Formation of N-coordinated 4H-benzo[d][1,3]oxazine from 2-(trimethylsiloxymethyl)- and 2-(hydroxymethyl)-phenyl isocyanides promoted by dirhodium(II) acetate

Article abstract of DOI:10.1016/j.inoche.2006.11.019

The neutral dirhodium(II) acetate [Rh₂(OAc)₄] and the related cationic complex [Rh₂(OAc)₂(MeCN)₆](BF₄)₂ react with 2-(trimethylsiloxymethyl)phenyl isocyanide (I) and/or 2-(hydroxymethyl)phenyl isocyanide (II) to give the corresponding adducts, in which the entering isocyanide occupies the axial position at the two rhodium centres. These complexes are stable in toluene, but they evolve in chloroform as a consequence of an intramolecular ligand rearrangement via oxygen attack to the coordinated carbon atom. With the neutral dirhodium acetate species the cyclization reaction affords the well characterized benzoxazine complex {A figure is presented} (4), whereas the cationic acetonitrile derivative rapidly evolves into a complex mixture.

Full text of DOI:10.1016/j.inoche.2006.11.019

Inorganic Chemistry Communications 10 (2007) 407–409
www.elsevier.com/locate/inoche
Formation of N-coordinated 4H-benzo[d][1,3]oxazine from
2-(trimethylsiloxymethyl)- and 2-(hydroxymethyl)-phenyl
isocyanides promoted by dirhodium(II) acetate
a,
a
a
a
a
Marino Basato *, Andrea Biffis , Gianluca Martinati , Mauro Polo , Luca Ronconi ,
a
b,
b
b
Cristina Tubaro , Rino A. Michelin *, Paolo Sgarbossa , Silvia Mazzega Sbovata
a
`
Dipartimento di Scienze Chimiche, Universita degli Studi di Padova, via Marzolo 1, 35131 Padova, Italy
`
Dipartimento di Processi Chimici dell’Ingegneria, Universita degli Studi di Padova, via Marzolo 9, 35131 Padova, Italy
b
Received 2 October 2006; accepted 30 November 2006
Available online 12 December 2006
Abstract
The neutral dirhodium(II) acetate [Rh2(OAc)4] and the related cationic complex [Rh2(OAc)2(MeCN)6](BF4)2 react with 2-(trimethyl-
siloxymethyl)phenyl isocyanide (I) and/or 2-(hydroxymethyl)phenyl isocyanide (II) to give the corresponding adducts, in which the enter-
ing isocyanide occupies the axial position at the two rhodium centres. These complexes are stable in toluene, but they evolve in
chloroform as a consequence of an intramolecular ligand rearrangement via oxygen attack to the coordinated carbon atom. With the
neutral dirhodium acetate species the cyclization reaction affords the well characterized benzoxazine complex
(4), whereas the cationic acetonitrile derivative rapidly evolves into a complex mixture.
[Rh2(OAc)4(NC6H4-2-CH2OC(H))2]
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Rhodium(II); Functionalized isocyanides; Benzo[d][1,3]oxazine complex
The reactivity of functionalized isocyanides towards
metal complexes has been widely studied in the last years
[1] and it has been demonstrated that it depends on several
factors such as the nature of the functional group, the type
of transition metal and its oxidation state. In this con-
tribution we wish to report some results on the coordina-
tion of 2-(trimethylsiloxymethyl)phenyl isocyanide (I) and
2-(hydroxymethyl)phenyl isocyanide (II) (Fig. 1) to [Rh2
(OAc)4] and of (II) to the cationic complex [Rh2(OAc)2
(MeCN)6](BF4)2.
to the C„N triple bond, would generate the N,O-heterocy-
clic carbene benzo[1,3]oxazin-2-ylidene, as it has been
already reported with several palladium(II) and plati-
num(II) complexes [2]. In this context we have tested this
particular ability of ligands (I) and (II) with dirhodium(II)
metal centres, for which up to now only a single example of
carbene complex has been reported in the literature [3].
The complexes [Rh2(OAc)4(CNC6H4-2-CH2OR)2] (1, R
= SiMe3; 2, R = H) and [Rh2(OAc)2(CH3CN)4(CNC6H4-
2-CH2OSiMe3)2] (BF4)2 (3) were obtained in good yield
by reaction of [Rh2(OAc)4] or [Rh2(OAc)2(MeCN)6](BF4)2,
respectively, with the functionalized isocyanides in 1/1 Rh/
isocyanide ratio (Scheme 1) [4].
The investigated isocyanides are interesting ligands
because they present both the isocyanide group and an eas-
ily accessible hydroxyl function, which upon 1,2-addition
1
Their H and13C NMR spectra show the expected reso-
nances for the functional groups present in the molecule
and they indicate the C-coordination of the isocyanide
function to the free apical positions of the metal precursor;
the signals relative to the bridging acetate (or to the aceto-
nitrile molecules) are in fact almost unaffected by coordina-
*
Corresponding authors. Tel.: +39 049 8275217; fax: +39 049 8275223
(M. Basato); Tel.: +39 049 8275522; fax: +39 049 8275525 (R.A.
Michelin).
E-mail addresses: marino.basato@unipd.it (M. Basato), rino.michelin
@unipd.it (R.A. Michelin).
1387-7003/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2006.11.019

408
M. Basato et al. / Inorganic Chemistry Communications 10 (2007) 407–409
Complex (1) is converted in methanol in the presence of
OR
C
a catalytic amount of Fꢀ anions (tetrabutyl ammonium
fluoride, TBAF) to the corresponding benzoxazine deriva-
tive (4), which was isolated in good yield as a purple solid
(Scheme 2) [10].
N
(I): R = SiMe3
(II): R = H
Fig. 1. Functionalized isocyanides employed in this study [2].
The IR spectrum of complex (4) is characterized by the
absence of the absorption band at 2125 cmꢀ1, relative to
the N„C bond stretching, indicating an evolution of the
system. However, the expected carbene derivative has not
been obtained (Scheme 3): in the IR spectrum in fact the
absorption band at ca. 3300 cmꢀ1 relative to the N–H bond
CH3
C
CH3
C
[Rh2(OAc)4]
OR
O
O
O
O
toluene
r.t.
+
N
C
Rh Rh
C N
OR
C
1
2
O
stretching is absent; moreover, in the H NMR spectrum
O
O
RO
O
C
N
there is no signal attributable to the N–H proton.
C
At the same time, the 1H NMR spectrum of complex (4)
exhibits a signal at 7.72 ppm attributable to the N@CH
hydrogen and the 13C NMR spectrum shows the corre-
sponding carbon resonance at 157.2 ppm.
CH3
CH3
(1): R = SiMe3
(2): R = H
In order to shed light on the mechanism of this cycliza-
tion reaction and to identify possible carbene intermedi-
ates, the stability of complex [Rh2(OAc)4(CNC6H4-2-
CH2OH)2] (2) has been studied in CDCl3 solution. During
the 1H NMR investigation a slow evolution of complex (2)
to the benzoxazine complex (4) has been observed, but we
were unable to detect any carbene intermediate. Attempts
to trap the carbene species by transfer to other substrates
such as styrene or diazoacetate, were unsuccessful: in fact
no cyclopropanation or coupling products were observed.
The chemical behaviour of the hydroxymethyl func-
tional isocyanide (II) coordinated to dirhodium(II) metal
species can be compared with that already described for a
series of metal centers like Cr(0), W(0), Pd(II), Pt(II)
[1b,2]. While with the first two electron-rich metals only
coordination of the isocyanide is observed, without any
activation towards nucleophilic attack of the pending
OH, with the less electron rich Pd(II) and Pt(II) metal cen-
ters activation occurs to give intramolecular cyclization
and final formation of the N,O-heterocyclic carbene com-
[Rh2(OAc)2(CH3CN)6](BF4)2
OR
toluene
r.t.
+
(BF4)2
N
C
N
C
Rh Rh
O
OR
C
2
O
RO
O
O
N
C
C
CH3
CH3
(3): R = SiMe3
Scheme 1.
tion of the isocyanide ligand. Rh(II)–Cax coordination is
rather rare and involves as axial ligands isocyanides [5],
CO [6], carbenes [3], alkenes [7], alkynes [8] or arenes [9].
In this context complex (3) represents the first example of
Cax coordination in a dicationic rhodium(II) complex.
The infrared spectra of complexes (1)–(3) confirm the pro-
posed structure. In particular, complex (1) exhibits the
N„C absorption at 2128 cmꢀ1 (force constant k(N„C)
~
~
ca.
1730 N/m)
with
a
Dm ¼ ðmðNBCÞcoordinatedꢀ
mðNBCÞfreeÞ [1a] of ca. 10 cmꢀ1, whereas complex (3) shows
~
an absorption at 2176 cmꢀ1 (k(N„C) = 1810 N/m) and
CH3
C
CH3
CH3
C
CH3
the corresponding Dm is 45 cmꢀ1. As already observed, a
~
C
C
H
OR
O
O
O
O
O
O
~
O
higher value of Dm reflects a greater electrophilicity of the
O
C
O
MeOH
TBAF
N
Rh Rh
N C Rh Rh
C
N
N
isocyanide carbon, which is therefore more susceptible of
nucleophilic attack [1a]. As expected the highest value is
observed for the cationic dirhodium(II) complex, as a con-
sequence of the positive charge localized on the metal cen-
ter. This fact explains the instability in solution of complex
(3), whose evolution is already observed during the record
of the NMR spectrum in deuterated chloroform.
O
C
H
O
O
O
O
O
RO
O
C
CH3
O
C
CH3
O
C
C
CH3
CH3
(4)
(1): R = SiMe3
Scheme 2.
Complex (2) is characterized in the solid state by two
N„C absorptions at 2154 and 2138 cmꢀ1; furthermore
the solid state 13C NMR spectrum shows two sets of signals
for the carboxylato and isocyanide ligands. This non-
equivalence of the axial ligands may be attributable to an
intra- or intermolecular interaction (possibly via –OH
hydrogen bonding) involving one of the two coordinated
isocyanides, which is effective only in the solid state. In
fact, both infrared and NMR spectra in solution shows
the presence of a single set of signals.
O
CH
CH2
N
H
H
N
O
O
C
M
N
M
C
M
O
C
N
H
M
Scheme 3.

M. Basato et al. / Inorganic Chemistry Communications 10 (2007) 407–409
409
ing to the isocyanide carbon C„N was not detected. FT IR (KBr,
cmꢀ1): 2953-2876, 2128 [m(C„N)], 1591, 1429, 1346, 1248, 1115, 1076,
880, 843, 750 and 696. [Rh2(OAc)4 (CNC6H4-2-CH2OH)2] (2). This
compound was obtained from [Rh2(OAc)4] (200 mg, 0.45 mmol) and
(II) (0.12 g, 0.9 mmol), as a yellow solid (52% yield). Anal. Calc. for
C24H26N2O10Rh2 (M = 708.29): C, 40.70; H, 3.70; N, 3.96. Found: C,
40.27; H, 3.40; N, 3.70%. 1H NMR (CDCl3): d 1.96 (s, 6H, CH3COꢀ2 ),
3.80 (br, 1H, OH), 5.00 (2s, 2H, CH2O), 7.45–7.75 (m, 4H,
C6H4).13C{1H}NMR (CDCl3): d 24.0 ðCH3CO2ꢀÞ, 61.8 (CH2O),
126.3, 126.5, 128.8, 129.8, 130.5, 139.5 (C6H4), 188.5 (C„N), 194.7
ðCH3COꢀ2 Þ.13C CP MAS NMR: d 25.3 and 26.4 ðCH3COꢀ2 Þ, 61.2 (br,
CH2O), 120.0–139.0 (br, C6H4), 140.0–148.0 (br, C6H4), 190.0–198.0
(br, C„N and CH3COꢀ2 ). FT IR (KBr, cmꢀ1): 3484, 3000–2930, 2154
and 2138 [m(C„N)], 1595, 1436, 981, 476, 460. FT IR (CHCl3, cmꢀ1):
2143 [m(C„N)]. [Rh2(OAc)2(MeCN)6(CNC6H4-2-CH2OSiMe3)2]-
(BF4)2 (3). This compound was obtained from [Rh2(OAc)2(MeCN)6]
(BF4)2 (250 mg, 0.34 mmol) and (I) (0.14 g, 0.74 mmol), as a light
brown solid (76% yield). Anal. Calc. for C34H48B2F8N6O6Rh2Si2
(M = 1072.39): C, 38.08; H, 4.51; N, 7.84. Found: C, 38.40; H, 4.43;
N, 7.59%. 1H NMR (CDCl3): d 0.17 (s, 9H, Si(CH3)3), 2.09 (s, 3H,
CH3COꢀ2 ), 2.66 (s, 6H, CH3CN), 4.97 (s, 2H, CH2OSi), 7.47–7.68 (m,
4H, C6H4). 13C{1H} NMR (CDCl3): d ꢀ 0.5 (Si(CH3)3), 3.9
(CH3CN), 23.3 ðCH3COꢀ2 Þ, 60.9 (CH2OSi), 123.4-138.6 (m, C6H4),
193.8 ðCH3COꢀ2 Þ, the signal corresponding to the isocyanide carbon
C„N was not detected. FT IR (KBr, cmꢀ1): 3072–2955, 2336,
2176[m(C„N) isocyanide], 1566, 1448, 1254, 1057, 870, 843, 762 and
714.
plexes. In the case of Rh(II) the activation gives rise to a
final product involving the coordination of the N,O-hetero-
cyclic ligand through the imino nitrogen instead of the car-
bene carbon. It has been recently reported for N,N-
heterocyclic carbenes that N- or C-coordination markedly
depends on the nature of the metal center [11] and the
observed behaviour confirms the well known reluctance
of dirhodium(II) towards C-carbene bonding [3]. The
importance of the electron density at the metal on the
degree of activation of the coordinated isocyanide is well
demonstrated in the case of the cationic complex
[Rh2(OAc)2(CH3CN)4(CNC6H4-2-H2OSiMe3)2](BF4)2 (3).
The positive charge makes the metal more electrophilic
and, as a consequence, the isocyanide carbon becomes
more susceptible of nucleophilic attack even by the
SiMe3-protected oxygen atom.
Acknowledgements
Financial support from the University of Padova is
gratefully acknowledged. We thank Mr. A. Ravazzolo for
skilled technical assistance.
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Organometallics 17 (1998) 4523.
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[10]
(4). To a solution of complex
[Rh2(OAc)4(NC6H4-2-CH2OC(H))2]
[4] [[Rh2(OAc)4(CNC6H4-2-CH2OSiMe3)2] (1). In this typical reaction
to a suspension of [Rh2(OAc)4] (200 mg, 0.45 mmol) in toluene
(25 mL) was added slowly and under vigorous stirring (I) (0.19 g,
1.0 mmol) dissolved in toluene (5 mL). The reaction mixture was
stirred for 3 h at room temperature and then evaporated to small
volume under reduced pressure. Treatment with n-hexane (10 mL)
gave a light brown solid, which was filtered and dried under vacuum
(73% yield). Anal. Calc. for C30H42N2O10Rh2Si2 (M = 852.65): C,
42.26; H, 4.97; N, 3.29. Found: C, 43.04; H, 4.94; N, 3.30%.1H NMR
(CDCl3): d 0.18 (s, 9H, Si(CH3)3), 1.96 (s, 6H, CH3COꢀ2 ), 5.17 (s, 2H,
CH2OSi), 7.41–7.74 (m, 4H, C6H4). 13C{1H} NMR (CDCl3): d ꢀ 0.6
(Si(CH3)3), 24.2 ðCH3COꢀ2 Þ, 60.9 (CH2OSi), 124.2, 126.7, 127.3,
127.7, 130.1, 139.1 (C6H4), 194.4 ðCH3COꢀ2 Þ, the signal correspond-
(1) (0.13 mg, 0.15 mmol) in dichloromethane (10 mL) and methanol
(10 mL) was added TBAF (two drops of 1 M solution in THF); the
reaction mixture was stirred at room temperature for 12 h, giving a
purple solid suspended in a green solution. The solid was filtered and
dried under vacuum (57% yield). Anal. Calc. for C24H28N2O11R-
h2 Æ H2O (M = 726.30): C, 39.69; H, 3.88; N, 3.86. Found: C, 39.32;
H, 3.57; N, 3.68. 1H NMR (CDCl3): d 1.85 (s, 6H, CH3COꢀ2 Þ, 5.56 (s,
2H, CH2O), 6.96–7.81 (m, 4H, C6H4), 7.72 (s, 1H, NCH). 13C{1H}
NMR (CDCl3): d 24.4 ðCH3COꢀ2 Þ, 66.9 (CH2O), 122.8, 124.0, 125.9,
128.1, 129.7, 137.8 (C6H4), 157.8 (NCH), 192.2 ðCH3COꢀ2 Þ. FT IR
(KBr, cmꢀ1): 3067–2886, 1589, 1618, 1493, 1233, 768 and 694.
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