
1
certainly not stereospecific. Full H- and 13C-NMR data of the new b-stannylketones are given. © 1999 Elsevier Science S.A. All
rights reserved.
Keywords: Organotin anions; 1,4-Addition; Single electron transfer
The 1,4-addition of trialkylstannyllithium to a,b-un-
saturated carbonyl compounds in THF [1,2] is known
1. Introduction
to be a simple and efficient way to functionalize
organostannanes. The subsequent treatment of the in-
termediate enolates with electrophiles such as a proton
or alkyl halides results in high anti selectivity [3]. Tak-
In the course of our studies we found it necessary to
prepare diastereoisomers of the series of b-triphenyl-
and b-trimethylstannyl-a,b-disubstituted ketones shown
in Fig. 1.
ing into account these antecedents, we carried out the
reaction of triphenylstannyl- and trimethylstannyl-
lithium [1] with enones 1–5, quenching the enolates
with the appropriate electrophile using THF as solvent.
We observed that although the stereochemical results
were promising, the reactions always gave rather low
yields of 1,4-addition products (0–51%) except with
(E)-1,2,3-triphenylpropenone (3) (78%).
Previous results obtained in our laboratory regarding
the reactivity of triorganostannyl anionoids in polar
aprotic solvents such as acetonitrile (ACN) and
Fig. 1. Stereochemistry of compounds 6–16.
dimethylsulphoxide (DMSO) toward different sub-
strates [4] encouraged us to carried out the addition
* Corresponding author. Tel.: +54-291-4595-187; fax: +54-291-
reactions in ACN. We prefer to use the latter solvent
4595-187.
over DMSO because ACN is easier to purify and
E-mail address: achopa@criba.edu.ar (A.B. Chopa)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00186-2

36
A.B. Chopa et al. / Journal of Organometallic Chemistry 585 (1999) 35–42
handle. As far as we know there are no reports about
the generation of stannylanionoids in ACN [5]. In the
present paper we report the results obtained in the
course of these studies.
2. Results and discussion
Fig. 2. Results obtained in the generation of triphenyl- and trimethyl-
stannylpotassium.
We generated triphenyl- and trimethylstannylpotas-
sium by reaction of potassium t-butoxide with the
corresponding triorganostannanes (R3SnH) [6]. The re-
sulting anions were trapped with n-butyl iodide and
quantified by GLC (Fig. 2).
of the reduction potential of the ketone. Thus, while
ketones 2–5 (Ered within the range −1.0 to −1.4 V
[8]) give high yields, ketone 1 (Ered= −2.2 V [8]) gives
low yields or no product either with triphenyl- or
trimethylstannylpotassium.
Moreover, the reaction of triphenylstannylpotassium
with butenone (Ered= −2.25 V [8]) in ACN led (1 h) to
4-(triphenyltin)butanone in 76.6% yield. This reaction
was not inhibited by the addition of galvinoxyl or
p-DNB. Probably, the reaction takes place by a direct
nucleophilic addition which is favoured in less hindered
ketones [9]. It is to be noted that we could not detect
any of di-addition product (conjugate addition of the
enolate anion to a second molecule of ketone) as had
been observed when the reaction was carried out in
THF [10].
The reaction of the enones 2–5 with the triphenyl-
stannylpotassium followed by quenching of the inter-
mediate enolates with methyl iodide or water led in
nearly quantitative yields either to a mixture of
diastereomers or to a pure diastereomer of the desired
b-stannylketones in a rather instantaneous reaction
(Table 1, Entries 3,7, 11 and 14). For example, the
addition at room temperature of (E)-1,2,3-triphenyl-
propenone (3) to a solution of 1.2 equivalents of
triphenylstannylpotassium in ACN followed after a few
seconds by quenching with water, afforded a mixture
(98.3%) of diastereomers 11 (75%) and 12 (25%) (Entry
7). The reaction with enone 1 led to the pure
diastereomer 6 but in rather low yield (18,2%) (Entry
1). Similar reactions involving enones 1–4 and
trimethylstannylpotassium in ACN were also carried
out, showing that while ketones 2 and 3 gave mixtures
of diastereomers in high yields (Table 1, Entries 5 and
9), ketone 4 gave only adduct 16 in 45% yield (Entry
13), and ketone 1 failed to react (Entry 2). Attempts to
increase the yields by either increasing reaction times or
lowering the temperature proved to be unfruitful.
Moreover, the results summarized in Table 1 show
that trimethylstannylpotassium is less reactive than
triphenylstannylpotassium toward the substrates stud-
ied under these reaction conditions.
Triorganostannyl anions have been proven to be
excellent one-electron donors toward alkyl halides [7].
Nevertheless, as far as we know there are no examples
in the literature of a single electron transfer (SET)
mechanism in the reaction between triorganostannyl
anions and enones. In our view 1,4-conjugate addition
of triphenyl- and trimethylstannylpotassium to enones
1–5 in ACN takes place clearly via a SET mechanism.
The partial or total inhibition of the reactions by
addition of a free radical scavenger (galvinoxyl) or a
radical anion scavenger (p-dinitrobenzene, p-DNB)
(Table 1, Entries 4, 6, 8, 10, 12 and 15) leads us to
believe that the conjugate addition of triphenyl- and
trimethylstannylpotassium to the enones studied could
be an example of a two-stage reaction involving an
initial electron transfer step (Scheme 1). The electron
transfer nature of this reaction appears to be a function
The analysis of the diastereomeric mixtures gave
valuable information about the stereochemistry of these
reactions. Product analysis showed that one dia-
stereomer or mixtures of diastereomers with a relatively
high predominance of one of them were always ob-
tained (Table 1). The diastereomeric ratios in the
product mixtures were identical independently of the
starting olefin configuration. Consequently, these reac-
tions are stereoselective but certainly not stereospecific.
When the reactions were carried out using limiting
amounts of stannyl anion (olefin/anion: 1/0.5), the ad-
dition products were detected together with the starting
olefin which showed no appreciable isomerization.
These stereochemical results indicate that the collapsing
rate of the SET intermediates (radical–radical anion
pair) to give products is faster than the isomerization
rate of cis and trans ketyl and that the diastereomeric
ratios of the products would depend only on the stereo-
chemistry of the electrophilic attack (Scheme 1).
The 1H-and 13C-NMR characteristics of the new
b-stannylketones are summarized in Tables 2 and 3.
The 13C-NMR chemical shifts were assigned by means
of DEPT experiments and taking into account the
n
magnitude of J (13C, 119Sn) coupling constants.
The configuration of the diastereoisomers was
1
defined unambiguously on the basis of 13C- and H-
NMR data. Thus, the correlation existing between the
coupling constants and the dihedral angles [11] enabled
us to assign the configuration threo to compounds 8,

A.B. Chopa et al. / Journal of Organometallic Chemistry 585 (1999) 35–42
37
10, 12, 14 and 15 and erythro to compounds 6, 7, 9, 11
and 13.
3, eclipsed-conformer I) and/or of electronic factors (Fig.
3, conformer II). As for the electronic factors, theoretical
studies carried out by Houk et al. [13] and experimental
results obtained by Fleming et al. [14] and McGarvey
et al. [3] showed that the lower energy transition states
are the ones originated from those conformers in
which the electron donor substituent (R3Sn) is lo-
cated in the least crowded anti-conformational posi-
The diastereoselectivity of the additions has been
explained taking into account the stability of the enolate
intermediates as well as the stereoselection of the elec-
trophilic attack. The stability of the enolate intermediates
could be rationalized in terms of the steric requirements
of the substituents attached to the chiral carbon [12] (Fig.
Table 1
Reaction of trimethyl- and triphenyltin–potassium with enones 1–5 e
Entry
Enone
Reaction conditions
Erythro/threo No. (%)
Yield (%) a
1
2
Ph3Sn− K+, H2O
Me3Sn− K+, H2O
6 (100)/0
–
18
0
3
4
5
6
Ph3Sn− K+, H2O
Ph3Sn− K+, H2O b,c
Me3Sn− K+, H2O
Me3Sn− K+, H2O b
7 (97)/8 (3)
–
9 (67)/10 (33)
9 (67)/10 (33)
97
0
77
8
7
Ph3Sn− K+, H2O
11 (75)/12 (25)
98
8
9
10
Ph3Sn− K+, H2O b,c
Me3Sn− K+, H2O
Me3Sn− K+, H2O b
–
0
86
41
13 (43)/14 (57)
13 (43)/14 (57)
11
12
13
Ph3Sn− K+, MeI
Ph3Sn− K+, MeI b
Me3Sn− K+, MeI
0/15 (100)
0/15 (100)
0/16 d (100)
99
34
45
14
15
Ph3Sn− K+, MeI
Ph3Sn− K+, MeI b
0/8 (100)
0/8 (100)
99
27
a Yields refer to isolated, purified material by column chromatography.
b Addition of 20% mol of galvinoxyl.
c The same results were obtained in the presence of 20% mol of p-DNB.
d Erythro/threo: 1/10, 60%, in THF [2].
e Stereochemical results and yields.

Table 2
1H-NMR data of compounds 6–15 a
/
Number
R
R1 R2 R3 Ha 3J(Sn, H)
Hb 2J(Sn, H)
J(Ha, Hb)
Others
6
7
Ph Me Me Me 2.72 (59.7)
Ph Me Ph Ph 4.43 (35.1)
4.17 (81.3)
2.20 (62.0)
3.92 (61.7)
3.37 (71.8)
3.13 (58.4)
7.3
11.0
5.3
1.15 (3H, d, 3J(H,H) 7.0); 1.26 (3H, d, 3J(H,H) 7.8); 1.95 (3H, s); 7.15–7.28 (9H, m); 7.36–7.56, (6H, m)
1.38 (3H, d, 3J(H,H) 7.0); 6.94–7.11 (5H, m); 7.31–7.47 (18H, m); 7.33 (2H, d, 3J(H,H) 7.2)
1.13 (3H, d, 3J(H,H) 7.3); 6.83–7.45 (23H, m); 7.77 (2H, d, 3J(H,H) 7.5)
0.07 (9H, s, 2J(Sn,H) 51.9); 1.35 (3H, d, 3J(H,H) 6.8); 6.88–7.14 (3H, m); 7.34–7.54 (5H, m); 7.81–7.88
(2H, m)
8
9 b
Me Me Ph Ph 4.23 (NO)
10.6
10 b
4.21 (50.5)
2.80 (64.8)
8.0
−0.03 (9H, s, 2J(Sn,H) 52.0); 1.26 (3H, d, 3J(H,H) 7.1); 6.99–7.06 (3H, m); 7.34–7.54 (5H, m); 7.81–
7.88 (2H, m)
5
11
12
Ph Ph Ph Ph 5.55 (29.7)
5.20 (54.4)
Me Ph Ph Ph 5.37 (23.4)
3.88 (34.1)
4.36 (57.4)
3.77 (70.8)
3.68 (56.0)
3.34 (60.6)
3.14 (72.3)
12.8
9.2
12.3
12.1
8.3
6.94–7.43 (28H, m); 7.76 (2H, d, 3J(H,H) 8.2)
6.84–7.39 (28H, m); 7.58 (2H, d, 3J(H,H) 7.2)
(
13 b
14 b
15
0.13 (9H, s, 2J(Sn,H) 50.0); 6.83–7.51 (15H, m)
0.12 (9H, s, 2J(Sn,H) 51.6); 6.74–7.21 (15H, m)
3
Ph Me Ph Me 3.28 (59.7)
1.02 (3H, d, 3J(H,H) 7.0); 1.77 (3H, s); 6.84–7.04 (5H, m); 7.15–7.33 (15H, m)
a Chemical shifts in ppm versus TMS, nJ in Hz. In CDCl3. Multiplicity: s stands for singlet, d for doublet and m for multiplet.
b From Ref. [15].

Table 3
13C-NMR data of compounds 6–15 a
Number
R
R1 R2 R3 C-1
C-2
C-3
C-1%
C-2%
Others
6
7
Ph Me Me Me 213.38 (37.9)
Ph Me Ph Ph 204.35 (50.1)
51.57 (14.6)
43.95 (NO)
26.19 (439.3)
41.69 (NO)
16.64 (15.5)
20.67 (16.5)
17.44 (35.9)
143.29 (28.8)
139.72, 137.27 (33.0), 128.64, 128.58, 128.33 (46.6)
138.74 (484.0), 137.15 (35.0), 132.70, 128.84 (10.7), 128.48, 128.43
(47.6), 127.93, 124.85 (13.6)
/
8
204.43 (20.3)
Me Me Ph Ph 204.40 (50.9)
204.28 (13.3)
44.91 (16.9)
43.51 (NO)
44.25 (13.6)
55.88 (NO)
58.27 (11.9)
55.52 (NO)
59.68 (11.0)
50.91 (14.6)
40.70 (NO)
19.91 (28.8)
19.61 (19.5)
20.11 (33.9)
137.39 (10.2)
139.41 (44.9)
138.62 (11.0)
140.69 (56.8)
18.26 (40.8)
143.62 (35.6)
144.90 (29.6)
145.12 (31.7)
143.22 (33.9)
142.76 (31.4)
144.55 (29.7)
145.74 (33.1)
143.38 (31.0)
140.77 (495.1), 137.25 (34.8), 133.01, 128.41, 128.36, 128.09 (11.9),
127.82 (49.2), 127.14 (25.4), 124.38 (15.3)
9 b
10 b
11
39.70 (332.3)
38.75 (334.8)
43.24 (362.8)
42.53 (377.2)
40.07 (337.4)
35.39 (373.8)
39.99 (384.8)
136.94, 132.35, 130.00, 129.10, 128.21, 128.08, 127.76, 126.54 (24.2),
123.86 (13.1), −9.16 (323.0).
135.89, 132.93, 128.56, 128.26, 126.42 (23.2), 123.76 (13.5), −8.40
(328.1)
Ph Ph Ph Ph 199.90 (68.7)
200.54 (14.4)
138.74 (487.4), 137.19 (34.8), 132.64, 129.44, 128.95, 128.58 (11.0),
128.39, 128.23, 128.13, 127.99, 127.89, 124.98 (14.4)
5
12
142.76 (31.3), 140.15 (493.4), 137.35 (34.7), 132.60, 128.71, 128.51,
128.27, 128.24, 128.15, 126.67, 124.52 (14.4)
)
13 b
14 b
15
Me Ph Ph Ph 199.87 (56.0)
200.37 (9.2)
137.39, 132.30, 128.97, 128.72, 128.19, 128.14, 127.99, 127.50,
126.34, (22.9), 123.97 (12.7), −10.06 (324.7)
3
143.69, 140.49, 128.63, 128.34, 127.13, 126.78, 126.22, 125.58, 122.54
(15.3), −8.45 (326.8)
Ph Me Ph Me 213.55 (17.5)
140.30 (490.8), 137.36 (34.0), 128.57(11.7), 128.35, 128.04 (48.6),
127.59 (26.2), 124.57 (14.6)
a Chemical shifts in ppm with respect TMS; nJ, in brackets, in Hz; solvent, CDCl3; NO: not observed. C-1% and C-2% are the ‘first’ carbons in R1 and R2, respectively.
b From Ref. [15].

40
A.B. Chopa et al. / Journal of Organometallic Chemistry 585 (1999) 35–42
Scheme 1. Reaction of triphenyl- and trimethylstannyl–potassium with enones 1–5.
tion, i.e. perpendicular to the double bond. Then in
order to minimize torsional strains in the transition state,
the electrophile would attack on the same side of the
organotin moiety in conformer I and on the opposite
side in conformer II, leading to the syn and anti addition
products, respectively.
Our experimental results indicate that in the reactions
of the triphenyltin anion with activated ketones the
products are predominantly those resulting from an anti
attack, i.e. the attack on the preferred conformation II
(Scheme 2). The rather diminished stereoselection found
in the reaction of this anion with 1,2,3-triphenyl-
propenone (3) (erythro/threo: 75/25), could be explained
taking into account that due to the larger steric require-
ments of the phenyl groups in conformation II (R1=
R2=Ph), the attack of the electrophile to conformations
I and II would lead to transition states of comparable
energy.
The reaction of the trimethyltin anion with 4-phenyl-
butenone followed by quenching with methyl iodide
gave as only product the threo diastereomer (45%),
confirming the preference of electrophilic attack to
conformer II. However, whereas the reaction of
trimethylstannylpotassium with 1,3-diphenyl-2-methyl-
propenone (2) led also to the anti addition but with
diminished stereoselection (erythro/threo, 67/33), the
reaction with 1,2,3-triphenylpropenone (3) gave the syn
addition product (erythro/threo, 43/57) in higher yield.
The observed stereoselection could be explained by the
attack of the electrophile to conformer I shown in
Scheme 3.
reaction conditions, the triphenyltin group acts as a
better donor electron group than the trimethyltin group.
This study provides experimental support for the
existence of a single electron transfer mechanism in the
1,4-addition of triphenyl- and trimethyltinpotassium
over different enones, in ACN as solvent. The possibility
of SET depends on the one-electron donor ability of the
nucleophile and the electron-acceptor ability of the
ketone. Sterically hindered molecules with low reduction
potential (enone 1) lead to low or null yields. On the
other hand, uncrowded ketones yield the 1,4-addition
products through a direct nucleophilic attack.
3. Experimental
3.1. General
1H and 13C-NMR spectra were determined with a
Bruker AC200 instrument at IQUIOS (Rosario, Ar-
gentina) and a Bruker AM300 instrument at Dortmund
University (Germany). Ph3SnH and Me3SnH were pre-
pared by the reaction of Ph3SnCl (Fluka) and Me3SnCl
(Fluka) and LiAlH4 as reported [16]. Acetonitrile was
vacuum distilled from H2Ca and put in storage over
molecular sieves. Commercial 1,3-diphenylpropenone
and 4-phenyl-3-buten-2-one were recrystallized before
used. The olefins used were synthesized by known
procedures [17–19].
All the reactions were monitored by thin-layer chro-
matography (TLC) and judged complete when starting
These results suggest that the diastereocontrol ob-
served in the addition of stannyl anions to the acyclic
activated ketones studied arises from stereoelectronic
and steric effects. Thus, stereoelectronic effects highly
prevail over the steric hindrance when the triphenyltin
group acts as a donor group, while the opposite situation
exists in the case of the trimethyltin group. On the basis
of these results, we could also assume that under these
Fig. 3. Eclipsed and perpendicular models for enolate conformations.

A.B. Chopa et al. / Journal of Organometallic Chemistry 585 (1999) 35–42
41
and the solvent was removed by distillation under
reduced pressure. Column chromatography on silica gel
60 of the crude mixture yielded 0.35 g (0.61 mmol, 94%)
of compound 7 and 0.011 g (0.019 mmol, 2.9%) of 8.
The reaction of triphenylstannylpotassium with 1,3-
diphenylpropenone (5) was carried out under similar
conditions. The resulting mixture was quenched by
adding 0.1 ml (0.22 g, 1.6 mmol) of methyl iodide
instead of water and was stirred during 15 min. After
usual work up the reaction conduced to a quantitative
yield of isomer 8.
Addition reactions in the presence of galvinoxyl or
p-DNB. The procedure was similar to that for the
previous reactions, except that 20 mol% of galvinoxyl
or p-DNB was added to the solution of the anion prior
to the substrate addition.
Scheme 2. Preferred intermediate enolates and major products in the
reaction of enones 1–5 with triphenylstannyl–potassium.
Acknowledgements
This work was partially supported by CIC, CON-
ICET and UNS (Argentina). The CIC is thanked for a
research fellowship to A.P.M. The authors thank Dr
T.N. Mitchell (University of Dortmund, Germany) and
Dr M. Gonza´lez Sierra (IQUIOS, Argentina) for the
NMR spectra.
material was no longer visible in the reaction mixture.
Analytical TLC was performed using precoated silica
gel plates with phosphomolibdic acid in ethanol as
indicator. The diastereoisomers ratios were determined
by GLC.
All the reactions were carried out following the same
procedure. One experiment is described in detail in each
case in order to illustrate the method used.
References
3.2. Reaction of triphenylstannylpotassium with 1,3-
diphenyl-2-methylpropenone (2). Synthesis of erythro-
and threo-1,3-diphenyl-2-methyl-3-(triphenylstannyl)
propanone (7 and 8)
[1] W.C. Still, J. Am. Chem. Soc. 99 (1977) 4836.
[2] I. Fleming, C.J. Urch, J. Organomet. Chem. 285 (1985) 173.
[3] G.J. Mc Garvey, J.M. Williams, J. Am. Chem. Soc 107 (1985)
1435.
[4] M.T. Lockhart, Thesis (1998). Results to be published
[5] A.G. Davies, Organotin Compounds, VCH, Weinheim, 1997.
E.W. Abel, F.G.A. Stone, G. Wilkinson (Eds.), Comprehensive
Organometallic Chemistry II, vol. 11, Elsevier, Amsterdam,
1995, p. 356. M. Pereyre, J.P. Quintard, A. Rahm, Tin in
Organic Synthesis, Butterworths, London, 1987.
[6] The pKa of Ph3SnH is ca. 16 (E.S. Petrov, M.I. Terekhova, R.G.
Mirskov, M.G. Voronkov, A.I. Shatenshtein, Dokl. Akad.
Nauk. SSSR 221 (1975) 111; Chem. Abstr. 82 (1975) 155040e;
Me3SnH ca. 25 (M.S. Alnajjar, H.G. Kuivila, J. Am. Chem.
Soc., 107 (1985) 416), lower than t-BuOH (32.2) (F.G. Bordwell,
Acc. Chem. Res. 21 (1988) 456).
The reactions were carried out in a two-necked, 25
ml, round-bottomed flask equipped with a nitrogen
inlet, a magnetic stirrer and septums. To a solution of
0.088 g (0.78 mmol) of t-BuOK in 5 ml of ACN was
added, by syringe, a solution of 0.27 g (0.2 ml, 0.78
mmol) of Ph3SnH in 4.8 ml of ACN. After 5 min, a
solution of 0.144 g (0.65 mmole) of 2 in 5 ml of ACN
was added. The resulting mixture was quenched imme-
diately by adding 5 ml of water. The mixture was
extracted with ether, dried over magnesium sulphate
[7] E.C. Ashby, Acc. Chem. Res. 21 (1988) 414 and references cited
therein.
[8] H.O. House, L.E. Huber, M.J. Umen, J. Am. Chem. Soc. 94
(1972) 8471.
[9] A. Pross, Acc. Chem. Res. 18 (1985) 212.
[10] E. Piers, H.E. Morton, J.M. Chong, Can. J. Chem. 65 (1987) 78.
[11] T.N. Mitchell, J.C. Podesta´, A. Ayala, A.B. Chopa, Magn.
Reson. Chem. 26 (1988) 497.
[12] The A value of the phenyl group is 2.8 kcal mol−1: M.E.
Squillacote, J.M. Neth, J. Am. Chem. Soc. 109 (1987)198. The A
value of the methyl group is 1.74 kcal mol−1: H. Book, J.R.
Everett, J. Chem. Soc. Chem. Commun. (1976) 278. The A
values of Me3Sn- and Ph3Sn- are 1.0 kcal mol−1 and 1.44 kcal
mol−1, respectively: W. Kitching, H.A. Olszowy, K. Harvey, J.
Org. Chem. 47 (1982) 1893.
Scheme 3. Preferred intermediate enolate and major product in the
reaction of enone 3 with trimethylstannyl–potassium.

42
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