γ-silyl group effect in hydroalumination and carbolithiation of propargylic alcohols

Article abstract of DOI:10.1002/anie.200503176

(Chemical Equation Presented) Making the right substitution: A remarkable acceleration effect of phenyl-substituted silyl groups in hydroalumination and carbolithiation reactions of propargylic alcohols (see formula) is described. The results show that an

Full text of DOI:10.1002/anie.200503176

Communications
Hydroalumination
group-selective functionalization of intermediate 1, which has
three alkynyl groups on C4 and C5 [Eq. (3); Bn = benzyl,
TBDPS = tert-butyldiphenylsilyl, TBS = tert-butyldimethyl-
silyl, TMS = trimethylsilyl]. Surprisingly enough, hydroalu-
mination of alcohol 1 with sodium bis(2-methoxyethoxy)
aluminum hydride (Red-Al) provides vinylsilane 2 exclu-
sively.[5]
DOI: 10.1002/anie.200503176
g-Silyl Group Effect in Hydroalumination and
Carbolithiation of Propargylic Alcohols**
Kazunobu Igawa and Katsuhiko Tomooka*
The hydrometalation reaction of an alkynyl group is a
valuable process, not only for functionalized alkene synthesis
but also for alkenyl metal preparations.[1] Among the
numerous variants of this class of reaction, hydroalumination
of propargylic alcohols is especially well utilized in many
facets of organic synthesis owing to its promising chemo- and
stereoselectivity [Eq. (1); L = ligand, E = electrophile].[2,3]
Our purpose for the total synthesis was achieved by this
group-selective transformation; however, a question that
immediately arose was why the reaction preferentially
occurred at the alkyne substituted with the bulkier TBDPS
group, rather than one substituted with the less bulky TMS
group.[6] At an earlier stage, we suspected an influence of the
multiple oxy functionalities on 1 to be the origin of the
peculiar selectivity; these functionalities may exert chelation
control on the aluminum reagent.[7] Thus, in order to
eliminate this confusing factor, we examined a competitive
hydroalumination in a series of simplified 1,1-bis(alkynyl)
alcohols 3 bearing TBDPS and TMS groups at the g and g’
positions (Table 1).
Herein, we report a noticeable accelerating effect of a g-
silyl substituent on the hydroalumination reaction, an effect
that can substantially enhance the synthetic utility of this
classical transformation. By proper choice of the silyl group,
highly group-selective functionalization in polyalkynyl alco-
hols A has been accomplished by overcoming the steric
influences, to provide valuable multifunctionalized alcohols B
through hydroalumination and also carbolithiation [Eq. (2)].
The reaction of sec-alcohol 3a (R = H) with Red-Al in
toluene at 08C provides vinylsilane (E)-4a (95%) predom-
inantly, along with a trace amount of (E)-5a (3%; TBDPS
side/TMS side 97:3; Table 1, entry 1).[8] Similar selectivity was
Table 1: Reduction of 1,1-bis(alkynyl) alcohol 3.
We have found this unprecedented silyl effect during the
course of our synthetic study of zaragozic acid A.[4] As one of
the key steps in this total synthesis, we faced the problem of
Entry
3
R
Reagent Solvent, T
(equiv)
Yield [%][a]
TBPS side/
TMS side
4/(5+6)
4[b]
5[b]
6
[*] Dr. K. Igawa, Prof. Dr. K. Tomooka
Department of Applied Chemistry
Graduate School of Science and Engineering
Tokyo Institute of Technology
1
2
3a
3a
H
H
Red-Al
(1.1)
toluene,
08C
95[c]
3
n.d.[d] 97:3
LiAlH4
(3.0)
THF,
58[c]
6
n.d.[d] 91:9
À78 to
À108C[e]
toluene,
08C
Meguro-ku, Tokyo 152-8552 (Japan)
Fax: (+81)3-5734-3931
E-mail: ktomooka@apc.titech.ac.jp
3
4
3b Me Red-Al
88
5
n.d.[d] 95:5
(1.1)
[**] This research was supported in part bya Grant-in-Aid for Scientific
Research on PriorityAreas (A) “Creation of BiologicallyFunctional
Molecules” (Grant No. 16073209), byan ExploratoryResearch grant
(Grant No. 17655038), and byThe 21st CenturyCOE Program
“Creation of Molecular Diversityand Development of Function-
alities” of the Ministryof Education, Culture, Sports, Science, and
Technology, Japan.
3b Me H2, Lind- MeOH, RT n.d.[d] 70[f] 19
<1:99
lar cat.
(0.11)
1
[a] Yield of isolated product. [b] >95% E by H NMR analysis. [c] The
product with the TMS group removed (X=H) was obtained in 7–16%
yield. [d] n.d.=not detected by 1H NMR and TLC analyses. [e] THF=
tetrahydrofuran. [f] The E/Z ratio was determined by 1H NMR analysis to
be 83:17.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
232
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 232 –234

Angewandte
Chemie
observed when LiAlH4 was employed instead of Red-Al
(TBDPS side/TMS side 91:9; entry 2) and also in the reaction
of tert-alcohol 3b (TBDPS side/TMS side 95:5; entry 3). From
these results, it was clarified that the peculiar selectivity of the
reaction of 1 was caused by the g-silyl group of the propargylic
moiety. In sharp contrast with the above-mentioned hydro-
alumination, a partial reduction of 3b by catalytic hydro-
genation (H2/Lindlar catalyst) preferentially gave product 5b,
from reduction of the less-hindered TMS-substituted alkyne,
and the overreduced product 6b (TBDPS side/TMS side
< 1:99; entry 4). These results obviously show that the
TBDPS substituent, rather than the TMS group, accelerates
hydroalumination on the attached ethyne functionality,
despite its steric disadvantage. A stereoelectronic effect of
the phenyl group on silicon (see below) is regarded as the
origin of this TBDPS effect. Based on this postulate, we
anticipated that a triphenylsilyl group (TPS) would have a
much higher ability to accelerate hydroalumination than that
of TBDPS. To confirm this idea and to determine the order of
magnitude of the rate acceleration by various substituents, we
examined the competitive hydroalumination of 1,1-bis-
(alkynyl) alcohols 7a–g, which have various combinations of
g substituents; the results are summarized in Table 2.[8]
supported by DFT calculations of the simple TPS- or
TBDPS-substituted propargyl alcohols 10 and 11, in which
the lowest unoccupied molecular orbital (LUMO) of the
alcohol consists of the p* orbital of the phenyl groups, the s*
À
orbital of the Si C bond, and the p* orbital of the ethyne
moieties (Figure 1).[11] The calculated LUMO energy levels of
10 and 11 are significantly lower than that of the TMS-
substituted counterpart 12 (10: À0.9965 eV, 11: À0.9954 eV,
12: À0.4438 eV).[12–15]
Table 2: Hydroalumination of 1,1-bis(alkynyl) alcohol 7.
Entry
7
X
Y
Yield
8/9[b]
(8+9) [%][a]
1
2
3
4
5
6
7
7a
7b
7c
7d
7e
7 f
7g
tBu
TMS
TIPS[c]
TBDPS
TPS
Ph
Ph
91
97
100
93
>99:1
55:45
88:12
72:28
94:6
TMS
TIPS[c]
TBDPS
TMS
TBDPS
Ph
Figure 1. LUMOs of silyl-substituted propargyl alcohols 10–12 and
Mulliken charges on the alkyne carbon atoms.
89
40[d]
79
55:45
78:22
TPS
A similar accelerating effect was observed in an inter-
molecularly competitive variant, in which the reaction of an
equimolar mixture of TBDPS- and TMS-substituted prop-
argylic alcohols 13 and 14 with Red-Al gave vinylsilane 15 as
the major product after 5 min (TBDPS side/TMS side 92:8)
[Eq. (4)].
[a] Yield of isolated product. [b] Determined by 1H NMR analysis.
[c] TIPS=triisopropylsilyl. [d] 1,1-Bis(alkenyl) alcohol (doubly reduced
product) was obtained as a byproduct in 18% yield.
As can be seen from the results of 7a and 7e (Table 2,
entries 1 and 5), the acceleration effect of TMS is higher than
that of the alkyl group, but lower than that of the phenyl
group.[9] Among the examined substituents, the TPS group
shows the highest aptitude, as we expected (see results for 7d
and 7g; entries 4 and 7). From these results, the order of the
magnitude of the rate enhancement was determined to be:
TPS > Ph ꢀ TBDPS > TIPS ꢀ TMS @ alkyl. Thus, the
observed significant acceleration effect of TBDPS and TPS is
explainable by the increase of electrophilicity of the b-carbon
atom on the alkynyl group caused by the phenyl groups on the
silicon atom. The alkynyl group acts as an electrophile
(hydride acceptor) in a hydroalumination with an aluminum
ate complex,[2a,3i] and the phenyl groups on the TBDPS and
TPS moieties should raise the electrophilicity of the ethyne
due to a hyperconjugation of the phenyl and ethyne p orbitals
Moreover, proper combination of silyl groups may also
enable group-selective functionalization in 1,1,1-tris-
(alkynyl)alcohol systems. For instance, the reaction of alcohol
17, having TPS, TMS, and methyl groups at the g positions,
with Red-Al provided vinylsilane 18 as the sole product in
81% yield [Eq. (5)].
[10]
À
through a Si C s* bond.
This supposition was well
Angew. Chem. Int. Ed. 2006, 45, 232 –234
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
233

Communications
4901; g) B. Grant, C. Djerassi, J. Org. Chem. 1974, 39, 968 – 970;
h) S. E. Denmark, T. K. Jones, J. Org. Chem. 1982, 47, 4595 –
4597; i) K. Kakinuma, T. Matsuzawa, T. Eguchi, Tetrahedron
1991, 47, 6975 – 6982; j) B. Caro, M.-C. SØnØchal-Tocquer, F. R.-
L. Guen, P. LePoul, J. Organomet. Chem. 1999, 585, 43 – 52; k) K.
Ohmori, T. Suzuki, K. Taya, D. Tanabe, T. Ohta, K. Suzuki, Org.
Lett. 2001, 3, 1057 – 1060; l) T. Suzuki, K. Ohmori, K. Suzuki,
Org. Lett. 2001, 3, 1741 – 1744; m) K. Ohmori, Y. Hachisu, T.
Suzuki, K. Suzuki, Tetrahedron Lett. 2002, 43, 1031 – 1034.
[4] K. Tomooka, M. Kikuchi, K. Igawa, M. Suzuki, P.-H. Keong, T.
Nakai, Angew. Chem. 2000, 112, 4676 – 4679; Angew. Chem. Int.
Ed. 2000, 39, 4502 – 4505.
[5] It is well known that hydroalumination of a propargylic alcohol
system proceeds significantly faster than that of a homopropar-
gylic alcohol system; a) S. Ma, F. Liu, E. Negishi, Tetrahedron
Lett. 1997, 38, 3829 – 3832; b) B. Crousse, M. Alami, G.
Linstrumelle, Synlett 1997, 992 – 994.
Finally, we examined expansion of this methodology to a
carbon–carbon bond-forming reaction, that is, carbolithiation
of propargylic alcohol.[16] The reaction of alcohol 7c with
nBuLi in the presence of N,N,N’,N’-tetramethylethylenedi-
amine (TMEDA) gave allylic alcohol 19, with a trisubstituted
alkene moiety, in 80% yield with excellent group selectivity.
The stereochemistry of the resulting alkene was determined
as the E configuration by transformation to the g-lactone 20
through CO2 trapping of vinyl lithium [Eq. (6)].
[6] The difference in bulkiness between TMS and TBDPS plays an
important role in the diastereoselectivity of the [1,2]-Wittig
À
rearrangement which forms a C C bond between C4 and C5; see
ref. [4].
[7] Quite recently, Suzukiꢀs group has developed a chelation-
controlled diastereoselective hydroalumination in a 1,1-bis-
(alkynyl) alcohol system. They performed stereoselective con-
struction of continuous pseudoquaternary chiral centers by using
this method; see ref. [3k–m].
[8] The produced alcohols (that is, 4 and 5) were easily distinguished
1
by H and 13C NMR analysis after selective desilylation at the
alkyne terminal.
[9] It has been reported that a phenyl substituent increases the
electrophilicity of alkynes. For example, Slaughꢀs group reported
that the reaction of diphenylacetylene with LiAlH4 proceeds
markedly faster than the same reaction of dialkylacetylene: E. F.
Magoon, L. H. Slaugh, Tetrahedron 1967, 23, 4509 – 4515.
In summary, we have described a remarkable acceleration
effect of phenyl-substituted silyl groups in hydroalumination
and carbolithiation reactions of propargylic alcohols. These
results clearly show that an arylsilyl group can act not only as
a protecting group but also as an activating group of the
alkynyl moiety. This work provides an efficient group-
selective approach to multifunctionalized alkenes. Further
work is underway in our group to expand the utility of this
interesting phenomenon.
À
[10] Hyperconjugative interaction between the Si C s* orbitals and
the p* orbitals of the phenyl and alkynyl moieties was observed
by electron transmission spectroscopy in trimethylsilylbenzene
and trimethylsilylacetylene: a) A. Modelli, D. Jones, G. Diste-
fano, Chem. Phys. Lett. 1982, 86, 434 – 437; b) J. C. Giordan, J. H.
Moore, J. Am. Chem. Soc. 1983, 105, 6541 – 6544; c) J. C.
Giordan, J. Am. Chem. Soc. 1983, 105, 6544 – 6546.
[11] It has been reported that hydroalumination of alkynes with an
ate-complex aluminum reagent is strongly dependent on the
electrophilicity of the alkyne. Kakinuma, Matsuzawa, and
Eguchi discussed the relationship between the LUMO level of
the alkyne and the reactivity of the propargylic alcohols in
hydroalumination with LiAlH4; see ref. [3i].
Received: September 7, 2005
Published online: November 28, 2005
Keywords: alkenes · carbolithiation · hydroalumination ·
.
hyperconjugation · substituent effects
[12] These calculations were performed at the B3LYP/6À31 + G(d)//
B3LYP/6À31G(d) level with Gaussian 98.
[13] For more detailed discussion, calculation of a transition-state
model is needed.
[1] For leading reviews of hydrometalation, see: a) J. A. Labinger in
Comprehensive Organic Synthesis, Vol. 8 (Eds.: B. M. Trost, I.
Fleming), Pergamon, Oxford, 1991, pp. 667 – 702; b) N. Asao, Y.
Yamamoto, Bull. Chem. Soc. Jpn. 2000, 73, 1071 – 1087.
[2] For reviews of hydroalumination, see: a) G. Zweifel, J. A. Miller
in Organic Reactions, Vol. 32 (Ed.: W. G. Dauben), Wiley, New
York, 1984, pp. 375 – 517; b) J. J. Eisch in Comprehensive
Organic Synthesis, Vol. 8 (Eds.: B. M. Trost, I. Fleming),
Pergamon, Oxford, 1991, pp. 733 – 761.
[3] For representative studies on hydroalumination of propargylic
alcohols, see: a) J. D. Chanley, H. Sobotka, J. Am. Chem. Soc.
1949, 71, 4140 – 4141; b) E. B. Bates, E. R. H. Jones, M. C.
Whiting, J. Chem. Soc. 1954, 1854 – 1860; c) B. Franzus, E. I.
Snyder, J. Am. Chem. Soc. 1965, 87, 3423 – 3429; d) E. I. Snyder,
J. Org. Chem. 1967, 32, 3531 – 3534; e) E. J. Corey, J. A.
Katzenellenbogen, G. H. Posner, J. Am. Chem. Soc. 1967, 89,
4245 – 4247; f) W. T. Borden, J. Am. Chem. Soc. 1970, 92, 4898 –
[14] We have found that measurement of the Dd value of the ethyne
carbon atoms (db-carbonÀdg-carbon) in 13C NMR analysis can predict
the reactivity of silyl-substituted propargylic alcohols. Actually,
the Dd value of 10 was higher than that of the less reactive one
(10: 22.4; 11: 21.8; 12: 13.1), which means that the Dd value
shows a polarization of the ethyne carbon atoms and electro-
deficiency on the b-carbon atom.
[15] Recently, Gevorgyanꢀs group reported that regiochemistry of the
palladium-catalyzed hydrostannation is predictable from the Dd
value of the ethyne carbon atoms in a diarylacetylene system; M.
Rubin, A. Trofimov, V. Gevorgyan, J. Am. Chem. Soc. 2005, 127,
10243 – 10249.
[16] Carbolithiation of propargylic alcohols has been reported in only
a few cases of highly activated alkynyl systems. For the reaction
of g-phenylpropargyl alcohol, see: L.-I. Olsson, A. Claesson,
Tetrahedron Lett. 1974, 2161 – 2162.
234
www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 232 –234