been desired. In addition, differentiation between allyl and
propargyl ethers has been scarcely investigated.5
Scheme 2
Herein, we wish to report facile deallylation and depro-
pargylation reactions catalyzed by a low valent titanium
generated from Ti(O-i-Pr)
source such as TMSCl (TMSdSiMe
4
/Mg in the presence of a halogen
) and MgBr
3
2
.
In the course of our study for developing a catalytic
6
cyclotrimerization of alkynes to substituted benzenes and
7
also in low-valent titanium chemistry, we found that Ti(O-
4
i-Pr) promoted cyclization of triynes 1a and 1b to the
corresponding annulated benzene 2 in the presence of Mg
With these assumptions in mind, we carried out the
reactions of 3-aryloxypropyne and -propene, 2-naphthyl
propargyl ether (3a), and 2-naphthyl allyl ether (4a) with
powder and TMSCl in THF and the amount of the titanium
8
could be reduced to be catalytic (Scheme 1). Similarly,
4
this Ti(O-i-Pr) /TMSCl/Mg reagent (Scheme 3). As expected,
Scheme 1
Scheme 3
the ethers reacted within 12 h to provide 2-naphthol in nearly
quantitative yield after aqueous acidic workup. Herein, we
wish to focus on these deallylation and depropargylation
reactions, expecting a new deprotective protocol, although
the investigation on benzene formation shown in Scheme 1
is of interest and is underway in this laboratory.
To clarify the requirement of reagents and conditions for
these reactions, the reactions of allyl and propargyl ethers
of 3-phenyl-1-propanol, 3b and 4b, were carried out, and
the results are summarized in Table 1. Under reaction
trisubstituted benzenes 2c and 2d were obtained as a
regioisomeric mixture from the corresponding 1-alkynes
by the reaction with a stoichiometric amount of the rea-
8
gent.
It was noteworthy that the yields of 2a and 2b were not
necessarily high (40-60%), although the reaction consumed
1
completely and the concentrated crude residue had no other
Table 1. Reaction Conditions
product than 2. The loss of 1 may be explained by assuming
that the â-elimination reactions from the possible intermedi-
2
ates, η -alkyne complex(es) and/or titanacyclopentadiene,
occur as a side reaction3
d-f,9
as illustrated in Scheme 2.
(
4) (a) Crich, D.; Jayalath, P. Org. Lett. 2005, 7, 2277. (b) Crich, D.;
equiv
Jayalath, P.; Hutton, T. K. J. Org. Chem. 2006, 71, 3064. Crich, D.; Wu,
B. Org. Lett. 2006, 8, 4879. (c) Nayak, S. K.; Kadam, S. M.; Banerji, A.
Synlett 1993, 581. (d) Swamy, V. M.; Ilankumaran, P.; Chandrasekaran, S.
Synlett 1997, 513.
entry ether Ti(O-i-Pr)4 Mga additive (equiv)
h
yield, %
b
1
2
3
4
5
6
7
8
3b
3b
3b
3b
3b
3b
4b
4b
0.2
0.2
0.2
0.05
0.05
0.05
0.05
0.05
3
no reaction
no reaction
3 91
(
5) For selective cleavage of allyl propargyl ether, see ref 4c,d.
TMSCl (2)
TMSCl (1)
(6) Saino, N.; Kogure, D.; Kase, K.; Okamoto, S. J. Organomet. Chem.
2
2
2
2
2
2
2
006, 691, 3129. Saino, N.; Amemiya, F.; Tanabe, E.; Kase, K.; Okamoto,
S. Org. Lett. 2006, 8, 1439. Saino, N.; Kogure, D.; Okamoto, S. Org. Lett.
005, 7, 3065.
7) Sato, F.; Urabe, H.; Okamoto, S. Chem. ReV. 2000, 100, 2835.
TMSCl (0.15)
MgBr2 (0.15)
5
8
86
c
d
2
87 (85)
(
MgCl (0.15)e
24 96
88
11 100
2
Kulinkovich, O. G.; de Meijere, A. Chem. ReV. 2000, 100, 2789. Eisch, J.
J. J. Organomet. Chem. 2001, 617-618., 148. Sato, F.; Okamoto, S. AdV.
Synth. Catal. 2001, 343, 759. Sato, F.; Urabe, H. In Titanium and Zirconium
in Organic Synthesis; Marek, I., Ed.; Wiley-VCH: Weinheim, Germany,
TMSCl (0.15)
MgBr2 (0.15)
8
c
a
Other metal powders such as Zn, Mn, and Al were not effective.
2
002; pp 319-354.
8) Low-valent titanium-promoted or -catalyzed benzene formation from
alkynes: Suzuki, D.; Urabe, H.; Sato, F. J. Am. Chem. Soc. 2001, 123,
925. Tanaka, R.; Nakano, Y.; Suzuki, D.; Urabe, H.; Sato, F. J. Am. Chem.
Soc. 2002, 124, 9682. Eisch, J. J.; Gitua, J. N. Organometallics 2003, 22,
b
Determined by 1H NMR analysis using an internal standard. A THF
c
(
solution prepared by the reaction of 1,2-dibromoethane with Mg was used.
d
Commercial solid MgBr2 was used. e Commercial solid MgCl2 was used.
7
2
1
4. Xi, Z.; Sato, K.; Gao, Y.; Lu, J.; Takahashi, T. J. Am. Chem. Soc. 2003,
25, 9568. Morohashi, N.; Yokomakura, K.; Hattori, T.; Miyano, S.
Tetrahedron Lett. 2006, 47, 1157 and references therein.
9) Takayama, Y.; Gao, Y.; Sato, F. Angew. Chem., Int. Ed. Engl. 1997,
6, 851. See also ref 7.
conditions identical to those for the reaction of aryl ethers
3a and 4a shown in Scheme 3, alkyl ether 3b was effectively
(
3
774
Org. Lett., Vol. 9, No. 5, 2007