Competing reactions of hypercoordinate silicon dichelates

Article abstract of DOI:10.1002/poc.1391

Neutral hexacoordinate silicon complexes derived from hydrazide chelating ligands with imino-donor groups, and their pentacoordinate ionic dissociation products, undergo facile intramolecular aldol-type condensation catalyzed by their chloride counterion leading to formation of a third chelate ring. In analogous silacyclobutane dichelates, in the absence of halide counterion, a similar uncatalyzed rearrangement takes place, accompanied by opening of the four-membered ring. In the absence of a-protons necessary for the condensation, the four-membered ring residue adds directly to one of the imino-carbon atoms forming a new C-C bond and closing a different chelate ring. This latter addition to the imino carbon is the preferred reaction pathway, even in the presence of 12 α-protons, when cyanide ion replaces the chloride counterion and acts as nucleophile. The cyanide reactivity is rationalized in terms of the HSAB concept. An unusual intramolecular rearrangement involving the migration of a t- butyl group from silicon to carbon, while enabling the unprecedented attachment of a third hydrazide chelating agent, leading to a hexacoordinate trichelate complex, is presented. Copyright

Full text of DOI:10.1002/poc.1391

Special Issue
Received: 10 January 2008,
Revised: 9 April 2008,
Accepted: 15 April 2008,
Published online in Wiley InterScience: 9 July 2008
(www.interscience.wiley.com) DOI 10.1002/poc.1391
Competing reactions of hypercoordinate
silicon dichelates
Inna Kalikhmana, Boris Gostevskiia, Evgenia Kertsnusa, Stephan Deuerleinb,
b
c
a
*
Dietmar Stalke , Mark Botoshansky and Daniel Kost
Neutral hexacoordinate silicon complexes derived from hydrazide chelating ligands with imino-donor groups, and
their pentacoordinate ionic dissociation products, undergo facile intramolecular aldol-type condensation catalyzed
by their chloride counterion leading to formation of a third chelate ring. In analogous silacyclobutane dichelates, in
the absence of halide counterion, a similar uncatalyzed rearrangement takes place, accompanied by opening of the
four-membered ring. In the absence of a-protons necessary for the condensation, the four-membered ring residue
adds directly to one of the imino-carbon atoms forming a new C—C bond and closing a different chelate ring. This
latter addition to the imino carbon is the preferred reaction pathway, even in the presence of 12 a-protons, when
cyanide ion replaces the chloride counterion and acts as nucleophile. The cyanide reactivity is rationalized in terms of
the HSAB concept. An unusual intramolecular rearrangement involving the migration of a t-butyl group from silicon to
carbon, while enabling the unprecedented attachment of a third hydrazide chelating agent, leading to a hexacoor-
dinate trichelate complex, is presented. Copyright ß 2008 John Wiley & Sons, Ltd.
Keywords: hypercoordinate silicon; molecular rearrangement; silacyclobutane complex; aldol condensation of imines;
silicon trichelate
high field 29Si NMR chemical shift: ꢀ134.7 ppm for
INTRAMOLECULAR ALDOL CONDENSATION
OF IMINES
R ¼ Me, R1 ¼ H, and from the lack of easily ionizable halide
ligands), a very similar molecular rearrangement took place Eqn
(2).[6] There are two distinct differences between these
Pentacoordinate hydrazide-based siliconium chloride comp-
rearrangements: the absence of apparent catalysis, and the
accompanying opening of the four-membered ring in the latter
reaction. Clearly, the fact that the silacyclobutane ring opens
and forms an n-propyl ligand requires that a proton be
abstracted by the terminal carbon atom of the opening ring. It
follows that the ring very likely opens spontaneously due to ring
strain resulting in a positively charged pentacoordinate silicon
center and a primary carbanion. The latter rapidly abstracts one
of the allylic protons, initiating the interchelate aldol conden-
sation. Because of the high energy generally attributed to
primary carbanions,[7] it seems more likely that ring opening
and proton abstraction take place simultaneously, during
collision of a methyl group with the four-membered ring
carbon, such that no free carbanion actually exists at any point
in time.
lexes were recently shown to undergo a facile base-catalyzed
molecular rearrangement Eqn (1), forming a new carbon–
carbon bond and closing a third chelate ring.[1] A closer
examination reveals that the rearrangement is equivalent to an
intramolecular aldol-type condensation between two adjacent
imine groups,[2,3,4,5] catalyzed by its own counterion, chloride,
acting as a base to abstract an allylic proton. The catalysis by the
counterion was recognized from the observation that chloride
reacted faster than bromide, which in turn reacted faster than
iodide in this reaction, under otherwise similar conditions
(boiling chloroform solution).
*
Correspondence to: D. Kost, Department of Chemistry, Ben-Gurion University,
Beer-Sheva 84105, Israel.
E-mail: kostd@bgu.ac.il
a
b
c
I. Kalikhman, B. Gostevskii, E. Kertsnus, D. Kost
Department of Chemistry, Ben-Gurion University, Beer-Sheva 84105, Israel
S. Deuerlein, D. Stalke
Institut fu¨r Anorganische Chemie, Universita¨t Gottingen, Gottingen, Germany
¨
¨
It was therefore quite surprising to find later that even in the
absence of any counterions in the silacyclobutane complexes 3,
which are purely hexacoordinate in solution (judging from the
M. Botoshansky
Department of Chemistry, Technion – Israel Institute of Technology, Haifa
32000, Israel
J. Phys. Org. Chem. 2008, 21 1029–1034
Copyright ß 2008 John Wiley & Sons, Ltd.

I. KALIKHMAN ET AL.
It is obvious that both of the reactions described in Eqs. 1 and 2
require the presence of activated (allylic) a-protons, to initiate the
condensation. In the absence of a-protons, the reaction takes a
different course: the transient carbanion intermediate, unable to
abstract a proton, acts as a nucleophile and attacks the most
electrophilic carbon atom in the system, the imino carbon
Eqn (3).[6] As a result a different third chelate ring is closed, with
the formation of a new C—C bond, and the conversion of one of
the N ! Si dative bonds to a substantially shorter covalent bond
Traces of the intramolecular condensation product 10 are
always found as a by-product along with 9 in the reaction shown
in Eqn (4). 10 could, in principle, be either the result of chloride
reacting with the reactant 7, as in Eqn (1), or could result from
cyanide acting as a base, abstracting a proton and initiating
the condensation, or could be formed by rearrangement of 9.
When carefully purified 9 was heated for several hours in boiling
chloroform, it eventually produced the rearrangement product
10 Eqn (5).[8] 10 was identified by its 1H, 13C and 29Si NMR spectral
analogy with the corresponding spectra of an authentic
sample,[1] prepared and isolated from the rearrangement of 7,
and by the appearance of a distinct HCN signal at 108.8 ppm in
the 13C NMR spectrum of the reaction mixture.
˚
(1.758(1) vs. 2.044(2) A, respectively, in 6a). In this case the
alternative reaction pathways are dictated by the presence or
absence of a-protons; the aldol condensation Eqn (2) takes place
preferentially over addition to the imino double bond Eqn (3),
and the latter is only observed when the first pathway is
unavailable.
The reaction takes a slightly different course when the dichloro
complex 11 reacts with one molar equivalent of Me3SiCN
Eqn (6);[8] in addition to the major product (12) in which the
cyanide group has added to the imino carbon, also the
hexacoordinate complex (13) is found as a minor product in
contrast to the total absence of 8 in the analogous reaction of the
monochloro complex 7, Eqn (4). The formation of 13 as a
by-product in Eqn (6), and the lack of a similar product in Eqn (4),
probably reflect the additional electron-withdrawal from silicon
by the additional chloro ligand in 11, resulting in a greater
tendency of the silicon to attract donor ligands and, hence, a
relatively more stable hexacoordinate complex 13.
REARRANGEMENTS INVOLVING THE
CYANIDE GROUP
The selectivity order described above (preference for aldol
condensation over addition to imino carbon) is completely
reversed in the following reactions involving the cyanide group.
Attempts to replace chloride by cyanide in a silicon complex via
transsilylation, using Me3SiCN Eqn (4), did not result in the
expected hexacoordinate cyano-complex 8, but in spontaneous
addition of the cyano group to the imino carbon of one of the
chelate rings (9), despite the availability of no less than 12
a-protons![8] Thus, when instead of the presumed alkyl carbanion
in Eqn (2) a cyanide ion (also a carbon base) is present in the
reaction, addition to the imino carbon becomes the over-
whelmingly preferred reaction.[8]
This greater tendency of silicon to form hexacoordinate
complexes in the presence of electron-withdrawing ligands
becomes even more evident when 11 is treated with two molar
equivalents Me3SiCN Eqn (7). The hexacoordinate dicyano
complex (14) is now the major product.[8] Only upon prolonged
heating (20 h) in boiling chloroform it eventually rearranges to
the tricyclic cyano complex 16. However, upon attempts to
The preference of the cyanide ion to add to the imino carbon,
in contrast to the condensations of Eqs. 1 and 2, may be
rationalized by reference to the Hard Soft Acid Base (HSAB)
concept:[9,10,11] the cyanide is a soft base, and therefore prefers to
react with the soft imino-carbon acid, rather than to abstract a
hard proton acid.
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 1029–1034

REARRANGEMENTS IN SILICON COMPLEXES
crystallize 14, by keeping its chloroform solution at 4 8C for
several days, the doubly rearranged 15 was obtained, with no
trace of the pentacoordinate, singly rearranged, presumed
intermediate.
Formation of 14 in the presence of excess Me3SiCN provides
indirect evidence that 12 does not form directly from 11, but
probably via initial formation of 13: the latter is required for the
formation of 14, and thus the reactions leading from 13 to either
12 or 14 are competitive, and the outcome is dictated by the
concentration of the reactant Me3SiCN. If 13 were not the
intermediate during formation of 12, 14 would probably not form.
REACTIONS INVOLVING A t-BUTYL LIGAND
AT HEXACOORDINATE SILICON
It has been shown previously that the bulky t-butyl ligand, when
attached to silicon, causes an adjacent chloro ligand to dissociate
already at room temperature Eqn (8), in contrast to less
sterically-demanding ligands which cause significant dissociation
only at lower temperatures (in CD2Cl2 solutions).[12] However,
depending on the nature of the chelating ligand, a t-butyl ligand
attached to silicon can act in a variety of different ways, described
below. When the chelating ligand is substituted with relatively
strong electron-withdrawing groups, such as in the benzylide-
neimino complex 17 (having phenyl and H groups pulling
electrons from the donor-nitrogen through the imino double
bond), silicon becomes sufficiently electron poor to resist ionic
dissociation Eqn (9). 17 is the first undissociated hexacoordinate
chlorosilicon complex with a t-butyl ligand. The solution (CDCl3)
29Si NMR spectrum of 17 showed two stereoisomers (ꢀ132.1 and
ꢀ135.4 ppm) both of which were well within the hexacoordinate
silicon resonance range. The hexacoordinate nature of one of the
isomers was confirmed by an X-ray crystal analysis.[8]
Figure 1. Molecular structure of 18 in the crystal, depicted at the 50%
probability level. Hydrogen atoms omitted for clarity
Perhaps the most striking case of intramolecular rearrange-
ment, involving the t-butyl ligand, is the following: synthesis of
the dichelate 20 was attempted by transsilylation[13] of 19 with
t-BuSiCl3 as shown in Eqn (10). However, instead of the expected
20 a red crystalline trichelate 21 was obtained. The latter was
subjected to single crystal X-ray analysis, which confirmed its
structure (Figure 2). Selected bond lengths and angles are
presented in Table 1. 21 is the first of the hydrazide-derived
silicon complexes[13] in which all three chlorine atoms of the
XSiCl3 precursor (X ¼ t-Bu in the present case, and alkyl, aryl or
halogens in others) have been replaced by the chelate-forming
bidentate O—Si ligands.
This situation can easily be reversed by substitution of the
chloro by the bulkier and better leaving group bromo ligand.
Transsilylation of 17 with Me3SiBr results in the dissociated
pentacoordinate t-butylsiliconium bromide (18). 18 was charac-
terized by its single crystal X-ray analysis (Fig. 1, Table 1), that
features a distorted trigonal bipyramid (TBP) geometry about the
pentacoordinate silicon, and a well separated bromide counter-
ion. The solid state structure agrees well with the solution
29Si NMR chemical shift (CDCl3 solution 300 K) of ꢀ79.6 ppm,
characteristic of pentacoordination. Clearly, the tendency of
silicon to keep the halogeno ligand attached, due to weak
coordination by the relatively weak nitrogen donors, is counter-
balanced by the steric bulk of the halogen, which in 18 is
dominant and causes dissociation.
Why is 21 different from so many previously reported
hexacoordinate silicon dichelates of this family? The answer
must relate to the exceptionally electron-withdrawing groups
attached to the chelating (hydrazide derived) ligands. In each
J. Phys. Org. Chem. 2008, 21 1029–1034
Copyright ß 2008 John Wiley & Sons, Ltd.
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I. KALIKHMAN ET AL.
probably the weakest N-donor ligands of the hydrazide family
prepared and reported so far.[13] As a result of this weak donor
property, the coordination of nitrogen to silicon is weak, resulting
in a relatively electron-poor silicon atom. This, in turn, causes the
residual chloro ligand in 20 to resist ionic dissociation (in contrast
to Eqn (8)).[12] Rather than dissociate, the chloro ligand of 20 is
readily exchanged by the less-electronegative oxygen atom of a
third chelating agent 19, which also forms a substantially
stronger bond to silicon.[14] Therefore, 20 quickly forms 21, in
which three of the hydrazide residues are ligated. Formation of 21
by attachment of a third hydrazide residue is accompanied by an
intramolecular 1,3-t-butyl shift from silicon to the adjacent
imino-carbon, replacing the relatively weak carbon–silicon bond
by the stronger carbon–carbon bond,[14] and releasing the steric
congestion caused by the t-butyl ligand, while closing a third
chelate ring by attachment of the N-donor group to silicon. Thus
the unusual 21 is formed, with the (unprecedented) three
hydrazide-derived chelate rings, two of which have the
benzylideneimino group, while the third imino double bond
has been saturated by addition of a t-butyl group to the imino
carbon.
˚
Table 1. Selected bond lengths (A) and angles (8) for 18 and
21
18
21
Si–O1
Si–O2
Si–C19
Si–N3
Si–N1
C12–N3
C1–N3
1.695(2)
1.701(3)
1.894(2)
1.908(3)
1.908(2)
Si–O1
Si–O2
Si–O3
Si–N2
1.7317(16)
1.8054(15)
1.7781(16)
1.7873(18)
1.9424(18)
1.9495(19)
1.466(3)
Si–N4
Si–N6
C3–N2
C16–N4
C25–N6
N4–Si–N6
O1–Si–O3
N2–Si–O2
O1–Si–N4
O2–Si–N4
1.302(3)
1.310(4)
O1–Si–O2
O1–Si–C19
O2–Si–C19
O1–Si–N3
N3–Si–N1
O2–Si–N1
O1–Si–N2
132.73(11)
113.60(13)
113.65(13)
87.75(11)
157.94(11)
88.73(10)
82.81(11)
1.291(3)
1.280(3)
162.59(8)
175.52(8)
175.66(8)
96.74(8)
80.82(7)
The results described above demonstrate that in the same
basic molecular system all three different species can be
observed: the undissociated, hexacoordinate t-butylchlorosilicon
dichelate 17, the dissociated ionic (pentacoordinate)
t-butylsiliconium bromide 18, and when additional
electron-withdrawing CF3 substituents are introduced in the
chelate rings, the t-butyl group migrates to the imino-carbon,
making room for the attachment of a third chelating ligand and
formation of the rearranged trichelate 21.
chelate ring in the presumed intermediate 20 (and in two of
the three in 21) the donor nitrogen atom is part of a
benzylideneimino group, with its relatively strong electron-
withdrawing hydrogen and phenyl groups. In addition, the
powerful electron-withdrawing CF3 substituents in each ring
make the donor nitrogen a very weak electron donor, even
weaker than in 17 and 18 above. These chelating ligands are
Figure 2. Molecular structure of 21 in the crystal, depicted at the 50% probability level. Hydrogen atoms, except the two imino hydrogens and one
saturated imino hydrogen, were omitted for clarity
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J. Phys. Org. Chem. 2008, 21 1029–1034

REARRANGEMENTS IN SILICON COMPLEXES
The product was stirred with 0.319 g (2.09 mmol) of Me3SiBr in
5 mL of chloroform for 24 h. The white solid was isolated by
decantation and vacuum drying to yield 0.926 g (90%), mp
173–174 8C. Crystals for X-ray diffraction analysis were grown
from acetonitrile solution. Anal. Calcd for C22H27BrN4O2Si: C,
54.21; H, 5.58; N, 11.49. Found: C, 54.30; H, 5.73; N, 11.35. 1H NMR
(CDCl3, 300 K): d 1.02 (s, 9H, C4H9), 2.53 (s, 6H, Me), 7.56–8.73 (m,
10H, Ph). 13C NMR (CDCl3, 300 K): d 18.3 (Me), 19.8 (C(CH3)3), 27.0
EXPERIMENTAL SECTION
The reactions were carried out under dry argon using Schlenk
techniques. Solvents were dried and purified by standard
methods. NMR spectra were recorded on a Bruker Avance
DMX-500 spectrometer operating at 500.13, 125.76, and
99.36 MHz, respectively, for 1H, 13C and 29Si spectra. Spectra
are reported in d (ppm) relative to TMS, as determined from
1
—
standard residual solvent-proton (or carbon) signals for H and
(C(CH ) ), 128.9, 129.2, 135.9, 136.4 (Ph), 161.7, 171.4 (C N).
—
3 3
13C and directly from TMS for 29Si. Melting points were measured
in sealed capillaries using a Buchi melting point instrument,
and are uncorrected. Elemental analyses were performed by
29Si NMR (CDCl3, 300 K): d -79.6.
¨
Mikroanalytisches Laboratorium Beller, Gottingen, Germany.
Bis[N-(benzylideneimino)trifluoroacetimidato-N’, O]
[N-(1-(tert-butyl)benzylamino)trifluoroacetimidato-N’, O]
silicon(IV) (21)
Single crystal X-ray diffraction measurements were performed
on a Nonius Kappa-CCD Diffractometer (18), and a Bruker Smart
Apex on a D8-Goniometer (21). Crystallographic details are listed
in Table 2. Crystallographic data for 18 and 21 have been
deposited with the Cambridge Crystallographic Data Centre. The
CCDC numbers are listed in Table 2.
A mixture of 1.292 g (4.48 mmol) of N-(benzylideneimino)O-
(trimethylsilyl)trifluoroacetimidate (19) and 0.423 g (2.21 mmol)
of t-BuSiCl3 in 5 mL of chloroform was kept at 100 8C for 170 h. The
volatiles were removed under reduced pressure leaving a viscous
orange residue that formed red crystals after treating with 5 mL
n-hexane. 0.48 g (41%) of 21 was isolated, mp. 108–110 8C. Anal.
Calcd for C31H27F9N6O3Si: C, 50.96; H, 3.72; N, 11.50. Found: C,
Bis[N-(benzylideneimino)acetimidato-N’, O]tert-
butylsiliconium bromide(18)
A mixture of 1.013 g (4.32 mmol) of N-(benzylideneimino)O-
(trimethylsilyl)acetimidate[6] and 0.458 (2.39 mmol) of t-BuSiCl3 in
5 mL of chloroform was kept at room temperature for 48 h,
followed by removal of volatiles under 0.2 mmHg. The white solid
was washed in 10 mL of n-hexane to yield 0.895 g (94%) of 17.[8]
50.65; H, 3.74; N, 11.38. 1H NMR (CDCl3, 300 K): d 0.86 (s, 9H, C4H9),
—
3.89 (s, 1H, N–CH) 7.06 - 8.35 (m, 15H, Ph), 7.24, 8.11 (2s, N CH).
—
13C NMR (CDCl3, 300 K): d 28.4 (C(CH3)3), 37.5 (C(CH3)3), 117.5 (q,
1
—
JC–F ¼ 280 Hz, CF ), 126.2–142.8, (Ph), 159.2, 160.5 (C N), 153.0,
—
3
158.3 (CF C N). 29Si NMR (CDCl3, 300 K) d -149.8.
—
—
3
Table 2. Crystal data and experimental parameters for the structure analyses of 18 and 21
18
21
CCDC number
Empirical formula
Form mass (g molꢀ1
Collection T, K
Cryst. syst.
681007
C46H57Br2N9O4Si2
1016.01
240(1)
681008
C31H27F9N6O3Si
730.68
133(2)
Orthorhombic
Pbca
)
Triclinic
P1
Space group
˚
a (A)
˚
10.232(2)
10.486(2)
26.163(5)
94.97(2)
92.14(2)
118.70(3)
2442.9(8)
2
18.367(1)
18.285(1)
19.431(1)
90
90
90
6525.6(6)
8
1.487
b (A)
˚
c (A)
a (8)
b (8)
g (8)
3
˚
V (A )
Z
rcalc, (Mg/m3)
F (000)
1.360
1052
2992
u range (8)
2.27–25.02
16355
8409
0.0406
8409
1.89–24.85
73735
5641
0.1048
5641
No. of coll. reflns
No. of indep. reflns
Rint
No. of reflns used
No. params.
Goof
570
0.968
514
0.927
R1 wR2[I > 2s(I)]
R1 wR2(all data)
Max./min. res electron dens (eA
0.0423 0.1028
0.0774 0.1104
0.481/0.386
0.0388 0.0773
0.0704 0.0846
0.373/0.172
ꢀ3
˚
)
J. Phys. Org. Chem. 2008, 21 1029–1034
Copyright ß 2008 John Wiley & Sons, Ltd.
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I. KALIKHMAN ET AL.
[5] N. F. Curtis, H. K. J. Powell, H. Puschmann, C. E. F. Rickard, J. M. Waters,
Inorg. Chim. Acta 2003, 355, 25.
Acknowledgements
Financial support from the Israel Science Foundation, grant no.
ISF-139/05, and from INTAS, project 03-51-4164, is gratefully
acknowledged.
[6] B. Gostevskii, I. Kalikhman, C. A. Tessier, M. J. Panzner, W. J. Youngs, D.
Kost, Organometallics 2005, 24, 5786.
[7] A. C. Knipe, Org. React. Mech. 2004, 307–349.
[8] I. Kalikhman, B. Gostevskii, E. Kertsnus, M. Botoshansky, C. A. Tessier,
W. J. Youngs, S. Deuerlein, D. Stalke, D. Kost, Organometallics 2007, 26,
2652.
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J. Phys. Org. Chem. 2008, 21 1029–1034