Titanium(II) Porphyrin Complexes: Versatile One- And Two-Electron Reducing Agents. Reduction of Organic Chlorides, Epoxides, and Sulfoxides

Article abstract of DOI:10.1021/jo971893p

Treatment of the well-defined complexes (TTP)Ti(η²-EtC≡CEt) or transs-(TTP)Ti(THF)₂ with vicinal dichloroalkanes or dichloroalkenes results in the production of alkenes or alkynes and 2 equiv of (TTP)TiCl. This net two-electron redox

Full text of DOI:10.1021/jo971893p

356
J . Org. Chem. 1998, 63, 356-360
Tita n iu m (II) P or p h yr in Com p lexes: Ver sa tile On e- a n d
Tw o-Electr on Red u cin g Agen ts. Red u ction of Or ga n ic Ch lor id es,
Ep oxid es, a n d Su lfoxid es
Xiaotai Wang† and L. Keith Woo*,1
Department of Chemistry, Iowa State University, Ames, Iowa 50011-3111
Received October 14, 1997
Treatment of the well-defined complexes (TTP)Ti(η2-EtCtCEt) or trans-(TTP)Ti(THF)2 with vicinal
dichloroalkanes or dichloroalkenes results in the production of alkenes or alkynes and 2 equiv of
(TTP)TiCl. This net two-electron redox reaction arises from two formal one-electron reduction
processes mediated by chlorine atom transfer. Oxygen atom transfer occurs when the Ti(II)
porphyrins are treated with several different sulfoxides or epoxides, resulting in two-electron redox
products, (TTP)TidO, the sulfide or alkene, and EtCtCEt or THF. The electronic properties of
the substituents on the sulfoxides or epoxides correlate with the yield and rate of the deoxygenation
reactions.
were subsequently degassed on a vacuum line (10-5 Torr) with
three successive freeze-pump-thaw cycles. Benzyl sulfoxide,
In tr od u ction
Numerous studies have demonstrated the versatility
of metalloporphyrin complexes as reagents for a wide
range of inner-sphere redox processes, including halogen
and oxygen atom-transfer reactions.2 We have been
investigating the reactivity of Ti(II) porphyrin complexes
since first reporting their synthesis and characterization
in 1991.3 Our findings indicate that Ti(II) tetratolylpor-
phyrinato (TTP) complexes function as good oxygen,
sulfur, or selenium atom acceptors in inner-sphere
electron-transfer reactions.,4,5 These atom-transfer reac-
tions involving Ti(II) porphyrins were centered around
reactions between two metal complexessintermetal atom-
transfer processes.
Low-valent titanium species have great utility in
organic synthesis.6 As part of our continuing program
on early transition metal porphyrin chemistry, we re-
cently examined the chlorine and oxygen atom-transfer
reactions between some well-defined Ti(II) porphyrin
complexes and nonmetal substrates such as organic
halides, epoxides, and sulfoxides, in the hope that this
effort would lead to useful methods for organic synthesis.
In this report, we wish to present a detailed account of
these reactions.
4-tolyl sulfoxide, 4-tolyl sulfone, and Ph3CH were purchased
from Aldrich and used without further purification. 1,2-
Dichloroethane, trans-1,2-dichlorocyclohexane, trans-1,2-dichlo-
roethylene, methyl sulfoxide, styrene oxide, and cyclohexene
oxide were purchased from Aldrich and vacuum distilled for
glovebox use. The Ti(II) tetratolylporphyrinato (TTP) com-
plexes (TTP)Ti(η2-EtCtCEt) and trans-(TTP)Ti(THF)2 were
prepared as described elsewhere.7
All manipulations were performed either in a Vacuum
Atmosphere glovebox equipped with a Model HE-553-2 Dri-
Train gas purifier or on a vacuum line using standard Schlenk
techniques. UV-visible data were obtained using a Hewlett-
Packard HP 8452A diode-array spectrophotometer. 1H NMR
spectra were recorded on a Varian 300-MHz spectrometer,
with Ph3CH used as an internal standard for quantitative
analyses. GC-MS analyses were performed on a Finnigan
TSQ 700 mass spectrometer coupled to a Varian GC 3400
chromatograph using a 30-m DB5 column.
Rea ction of (TTP )Ti(η2-EtCtCEt) w ith 1,2-Dich lor o-
eth a n e. To an NMR tube containing a C6D6 solution (0.65
mL) of (TTP)Ti(η2-EtCtCEt) (1.9 mg, 2.38 × 10-3 mmol) and
Ph3CH (1.55 mg, 6.34 × 10-3 mmol) was added ca. 0.5 µL (6.34
× 10-3 mmol) of 1,2-dichloroethane. The tube was sealed
under nitrogen. After standing at ambient temperature for 1
h, the sample was monitored by 1H NMR and the spectrum
showed no reaction. Subsequently, it was heated at 80 °C for
6 h and monitored by 1H NMR again. The spectrum indicated
the formation of (TTP)TiCl (2.1 × 10-3 mmol, 88%), free
EtCtCEt (2.1 × 10-3 mmol, 88%), and ethylene (3.5 × 10-4
mmol in solution), with no (TTP)Ti(η2-EtCtCEt) left. The
product ethylene was further confirmed by its molecular peak
(m/e ) 28) by GC-MS obtained from a headspace sample. The
Ti(III) complex, (TTP)TiCl, was also identified by UV-vis in
toluene (428 nm, Soret; 552 nm).
Exp er im en ta l Section
Gen er a l Con sid er a tion s. Toluene, benzene-d6, tetrahy-
drofuran, and hexanes for glovebox use were distilled from
purple solutions of sodium benzophenone ketyl. Dry solvents
† Present address: Department of Chemistry, University of Colorado-
Denver, Campus Box 194, P.O. Box 17 3364, Denver, CO 80204.
(1) Camille and Henry Dreyfus Teacher-Scholar.
(2) Woo, L. K. Chem. Rev. 1993, 93, 1125 and references therein.
(3) Woo, L. K.; Hays, J . A.; J acobson, R. A.; Day, C. L. Organome-
tallics 1991, 10, 2102.
(4) (a) Woo, L. K.; Hays, J . A. Inorg. Chem. 1993, 32, 2228. (b) Woo,
L. K.; Hays, J . A.; Young, V. G., J r.; Day, C. L.; Caron, C.; D’Souza F.;
Kadish, K. M. Inorg. Chem. 1993, 32, 4186.
(5) Hays, J . A.; Day, C. L.; Young, V. G., J r.; Woo, L. K. Inorg. Chem.
1996, 35, 7601.
R ea ct ion of (TTP )Ti(η2-E t CtCE t ) w it h tr a n s-1,2-
Dich lor ocycloh exa n e. This experiment was conducted in
the same manner as above, using 1.1 mg (1.4 × 10-3 mmol) of
(TTP)Ti(η2-EtCtCEt), 3.7 mg (1.5 × 10-2 mmol) of Ph3CH, and
0.4 µL (3.0 × 10-3 mmol) of trans-1,2-dichlorocyclohexane.
After being heated at 80 °C for 6 h, the sample was monitored
1
by H NMR, and the spectrum showed three products: cyclo-
hexene (6.5 × 10-4 mmol, 93%), (TTP)TiCl (1.3 × 10-3 mmol,
93%), and free EtCtCEt (1.3 × 10-3 mmol, 93%). The product
(6) (a) Lenoir, D. Synthesis 1989, 883. (b) Dang, Y.; Geise, H. J . J .
Organomet. Chem. 1991, 405, 1. (c) Pons, J . M.; Santelli, M. Tetrahe-
dron 1988, 44, 4295. (d) Furstner, A.; Bogdanovic, B. Angew. Chem.,Int.
Ed. Engl. 1996, 35, 2442.
(7) Wang, X.; Gray, S. D.; Chen, J .; Woo, L. K. Inorg. Chem., in press.
S0022-3263(97)01893-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/07/1998

Titanium(II) Porphyrin Complexes
J . Org. Chem., Vol. 63, No. 2, 1998 357
cyclohexene was further confirmed by its molecular peak (m/e
) 80) using GC-MS.
Rea ction of tr a n s-(TTP )Ti(THF )2 w ith Styr en e Oxid e.
To an NMR tube containing a C6D6 solution (0.65 mL) of (TTP)-
Ti(THF)2 (2.1 mg, 2.4 × 10-3 mmol) was added ca. 0.5 µL (4.4
× 10-3 mmol) of styrene oxide. The tube was sealed under
nitrogen. After standing at ambient temperature for 20 min,
the sample was monitored by 1H NMR and the spectrum
showed three products: (TTP)TidO, styrene, and free THF,
with no trans-(TTP)Ti(THF)2 left. The ratio of (TTP)TidO to
styrene was 1:1, as was expected.
Rea ction of (TTP )Ti(THF )2 w ith Cycloh exen e Oxid e.
To an NMR tube containing a C6D6 solution (0.65 mL) of (TTP)-
Ti(THF)2 (1.9 mg, 2.2 × 10-3 mmol) and was added ca. 0.8 µL
(7.9 × 10-3 mmol) of cyclohexene oxide. The tube was sealed
under nitrogen. After standing at ambient temperature for
20 min, the sample was monitored by 1H NMR and the
spectrum showed three products: (TTP)TidO, cyclohexene,
and free THF, with no trans-(TTP)Ti(THF)2 left. The ratio of
(TTP)TidO to styrene was 1:1, as was expected.
R ea ct ion of (TTP )Ti(η2-E t CtCE t ) w it h tr a n s-1,2-
Dich lor oeth ylen e. This experiment was conducted in the
same manner as above, using 1.0 mg (1.2 × 10-3 mmol) of
(TTP)Ti(η2-EtCtCEt), 2.5 mg (1.0 × 10-2 mmol) of Ph3CH, and
0.3 µL (3.9 × 10-3 mmol) of trans-1,2-dichloroethylene. After
being heated at 80 °C for 2 h, the sample was monitored by
1H NMR and the spectrum showed three products: acetylene
(2.2 × 10-4 mmol in solution), (TTP)TiCl (1.0 × 10-3 mmol,
83%), and free EtCtCEt (1.1 × 10-3 mmol, 92%). The product
acetylene was further confirmed by its molecular peak (m/e )
26) by GC-MS obtained from a headspace sample.
R ea ct ion of tr a n s-(TTP )Ti(TH F )2 w it h 1,2-Dich lor o-
eth a n e. To an NMR tube containing a C6D6 suspension of
trans-(TTP)Ti(THF)2 (5.6 mg, 6.5 × 10-3 mmol) was added ca.
0.9 µL (1.1 × 10-2 mmol) of 1,2-dichloroethane. The tube was
sealed under nitrogen. After being heated at 80 °C for 3 h,
the sample was monitored by 1H NMR and showed three
products: ethylene, (TTP)TiCl, and free THF, with no trans-
(TTP)Ti(THF)2 left.
Reaction of tr a n s-(TTP )Ti(THF)2 with tr a n s-1,2-Dich lo-
r ocycloh exa n e. This experiment was conducted in the same
manner as above, using 2.3 mg (2.7 × 10-3 mmol) of (TTP)-
Ti(THF)2 and 0.5 µL (3.8 × 10-3 mmol) of trans-1,2-dichloro-
cyclohexane. After being heated at 80 °C for 2 h, the sample
was monitored by 1H NMR and the spectrum showed three
products: cyclohexene, (TTP)TiCl, and free THF, with no
trans-(TTP)Ti(THF)2 left.
Reaction of tr a n s-(TTP )Ti(THF)2 with tr a n s-1,2-Dich lo-
r oeth ylen e. This experiment was conducted in the same
manner as above, using 2.8 mg (3.3 × 10-3 mmol) of (TTP)-
Ti(THF)2 and 0.5 µL (6.5 × 10-3 mmol) of trans-1,2-dichloro-
ethylene. After being heated at 80 °C for 2 h, the sample was
monitored by 1H NMR and the spectrum showed three
products: HCtCH, (TTP)TiCl and free THF, with no trans-
(TTP)Ti(THF)2 left.
Rea ction of (TTP )Ti(η2-EtCtCEt) w ith Styr en e Oxid e.
To an NMR tube containing a C6D6 solution (0.65 mL) of (TTP)-
Ti(η2-EtCtCEt) (1.0 mg, 1.25 × 10-3 mmol) and Ph3CH (3.0
mg, 1.2 × 10-2 mmol) was added ca. 0.4 µL (3.5 × 10-3 mmol)
of styrene oxide. The tube was sealed under nitrogen. After
standing at ambient temperature for 30 min, the sample was
monitored by 1H NMR and the spectrum showed three
products: (TTP)TidO (3.8 × 10-4 mmol), styrene (9.8 × 10-4
mmol), and free EtCtCEt (1.2 × 10-3 mmol, 96%), with no
(TTP)Ti(η2-EtCtCEt) left. In addition, a broad peak appeared
at 2.5 ppm. As the reaction proceeded at ambient tempera-
ture, the quantities of styrene and (TTP)TidO kept growing
to reach ca. 1.2 × 10-3 mmol (96%) after 9 h, whereas the peak
at 2.5 ppm decreased to <5% intensity. The product styrene
oxide was further confirmed by its molecular peak (m/e ) 104)
by GC-MS. Within experimental error, the reaction was
quantitative.
Rea ction of (TTP )Ti(η2-EtCtCEt) w ith Meth yl Su lfox-
id e. To an NMR tube containing a C6D6 solution (0.65 mL) of
(TTP)Ti(η2-EtCtCEt) (2.4 mg, 3.0 × 10-3 mmol) and Ph3CH
(3.3 mg, 1.35 × 10-2 mmol) was added ca. 0.6 µL (8.5 × 10-3
mmol) of methyl sulfoxide. The tube was sealed under
nitrogen. After standing at ambient temperature for 2 h, the
1
sample was monitored by H NMR and the spectrum showed
three products: (TTP)TidO (4.7 × 10-4 mmol), methyl sulfide
(1.7 × 10-3 mmol), and free EtCtCEt (3.0 × 10-3 mmol), with
no (TTP)Ti(EtCtCEt) left. In addition, a broad peak appeared
at 2.5 ppm. As the reaction proceeded at ambient tempera-
ture, the quantities of methyl sulfide and (TTP)TidO kept
growing over a period of 49 h to reach ca. 1.9 × 10-3 mmol
(79%), whereas the peak at 2.5 ppm decreased to <5%. The
product methyl sulfide was further confirmed by its molecular
peak (m/e ) 62) by GC-MS.
Rea ction of (TTP )Ti(η2-EtCtCEt) w ith Ben zyl Su lfox-
id e. The two reagents TTPTi(η2-EtCtCEt) (0.7 mg, 9.0 × 10-4
mmol) and benzyl sulfoxide (0.5 mg, 2.2 × 10-3 mmol) and the
internal standard Ph3CH (2.7 mg, 1.1 × 10-2) were transferred
into a 10-mL beaker. About 1 mL of C6D6 was added to the
beaker. The mixture was agitated for 5 min and then
transferred to a NMR tube and sealed under nitrogen. After
standing at ambient temperature for 30 min, the sample was
monitored by 1H NMR and the spectrum showed three
products: (TTP)TidO (8.3 × 10-5 mmol), benzyl sulfide (5.6
× 10-4 mmol), and free EtCtCEt (9.0 × 10-4 mmol, 100%),
with no (TTP)Ti(η2-EtCtCEt) left. In addition, a broad peak
appeared at 2.5 ppm. As the reaction proceeded at ambient
temperature, the quantities of benzyl sulfide and (TTP)TidO
kept growing over a period of 57 h to reach ca. 9.0 × 10-4 mmol
(100%), whereas the peak at 2.5 ppm disappeared. Within
experimental error, the reaction was quantitative.
Rea ction of (TTP )Ti(η2-EtCtCEt) w ith 4-Tolyl Su lfox-
id e. This experiment was conducted in the same manner as
above, using 0.9 mg (1.1 × 10-3 mmol) of (TTP)Ti(η2-EtCtCEt),
2.2 mg (9.0 × 10-3 mmol) of Ph3CH, and 0.5 mg (2.1 × 10-3
mmol) of 4-tolyl sulfoxide. After standing at ambient temper-
Rea ction of (TTP )Ti(η2-EtCtCEt) w ith Cycloh exen e
Oxid e. To an NMR tube containing a C6D6 solution (0.65 mL)
of (TTP)Ti(η2-EtCtCEt) (3.9 mg, 4.9 × 10-3 mmol) and Ph3-
CH (2.6 mg, 1.1 × 10-2 mmol) was added ca. 1.5 µL (1.5 ×
10-2 mmol) of cyclohexene oxide. The tube was sealed under
nitrogen. After standing at ambient temperature for 1 h, the
1
ature for 25 min, the sample was monitored by H NMR and
the spectrum showed three products: (TTP)TidO (3.1 × 10-4
mmol), 4-tolyl sulfide (7.9 × 10-4 mmol), and free EtCtCEt
(1.1 × 10-3 mmol), with no (TTP)Ti(EtCtCEt) left. In addi-
tion, a broad peak appeared at 2.5 ppm. As the reaction
proceeded at ambient temperature, the quantities of 4-tolyl
sulfide and (TTP)TidO kept growing over a period of 22 h to
reach 1.1 × 10-3 mmol (100%), whereas the peak at 2.5 ppm
disappeared. The product 4-tolyl sulfide was further confirmed
by its molecular peak (m/e ) 214) by GC-MS. Within
experimental error, the reaction was quantitative.
1
sample was monitored by H NMR and the spectrum showed
two products: cyclohexene (2.3 × 10-3 mmol, 47%) and free
EtCtCEt (4.9 × 10-3 mmol, 100%), with no (TTP)Ti(EtCtCEt)
left. In addition, a broad peak appeared at 2.5 ppm. No
1
appreciable change in the H NMR spectrum was observed as
the sample stood at ambient temperature over a period of 7
h. Thus, it was heated at 80 °C and monitored by 1H NMR
every 24 h. The spectra showed the appearance of (TTP)TidO
and that both it and cyclohexene were growing continuously
over a period of 6 days to reach 3.0 × 10-3 (77%) and 2.9 ×
10-3 mmol (60%), respectively. In the meantime, the peak at
2.5 ppm decreased by 88%. The product cyclohexene was
further confirmed by its molecular peak (m/e ) 82) by GC-
MS.
Rea ction of tr a n s-(TTP )Ti(THF )2 w ith Meth yl Su lfox-
id e. To an NMR tube containing a C6D6 solution (0.65 mL) of
trans-(TTP)Ti(THF)2 (1.1 mg, 1.2 × 10-3 mmol) and was added
ca. 0.4 µL (5.6 × 10-3 mmol) of methyl sulfoxide. The tube
was sealed under nitrogen. After standing at ambient tem-
perature for 20 min, the sample was monitored by 1H NMR
and the spectrum showed three products: (TTP)TidO, methyl
sulfide, and free THF, with no trans-(TTP)Ti(THF)2 left. In

358 J . Org. Chem., Vol. 63, No. 2, 1998
Wang and Woo
addition, a broad peak appeared at 2.5 ppm. As the reaction
proceeded to 36 h, this broad peak disappeared. The final ratio
of (TTP)TidO to methyl sulfide was 10:8. The slightly lower
than expected observed amount of methyl sulfide is presum-
ably due to its volatility.
Rea ction of tr a n s-(TTP )Ti(THF )2 w ith Ben zyl Su lfox-
id e. The two reagents, trans-(TTP)Ti(THF)2 (2.0 mg, 2.3 ×
10-3 mmol) and benzyl sulfoxide (1.0 mg, 4.3 × 10-3 mmol),
were transferred into a 10-mL beaker. About 1 mL of C6D6
was added to the beaker. The mixture was agitated for 5 min,
and then transferred to an NMR tube and sealed under
nitrogen. After standing at ambient temperature for 20 min,
the sample was monitored by 1H NMR and the spectrum
showed three products: (TTP)TidO, benzyl sulfide, and free
EtCtCEt, with no trans-(TTP)Ti(THF)2 left. The final ratio
of (TTP)TidO to benzyl sulfide was 1:1 as expected.
determined by comparing the product integrals with that
of the internal reference. Yields were relatively high,
ranging from 88% to 94%.
Another well-defined Ti(II) porphyrin complex, trans-
(TTP)Ti(THF)2, demonstrated analogous reactivity with
organic vicinal dichlorides.
Rea ction s of Ti(II) P or p h yr in s w ith Ep oxid es a n d
Su lfoxid es. When (TTP)Ti(η2-EtCtCEt) was treated
with styrene oxide in C6D6 solution at ambient temper-
1
ature, facile reactions were recorded by H NMR after
20 min. The release of free EtCtCEt was quantitative,
some (TTP)TidO9 and styrene were produced, and a new
species with a broad 1H NMR peak at 2.5 ppm was
observed. Quantitative yields for styrene and (TTP)TidO
were achieved after 9 h, while the signal for the inter-
mediate species disappeared. Similarly, the reaction of
(TTP)Ti(η2-EtCtCEt) with cyclohexene oxide at ambient
temperature quantitatively released free EtCtCEt, pro-
duced cyclohexene in 47% yield, and gave a new species
Resu lts
R ea ct ion s of Ti(II) P or p h yr in s w it h Vicin a l
Dich lor oa lk a n es a n d Dich lor oa lk en es. In the pro-
cess of characterizing (TTP)Ti(η2-EtCtCEt) and trans-
(TTP)Ti(THF)2, we noticed that these Ti(II) porphyrins
react with chlorinated NMR solvents such as CDCl3 and
CD2Cl2, yielding the known Ti(III) complex (TTP)TiCl.
To unravel the reactivity of Ti(II) porphyrins with organic
halides, the diamagnetic (TTP)Ti(η2-EtCtCEt) and a few
vicinal dichloroalkanes and dichloroalkenes were selected
as reactants, and exploratory reactions were conducted
in C6D6 using Ph3CH as internal reference to quantify
product formation.
1
indicated by a 2.5 ppm peak in the H NMR spectrum.
However, no (TTP)TidO was afforded until the reaction
was heated to 80 °C. At this elevated temperature, both
(TTP)TidO and cyclohexene slowly grew in quantity over
the course of 6 days to reach a maximum yield of 60%,
1
while the H NMR peak at 2.5 ppm associated with the
intermediate species decreased by 88%. These oxygen-
transfer reactions are illustrated in eqs 4 and 5.
When (TTP)Ti(η2-EtCtCEt) was treated with 1,2-
dichloroethane in slight excess (ca. 2.7-fold), no ap-
1
preciable reaction was detected by H NMR at ambient
temperature up to 1 h. At 80 °C for 6 h, the reaction
1
was complete as monitored by H NMR. The spectrum
clearly indicated the formation of ethylene (δ ) 5.25 ppm
in C6D6), (TTP)TiCl,8 and free EtCtCEt (eq 1). More-
Treatment of (TTP)Ti(η2-EtCtCEt) with methyl, ben-
zyl, and 4-tolyl sulfoxides at ambient temperature also
resulted in oxygen-transfer reactions, affording (TTP)-
TidO and the corresponding sulfides (eqs 6-8). The
reaction with methyl sulfoxide afforded dimethyl sulfide
in a yield of 64% over a period of 49 h, and those with
benzyl and 4-tolyl sulfoxides were quantitative and
complete after 57 and 22 h, respectively. Furthermore,
1
an intermediate with an H NMR signal at 2.5 ppm was
observed during the course of all three reactions.
The Ti(II) porphyrin complex trans-(TTP)Ti(THF)2 also
undergoes oxygen atom-transfer reactions with epoxides
or sulfoxides, affording (TTP)TidO, THF, alkenes, or
sulfides. These reactions are qualitatively faster than
those involving (TTP)Ti(η2-EtCtCEt). In general, an
over, the identities of (TTP)TiCl and ethylene were
further confirmed by UV-vis and GC-MS analyses,
respectively. When treated with (TTP)Ti(η2-EtCtCEt)
at 80 °C, trans-1,2-dichlorocyclohexane afforded analo-
gous products: cyclohexene, (TTP)TiCl, and free EtCtCEt
(eq 2). This newly discovered reactivity was also ex-
tended to 1,2-trans-dichloroethylene, a vicinal dichloro-
alkene, whose reaction with (TTP)Ti(η2-EtCtCEt) at 80
°C gave acetylene, (TTP)TiCl, and free EtCtCEt (eq 3).
The NMR yields of the aforementioned reactions were
1
intermediate with an H NMR signal at 2.5 ppm was also
observed with reactions involving trans-(TTP)Ti(THF)2.
Discu ssion
Dech lor in a tion of vic-Dich lor id es th r ou gh Ti(II)
P or p h yr in s. Dechlorination of vic-dichlorides to alkenes
can be useful in organic synthesis. Although a variety
(8) Berrreau, L. M.; Hays, J . A.; Young, V. G., J r.; Woo, L. K. Inorg.
Chem. 1994, 33, 105.
(9) Fournari, C.; Guilard, R.; Fontesse, M.; Latour, J .-M.; Marchon,
J .-C. J . Organomet. Chem. 1976, 110, 205.

Titanium(II) Porphyrin Complexes
J . Org. Chem., Vol. 63, No. 2, 1998 359
Sch em e 1
The initial step in the reactions of vic-dichloroalkanes
with (TTP)Ti(η2-EtCtCEt) appears to be the reduction
of the dichlorides, as represented by eq 9. This step is
presumably inner- sphere in nature. Two possibilities
•
can explain the fate of the radical CH2CH2Cl. They are
illustrated in Scheme 1.
Of the two options, pathway ii is more likely because
it is thermodynamically favored with formation of a
strong Ti-Cl bond. Moreover, there is a precedent in
which a similar pathway was demonstrated for the
reaction between ClCH2CH2Cl and a Ni(I) complex that
yielded ethylene and a Ni(II) species.18 For the reaction
of (TTP)Ti(η2-EtCtCEt) with trans-1,2-dichloroethylene
that produces (TTP)TiCl and acetylene, we propose a
mechanism analogous to that for 1,2-dichloroethane.
This mechanism appears to be in contrast to the E2
pathway (halonium ion abstraction) proposed by Save´ant,
Scha¨fer, and co-workers for the electrochemical reduction
of vicinal dibromides mediated by iron and cobalt por-
phyrin complexes.19 However, in Save´ant’s system,
electrochemically generated Fe(I)- and Co(I)- complexes
serve as two-electron reductants that form inert Fe(III)
and Co(III) halide complexes on halonium ion transfer.
While a two-electron process with our Ti(II) complex is
possible (vida infra), this would produce a diamagnetic
Ti(IV) product that we do not observe.
Deoxygen a t ion of E p oxid es a n d Su lfoxid es
th r ou gh Ti(II) P or p h yr in s. Both deoxygenation of
epoxide to alkene20 and sulfoxide to sulfide21 are useful
organic transformations. For the deoxygenating reagents
derived from low-valent transition metal compounds, the
exact nature of the active reductant is poorly understood
and the reaction is usually conducted under heteroge-
neous conditions, giving intractable metal oxo species.
Recently, Chan and co-workers reported the deoxygen-
of methods have been developed to afford such transfor-
mations,10 examples involving the use of well-defined
transition metal complexes are rare. For instance, a
system comprising TiCl3- or TiCl4/LiAlH4 in tetrahydro-
furan was utilized to convert trans-1,2-dichlorocyclohex-
ane to cyclohexene in 72% yield, but the identity of the
active Ti reagent was unclear.11 Chromium(II) reagents
are capable of reducing 1,2-dihalides to olefins, but 1,2-
dichlorides are reportedly inert or very slow toward this
process.12 In addition, Zn reagents will also dechlorinate
1,2-dihalides, albeit in modest yields.13
When treated with vic-dichloroalkanes in homogeneous
solution at elevated temperature (80 °C), the well-defined
Ti(II) porphyrin complexes such as (TTP)Ti(η2-EtCtCEt)
generated alkenes and (TTP)TiCl in good yield. These
are chlorine-transfer reactions in which every product can
be conclusively identified. It is interesting to note that
these reactions are single-electron oxidations rather than
two-electron oxidations that would give six-coordinate
Ti(IV) porphyrin alkyl chlorides. Two-electron processes
are characteristic of the oxidation chemistry of some
cyclopentadinenyl-based Ti(II) complexes,14 as manifested
by the reaction of Cp2Ti(PMe3)2 with MeI, which yields
Cp2Ti(Me)(I).,15,16 However, six-coordinate Ti(IV) por-
phyrin complexes (ionic radius 0.605 Å) have trans
geometries except in cases with bidentate ligands.17 The
rigid macrocyclic porphyrin ligand may prevent Ti(II)
porphyrins from undergoing a cis-constrained oxidative
addition reaction to give six-coordinate Ti(IV) complexes.
(16) Treatment of (TTP)Ti(EtC≡CEt) with MeI in C6D6 at 80 °C
resulted in what appears to be two paramagnetic species with broad
1H NMR peaks at 2.36 and 2.48 ppm, respectively. No Ti(IV) com-
pounds were observed. In comparison with the 1H NMR spectrum of
(TTP)TiCl, we tentatively assign these two products as Ti(III) com-
pounds (TTP)TiI and (TTP)TiMe. Further studies of this reaction will
be undertaken.
(10) Larock, R. C. Comprehensive Organic Transformations; VCH
Publishers Inc.: New York, 1989; p 133.
(11) Olah, G. A.; Prakash, G. K. S. Synthesis 1976, 607.
(12) Kray, W. C., J r.; Castro, C. E. J . Am. Chem. Soc. 1964, 86, 4603.
(13) (a) Butenandt, A.; Schmidt-Thome´, J . Ber. Otsch. Ges. 1936,
69, 882. (b) Bohlmann, F.; Kieslich, K. Chem. Ber. 1954, 87, 1363. (c)
McCutcheon, J . W. Organic Syntheses; Wiley: New York, 1955; Collect.
Vol. III, p 526. (d) Smith, C. W.; Norton, D. G. Organic Syntheses;
Wiley: New York, 1963; Collect. Vol. IV, p 348. (e) Allred, E. L.; Beck,
B. R.; Voorhees, K. J . J . Org. Chem. 1974, 39, 1426.
(14) Cloke, F. G. N. Titanium Complexes in Oxidation States +2
and +3”; In Comprehensive Organometallic Chemistry II; Lappert, M.
F., Ed.; Elsevier Science Inc.: New York, 1995; Vol. 4, p 221.
(15) Kool, L. B.; Rausch, M. D.; Alt, H. G.; Herberhold, M.; Honold,
B.; Thewald, U. J . Organomet. Chem. 1987, 320, 37.
(17) Brand, H.; Arnold, J . Coord. Chem. Rev. 1995, 140, 137.
(18) Ram, M. S.; Bakac, A.; Espenson, J . H. Inorg. Chem. 1988, 27,
4231.
(19) (a) Lexa, D.; Save´ant, J .-M.; Scha¨fer, H. J .; Su, K.-B.; Vering,
B.; Wang, D. L. J . Am. Chem. Soc. 1990, 112, 6162. (b) Lexa, D.;
Save´ant, J .-M.; Su, K.-B.; Wang, D. L. J . Am. Chem. Soc. 1987, 109,
6464.
(20) Murai, S.; Murai, T.; Kato, S. Reduction of Epoxides. In
Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon Press: Inc.; New York, 1991; Vol. 8, p 871.
(21) (a) Madesclaire, M. Tetrahedron 1988, 44, 6537. (b) Kukushkin,
V. Y. Russ. Chem. Rev. 1990, 59, 844.

360 J . Org. Chem., Vol. 63, No. 2, 1998
Wang and Woo
Sch em e 2
ation of sulfoxides by the well-defined Cp2Ti(CO)2 to give
sulfides. However, the titanium oxo products were
uncharacterized, with partial loss of Cp.22 Other ex-
amples of oxygen atom abstraction from sulfoxides are
reported in a review by Kukushkin.23 Bergman has
shown that photolysis of Cp2TaCH3(η2-CH2dCH2) gener-
ates a transient species that behaves as Cp2TaCH3 and
that efficiently abstracts heteroatoms from epoxides,
thiiranes, and aziridines.24 In this system, the metal
product is Cp2Ta()X)(CH3), where X ) O, S, NR. Mayer
has also shown that Mo(II) and W(II) complexes are also
capable of abstracting oxygen or sulfur from epoxides and
thiiranes.25 To our knowledge, no reactions of epoxides
with well-defined Ti(II) complexes have been described
in the literature.
Because of the extreme oxophilicity of the Ti(II) center,
upon exposure to oxygen, TTPTi(II)-based complexes
rapidly convert to the fully characterized (TTP)TidO.
Thus, these Ti(II) porphyrins may serve as efficient
deoxygenating agents. The reaction of (TTP)Ti(η2-
EtCtCEt) with styrene oxide was quantitative after 9 h
at ambient temperature, yielding styrene and (TTP)-
TidO. In contrast, the reaction between (TTP)Ti(η2-
EtCtCEt) and cyclohexene oxide required higher tem-
perature and was incomplete even after 6 days. We
attribute the significant differences in reaction rates and
yields to the electronic effect induced by the R-carbon
substituents of the epoxide substrate. With an electron-
withdrawing phenyl group,26 styrene oxide is more
susceptible to reduction than is cyclohexene oxide, in
which the R-carbon substituents are methylenes. This
observation, based on electronic effects, was corroborated
by the reactions of (TTP)Ti(η2-EtCtCEt) with various
sulfoxides (R2SdO), where R ) Me, benzyl, and 4-totyl,
which generate the corresponding sulfides and (TTP)-
TidO. As the electronic withdrawing effect of R increases
in the order of Me, benzyl, and 4-tolyl, the deoxygenation
reaction of R2SdO with (TTP)Ti(η2-EtCtCEt) becomes
faster and gives higher yields.
slow decay correlated with the growth of (TTP)TidO and
additional alkene or sulfide. On the basis of these
experimental observations, we suggest that this inter-
mediate is the known Ti(III) µ-oxo complex [(TTP)Ti]2O.
This complex has been synthesized independently from
a
reaction between (TTP)TidO and (TTP)Ti(η2-
PhCtCPh).5 Moreover, we propose a mechanism for
these oxygen-transfer reactions, as represented by Scheme
2. The first two steps are facile, but the third step
involves oxidizing a Ti(III) complex and is slow. The
reactivity of [(TTP)Ti]2O with oxygen atom donors is
currently under investigation.
Attempts to reduce sulfones to sulfoxides by Ti(II)
porphyrins at ambient or elevated temperature were
unsuccessful, resulting in either no reaction or decom-
position of the Ti(II) starting materials.
Con clu d in g Rem a r k s
A series of new reactions of Ti(II) porphyrin complexes
has been demonstrated. Of particular interest is the
observation that when treated with organic electrophiles,
(TTP)Ti(II)-based complexes may undergo both one- and
two-electron processes depending on the substrate. Ti(II)
reductions with vicinal dichloroalkanes or dichloroal-
kenes are tandem single-electron oxidation processes
mediated by chlorine atom transfer, affording (TTP)TiCl
and alkenes or alkynes. We believe that this novel
reactivity for Ti(II) complexes is enforced by the porphy-
rin ligand geometry. In addition, reactions of (TTP)Ti(II)
complexes with epoxides or sulfoxides are two-electron
processes mediated by oxygen atom transfer. These
deoxygenation reactions lead to (TTP)TidO and alkenes
or sulfides. The electronic effects of the substituents
account for the difference in the yield and rate of these
reactions. The extent of these reactions is consistent with
the nature of both the reactants and products. The scope
and mechanistic details of these reactions are being
studied further.
In all reactions of (TTP)Ti(η2-EtCtCEt) with the
epoxides or sulfoxides, consumption of (TTP)Ti(η2-
EtCtCEt) was fast and complete in less than 1 h,
accompanied by formation of (TTP)TidO, the alkene or
sulfide, and a paramagnetic intermediate. This inter-
mediate shows the same resonance in the 1H NMR
spectra (δ ) 2.5 ppm) in all reactions. Hence, it is
unlikely that this intermediate is an adduct of the (TTP)-
Ti(II) fragment with various epoxides or sulfoxides. In
addition, after initial formation of this intermediate, its
(22) Chen, T. L.; Shaver, A.; Chan, T. H. J . Organomet. Chem. 1989,
367, C5.
Ack n ow led gm en t. We thank the Petroleum Re-
search Fund, administered by the American Chemical
Society, and the Iowa Soybean Promotion Board for
partial support of this work.
(23) Kukushkin, V. Y. Coord. Chem. Rev. 1995, 139, 375.
(24) (a) Proulx, G.; Bergman, R. G. J . Am. Chem. Soc. 1994, 116,
7953. (b) Proulx, G.; Bergman, R. G. Organometallics 1996, 15, 133.
(25) Hall, K. A.; Mayer, J . M. J . Am. Chem. Soc. 1992, 114, 10402.
(26) March, J . Advanced Organic Chemistry, 4th ed.; J ohn Wiley &
Sons, Inc.: New York, 1992; p 15.
J O971893P