Synthesis, Structure, and Thermolysis of a Pentacoordinate 1,2-Oxastibetane

Article abstract of DOI:10.1021/ja037586k

Isomeric pentacoordinate 1,2-oxastibetanes bearing the Martin ligand have been synthesized by the reactions of bromo(2-hydroxyalkyl)stiboranes with sodium hydride of which one of the isomers was characterized by X-ray crystallographic analysis and NMR spectroscopy. Thermolysis of the isomer shown (CD₃CN, 140 °C) afforded the oxirane with retention of configuration, while its thermolyses in the presence of lithium bromide and LiBPh₄·3DME provided the oxirane with inversion of configuration and the olefin. The addition of the salts in the thermolyses of this pentacoordinate 1,2-oxastibetane controls both the stereochemistry of the oxirane formed and the ratios of the three main products. Copyright

Full text of DOI:10.1021/ja037586k

Published on Web 10/10/2003
Synthesis, Structure, and Thermolysis of a Pentacoordinate 1,2-Oxastibetane
Yosuke Uchiyama, Naokazu Kano, and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan
Received July 29, 2003; E-mail: takayuki@chem.s.u-tokyo.ac.jp
Scheme 1 a
A pentacoordinate 1,2-oxaphosphetane has been widely accepted
as the intermediate of the Wittig reaction, which is well-known as
a useful method for olefin formation.1 On the other hand, reactions
of stibonium ylides (heavier congeners of phosphonium ylides) with
carbonyl compounds afford olefins and oxiranes, depending on the
ylide used.2 The difference in reactivity between phosphonium and
stibonium ylides toward carbonyl compounds has been ascribed to
different reaction intermediates. The olefin and the oxirane are
believed to be formed from the corresponding oxetane containing
a pentacoordinate group 15 element at the position adjacent to the
oxygen atom and from the corresponding anti-betaine, respectively.
Although the reaction of a stibonium ylide with a carbonyl
compound has been investigated by theoretical calculations based
on the above assumptions,3 there has been no report of the isolation
of an intermediate. We herein describe the synthesis and the
properties of a pentacoordinate 1,2-oxastibetane, which is an
antimony analogue of a pentacoordinate 1,2-oxaphosphetane and
a formal [2 + 2] cycloadduct of a stibonium ylide and a carbonyl
compound.
a Reaction conditions: Ar ) 4-t-Bu-C6H4, a: R1 ) Ph, R2 ) CF3, b:
R1 ) CF3, R2 ) Ph. i) PhCH2MgCl (1.2 equiv), Et2O, 0 °C, 2 h; ii) LiTMP
(2.2 equiv), benzene, rt, 12 h; iii) Ph(F3C)CdO (2.2 equiv), rt, 30 min; iv)
H3O+; v) SiO2 column chromatography; vi) Br2 (2.0 equiv), CHCl3, rt, 1
h; vii) NaH (4.0 equiv), THF, rt, 2 h; viii) PhLi, THF, rt, 1 h; ix) H3O+.
Benzylstiborane 1 bearing the Martin ligand,4 which was
synthesized by the reaction of chlorostiborane 25 with benzylmag-
nesium chloride, was allowed to react successively with LiTMP
and trifluoroacetophenone in benzene at room temperature to give
a mixture of 2-hydroxyalkylstiboranes 3a and 3b. After separation
by silica gel column chromatography, treatment of 3a and 3b with
bromine gave bromo(2-hydroxyalkyl)stiboranes 4a and 4b, which
were allowed to react with NaH to afford the isomeric pentacoor-
dinate 1,2-oxastibetanes 5a and 5b, respectively (Scheme 1).
Although 5b was unstable toward moisture and could not be
isolated, 5a was successfully isolated by recrystallization from
hexane as colorless crystals.6
Figure 1. ORTEP drawing of 5a with thermal ellipsoids plotted at 50%
probability. Selected bond lengths (Å) and angles (deg): Sb1-O1, 2.054(7);
Sb1-O2, 2.038(7); Sb1-C1, 2.144(9); Sb1-C3, 2.103(10); Sb1-C4,
2.106(10); C1-C2, 1.575(13); O1-C2, 1.440(11); O1-Sb1-O2, 165.4(3);
O1-Sb1-C1, 69.7(3); Sb1-O1-C2, 96.2(5); Sb1-C1-C2, 88.7(5); C1-
C2-O1, 105.4(7). One of the independent molecules of 5a and the hydrogen
atoms are omitted for clarity.
that the reaction occurs via apical-equatorial ligand coupling.9
Although the mode of such ligand coupling is thermally forbidden
according to the Woodward-Hoffmann rules, Morokuma et al.
reported that this process is thermally favored over other symmetry-
allowed ligand coupling processes in the case of BiH5, which
involves a zwitterionic transition state.8,10 Since a polar transition
state was strongly supported by the solvent effect (entries 2 and
3), it is most likely that the formation of 6 proceeds via apical-
equatorial ligand coupling through polar transition state A (Scheme
2).
Thermolysis of 5a in the presence of LiBr or LiI in CD3CN at
140 °C provided a mixture of 6, 7, and 9 (entries 5 and 6). The
formation of 6 and 9 contrasts the reaction without the salts which
affords only 6 (entry 4). Taking into consideration the results of
the thermolyses with LiI and (n-Bu)4NBr (entries 6 and 7), it is
concluded that both lithium cation and bromide ion play an
important role in the formation of 9, which requires the interaction
X-ray crystallographic analysis of 5a7 revealed that it has a
distorted trigonal bipyramidal structure with two oxygen atoms at
the apical positions and three carbon atoms at the equatorial
positions. Moreover, the relative configuration of the phenyl group
at the 3-position is cis to both the 4-tert-butylphenyl group at the
2-position and the phenyl group at the 4-position of the 1,2-
1
oxastibetane ring (Figure 1). The H, 13C, and 19F NMR spectra
(CDCl3) of 5a were consistent with the crystal structure. At room
temperature, 5a easily undergoes epimerization at the Sb atom by
pseudorotation to give the equilibrium mixture of 5a and its epimer
in a ratio 5:1.8
Thermolysis of 5a in o-xylene-d10 at 220 °C for 17 h gave the
corresponding oxirane 6 (90%) with retention of configuration and
stibine 7 (89%) along with a small amount of 1,1,1-trifluoro-3,3-
diphenyl-2-propanone (5%) and tert-butylbenzene (5%), the latter
two of which were omitted in Table 1 (entry 1). Similar results
were obtained when thermolysis was carried out in CD3CN (entry
4). In both cases olefin 8 was not detected, which is in marked
contrast to the thermolysis of pentacoordinate 1,2-oxaphosphetanes.1
The formation of 6 with retention of configuration strongly suggests
9
13346
J. AM. CHEM. SOC. 2003, 125, 13346-13347
10.1021/ja037586k CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Thermolyses of 5a in the Presence of Saltsa,b
oxirane, with both retention and inversion of configuration, and
the corresponding olefin. It is interesting that three different products
can be selectively obtained from a single compound by ap-
propriately tuning the conditions.
Acknowledgment. This work was partially supported by Grants-
in-Aid for The 21st Century COE Program for Frontiers in
Fundamental Chemistry (T.K.) and for Scientific Research (T.K.)
from the Ministry of Education, Culture, Sports, Science and
Technology of Japan. We thank Central Glass and Tosoh Finechem
Corporation for the gifts of organofluorine compounds and alkyl-
lithiums, respectively.
yieldsc (%)
entry
solvent
time (h)
additive
none
none
none
5a
6
7
8
9
1
2
3
4
5
6
7
8
9
o-xylene-d10
o-xylene-d10
CD3CN
CD3CN
CD3CN
CD3CN
CD3CN
CD3CN
PhCN
17
48
48
5
98
90 89
0
0
0
0
2
2
0
0
0
0
0
2
52 46 48
19 80 80
12 10 88
56 18 44
68 13 32
112.5 none
0
10
10
10
10
6
LiBr
LiI
(n-Bu)4NBr
71
19
18
Supporting Information Available: Experimental procedures and
spectral data for 1, 3a, 3b, 4a, 4b, 5a, 5b, and 10a (PDF), and data for
the X-ray crystallographic analysis of 5a (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
LiBPh4‚3DME 10 <1 <1 85 <1
PhLi 74 0
0
0
0
References
a Thermolyses were carried out at 220 °C for entry 1 and 140 °C for
entries 2-9. b Rearranged ketone and tert-butylbenzene were omitted for
clarity in Table 1. c Yields were calculated on the basis of the molar ratios
with respect to 5a. Unreacted starting material 5a was recovered.
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Scheme 2
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of a lithium cation and an oxygen atom of 5a, and the nucleophilic
attack of bromide ion on the antimony atom. Oxirane 9 can be
obtained by the backside attack of the oxide anion of the anti-
betaine type intermediate B (Scheme 2).
(6) 5a: colorless crystals from hexane; mp 189-190 °C dec; 1H NMR (500
3
MHz, CDCl3) δ 1.30 (s, 9H), 6.09 (s, 1H), 6.65 (d, JHH ) 7.5 Hz, 2H),
3
6.94 (t, 3JHH ) 7.4 Hz, 2H), 7.04 (t, JHH ) 7.5 Hz, 1H), 7.16-7.19 (m,
3
3
5H), 7.30 (br d, JHH ) 6.4 Hz, 2H), 7.55 (d, JHH ) 8.4 Hz, 2H), 7.68
Interestingly, thermolysis of 5a in the presence of lithium
tetraphenylborate‚3dimethoxyethane (LiBPh4‚3DME) gave olefin
8 (85%) selectively, together with 5a (10%) and trace amounts of
6, 7, and 9 (entry 8). Other products containing antimony and the
Martin ligand were obtained as a complex mixture, and they could
3
3
3
(t, JHH ) 7.9 Hz, 1H), 7.74 (t, JHH ) 7.1 Hz, 1H), 7.87 (br d, JHH
)
7.3 Hz, 1H), 8.19 (d, JHH ) 7.4 Hz, 1H); 13C{1H} NMR (126 MHz,
3
2
CDCl3) δ 31.1 (s, CH3), 35.1 (s), 78.7 (s, CH), 81.7 (q, JCF ) 28 Hz),
2
1
1
82.1 (sept, JCF ) 30 Hz), 123.38 (q, JCF ) 288 Hz), 123.41 (q, JCF
)
1
288 Hz), 124.5 (q, JCF ) 288 Hz), 126.2 (s), 126.4 (s), 127.1 (s), 127.3
(s), 127.4 (s), 127.8 (s), 128.2 (s), 128.3 (s), 128.5 (s), 130.1 (s), 131.3
(s), 132.3 (s), 132.4 (s), 133.5 (s), 134.1 (s), 136.5 (s), 137.2 (s), 156.4
1
not be identified by H and 19F NMR spectroscopy, but FAB-MS
(s); 19F NMR (470 MHz, CDCl3) δ -79.3 (s, 3F), -76.8 (q, JFF ) 8.2
4
Hz, 3F), -74.3 (q, JFF ) 8.2 Hz, 3F); MS (FAB) m/z 761 (M + H+),
4
of the reaction mixture showed a peak at m/z 573, which could be
assigned to the tert-butylphenyl(phenyl)benzoxastibonium ion. It
is expected that formation of 8 involves the migration of a phenyl
group from the boron atom of tetraphenylborate to the antimony
atom of 5a, followed by thermal decomposition of the hexacoor-
dinate 1,2-oxastibetanide C (Scheme 2).11 Actually, C (which was
alternatively generated by the reaction of 5a with PhLi in THF at
0 °C) gave 8 (74%) upon heating at 140 °C after changing the
solvent from THF to benzonitrile (entry 9). Hydrolysis of C
provided 2-hydroxyalkylstiborane 10a (94%) (Scheme 1), and the
reaction of 10a with LiH gave C at room temperature quantitatively.
These results suggest that hexacoordinate 1,2-oxastibetanide C is
an intermediate in the olefin formation reaction from 5a in the
presence of LiBPh4‚3DME.
691 (M+ - CF3), 648, 586, 513, 496, 481, 427, 335, 273, 185, 115, 57;
Anal. Calcd for C34H28F9O2Sb: C, 53.64; H, 3.71. Found C, 53.68; H,
3.89.
(7) Crystal data for 5a: C34H28F9O2Sb, FW ) 761.31, triclinic, space group
P-1, a ) 9.733(2) Å, b ) 10.284(5) Å, c ) 36.391(7) Å, R ) 97.523(6)°,
â ) 90.015(13)°, γ ) 118.133(7)°, V ) 3176.6(17) Å3, Z ) 4, Dc
)
1.592 g cm-3. The final cycle of full-matrix least-squares refinement was
based on 10995 observed reflections and 835 variable parameters and
converged at R1 ) 0.0988 (I > 2σ(I)) and wR2 (all data) ) 0.1917 with
a GOF ) 1.382.
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Wiley-VCH: New York, 1999; Chapter 2, pp 9-47.
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(11) Olefin synthesis via a hexacoordinate 1,2-oxastibetanide, see: Kojima,
S.; Takagi, R.; Akiba, K.-y. J. Am. Chem. Soc. 1997, 119, 5970.
We have found that thermolyses of a pentacoordinate 1,2-
oxastibetane under suitable conditions give the corresponding
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J. AM. CHEM. SOC. VOL. 125, NO. 44, 2003 13347