Formation of aluminacyclobutenes via carbon monoxide and isocyanide insertion

Article abstract of DOI:10.1039/b601056c

Reaction of LAl[η²-(CSiMe₃)₂] (L = HC[(CMe)(NAr)]₂, Ar = 2,6-iPr₂C₆H₃) with carbon monoxide and tert-butyl isocyanide afforded unique AlC₃ aluminacyclobutenes via inse

Full text of DOI:10.1039/b601056c

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Formation of aluminacyclobutenes via carbon monoxide and
isocyanide insertion
Xiaofei Li, Chengbao Ni, Haibing Song and Chunming Cui*
Received (in Cambridge, UK) 23rd January 2006, Accepted 3rd March 2006
First published as an Advance Article on the web 15th March 2006
DOI: 10.1039/b601056c
Reaction of LAl[g2-(CSiMe3)2] (L = HC[(CMe)(NAr)]2, Ar =
2,6-iPr2C6H3) with carbon monoxide and tert-butyl isocyanide
afforded unique AlC3 aluminacyclobutenes via insertion into
one of the aluminium–carbon bonds.
There is increasing interest in strained ring compounds that
incorporate heavier main group elements because of their unique
structures and high reactivity.1 In this context, we are interested in
the strained aluminium ring systems stabilized by sterically
demanding ligands. Recently, several three-membered AlC2 ring
compounds have been reported.2 It has been demonstrated that
the AlC2 ring compound, LAl[g2-(CSiMe3)2] (1, L = HC[(CMe)-
(NAr)]2, Ar = 2,6-iPr2C6H3), can react with a range of small
molecules such as carbon dioxide, ketones, nitriles, organic azides,
dioxygen and CS2, leading to either ring expansion or elimination
of bis(trimethylacetylene).2b,3 These studies prompted us to explore
the reaction of 1 with carbon monoxide and isocyanides since they
are versatile synthons in organic and organometallic synthesis.
Although the insertion of CO and isocyanides into transition
metal–carbon bonds has been well established,4 similar studies on
the insertion into aluminium–carbon bonds are much less
common.5 To the best of our knowledge, there are only two
reports concerning the insertion of CO and an isocyanide into
aluminium–carbon bonds, namely the insertion of CO into the
aluminium–carbon bond of tri-tert-butylaluminium6 and the
double insertion of tert-butyl isocyanide into the Al–C bond of
Cp93Al (Cp9 = C5Me4H).7 The facile insertion of CO and tert-
butyl isocyanide in these two cases may result from the three-
coordinate environment of the aluminium centers. Herein we
report on the syntheses and characterization of the AlC3
aluminacyclobutene rings generated from the insertion reactions
of CO and tert-butyl isocyanide with 1.
Scheme 1 Reagents and conditions: i. CO, n-hexane, r.t.; ii. CNBut,
n-hexane, 278 uC; iii. O2, diethyl ether, 278 uC.
NMR spectrum of 2 shows a singlet at d 234.8 ppm, attributed to
the resonance of Al–C(O), which is downfield shifted from those of
ketones (197–220 ppm)8 but significantly upfield from that
reported for the bridging acyl group of [t-Bu2AlC(O)Bu-t]2,6
indicating terminal acyl coordination9 and the existence of an
a,b-unsaturated acyl unit.2b The infrared spectrum of 2 exhibits a
band at 1677 cm21 as expected for a terminal acyl moiety.9
Unfortunately we were unable to obtain crystals suitable for single
crystal X-ray analysis for further confirmation because of its
extremely high sensitivity to the air and moisture although many
attempts have been made. Interestingly, reaction of 2 with
dioxygen at 278 uC in diethyl ether results in the selective
insertion of one oxygen atom into the Al–C(O) bond with the
formation of LAl[OC(O)C(SiMe3)(CSiMe3)] (4), the same product
as reported for the reaction of 1 with CO2.2b The formation of 4
When a red black solution of 1 was exposed to a purified CO
atmosphere (1 atm, moisture and oxygen should be strictly
excluded) in n-hexane at room temperature, the colour change of
the solution to yellow was observed within 2 min. Compound 2
was obtained in high yield after additional stirring for 20 min
followed by a standard workup (Scheme 1). The reaction of 1 with
tert-butyl isocyanide in n-hexane proceeds rapidly at 278 uC to
give orange crystals of 3 after crystallization from diethyl ether.
Compounds 2 and 3 have been characterized by 1H and 13C NMR
1
has been confirmed by its H NMR spectrum and single crystal
X-ray analysis.{ 4 crystallises in a monoclinic space group,
different from that reported previously, but the structural
parameters are only marginally different from those reported.3a
The NMR spectrum of 3 indicates the formation of two isomers
because of the two possible orientations of the tert-butyl group
(Scheme 1). The two isomers are stable, and cannot be
interconverted in hot toluene. It should be noted that the ratio
(ca. 2 : 1 estimated from the 1H NMR spectrum) of the two
1
spectroscopy, IR spectroscopy and elemental analysis.{ The H
NMR spectrum of 2 displays four doublets for the CHMe2 groups
on the Ar rings because of the non-symmetric AlC3 ring. The 13
C
State Key Laboratory of Elemento-Organic Chemistry, Nankai
University, Tianjin 300071, P. R. China. E-mail: cmcui@nankai.edu.cn;
Fax: +86-22-23503461; Tel: +86-22-23503461
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isomers is independent on the reaction temperatures (278 uC to
room temperature). Attempts to separate 3a and 3b by repeated
crystallisation failed because of their very similar physical proper-
ties (solubility and appearance). However, the 1H NMR spectrum
of the mixture could be partially distinguished despite the overlaps
of the resonances of the aromatic protons and CHMe2 groups of
the two molecules. The proton NMR spectrum of the major
isomer shows three singlets at d 0.48, 0.55 and 0.80 ppm with the
same integration, attributed to the resonances of the two SiMe3
groups and the t-butyl group while the spectrum of the minor one
shows resonances at d 20.14, 0.52 and 1.42 ppm. The down and
upfield shifts of one of the SiMe3 groups and the t-butyl group in
the 1H NMR spectrum of the major isomer may result from
two fused rings around the aluminium atom. The iminoacyl group
is coordinated to the aluminium atom in an g1 fashion and the
2
˚
Al1–C30 bond length (1.9842(19) A) is in line with a Al–C (sp
hybrid) single bond.2b The N3–C30 bond length (1.285(2) A) is in
˚
the range for iminoacyl groups. The C30–C31 bond length
˚
(1.519(2) A) is consistent with a C–C single bond, and the C31–
˚
C32 bond length (1.374(3) A) indicates double bonding. The trans
a,b-unsaturated iminoacyl moiety N3–C30–C31–C32 is not planar
(torsion angle: 10.8u) probably due to the sterically demanding
ligands on the AlC3 four-membered ring.
In summary, the facile insertion of CO and isocyanide into an
Al–C bond of the four coordinate aluminium AlC2 ring is
remarkable, demonstrating that ring strain could induce new
reactivity for higher coordinate aluminium species. The formed
AlC3 aluminacyclobutenes are unique and new additions to
strained aluminium ring systems. Compounds 2 and 3 may
…
intramolecular H H interactions among the two groups. The
chemical shifts for the other protons and the resonances found in
the 13C NMR spectrum of the two isomers are quite close to each
other. Based on these observations, we, tentatively, assign 3a as the
major isomer because of the existence of steric repulsion between
the t-butyl and its neighboring SiMe3 group. The iminoacyl
resonances in the 13C NMR spectrum of 3 appear at d 197.5 and
194.6 ppm, which are in the reported range for those found in
terminal iminoacyl complexes.9 The molecular structure of one
isomer has been characterized by single crystal X-ray analysis,
which disclosed that the molecular array corresponds to 3b.{
The structure of 3b is shown in Fig. 1. The most striking feature
of 3b is the almost planar AlC3 ring (mean deviation from the
undergo interesting insertion reactions with
a number of
unsaturated small molecules to generate novel aluminium ring
systems because of the two distinct Al–C bonds and the ring strain.
Investigation of new reactions of 2 and 3 is currently being
undertaken.
We are grateful to the National Natural Science Foundation of
China (Project No. 20572050 and 20421202) and the NCET for
support of this work.
Notes and references
˚
plane: 0.0236 A), which is arranged nearly perpendicular to the
{ 2: A solution of 1 (0.25 g, 0.4 mmol) in n-hexane at room temperature
was exposed to dry carbon monoxide under normal pressure. A colour
change to yellow was observed within 2 min. It was stirred at room
temperature for an additional 20 min under a CO atmosphere. All volatiles
were removed to give a pale yellow solid, which was washed with n-hexane
to afford 2: yield 72%; mp 126 uC (dec.); anal. found for C38H59AlN2OSi2:
N1–Al1–N2 plane (the angle between the C30–Al1–C32 and N1–
Al1–N2 planes: 91.6u). The aluminium atom adopts a distorted
tetrahedral geometry because of the geometric constraints of the
1
C 70.63, H, 9.53. Calc. C 70.98, H, 9.41; H NMR (C6D6, 400 MHz): d
20.07 (s, 9H, SiMe3), 0.43 (s, 9H, SiMe3), 1.03 (d, 6H, J = 6.80 Hz,
CHMe2), 1.08 (d, 6H, J = 6.80 Hz, CHMe2), 1.19 (d, 6H, J = 6.80 Hz,
CHMe2), 1.36 (d, 6H, J = 6.80 Hz, CHMe2), 1.43 (s, 6H, b-CMe), 3.13
(sept, 2H, CHMe2), 3.26 (sept, 2H, CHMe2), 4.74 (s, 1H, c-CH), 6.97 (m,
2H, Ar–H), 7.04 (m, 4H, Ar–H); 13C NMR (C6D6): d 1.23 (SiMe3), 2.66
(SiMe3), 22.13 (CHMe2), 23.02 (CHMe2), 25.86 (CHMe2), 26.80 (CHMe2),
30.21 (CMe), 31.07 (CHMe2), 123.4, 126.8, 128.6, 129.6, 142.3, 145.5, 146.8,
148.9, 150.4 (AlCC, Ar–C), 234.8 (CO); IR n/cm21: 1677 (s, AlCO). 3: To a
solution of 1 (0.20 g, 0.32 mmol) in n-hexane (15 mL) was added neat tert-
butyl isocyanide (0.027 g, 0.32 mmol) at 278 uC. The colour of the solution
turned into orange immediately. The resulting mixture was allowed to
warm up to room temperature and was stirred for 30 min. All volatiles
were removed and the remaining solid was crystallised from diethyl ether at
225 uC to give orange crystals of 3: yield 57%; mp 145 uC (dec.); anal.
found for C42H68AlN3Si2: C 72.26, H 9.96. Calc. C 72.25, H 9.82; 1H
NMR (C6D6): d 20.14, 0.48 (s, 1 : 2, 9H, SiMe3), 0.52, 0.55 (s, 1 : 2, 9H,
SiMe3), 0.80 (s 6H, CMe3), 1.01 (d, 2H, J = 6.40 Hz, CHMe2), 1.04 (d, 2H,
J = 6.80 Hz, CHMe2), 1.13 (d, 6H, J = 6.80 Hz, CHMe2), 1.17 (d, 4H, J =
6.80 Hz, CHMe2), 1.20 (d, 2H, J = 6.80 Hz, CHMe2), 1.26 (d, 4H, J =
6.80 Hz, CHMe2), 1.39 (d, 2H, J = 6.80 Hz, CHMe2), 1.42 (s, 3H, CMe3),
1.47 (d, 6H, CMe), 2.29 (m, 4H, CHMe2), 4.96, 5.09 (s, 1H, 1 : 2, c-CH),
7.04–7.09 (m, 6H, Ar–H); 13C NMR (C6D6): d 2.04, 2.73, 3.28 (SiMe3),
23.90, 24.33, 24.54, 24.72, 24.85, 25.07, 25.15, 25.85, 26.64 (Me, CHMe2),
27.81, 29.03, 29.11, 29.24, 30.30 (CHMe2 and CMe3), 55.75, 55.48 (CMe3),
100.48, 101.42 (c-C), 124.23, 124.29, 125.12, 125.29, 127.41, 127.47, 139.00,
140.57, 140.82, 142.94, 144.91, 145.40 (Ar–C), 170.75, 171.27, 172.54
(NCMe and CSiMe3), 194.62, 197.47 (AlC(N)). IR n/cm21: 1587 (s, AlCN)
{ Crystal data for 3b?Et2O: C46H78AlN3OSi2, M = 772.27, triclinic, space
Fig. 1 Thermal ellipsoid drawing of 3b (30% probability). Hydrogen
atoms and iPr groups on the Ar rings have been omitted. Selected bond
¯
˚
group P1, a = 10.672(3), b = 13.588(4), c = 18.468(6) A, a = 72.022(4), b =
˚
distances (A) and angles (u): Al1–C30 1.9842(19), Al1–C32 1.989(2), Al1–
3
23
˚
87.112(4), c = 80.639(4)u, V = 2513.3(14) A , Z = 2, Dc = 1.020 g cm
,
N1 1.9020(16), Al1–N2 1.9141(15), C30–C31 1.519(2), C31–C32 1.374(3),
N3–C30 1.285(2), N3–C39 1.480(2); N1–Al1–N2 95.88(7), C30–Al1–C32
72.94(8), N1–Al1–C30 125.36(7), N2–Al1–C32 117.81(7), C30–C31–C32
109.31(16), C30–N3–C39 119.71(16).
F(000) = 848, 13739 reflections measured (8763 unique). R1 [I > 2s(I)] =
0.0442, wR2 (all data) = 0.1361, GOF = 1.019 for 497 parameters and 0
restraints. CCDC 296206. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b601056c 4: C38H59AlN2O2Si2,
1764 | Chem. Commun., 2006, 1763–1765
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M = 659.03, monoclinic, space group P21/n, a = 16.1975(18), b =
H. Song, L. Duan, C. Cui and H. W. Roesky, Inorg. Chem., 2006, 45,
1912.
3
˚
˚
11.3234(12), c = 22.709(2) A, b = 104.540(2)u, V = 4031.7(8) A , Z = 4,
Dc = 1.086 g cm23, F(000) = 1432, 22056 reflections measured
(8091 unique). R1 [I > 2s(I)] = 0.0537, wR2 (all data) = 0.1552, GOF =
0.995 for 422 parameters and 0 restraints The X-ray data were collected on
a Bruker Smart-Apex II diffractometer using graphite monochromated
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