Ambient-temperature carbon-oxygen bond cleavage of an α-aryloxy ketone with Cp2Ti(BTMSA) and selective protonolysis of the resulting Ti-OR bonds

Article abstract of DOI:10.1021/om300950c

Reaction of Cp₂Ti[η²-(CSiMe₃) ₂] with an α-aryloxy ketone produces a Ti(IV) enolate aryloxide complex. Selective protonolysis of the enolate ligand or both Ti-OR bonds can be achieved with various acids. The reaction of the enolate aryloxide with 1-phenyl-2-phenoxyethanol is catalyzed by a mixture of NEt₃ and [HNEt₃]X (X = OTf, BPh₄).

Full text of DOI:10.1021/om300950c

Note
pubs.acs.org/Organometallics
Ambient-Temperature Carbon−Oxygen Bond Cleavage of an
α‑Aryloxy Ketone with Cp2Ti(BTMSA) and Selective Protonolysis of
the Resulting Ti−OR Bonds
Addison N. Desnoyer,† Beata Fartel,† K. Cory MacLeod,† Brian O. Patrick,‡ and Kevin M. Smith*,†
†Department of Chemistry, University of British Columbia Okanagan, 3333 University Way, Kelowna, British Columbia, Canada V1V
1V7
‡Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
S
* Supporting Information
ABSTRACT: Reaction of Cp2Ti[η2-(CSiMe3)2] with an α-
aryloxy ketone produces a Ti(IV) enolate aryloxide complex.
Selective protonolysis of the enolate ligand or both Ti−OR
bonds can be achieved with various acids. The reaction of the
enolate aryloxide with 1-phenyl-2-phenoxyethanol is catalyzed by
a mixture of NEt3 and [HNEt3]X (X = OTf, BPh4).
he development of selective new catalysts for the cleavage
earth-abundant transition metal with low inherent toxicity.7
The synthesis and reactivity of Cp2Ti complexes have been
extensively investigated for the Ti(IV), Ti(III), and Ti(II)
oxidation states.8 These oxophilic early-transition-metal organo-
metallic compounds can be remarkably water tolerant,7,9 and
subtle modifications of the ancillary Cp ligands can have
T
of carbon−oxygen bonds is a critical challenge for
applications from biomass conversion1 to organic synthesis.2
Recently reported reactions based on ruthenium,3 nickel,2,4 and
vanadium5 typically require high temperatures for effective
catalysis.2−5 For example, Scheme 1a illustrates the mechanism
dramatic reactivity consequences.10 Importantly, Gansauer has
̈
demonstrated the use of lutidinium chloride to break TiIV−OR
bonds under catalytic conditions,11 potentially allowing for
release of acetophenone and phenol from the Cp2Ti fragment.
Scheme 1
RESULTS AND DISCUSSION
■
Cp2Ti(BTMSA) (5; BTMSA = bis(trimethylsilyl)acetylene) is
a well-defined organometallic complex used extensively as a
source of Cp2TiII.12 Complex 5 reacted readily with α-aryloxy
ketone 2 at ambient temperature to cleanly form the desired
Ti(IV) enolate phenoxide 4 (eq 1).13 As shown in eq 2, 4 was
independently synthesized from Cp2Ti(OPh)Cl (6) via the in
situ generated triflate 7. Luinstra has previously demonstrated
the utility of Cp2Ti(X)(OTf) precursors for the synthesis of
TiIV−OR complexes via salt metathesis reactions.14 Following
Luinstra’s general procedure,14 pure 7 was readily isolated from
the comproportionation reaction of Cp2Ti(OTf)2 and Cp2Ti-
proposed for the Ru-catalyzed conversion of 1-phenyl-2-
phenoxyethanol (1), a model for the β-O-4 linkage in lignin,1
to acetophenone (3) and phenol at 135 °C.3 Notably, the C−O
bond-cleavage step is preceded by the Ru-catalyzed dehydro-
genation of 1 to α-aryloxy ketone 2.
We hypothesized that, with an appropriate source of “Cp2Ti”,
α-aryloxy ketone 2 might react via intermediate A to give the
Ti(IV) enolate phenoxide 4 (Scheme 1b).6 Titanium is
particularly attractive for these applications as an inexpensive,
Received: October 10, 2012
Published: October 31, 2012
© 2012 American Chemical Society
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Organometallics
Note
or various [HNR3]X reagents also cleanly generate acetophe-
none and the corresponding Cp2Ti(OPh)X complexes.15 The
selectivity of the [HNR3]X salts is consistent with the
previously noted sensitivity of Ti−enolate complexes to
protonolysis.17,21 In contrast, the Cp2Ti(OPh)X products are
expected to be more robust, since 6 and related Ti(IV)
aryloxides can be prepared from Cp2TiCl2, NEt3, and
HOAr.18,22 However, the analogous reactions employing
HOCH(Me)Ph and other secondary alcohols can lead to
displacement of a Cp ligand and formation of CpTi(OCHR2)-
Cl2 compounds.23
(OPh)2. The 1H NMR spectrum of isolated enolate phenoxide
4 prepared according to eq 2 was identical with that synthesized
from Cp2Ti(BTMSA) and 2.15
The molecular structure of 4 was confirmed by single-crystal
X-ray diffraction (Figure 1a). In comparison to mono(alkoxide)
As shown in eq 3, the Cp2TiIV alkoxide 8 derived from
secondary alcohol 1 was obtained through the use of DBU. In
contrast, the use of NEt3 resulted in 8 contaminated with
Cp2TiCl2, even after recrystallization.24 The molecular structure
of 8 determined by X-ray diffraction is shown in Figure 1b. In
comparison to 4, the Ti−OR bonds in mono(alkoxide) 8 are
significantly shorter for both independent molecules in the unit
cell, at 1.8440(9) and 1.8438(9) Å.16 The ether oxygen does
not significantly interact with the Ti center in 8, with Ti1···O2
and Ti2···O4 distances both above 4.0 Å.17
Although 4 does not react directly with 1 over 2 days to
produce acetophenone, the reaction does proceed at ambient
temperature in the presence of [HNEt3]X salts (X = OTf,
BPh4),25 as shown in Scheme 2. High conversions of 4 and 1 to
Figure 1. Molecular structures (50% probability ellipsoids) of 4 (left)
and 8 (right). For 8, only one of the two independent molecules in the
unit cell is shown.
Cp2TiIV complexes,16 both of the Ti−O bonds in 4 are quite
long, at 1.9269(16) and 1.9325(17) Å for Ti1−O1 and Ti1−
O2, respectively. A similar trend has previously been observed
for Cp2Ti(OR)2 structures, where OR = enolate,17 aryloxide.18
The C7−C8 distance of 1.337(3) Å indicates significant C−C
double-bond character for the enolate ligand.17
Scheme 2
Prompted by the demonstrated stability of the Cp2TiΙV
framework to triflic acid,14 the protonolysis reactions of 4
with HOTf, HOTs, and HCl were investigated (Figure 2).
Figure 2. Reactivity of enolate phenoxide 4 with various protonolysis
reagents, HX.
9 and 3 can be achieved with a mixture of 0.5 equiv of NEt3 and
0.1 equiv of HOTf. As described in the Supporting
Information,15 the conversion of 4 to 9 is sensitive to both
the nature of the added base and the identity of the anion of the
conjugate acid. The phenoxide alkoxide 9 can be independently
synthesized from 7, NEt3, and 1.
Triflic acid reacts with 4 to cleanly generate Cp2Ti(OTf)2 and
3. Although the presence of water can lead to unexpected
outcomes in Cp2Ti chemistry,19 the reactions of 4 with p-
toluenesulfonic acid monohydrate or aqueous HCl also
provided Cp2TiX2 and acetophenone. While 4 did not react
appreciably with Me3SiCl under strictly anhydrous conditions,
intentional addition of 2 equiv of H2O to the reaction mixture
led to Cp2TiCl2 and 3. Similar reactivity was recently reported
for CrIII−OR complexes20d and may be significant in other
multicomponent catalytic reactions that employ Me3SiCl to
cleave metal−OR bonds.20
The use of milder acids permits the selective protonolysis of
the Ti−enolate bond of 4 while retaining the phenoxide group
(Figure 2). Reaction of 4 with lutidinium chloride produced
Cp2Ti(OPh)Cl (6) and 3.11 Similar reactions of 4 with other
lutidinium salts, commercial collidinium tosylate or [HNEt3]Cl,
CONCLUSION
■
The anion sensitivity of the formation of 9 is not unexpected,
given the critical role played by weakly coordinating anions in
polymerization catalysts based on cationic group 4 metal-
locenes.26 More surprising is that all the reactions presented
here proceed readily at ambient temperature, including the
crucial C−O bond cleavage step (eq 1), which is complete
within 20 min. The sequential reactions of Cp2TiCl2 with Mg
and BTMSA,12,27 alkyne complex 5 with 2 (eq 1), and enolate
phenoxide 4 with aqueous HCl (Figure 2) constitute a
stoichiometric cycle for the conversion of the α-aryloxy ketone
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Organometallics
Note
(dd, J = 9.6, 6.8 Hz, 1H), 3.95 (dd, J = 9.6, 4.8 Hz, 1H). 13C NMR
(100 MHz, CD2Cl2): δ 159.37, 142.74, 130.07, 128.75, 128.07, 127.01,
121.30, 117.38, 117.32, 115.04, 90.51, 73.74. Anal. Calcd for
C24H23O2ClTi: C, 67.55; H, 5.43. Found: C, 67.25; H, 5.45.
to acetophenone and phenol.28 Similarly, deprotonation of 8
and loss of chloride should also generate enolate phenoxide 4
through intermediate A, although our initial attempts to achieve
this transformation have been unsuccessful. Finally, elimination
of PhOH from 9 would also be expected to regenerate 4
(Scheme 2). Developing a catalytic system capable of operating
at ambient temperatures will require an ancillary ligand
framework that can address the reactivity challenges identified
in this study.
Synthesis of Cp2Ti[OCH(Ph)CH2OPh](OPh) (9). To a solution of
AgOTf (41.8 mg, 0.163 mmol) in 3 mL of toluene was added a
solution of 6 (50.0 mg, 0.163 mmol) dissolved in 3 mL of toluene,
resulting in an immediate color change to dark red and formation of a
pale precipitate. The mixture was wrapped in foil and stirred for 1 h,
after which the mixture was filtered through Celite, removing the white
precipitate and giving a dark red solution of 7. A solution of 1 (34.9
mg, 0.163 mmol) in 3 mL of toluene and NEt3 (45 μL, 0.323 mmol)
were added to the filtrate, and the mixture was stirred for 72 h,
resulting in an orange solution. The solvent was removed in vacuo to
give an orange residue. The residue was extracted with Et2O and
filtered through Celite. Recrystallization from 1/2 Et2O/hexanes at
−30 °C yielded a dark orange powder of 9 (38.0 mg, 48%). NMR
analysis indicated that the product obtained by recrystallization was
contaminated with a small amount (∼7%) of the intermediate
phenoxide triflate 7. 1H NMR (400 MHz, CD2Cl2): δ 7.48 (dt, J = 0.8,
8.0 Hz, 2H), 7.41 (t, J = 7.6 Hz, 2H), 7.26−7.34 (m, 4H), 7.07 (dd, J =
7.2, 8.4 Hz, 2H), 6.93−6.97 (m, 4H), 6.67 (dd, 7.6, 8.0 Hz, 1H), 6.26
(s, 5H), 6.23 (s, 5H), 5.70 (dd, J = 4.4, 7.2 Hz, 1H), 4.09 (dd, J = 7.2,
9.6 Hz, 1H), 4.01 (dd, J = 4.4, 9.6 Hz, 1H). 13C NMR (100 MHz,
CD2Cl2): δ 170.81, 159.63, 144.12, 130.04, 129.401, 128.77, 127.94,
127.27, 121.17, 118.55, 118.37, 115.94, 115.77, 115.05, 88.60, 68.65.
EXPERIMENTAL SECTION
■
Synthesis of Cp2Ti[OC(CH2)Ph](OPh) (4). Method A: From
Cp2Ti(OPh)Cl (6). To an orange solution of Cp2Ti(OPh)Cl (50.3 mg,
0.164 mmol) in 6 mL of Et2O was added AgOTf (42.3 mg, 0.165
mmol) in 3 mL of Et2O, resulting in a rapid color change to pink and
the formation of a pale precipitate. The mixture was protected from
light by wrapping with foil and was stirred for 45 min. The mixture was
then filtered through Celite, stranding a pink solid and giving a dark
red filtrate. The solvent was removed in vacuo to give a dark red
residue, which was then taken up in 3 mL of THF. A solution of
KOC(CH2)Ph (28.6 mg, 0.181 mmol) in 3 mL of THF was then
added, resulting in an instant color change from red to light orange.
The solution was stirred for 14 h before the solvent was removed in
vacuo. The residue was extracted with Et2O, filtered through Celite,
and then again taken to dryness in vacuo. Recrystallization of this
crude product from 2 mL of 1/1 Et2O/hexanes at −30 °C yielded red
X-ray-quality crystals of 4 (34.6 mg, 54%). 1H NMR (400 MHz,
CD2Cl2): δ 7.60 (d, J = 7.2 Hz, 2H), 7.35 (t, J = 7.6 Hz, 2H), 7.30 (d, J
= 7.2 Hz, 1H) 7.21 (dd, J = 8.4, 7.6 Hz, 2H), 6.78 (tt, J = 7.2, 1.2 Hz,
1H), 6.60 (dd, J = 8.4, 1.2 Hz, 2H), 6.35 (s, 10H), 4.72 (s, 1H), 4.09
(s, 1H). 13C NMR (100 MHz, CD2Cl2): δ 171.04, 140.03, 129.59,
128.61, 128.18, 125.77, 119.19, 118.45, 116.57, 116.36, 86.72. Anal.
Calcd for C24H22O2Ti: C, 73.86; H, 5.68. Found: C, 73.81; H, 5.66.
Method B: From Cp2Ti(BTMSA) (5). A sample of Cp2Ti(BTMSA)
in C6D6 was found to contain 0.0232 mmol of Cp2Ti(BTMSA) via 1H
NMR with 1,3,5-tBu3C6H3 as an internal standard. To this yellow-
brown solution was added 2 (6.0 mg, 0.0283 mmol), resulting in an
immediate color change to red-orange. Subsequent NMR analysis
showed 80% conversion of Cp2Ti(BTMSA) to 4 after 20 min at
ambient temperature.
1
General Procedure for H NMR Monitoring of Protonolysis
Reactions of Cp2Ti[OC(CH2)Ph](OPh) (4). In an inert-atmos-
phere glovebox, 4 (8.0 mg, 0.020 mmol) and the internal standard
1,3,5-tBu3C6H3 (2.0 mg, 0.0081 mmol) were dissolved in CD2Cl2. An
appropriate amount of an acid (Figure 2) and/or 1-phenyl-2-
phenoxyethanol (1) was dissolved in CD2Cl2. The solutions were
combined in a J. Young or Wilmad screw-cap NMR tube and allowed
to react before recording a 1H NMR spectrum. The yields of
acetophenone (3) and the Cp2Ti product(s) were determined at
various reaction times.
X-ray Diffraction Studies: Crystal Data. 4: C24H22O2Ti,
monoclinic, space group P21/c, a = 13.3250(19) Å, b = 9.4540(13)
Å, c = 14.454(2) Å, β = 92.884(4)°, V = 1830.0(4) Å3, Z = 4, μ = 4.84
cm−1, F000 = 816, 11 999 reflections collected, 4030 unique reflections,
Rint = 0.0431, GOF = 1.022, R1 (I > 2σ(I)) = 0.0465, wR2 (all data) =
0.1209.
Synthesis of Cp2Ti(OPh)(OTf) (7). Cp2Ti(OTf)2 (130.7 mg,
0.275 mmol) and Cp2Ti(OPh)2 (99.9 mg, 0.274 mmol) were
dissolved in 10 mL of toluene. The mixture was stirred for 24 h,
and the dark cherry red solution was then filtered through Celite. The
solvent was removed in vacuo, and the residue was dissolved in a
minimum amount of toluene. Storage at −30 °C for 2 days resulted in
8: C H O ClTi, triclinic, space group P1, a = 9.1320(11) Å, b =
̅
24 23
2
13.5283(16) Å, c = 16.404(2) Å, α = 88.597(3)°, β = 83.995(3)°, γ =
85.320(3)°, V = 2008.5(5) Å3, Z = 4, μ = 5.76 cm−1, F000 = 888, 42 998
reflections collected, 11 812 unique reflections, Rint = 0.0236, GOF =
1.05, R1 (I > 2σ(I)) = 0.0303, wR2 (all data) = 0.0794.
1
very dark red crystals of 7 (89.9 mg, 39%). H NMR (400 MHz,
ASSOCIATED CONTENT
CD2Cl2): δ 7.28 (dd, J = 7.6, 8.8 Hz, 2H), 6.92 (tt, J = 7.6, 1.36 Hz,
1H), 6.61 (dd, J = 8.8, 1.24 Hz, 2H), 6.54 (s, 10H). 13C NMR (100
MHz, CD2Cl2): δ 171.91, 129.87, 121.65, 119.40, 116.80. Anal. Calcd
for C17H15F3O4STi: C, 48.59; H, 3.60. Found: C, 48.94; H, 3.70.
Synthesis of Cp2Ti[OCH(Ph)CH2OPh]Cl (8). To a suspension of
Cp2TiCl2 (162.9 mg, 0.654 mmol) in 50 mL of toluene were added 1
(141.8 mg, 0.662 mmol) and DBU (48 μL, 0.321 mmol), and the
mixture was stirred at room temperature for 10 min, during which time
a pale precipitate formed. More DBU (50 μL, 0.334 mmol) was then
added, and the orange mixture was stirred for an additional 80 min
before a final aliquot of DBU (10 μL, 0.067 mmol) was added. This
bright yellow mixture was then stirred for 75 min before being filtered
through Celite. The solvent was removed in vacuo, the resulting solid
residue was extracted with 3 mL of 10/1 Et2O/hexanes, and the extract
was filtered through Celite and then cooled to −30 °C for 15 min,
during which time an oily residue formed. The amber supernatant was
decanted and diluted with 1 mL of Et2O. Chilling this solution at −30
°C yielded 8 (132.5 mg, 48%) as orange crystals isolated in two
■
S
* Supporting Information
Text, tables, figures, and CIF files giving crystallographic data
for 4 and 8, complete experimental details, and characterization
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kevin.m.smith@ubc.ca.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1
We are grateful to the Natural Sciences and Engineering
Research Council of Canada (NSERC), the Canadian
Foundation for Innovation, and the University of British
Columbia for financial support. K.M.S. thanks Anita Lam and
different fractions. X-ray-quality crystals were grown from Et2O. H
NMR (400 MHz, CD2Cl2): δ 7.40−7.32 (m, 4H), 7.30 (dd, J = 8.8,
7.6 Hz, 3H), 6.96 (dt, J = 7.6, 1.2 Hz, 1H), 6.92 (dt, J = 7.6, 1.2 Hz,
2H), 6.33 (s, 5H), 6.28 (s, 5H), 5.76 (dd, J = 6.8, 4.8 Hz, 1H), 4.03
7627
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