Conversion of propargyl alcohols to chloroallenes and arylalkynes using the TiCl4/R3N reagent system

Article abstract of DOI:10.1021/jo060683m

Whereas the reaction of certain propargyl alcohols with TiCl₄ in the presence of tertiary alkylamines gives the corresponding chloroallenes in 37-58% yields, reaction with the tertiary arylamines gives the corresponding arylalkynes in 68-77% yi

Full text of DOI:10.1021/jo060683m

fatty acids,9 enediynes (Taxamycins),10 (Z)-tamoxifen,11 AB-
taxane ring with enone,12 Taxol and taxotere,13 vitamin A,14
â-lactones,15 (+)-parviflorin, and (+)-5S-hydroxyparviflorin.16
During the investigations on the development of new synthetic
methods using the TiCl4/R3N reagent system,17 we have
observed that certain propargyl alcohols 1 are readily con-
verted to the corresponding chloroallene derivatives and aryl
substituted alkynes. Initially, we have carried out the preparation
of the alkynyltitanium reagent in situ for reaction with benzal-
dehyde (1 equiv) using 1-heptyne (1 equiv), TiCl4 (1 equiv),
and Et3N (1.5 equiv) in dichloromethane. In this experiment,
the corresponding chloroallene was isolated in 21% yield
(Scheme 1).
Conversion of Propargyl Alcohols to
Chloroallenes and Arylalkynes Using the
TiCl4/R3N Reagent System
Galla V. Karunakar and Mariappan Periasamy*
School of Chemistry, UniVersity of Hyderabad,
Central UniVersity P. O., Hyderabad-500 046, India
mpsc@uohyd.ernet.in
ReceiVed March 30, 2006
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arylamines gives the corresponding arylalkynes in 68-77%
yields.
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10.1021/jo060683m CCC: $33.50 © 2006 American Chemical Society
Published on Web 08/23/2006
J. Org. Chem. 2006, 71, 7463-7466
7463

SCHEME 1. Formation of Chloroallene 2a in the Reaction
of 1-Heptyne with Benzaldehyde
SCHEME 2. Formation of Chloroallenes from Propargyl
Alcohols
FIGURE 1. ORTEP diagram of the allenyltriethylamine salt 3 (thermal
ellipsoids are drawn at 20% probability).
TABLE 1. Conversion of Propargyl Alcohols to Chloroallenes
Using the TiCl4/R3N Reagent Systema
SCHEME 3. Allenyltriethylamine Salt 3 Formation in the
Reaction of TiCl4/Et3N Reagent System with the Propargyl
Alcohol 1d
SCHEME 4. Mechanism for the Formation of
Chloroallenes and the Allenylammonium Salt
a The reagents were used in the following quantities: propargyl alcohol
(1 mmol), TiCl4 (1 mmol), and alkylamines (2 mmol). b The products were
identified by 1H NMR, 13C NMR, mass spectral data, and elemental analysis.
c Yields of isolated products.
Since this transformation would most probably involve the
intermediacy of the corresponding propargyl alcohol deriv-
ative formed in situ, we have examined the reaction of 1-phenyl-
2-octyn-1-ol 1a with triethylamine and TiCl4. In this case,
the corresponding chloroallene was obtained in 56% yield
(Scheme 2).
We have examined this reaction using different amines. It
was found that the Et3N gave better yields (Table 1). In the
case of Bu3N and i-Pr2NEt, the chloroallene was isolated in
45% and 37% yields, respectively (Table 1). The chloroallenes
were also isolated in reasonable yields from the propargyl
alcohols derived from benzil (Table 1). However, the propargyl
alcohols derived from aliphatic aldehydes and ketones (e.g.,
butyraldehyde, acetophenone, and cyclohexanone) led to unclean
reactions, and the corresponding chloroallenes were not obtained
in these cases. It is of interest to note here that the Ti(III) species,
produced in situ by first reacting the TiCl4 with the Et3N at 0
°C for 1 h, upon subsequent addition of with certain propargyl
alcohols including those derived from aliphatic aldehydes, give
the corresponding 1,5-diynes through the intermediacy of the
corresponding radical species.17a
(Scheme 3). This salt 3 was characterized by single-crystal X-ray
analysis (Figure 1).18
The formation of the chloroallene derivatives may be ex-
plained by the tentative mechanism outlined in Scheme 4
involving the intermediacy of the corresponding allenyl cations.
However, as mentioned earlier, the propargyl alcohols react
with the Ti(III) species, produced in situ using the TiCl4/Et3N
reagent system under slightly different conditions, to give the
corresponding 1,5-diynes or enynes via the coupling of the
corresponding radical intermediates.17a
We have observed when the propargyl alcohol 1a was reacted
with N,N-diethylaniline at 0-25 °C the chloroallene 2a was
(18) Crystal data for allenyltriethylamine salt (3): C27H36ClNO3, MW
) 458.03, monoclinic, space group: P21/n, a ) 9.1453(18) Å, b ) 9.8672-
(5) Å, c ) 17.0435(9) Å, â ) 99.15(3)°, V ) 2606.5(2) Å3, Z ) 42, Fc )
1.143 Mg m-3, µ ) 0.100 mm-1, T ) 298 K. Of the 24177 reflections
collected, 4574 were unique [Rint ) 0.0401]. Refinement on all data
converged at R1 ) 0.0821, wR2 ) 0.2610.
An interesting observation was made in the case of the
propargyl alcohol 1d prepared from benzophenone and phenyl-
acetylene (1,1,3-triphenyl-2-propyn-1-ol). In this case, the cor-
responding allenyl quaternary salt 3 was isolated in 68% yield
7464 J. Org. Chem., Vol. 71, No. 19, 2006

SCHEME 5. Formation of Chloroallene 2a from Propargyl
Alcohol 1a
SCHEME 7. Arylamine-Substituted Allenic Product 6 from
Propargyl Alcohol 1d
SCHEME 6. Arylated Alkynes from Propargyl Alcohols
SCHEME 8. Tentative Mechanism for the Arylation of
Propargyl Alcohols Using ArNR2
TABLE 2. Arylation of Propargyl Alcohols Using the TiCl4/R2NAr
Reagent Systema
Interestingly, in the case of the propargyl alcohol 1d prepared
from benzophenone, the corresponding allenic product 6 was
obtained in 78% yield (Scheme 7). Presumably, in this case
the allene is formed as there would be tremendous steric
hindrance toward attack of the propargyl cation that would be
formed in situ (Scheme 8).
These transformations leading to arylation of the propargyl
derivatives may be explained by the mechanism outlined in
Scheme 8. Although the transformation can be rationalized by
the electrophilic substitution reaction of the allenyl cation formed
in situ with the aniline derivatives, the alternative mechanism
involving the reaction of aryltitanium species cannot be ruled
out as in all these cases the corresponding benzidine derivatives
are isolated in <12% yields. Previously, the formation of
benzidine from N,N-dialkylarylamines and TiCl4 was explained
by considering the coupling of the radical cations and/or
aryltitanium species formed in situ.19
To further examine the intermediacy of aryltitanium species
in this transformation, we have quenched the reaction (Scheme
6) with D2O in the case of N,N-dialkylaniline after 6 h. In this
case, the benzidine was isolated in 10% yield and the recovered
N,N-dialkylaniline did not contain deuterium in the 4 position.
Presumably, this transformation (Scheme 8) does not go through
the corresponding aryltitanium intermediate or the aryltitanium
formed in situ is not stable and undergoes coupling to give the
corresponding benzidine before quenching with D2O.
a The reagents were used in the following quantities: propargyl alcohol
(1 mmol), TiCl4 (1 mmol), and of arylamines (1.5 mmol). b The products
1
were identified by H NMR, 13C NMR, mass spectral data, and elemental
analysis. c Yields of isolated products. d The corresponding benzidine was
isolated with <12% yield.
isolated in 47% yield, besides the corresponding benzidine 4
(22% yield) (Scheme 5).19
However, when the reaction was carried out -40 °C, the
corresponding arylated alkynyl product 5a was isolated in 68%
yield as major product besides the corresponding benzidine (12%
yield) (Scheme 6). This transformation was found to be general
for other tertiary aromatic amines and the results are summarized
in Table 2.
In recent years, several methods have been reported for the
preparation of allene derivatives from propargyl alcohol deriva-
tives.20 The simple method of synthesis of chloroallenes
described here is a good addition to this pool of procedures. In
addition, the interesting reactivity pattern uncovered for the
(19) Periasamy, M.; Jayakumar, K. N.; Bharathi, P. J. Org. Chem. 2000,
65, 3548-3550.
J. Org. Chem, Vol. 71, No. 19, 2006 7465

Presence of TiCl4/Et3N Reagent System. In CH2Cl2 (35 mL),
propargyl alcohol 1d (0.284 g, 1 mmol), TiCl4 (0.19 g, 0.11 mL,
1 mmol), and Et3N (0.3 g, 0.42 mL, 3 mmol) were taken at -40
°C under N2. The reaction mixture was stirred for 6 h. The reaction
mixture was quenched with water (10 mL). The organic layer was
separated, and the aqueous layer was extracted with CH2Cl2 (2 ×
5 mL). The solvent was removed, and the residue was concentrated.
The residue was crystallized from ethyl acetate. The allenyl
triethylamine salt 3 was isolated in 68% yield (0.27 g). Spectral
data for product 3: IR (neat) 3393, 1946, 1454 cm-1;13C NMR
(100 MHz, CDCl3, δ ppm) 8.8, 53.1, 115.1, 121.6, 126.1, 127.4,
128.2, 128.6, 129.2, 129.7, 130.1, 131.2, 200.1; 1H NMR (400 MHz,
CDCl3, δ ppm) 7.56-7.29 (m, 15H), 3.75-3.7 (q, 6H, J ) 6.8
Hz), 1.48 (t, 9H, J ) 6.8 Hz); MS (EI) m/z 403 (M+). Anal. Calcd
for C27H30ClN: C, 80.27; H, 7.48; N, 3.47. Found: C, 80.31; H,
7.25; N, 3.56.
TiCl4/tertiary arylamine reagent system should be useful for
further synthetic exploitations.
Experimental Section
Typical Experimental Procedure for the Conversion of
1-Heptyne and Benzaldehyde in the Presence of TiCl4/Et3N
Reagent System. In CH2Cl2 (35 mL), 1-heptyne (0.96 g, 1.3 mL,
10 mmol), TiCl4 (1.9 g, 1.1 mL, 10 mmol), Et3N (1.5 g, 2.1 mL,
15 mmol), and benzaldehyde (1.06 g, 1 mL, 10 mmol) were taken
at -40 °C under N2 and stirred for 2 h at -40 °C. It was stirred
further at 25 °C for another 6 h. A saturated NH4Cl solution (20
mL) was added and the mixture stirred for 30 min. The organic
layer was separated, and the aqueous layer was extracted with
CH2Cl2 (2 × 15 mL). The combined organic extract was washed
with brine solution (10 mL) and dried over anhydrous Na2SO4. The
solvent was removed, and the residue was chromatographed on a
silica gel column with hexane as eluent to isolate the chloroallene
2a (0.46 g, 21%). Spectral data for product 2a: IR (neat) 2957,
Typical Experimental Procedure for the Conversion of
Proparyl Alcohols to Arylalkynes in the Presence of TiCl4/Et3N
Reagent System. In dichloromethane (35 mL), propargyl alcohol
1a (0.20 g, 1 mmol), TiCl4 (0.19 g, 0.1 mL, 1 mmol), and N,N-
diethylaniline (0.3 g, 0.32 mL, 2 mmol) were taken at -40 °C under
N2. The reaction mixture was stirred for 6 h at -40 °C. A saturated
K2CO3 solution (20 mL) was added and the mixture stirred for 10
min. The organic layer was separated, and the aqueous layer was
extracted with dichloromethane (2 × 15 mL). The combined organic
extract was washed with brine solution (10 mL) and dried over
anhydrous Na2SO4. The solvent was removed, and the residue was
chromatographed on a silica gel column using 1% ethyl acetate in
hexane to isolate the aryl alkyne 5a (0.26 g, 68%) and benzidine
(0.03 g, 12%). Spectral data for 5a: IR (neat) 3412, 3067, 2069,
1930, 1493 cm-1 13C NMR (100 MHz, CDCl3, δ ppm) 14.0, 22.3,
;
26.8, 31.0, 36.5, 101.7, 108.8, 127.7, 128.2, 128.7, 133.3, 200.4;
1H NMR (400 MHz, CDCl3, δ ppm) 7.5-7.2 (m, 5H), 6.48 (s,
1H), 2.53 (t, 2H, J ) 6.8 Hz), 1.67-1.38 (m, 6H), 0.95 (t, 3H, J
) 6.8 Hz); MS (EI) m/z 220 (M+). Anal. Calcd for C14H17Cl: C,
76.18; H, 7.76. Found: C, 76.21; H, 7.78.
Typical Experimental Procedure for the Conversion of
Proparyl Alcohols to Chloroallenes in the Presence of TiCl4/
Et3N Reagent System. In CH2Cl2 (35 mL), propargyl alcohol 1a
(0.20 g, 1 mmol), TiCl4 (0.19 g, 0.11 mL, 1 mmol), and Et3N (0.2
g, 0.28 mL, 2 mmol) were taken at 0 °C under N2. The reaction
mixture was stirred for 30 min at 0 °C. It was stirred further at 25
°C for 6 h. A saturated NH4Cl solution (20 mL) was added and
stirred for 30 min. The organic layer was separated, and the aqueous
layer was extracted with CH2Cl2 (2 × 15 mL). The combined
organic extract was washed with brine solution (10 mL) and dried
over anhydrous Na2SO4. The solvent was removed, and the residue
was chromatographed on a silica gel column using hexane as eluent
to isolate chloroallene 2a (0.14 g, 56%). The same procedure was
followed to obtain the compound 2b (0.17 g, 54%) and 2c (0.19 g,
58%).
1634 cm-1 13C NMR (100 MHz, CDCl3, δ ppm) 12.5, 14.0, 18.9,
;
22.2, 28.7, 30.9, 42.2, 44.3, 84.4, 96.0, 111.8, 123.3, 126.3, 128.0,
1
128.6, 129.1, 133.2, 143.3, 146.5; H NMR (400 MHz, CDCl3, δ
ppm) 7.13-7.39 (m, 9H), 4.48 (s, 1H), 3.12-3.32 (m, 2H), 2.2-
2.24 (m, 4H), 1.48-1.56 (6H), 1.41 (t, 6H, J ) 7.3 Hz), 0.92 (t,
3H, J ) 7.2 Hz); MS (EI) m/z 333 (M+). Anal. Calcd for C24H31N:
C, 86.43; H, 9.37; N, 4.27. Found: C, 86.45; H, 9.39; N, 4.23.
Acknowledgment. G.V.K. is thankful to the ILS-UOH for
a fellowship and M.P. is thankful to ILS-UOH for a research
grant. UGC support under the “University with Potential for
Excellence (UPE)” and “Centre for Advance Study (CAS-SAP)”
programs is gratefully acknowledged. We are thankful to the
DST for 400 MHz NMR spectrometer and National XRD-CCD
facilities in the School of Chemistry, University of Hyderabad.
Typical Experimental Procedure for the Conversion of
Proparyl Alcohols 1d to Allenyltriethylamine Salt 3 in the
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Supporting Information Available: Spectral data for com-
pounds 2b,c, 5b-e, and 6, 13C NMR spectra of products, crystal-
lographic diagrams and crystallographic information files (CIF) for
compound 3. This material is available free of charge via the
Internet at http://pubs.acs.org.
JO060683M
7466 J. Org. Chem., Vol. 71, No. 19, 2006