Intermolecular C-H bond activation promoted by a titanium alkylidyne

Article abstract of DOI:10.1021/ja0556934

The transient titanium alkylidyne complex (PNP)Ti_≡C^tBu (PNP = N-[2-P(CHMe₂)₂-4-methylphenyl]₂^-), prepared from α-hydrogen abstraction of the corresponding alkylidene-alkyl species (PNP)Ti=CH^tBu(CH₂^tBu), can readily undergo intermolecular 1,2-addition of C-H bonds of benzene and SiMe₄. Synthesis and reactivity, isotopic labeling, kinetics, and theoretical studies strongly favor an alkylidyne pathway and the α-H abstraction step to be the rate-determining step. Copyright

Full text of DOI:10.1021/ja0556934

Published on Web 10/28/2005
Intermolecular C-H Bond Activation Promoted by a Titanium Alkylidyne
Brad C. Bailey, Hongjun Fan, Erich W. Baum, John C. Huffman, Mu-Hyun Baik,* and
Daniel J. Mindiola*
Department of Chemistry, Molecular Structure Center and School of Informatics,
Indiana UniVersity, Bloomington, Indiana 47405
Received August 26, 2005; E-mail: mbaik@indiana.edu; mindiola@indiana.edu
The intermolecular activation of inert C-H bonds by organo-
transition metal complexes remains one of the most intensely studied
areas in chemistry since these systems may allow for catalytically
converting hydrocarbons from cheap sources to valuable, function-
alized organic precursors.1 Several strategies can lead to C-H bond
activation reactions and include discrete oxidative addition, σ-bond
metathesis, radical bond homolysis, electrophilic reactions, and 1,2-
addition reactions across metal-ligand multiple bonded function-
alities.2 For the latter class of reactions, only a handful of M-L
multiple bonded functionalities have been observed to perform
intermolecular activation of inert C-H bonds.3-7 We now report
that transient titanium alkylidynes, which are generated by an
abstraction reaction, can activate sp2 and sp3 C-H bonds under
mild conditions. Experimental and theoretical studies were con-
ducted in order to understand the mechanism and to deduce an
energy profile for the C-H bond activation reaction.
∆Gq of 27.8 kcal/mol (1-TS), whereas C-H activation of the
solvent is not predicted to be rate-limiting with the transition state,
A-TS, found at 21.0 kcal/mol. Thus, the C-H activation barrier
relative to the reactive alkylidyne intermediate is only 16.4 kcal/
mol, a value which is remarkably low and justifies the titanium
alkylidyne complex as a reactive intermediate. Both 1-TS and A-TS
feature structural elements that are intuitively expected for σ-bond
metathesis transition states invoking the structural motif of a
transient metallacycle.9 We have also explored a number of
plausible mechanistic alternatives theoretically and identified them
all to not be viable.9
The proposal of the titanium alkylidyne intermediate and the
reaction profile shown have been explored in a series of experi-
mental studies. First, the reaction was carried out in C6D6 to confirm
that 1 is quantitatively transformed to (PNP)TidCDtBu(C6D5)
2
2
(2-d6). Both H and H{1H} NMR spectra of 2-d6 reveal >99+%
Recently, our group reported the synthesis of a titanium alkyli-
dene complex (PNP)TidCHtBu(OTf) supported by a pincer ligand
(PNP ) N-[2-P(CHMe2)2-4-methylphenyl]2-).8 The triflate group
2H incorporation into the R-carbon without any 2H scrambling into
the aryl or Pr motifs of the PNP ligand. Most importantly, when
complex 1 is thermolyzed in C6D6, we observe CH3 Bu as the sole
byproduct (via 13C and H NMR spectroscopy), supporting our
i
t
t
1
can be readily substituted with stoichiometric LiCH2 Bu to afford
the titanium alkylidene-alkyl (PNP)TidCHtBu(CH2tBu) (1) (Scheme
1).9,10 Compound 1 displays a 13C NMR resonance at 260 ppm
with a low JC-H coupling of 86 Hz, consistent with an agostic R-H
interaction taking place with the electron-deficient metal center.7,11
Compound 1 is a kinetic product inasmuch as it reacts with
benzene over several hours at room temperature. As a result,
compound 1 must be prepared in cyclohexane in order to avoid
rapid C-H activation of benzene at 27 °C over 11.9 h (4.5 half-
lives) to afford (PNP)TidCHtBu(C6H5) (2) in quantitative
yield, and such a system can be independently prepared from
(PNP)TidCHtBu(OTf) and ClMgPh (Scheme 1).9
To establish the connectivity and degree of R-H agostic interac-
tion in these alkylidenes, we collected single-crystal X-ray diffrac-
tion data for compounds 1 and 2 (Scheme 1).9 The structure for
each reveals short TidC bonds (1.790(5) Å for 1; 1.873(5) Å for
2)12 and a metal center residing in a pseudo trigonal bipyramidal
geometry with “trans-like” phosphines (∼149°). These systems are
impervious to intramolecular ancillary ligand degradation pathways
when compared to other titanium alkylidene alkyl derivatives
prepared by our group.13
proposal that R-abstraction in 1 precedes the intermolecular C-H
activation of benzene. Interestingly, the C-H and C-D activation
steps are reversible since 2-d6 undergoes clean conversion to 2 in
C6H6 (80 °C, 48 h, Scheme 1) and vice versa, thus linking
intermediate A to 2.9 Our proposed reaction energy profile in
Scheme 1 suggests that the reverse step of the C-H activation,
that is, 2fA, is associated with a barrier of 32.8 kcal/mol, which
can be overcome at a slow, but reasonable rate under elevated
temperature conditions.
Next, the kinetic parameters for the conversion of 1 to 2 were
assessed experimentally by monitoring the decay of 1 using 31P
NMR spectroscopy in C6D6 to reveal a kaverage ) 6.5(4) × 10-5 s-1
at 27 °C. The reaction was found to be pseudo-first-order in
titanium, which is in good agreement with the reaction energy
profile proposed based on DFT calculations. Under a depleted
amount of benzene (e.g., C6D12), formation of 3 is not clean given
the promiscuity of A to react with other solvent media. Temperature
dependence studies of the 1f2 conversion (15-40 °C) allowed
for extraction of the activation parameters9 ∆Hq ) 24.1(4) kcal/
mol and ∆Sq ) -2(1) cal/mol‚K-1, yielding a ∆Gq value of ∼24.7
kcal/mol at 298.15 K, which is in good agreement with the DFT
prediction of 27.8 kcal/mol. There is insignificant change in the
kH/kD (1.04 at 27 °C) when examining the C-H activation reaction
in C6H6 versus C6D6, but substantial kH/kD discrepancy (3.7 at 27
To better understand the mechanism for conversion of 1 to 2,
by which the C-H bond of benzene is activated, we simulated the
reaction using high-level density functional theory (DFT) at the
B3LYP/cc-pVTZ(-f) level of theory14 as implemented in the Jaguar
program (Scheme 1).15 Complex 1 first undergoes R-H abstraction
t
t
°C) associated with the -CD2 Bu versus -CH2 Bu abstraction,
thereby providing credence to a rate-determining step involving
R-abstraction to generate a titanium alkylidyne linkage.
t
concomitant with elimination of CH3 Bu to afford the titanium
alkylidyne intermediate A, which embarks on an intermolecular
1,2-addition of the benzene C-H bond across the reactive TitC
linkage to provide product 2. As shown in Scheme 1, the R-H
abstraction step is rate-determining with a computed solution phase
When compound (PNP)TidCHtBu(OTf) is treated with
LiCH2SiMe3 in C6D6 at 40 °C for 24 h, complex (PNP)TidCDSiMe3-
t
(C6D5) (3-d6) is formed quantitatively along with CH3 Bu. As
9
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J. AM. CHEM. SOC. 2005, 127, 16016-16017
10.1021/ja0556934 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Reactivity Studies of Titanium Alkylidene Complexes along with the Reaction Coordinate for the 1f2 Conversion in Benzene (ꢀ
) 2.284) (relative energies in kcal/mol are given in parentheses)
Supporting Information Available: Experimental preparation
(compounds 1-9), kinetic (conversion of 1 to 2), crystallographic data
(compound 1, 2, (PNP)Ti(CH2Si(CH3)3)2, and 6), computational details,
all calculated structures, and additional discussions. This material is
available free of charge via the Internet at http://pubs.acs.org.
observed with isotopomer 2-d6, complex 3-d6 also experiences
exchange with excess C6H6 to afford proteo 3 (Scheme 1). By
analogy to the 1f2 conversion, we propose that the intermediate
(PNP)TidCHtBu(CH2SiMe3) (4) undergoes R-H migration to
t
generate (PNP)TidCHSiMe3(CH2 Bu) (5), which then participates
in C-H/C-D activation of solvent via the putative alkylidyne
intermediate (PNP)TitCSiMe3 (B) (Scheme 1).9 If the reaction is
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therein.
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In summary, we combined extensive experimental and theoretical
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titanium alkylidene complex 1. We found strong evidence for the
existence of a thus far elusive terminal titanium alkylidyne
intermediate, which has been shown to promote a number of diverse
C-H activation reactions. Some of the transformations highlighted
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steps in thermodynamic equilibrium scenarios that should allow
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Acknowledgment. This paper is dedicated to Prof. R. R.
Schrock on the occasion of his winning the 2005 Nobel Prize in
Chemistry. We thank Indiana UniversitysBloomington, the Dreyfus
Foundation, the Alfred P. Sloan Foundation, the NSF (CHE-
0348941, PECASE Award, 0116050), and NIH (HG003894) for
financial support. B.C.B. acknowledges the Department of Educa-
tion for a GAANN Fellowship.
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