Rhodium(II)-catalyzed stereoselective synthesis of allylsilanes

Article abstract of DOI:10.1021/ol4028978

The rhodium-catalyzed decomposition of 2-(triisopropylsilyl)ethyl aryl- and vinyldiazoacetates results in the stereoselective formation of Z-allylsilanes. The transformation is considered to proceed by silyl-directed intramolecular C-H functionalization to form a β-lactone intermediate followed by a silyl-activated extrusion of carbon dioxide.

Full text of DOI:10.1021/ol4028978

ORGANIC
LETTERS
2013
Vol. 15, No. 24
6120–6123
Rhodium(II)-Catalyzed Stereoselective
Synthesis of Allylsilanes
David M. Guptill, Carolyn M. Cohen, and Huw M. L. Davies*
Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta,
Georgia 30322, United States
hmdavie@emory.edu
Received October 8, 2013
ABSTRACT
The rhodium-catalyzed decomposition of 2-(triisopropylsilyl)ethyl aryl- and vinyldiazoacetates results in the stereoselective formation of Z-allylsilanes.
The transformation is considered to proceed by silyl-directed intramolecular CÀH functionalization to form a β-lactone intermediate followed by a silyl-
activated extrusion of carbon dioxide.
Allylsilanes represent a privileged class of reagents in
organic synthesis. They undergo a wide variety of chemical
transformations1,2 and have been used extensively as
building blocks in the synthesis of complex molecules.3
As such, there are many methods for the synthesis
of allylsilanes.4,5 As with any alkene, the preparation of
allylsilanes of defined geometry can be challenging. New,
complementary methods for the stereoselective generation
of allylsilanes are therefore desirable. Herein we describe a
stereoselective synthesis of Z-allylsilanes by means of
rhodium-catalyzed reactions of 2-(triisopropylsilyl)ethyl
aryl- and vinyldiazoacetates.
Recently, while investigating the use of 2-(trimethylsilyl)-
ethyl diazoacetate 1a in carbenoid transformations, we
observed an unexpected product, allylsilane 2a (Scheme 1),
formed in 29% yield as an 80:20 mixture of Z/E alkene
isomers with Rh2(S-DOSP)4 as the catalyst. While rhodium-
carbenoid chemistry has been used previously to prepare
allylsilanes by SiÀH insertion,6,2a the potential utility of
a novel method for the synthesis of allylsilanes led us to
explore the reaction further.
(1) For reviews, see: (a) Brook, M. A. Silicon in Organic, Organome-
tallic and Polymer Chemistry; Wiley Interscience: New York, 2000. (b)
Fleming, I. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I.,
Eds.; Pergamon Press: Oxford, 1991; Vol. 2, pp 563À593. (c) Sarkar, T. K.
In Science of Synthesis, Houben-Weyl Methods of Molecular Transfor-
mations; Georg Thieme Verlag: Stuttgard, 2001; Vol. 4, pp 901À918. (d)
Chan, T. H.; Wang, D. Chem. Rev. 1995, 1279–1292. (e) Masse, C. E.;
Panek, J. S. Chem. Rev. 1995, 95, 1293–1316.
(2) For some recent examples, see: (a) Wu, J.; Chen, Y.; Panek, J. S.
Org. Lett. 2010, 12, 2112–2115. (b) Grote, R. E.; Jarvo, E. R. Org. Lett.
2009, 11, 485–488. (c) Kinnaird, J. W. A.; Ng, P. Y.; Kubota, K.; Wang,
X.; Leighton, J. L. J. Am. Chem. Soc. 2002, 124, 7920–7921. (d) Panek,
J. S.; Yang, M. J. Am. Chem. Soc. 1991, 113, 9868–9870.
(3) For reviews, see: (a) Langkopf, E.; Schinzer, D. Chem. Rev. 1995,
95, 1375–1408. (b) Chabaud, L.; James, P.; Landais, Y. Eur. J. Org.
Chem. 2004, 3173–3199.
(4) For reviews, see: (a) Sarkar, T. K. In Science of Synthesis, Houben-
Weyl Methods of Molecular Transformations; Georg Thieme Verlag:
Stuttgart, 2001; Vol. 4, pp 837À900. (b) Sarkar, T. K. Synthesis 1990,
1101–1111.
We first examined the influence of a variety of rhodium
catalysts (Table 1) and found that the catalyst has a
marked effect on both the yield and the Z/E ratio of the
allylsilane products. The somewhat bulky chiral catalysts
(entries 1À3) gave moderate Z/E ratios (74:26 to 83:17)
and moderate yields (42À57%). The majority of achiral
catalysts (entries 4À8) showed essentially no stereochemical
preference and gave low to moderate yields of the product.
The one exception was the bulky catalyst, rhodium(II)
tetrakis(triphenylacetate),7 Rh2(TPA)4 (entry 9), which
gave the allylsilane 2a with a Z/E ratio of 84:16 and in
70% yield. With this catalyst, the optimal conditions were
(6) (a) Wu, J.; Panek, J. S. J. Org. Chem. 2011, 76, 9900–9918. (b)
Davies, H. M. L.; Hansen, T.; Rutberg, J.; Bruzinski, P. R. Tetrahedron
Lett. 1997, 38, 1741–1744. (c) Bulugahapitiya, P.; Landais, Y.;
Parra-Rapado, L.; Planchenault, D.; Weber, V. J. Org. Chem. 1997,
62, 1630–1641.
(7) Hashimoto, S.; Watanabe, N.; Ikegami, S. Tetrahedron Lett.
1992, 33, 2709–2712.
(5) For some recent methods, see: (a) Takeda, M.; Shintani, R.;
Hayashi, T. J. Org. Chem. 2013, 78, 5007–5017. (b) Ito, H.; Horita, Y.;
Sawamura, M. Adv. Synth. Catal. 2012, 354, 813–817. (c) McLaughlin,
M. G.; Cook, M. J. J. Org. Chem. 2012, 77, 2058–2063. (d) Selander, N.;
ꢀ
Paasch, J. R.; Szabo, K. J. J. Am. Chem. Soc. 2011, 133, 409–411.
r
10.1021/ol4028978
Published on Web 11/27/2013
2013 American Chemical Society

Table 2. Optimization of Substrate
Scheme 1. Discovery of Allylsilane Reaction
Z/E
ratioa
yield
(%)b
entry
R
product
1
2
3
4
5
6
SiEt3
2b
2c
2d
2e
2f
91:9
95:5
76
79
82
80
70
68
SiMe2t-Bu
Si(i-Pr)3
SiMe2Ph
SiMePh2
SiPh2t-Bu
Table 1. Optimization of Allylsilane Formationc
>97:3
88:12
89:11
96:4
2g
a Determined by 1H NMR analysis of the crude reaction mixture.
b Isolated yield of the mixture of isomers.
Z/E
ratioa
yield
(%)b
entry
Rh(II)
solvent
PhCF3
temp
the allylsilanes in g97:3 Z/E ratios and good yields
(64À74%). The Z/E ratio dropped slightly with an ortho-
chloro substituent, and allylsilane 4h was formed in 77%
yield and with a 94:6 Z/E ratio (entry 8). A 2-naphthyl aryl
group was effective in this chemistry (entry 9), and 4i was
formed in 74% yield with a >97:3 Z/E ratio. It was also
possible to form conjugated polyenes 4j and 4k by using
styryldiazoacetate 3j and phenyldienyldiazoacetate 3k as
substrates (entries 10À11) with control over alkene geo-
metry of each alkene (93:7 to95:5 Z/E ratio). The relatively
low isolated yield (37%) of 4k was at least partially due to
product instability. The reaction could be extended to the
formation of the trisubstituted allylsilane4l, but inthis case
competing side reactions were observed.9
1
Rh2(S-DOSP)4
Rh2(S-BTPCP)4
Rh2(S-PTAD)4
Rh2(Oct)4
70
70
70
70
70
70
70
70
70
40
81
83
80:20
74:26
83:17
47:53
48:52
50:50
51:49
46:54
84:16
85:15
87:13
89:11
57
45
42
48
58
17
25
42
70
36
64
76
2
PhCF3
3
PhCF3
4
PhCF3
5
Rh2(Piv)4
PhCF3
6
Rh2(pfb)4
PhCF3
7
Rh2(TFA)4
Rh2(esp)2
PhCF3
8
PhCF3
9
Rh2(TPA)4
Rh2(TPA)4
Rh2(TPA)4
Rh2(TPA)4
PhCF3
10
11
12
PhCF3
cyclohexane
1,2-DCE
a Determined by 1H NMR analysis of the crude reaction mixture.
b Isolated yield of the mixture of isomers. c See references for preparation
and structures of Rh2(DOSP)4, Rh2(BTPCP)4, and Rh2(PTAD)4.8
The reaction can also be extended to form chiral allylsi-
lanes by using esters derived from chiral alcohols. As can
be seen from Scheme 3, yields and selectivities are respect-
able for the formation of allylsilanes 6aÀc (47À68% yield
and 90:10 to >97:3 Z/E ratio).
found to be 1,2-dichloroethane (1,2-DCE) at reflux
(entry 12) and allylsilane 2a was isolated in 76% yield
with a Z/E ratio of 89:11.
Having established the optimal conditions, we next
explored the influence of the silyl group on the reaction
(Table 2). The use ofbulkier alkyl silyl groups(entries 1À3)
resulted in improved Z/E ratios of the products 2bÀ2d,
without decreasing of the yield. In the case of the
triisopropylsilyl diazo 1d, the allylsilane 2d was isolated
as essentially a single isomer in 82% yield. Aryl-substituted
silyl groups generally did not perform as well as their alkyl-
substituted counterparts (entries 4 and 5), giving 2e and 2f
in good yields (70À80%) but without improvement of the
Z/E ratio relative to 2a. The tert-butyldiphenylsilyl deriv-
ative 1g, however, resulted in the formation of allylsilane
2g in 68% yield as a 96:4 mixture of Z and E isomers,
respectively.
With the knowledge that the bulky triisopropylsilyl
group is optimal for formation of Z-allylsilanes, we next
evaluated the scope of the reaction (Scheme 2) relative to
the diazo donor group. Both elecron-rich (entries 1À2) and
electron-poor (entries 3À4) aryl groups performed well,
giving the allylsilane products in good yields (60À77%),
and with good Z/E ratios (95:5 to >97:3). Halogenated
aryl groups were also well tolerated (entries 5À7), giving
Enantiomerically enriched allyl silanes can be formed
by a sequence using two rhodium-catalyzed reactions
(Scheme 4). The Rh2(S-DOSP)4-catalyzed reaction of
phenyldiazoacetate 7a with tert-butyldimethylsilane re-
sults in SiÀH insertion and the formation of 8a in 88%
yield and 86% ee. The stereochemistry of 8a, and products
derived from it, were tentatively assigned by comparison to
previously reported results in a similar system.6b Reduc-
tion of 8a with DIBAL-H gave 9a in 89% yield. Esterifica-
tion and diazo transfer produced (R)-5a in 39% yield over
two steps. The Rh2(TPA)4 catalyzed conversion of (R)-5a
to the allylsilane 6a required slightly more vigorous con-
ditions than those used for the simpler systems, but still 6a
was formed with retention of the enantioenrichment.
(8) (a) For DOSP: Davies, H. M. L.; Bruzinski, P. R.; Lake, D. H.;
Kong, N.; Fall, M. J.; York, N.; Forest, W.; Uni, V.; Carolina, N. J. Am.
Chem. Soc. 1996, 118, 6897–6907. (b) For BTPCP: Qin, C.; Boyarskikh,
V.; Hansen, J. H.; Hardcastle, K. I.; Musaev, D. G.; Davies, H. M. L.
J. Am. Chem. Soc. 2011, 133, 19198–19204. (c) For PTAD: Reddy, R. P.;
Lee, G. H.; Davies, H. M. L. Org. Lett. 2006, 8, 3437–3440.
(9) Triisopropylsilylacetone was also formed, in 39% yield, presum-
ably by a hydride abstraction mechanism.
Org. Lett., Vol. 15, No. 24, 2013
6121

Scheme 2. Scope of the Donor Groupa,b
Scheme 4. Synthesis of Enantioenriched Allylsilane 6a
Scheme 5. Control Reactions
1
a Z/E ratios determined by H NMR analysis of the crude reaction
mixtures. b Yields refer to isolated yields.
Scheme 3. Synthesis of Chiral Allylsilanesa,b
be isolated as a single, cis diastereomer. Further, when 12 was
heated in toluene at reflux, it was smoothly converted to the
allylsilane 6a, as exclusively the Z isomer.
From the above experiments, and other observa-
tions noted below, we propose the following conclusions:
(1) β-lactones are likely intermediates in the formation of the
allylsilanes. The isolation and characterization of 12 is the
primary evidence.10 Additionally, when nonpolar solvents
are used, β-lactones can be observed in the crude mixture
1
by H NMR, though they are unstable and proved not
to be isolable. (2) The presence of the silicon is important
for a facile rearrangement of the β-lactone intermediates.11
This is supported by the isolability and thermal stability
of 10. Since a TMS and a t-Bu group are similar sterically,
this could be due to an electronic effect. The solvent effect
mentioned above supports this as well. (3) The rearrangement
1
a Z/E ratios determined by H NMR analysis of the crude reaction
mixtures. b Yields refer to isolated yields.
In the interest of elucidating the mechanism of this
transformation, we ran a series of control reactions
(Scheme 5). For reference, the reaction of trimethylsilyl-
ethyl derivative 1a to give 2a is shown. First, to determine
the effect of the silicon group, we replaced the TMS with
a tert-butyl group. Under the standard conditions the
β-lactone 10 was formed in 84% yield. β-Lactone 10 showed
no signs of decomposition after 16 h at reflux in PhCF3. When
an n-Bu ester was used, however, a mixture of β-lactone 11a
and γ-lactone 11b was formed in a 1:1 ratio. Finally, with the
more highly funtionalized diazoacetate 5a, β-lactone 12 could
(10) Some examples of β-lactone formation by intramolecular carbe-
noid CÀH insertion: (a) Doyle, M. P.; May, E. J. Synlett 2001, 967–969.
(b) Doyle, M. P.; Davies, S. B.; May, E. J. J. Org. Chem. 2001, 66, 8112–
8119. (c) Balaji, B. S.; Chanda, B. M. Tetrahedron Lett. 1998, 39, 6381–
6382.
(11) Rearrangements of β-lactones to give alkenes are known, though
harsh conditions are required: (a) Adam, W.; Nava-Salgado, V. O.
J. Org. Chem. 1995, 60, 578–584. (b) Fleming, I.; Sarkar, A. K. J. Chem.
Soc., Chem. Commun. 1986, 1199–1201. (c) Mulzer, J.; Pointner, A.;
Chucholowski, A.; Bruntrup, G. J. Chem. Soc., Chem. Commun. 1979,
52–54.
6122
Org. Lett., Vol. 15, No. 24, 2013

of the intermediate β-lactones is stereospecific. This is sup-
ported by the rearrangement of a single diastereomer of 12 to
a single isomer of 6a and the dramatic effect of the rhodium
catalyst on the Z/E selectivity (Table 1). The stereospecific
rearrangement of β-lactones is precedented in the literature
as well.11 (4) The silicon directs the CÀH insertion to the
β-position. Since 10 is formed from a bulky ester while the less
crowded n-Bu ester leads to a mixture of 11a and 11b,
the silicon is likely exhibiting a steric effect: directing
the CÀH insertion away from the more crowded R-position
(and the formation of a 5-membered ring such as 11b).
Nevertheless, an electronic directing effect cannot necessarily
be ruled out: The silicon could electronically disfavor CÀH
insertion (and the resulting build-up of positive charge
character12) at the R position and/or favor CÀH insertion
β to silicon.
Scheme 7. Stereochemical Rationale
With a reasonable mechanistic proposal in place, we
next provide a rationale for the stereochemical outcome
of this reaction (Scheme 7). As previously discussed, it is
likely the diastereoselectivity of the CÀH insertion step is
responsible for the stereochemistry of the allylsilane pro-
duct. We considered two diastereomeric transition states for
this step (TS1 and TS2). In transition state TS1, the bulky
trialkylsilyl group is positioned away from the triphenylace-
tate ligands. In transition state TS2, however, the trialkylsilyl
group would be expected to experience a disfavorable steric
interaction with the ligands. The preferred pathway (TS1)
ultimately leads to the observed major Z product.
Scheme 6. Proposed Mechanism for Allylsilane Formation
In summary, these studies demonstrate that 2-(trialkylsilyl)-
ethyl aryl- and vinyldiazoacetates are effective reagents for
the synthesis of allylsilanes. With 2-(triisopropylsilyl)ethyl
esters and Rh2(TPA)4 as the catalyst, Z-allylsilanes are
produced exclusively. The reaction is considered to pro-
ceed by an intramolecular CÀH insertion followed by a
β-lactone rearrangement. The high selectivity and novelty
of this method for the synthesis of allylsilanes is likely to be
of utility to the synthetic community.
With these observations, we propose a mechanism that
occurs in three steps (Scheme 6). First, rhodium-catalyzed
extrusionofnitrogenfromdiazo 13generatestherhodium-
carbenoid intermediate 14. Second, the carbenoid under-
goes an intramolecular CÀH insertion β to the silicon,
forming the intermediate β-lactone 16.13 Third, the silicon
facilitates the polarization of the lactone CÀO bond (17),
from which the loss of CO2 and concomitant double bond
formation gives the observed allylsilane 18.
Acknowledgment. This work was supported by NSF
under the CCI Center for Selective CÀH Functionalization,
CHE-1205646.
(12) Hansen, J.; Autschbach, J.; Davies, H. M. L. J. Org. Chem. 2009,
74, 6555–6563.
(13) For some reviews concerning intramolecular CÀH insertion,
see the following, and references contained therein: (a) Doyle, M.;
McKervey, M.; Ye, T. Modern Catalytic Methods for Organic Synthesis
with Diazo Compounds: From Cyclopropanes to Ylides; Wiley: New York,
1998. (b) Taber, D. F. InComprehensive Organic Synthesis; Pattenden, G.,
Ed.; Pergamon Press: Oxford, U.K., 1991; Vol. 3, Chapter 4.2. (c) Davies,
H. M. L.; Dai, X. In Comprehensive Organometallic Chemistry III;
Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier, 2007; pp 167À212.
Supporting Information Available. Full experimental
data. This material is available free of charge via the
Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 24, 2013
6123