Rhodium(l)-catalyzed carbonyl allenylation versus propargylation via redox transmetalation across tetragonal tin(ll) oxide

Article abstract of DOI:10.1021/ol0493352

A reagent combination of β-SnO and catalytic [Rh(COD)Cl]₂ in THF-H₂O promotes the reaction of propargyl bromides and aldehydes and directs the regioselectivity toward the formation of either allenic alcohols or homopropargylic alcohols. This highly regioselective either/or transformation proceeds via a transmetalation from rhodium to tin, in which metallotropic rearrangement between a propargylmetal and allenylmetal is arrested.

Full text of DOI:10.1021/ol0493352

ORGANIC
LETTERS
2004
Vol. 6, No. 13
2137-2140
Rhodium(I)-Catalyzed Carbonyl
Allenylation versus Propargylation via
Redox Transmetalation Across
Tetragonal Tin(II) Oxide†
Moloy Banerjee and Sujit Roy*
Organometallics & Catalysis Laboratory, Chemistry Department,
Indian Institute of Technology, Kharagpur 721302, India
sroy@chem.iitkgp.ernet.in
Received April 12, 2004
ABSTRACT
A reagent combination of â-SnO and catalytic [Rh(COD)Cl]2 in THF−H O promotes the reaction of propargyl bromides and aldehydes and
2
directs the regioselectivity toward the formation of either allenic alcohols or homopropargylic alcohols. This highly regioselective either/or
transformation proceeds via a transmetalation from rhodium to tin, in which metallotropic rearrangement between a propargylmetal and
allenylmetal is arrested.
An attractive choice in carbon-carbon bond forming strate-
gies is to exploit the nucleophilic reactivity of an organo
main group metal reagent, generated in situ via transmeta-
lation from a catalytic organotransition metal partner. The
selectivity in the end organic product is determined by the
degree of control in all three reactions occurring in tandem.1
Deployment of this strategy for the generation of a propar-
gylmetal (Scheme 1) is expected to be complicated by
metallotropic rearrangement (A to A′, B to B′), transmeta-
lation selectivity (A to B/B′; A′ to B′/B), and selectivity
during the reaction with electrophile (B to C/C′; B′ to C′/
C).
the synthesis of homopropargyl alcohols. Tuning the regio-
selectivity to either allenic or homopropargyl alcohol (either
C or C′) by the same set of transition metal and main group
metal partners remains a pertinent challenge. We are pleased
to achieve such an either/or regioselective transformation via
redox-transmetalation from rhodium to tetragonal tin(II)-
oxide (hereafter â-SnO) in the presence of water and to report
our preliminary finding here.4
Enhancement of nucleophilic reactivity of organostannanes
by water continues to evoke interest. Most recently, Li et
For carbonyl compounds as the electrophile, Tamaru2 and
Marshall3 elegantly demonstrated pathway A′ f B f C
using a palladium to zinc/indium transmetalation strategy for
Scheme 1
† Presented in part at the Indo-US Conference on Recent Advances in
Organometallic Catalysis & Olefin Polymerization, IIT Madras, Dec 10-
12, 2003.
(1) Marshall, J. A. Chem. ReV. 2000, 100, 3163.
(2) Tamaru, Y.; Goto, S.; Tanaka, A.; Shimuzu, M.; Kimura, M. Angew.
Chem., Int. Ed. Engl. 1996, 35, 878.
(3) Marshall, J. A.; Grant, C. M. J. Org. Chem. 1999, 64, 696.
10.1021/ol0493352 CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/22/2004

al. speculated that an alkoxy or hydroxy pendant in R-Sn-
(OH)3 exerts electron donation to the vacant d orbital of tin,
thereby stabilizing the species and consequently enhancing
its reactivity.5 Drawn by the bonding and structural similari-
ties of â-SnO and hydrated tin(II) halides and the trihalo-
stannous ion, we reasoned that transmetalation over â-SnO
in the presence of water may generate intermediates that
could be considered as surrogates of R-Sn(OH)3 (Scheme
2).6 In the present study we sought to explore whether such
an intermediate is capable of arresting the metallotropic
rearrangement in the case of an ambident nucleophile like
propargylmetal.7
Table 1. Activation of Phenylpropargyl Bromide 1 over
Tin(II): Effect of Catalyst and Reaction Conditions
entry
conditions
yield (%)
1
2
3
4
5
6
7
8
Pd2(dba)3, SnCl2, THF-H2O (9:1)
Pd2(dba)3, â-SnO, THF-H2O (9:1)
PdCl2(PPh3)2, SnCl2, THF-H2O (9:1)
PtCl2(PPh3)2, SnCl2, THF-H2O (9:1)
NiCl2(PPh3)2, SnCl2, THF-H2O (9:1)
RhCl(PPh3)3, SnCl2, THF-H2O (9:1)
[Rh(COD)Cl]2, SnCl2, THF-H2O (9:1)
[Rh(COD)Cl]2, SnBr2, THF-H2O (9:1)
[Rh(COD)Cl]2, â-SnO, THF-H2O (9:1)
[Rh(COD)Cl]2, â-SnO, THF
0
0
<5
<5
0
37
38
41
77
0
Scheme 2. Transmetalation from R-Tm to â-SnO
9
10
11
12
13
14
15
[Rh(COD)Cl]2, â-SnO, DCM
0
[Rh(COD)Cl]2, â-SnO, DCM-H2O (9:1)
[Rh(COD)Cl]2, â-SnO, MeOH-H2O (9:1)
â-SnO, THF-H2O (9:1)
37
46
0
[Rh(COD)Cl]2, THF-H2O (9:1)
0
Taking phenylpropargyl bromide 1 as the model substrate,
a number of optimization experiments were carried out,
varying the transition metal catalyst and tin(II) partner.
Palladium(0/II), platinum(II), and nickel(II) catalysts resulted
in no or negligible amount of desired product (either allenic
or homopropargyl alcohol) (Table 1 entries 1-5). Since
reaction of 1 with d8/d10 complexes of palladium and
platinum is well documented,8 we ascribe the above failure
to the poor ability of the organotransition metal species to
transmetalate to the tin(II) partner. Gratifyingly transmeta-
lation from Rh(I) to Sn(II) provided the desired allenic
alcohol (entries 6-9, 12, and 13). Compared to other tin(II)
partners, â-SnO was the best, affording exclusively the
allenic alcohol in 77% yield (entry 9). Among the Rh(I)-
complexes screened, [Rh(COD)Cl]2 (where COD is cyclo-
octadiene) is found to be the best catalyst. The profound
influence of water in the reaction may be noted: no reaction
occurred in the absence of water (entries 10 and 11). Further,
the carbonyl addition reaction between 1 and aldehyde is
not promoted by either [Rh(COD)Cl]2 or â-SnO alone
(entries 14 and 15).
Motivated by these results, we extended the rhodium-
catalyzed reaction of propargyl bromides 1-4 with various
aldehydes to obtain the corresponding allenic alcohols in
moderate to excellent yields (Table 2, entries 1-10).9 No
homopropargylic alcohols were observed in any of these
cases.
It may be noted that allenic alcohols are versatile synthons
in various stereoselective transformations leading to impor-
tant organic intermediates such as amino alcohols, 1,2-diols,
vinyl epoxides, vinyl cyclopropanes, and dihydrofurans.10
As reactions of propargyl and allenylstannanes with
carbonyl compounds follow an SE2′ pathway, the allenic
alcohol in the present case will arise from a propargyltin
intermediate. The latter may originate from either an allenyl
1
or propargylrhodium species. H NMR monitoring of the
reaction of 1 (0.02 mmol) with [Rh(COD)Cl]2 (0.01 mmol)
in CDCl3 at 50 °C indicated a clear shift of the methylene
signal of free bromide from δ (ppm) 4.17 to 2.17, indicating
the formation of a propargylrhodium(III) intermediate.
Wojcicki et al. also reported the formation of propargyl-
(4) For allylic, aryl, and propargylic activation by Rh(I) complexes,
see: (a) Chin, C. S.; Shin, S. Y.; Lee, C. J. Chem. Soc., Dalton Trans.
1992, 1323. (b) Li, C. J.; Meng, Y. J. Am. Chem. Soc. 2000, 122, 9538. (c)
Kayan, A.; Gallucci, J. C.; Wojcicki, A. J. Organomet. Chem. 2001, 630,
44.
(5) Huang, T.; Meng, Y.; Venkatraman, S.; Wang, D.; Li, C. J. J. Am.
Chem. Soc. 2001, 123, 7451.
(6) (a) Sinha, P.; Roy, S. Organometallics 2004, 23, 67. (b) Banerjee,
M.; Roy, S. Chem. Commun. 2003, 534. (c) Sinha, P.; Roy, S. Chem.
Commun. 2001, 1798.
(7) Earlier attempts in this regard have been to vary process parameters
such as temperature, solvent, or additive or to execute a further transmeta-
lation involving tin-tin, tin-indium, tin-lithium, and tin-boron. See: (a)
Masuyama, Y.; Watabe, A.; Ito, A.; Kurusu, Y. Chem. Commun. 2000,
2009. (b) Marshall, J. A.; Yu, R. H.; Perkins, J. F. J. Org. Chem. 1995, 60,
5550. (c) Marshall, J. A.; Grant, C. M. J. Org. Chem. 1999, 64, 696. (d)
Suzuki, M.; Morita, Y.; Noyori, R. J. Org. Chem. 1990, 55, 441. (e) Corey,
E. J.; Yu, C. M.; Lee, D. H. J. Am. Chem. Soc. 1990, 112, 878.
(8) (a) Ogoshi, S.; Nishida, T.; Shinagawa, T.; Kurosawa, H. J. Am.
Chem. Soc. 2001, 123, 7164. (b) Ogoshi, S.; Fukunishi, Y.; Tsutsumi, K.;
Kurosawa, H. Chem. Commun. 1995, 2485.
(9) Representative Experimental Procedure. A mixture of 4-chloro-
benzaldehyde (71 mg, 0.5 mmol) and (3-bromo-prop-1-ynyl)-benzene (195
mg, 1 mmol) in THF (2 mL) was slowly added to a stirred solution
containing â-SnO (101 mg, 0.75 mmol) and [Rh(COD)Cl]2 (5 mg, 0.02
mmol) in THF (2.5 mL) and H2O (0.5 mL), which was previously refluxed
for 30 min. The suspension was refluxed at 70 °C under an inert atmosphere
for 14 h (TLC monitoring on silica gel, eluent ethyl acetate-hexane 1:9
v/v). An aqueous solution of NH4F (15%, 10 mL) was added to the reaction
mixture, and the organic layer was extracted with diethyl ether (3 × 10
mL), washed with water (2 × 10 mL) and brine (2 × 10 mL), and dried
over magnesium sulfate. Solvent removal followed by column chromatog-
raphy (eluent n-hexanes-ethyl acetate; gradient elution) afforded pure 1-(4-
chloro-phenyl)-2-phenyl-buta-2,3-dien-1-ol (99 mg, 77% with respect to
aldehyde).
(10) (a) Ma, S. Acc. Chem. Res. 2003, 36, 701. (b) Hashmi, A. S. K.
Angew. Chem., Int. Ed. 2000, 39, 3590.
2138
Org. Lett., Vol. 6, No. 13, 2004

Table 2. Rh(I)-Catalyzed Carbonyl Allenylation/Propargylation
Org. Lett., Vol. 6, No. 13, 2004
2139

rhodium(III) products from the oxidative addition reactions
of Rh(SbPh3)3(CO)X (X ) Cl, Br) with phenyl propargyl
halides.11 We therefore propose that the carbonyl allenylation
reactions proceed by the pathway A f B′ f C′ (Scheme
1).
Scheme 3
The Rh(I) to â-SnO transmetalation selectivity from
propargylrhodium to propargyltin prompted us to explore
next the fate of an allenylrhodium(III) in the reaction.
Reactions of trimethylsilylpropargyl bromide 5 with Wilkin-
son’s catalyst at ambient temperature in methylene chloride
showed the formation of an allenylrhodium(III) species.12
In a representative experiment trimethylsilyl propargyl
bromide 5 (0.05 mmol) and Wilkinson’s catalyst (0.01 mmol)
were allowed to react for 6 h in methylene chloride, followed
by concentration and addition of hexane to precipitate a
yellow-brown solid. 1H NMR of the solid showed the signal
due to the allenyl proton of the intermediate at δ 5.3, in
contrast to the methylene proton signal of free bromide 5 at
δ 3.9. Most interestingly, further reaction of 5 and 6 with
aldehydes in the presence of â-SnO and catalytic [Rh(COD)-
Cl]2 afforded the corresponding homopropargylic alcohols
in moderate to good yields (Table 2, entries 11-16). No
allenic alcohols were observed in any of these cases. This
suggests a transmetalation selectivity from allenyl rhodium-
(III) to allenylltin(IV) and the likely reaction pathway to be
A′ f B f C (Scheme 1).
one of major factors in controlling the regioselectivity in the
end organic product. Equally important is the redox trans-
metalation step from rhodium to â-SnO (step B) to generate
the corresponding organotin(IV) intermediate (III or IV) in
which the metallotropic rearrangement is completely arrested.
Further work is underway to establish the exact nature of
the tin(IV) intermediate and to understand the stereoelectronic
factors that stabilize such intermediates and thereby arrest
the metallotropic rearrangement between propargylmetal and
allenylmetal.
In summary, we have developed a rhodium(I)-catalyzed
highly regioselective either/or transmetalation strategy for
the synthesis of allenic and homopropargyl alcohols, which
are key intermediates in organic synthesis. Although a
mechanistic rationale is yet to develop, the likely bond-
forming steps are shown in Scheme 3.
Acknowledgment. We thank CSIR for financial support
The remarkable ability of Rh(I) to facilitate a regioselective
oxidative addition reaction (step A) generating either a
propargylrhodium or an allenylrhodium species (I or II) is
and a senior research fellowship to M.B.
Supporting Information Available: Experimental pro-
cedures including spectral data. This material is available
free of charge via the Internet at http://pubs.acs.org.
(11) Kayan, A.; Wojcicki, A. Inorg. Chim. Acta 2001, 319, 187.
(12) Kayan, A.; Galluci J. C.; Wojcicki, A. Inorg. Chem. Commun. 1998,
1, 446.
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