Unique reactivities of new highly coordinated ate complexes of organozinc derivatives

Full text of DOI:10.1021/ja961320e

J. Am. Chem. Soc. 1996, 118, 8733-8734
Table 1. 1H-NMR of Zincates in THF (-20 °C)
8733
Unique Reactivities of New Highly Coordinated Ate
Complexes of Organozinc Derivatives
Me (ppm)a
δ
entry
metal reagent
1
2
3
4
5
6
7
8
MeLi
-1.96
-0.84
-1.08
-1.44
-1.23
-1.31
-1.20
-1.95
Me
Me
Me
Me
2
3
4
3
Zn
ZnLi
ZnLi
Masanobu Uchiyama, Minako Koike, Mitsuyoshi Kameda,
Yoshinori Kondo,* and Takao Sakamoto*
2
Zn(SCN)Li
2
Faculty of Pharmaceutical Sciences
Tohoku UniVersity, Aobayama, Aoba-ku
Sendai 980-77, Japan
Zn(SCN)
2
+ 3MeLi
Me Zn(CN)Li2
3
Zn(CN) + 3MeLi
2
a
The δ values are relative to â methylene proton (1.85 ppm) to
THF.
ReceiVed April 22, 1996
Organozinc reagents have been extensively used in organic
synthesis. The most important classes of organozinc derivatives
1
First, preparation of highly coordinated zincates were inves-
tigated, and H-NMR spectra of the zincates were measured
1
have been organozinc halides and diorganozincs, and these have
been prepared either by oxidative addition reaction of zinc metal
to organic halides or by transmetalation reaction of organolithio
or organomagnesio compounds with zinc chloride. As a new
type of synthetic reagent, triorganozincates were reported to be
convenient and synthetically useful for transferring various
for the preliminary estimation of the component of the zincate
solution. Dilithium tetramethylzincate was prepared by the reac-
tion of zinc chloride with 4 equiv of methyllithium in THF using
4
a
the modified reported procedure. The methyl signal of Me4-
ZnLi2 in THF (-1.44 ppm) was observed midway as a sharp
2
singlet between the signals of Me ZnLi (-1.08 ppm) and MeLi
organic moieties to R,â-unsaturated ketones in a 1,4-fashion.
3
(
-1.96 ppm). High-field shift from the value of Me3ZnLi is
Lithium trialkylzincates were also reported to be useful as
metalating reagents of aromatic halides or vinyl halides. The
3
considered to indicate the more anionic character of the zincates.
Measurement at -78 °C gave very similar results with a slight
high-field shift of each signal. Addition of one more equivalent
of MeLi to the Me4ZnLi2 solution resulted in a broad singlet
signal at -1.49 ppm at -20 °C which separated at -78 °C
into two signals corresponding to the signals of MeLi and those
of Me4ZnLi2. Addition of 1 equiv of Me3SiCN or Me3SiNCS
to the Me4ZnLi2 solution should give the zincates with methyl
group replacement by the CN or SCN ligand. As we expected,
the reaction of the tetramethylzincate with Me SiCN and Me -
outer shell of the zinc atom in a lithium trialkylzincate is filled
with 16 electrons, and there is a room for an additional ligand
to coordinate to form a favorable 18-electron state. There are
4
some reports on tetraalkylzincates and X-ray studies of the
complexes which have disclosed that in the crystal structure of
tetraalkylzincates there is a tetrahedral arrangement about the
zinc atom.5 However the reactivities of tetraalkylzincates have
not been well studied. The unique tetrahedral form of these
complexes made us study the reactivity of these tetraalkylor-
3
3
1
ganozincates and related modified organozincates. The H-
3 2 3
SiNCS gave the new zincates, Me Zn(CN)Li (3) and Me Zn-
NMR study of organozincates indicated the difference between
these highly coordinated zincates and lithium trimethylzincate.
Studies to compared the reactivity of these reagents disclosed
a different regioselectivity for epoxide opening and a different
reactivity toward halogen-metal exchange. These results
support that these highly coordinated zincates should be
distinguished from ordinary lithium trialkylzincates in structure
and reactivity.
2
(SCN)Li (4a), of which methyl signals were observed as sharp
6a
singlets (-1.20 ppm for 3 and -1.23 ppm for 4a). As an
alternative way to prepare CN- or SCN-ligated zincates, the
reaction of Zn(CN) or Zn(SCN) with 3 equiv of methyllithium
2
2
1
was examined. From the preliminarly H-NMR study, the reac-
tion of Zn(CN)2 with MeLi is sluggish and no formation of a
zincate was observed, while the reaction of Zn(SCN)2 with
6b
MeLi gave a zincate (-1.31 ppm) similar to 4a (Table 1). These
results indicate that the different species of zincates are formed
except for the reaction of Zn(CN)2 with MeLi, and the more
anionic character of these highly coordinated zincates than that
of Me3ZnLi encouraged us to investigate the following reactions.
Regioselectivity of intermolecular ring-opening reaction of
epoxides has been investigated focusing on the nature of the
organometallic reagent, such as the Lewis acidity or the
7
basicity. Styrene oxide was reacted with various organome-
(
1) (a) Knochel, P.; Singer, R. D. Chem. ReV. 1993, 93, 2117-2188.
tallic derivatives, and the ratio of two isomeric compounds,
1-phenyl-1-propanol (6) and 2-phenyl-1-propanol (7) was
estimated. The results are as listed in Table 2. Reaction with
Me3ZnLi gave almost equal amounts of 6 and 7. Me4ZnLi2
also showed similar selectivity, while the reaction with Me3-
Zn(CN)Li2 and Me3Zn(SCN)Li2 gave 7 as a major product, and
the reaction with MeLi and Me2Cu(CN)Li2 gave 6 as a major
product. Although the reasons for the divergent selectivity are
not clear at present, the inverse selectivity between Me3Zn(CN)-
Li2 and Me2Cu(CN)Li2 is attractive from a synthetic viewpoint.
(
b) Erdik, E. Tetrahedron 1987, 43, 2203-2212. (c) Erdik, E. Tetrahedron
1
992, 48, 9577-9648. (d) Knochel, P. In ComprehensiVe Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, 1991;
Vol. 1, pp. 211-229.
(2) (a) Isobe, M.; Kondo, S.; Nagasawa, N.; Goto, T. Chem. Lett. 1977,
6
1
1
1
79-682. (b) Tuckmantel, W.; Oshima, K.; Nozaki, H. Chem. Ber. 1986,
19, 1581-1593. (c) Jansen, J. F. G. A.; Feringa, B. L. Tetrahedron Lett.
988, 29, 3593-3596. (d) Kjonaas, R. A.; Hoffer, R. K. J. Org. Chem.
988, 53, 4133-4135.
(
3) (a) Kondo, Y.; Takazawa, N.; Yamazaki, C.; Sakamoto, T. J. Org.
Chem. 1994, 59, 4717-4718. (b) Harada, T.; Katsuhira, K.; Hattori, K.;
Oku, A. J. Org. Chem. 1993, 58, 2958-2965. (c) Harada, T.; Katsuhira,
K.; Hara, D.; Kotani, Y.; Maejima, K.; Kaji, R.; Oku, A. J. Org. Chem.
1
993, 58, 4897-4907.
4) (a) Hurd, D. T. J. Org. Chem. 1948, 13, 711-713; (b) Nast, R. Angew.
Chem. 1960, 72, 26-31. (c) Seitz, L. M.; Brown, T. L. J. Am. Chem. Soc.
Our next interest was focused on the intramolecular ring-
opening reaction of epoxide 8 using the zincates as metalating
(
1
966, 88, 4140-4147. (d) Toppet, S.; Slinckx, G.; Smets, G. J. Organomet.
Chem. 1967, 9, 205-213. (e) Kaufmann, F.; Geraudelle, A.; Kaempf, B.;
(6) (a) The signal of tetramethylsilane newly generated in the mixture
was observed at 0.00 ppm together with disappearance of the signal of Me3-
SiCN (0.38 ppm) or Me3SiNCS (0.40 ppm). (b) For recent discussion on
the structure of Zn(CN)2, see: Hoskins, B. F.; Robson, R. J. Am. Chem.
Soc. 1990, 112, 1546-1554.
Schue, F.; Deluzarche, A.; Maillard, A. J. Organomet. Chem. 1974, 71,
1
1
1-15. (f) Chastrette, M.; Gauthier, R. J. Organomet. Chem. 1974, 71,
1-15.
(
5) (a) Weiss, E.; Wolfrum, R. Chem. Ber. 1968, 101, 35-40. (b) Weiss,
E.; Plass, H. J. Organomet. Chem. 1968, 14, 21-31.
(7) Smith, J. G. Synthesis 1984, 629-656.
S0002-7863(96)01320-0 CCC: $12.00 © 1996 American Chemical Society

8734 J. Am. Chem. Soc., Vol. 118, No. 36, 1996
Communications to the Editor
Table 3. Intramolecular Epoxide-Opening Reaction
entry
metalating reagent
yield (%) 9 + 10
9:10
1
2
3
4
5
6
Me
Me
Me
Me
Zn(SCN)
Me
3
4
3
3
ZnLi
ZnLi
Zn(CN)Li
Zn(SCN)Li
86
67
57
92
80
50
4:96
87:13
80:20
97:3
81:19
12:88
Table 2. Intermolecular Epoxide-Opening Reactions
2
2
entry
metal reagent
yield (%) 6 + 7
6:7
2
2
+ 3MeLi
1
2
3
4
5
6
7
8
Me
Me
Me
Me
Zn(SCN)
MeLi
3
4
3
3
ZnLi
ZnLi
Zn(CN)Li
Zn(SCN)Li
70
93
41
trace
trace
trace
93
56:44
51:49
29:71
37:63
32:68
71:29
68:32
61:39
2
Cu(CN)Li
2
2
2
2
lation was observed. On the other hand, the reaction of 13 with
Me4ZnLi2 followed by hydrolysis gave 3-methyldihydrobenzo-
b]furan (15) in 42% yield, which probably results from
intramolecular carbozincation.
2
+ 3MeLi
[
Me
Me
2
Cu(CN)Li
Cu(SCN)Li
2
2
2
100
agents.8 Interestingly, Me3ZnLi showed almost completely
opposite regioselectivity from those of the other modified
zincates for the ring-opening reaction (Table 3). Especially,
the reaction with Me3ZnLi and the reaction with Me3Zn(SCN)-
Li2 showed almost perfect reverse regioselectivity. The reaction
with Me3ZnLi gave endo-cyclized 1,2,3,4-tetrahydroquinoline
derivative (10) as a major product, while the reaction with Me3-
Zn(SCN)Li2 gave exo-cyclized indoline derivative (9). Interest-
ingly, the cyano-modified reagents, Me3Zn(CN)Li2 and Me2Cu-
We have reported that Me3ZnLi is useful for iodine-zinc
exchange reaction of functionalized aromatic iodides; however,
aromatic bromides were unreactive to this metalation process.3a
Since Me4ZnLi2 is considered to have more anionic character,
the bromine-zinc exchange reaction of bromobenzene was
then investigated. The reaction was carried out at -20 °C, and
the resulting metal species was trapped with benzaldehyde to
give benzhydrol in 47% yield (Table 4). Under the same
reaction conditions, the reaction with Me3ZnLi did not proceed
at all, and MeLi was also found inactive under the same
conditions. The modified zincates, Me3Zn(CN)Li2 and Me3-
Zn(SCN)Li2, showed moderate reactivity toward the bromine-
zinc exchange reaction, and benzhydrol was obtained in 22%
and 23% yields, respectively. These results also support the
structural difference between the highly coodinated zincates and
Me3ZnLi.
(
CN)Li2, showed completely opposite regioselectivity in this
system. R2Cu(CN)Li2 compounds have been called “higher
order cuprates”, and the structure of these cuprates has been a
controversial issue. However recent studies on the structure
9
1
0
revealed that the CN ligand is not on the Cu atom but on the Li
atom.11 The structures of highly coordinated zincates with the
CN or SCN ligand have never been studied. Taking the
tendency of zinc derivatives to form complexes with tetrahedral
3
configuration (sp -hybridized structure) into account, we cannot
omit the possibility of the ligation of CN or SCN to the Zn
atom.
Table 4. Bromine-Zinc Exchange Reaction
entry
metalating reagent
yield (%)
Intramolecular carbometalation was then examined using
iodine-zinc exchange reaction of allyl 2-iodophenyl ether (13).
When 13 was treated with Me3ZnLi, halogen-metal exchange
reaction proceeded smoothly, but no intramolecular carbometa-
1
2
3
4
5
MeLi
0
0
47
22
23
Me
Me
3
4
ZnLi
ZnLi
2
Me Zn(CN)Li2
3
3 2
Me Zn(SCN)Li
(8) We recently reported a regioselective ring-opening reaction of
epoxyorganometallic compounds which derived from halogen-metal
exchange reaction using ate complexes. Kondo, Y.; Matsudaira, T.; Sato,
J.; Murata, N.; Sakamoto, T. Angew. Chem., Int. Ed. Engl. 1996, 35, 736-
Further studies on the structural analyses of the new highly
coordinated organozincates are underway together with their
applications for organic syntheses.
7
38.
(
9) (a) Lipshutz, B. H.; Sengupta, S. Org. React. 1992, 41, 135-631.
(
b) Lipshutz, B. H.; Wilhelm, R. S.; Kozlowski, J. A. Tetrahedron 1984,
4
0, 5005-5038. (c) Lipshutz, B. H. Synthesis 1987, 325-341.
10) (a) Snyder, J. P.; Spangler, D. D.; Behling, J. R. J. Org. Chem.
Acknowledgment. We are grateful to the reviewers for their useful
and valuable comments.
(
1
994, 59, 2665-2667. (b) Lipshutz, B. H.; Ellsworth, E. L.; Siahaan, T.
J. J. Am. Chem. Soc. 1988, 110, 4834-4835. (c) Bertz, B. H. J. Am. Chem.
Soc. 1990, 112, 4031-4032. (d) Lipshutz, B. H.; Sharma, S.; Ellsworth,
E. J. Am. Chem. Soc. 1990, 112, 4032-4034.
Supporting Information Available: Representative experimental
1
procedures and H-NMR spectra for the zincates (6 pages). See any
(
11) Stemmler, T. L.; Barnhart, T. M.; Penner-Hahn, J. E.; Tucker, C.
current masthead page for ordering and Internet access instructions.
E.; Knochel, P.; Boehme, M.; Frenking, G. J. Am. Chem. Soc. 1995, 117,
2489-12497.
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JA961320E