First oxazagermete: Synthesis, structure and thermal cycloreversion into a germanone

Article abstract of DOI:10.1039/a703241b

A novel four-membered ring compound, 1,2,4-oxazagermete, is obtained by the reaction of diarylgermylene tbt(tip)Ge: with mesitonitrile oxide; its thermolysis generates germanone tbt(tip)Ge=O as evidenced by trapping reactions with dimethylbutadiene and et

Full text of DOI:10.1039/a703241b

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First oxazagermete: synthesis, structure and thermal cycloreversion into a
germanone
Tsuyoshi Matsumoto, Norihiro Tokitoh and Renji Okazaki*
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
A novel four-membered ring compound, 1,2,4-oxazager-
mete, is obtained by the reaction of diarylgermylene
tbt(tip)Ge: with mesitonitrile oxide; its thermolysis generates
germanone tbt(tip)GeNO as evidenced by trapping reactions
with dimethylbutadiene and ethanol.
Thermolysis of 1 was carried out in the expectation that it
would dissociate into a germanium–oxygen double-bond spe-
cies (germanone) tbt(tip)GeNO 4 and mesitonitrile, since an
1,2-oxazete is known to undergo thermal cycloreversion into
the corresponding ketone and nitrile.3,4 The chemistry of
germanones has not been fully disclosed yet, although they have
been a fascinating subject of considerable attention in recent
years.1b,8
Small-ring compounds containing a group 14 element have
been of current interest because of their intriguing structural
properties and potential utility as precursors of reactive
intermediates taking advantage of their ring strain. Since many
such small-ring compounds are highly reactive under ambient
conditions, some stabilization is often necessary for their
isolation, for example, the use of a very bulky substituent.1
Here we report the synthesis of kinetically stabilized
1,2,4-oxazagermete 1, a novel four-membered ring compound
containing germanium, by an efficient steric protecting group,
2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (tbt).2 There have
been several reports on 1,2-oxazetes. They were observed as
reactive intermediates in the thermal and photochemical
reactions of a,b-unsaturated nitro compounds or in the reaction
of vinyl radicals and NO,3 and some oxazetes having bulky
substituents have been isolated although their molecular
structures were not established.4 In contrast, there has been no
report so far on the heavier group 14 element analogue of an
1,2-oxazete.
When a [2H6]benzene solution of 1 and 2,3-dimethylbuta-
1,3-diene in a sealed NMR tube was heated at 120 °C for 12 h,
the starting materials were consumed completely (1H NMR) to
afford 5–7 and mesitonitrile (Scheme 2).§ Formation of
compounds 5–7 and an almost quantitative yield of mesitonitrile
clearly indicates the generation of intermediary germanone 4,
since we previously reported the formation of 7 from germa-
none 4 generated from by the reaction of germylene 3 with
tribenzylamine N-oxide.8
Compounds 6 and 7 are most likely produced from
germanone 4 via intramolecular cyclization,9 while the forma-
tion of compound 5 is reasonably explained in terms of an ene
Oxazagermete 1 was synthesized as stable colourless crystals
in 87% by the reaction of diarylgermylene tbt(tip)Ge: 3
(tip = 2,4,6-triisopropylphenyl), generated from dibromo-
germane 2 and lithium naphthalenide in thf,5 with mesitonitrile
oxide (Scheme 1). This represents the first synthesis of an
oxazagermete as well as the first isolation of a [1 + 3]-
cycloaddition reaction product of a germylene with 1,3-dipolar
reagents.6
C(1)
Oxazagermete 1 showed characteristic spectral data.† The
CNN bond stretching was observed at 1612 cm21 in the IR
spectrum, which is similar to those reported for 1,2-oxazetes.4
In the 13C NMR spectrum, a singlet peak was observed at
d 174.8 assignable to the carbon in the four-membered ring,
which is more shielded than that of an oxazete owing to the
more electropositive Ge atom on the carbon.4c
C(19)
Ge
O
C(34)
N
The molecular structure of 1 has been determined by X-ray
crystallographic analysis and an ORTEP drawing is shown in
Fig. 1.‡ The bond angle C(34)–Ge–O considerably shrinks from
that of normal sp3 configuration to 68.7(2)°, suggesting the
large ring strain of the four-membered ring, which is also
reflected on the long Ge–O bond length [1.872(4) Å] compared
with that of a typical Ge–O single bond (1.75–1.85 Å).7 The
four-membered ring of 1 is perfectly planar with the sum of the
bond angles being 360°.
C(10)
Fig. 1 ORTEP drawing of oxazagermete 1 with thermal ellipsoid plots (30%
probability). Selected bond lengths (Å) and angles (°): Ge–O 1.872(4), Ge–
C(1) 1.968(6), Ge–C(19) 1.974(6), Ge–C(34) 2.001(6), C(34)–N 1.291(9),
N–O 1.418(8), C(10)–C(34) 1.459(10); C(1)–Ge–O 105.9(2), C(1)–Ge–
C(19) 114.9(2), C(1)–Ge–C(34) 123.0(3), C(19)–Ge–O 112.9(3),
C(19)–Ge–C(34) 119.1(3), O–Ge–C(34) 68.7(2), Ge–O–N 92.7(3), O–N–
C(34) 107.6(5), Ge–C(34)–N 91.1(5), Ge–C(34)–C(10) 146.2(5),
N–C(34)–C(10) 122.7(6).
mes
tbt
tip
tbt
tip
tbt
C
LiC10H7
thf
mesCNO
GeBr2
Ge:
Ge
N
tip
O
2
3
1 (86%)
Scheme 1
Chem. Commun., 1997
1553

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‡ Crystallographic data for 1: C52H93GeNOSi6, M = 989.42, triclinic,
mes
C
mesCN (93%)
¯
space group P1, a
= 13.002(3), b = 19.433(4), c = 12.503(5) Å,
a = 101.48(2), b = 91.22(2), g = 77.17(1)°, U = 3018(1) Å3, Z = 2,
tbt
+
C6D6
Dc = 1.083 g cm23, T = 296 K, F(000) = 1072, colourless prism with
dimensions 0.60
Ge
N
tbt
120 °C
6.48 cm21
,
tip
Ge
O
3
0.50
3
0.40, m(Mo-Ka)
=
O
R(Rw) = 0.056(0.062). Weighting scheme; w = 1/s2(Fo). The intensity
data (2q < 55°) for 1 were collected on a Rigaku AFC5R diffractometer
with graphite-monochromated Mo-Ka radiation (l = 0.71069 Å), and
14509 reflections (13892 unique) were measured. The structure of 1 was
solved by direct methods (structure solution methods; MITHRIL10 and
DIRDIF11), and refined by full-matrix least squares using the TEXSAN
crystallographic software package.12 All the non-hydrogen atoms were
refined anisotropically, while the hydrogen atoms were placed in calculated
positions. The final cycles of the least-squares refinement were based on
5387 observed reflections [I > 3sıIı] and 550 variable parameters. The
maximum and minimum peaks on the final difference Fourier map
correspond to 0.40 and 20.37 e Å23, respectively. CCDC 182/526.
§ New compounds 5 and 6 here obtained gave satisfactory spectral and
analytical data. For 5: white crystals; mp 189–191 °C (deccomp.) (CH2Cl2–
EtOH); 1H NMR (CDCl3, 500 MHz, 300 K) d 20.17 (s, 9 H), 0.00 (s, 9 H),
0.03 (s, 9 H), 0.05 (s, 9 H), 0.06 (s, 9 H), 0.13 (s, 9 H), 0.82 (d, J 6.9 Hz, 3
H), 1.02 (d, J 6.9 Hz, 3 H), 1.13 (s, 1 H), 1.173 (d, J 6.9 Hz, 3 H), 1.174 (d,
J 6.9 Hz, 3 H), 1.27 (br s, 6 H), 1.84 (s, 3 H), 2.09 (s, 1 H), 2.13 (s, 1 H),
2.37 (d, J 12.3 Hz, 1 H), 2.66 (br s, 1 H), 2.79 (spt, J 6.9 Hz, 1 H), 2.85 (d,
J 12.3 Hz, 1 H), 3.95 (br s, 1 H), 4.28 (s, 1 H), 4.86 (s, 1 H), 5.00 (s, 1 H),
5.17 (s, 1 H), 6.29 (s, 1 H), 6.40 (s, 1 H), 6.89 (s, 1 H), 6.93 (s, 1 H). High-
resolution FABMS: observed m/z 927.5016 ([M + H]+). Calc. for
C48H9274GeOSi6 927.5054. For 6: white crystals; mp 177–179 °C (CH2Cl2–
EtOH); 1H NMR (CDCl3, 500 MHz, 300 K) d 20.09 (s, 9 H), 0.04 (s, 18 H),
0.06 (s, 9 H), 0.16 (s, 9 H), 0.17 (s, 9 H), 1.24 (d, J 6.9 Hz, 6 H), 1.29 (br
s, 6 H), 1.33 (d, J 6.9 Hz, 6 H), 1.66 (s, 1 H), 1.72 (br s, 1 H), 2.87 (spt, J
6.9 Hz, 1 H), 3.29 (br s, 2 H), 6.44 (s, 1 H), 6.46 (s, 1 H). High-resolution
FABMS: observed m/z 845.4339 ([M+H]+); calc. for C42H8374GeOSi6
845.4271.
tip
1
4
tip
OH
Me3Si
Ge
SiMe3
SiMe3
tbt
OH
Ge
Me3Si
+
tip
5 (10%)
Me3Si
SiMe3
6 (24%)
tip
OSiMe3
H
tip
OSiMe3
H
Me3Si
Me3Si
Me3Si
Me3Si
Ge
Ge
SiMe3
SiMe3
+
+
Me3Si
SiMe3
7a (35%)
Me3Si
SiMe3
7b (8%)
Scheme 2
reaction of germanone 4 with the butadiene. In the cases of
chalcogen double-bond species, tbt(tip)GeNX (X = S, Se, Te),
the corresponding [4 + 2] cycloadducts were obtained in the
reactions with dienes.2 The reason for this difference is not clear
at present.
The total yield of the silyl-migrated compounds 7a and 7b is
higher than that of the hydrogen-migrated compound 6, which
is probably due to the higher affinity of silicon for oxygen. The
product ratio of 7a to 7b is considered to be kinetically
determined since no isomerization from 7a to 7b is observed
even at 180 °C.
The generation of germanone 4 was also confirmed by the
thermal reaction of 1 in benzene (at 120 °C, in a sealed tube) in
the presence of ethanol, in which ethoxy(hydroxy)germane
tbt(tip)Ge(OH)(OEt) 8, an ethanol adduct of 4, was obtained in
82%. This finding indicates that the intermolecular reaction
with ethanol was faster than the intramolecular silyl or
hydrogen migration mentioned above. It was also confirmed by
thermolysis (120 °C) of a [2H6]benzene solution of oxaza-
germete 1 in the presence of 0.3 equiv of ethanol while being
monitored by 1H NMR. In the initial stage, the ethanol adduct 8
was observed exclusively, and, after consumption of ethanol,
the rearrangement reaction to 6, 7a and 7b occurred.
In summary, we have succeeded in the isolation and
crystallographic analysis of the first 1,2,4-oxazagermete 1,
which undergoes thermal cycloreversion into mesitonitrile and
germanone tbt(tip)GeNO.
1 Recent reviews; see (a) T. Tsumuraya, S. A. Batcheller and S.
Masamune, Angew. Chem., Int. Ed. Engl., 1991, 30, 902; (b) J. Barrau,
J. Escudie´ and J. Satge´, Chem. Rev., 1990, 90, 283; (c) M. Driess and
H. Gru¨tzmacher, Angew. Chem., Int. Ed. Engl., 1996, 35, 828; (d) K. M.
Baines and W. G. Stibbs, Adv. Organomet. Chem., 1996, 39, 275.
2 For the other examples of kinetic stabilization by tbt, see: H. Suzuki,
N. Tokitoh, S. Nagase and R. Okazaki, J. Am. Chem. Soc., 1994, 116,
11 578; N. Tokitoh, T. Matsumoto, K. Manmaru and R. Okazaki, J. Am.
Chem. Soc., 1993, 115, 8855; T. Matsumoto, N. Tokitoh and
R. Okazaki, Angew. Chem., Int. Ed. Engl., 1994, 33, 2316; N. Tokitoh,
T. Matsumoto and R. Okazaki, J. Am. Chem. Soc., 1997, 119, 2337;
N. Tokitoh, M. Saito and R. Okazaki, J. Am. Chem. Soc., 1993, 115,
2065.
3 T. H. Kinstle and J. G. Stam, J. Org. Chem., 1970, 35, 1771; J. T. Pinhey
and E. Rizzardo, Tetrahedron Lett., 1973, 4057; A. G. Sherwood and
H. E. Qunning, J. Am. Chem. Soc., 1963, 85, 3506; J. M. Surzur,
C. Dupy, M. P. Bertrand and R. Nousier, J. Org. Chem., 1972, 37,
2782.
4 (a) K. Wieser and A. Berndt, Angew. Chem., Int. Ed. Engl., 1975, 14, 69;
(b) 1975, 14, 70; (c) H. G. Corkins, L. Storace and E. R. Osgood,
Tetrahedron Lett., 1980, 21, 2025.
5 N. Tokitoh, K. Kishikawa, T. Matsumoto and R. Okazaki, Chem. Lett.,
1995, 827.
6 For the reaction of a germylene with 1,3-dipolar reagents, see, for
example: C. Glidewell, D. Lloyd and K. W. Lumbard, J. Chem. Soc.,
Dalton Trans., 1987, 501.
7 K. M. Baines and W. G. Stibbs, Coord. Chem. Rev., 1995, 145, 157.
8 N. Tokitoh, T. Matsumoto and R. Okazaki, Chem. Lett., 1995, 1087.
9 For a similar cyclization of bis(2,4,6-tri-tert-butylphenyl)germanone,
see: P. Jutzi, H. Schmidt, B. Neumann and H.-G. Stammler, Organome-
tallics, 1996, 15, 741.
10 C. J. Gilmore, MITHRIL. An integrated direct methods computer
program, J. Appl. Crystallogr., 1984, 17, 42 (University of Glasgow).
11 P. T. Beuskens, DIRDIF. Direct methods for difference structures—an
automatic procedure for phase extension and refinement of difference
structure factors, Technical Report 1984/1 Crystallography Laboratory,
Toernooiveld, 6525 Ed Nijmegen, 1984.
Footnotes and References
* E-mail: okazaki:@chem.s.u-tokyo.ac.jp
† Spectral data for 1: white crystals; mp 196–197 °C (decomp.) (CH2Cl2–
EtOH); 1H NMR (CDCl3, 500 MHz, 350 K) d 20.12 (s, 18 H), 0.07 (s, 18
H), 0.09 (s, 18 H), 1.03 (br s, 6 H), 1.24 (d, J 6.9 Hz, 6 H), 1.25 (br s, 6 H),
1.39 (s, 1 H), 1.58 (br s, 2 H), 2.25 (s, 3 H), 2.39 (s, 6 H), 2.87 (spt, J 6.9
Hz, 1 H), 3.37 (br s, 2 H), 6.39 (br s, 2 H), 6.81 (s, 2 H), 7.06 (s, 2 H); 13
C
NMR (CDCl3, 125 MHz, 340 K) d 1.00 (q), 1.08 (q), 1.97 (q), 2.21 (q), 2.23
(q), 20.86 (q), 23.68 (q), 23.73 (q), 24.03 (q), 24.93 (q), 26.94 (q), 29.36 (d),
31.04 (d), 33.99 (d), 34.32 (d), 123.41 (d), 128.71 (d), 129.86 (d), 133.43 (s),
134.28 (s), 137.49 (s), 137.90 (s), 145.58 (s), 149.96 (s), 151.69 (s), 155.17
(s). High-resolution FABMS: observed m/z 990.5158 ([M + H]+). Calc. for
12 TEXSAN: TEXRAY Structure Analysis Package, Molecular Structure
Corporation, 1985.
C
52H9474GeNOSi6 990.5163.
Received in Cambridge, UK, 12th May 1997; 7/03241B
1554
Chem. Commun., 1997