Cyclization/Hydrosilylation of Functionalized Diynes Catalyzed by a Cationic Platinum Phenanthroline Complex

Article abstract of DOI:10.1021/ol006901u

(Equation Presented) A 1:1 mixture of the platinum phenanthroline complex (phen)PtMe₂ and B(C₆F₅)₃ catalyzed the cyclization/hydrosilylation of functionalized 1,6- and 1,7-diynes to form silylated 1,2-dialkylide

Full text of DOI:10.1021/ol006901u

ORGANIC
LETTERS
2001
Vol. 3, No. 3
385-388
Cyclization/Hydrosilylation of
Functionalized Diynes Catalyzed by a
Cationic Platinum Phenanthroline
Complex
James W. Madine, Xiang Wang, and Ross A. Widenhoefer*
Duke UniVersity, P. M. Gross Chemical Laboratory,
Durham, North Carolina 27708-0346
rwidenho@chem.duke.edu
Received November 20, 2000
ABSTRACT
A 1:1 mixture of the platinum phenanthroline complex (phen)PtMe2 and B(C F5)3 catalyzed the cyclization/hydrosilylation of functionalized 1,6-
6
and 1,7-diynes to form silylated 1,2-dialkylidenecycloalkanes in good yield and with high Z-selectivity.
The cyclization/hydrosilylation of dienes, catalyzed by both
cationic palladium1 and neutral yttrocene2 complexes, has
emerged as a useful method for the synthesis of silylated
cycloalkanes. Similarly, both yttrocene3 and low-valent
rhodium complexes4 catalyze the cyclization/hydrosilylation
of enynes to form silylated alkylidenecycloalkanes. In
addition, several cyclization/addition protocols employing
diynes are known.5 In contrast, the cyclization/hydrosilylation
of diynes, particularly 1,6-diynes, to form 1,2-dialkyl-
idenecycloalkanes remains problematic. For example, Ni(0)
complexes catalyze the cyclization/hydrosilylation of 1,7-
diynes to form silylated (Z)-1,2-dialkylidenecyclohexanes but
do not cyclize 1,6-diynes.6 Rhodium phosphine7 and carbo-
nyl8 complexes catalyze the cyclization/hydrosilylation of
1,6-diynes but these procedures suffer from several key
limitations.9 The absence of a selective and general catalyst
system for the cyclization/hydrosilylation of diynes is
unfortunate as the 1,2-dialkylidenecycloalkanes formed in
these transformations are useful synthetic intermediates.10 For
this reason, we have been working toward the development
of an efficient and general catalyst system for the cyclization/
hydrosilylation of diynes. Here we report the platinum-
catalyzed cyclization/hydrosilylation of functionalized 1,6-
(1) (a) Widenhoefer, R. A.; DeCarli, M. A. J. Am. Chem. Soc. 1998,
120, 3805. (b) Stengone, C. N.; Widenhoefer, R. A. Tetrahedron Lett. 1999,
40, 1451. (c) Widenhoefer, R. A.; Stengone, C. N. J. Org. Chem. 1999, 64,
8681. (d) Widenhoefer, R. A.; Vadehra, A. Tetrahedron Lett. 1999, 40,
8499. (e) Pei, T.; Widenhoefer, R. A. Org. Lett. 2000, 2, 1469. (f) Perch,
N. S.; Widenhoefer, R. A. J. Am. Chem. Soc. 1999, 121, 6960. (g) Perch,
N. S.; Pei, T.; Widenhoefer, R. A. J. Org. Chem. 2000, 65, 3836. (h) Pei,
T.; Widenhoefer, R. A. Tetrahedron Lett. 2000, 41, 7597.
(2) (a) Molander, G. A.; Nichols, P. J. J. Am. Chem. Soc. 1995, 117,
3705. (b) Molander, G. A.; Dowdy, E. D.; Schumann, H. J. Org. Chem.
1998, 63, 3386.
(3) (a) Molander, G. A.; Retsch, W. H. J. Am. Chem. Soc. 1997, 119,
8817. (b) Molander, G. A.; Corrette, C. P. J. Org. Chem. 1999, 64, 9697.
(4) Ojima, I.; Donovan, R. J.; Shay, W. R. J. Am. Chem. Soc. 1992,
114, 6580.
(5) (a) Trost, B. M.; Lee, D. C. J. Am. Chem. Soc. 1988, 110, 7255. (b)
Onozawa, S.; Hatanaka, Y.; Tanaka, M. Chem. Commun. 1997, 1229. (c)
Matsuda, I.; Eshibashi, H.; Ii, N. Tetrahedron Lett. 1995, 36, 241. (d)
Onozawa, S.; Hatanaka, Y.; Choi, N.; Tanaka, M. Organometallics 1997,
16, 5389. (e) Lautens, M.; Smith, N. D.; Ostrovsky, D. J. Org. Chem. 1997,
62, 8970.
(6) (a) Tamao, K.; Kobayashi, K.; Ito, Y. J. Am. Chem. Soc. 1989, 111,
6478. (b) Tamao, K.; Kobayashi, K.; Ito, Y. Synlett 1992, 539.
(7) Muraoka, T.; Matsuda, I.; Itoh, K. Tetrahedron Lett. 1998, 39, 7325.
(8) (a) Ojima, I.; Zhu, J.; Vidal, E. S.; Kass, D. F. J. Am. Chem. Soc.
1998, 120, 6690. (b) Ojima, I.; Donovan, R. J.; Banerji, P. J. Org. Chem.
1994, 59, 7594. (c) Ojima, I.; Kass, D. F.; Zhu, J. Organometallics 1996,
15, 5191.
(9) Rhodium phosphine complexes catalyze the cyclization/hydrosily-
lation of 1,6-diynes to form (E)-1,2-dialkylidenecyclopentanes but suffer
from limited substrate scope and low yield. In addition, the E/Z-selectivity
of these cyclizations was not reported.7 Rhodium carbonyl complexes also
catalyze the cyclization/hydrosilylation of 1,6-diynes but form primarily
disilylated mono alkylidenecyclopentanes and silylbicyclization products.8
(10) Schore, N. E. Chem. ReV. 1988, 88, 1081. (b) Vollhardt, K. P. C.
Angew. Chem., Int. Ed. Engl. 1984, 23, 539. (c) Trost, B. M.; Hipskind, P.
A.; Chung, J. Y. L.; Chan, C. Angew. Chem., Int. Ed. Engl. 1989, 28, 1502.
10.1021/ol006901u CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/06/2001

and 1,7-diynes to form silylated 1,2-alkylidenecycloalkanes
in good yield with high Z-selectivity.
The 1,2-dialkylidenecycloalkanes formed via platinum-
catalyzed diyne cyclization/hydrosilylation are active sub-
strates for Diels-Alder cyclization.17 For example, cyclo-
pentane 2 (20:1, Z/E) reacted with N-phenylmaleimide in
refluxing toluene overnight to form the tricyclic compound
3 in quantitative yield (102 ( 2%) as a single diastereomer
Cationic platinum complexes containing bidentate nitrogen
ligands have attracted recent interest as model compounds
for the study of the platinum-catalyzed oxidation of alkanes.11
These cationic platinum complexes are structurally and
electronically similar to the cationic palladium complexes
which we have employed as catalysts for the cyclization/
hydrosilylation of functionalized dienes.1 Because of this,
we considered that cationic platinum complexes might also
serve as active cyclization/hydrosilylation catalysts. To this
end, a solution of dimethyl dipropargylmalonate (1), HSiEt3,
and a catalytic 1:1 mixture of the platinum phenanthroline
complex (phen)Pt(Me)2 and the Lewis acid B(C6F5)3 in
toluene was heated at 110 °C and monitored periodically by
GC analysis.12-14 Diyne 1 reacted over the course of 1.5 h
to form a 20:1 mixture of (Z)- and (E)-1,1-dicarbomethoxy-
3-methylene-4-(triethylsilylmethylene)cyclopentane (2) which
together accounted for ∼95% of the products.15 Evaporation
of solvent and flash chromatography of the residue on neutral
alumina gave silylated cyclopentane 2 in 74% yield as an
18:1 mixture of Z:E isomers (Scheme 1).16
1
by H and 13C NMR spectroscopy (g20:1) (Scheme 2).
Scheme 2
Noteworthy is that Diels-Alder adducts such as 3 possess
an allylic silyl group and are therefore potentially reactive
toward a range of electrophiles.18
Scheme 1
We propose a working mechanism for platinum-catalyzed
diyne cyclization/hydrosilylation involving â-migratory in-
sertion of an alkyne into the Pt-Si bond of platinum silyl
alkyne complex I to form platinum alkenyl alkyne complex
II (Scheme 3). â-Migratory insertion of the coordinated
Scheme 3
A number of tertiary silanes reacted with diene 1 under
platinum catalysis to give the corresponding 1,2-dialkyl-
idenecyclopentanes in good yield and with good stereose-
lectivity (g12:1) (Table 1, entries 1-3). The procedure
tolerated a range of functional groups including esters, benzyl
and silyl ethers, acetals, sulfonates, and amides (Table 1,
entries 4-10). The procedure also tolerated propargyl
substitution to give products resulting from silyl transfer to
the less hindered alkyne as a mixture of Z/E isomers (Table
1, entry 11). Mixtures of (phen)Pt(Me)2 and B(C6F5)3 also
catalyzed the cyclization/hydrosilylation of a 1,7-diyne to
form a 1,2-dialkylidenecyclohexane in good yield and with
high (26:1) Z-selectivity (Table 1, entry 12).
alkyne into the Pt-C bond of II could form the platinum
dienyl species III which could oxidatively add silane to form
the platinum silyl hydride intermediate IV.19 C-H reductive
elimination from IV coupled with alkyne coordination would
then form diene 2 with regeneration of palladium silyl alkyne
(11) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am. Chem. Soc. 1999,
121, 1974. (b) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 1997, 119, 848. (c) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E.
J. Am. Chem. Soc. 1996, 118, 5961. (d) Holtcamp, M. W.; Henling, L. M.;
Day, M. W.; Labinger, J. A. Bercaw, J. E. Inorg. Chim. Acta 1998, 270,
467.
386
Org. Lett., Vol. 3, No. 3, 2001

Table 1. Cyclization/Hydrosilylation of Functionalized Diynes Catalyzed by a 1:1 Mixture of (phen)PtMe2 and B(C6F5)3 in Toluene at
110 °C
a Yields refer to isolated material of g95% purity. b E/Z ratio determined by GC analysis of the purified material. c Refers to isolated yield of the Diels-
Alder adduct with 4-phenyl[1,2,4]triazole-3,5-dione. d E/Z ratio determined by GC analysis of the crude reaction mixture.
intermediate I.20 Consistent with the proposed mechanism,
platinum-catalyzed reaction of 1 and DSiEt3 led to the
isolation of 2-d1 with exclusive incorporation of deuterium
into the Z position of the exocyclic methylene group (Scheme
4).21
cyclization/hydrosilylation of both 1,6- and 1,7-diynes to
form silylated 1,2-dialkylidenecyclopentanes and 1,2-dialkyl-
idenecyclohexanes, respectively, in good yield and with high
Z-selectivity. These 1,2-dialkylidenecycloalkanes are reactive
(12) The mixture of (phen)Pt(Me)2 and B(C6F5)3 forms the active cationic
catalyst in situ.13 Comparable results were observed when the preformed
cationic complex [(phen)Pt(Me)CH3CN]+ [MeB(C6F5)3]- was employed
as the catalyst. The individual precatalysts employed separately gave no
cyclization under reaction conditions.
(13) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. J. Chem. Soc., Dalton
Trans. 1996, 1809.
Scheme 4
(14) These catalysts were inactive toward both dienes and enynes.
(15) Addition of toluene to the precatalyst mixture prior to the diyne
led to immediate darkening of the solution and low subsequent conversion.
This observation suggests that the cationic platinum complex formed via
methyl abstraction is unstable in the absence of a coordinating ligand such
as an alkyne.
(16) The stereochemistry of both E-2 and Z-2 was determined by 1H
NOE. The remainder of the products consisted of disilylated compounds.
(17) (a) Craig, D. Chem. Soc. ReV. 1987, 16, 187. (b) Brieger, G.; Bennett,
J. N. Chem. ReV. 1980, 80, 63.
(18) (a) Yamamoto, Y.; Asao, N. Chem. ReV. 1993, 93, 2207. (b) Masse,
C. E.; Panek, J. S. Chem. ReV. 1995, 95, 1293.
In summary, cationic platinum complexes generated in situ
from mixtures of (phen)PtMe2 and B(C6F5)3 catalyzed the
Org. Lett., Vol. 3, No. 3, 2001
387

substrates for subsequent Diels-Alder cyclizations to form
fused polycyclic compounds which possess an allylic silyl
group.
thanks the Camille and Henry Dreyfus Foundation and
DuPont for young faculty awards and the Alfred P. Sloan
Foundation for a Research Fellowship. We thank Ms. H.
Annita Zhong for helpful discussions.
Acknowledgment is made to the National Institutes of
Supporting Information Available: Experimental pro-
cedures and spectroscopic and analytical data for relevant
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
Health (GM59830-01) for support of this research. R.W.
(19) Oxidative addition of hydrosilanes to both neutral Pt(II) diphosphine19a
and cationic Pt(II) diimine19b complexes has been demonstrated. (a) Pfeiffer,
J.; Kickelbick, G.; Schubert, U. Organometallics 2000, 19, 62. (b) Fang,
X.; Scott, B. L.; Watkins, J. G.; Kubas, G. J. Organometallics 2000, 19,
4193.
OL006901U
(20) Diyne 1 could also enter the catalytic cycle in the conversion of II
to III. Initial formation of I presumably occurs via H-Si oxidative addition
to a cationic Pt-Me complex followed by C-H reductive elimination.
(21) Determined by 1H and 13C NMR spectroscopy. Chromatographic
purification of 2-d1 on SiO2 led to diminished yield.
388
Org. Lett., Vol. 3, No. 3, 2001