Trans- and cis-selective Lewis acid catalyzed hydrogermylation of alkynes

Article abstract of DOI:10.1021/ol0521026

(Chemical Equation Presented) The first examples of Lewis acid catalyzed hydrogermylation of alkynes have been demonstrated. It was found that this method has much higher functional group compatibility compared to the known Lewis acid catalyzed hydrosilyl

Full text of DOI:10.1021/ol0521026

ORGANIC
LETTERS
2005
Vol. 7, No. 23
5191-5194
Trans- and Cis-Selective Lewis Acid
Catalyzed Hydrogermylation of Alkynes
Todd Schwier and Vladimir Gevorgyan*
Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor Street,
Chicago, Illinois 60607-7061
Vlad@uic.edu
Received September 1, 2005
ABSTRACT
The first examples of Lewis acid catalyzed hydrogermylation of alkynes have been demonstrated. It was found that this method has much
higher functional group compatibility compared to the known Lewis acid catalyzed hydrosilylation and hydrostannation reactions. Remarkably,
the stereochemical outcome of this hydrogermylation reaction depends on the nature of the alkyne used: proceeding via a trans-addition
pathway with simple alkynes and cis-addition with propiolates. Mechanistic studies strongly support the proposed rationale on the origins of
cis-selectivity in the hydrogermylation of activated alkynes.
Hydrometalation of alkynes is a simple and efficient ap-
proach to vinylmetals, useful synthetic intermediates in
organic chemistry.1 Normally, transition-metal-catalyzed
hydrometalation serves as a mild protocol toward cis-addition
products.2,3 Alternatively, trans-addition products can be
obtained via Lewis acid catalyzed hydrometalations of C-C
multiple bonds (eq 1).4,5 Unfortunately, these methods exhibit
low functional group tolerance,6,7 thus justifying development
of milder trans-selective general hydrometalation.8 Herein,
we report the first Lewis acid catalyzed hydrogermylation
reaction,9 which not only exhibits higher functional group
tolerance in comparison to the previously reported Lewis
acid catalyzed hydrometalation reactions but also exhibits
perfect trans-addition for simple alkynes and cis-addition for
propiolates (eq 2).
(1) Langkopf, E.; Schinzer, D. Chem. ReV. 1995, 95, 1375.
(2) (a) Ojima, I. In The Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, Z., Eds.; John Wiley: Chichester, 1989; p 1479. (b)
Marciniec, B. ComprehensiVe Handbook on Hydrosilylation; Pergamon
Press: Oxford, 1992. (c) Hiyama, T.; Kusumoto, T. In ComprehensiVe
Organic Synthesis; Trost, B. M., Fleming, I., Eds. Pergamon Press: Oxford,
1991; Vol. 8, p 763.
(3) An isolated case of transition-metal-catalyzed trans-addition has
recently been disclosed. See: (a) Trost, B. M.; Ball, Z. T. J. Am. Chem.
Soc. 2001, 123, 12726. (b) Trost, B. M.; Ball, Z. T.; Jo¨ge, T. J. Am. Chem.
Soc. 2002, 124, 7922.
(4) For Si, see: (a) Oertle, K.; Wetter, H. Tetrahedron Lett. 1985, 26,
5511. (b) Asao, N.; Sudo, T.; Yamamoto, Y. J. Org. Chem. 1996, 61, 7654.
(c) Sudo, T.; Asao, N.; Gevorgyan, V.; Yamamoto, Y. J. Org. Chem. 1999,
64, 2494. (d) Song, Y.-S.; Yoo, B. R.; Lee, G.-H.; Jung, I. N. Organo-
metallics 1999, 18, 3109. (e) Rubin, M.; Schwier, T.; Gevorgyan, V. J.
Org. Chem. 2002, 67, 1936.
During our continuing studies on B(C6F5)3-catalyzed
reductions with hydrosilanes,10 we found this system to be
(6) The harsh conditions employed limited the functional group compat-
ibility to bulky TBS- or TIPS-protected alcohols; see refs 4 and 5.
(7) (Me3Si)3SiH was used in the radical and Lewis acid catalyzed hydro-
silylation of of terminal propiolates, though the stereochemistry of addition
is not established. See: Liu, Y.; Yamazaki, S.; Yamabe, S. J. Org. Chem.
2005, 70, 556.
(5) For Sn, see: Asao, N.; Liu, J.-X.; Sudoh, T.; Yamamoto, Y. J. Org.
Chem. 1996, 61, 4568.
10.1021/ol0521026 CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/12/2005

highly efficient for the cleavage of alkyl-aryl ethers10b and
for the hydrosilylation of olefins.4e While investigating the
functional group compatibility of these reactions, we dis-
covered that treatment of ambident substrate 1a with 1 equiv
of triethylsilane resulted in quantitative formation of the
cleavage product 1b with no traces of hydrosilylation
products detected (eq 3). It was hypothesized that the
chemoselectivity of the reaction can be affected by switching
silicon to a metal of lower oxophilicity. To this end, we
examined the reactivity of 1a with triethylgermane. To our
great delight, this reaction resulted in clean hydrogermyla-
tion without disturbing the methoxy moiety, eVen with excess
germane (eq 3)!
Table 1. Trans-Hydrogermylation of Simple Alkynes
Encouraged by this result, we tested hydrogermylation of
different alkynes under these reaction conditions (Table 1).
It was found that, regardless of the hydrogermane used, the
reaction with 1c proceeded with perfect regio- and stereo-
selectivity, providing the corresponding (Z)-vinylgermanes
in virtually quantitative yield (entries 1-4). Hydrogermy-
lation of symmetrically substituted alkynes 1d and 1e
proceeded uneventfully to give the corresponding products
in excellent yields (entries 5 and 6). Reaction with unsym-
metrical tolanes 1f and 1g provided the stilbene derivatives
with high regioselectivity and high yields (entries 7 and 8).
Likewise, terminal alkyne 1h gave (Z)-vinylgermane 3hb
in 96% yield (entry 9). Aryl alkynes possesing halogen at
the aromatic ring reacted uneventfully to produce the
corresponding vinylgermanes in quantitative yield (entries
10 and 11). Furyl-containing alkyne 1k reacted smoothly to
give the (Z)-vinylgermane 3kb in 89% yield (entry 12).
Remarkably, the pinacolborane moiety did not interfere with
the hydrogermylation reaction: the corresponding vinyl-
germane was obtained quantitatively (entry 13).
It deserves mention that this hydrometalation methodology
exhibits wider functional group tolerance compared to that
displayed in previously known Lewis acid catalyzed meth-
odologies. Indeed, anisole derivative 1f and furyl derivative
1k would not be tolerated in any previous Lewis acid
catalyzed hydrometalation reactions,4 whereas employment
of aryl-containing metal hydride 2d (entry 4) would lead to
a decrease in yields.11 Furthermore, it should be emphasized
that this method offers wider functional group tolerance vs
a Isolated yields.
radical hydrogermylation,8 which would not tolerate aryl
halides 1i and 1j.12
We propose the following mechanism to account for the
trans-selective hydrogermylation of simple alkynes (Scheme
1). Equilibration between germane 2 and B(C6F5)3 produces
ate-complex a, analogous to that formed from hydrosilane
(8) For trans-selective radical hydrogermylation of alkyl-substituted
alkynes with triphenylgermane, see: Ichinose, Y.; Nozaki, K.; Wakamatsu,
K.; Oshima, K. Tetrahedron Lett. 1987, 28, 3709.
(9) EtAlCl2 was used in the functionalization of Ge(100) surfaces, though
the stereoselectivity of this transformation was not investigated. See: Choi,
K.; Buriak, J. M. Langmuir 2000, 16, 7737.
(10) (a) Gevorgyan, V.; Liu, J.-X.; Rubin, M.; Benson, S.; Yamamoto,
Y. Tetrahedron Lett. 1999, 40, 8919. (b) Gevorgyan, V.; Rubin, M.; Benson,
S.; Liu, J.-X.; Yamamoto, Y. J. Org. Chem. 2000, 65, 6179. (c) Gevorgyan,
V.; Rubin, M.; Liu, J.-X.; Yamamoto, Y. J. Org. Chem. 2001, 66, 1672.
(11) It was observed that employment of arylsilanes led to decreased
reaction yields in the hydrosilylation of alkynes (see ref 4b) and Friedel-
Crafts alkylations of the aryl groups at silicon in the hydrosilylation of
olefins (see ref 4d).
(12) Nakamura, T.; Yorimitsu, H.; Shinokubo, H.; Oshima, K. Bull.
Chem. Soc. Jpn. 2001, 74, 747.
5192
Org. Lett., Vol. 7, No. 23, 2005

and B(C6F5)3, proposed by Piers.13 Direct reversible addition
of coordinated germylium14 to alkyne produces vinyl cation
b. The regiochemistry of this step is governed by the stability
It is believed that hydrogermylation of propiolates follows
a different mechanistic path than that for nonactivated
alkynes (Scheme 2). Reversible coordination of propiolate
Scheme 1. Mechanism for Trans-Hydrogermylation
Scheme 2. Mechanism for Cis-Hydrogermylation
of the forming cation, which is in perfect agreement with
to B(C6F5)3 produces zwitterionic complex c, which abstracts
hydride from germane 216 to produce allenolate d. Trapping
of the latter with germylium-type species occurs from the
less hindered face, cis to H, thus leading to cis hydrogemy-
lation products 4.17
To gain additional support for this mechanism, we tested
the hydrogermylation of deuterium analogue d-1i (eq 4). If
the above idea on the origins of cis-selectivity is correct,
trapping allenolate d generated from d-1i (R1 ) D) with
germylium species should have no facial preference, thereby
generating an equimolar mixture of cis- and trans-addition
products d-3ib and d-4ib. Indeed, the experiment resulted
in a 1:1 mixture of d-3ib and d-4ib, thus confirming the
proposed rationale.
the observed regiochemistry of the products. Vinyl cation b
-
is quenched by bulky hydride, HB(C6F5)3, from the least
hindered site to give trans-addition product 3 and regenerate
the catalyst.15
Investigating the scope of hydrogermylation, we found that
the ester group can be perfectly tolerated in this reaction!
Thus, hydrogermylation of 2 with propiolates proceeded with
very good yields (Table 2). Remarkably, in striking contrast
Table 2. Cis-Hydrogermylation of Differently Substituted
Propiolates
To test the synthetic utility of hydrogermylation, we
pursued transformation of the obtained vinylgermanes into
synthetically useful vinylhalides. As expected, halode-
germylation of 3cb proceeded efficiently, producing vinyl
halides 5 in excellent yields (eq 5).18 Iododegermylation of
4 under slightly modified conditions proceeded smoothly,
affording vinyl iodides 6 in good yields (eq 6). Importantly,
all halodegermylation reactions proceeded with perfect
retention of the double-bond geometry.19
a Isolated yields.
to all known Lewis acid catalyzed hydrometalations, this
reaction proceeded with cis-selectiVity! A variety of terminal
(entry 1), alkyl- (entries 2-3), and aryl-substituted propi-
olates (entries 4-5) reacted to give vinyl germanes 4 in high
yields.
(13) Parks, D. J.; Blackwell, J. M.; Piers, W. E. J. Org. Chem. 2000,
65, 3090.
(14) For a stoichiometric reaction between solvated germylium cation
and alkene, see: Lambert, J. B.; Zhao, Y.; Wu, H. J. Org. Chem. 1999, 64,
2729.
(15) This mechanism is analogous to that proposed by Yamamoto for
Zr-catalyzed hydrostannation of alkynes (see ref 5) and by us for the
hydrosilylation of alkenes (see ref 4e).
(16) It was shown that hydrogermanes are excellent hydride donors for
carbocations. See: Mayr, H.; Basso, N. Angew. Chem., Int. Ed. Engl. 1992,
31, 1046.
(17) An analogous mechanism was proposed for the AlCl3-catalyzed
hydrosilylation of terminal propiolates. See ref 6.
Org. Lett., Vol. 7, No. 23, 2005
5193

In summary, we have demonstrated the first examples of
Lewis acid catalyzed hydrogermylation of alkynes. The
hydrogermylation reaction exhibits excellent chemoselectivity
compared to the analogous hydrosilylation reaction owing
to the lower oxophilicity of germanium relative to silicon.
Depending on the nature of alkyne, the reaction proceeds
with high trans- or cis-selectivity. It was also shown that
halodegermylation of obtained vinylgermanes proceeded
smoothly to afford the corresponding (Z)- and (E)-vinyl
halides in high chemical yield, with complete preservation
of the double-bond geometry. This methodology allowed for
efficient synthesis of bifunctional vinylgermanes 3ib, 3jb,
and 3lb which can serve as either electrophilic components
in transition-metal-catalyzed cross-coupling reactions at the
aryl halide moiety (3ib and 3jb) or as the nucleophilic
component in the Suzuki cross-coupling reaction (3lb)
followed by transformation to the respective vinyl halides
via halodegermylation.
Acknowledgment. The support of the Petroleum Re-
search Fund (AC1-40691), administered by the American
Chemical Society, and of the National Science Foundation
(CHE 0354613) is gratefully acknowledged.
Supporting Information Available: Experimental de-
tails. This material is available free of charge via the Internet
at http://pubs.acs.org.
(18) For halodegermylation reactions, see: Oda, H.; Morizawa, Y.;
Oshima, K.; Nozaki, H. Tetrahedron Lett. 1984, 25, 3221.
(19) GC/MS analysis of the reaction mixtures showed formation of a
single product 6. Chromatographic isolation of which results in formation
of a trace amount (<5%) of Z-vinyl iodide.
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Org. Lett., Vol. 7, No. 23, 2005