First synthesis, structure, and reactivity of (acylimino)triaryl-λ5-bismuthanes stabilized by ortho-substituted aryl ligands

Article abstract of DOI:10.1021/om990252c

The reactive Bi(V)=N bond in (acylimino)-triaryl-λ⁵-bismuthanes has been found to be stabilized effectively by ortho-substituted aryl ligands such as o-tolyl, o-anisyl, and mesityl groups. These imino-λ⁵-bismuthanes, in which the bismuth center possesses a distorted tetrahedral geometry, behave as a nitrene precursor as well as an oxidizing agent, depending on the substrates and conditions.

Full text of DOI:10.1021/om990252c

2580
Organometallics 1999, 18, 2580-2582
F ir st Syn th esis, Str u ctu r e, a n d Rea ctivity of
(Acylim in o)tr ia r yl-λ5-bism u th a n es Sta bilized by
or th o-Su bstitu ted Ar yl Liga n d s
Yoshihiro Matano,*,† Hazumi Nomura,† Motoo Shiro,‡ and Hitomi Suzuki†
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto
606-8502, J apan, and Rigaku Corporation, Akishima-shi, Tokyo 196-8666, J apan
Received April 9, 1999
Sch em e 1
Summary: The reactive Bi(V)dN bond in (acylimino)-
triaryl-λ5-bismuthanes has been found to be stabilized
effectively by ortho-substituted aryl ligands such as
o-tolyl, o-anisyl, and mesityl groups. These imino-λ5-
bismuthanes, in which the bismuth center possesses a
distorted tetrahedral geometry, behave as a nitrene
precursor as well as an oxidizing agent, depending on
the substrates and conditions.
Iminopnictoranes (R3MdNR′; M ) P, As, Sb, Bi) are
a class of compounds bearing a formal double bond
between the nitrogen and the pnictogen elements. They
have been receiving considerable attention due to the
chemical analogy to pnictogen ylides as well as for their
potential utility in organic synthesis.1 The properties
and reactivity of the MdN bonds are known to vary
considerably depending on the pnictogen atom. How-
ever, only little information is available concerning
imino-λ5-bismuthanes (R3BidNR′). All known imino-λ5-
bismuthanes are thermally stabilized by a highly elec-
tronegative sulfonyl group on the nitrogen atom.2-5
Modification of the N-substituent is expected to lead to
different stability and reactivity of the BidN bond, but
no imino-λ5-bismuthanes bearing a functional group
other than the sulfonyl have been reported to date. To
obtain a better understanding of this class of bismuth-
(V) compounds, we have prepared a different type of
imino-λ5-bismuthanes and examined their fundamental
properties. We report herein the first synthesis, the
structure, and the reactivity of (acylimino)triaryl-λ5-
bismuthanes, which are stabilized by ortho-substituents
on the aryl groups bonded to bismuth.
Ta ble 1. Syn th esis of
(Acylim in o)tr ia r yl-λ5-bism u th a n es 3
entry
1
2
3 (isolated yield)
1
2
3
4
5
1a
1a
1b
1b
1c
2a
2b
2a
2b
2a
3a a (93%; Ar ) 2-MeC6H4; R ) CF3)
3a b (77%; Ar ) 2-MeC6H4; R ) CCl3)
3ba (90%; Ar ) 2-MeOC6H4; R ) CF3)
3bb (96%; Ar ) 2-MeOC6H4; R ) CCl3)
3ca (86%; Ar ) 2,4,6-Me3C6H2; R ) CF3)
The preparation of such compounds was effected as
shown in Scheme 1. ortho-Substituted triarylbismuth
dichlorides 1a -c were reacted with trihaloacetamides
2a ,b in the presence of 2 equiv of potassium tert-
butoxide (KOt-Bu) in dichloromethane to give (acylimi-
no)triaryl-λ5-bismuthanes 3a a -ca as pale yellow solids
(Table 1).6 All attempts to isolate the N-aroyl counter-
parts formed from dichlorides 1a ,b and aromatic amides
2c,d were unsuccessful. The crude products readily
decomposed to the triarylbismuthane and the original
amide 2c,d during recrystallization. The KOt-Bu-
promoted reaction of 1a with pivalamide 2e at room
temperature resulted in formation of tris(2-methylphe-
nyl)bismuthane 4 and recovery of 2e. The similar
reaction between the para-substituted triarylbismuth
dichlorides 1d ,e and amide 2a appeared to afford the
expected imino-λ5-bismuthanes, but they were quite
sensitive to moisture and difficult to purify. These
results show that the stability of (acylimino)triaryl-λ5-
† Kyoto University: matano@kuchem.kyoto-u.ac.jp.
‡ Rigaku Corporation.
(1) J ohnson, A. W. Ylides and Imines of Phosphorus; Wiley: New
York, 1993.
(2) Wittig, G.; Hellwinkel, D. Chem. Ber. 1964, 97, 789.
(3) (a) Suzuki, H.; Nakaya, C.; Matano, Y.; Ogawa, T. Chem. Lett.
1991, 105. (b) Suzuki, H.; Ikegami, T. J . Chem. Res., Synop. 1996, 24.
(4) Pasenok, S. V.; Kirij, N. V.; Yagupolskii, Y. L.; Naumann, D.;
Tyrra, W. J . Fluorine Chem. 1993, 63, 179.
(5) Compound A was structurally characterized by X-ray diffraction
analysis, where the bismuth center adopts a distorted trigonal bipy-
ramidal geometry with a BidN bond length of 2.13(1) Å. The oxazoline
group at the ortho position intramolecularly coordinates to the bismuth.
Ikegami, T.; Suzuki, H. Organometallics 1998, 17, 1013.
(6) A typical procedure is as follows: to a mixture of tris(2-
methylphenyl)bismuth dichloride 1a (2.21 g, 4.00 mmol), trifluoroac-
etamide 2a (458 mg, 4.05 mmol), and dichloromethane (80 mL) was
added KOt-Bu (998 mg, 8.89 mmol) at -50 °C. The resulting mixture
was allowed to warm gradually to 10 °C with vigorous stirring. The
insoluble solid was filtered through Celite, and the filtrate was
concentrated under reduced pressure to leave an oily residue, which
was crystallized from dichloromethane/hexane to give [(trifluoroacetyl)-
imino]tris(2-methylphenyl)-λ5-bismuthane (3a a ) as a pale yellow solid
(2.205 g, 93%): mp 108-116 °C (decomp); 1H NMR (200 MHz, CDCl3)
δ 2.46 (s, 9H), 7.32-7.49 (m, 9H), 7.68-7.73 (m, 3H); 19F NMR (470
MHz, CDCl3) δ 9.92 (vs neat CF3CO2H); IR (KBr) ν 1561 (CdO) cm-1
;
Anal. Calcd for C23H21BiF3NO: C, 46.55; H, 3.57; N, 2.36. Found: C,
46.59; H, 3.47; N, 2.09. Compounds 3a b-ca also gave satisfactory
spectral and analytical data. See Supporting Information.
10.1021/om990252c CCC: $18.00 © 1999 American Chemical Society
Publication on Web 06/18/1999

Communications
Organometallics, Vol. 18, No. 14, 1999 2581
bismuthanes is strongly dependent on both steric and
electronic environments around the BidN bond; the
ortho-substituted aryl groups result in kinetic stabiliza-
tion,7 and the electron-withdrawing N-acyl group en-
hances the thermal stability.8
Compounds 3 can be stored in the solid state over a
month in a refrigerator. They were characterized by
NMR and IR spectroscopies as well as by elemental
analysis.6 Their IR spectra showed the characteristic Cd
O stretching band at rather low frequency (1561-1593
cm-1) as compared to the parent amides 2a ,b or a
phosphorus counterpart Ph3PdNCOCF3,9 suggesting
the important contribution of canonical form Z to the
stabilization of the acylimino-λ5-bismuthanes 3.
To obtain further structural information, the crystal
structure of 3bb was elucidated by X-ray diffraction
analysis.10 Figure 1 shows the ORTEP diagram together
with selected bond lengths and angles. The bismuth
center possesses a distorted tetrahedral geometry with
an average Bi-C bond length of 2.20 Å and an average
C-Bi-C bond angle of 108.3°. The large N-Bi-C(17)
bond angle of 116.1(1)° may be attributed to the
electronic repulsion between the aromatic moiety and
the carbonyl oxygen, the latter coordinating weakly to
the bismuth center with a Bi-O(1) bond distance of
2.877(3) Å. Due to this intramolecular coordination,
three atoms on the N-C(1) bond are almost in the same
plane with a small Bi-N-C(1)-O(1) torsion angle of
4.2(5)°. The observed Bi-N bond length, 2.125(3) Å, lies
close to the shorter end of the range of known Bi-N
single bond distances [2.101(7)-2.28(2) Å].11 Thus, the
BidN bond possesses little double-bond character,12 and
the canonical structure Z best represents the actual
bonding in 3bb, as had been inferred from the IR data.
F igu r e 1. ORTEP diagram of 3bb. Bond lengths (Å): Bi-
N, 2.125(3); Bi-C(3), 2.198(4); Bi-C(10), 2.204(4); Bi-
C(17), 2.196(4); N-C(1), 1.314(5); C(1)-O, 1.255(5). Bond
angles (deg): N-Bi-C(3), 106.9(1); N-Bi-C(10), 108.8-
(1); N-Bi-C(17), 116.1(1); C(3)-Bi-C(10), 109.4(1); C(3)-
Bi-C(17), 109.3(1); C(10)-Bi-C(17), 106.2(1); Bi-N-C(1),
104.0(3); N-C(1)-O, 130.9(4).
This is also in accord with the observed relatively short
N-C bond length [1.314(5) Å] and somewhat longer Cd
O bond length [1.255(5) Å].13 Three ortho-methoxyl
groups surround the BidN bond through a weak inter-
action between the bismuth and oxygen atoms, protect-
ing the reactive BidN bond both sterically and elec-
tronically.8
The reactivity of (acylimino)triaryl-λ5-bismuthanes
has been examined using 3a a (Scheme 2).14 When
heated in benzene at 60 °C for 48 h, 3a a decomposed
completely to give bismuthane 4 (61% isolated yield) and
2a .15 This mode of decomposition is in marked contrast
to that of the iminophosphorane Ph3PdNCOPh, which
decomposes at elevated temperatures to produce triph-
enylphosphine oxide and benzonitrile, presumably
through an enolate structure.16 In contrast to 3a a ,
[(trifluoromethanesulfonyl)imino]tris(2-methylphenyl)-
λ5-bismuthane (o-Tol3BidNSO2CF3)17 remained mostly
unchanged after a week-long heating in benzene-d6 at
60 °C, showing that acylimino-λ5-bismuthanes are much
less stable thermally than the N-sulfonyl analogues.
(7) Kinetic stabilization afforded by ortho-substituted aryl groups
has been applied for stabilizing highly reactive functionalities. For
example, see: (a) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.;
Higuchi, T. J . Am. Chem. Soc. 1981, 103, 4587. (b) West, R.; Fink, M.
J .; Michl, J . Science 1981, 214, 1343. (c) Cowley, A. H.; Kilduff, J . E.;
Newman, T. H.; Pakulski, M. J . Am. Chem. Soc. 1982, 104, 5820. (d)
Tokitoh, N.; Matsumoto, T.; Manmaru, K.; Okazaki, R. J . Am. Chem.
Soc. 1993, 115, 8855.
(8) ortho-Methoxy aryl groups have been shown to stabilize the
cationic heteroatom thermodynamically. For example, see: (a) McE-
wen, W. E.; Lau, K. W. J . Org. Chem. 1982, 47, 3595. (b) Wada, M.;
Kanzaki, M.; Fujiwara, M.; Kajihara, K.; Erabi, T. Bull. Chem. Soc.
J pn. 1991, 64, 1782. (c) Suzuki, H.; Ikegami, T.; Azuma, N. J . Chem.
Soc., Perkin Trans. 1 1997, 1609. Thus, in compounds 3ba and 3bb,
the ortho-methoxyl groups may also afford thermodynamic stabilization
to the BidN bond.
(9) νCdO (2a ): 1684 cm-1. νCdO (2b): 1698 cm-1. νCdO (Ph3Pd
NCOCF3): 1635 cm-1, Cristau, H. J .; Manginot, E.; Torreilles, E.
Synthesis 1991, 382.
(12) Koketsu et al. estimated the BidN/Bi-N bond length ratio
between H3BidNH and H2Bi-NH2 to be 0.927 from ab initio calcula-
tions (MP2/DZ-d level). Koketsu, J .; Ninomiya, Y.; Suzuki, Y.; Koga,
N. Inorg. Chem. 1997, 36, 694.
(13) Typical N-Csp2 and Csp2dO bond lengths are 1.36 and 1.20 Å,
respectively. See: March, J . Advanced Organic Chemistry, 4th ed.;
Wiley: New York, 1992; Table 1.5, p 21.
(14) All products were identified by comparison with the authentic
specimens. Amide 2a was an accompanying byproduct in the thermal
decomposition, oxidation, and acidolysis. See Supporting Information.
(15) At present, we assume that 3a a decomposes to generate a
nitrene or nitrenoid species, which would abstract hydrogens from the
solvent or the aryl ligands.
(16) (a) Staudinger, H.; Hauser, E. Helv. Chim. Acta 1921, 4, 861.
(b) Horner, L.; Gross, A. J ustus Liebigs Ann. Chem. 1955, 591, 117.
(17) This compound was obtained from the KOt-Bu-promoted reac-
tion of 1a and trifluoromethanesulfonamide. Matano, Y.; Nomura, H.;
Suzuki, H. Unpublished results.
(10) C23H21BiCl3NO4, monoclinic, space group P21/n, a ) 8.776(2)
Å, b ) 17.577(4) Å, c ) 15.875(4) Å, â ) 96.10(2)°, V ) 2434.9(10) Å3,
Z ) 4, Dc ) 1.884 g cm-3, T ) -130 ( 1 °C, 4787 independent and
4342 observed reflections (I > 2.00σ(I)) refined to R ) 2.5%, Rw ) 3.4%,
GOF ) 1.11. For further details, see Supporting Information.
(11) (a) Clegg, W.; Compton, N. A.; Errington, R. J .; Norman, N. C.;
Wishart, N. Polyhedron 1989, 8, 1579. (b) Clegg, W. Compton, N. A.;
Errington, R. J .; Fisher, G. A.; Green, M. E.; Hockless, D. C. R.;
Norman, N. C. Inorg. Chem. 1991, 30, 4680. (c) Wirringra, U.; Roesky,
H. W.; Noltemeyer, M.; Schmidt, H.-G. Inorg. Chem. 1994, 33, 4607.
(d) Edwards, A. J .; Beswick, M. A.; Galsworthy, J . R.; Paver, M. A.;
Raithby, P. R.; Rennie, M.-A.; Russell, C. A.; Verhorevoort, K. L.;
Wright, D. S. Inorg. Chim. Acta 1996, 248, 9. (e) Burford, N.;
Macdonald, C. L. B.; Robertson, K. N.; Cameron, T. S. Inorg. Chem.
1996, 35, 4013.

2582 Organometallics, Vol. 18, No. 14, 1999
Communications
Sch em e 2
the good nucleofugal ability of the triarylbismuthonio
moiety as a bismuthane. Treatment of 3a a with an
aqueous HCl solution in CH2Cl2 at room temperature
yielded tris(2-methylphenyl)bismuth dichloride quan-
titatively. In the presence of a catalytic amount of
copper(II) triflate, N-arylation proceeded smoothly to
give N-tolylamide 6 with a moderate recovery of 4. The
yield of 6 was not affected by the presence or absence
of excess styrene, ruling out the possible involvement
of the nitrenoid species that would undergo aziridina-
tion. No C-N bond forming reaction was observed
between 3a a and benzaldehyde in boiling benzene.
Further studies on the chemistry of this class of
bismuth(V) compounds are in progress.
Ack n ow led gm en t. This work was supported by a
Grant-in-Aid for Scientific Research on Priority Areas
(A), “The Chemistry of Inter-element Linkage”
(No. 10133225), from the Ministry of Education, Science,
Sports and Culture of J apan. We thank Dr. M.
Ogasawara for 19F NMR measurements.
Su p p or tin g In for m a tion Ava ila ble: Spectroscopic and
analytical data for compounds 3, experimental details of the
reactions in Scheme 2, and X-ray diffraction data (ORTEP
diagrams, crystal and data collection parameters, positional
parameters, and bond distances and angles) for 3bb. This
material is available free of charge via the Internet at
http://pubs.acs.org.
When treated with an excess of triphenylphosphine in
refluxing benzene, 3a a transferred the nitrene unit to
the phosphine to give the iminophosphorane 59 and
bismuthane 4. Imide 3a a also exhibited a mild oxidizing
ability; benzpinacol and benzenethiol were oxidized to
benzophenone and diphenyl disulfide, respectively, on
treatment with an equimolar quantity of 3a a at room
temperature. The quantitative formation of 4 suggests
the concerted elimination of the bismuthane from a
pentavalent intermediate of the type o-Tol3Bi(ER)2 (E
) O, S) during oxidation.18 These findings demonstrate
OM990252C
(18) For reviews on the ligand coupling of hypervalent compounds,
see: (a) Barton, D. H. R.; Finet, J .-P. Pure Appl. Chem. 1987, 59, 937.
(b) Finet, J .-P. Chem. Rev. 1989, 89, 1487. (c) Oae, S. Pure Appl. Chem.
1996, 68, 805, and references therein.