A fluorobenzene adduct of Ti(IV), and catalytic carboamination to prepare α,β-unsaturated imines and triaryl-substituted quinolines

Article abstract of DOI:10.1021/ja0566026

A rare fluorobenzene adduct of group 4, [(nacnac)Ti=NAr(FC₆H₅)][B(C₆F₅)₄] (nacnac- = [ArNC(^tBu)]₂CH, Ar = 2,6-ⁱPr₂C₆H₃), has been isol

Full text of DOI:10.1021/ja0566026

Published on Web 12/02/2005
A Fluorobenzene Adduct of Ti(IV), and Catalytic Carboamination to Prepare
r,â-Unsaturated Imines and Triaryl-Substituted Quinolines
Falguni Basuli, Halikhedkar Aneetha, John C. Huffman, and Daniel J. Mindiola*
Department of Chemistry and Molecular Structure Center, Indiana UniVersity, Bloomington, Indiana 47405
Received September 30, 2005; E-mail: mindiola@indiana.edu
Group 4 imido1 complexes play a critical role in important
catalytic processes, such as the intermolecular hydroamination of
alkynes1b,2-6 and alkenes,7 hydrohydrazination of alkynes,8 three-
component coupling reactions to form R,â-unsaturated â-ami-
noimines,9 and guanylation of amines,10 among several other
transformations.1e More recently, zirconocene imido systems have
been implicated in carboamination reactions to produce R,â-
unsaturated imines.11 This carboamination process involves the
insertion of aldimines into azametallacyclobutene intermediates
generated by [2 + 2] cycloaddition reactions of an internal alkyne
Figure 1. Molecular structure of [(nacnac)TidNAr(FC6H5)]+ (left) depict-
ing thermal ellipsoids at the 35% probability level. A simplified structure
of the cationic skeleton of 1 is depicted on the right with omitted aryl groups
for N6, N2, and N39. Selected metrical parameters (lengths in angstroms,
angles in degrees): Ti1-N2, 1.973(7); Ti1-N6, 2.043(9); Ti1-N39, 1.707-
(2); Ti1-F, 2.113(7); Ti1-C3, 2.476(2); Ti1-C4, 2.518(3); Ti1-C5, 2.612-
(2); F-C53, 1.417(3); Ti1-N39-C40, 175.7(8); N2-Ti1-N6, 97.23(7);
Ti1-F-C53, 177.5(6).
with the corresponding metal imido.11 The latter reaction is
particularly attractive since a new CdC bond is formed while Cd
N bonds are both cleaved and generated in such a process.
In this work, we report the synthesis and isolation of a rare
example of a group 4 fluorobenzene adduct,12 [(nacnac)TidNAr-
(FC6H5)][B(C6F5)4] (nacnac- ) [ArNC(tBu)]2CH, Ar ) 2,6-i-
Pr2C6H3). This electron-deficient titanium imide can catalyze, under
low catalyst loadings, carboamination reactions involving diphen-
ylacetylene and a series of aryl aldimines to form R,â-unsaturated
imines. Depending on the nature of the aldimine, the catalytic
process can lead to formation of triaryl-substituted quinolines, the
product resulting from a cyclization of the electron-rich R,â-
unsaturated imine.
Recently, our group reported the synthesis of the p-dimethyl-
aminoarene adduct, [(nacnac)TidNAr(η1-C6H5NMe2)][B(C6F5)4].13
FC6H5 solutions of the latter over an extended period of time
gradually transform to a new complex, [(nacnac)TidNAr(FC6H5)]-
[B(C6F5)4] (1), which has been characterized on the basis of
elemental analysis and 1H, 13C, 11B, and 19F NMR spectroscopy.14
Pure samples of 1, however, require several recrystallization steps
in FC6H5 in order to force the equilibrium to formation of such a
product. Fortunately, complex 1 can be prepared independently in
one single step (74% isolated yield) utilizing [Et3Si][B(C6F5)4]15
with (nacnac)TidNAr(Cl) in FC6H5.14
Single-crystal X-ray diffraction studies of 1 at room temperature
reveal coordination of FC6H5 to an electron-deficient [(nacnac)-
TidNAr]+ framework (Figure 1). The Ti-F interaction is strong,
thus elongating the F-C53 bond (1.417(3) Å) from that observed
for free fluorobenzene (∼1.36 Å). Most significantly is the
coordination mode of the â-diketiminate ligand, which displays η5
hapticity as a result of a deviated Ti atom above the NCCCN
imaginary mean plane (∼1.297 Å).16 If one ignores the Ti-Câ and
-Cγ interactions, the FC6H5 ligand occupies the fourth coordination
site in a highly distorted tetrahedral geometry.
Complex 1 is exceedingly reactive and rapidly coordinates traces
of THF or Et2O to form the stable cations [(nacnac)TidNAr(THF)]+
and [(nacnac)TidNAr(Et2O)]+, respectively.13 In the absence of
these poisons, complex 1 catalyzes carboamination reactions of
PhCCPh and aldimines to produce highly arylated R,â-unsaturated
imines with exclusive E,E configuration at the olefin and imine
residues according to Figure 2. Whereas electron-poor aldimines
fail to afford products, electron-rich p-aryl-substituted substrates
react smoothly to afford eneimines in >70% isolated yield using
low catalyst loads (5 mol %) and short time periods (24-36 h,
entries 1-4, Figure 2). Catalyst loadings as low as 2.5 mol % also
work, but reaction times extend to 84 h (Figure 2, entry 2).
Generation of the R,â-unsaturated imine is proposed to occur
initially by FC6H5 displacement with the aldimine to afford the
adduct, [(nacnac)TidNAr(aldimine)][B(C6F5)4] (2),14 which sub-
sequently undergoes imine metathesis with an aldimine (Figure 2,
entries 1-6) to yield a much more reactive and less sterically
encumbered imido cation, [(nacnac)TidNAr′(FC6H5)][B(C6F5)4].
The latter species rapidly [2 + 2] cycloadds the internal alkyne to
provide the azametallacyclobutene [(nacnac)TiNAr′CPhCPh]-
[B(C6F5)4]. As proposed previously by Bergman and co-workers,11
the azametallacyclobutene intermediate undergoes insertion of the
aldimine to yield a thermally unstable six-membered ring metal-
lacycle, which carries a [4 + 2] retrocycloaddition to regenerate
the TidNAr′ linkage and extrude the R,â-unsaturated imine (Figure
2).
Evidence for azametallacyclobutene formation as opposed to a
1,2-insertion mechanism8a is supplied by structural and spectro-
scopic data.2d,g,17 Accordingly, addition of 1 equiv of PhCCPh to 1
quantitatively generates [(nacnac)TiNArCPhCPh][B(C6F5)4] (3) on
1
the basis of H and 13C NMR spectra and single-crystal X-ray
diffraction (Figure 2).14 Imine metathesis taking place in the first
step is strongly supported by stoichiometric reactions involving 1
and the corresponding aldimine to provide a new titanium imide
cation and the hindered aldimine R2CHdNAr. The latter organic
product appears to be kinetically incompetent throughout the
catalytic process since complex 3 does not react with such an
aldimine in the catalytic reactions.14 Unfortunately, attempts to
isolate the less hindered imide have been plagued by its rapid
decomposition.
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C O M M U N I C A T I O N S
carboamination and cyclization process (entry 5), thus suggesting
that acid might not be playing a role in these catalytic reactions.
We are currently exploring the mechanism behind formation of
these quinolines since this type of reaction might involve, under a
catalytic process, selective C-H activation pathways to afford
multi-substituted N-heterocycles.
Acknowledgment. D.J.M. thanks Indiana UniversitysBloom-
ington, the Dreyfus Foundation, the Sloan Foundation, and the NSF
(CHE-0348941, PECASE) for financial support of this research.
The authors also thank Dr. Donald L. Morrison and Boulder
Scientific for a generous gift of [CPh3][B(C6F5)4].
Supporting Information Available: Complete experimental prepa-
ration (compounds 1-3 and organic products), and crystallographic
data (compounds 1-3, and the quinoline from entry 6, Figure 2). This
material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 2. Mechanism for a catalytic carboamination reaction, where the
[Ti] represents the (nacnac)Ti cation scaffold. Bottom tables depict
carboamination reactions to prepare R,â-unsaturated imines and quinolines,
respectively. In the tables, the isolated yields for the organic products are
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aldimines (entries 5 and 6) does not afford the corresponding R,â-
unsaturated imines. Instead, triaryl-substituted quinolines are ob-
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heterocycle product formation.14 Intuitively, quinoline production
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