1-Titanacyclobuta-2,3-diene-an elusive four-membered cyclic allene

Article abstract of DOI:10.1039/c9sc01002e

The synthesis of an unusual 1-metalla-2,3-cyclobutadiene complex [rac-(ebthi)Ti(Me₃SiC₃SiMe₃)] (rac-ebthi = rac-1,2-ethylene-1,1′-bis(η⁵-tetrahydroindenyl)), a formal metallacyclic analogue of a non-existent four-membered 1,2-cyclobutadiene, is described. By variation of the cyclopentadienyl ligand of the titanocene precursor it was possible to stabilise this highly exotic compound which selectively reacts with ketones and aldehydes to yield enynes by oxygen transfer to titanium. Analysis of the bonding and electronic structure of the metallacycle shows that the complex is best described as an unusual antiferromagnetically coupled biradicaloid system, possessing a formal Ti(iii) centre coordinated with a monoanionic radical ligand.

Full text of DOI:10.1039/c9sc01002e

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1-Titanacyclobuta-2,3-diene – an Elusive Four-membered Cyclic
Allene
Received 00th January 20xx,
Accepted 00th January 20xx
Fabian Reiß,*a Melanie Reiß,a Jonas Bresien,b Anke Spannenberg,a Haijun Jiao,a Wolfgang
Baumann,a Perdita Arndta and Torsten Beweries*a
DOI: 10.1039/x0xx00000x
The synthesis of an unusual 1-metalla-2,3-cyclobutadiene complex [rac-(ebthi)Ti(Me3SiC3SiMe3)] (rac-ebthi = rac-1,2-
ethylene-1,1’-bis(5-tetrahydroindenyl)), a metallacyclic analogue of a non-existent four-membered 1,2-cyclobutadiene is
described. By variation of the cyclopentadienyl ligand of the titanocene precursor it was possible to stabilise this highly
exotic compound which selectively reacts with ketones and aldehydes to yield enynes by oxygen transfer to titanium.
Analysis of the bonding and electronic structure of the metallacycle shows that the complex is best described as an
unusual antiferromagnetically coupled biradicaloid system, possessing a formal Ti(III) centre coordinated with a
monoanionic radical ligand.
catalyst deactivation in alkyne metathesis. Schrock reported
on two structurally characterised examples of "all-C-
Introduction
deprotiometallacyclobutadienes"
Mo{C3-
Driven by the “desire to make the molecule that violates the
norm”1 chemists have always looked into possibilities to
stabilise exotic molecules and explore the reasons for its
existence and reactivities. In this respect, a commonly used
approach is the incorporation of unsaturation into cyclic
structures that increases the ring strain and therefore in
principle decreases the likeliness of its existence.2 For a long
time, organometallic chemists have investigated metallacycles,
both, with respect to their unusual coordination environments
as well as potential applications in stoichiometric3,4 and
catalytic applications.4,5 In the past, a number of five-
membered metallacycles based on early and late transition
metals have been reported that possess rather unusual
structural motifs, including Rosenthal’s 1-metallacyclopenta-
2,3-4-trienes6, Suzuki’s 1-metallacyclopent-3-ynes7, Erker’s 1-
metallacyclopenta-3,4-dienes8 (all based on group 4 metals) or
Xi’s metalloles (Rh, Pd, Pt).9 Intuitively, the synthesis of four-
membered and highly unsaturated metallacycles should be
even more challenging and to date a common approach to
stabilise such structures is the incorporation of heteroatoms
into the ring system.10 Also, the parent 1,2-cyclobutadiene has
a calculated strain energy of 74.9 kcal∙mol-1,11 thus reflecting
the instability and the difficulty in isolating a molecule of this
type. The chemistry of 1-metallacyclobuta-2,3-dienes has been
investigated in the past as such structures play a role for
(tBu)2}{OCH(CF3)2}2(py)2] (A)12 and [CpW{C3(tBu)2}-Cl] (B)13 (Cp
= 5-cyclopentadienyl) that were formed during metathesis of
terminal alkynes (Figure 1). Later, Fürstner (C) and – very
recently Tamm (D)
– observed the formation of 1-
molybdacyclobuta-2,3-dienes as a product of reaction of an
alkylidene catalyst with a terminal alkyne.14
Schrock
Fürstner
Tamm
t-Bu
t-Bu
t-Bu
i-PrO
py
(F3C)2CO
OC(CF3)2
Ph
N
N
W
Mo
Ph3SiO
R
Mo
OSiPh3
R
Mo
py
i-PrO
Cl
Mes
t-Bu
A
B
C
, R = p-OMe-C6H4
D
Figure 1. Structurally characterised 1-metallacyclobuta-2,3-diene complexes.
Bonding situations in the metallacycle are depicted as shown in the literature.
To further explore the frontiers of the chemistry of group 4
metallacycles Jemmis, Schulz and Rosenthal have
computationally explored the possibilities to stabilise planar 1-
metalla-2,3-cyclobutadienes and found that the incorporation
of electron-donating substituents at -carbon atoms could be
beneficial.11 Later, our group has attempted to synthesise such
structures by coupling of alkynyl and isonitrile ligands at
titanocene, unfortunately this approach only resulted in redox-
disproportionation of the Ti centre.15 Also, deprotonation of -
CH2SiMe3 and -CH2N(SiMe3)2 substituted titanocene alkyne
complexes was found to be unsuccessful.16 A more promising
approach appeared to be the initial construction of a C3
framework prior to coordination to the metal. We have thus
revisited and optimised the synthesis of a previously reported
1,3-dilithioallene precursor [Li2(Me3SiC3SiMe3)]17 and reacted
a. Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str.
29a, 18059 Rostock, Germany.
b. Abteilung für Anorganische Chemie, Institut für Chemie, Universität Rostock,
Albert-Einstein-Straße 3a, D-18059 Rostock, Germany.
Electronic Supplementary Information (ESI) available: experimental, spectroscopic,
crystallographic and computational details (PDF); xyz coordinates. See
DOI: 10.1039/x0xx00000x
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this with [Cp2ZrCl2]. To our surprise, we obtained two unusual Me(Me3Si)C3(SiMe3)Me (G = -11.77 kcal∙mol-1). The fact that
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DOI: 10.1039/C9SC01002E
complexes a coupling reaction leading to 1 was observed in reactions with
allenediide
bridged
dizirconocene
[Cp2Zr(RC3R)2ZrCp2] and [Cp2Zr(Cl)RC3R(Cl)ZrCp2] (R = SiMe3)18, Cp and Cp* complexes indicates that at some stage of the
the latter being a promising precatalyst for amine borane reaction the interaction of an intermediately formed C3
dehydropolymerisation.19 Interestingly, we were able to show framework with the Ti centre occurred. ansa-Cp complexes in
that the dizirconacyclooctatetraene [Cp2Zr(RC3R)2ZrCp2] many cases show reactivities that are comparable to Cp
degrades under mass spectrometry conditions into the desired analogues and furthermore exhibit additional stabilisation of
mononuclear 1-zirconacyclobuta-2,3-diene compound. In this the metallocene through the bridging unit. Subtle differences
contribution, we report on the synthesis and characterisation in reactivity (e.g. formation of different products or faster
of the first 1-metalla-2,3-cyclobutadiene complex of a group 4 conversion) are often referred to as the ansa effect.23 In line
metal as well as the reactivity of this unusual complex.
with this, the reaction of [rac-(ebthi)TiMe2] and
Me(Me3Si)C3(SiMe3)Me is even more thermodynamically
favourable (G = -16.60 kcal∙mol-1).
Results and Discussion
Reaction of [Cp2TiCl2] and [Cp*2TiCl2] with the 1,3-dilithioallene
precursor
Synthesis and characterisation of 1-titanacyclobuta-2,3-diene (2)
We have thus next reacted [rac-(ebthi)TiCl2] and
[Li2(Me3SiC3SiMe3)] at room temperature in pentane (Scheme
2) and observed formation of a deep red solution, from which
compound 2 could be isolated as red crystals after removal of
LiCl. Moderate yields of 2 (58%) can be explained by the
formation of 1 as a byproduct which was identified by NMR
spectroscopy and this nicely demonstrates the subtle
differences in reactivity along the series Cp - rac-(ebthi) - Cp*.
Single crystals suitable for X-ray analysis were obtained from a
saturated pentane solution at room temperature.
To evaluate the influence of the metal centre we first adapted
our previously reported procedure18 and reacted [Cp2TiCl2]
1
with [Li2(Me3SiC3SiMe3)]. H NMR analysis of an aliquot taken
after a reaction time of 16 hours at room temperature shows
the formation of several Cp containing products along with a
large number of resonances in the SiMe3 region of the
spectrum, out of which we identified the coupling product 1
(cf. p S4, Figure S1). Even at low temperatures (-40 °C), only
similar product mixtures could be isolated. We next employed
more electron-donating, sterically more demanding [Cp*2TiCl2]
(Cp* = 5-pentamethylcyclopentadienyl) and observed a slow
colour change from red to brown. NMR analysis of the reaction
mixture shows the formation of unidentified titanocene
species along with the coupling product 1 as the main species,
which can be isolated after column chromatographic workup
in 90% yield (Scheme 1). The formation of 1 can be formally
seen as the dimerisation of two carbenes in the coordination
sphere of Ti. A similar type of carbene coupling to form alkynyl
functionalised symmetrical olefins was reported before by
Casey and co-workers for a Re system.20 Also, the formation of
SiMe3
Li
Li
pentane, r.t.
-2 LiCl
Cl
Cl
+
C
C
C
Ti
Ti
Me3Si
SiMe3
SiMe3
2
Scheme 2. Reaction of [rac-(ebthi)TiCl2] with [Li2(Me3SiC3SiMe3)] and formation
of 2.
Compound 2 shows the expected characteristic 1H NMR signals
at 0.35, 5.27, and 7.22 ppm, corresponding to the SiMe3
groups and the Cp protons of the rac-(ebthi) ligand. The 13C
NMR spectra show two signals at low field that we assign to
the internal ( 134.2 ppm) and metal-bound carbon atoms (
213.8 ppm) of the metallacycle. Notably, these values strongly
differ from those found for the previously reported Mo and W
complexes, pointing towards structural differences such as the
absence of a M-C2 interaction ( > 56 ppm, Table 1).
Ph4C6
by
thermal/radical
degradation
of
1,3-
bis(alkylseleno)allenes supports this proposal.21
SiMe3
Me3Si
Li
Li
benzene, r.t.
-2 LiCl
Cl
Cl
unidentifed
Ti species
0.5
+
Ti
C
C C
+
Me3Si
SiMe3
SiMe3
Me3Si
1
Table 1. Comparison of 13C NMR values for metallacyclic units in
complexes 2, A, B, and C.
Scheme 1. Reaction of [Cp*2TiCl2] with [Li2(Me3SiC3SiMe3)].
A13
220.4 257 205.3 213.8
C-C-C 197.1 196 190.6 134.2
B12
C14
2
[Li2(Me3SiC3SiMe3)]18
37.8, 44.6[a]
174.1
In a previous study we had computed the isodesmic exchange
reaction of Cp*2TiMe2 and the allene precursor
Me(Me3Si)C3(SiMe3)Me to form the desired four-membered
metallacycle and ethane. This reaction is slightly exergonic and
thus in principle feasible (G = -1.75 kcal∙mol-1,22). This
M-C
[a] tetrameric compound in the solid state, dynamic behaviour in
solution leads to two inequivalent 13C shifts.
approach was chosen as
a theoretical model for salt
The molecular structure of compound 2 (Figure 2) shows the
bent metallocene fragment as part of the four-membered ring
system. Ti-C bond lengths are significantly longer than typical
single bonds (Ti1-C1 2.229(1), Ti1-C3 2.235(2); rcov = 2.11 Å24).
That the Ti1-C2 distance (2.178(1) Å) is shorter than Ti1-C1/C3
indicates that the C3 unit is only slightly bent, which is
metathesis using a 1,3-dilithioallene to avoid solubility effects
and cluster formation that would complicate computational
analysis. For the reaction of [Cp2TiCl2] with [Li2(Me3SiC3SiMe3)]
at low temperature we proposed formation of a 1-titana-2,3-
cyclobutadiene (vide supra), which is in accordance with the
computed
exergonic
reaction
of
Cp2TiMe2
and
2 | J. Name., 2012, 00, 1-3
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corroborated by the angle C1-C2-C3 of 150.1(2)°. Compared to correlation. Therefore we employed the Complete Active
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the Mo and W complexes this C3 unit in 2 is less bent (cf. 130°- Space (CAS(8,9)) SCF method30 to obtain a multi-determinant
DOI: 10.1039/C9SC01002E
135°). The C-C bond distances are in the range of double bonds open-shell singlet wave function and describe the bonding
(C1-C2 1.303(2), C2-C3 1.308(2) Å; rcov = 1.34 Å24) and shorter situation in 2 appropriately. This calculation determined the
compared to values found for Mo and W complexes, most biradical character of 2 with  = 28%,31,32 and identified the
likely due to less electron donation to the metal centre. While largest contributions to the overall wave function as the two
the Ti1-C1-C2-C3 unit is planar (1.1(3)°) the SiMe3 groups are determinants placing two electrons either in the formal HOMO
located above and below this plane, which is in contrast to the (4) or LUMO (5, Figure 3).
structural
features
reported
for
group
6
CAS
MOs
Most important
determinants
Occupation
Number
metallacyclobutadienes (cf. p. S15 Table S4,). This description
of a C=C=C unit in 2, is supported by the observed in-phase
(1344 cm-1) and out-of-phase (1729 cm-1) vibrations which are
in good agreement with calculated values (1343 cm-1/1787
cm-1).22
9
8
7
6
0.01
0.06
0.06
0.07
orbital localization
5
4


0.34
1.65
anti-
bonding
bonding
Figure 2. Two views of the molecular structure of complex 2. Thermal ellipsoids
correspond to 30% probability. Hydrogen atoms are omitted for clarity.
1

 =  (4  5)
2
3
2
1
1.91
Analysis of the structure and bonding of 2
side view
1.94
To obtain a better understanding of the bonding situation in 1-
titanacyclobuta-2,3-diene 2, a series of density functional
theory (DFT) and wave function theory (WFT) calculations
were performed. Firstly, the structure of 2 was optimised at
the BP8625/LANL2DZ26/TZVP27 level of theory, showing good
agreement with structural data from single-crystal X-ray
diffraction (cf. p. S48, Table S12). At this point we noticed a
small energy gap between the HOMO and LUMO (1.0 eV),
which can be indicative of substantial biradical character. To
further evaluate this, the optimised structure was used for
4
5
1.95
relative weights ci2
coefficients: 0.86 0.35
:
74 % 12 %
CAS  c11 + c22
Figure 3. Schematic depiction of the active orbitals of a CAS(8,9) calculation. Only
contributions to the wave function with relative weights > 1 % are shown. The orbital
localisation scheme indicates that one of the radical centres is localised at Ti, while the
other is delocalised across the C3 backbone
.
several single point calculations employing the pure and hybrid The singlet state is calculated to be the ground state (ΔES-
density functionals (DF) BP86 and B3LYP25,28 to calculate the T = −9.3 kcal/mol);
i.e. the radical centres are
Kohn-Sham (KS) orbitals, as well as HF to compute the antiferromagnetically coupled and the calculated exchange
canonical MOs. All (KS) wave functions were tested with coupling constant33 is 2J = ES – ET = -3260 cm-1. The radical
respect to RHF/UHF or RKS/UKS instabilities, in order to centres are localised at Ti and on the C3 backbone of the ligand
analyse the biradical character of Ti complex 2. While the KS (Figure 3, right). Therefore, the electronic structure can be
wave function based on the pure DF (BP86) showed no understood as a complex possessing a formal Ti(III) fragment
instabilities, the hybrid DF (B3LYP) and HF solution exhibited a and an organic radical, where the “free” electrons are
low-lying, “broken-symmetry” open-shell singlet state. This antiferromagnetically coupled. As a consequence, no classical
kind of behaviour is often observed if the biradical character is Ti(III) chemistry would be expected as the complex is a singlet.
not too large,29 since part of the non-dynamic correlation is In conjunction with results from Natural Bond Orbital (NBO)
treated by the exchange-correlation functional of the (pure) analysis, the electronic situation is best described by the main
density functional. Mixing in exact exchange reduces the Lewis-type resonance structures depicted on the right side of
amount of correlation treated by the DF and thus the “broken- Scheme 3. Note that the electrons in both the formal πx and πz
symmetry” solution becomes more stable. While the bonding systems of the formal propadienylide ligand are
structures that were optimised using the BP86 functional show delocalized across the C3 unit and that each of these π-bonding
good agreement with experimental structures (Table S12), the systems can be interpreted independently of the other,
electronic energy should only be considered as a rough resulting in a variety of different Lewis resonance structures
approximation due to incorrect treatment of the non-dynamic (see also Figure S50). The calculated natural charge of the
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C3(SiMe3)2 ligand amounts to −0.39 e (CAS) or −0.64 e (BP86), determinant character (cf. p S53, Figure S52). The results from
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DOI: 10.1039/C9SC01002E
which is in the expected range of a formally anionic ligand. For QT-AIM analysis are corroborated by ELF analysis (cf. p S53,
a more detailed discussion of the CAS wave function and Figure S53). There is no localised electron density in the
bonding situation, please refer to the Supporting Information, valence region of C2 directed towards Ti1, whereas the
pp. S49.
bonding electrons between C1/C3 and Ti are localised in the
same area as indicated by the Laplacian of the electron
density. Notably, there is no localised electron density around
C2 pointing away from Ti1 either, i.e. there is no lone pair of
electrons at the central carbon atom. Consequently, the
electronic structure of the C3 scaffold is different from that of
structurally related bent allenes, such as so-called
“carbodicarbenes” (cf. p S54, Figure S54).36
With these results in mind we calculated the biradical
character of the hypothetical analogous Cp ( = 30%) and Cp*
( = 74%) substituted complexes and found that the singlet-
triplet gap and therefore the biradical character greatly
depend on the pyramidalisation (hybridisation) of the carbon
atoms C1 and C3 of the TiC3 ring. Since the coordination
environment around C1/C3 is nearly planar in
[Cp*2Ti(Me3SiC3SiMe3)], it shows the highest biradical
character. This trend is in agreement with previous
computations11 and provides a possible explanation for the
selective formation of 1 over a highly reactive planar biradical
complex.
SiMe3
SiMe3
SiMe3
SiMe3
Ti
Ti
SiMe3
SiMe3
Ti
SiMe3
SiMe3
SiMe3
SiMe3
simplified structure
Ti
Ti
leading Lewis resonance structures
Scheme 3. Main resonance structures which best describe the bonding situation
in 2 as singlet biradical. The antiferromagnetically coupled electron at the ligand
is delocalised within the out-of-plane πx bonding system (red), while the in-plane
πz orbitals (blue) contribute to the C–Ti bonding interactions.
In order to investigate the topology of the electron density, we
performed a QT-AIM analysis.34,35 This revealed two Ti–C bond
paths (Ti1–C1 and Ti1–C3), in agreement with the Lewis
resonance scheme (Scheme 3). Despite the short interatomic
distance between Ti1 and C2, there is no strong bonding
interaction between those atoms; on the contrary, a ring
critical point is found near the centre of the TiC3 ring system.
Moreover, the Laplacian of the electron density ∇2r indicates
that the Ti–C bonds are strongly polarised towards the C atoms
(Figure 4).
Reactions of 2 with carbonyl compounds
To gain first insights into the reactivity of the isolable and
unusual biradical [rac-(ebthi)Ti(Me3SiC3SiMe3)] complex we
have performed stability tests. Therefore, we exposed
solutions of 2 to air and moisture and found that in both cases,
slow formation of the propyne Me3SiC2CH2SiMe3 takes place
(see Supporting Information for details). To evaluate whether
our compound shows radical-type reactivity despite its
biradical character, we have added 9,10-dihydroanthracene as
a potential radical trap, however, we observed no conversion
at room temperature. In reactions with TEMPO ((2,2,6,6-
tetramethylpiperidin-1-yl)oxyl) formation of ill-defined
reaction mixtures that contain Ti(III) species takes place.
We next focused on classical biradical-type reactions that are
also known for inorganic cyclic four-membered 1,3-biradical
systems.37 In the reaction with CO2, a colour change from red
to teal and finally brownish was observed. The NMR spectra of
this sample indicate the formation of three so far unidentified
organometallic products and thus suggest a high reactivity
towards carbonyl substrates. However, these species could not
be separated and characterised yet. Based on this observation
we have next evaluated the reactivity of 2 towards simple
carbonyl compounds benzophenone, acetone, acetophenone,
and benzaldehyde (Scheme 4).
Figure 4. Contour plot of the Laplacian of the electron density ∇2r of Ti complex 2
in the TiC3 ring plane. Dashed lines indicate negative (local charge
concentration), solid lines indicate positive values (local charge depletion). The
Laplacian plot is overlaid with the molecular graph from QT-AIM analysis. Brown
lines indicate bonding paths, blue dots correspond to bond critical points, orange
points indicate ring critical points. Density from CAS(8,9)/def2-TZVP calculation.
The densities obtained from CAS(8,9) and BP86 calculations
are similar, indicating that the pure DFT method is suitable to
approximately describe the electron density despite its single-
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into a catalytic protocol might be hampered by theVieowxAortpichleiOlicnliitnye
of the Ti centre.
SiMe3
SiMe3
DOI: 10.1039/C9SC01002E
O
Me3Si
R
benzene, r.t.
SiMe3
Ti
+
R
R'
- Ti oxido species
R'
3
: R = R' = Ph
2
Conclusions
4
: R = R' = Me
: R = Ph, R' = Me (E/Z isomer mixture)
5
For the first time, we have shown that through combination of
suitable metal centre, cyclopentadienyl ligand and substituents
at the C3 unit the synthesis of an elusive 1-metallacyclobuta-
2,3-diene based on a group 4 metal is possible. Whereas for Cp
and Cp* substituted complexes only formation of an
unsaturated six-membered chain product was observed,
possibly due to coupling of two C3 carbene moieties, in case of
rac-(ebthi) formation of the desired 1-titanacyclobuta-2,3-
diene was possible. This compound represents a metallacyclic
analogue of non-existent 1,2-cyclobutadiene. Analysis of the
structure and bonding reveals a highly unusual interaction of a
formal Ti(III) fragment and an organic monoanionic radical
with antiferromagnetic coupling between both radical centres,
resulting in a singlet species. Reaction of the metallacycle with
carbonyl compounds yields enynes via an unusual [3+1]
coupling with formal insertion of the C=O bond into the M-C
bond being the first step of the reaction. Further studies to
understand the structural principles as well as further
investigations regarding the reactivity of 2 and related
complexes will be done in our lab in the future.
6
: R = Ph, R' = H (E/Z isomer mixture)
SiMe3
O
R
C1
C2
R'
SiMe3
R'
C3
C4
Ti
- Ti oxido
species
-78 to -15 °C
O
R
7
8
: R = R' = Me
: R = H, R' = Ph
Scheme 4. Reaction of 2 with ketones to yield enynes 3, 4, 5 and 6.
NMR analysis of reaction solutions showed that in all cases
formation of enynes occurred by formal oxygen transfer to Ti
and coupling of the remaining C1 unit with the C3 moiety of the
metallacyle. The molecular structure of compound 3 (cf. p S14,
Figure S7) confirms the assignment as an enyne.
The nature of the Ti species formed by oxygen transfer could
not be unequivocally clarified; however, formation of a Ti
oxido species is likely. It should however be noted that such
species require either kinetic stabilisation by bulky Cp
ligands38, or additional Lewis bases such as pyridines39 to
persist as monomeric well-defined complexes. Monitoring of
the reaction of 2 and acetone at low temperature, i.e. slow
warming from -78°C, showed that the red colour of 2 is
retained up to a temperature of -15°C, at which point the
solution turned teal. After workup at low temperature, the
dark-teal residue was analysed by low-temperature NMR
spectroscopy. 13C NMR spectra showed four new resonances
due to a new metallacyclic species 7 at 179.7 (C2), 151.9 (C1),
109.4 (C3), and 90.3 ppm (C4), thus suggesting that the well-
known insertion of the ketone40 is the first step of C-C bond
formation that ultimately yields the enyne (Scheme 4). It
should however be noted that this reaction of carbonyl
compounds with 2 can also be regarded as an addition
reaction to a 1,2-biradical with addition of the C=O bond to
both radical centres. Looking at the nature of the reaction
products and possible intermediates this cannot be
distinguished from classical Ti(IV) diyl insertion reactivity.
Remarkably, in a similar low temperature experiment with 2
and benzaldehyde we detected only the intermediate species
8 which is consistent with a structure of the type 7 (13C NMR of
8: 179.5 (C2), 154.2 (C1), 109.1 (C3), 91.8 ppm (C4)).
Thermodynamic calculations reveal intermediate 8 as the
sterically less stressed isomer which is preferred by -5.2
kcal/mol (cf. p S42, Figure S48).22 This [3+1] coupling mode is
rather unusual for a 1,3-enyne as these compounds are
typically prepared by dimerisation of terminal alkynes41, or by
Pd catalysed Sonogashira coupling reactions.42 Similar carbene
transfer reagents include Tebbe’s and Petasis’ Cp2Ti=CH2
reagent43, however, in the herein described case transfer of a
C3 unit is possible. Moreover, the present reaction pattern is
reminiscent of earlier work on retro-cycloaddition using group
4 metallocene oxido or imido complexes.44 It should however
be noted that transformation of the herein described reaction
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank our technical and analytical staff for assistance and
Prof. Axel Schulz (University of Rostock) for helpful discussions.
General support by LIKAT is gratefully acknowledged. We
thank the University of Rostock for access to the cluster
computer and especially Malte Willert for technical support.
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DOI: 10.1039/C9SC01002E
TOC entry
The synthesis and characterisation of a 1-titanacyclobuta-2,3-diene complex, an organometallic
analog of elusive 1,2-cyclobutadiene, is presented.