Titanocene catalyzed opening of oxetanes

Article abstract of DOI:10.1016/j.tet.2008.08.107

The reductive opening of oxetanes by Cp₂TiCl was investigated by a combined synthetic and computational study. The activation and reaction energies predict a more difficult reaction than the related epoxide opening. Synthetically, the γ-titanoxy radicals obtained behave like typical free radicals. Their reactions are not controlled by the metal and its ligands. This is highlighted by the dimerization of phenyl substituted oxetane derived radicals.

Full text of DOI:10.1016/j.tet.2008.08.107

Tetrahedron 64 (2008) 11839–11845
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Titanocene catalyzed opening of oxetanes
,
a
a
a
a
Andreas Gansa¨uer a , Noe¨llie Ndene , Thorsten Lauterbach , Jose Justicia , Iris Winkler , Christian
*
´
Mu¨ck-Lichtenfeld b, Stefan Grimme b
,
*
a
´
¨
¨
Kekule-Institut fur Organische Chemie und Biochemie der Universitat Bonn, Gerhard Domagk Strasse 1, 53121 Bonn, Germany
b Organisch-Chemisches Institut der Westfa¨lischen Wilhelms-Universita¨t, Correnstrasse 4, 49149 Mu¨nster, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The reductive opening of oxetanes by Cp2TiCl was investigated by a combined synthetic and computa-
tional study. The activation and reaction energies predict a more difficult reaction than the related ep-
Received 18 July 2008
Accepted 7 August 2008
Available online 20 September 2008
oxide opening. Synthetically, the
g-titanoxy radicals obtained behave like typical free radicals. Their
reactions are not controlled by the metal and its ligands. This is highlighted by the dimerization of phenyl
substituted oxetane derived radicals.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
opening. Typical radical reactions, such as 5-exo cyclizations,10a
intermolecular additions to acrylates,10b and H-atom abstraction
Strained heterocycles that can be opened through radical
chemistry are highly versatile and often readily available radical
precursors. This is because a large driving force for the ring opening
is provided by the release of strain. It is possible to generate three
important intermediates in this manner: heteroatom-centered
radicals, carbon-centered radicals, and radical anions. While nu-
merous efficient applications of classical radical chain reactions
have emerged, the opening of heterocycles through electron
transfer has attracted attention only more recently.1
In this context, epoxides were first opened by solvated elec-
trons.2 This concept has been further developed by Bartmann,3
Cohen,4 and Yus5 who have employed aromatic radical anions as
the reducing agents in many synthetically useful applications.
However, the initially formed radical is rapidly reduced further to
give carbanionic species. The mechanism of this epoxide opening
through electron transfer has been studied computationally.6
Oxetane opening proceeds much slower under these condi-
tions7,8 but can be accelerated through activation by Lewis acids.
The radicals initially formed are reduced to organolithium species.
These intermediates can be used in the typical manner. The ad-
vantages of radical chemistry remain untapped by this approach.
With respect to the utilization of the intermediate radicals for
organic synthesis the use of low valent metal complexes is more
promising.9 This was first demonstrated by Nugent and Rajan-
Babu,10 who introduced Cp2TiCl as a reagent for reductive epoxide
from 1,4-cyclohexadiene,10c were realized. The mechanistic idea
(Fig. 1) is based on the similarity of the metal bound epoxide with
the cyclopropylcarbinyl radical that swiftly opens the ring.11
After the introduction of reaction conditions catalytic in tita-
nocene12 the field has developed rapidly in the last decade1,13 and
a number of unusual and pertinent applications, such as enantio-
selective radical generation,14 unusual cyclizations,15 tandem
processes,16 and epoxypolyene cyclizations via radicals,17 have
appeared. Other reagents have, as yet, proven to be inferior.18
Moreover, Cp2TiCl has been used as reagent or catalyst in a number
of other important reactions, such as transformations of ketyl
radicals,19 allylations,20 benzylations,20,21 Reformatsky type trans-
formations,22 and reductive couplings of allyl halides.23
The mechanism of epoxide opening has been studied by
a combination of computational, electrochemical, and synthetic
methods.24 Surprisingly, the exothermicity of ring opening that
proceeds via a homolytic substitution is rather low. In the absence
of the release of strain as driving force for C–O bond cleavage, C–O
bonds can indeed be formed by attack of alkyl radicals on Ti–O
bonds as has been demonstrated by the synthesis of tetrahydro-
furan derivatives.25 This unusual homolytic substitution reaction
that is shown in Figure 2 may even be regarded as an organome-
tallic analog of the oxygen-rebound step of the P450 enzymes.26
Here, we report on the use of oxetanes as substrates in titano-
cene catalyzed electron transfer reactions as shown in Figure 3. This
* Corresponding authors. Tel.: þ49 228 732800; fax: þ49 228 734760 (A.G.); tel.:
þ49 251 83 3341; fax: þ49 251 83 36515 (S.G.).
O
M O
M
E-mail addresses: andreas.gansaeuer@uni-bonn.de (A. Gansa¨uer), grimmes@
uni-muenster.de (S. Grimme).
Figure 1. Analogy of epoxide opening to cyclopropylcarbinyl radical opening.
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.08.107

11840
A. Gansa¨uer et al. / Tetrahedron 64 (2008) 11839–11845
TiCp2Cl
ClCp2Ti
O
E
E
E
E
O
O
- Cp2TiCl
Figure 2. THF formation via homolytic substitution.
ClCp2Ti
ClCp2Ti
O
O
Ph
+THF
2a: ΔE = 0.4
+THF
1a: ΔE = -3.4
transformation is interesting for two reasons. First, it is important
to understand how oxetanes behave in metal mediated electron
transfer reactions compared to epoxides and tetrahydrofurans and
ClCp2Ti
O
second, for applications in synthesis, the g-titanoxy radicals gen-
R
erated through ring opening constitute a potentially interesting
class of functionalized radicals. To achieve both goals, we carried
out a combined experimental and theoretical study of oxetane
opening. To the best of our knowledge no such study has appeared
in the literature, as yet.
TiCp2Cl
O
TiCp2Cl
O
R
2. Results
R
1b: R = Me,
1c: R = Me,
2.1. Computational investigations
E
a = 19.0, ΔE = 2.8
E
a = 9.7, ΔE = -3.8
2b: R = Ph,
Ea = 17.1, ΔE = -0.2
2c: R = Ph,
Ea = 3.3, ΔE = -18.7
In order to obtain data that can be readily compared to our study
of the titanocene mediated epoxide opening, the same computa-
tional methods were employed. Thus, the complexation of two
oxetanes by Cp2TiCl, the activation and reaction energies of ring
opening, and the structures of all pertinent intermediates, transi-
tion states, and products were studied by density functional theory
(DFT) calculations with the BP functional and a TZVP basis set.27
This pure density functional is suited for the description of re-
actions of transition metal compounds and has been successfully
used in the literature for chemically similar systems.28 One must be
aware, however, that the absolute barriers of radical reactions
predicted by this method are usually too low.29 Since we are mainly
interested in the comparison of reactivities of various oxetanes, this
systematic error seems acceptable and does not affect the conclu-
sions of our work. We did not investigate the reactions of the half-
open Cp2TiCl dimer30 with oxetanes as the results with this species
did not provide additional qualitative insight into the mechanism of
epoxide opening.
The results of these DFT calculations of the complexation of 2,2-
dimethyl oxetane (1) and 2-methyl-2-phenyl oxetane (2) and the
ring opening of their respective complexes 1a and 2a are summa-
rized in Figure 4.
As expected, replacement of THF from Cp2TiCl is more favorable
for the sterically less demanding 1 than for 2 containing the larger
phenyl substituent.
The activation energies of ring opening to yield the primary
radicals 1b and 2b (19.0 and 17.1 kcal molꢀ1) are substantially
higher than those calculated for generation of primary radicals
from epoxides (about 9 kcal molꢀ1). This constitutes a clear re-
flection of the stronger steric interaction of the substituents of the
tertiary carbon with the cyclopentadienyl ligands in both transition
structures of oxetane opening. The activation energy for the for-
mation of the tertiary radical 1c (9.7 kcal molꢀ1) is much lower than
that for 1b but still noticeably higher for the corresponding epoxide
(7.0 kcal molꢀ1).24
Figure 4. Computational results of the complexation and opening of 1 and 2. All en-
ergies are in kcal molꢀ1
.
1c. The transition states TS2b and TS2c (Fig. 5) are relatively early
and hence the C–O bonds are to a large degree still intact. Of course,
in TS2b the C–O distance is longer (1.98 Å) than in TS2c (1.82 Å).
Thus, only some but not the full stabilization of the benzylic radical
is available for rendering TS2c more favorable. The second contri-
bution to the lowering of TS2c, that is shown in Figure 5, is con-
stituted by the reduction of steric interactions between the phenyl
group and the upper cyclopentadienyl ligand. At present it is not
possible to quantify the relative magnitude of the steric and elec-
tronic effects on TS2c.
In summary, oxetane opening is predicted to proceed slower but
more selectively than the corresponding reactions of the epoxides.
Moreover, under the reaction conditions used here oxetane cleav-
age will occur irreversibly as typical radical transformations are
much faster than ring closure of the g-titanoxy radicals.
Thermodynamically, formation of the primary radicals is clearly
disfavored and thermoneutral (1b, ꢀ0.2 kcal molꢀ1) or even en-
dothermic (2b, þ2.8 kcal molꢀ1). The generation of the tertiary
radical 1c is slightly exothermic (ꢀ3.8 kcal molꢀ1). As expected for
electronic reasons, formation of the benzylic radical 2c is sub-
stantially more advantageous. Again, ring opening of the corre-
sponding epoxides is more favorable. An important structural
Due to the very low activation energies, the generation of the
benzylic radical 2c will be substantially faster than the formation of
R1
O
ClCp2Ti
O
ClCp2Ti
R2
R2 R1
Figure 3. Titanocene mediated oxetane opening investigated in this study.
Figure 5. Transition states TS2b and TS2c.

A. Gansa¨uer et al. / Tetrahedron 64 (2008) 11839–11845
11841
X
X
X
X
X
HO
HO
O
a or b
+
X
a:
b:
11: 90%
11: 46%
12: 0%
12: 47%
Figure 6. Calculated structures of 1c and 2c.
X = H: 9
a:
b:
13: 93%
13: 42%
14: 0%
14: 48%
X = Cl: 10
difference of the
-titanoxy radicals is apparent from Figure 6.
The most favorable conformations of 1c and 2c are fairly
stretched out to avoid steric interactions between the cyclo-
pentadienyl ligands and the radicals. Hence, the -titanoxy radicals
investigated here are not shielded by titanocene as the -titanoxy
g-titanoxy radicals 1c and 2c to epoxide derived
b
Figure 8. Reductive opening of diaryl substituted oxetanes in the presence or absence
of collidine hydrochloride. (a) 20 mol % Cp2TiCl2, Mn, 2.5 equiv Coll$HCl; (b) 20 mol %
Cp2TiCl2, Mn, 7 equiv Coll, 4 equiv Me3SiCl.
g
b
radicals are. Therefore, their reactions will most likely resemble
those of classical free radicals and are predicted not to be controlled
by the metal and its ligands.
C–C bond. It should be noted that this order of events is taking place
under both conditions described for the regeneration of Cp2TiCl
from titanocene alkoxides.12
In order to understand the influence of the oxetane’s sub-
stitution pattern on this peculiar reaction, we submitted the diaryl
substituted oxetanes 9 and 10 to both catalytic conditions for ring
opening (Fig. 8).
With the protic system for Cp2TiCl generation (conditions a) the
saturated alcohols 11 and 13 were obtained in high yield. Thus, the
presence of the second aryl group effectively prevents di-
merization. We suggest reductive trapping of the intermediate
radical by a second equivalent of the titanocene and protonation of
the Ti–C bond as mechanism for product formation.
Yet another reaction takes place in the presence of Coll$SiMe3Cl
(conditions b), that silylates Ti–O bonds. In this case, mixtures of
the saturated alcohols 11 and 13 with the respective allylic alcohols
12 and 14 were obtained that could be separated by column
chromatography. As no source of protons is available under these
conditions, the saturated alcohols must have formed through a H-
atom abstraction. We suggest the mechanism shown in Figure 9 for
the formation of 11 and 13 from 9.
2.2. Synthetic investigations
As the computational results suggest that ring opening with aryl
substituted oxetanes proceeds swiftly, we decided to employ such
substrates first. However, the formation of benzylic radicals in
titanocene mediated or catalyzed reactions may also lead to prob-
lems. This is because the typically employed trapping reagents,
such as 1,4-cyclohexadiene or acrylates, react only slowly with
these stabilized intermediates.10,12 Hence, potentially undesired
pathways, such as radical reduction by a second equivalent of the
titanocene, may interfere.
Our results with the three aryl substituted oxetanes 3–5 are
summarized in Figure 7. In the absence of external trapping agents,
such as 1,4-cyclohexadiene or tert-butyl acrylate a reductive oxe-
tane dimerization to 6–8 was observed that proceeded with low
diastereoselectivity.
Thus, the intermediate g-titanoxy radicals are not reduced by
a second Cp2TiCl but combine fast enough to form a hexasubstituted
After the oxetane opening, that constitutes the slowest step in
the sequence, and reductive trapping of 15 by Cp2TiCl, the allylic
alkoxide 16 is formed through b-hydride elimination. During this
step, Cp2TiClH is formed that should constitute a very powerful and
fast H-atom donor. Reduction of radical 15 by this complex will be
faster than reduction of 15 with Cp2TiCl and generates 17. In sup-
port of this proposal, we note that the closely related Schwartz
reagent Cp2ZrClH31 has already been demonstrated to be a potent
HO
O
a or b
3
6
OH
a: 70%, d.r. = 50:50
b: 76%, d.r. = 50:50
Br
HO
OTiCp2Cl
Cp2TiCl
OTiCp2Cl
TiCp2Cl
O
a or b
Br
β-hydride
elimination
4
7
OH
Br
Ph
Ph
Ph
Ph
OTiCp2Cl
15
a: 67%, d.r. = 50:50
b: 74%, d.r. = 50:50
+ Cp2TiClH
Ph
Ph
O
a or b
16
HO
OH
OTiCp2Cl
+ Cp2TiClH
OTiCp2Cl
5
8
+ Cp2TiCl
Ph
a: 36%, d.r. = 50:50
b: 47%, d.r. = 50:50
Ph
Ph
Ph
17
Figure 7. Reductive dimerization of oxetanes. (a) 20 mol % Cp2TiCl2, Mn, 2.5 equiv
Coll$HCl; (b) 20 mol % Cp2TiCl2, Mn, 7 equiv Coll, 4 equiv Me3SiCl.
Figure 9. Mechanism of the formation of 16 and 17 from 15 in the absence of acid.

11842
A. Gansa¨uer et al. / Tetrahedron 64 (2008) 11839–11845
ring opening with the aid of computational chemistry. Oxetane
opening proceeds slower and less exothermic than the related
epoxide opening. The resulting -titanoxy radicals are not shielded
by the metal and its ligands and therefore behave like typical free
radicals. This was demonstrated by their dimerization reactions,
reductions, and additions to acrylates.
OH
Ph
Ph
g
O
a
20, 50%
21, 15%
Ph
OH
HO
18
4. Experimental section
O
b
All reactions were performed in oven-dried (100 ꢁC) glassware
under Ar. THF was freshly distilled from K. CH2Cl2 was freshly
distilled from CaH2. The products were purified by flash chro-
matography on Merck silica gel 50 (eluents given in brackets, EA
refers to ethyl acetate, CH to cyclohexane) according to the
procedure of Still.34 Yields refer to analytically pure samples.
Isomer ratios were determined by suitable 1H NMR integrals of
cleanly separated signals. NMR: Bruker DRX 300, AMX 300, AM
400; DRX500 1H NMR, CHCl3 (7.26 ppm) or C6D5H (7.16 ppm) in
the indicated solvent as internal standard in the same solvent; 13C
NMR, CDCl3 (77.16 ppm) or C6D6 (128.06 ppm) as internal stan-
dard in the same solvent;35 integrals in accordance with assign-
ments, coupling constants are measured in hertz and always
constitutes J(H,H) coupling constants. IR spectra: Perkin–Elmer
1600 series FT-IR and Thermo Nicolet 380 as neat films on KBr
plates or via ATR measurements. Mass Spectrometry: EI Thermo-
quest Finningan MAT 95 XL, calibration against PFK; ESI Bruker
Daltonics microTOF-Q, calibration against HCO2Na. Combustion
analytics was performed on a vario microcube from Elementar,
Hanau.
22, 71%
19
Figure 10. Reductive opening of the alkyl substituted oxetanes 18 and 19. (a) 20 mol %
Cp2TiCl2, Mn, 2.5 equiv Coll$HCl, 10 equiv 1,4-C6H8; (b) 20 mol % Cp2TiCl2, Mn, 7 equiv
Coll, 4 equiv Me3SiCl.
H-atom donor.32 Silylation and protic work-up yields 11 and 12. An
alternative mechanism based on the mixed disproportionation of
radicals has recently been proposed by Cuerva et al.33 Currently, we
are unable to rule out either of these two pathways.
The final reductive openings of oxetanes were carried out with
18 and 19 under the conditions shown in Figure 10.
In the case of the reaction of 18 that was carried out under
similar conditions as typical reductive epoxide openings a relatively
low yield of 50% 20 was obtained. Moreover, the product of the
b
-hydride elimination pathway 21 was also obtained in 15% yield
together with 20% of recovered starting material. Thus, even in the
presence of 10 equiv of 1,4-cyclohexadiene radical reduction by
Cp2TiCl is competing. With lower amounts of titanium the con-
version is even worse. This is a clear indication of the relatively
difficult oxetane opening as predicted by the computational results.
As reductive trapping of the radicals by titanium is rather efficient,
we also investigated the aprotic conditions for oxetane with the
sterically less hindered substrate 19. Gratifyingly, 22 could be
obtained as the sole product of the reaction in 71% isolated yield.
However, this result also suggests that the formation of C–C bonds
either by intermolecular addition to acrylates or by cyclization re-
actions will be difficult.
To test this notion, we investigated the reaction of 18 with tert-
butyl acrylate under the protic and silylating conditions for Cp2TiCl
regeneration as summarized in Figure 11.
Under both conditions the yield of the desired 23 is only modest
and 21 was also obtained in 10–20% yield. Even with 10 equiv of
radical trap the usually very swift addition to acrylates is not able to
fully suppress the reductive trapping of the intermediate radical.
Oxetanes 3,36 9,36 and 1936 were prepared according to litera-
ture procedures. Alcohol 11 is commercially available.
4.1. General procedures for the preparation of oxetanes
4.1.1. GP1
A
solution of t-BuOK (4.848 g, 40.0 mmol) and trime-
thylsulfoxonium iodide (8.803 g, 40.0 mmol) in dry t-BuOH (30 mL)
was stirred at 50 ꢁC for 1 h. Then, a solution of the ketone
(10.0 mmol) in 5 mL of dry t-BuOH was added, and the new mixture
was heated at 50 ꢁC for 2 days. Then, the solvent was removed,
water was added and the mixture was extracted with t-BuOMe,
dried over MgSO4, and the solvent was removed. The residue was
purified by flash chromatography (mixtures of CH–EA).
4.1.2. GP2
Thus, as opposed to the epoxide derived
b-titanoxy radicals, oxe-
To a stirred suspension of NaH (791 mg, 33.0 mmol) in 100 mL of
dry DMSO, S,S-dimethyl-N-(4-tolylsulfonyl)sulfoximine (8.155 g,
33.0 mmol) was added and the suspension was stirred for 2 h at
47 ꢁC. The ketone (10.0 mmol) was dissolved in 30 mL of dry DMSO
and then was added to the mixture, and the stirring was continued
for 48 h at 55 ꢁC. Then, the reaction was cooled at rt, water was
added, and the mixture was extracted with Et2O (3ꢂ50 mL). The
combined organic layers were dried over MgSO4 and the solvent
was removed. The residue was purified by flash chromatography
(mixtures of CH–EA).
tane derived -titanoxy radicals are not suitable for the efficient
g
formation of C–C bonds. Unfortunately, this was also observed in
a number of attempted 5-exo cyclizations.
OH
O
a or b
Ph
Ph
18
23
tBuO2C
a: 50%
b: 60%
4.1.3. 2-(4-Bromophenyl)-2-methyloxetane (4)
(CH–EA 90:10) GP1: (1.596 g, 7.0 mmol, 70%), colorless oil. 1H
Figure 11. Reductive opening of the alkyl substituted oxetanes 18 and 19. (a) 20 mol %
Cp2TiCl2, Mn, 2.5 equiv Coll$HCl, 10 equiv tert-butyl acrylate; (b) 20 mol % Cp2TiCl2,
Mn, 7 equiv Coll, 4 equiv Me3SiCl, 10 equiv tert-butyl acrylate.
NMR (300 MHz, CDCl3)
d
¼7.48 (d, J¼8.7 Hz, 2H), 7.26 (d, J¼8.7 Hz,
2H), 4.62 (ddd, J¼8.6, 6.4, 6.3 Hz, 1H), 4.50 (ddd, J¼8.6, 6.9, 6.0 Hz,
1H), 2.80 (ddd, J¼10.7, 8.6, 6.9 Hz, 1H), 2.69 (ddd, J¼10.7, 8.7, 6.6 Hz,
3. Conclusion
1H), 1.70 (s, 3H); 13C NMR (75 MHz, CDCl3)
d
¼147.3, 131.3, 125.6,
120.6, 86.2, 64.5, 35.6, 30.5; IR (film)
1260, 1085, 1010, 820; EIHRMS calcd for C10H11OBr 225.9993, found
225.9990.
n
¼2975, 2880, 1485, 1395,
In summary, we have described the first opening of oxetanes by
a low valent metal complex and have studied the mechanism of

A. Gansa¨uer et al. / Tetrahedron 64 (2008) 11839–11845
11843
4.1.4. 3,4-Dihydro-2H-spiro(naphthalene-1,20-oxetane) (5)
CDCl3)
23.2; IR (film)
d
¼145.3,131.6,131.5,129.0,128.9,127.9, 61.4, 48.5, 39.9, 23.6,
(CH–EA 90:10) GP1: (1.010 g, 5.8 mmol, 58%), colorless oil. 1H
n
3420, 3025, 2930, 1725, 1600, 1495, 1455, 1365,
NMR (400 MHz, C6D6)
d
¼8.01 (d, J¼7.8 Hz, 1H), 7.22 (dd, J¼7.6,
1150, 1060, 845, 750, 700; EIHRMS calcd for C20H26O2Na 321.1825,
found 321.1824.
7.6 Hz, 1H), 7.08 (dd, J¼7.5, 7.5 Hz, 1H), 6.88 (d, J¼7.8 Hz, 1H), 4.35–
4.46 (m, 2H), 2.35–2.53 (m, 3H), 2.14 (ddd, J¼10.8, 8.3 Hz, J¼6.2 Hz,
1H), 2.08 (ddd, J¼12.5, 6.3, 3.0 Hz, 1H), 1.97 (ddd, J¼12.3, 12.1,
3.1 Hz, 1H), 1.55–1.65 (m, 1H), 1.35–1.45 (m, 1H); 13C NMR
4.2.4. 3,4-Bis(4-bromophenyl)-3,4-dimethylhexane-1,6-diol (7)
(CH–EA 10:90) GP3: (153 mg, 0.34 mmol, 67%); GP4 (169 mg,
0.37 mmol, 74%), colorless solid; mp 199 ꢁC. 1H NMR (400 MHz,
(100 MHz, C6D6)
37.2, 32.2, 29.5, 19.7; IR (film)
d
¼142.6, 136.3, 128.5, 127.6, 127.2, 126.7, 84.9, 64.4,
n
¼2930, 1485, 1455, 1305, 1225, 980,
CDCl3)
d
¼7.35 (d, J¼6.5 Hz, 2H), 7.33 (d, J¼6.5 Hz, 2H), 6.95 (d,
850, 755. HRMS (EI, 70 eV) calcd for C15H14O [M] 174.1045, found
174.1047.
J¼8.2 Hz, 2H), 6.90 (d, J¼8.2 Hz, 2H), 3.40–3.28 (m, 2H), 3.15–3.06
(m, 2H), 2.38 (ddd, J¼13.8, 9.7, 4.2 Hz, 1H), 2.17 (ddd, J¼13.8, 9.7,
4.2 Hz, 1H), 1.96–1.76 (m, 2H), 1.28 (s, 3H), 1.27 (s, 3H); 13C NMR
4.1.5. 2,2-Bis(4-chlorophenyl)oxetane (10)
(100 MHz, CDCl3)
61.0, 60.9, 48.4, 48.3, 39.9, 39.6, 23.2, 22.8; IR (film)
d
¼144.2, 133.4, 133.2, 131.9, 131.9, 122.0, 122.0,
(CH–EA 90:10) GP1: (1.956 g, 7.0 mmol, 70%), colorless oil. 1H
n
3445, 3025,
NMR (300 MHz, CDCl3)
d
¼7.32 (s, 8H), 4.65 (t, J¼7.7 Hz, 2H), 3.17 (t,
2965, 2875, 1605, 1495, 1450, 1375, 1255, 1065, 960, 855, 750, 700;
EIHRMS calcd for C20H24O2Br2Na 479.0016, found 479.0025; ele-
mental analysis calcd (%) for C20H24O2Br2: C 52.65, H 5.30; found: C
52.67, H 5.51.
J¼7.7 Hz, 2H); 13C NMR (75 MHz, CDCl3)
d
¼144.9, 133.3, 128.7,
126.6, 88.1, 65.5, 36.0; IR (film)
1090, 1010, 975, 810; EIHRMS calcd for C15H12OCl2 278.0265, found
278.0270.
n
¼2965, 2895, 1490, 1400, 1235,
4.2.5. 2,20-(1,10,2,20,3,30,4,40-Octahydro-1,10-binaphthyl-1,10-
diyl)diethanol (8)
4.1.6. 2-Methyl-2-phenethyloxetane (18)
(CH–EA 85:15) GP2: (1.058 g, 6.0 mmol, 60%), colorless oil. 1H
(CH–EA 50:50) GP3: (63 mg, 18 mmol, 36%); GP4: (82 mg,
NMR (400 MHz, CDCl3)
d
¼7.31–7.26 (mdd, J¼7.5, 7.4 Hz, 2H), 7.25–
0.235 mmol, 47%), colorless solid; mp 184 ꢁC. 1H NMR (300 MHz,
7.16 (mdd, J¼7.7, 7.1 Hz, 3H), 4.53 (ddd, J¼12.9, 8.8, 6.2 Hz, 1H), 4.47
(ddd, J¼12.6, 8.8, 6.2 Hz,1H), 2.74 (ddd, J¼12.6, 6.2, 2.5 Hz,1H), 2.72
(ddd, J¼12.6, 6.2, 2.5 Hz,1H), 2.50 (ddd, J¼10.9, 8.8, 6.8 Hz,1H), 2.36
(ddd, J¼10.9, 8.8, 6.8 Hz, 1H), 2.03–1.97 (md, J¼6.7 Hz, 2H), 1.49 (s,
DMSO-d6)
d
¼7.02–7.33 (m, 5H), 4.21 (dd, J¼5.1, 5.1 Hz, 2H), 3.14–
3.27 (m, 4H), 2.73–2.86 (m, 4H), 2.55–2.66 (m, 8H), 2.01–2.33 (m,
2H), 2.63–2.83 (m, 8H), 0.92–1.27 (m, 8H); 13C NMR (75 MHz,
DMSO-d6)
40.8, 34.6, 31.9, 23.8; IR (film)
745, 655; EIHRMS calcd for C24H30O2Na 373.2138, found 373.2130.
d
¼141.8, 139.4, 130.0, 128.9, 125.8, 125.1, 59.0, 48.5, 42.5,
3H); 13C NMR (100 MHz, CDCl3)
d
¼142.3, 128.5, 128.4, 125.8, 86.3,
n
¼3340, 2925, 2880,1485,1445,1035,
64.3, 44.0, 32.4, 30.0, 27.4; IR (film)
n
3445, 3025, 2965, 2875, 1605,
1495, 1450, 1375, 1255, 1065, 960, 855, 750, 700; EIHRMS calcd for
C12H16O 176.1201, found 176.1197.
4.2.6. Diphenylpropan-1-ol (11)
(CH–EA 80:20) GP3: (190 mg, 0.9 mmol, 90%); GP4: (97 mg,
0.46 mmol, 46%).
4.2. General procedures for the opening of oxetanes
4.2.1. GP3
4.2.7. 3,3-Diphenylprop-2-en-1-ol (12)37
Strictly deoxygenated THF (15 mL) was added to a mixture of
Cp2TiCl2 (50 mg, 0.2 mmol), Mn dust (440 mg, 8.0 mmol), and
collidine hydrochloride (396 mg, 2.5 mmol) under Ar atmosphere
and the suspension was stirred at rt until it turned green (about
15 min). Then, a solution of oxetane (1.0 mmol) in THF (5 mL) was
added and the mixture was stirred at 60 ꢁC overnight. Then, a so-
lution of 2 N of HCl was added and the mixture was extracted with
AcOEt. The organic layer was dried over MgSO4 and the solvent
removed. The residue was purified by flash chromatography
(mixtures of CH–EA).
(CH–EA 85:15) GP4 (99 mg, 0.47 mmol, 47%).
4.2.8. 3,3-Bis-(4-chlorophenyl)propan-1-ol (13)
(CH–EA 90:10) GP4: (262 mg, mmol, 93%); GP4: (118 mg, mmol,
42%), colorless oil. 1H NMR (300 MHz, CDCl3)
d
7.25 (d, J¼8.5 Hz,
4H), 7.14 (d, J¼8.5 Hz, 4H), 4.12 (t, J¼7.0 Hz, 1H), 3.58 (t, J¼7.0 Hz,
2H), 2.24 (q, J¼6.6 Hz, 2H); 13C NMR (75 MHz, CDCl3)
d 142.82 (C),
132.61 (C), 129.48 (CH), 129.10 (CH), 60.86 (CH2), 46.20 (CH), 38.25
(CH2); IR (film)
n
¼3445, 3025, 2965, 2875, 1605, 1495, 1450, 1375,
1255, 1065, 960, 855, 750, 700; EIHRMS calcd for C15H14OCl2
280.0422, found 280.0426.
4.2.2. GP4
Strictly deoxygenated THF (15 mL) was added to a mixture of
Cp2TiCl2 (50 mg, 0.2 mmol) and Mn dust (440 mg, 8.0 mmol) under
Ar atmosphere and the suspension was stirred at rt until it turned
4.2.9. 3,3-Bis-(4-chlorophenyl)prop-2-en-1-ol (14)38
(CH–EA 90:10) GP4: (135 mg, 0.48 mmol, 48%).
green (about 15 min). Then, a solution of 2,4,6-collidine (925
m
l,
4.2.10. 3-Methyl-5-phenylpentan-1-ol (20)
7.0 mmol) and Me3SiCl (510 l, 4.0 mmol) in THF (4 mL) was added
m
(CH–EA 80:20) GP3: (90 mg, 0.50 mmol, 50%), colorless oil. 1H
and the mixture was stirred for 5 min. Then, a solution of oxetane
(1.0 mmol) in THF (5 mL) was added and the mixture was stirred at
60 ꢁC overnight. Then, a solution 2 N of HCl was added and the
mixture was extracted with AcOEt. The organic layer was dried over
MgSO4 and the solvent removed. The residue was purified by flash
chromatography (mixtures of CH–EA).
NMR (400 MHz, CDCl3)
(md, J¼7.0 Hz, 3H), 3.58 (dd, J¼7.8, 7.5 Hz, 2H), 2.46–2.65 (m, 2H),
1.30–1.66 (m, 5H), 0.89 (s, 3H); 13C NMR (100 MHz, CDCl3)
¼142.9,
128.4, 128.4, 128.4, 125.7, 61.1, 39.9, 39.0, 33.4, 29.3, 19.6; IR (film)
3375, 3025, 2930, 1600, 1495, 1455, 1055, 895, 745, 700; EIHRMS
calcd for C12H18O [Mþ] 178.1358, found 178.1357.
d
¼7.14–7.23 (md, J¼6.7 Hz, 2H), 7.06–7.14
d
n
4.2.3. 3,4-Dimethyl-3,4-diphenylhexane-1,6-diol (6)
4.2.11. 1-Phenethylbut-1-ene-4-ol (21)
(CH–EA 10:90) GP3: (105 mg, 0.35 mmol, 70%); GP4: (114 mg,
(CH–EA 80:20) GP3: (26 mg, 0.15 mmol, 15%), colorless oil. 1H
0.38 mmol, 76%), colorless solid. mp 159 ꢁC. 1H NMR (400 MHz,
NMR (400 MHz, CDCl3)
d
¼7.17–7.24 (md, J¼7.5 Hz, 2H), 7.08–7.14
CDCl3)
d
¼7.16–7.07 (md, J¼7.3 Hz, 6H), 6.99–6.94 (md, J¼8.2 Hz,
(md, J¼6.2 Hz, 3H), 4.84 (d, J¼1.4 Hz, 1H), 4.79 (d, J¼0.8 Hz, 1H),
3.65 (t, J¼6.4 Hz, 2H), 2.70 (dd, J¼9.8, 8.5 Hz, 2H), 2.28 (t, J¼8.9 Hz,
2H), 2.26 (t, J¼6.3 Hz, 2H), 1.34 (br s, 1H); 13C NMR (100 MHz,
4H), 3.39 (ddd, J¼10.1, 9.9, 6.0 Hz, 2H), 3.23 (ddd, J¼10.1, 10.1,
4.6 Hz, 2H), 2.24 (ddd, J¼13.5, 9.9, 4.6 Hz, 2H), 1.87 (ddd, J¼13.5,
10.1, 6.0 Hz, 2H), 1.21 (s, 6H), 0.87 (br s, –OH); 13C NMR (100 MHz,
CDCl3)
d
¼145.5, 141.9, 128.5, 128.4, 128.4, 125.9, 112.0, 60.4, 39.4,

11844
A. Gansa¨uer et al. / Tetrahedron 64 (2008) 11839–11845
8. Mudryk, B.; Cohen, T. J. Org. Chem. 1991, 56, 5760–5761.
37.6, 34.3; IR (film) n 3375, 3025, 2930, 1600, 1495, 1455, 1055, 895,
9. Kochi, J. K.; Singleton, D. M.; Andrews, L. J. Tetrahedron 1968, 24, 3503–3515.
10. (a) Nugent, W. A.; RajanBabu, T. V. J. Am. Chem. Soc. 1988, 110, 8561–8562; (b)
RajanBabu, T. V.; Nugent, W. A. J. Am. Chem. Soc. 1989, 111, 4525–4527; (c)
RajanBabu, T. V.; Nugent, W. A.; Beattie, M. S. J. Am. Chem. Soc. 1990, 112, 6408–
6409; (d) RajanBabu, T. V.; Nugent, W. A. J. Am. Chem. Soc. 1994, 116, 986–997.
11. Ja¨ger, C. M.; Hennemann, M.; Miesza1a, A.; Clark, T. J. Org. Chem. 2008, 73, 1536–
1545 and references cited therein.
745, 700; EIHRMS calcd for C12H16O [Mþ] 176.1201, found 176.1200.
4.2.12. 2-(4-tert-Butylcyclohex-1-enyl)ethanol (22)36
(CH–EA 94:6) GP4: (131 mg, 0.71 mmol, 71%).
12. (a) Gansa¨uer, A.; Pierobon, M.; Bluhm, H. Angew. Chem., Int. Ed. 1998, 37,
101–103; (b) Gansa¨uer, A.; Bluhm, H.; Pierobon, M. J. Am. Chem. Soc. 1998,
120, 12849–12859; (c) Gansa¨uer, A.; Bluhm, H. Chem. Commun. 1998, 2143–
2144; (d) Barrero, A. F.; Rosales, A.; Cuerva, J. M.; Oltra, J. E. Org. Lett. 2003,
5, 1935–1938.
4.3. Experimental procedure for the opening of 18 in the
presence of tert-butyl acrylate
4.3.1. GP5
13. (a) Gansa¨uer, A.; Bluhm, H. Chem. Rev. 2000, 100, 2771–2788; (b) Gansa¨uer, A.;
Narayan, S. Adv. Synth. Catal. 2002, 344, 465–475; (c) Cuerva, J. M.; Justicia, J.;
Strictly deoxygenated THF (15 mL) was added to a mixture of
Cp2TiCl2 (50 mg, 0.2 mmol) and Mn dust (440 mg, 8.0 mmol) under
Ar atmosphere and the suspension was stirred at rt until it turned
´
Oller-Lopez, J. L.; Oltra, J. E. Top. Curr. Chem. 2006, 264, 63–92; (d) Barrero, A. F.;
´lez del Moral, J. F.; Sa´nchez, E. M.; Arteaga, J. F. Eur. J. Org. Chem. 2006, 1627–
Quı
1641; (e) Gansa¨uer, A.; Justicia, J.; Fan, C.-A.; Worgull, D.; Piestert, F. Top. Curr.
Chem. 2007, 279, 25–52.
green (about 15 min). Then, a solution of 2,4,6-collidine (925
ml,
7.0 mmol) and Me3SiCl (510 l, 4.0 mmol) in THF (4 mL) was added
m
14. (a) Gansa¨uer, A.; Lauterbach, T.; Bluhm, H.; Noltemeyer, M. Angew. Chem., Int.
Ed. 1999, 38, 2909–2910; (b) Gansa¨uer, A.; Bluhm, H.; Lauterbach, T. Adv. Synth.
Catal. 2001, 343, 785–787; (c) Gansa¨uer, A.; Bluhm, H.; Rinker, B.; Narayan, S.;
Schick, M.; Lauterbach, T.; Pierobon, M. Chem.dEur. J. 2003, 9, 531–542; (d)
Gansa¨uer, A.; Fan, C.-A.; Keller, F.; Keil, J. J. Am. Chem. Soc. 2007, 129, 3484–3485;
(e) Gansa¨uer, A.; Fan, C. A.; Keller, F.; Karbaum, P. Chem.dEur. J. 2007, 13, 8084–
8090.
15. (a) Gansa¨uer, A.; Lauterbach, T.; Geich-Gimbel, D. Chem.dEur. J. 2004, 10, 4983–
4990; (b) Friedrich, J.; Dolg, M.; Gansa¨uer, A.; Geich-Gimbel, D.; Lauterbach, T.
J. Am. Chem. Soc. 2005, 127, 7071–7077; (c) Friedrich, J.; Walczak, K.; Dolg, M.;
Piestert, F.; Lauterbach, T.; Worgull, D.; Gansa¨uer, A. J. Am. Chem. Soc. 2008, 130,
1788–1796.
and the mixture was stirred for 5 min. Then, a solution of oxetane
(176 mg, 1 mmol) and tert-butyl acrylate (1.50 mL, 10 mmol) in THF
(5 mL) was added and the mixture was stirred at 60 ꢁC overnight.
Then, a solution of 2 N of HCl was added and the mixture was
extracted with AcOEt. The organic layer was dried over MgSO4 and
the solvent removed. The residue was purified by flash chromato-
graphy (mixtures of CH–EA).
4.3.2. GP6
16. (a) Gansa¨uer, A.; Pierobon, M. Synlett 2000, 1357–1359; (b) Gansa¨uer, A.;
Pierobon, M.; Bluhm, H. Synthesis 2001, 2500–2520; (c) Gansa¨uer, A.; Pierobon,
M.; Bluhm, H. Angew. Chem., Int. Ed. 2002, 41, 3206–3208.
Strictly deoxygenated THF (15 mL) was added to a mixture of
Cp2TiCl2 (50 mg, 0.2 mmol), Mn dust (440 mg, 8.0 mmol), and
collidine hydrochloride (396 mg, 2.5 mmol) under Ar atmosphere
and the suspension was stirred at rt until it turned green (about
15 min). Then, a solution of oxetane (177 mg, 1.0 mmol) and tert-
butyl acrylate (1.50 mL, 10 mmol) in THF (5 mL) was added and the
mixture was stirred at 60 ꢁC overnight. Then, a solution of 2 N of
HCl was added and the mixture was extracted with AcOEt. The
organic layer was dried over MgSO4 and the solvent removed. The
residue was purified by flash chromatography (mixtures of CH–EA).
17. (a) Justicia, J.; Rosales, A.; Bun˜uel, E.; Oller-Lo´ pez, J. L.; Valdivia, M.; Haı¨dour, A.;
´
Oltra, J. E.; Barrero, A. F.; Cardenas, D. J.; Cuerva, J. M. Chem.dEur. J. 2004, 10,
1778–1788; (b) Justicia, J.; Oller-Lopez, J. L.; Campan˜a, A. G.; Oltra, J. E.; Cuerva,
´
J. M.; Bun˜uel, E.; Cardenas, D. J. J. Am. Chem. Soc. 2005, 127, 14911–14921.
18. Gansa¨uer, A.; Rinker, B. Tetrahedron 2002, 58, 7017–7026.
19. (a) Handa, Y.; Inanaga, J. Tetrahedron Lett. 1987, 28, 5717–5718; (b) Gansa¨uer, A.
Chem. Commun. 1997, 457–458; (c) Gansa¨uer, A.; Bauer, D. J. Org. Chem. 1998, 63,
2070–2071; (d) Gansa¨uer, A.; Moschioni, M.; Bauer, D. Eur. J. Org. Chem. 1998,
1923–1927; (e) Gansa¨uer, A.; Bauer, D. Eur. J. Org. Chem. 1998, 2673–2676; (f)
Dunlap, M. S.; Nicholas, K. M. Synth. Commun. 1999, 27, 1097–1106; (g)
Halterman, R. L.; Zhu, C.; Chen, Z.; Dunlap, M. S.; Khan, M. A.; Nicholas, K. M.
Organometallics 2000, 19, 3824–3829; (h) Este´vez, R. E.; Oller-Lo´pez, J. L.;
Robles, R.; Melgarejo, C. R.; Gansa¨uer, A.; Cuerva, J. M.; Oltra, J. E. Org. Lett. 2006,
8, 5433–5436.
20. Rosales, A.; Oller-LopezJusticia, J.; Gansa¨uer, A.; Oltra, J. E.; Cuerva, J. M. Chem.
Commun. 2004, 2628–2629.
21. Mandal, S. K.; Roy, S. C. Tetrahedron Lett. 2007, 48, 4131–4134.
22. (a) Parrish, J. D.; Shelton, D. R.; Little, R. D. Org. Lett. 2003, 5, 3615–3617; (b)
Sgreccia, L.; Bandini, M.; Morganti, S.; Quintavalla, A.; Umani-Ronchi, A.; Cozzi,
P. G. J. Organomet. Chem. 2007, 692, 3191–3197; (c) Este´vez, R. E.; Paradas, M.;
4.3.3. tert-Butyl-6-hydroxy-4-methyl-4-phenethyl-hexanoate (23)
(CH–EA 80:20) GP5: (155 mg, 0.50 mmol, 50%); GP6 (182 mg,
0.60 mmol,60%);colorlessoil.1HNMR (400 MHz, CDCl3)
d¼7.22–7.15
(m, 2H), 7.12–7.04 (m, 3H), 3.65 (t, J¼7.6 Hz, 2H), 2.52–2.44 (m, 2H),
2.17–2.11 (m, 2H), 1.58–1.50 (m, 4H), 1.47–1.40 (m, 2H), 1.37 (s, 9H),
0.89 (s, 3H); 13C NMR (100 MHz, CDCl3)
128.3, 125.7, 80.3, 59.3, 41.9, 41.7, 34.6, 34.4, 30.4, 30.2, 28.1, 24.9; IR
d
¼173.6, 142.9, 128.4, 128.4,
´
´
T.; Robles, R.; Cuerva, J. M.; Oltra, J. E. J. Org. Chem. 2008, 73,
Millan, A.; Jimenez,
1616–1619.
(film) n 3435, 2935, 1725,1455, 1370,1250,1150,1030, 845, 745, 700;
23. Barrero, A. F.; Herrador, M. M.; Del Moral, J. F. Q.; Arteaga, P.; Dieguez, H. R.;
Sanchez, E. M. J. Org. Chem. 2007, 72, 2988–2995.
EIHRMS calcd for C15H20O2 [MþꢀC4H10O] 232.1463, found 232.1197.
24. (a) Daasbjerg, K.; Svith, H.; Grimme, S.; Gerenkamp, M.; Mu¨ck-Lichtenfeld, C.;
Gansa¨uer, A.; Barchuk, A.; Keller, F. Angew. Chem., Int. Ed. 2006, 45, 2041–2044;
(b) Gansa¨uer, A.; Barchuk, A.; Keller, F.; Schmitt, M.; Grimme, S.; Gerenkamp,
M.; Mu¨ck-Lichtenfeld, C.; Daasbjerg, K.; Svith, H. J. Am. Chem. Soc. 2007, 129,
1359–1371.
25. (a) Gansa¨uer, A.; Rinker, B.; Pierobon, M.; Grimme, S.; Gerenkamp, M.; Mu¨ck-
Lichtenfeld, C. Angew. Chem., Int. Ed. 2003, 42, 3687–3690; (b) Gansa¨uer, A.;
Rinker, B.; Ndene-Schiffer, N.; Pierobon, M.; Grimme, S.; Gerenkamp, M.; Mu¨ck-
Lichtenfeld, C. Eur. J. Org. Chem. 2004, 2337–2351.
Acknowledgements
We are indebted to the DFG (‘Gerhard Hess-Programm’) and the
Fonds der Chemischen Industrie (‘Sachbeihilfen’) for continuing
financial support. J.J. is grateful to the University of Granada and
´
Ministerio de Educacion y Ciencia for postdoctoral grants.
26. For some recent reviews see: (a) Newcomb, M.; Toy, P. H. Acc. Chem. Res. 2000,
33, 449–455; (b) Shaik, S.; Cohen, S.; de Visser, S. P.; Sharma, P. K.; Kumar, D.;
Kozuch, S.; Ogliaro, F.; Danovich, D. Eur. J. Inorg. Chem. 2004, 207–226; (c)
Meunier, B.; de Visser, S. P.; Shaik, S. Chem. Rev. 2004, 104, 3947–3980; (d)
Shaik, S.; Kumar, D.; de Visser, S. P.; Thiel, W. Chem. Rev. 2005, 105, 2279–2328;
(e) Denisov, I. G.; Makris, T. M.; Sligar, S. G.; Schlichting, I. Chem. Rev. 2005, 105,
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