Facile Alder-Ene Reactions of Silylallenes Involving an Allenic C(sp2)-H Bond

Article abstract of DOI:10.1002/chem.201503243

Facile and selective Alder-ene reactions of silylallenes involving the activation of an allenic C(sp²)-H over an allylic C(sp³)-H bond is described. In this ene reaction, the presence of a silyl substituent was found to be critical for the observed reactivity and selectivity since the corresponding alkyl-substituted allenes show different reaction profiles. Computational studies show that the origin of this unusual reactivity is the lower bond dissociation energy of the α-C(sp²)-H bond in silylallenes compared to the corresponding nonsilylated allenes.

Full text of DOI:10.1002/chem.201503243

DOI: 10.1002/chem.201503243
Communication
&
Selective CÀH Activation
Facile Alder-Ene Reactions of Silylallenes Involving an Allenic
C(sp2)ÀH Bond
Venkata R. Sabbasani,[a] Genping Huang,[c] Yuanzhi Xia,*[b] and Daesung Lee*[a]
Based on the reported ene reaction of simple allenes,[5] we
Abstract: Facile and selective Alder-ene reactions of silyl-
surmised that the corresponding ene reaction of silylallenes[6]
allenes involving the activation of an allenic C(sp2)ÀH over
might promote an uncommon reaction involving an allenic
an allylic C(sp3)ÀH bond is described. In this ene reaction,
C(sp2)ÀH.[7] To explore this possibility, we systematically exam-
the presence of a silyl substituent was found to be critical
ined various silylallenes for their ene reaction behavior,[8] and
for the observed reactivity and selectivity since the corre-
herein we report the general trend of the ene reactivity and
sponding alkyl-substituted allenes show different reaction
selectivity of silylallenes, which unexpectedly favors the allenic
profiles. Computational studies show that the origin of
C(sp2)ÀH bond over an allylic C(sp3)ÀH bond.
this unusual reactivity is the lower bond dissociation
We commenced our investigation engaging alkyl-substituted
energy of the a-C(sp2)ÀH bond in silylallenes compared to
silylallenes 1a–g (Table 1). Under typical conditions (1.2 equiv
the corresponding nonsilylated allenes.
Table 1. Alder-ene reaction of silylallenes with azodicarboxylate.
In the ene reactions of an allylic CÀH bond with an enophile,[1]
a good orbital alignment of an allylic C(sp3)ÀH bond of ene-
donors and the p-systems of enophiles[1,2] is required
[Eq. (1)].[3] Although less common, ene reactions involving
a C(sp2)ÀH bond to generate allene or alkyne products are re-
Entry Silylallene
Product
Ratio 2/3 Yield
[%][b]
ported [Eq. (2)].[4] To the best of our knowledge, the favorable
participation of an allenic C(sp2)ÀH over an allylic C(sp3)ÀH
bond in the Alder-ene reaction has not been reported [Eq. (3)].
1
2
1a R=Me
1b R=H
2a
2b
1:0
1:0
76
63
3
4
1c n=1
1d n=2
2c
2d
1:0
1:0
68
55
5
6
1e Silyl=SiEt3
1 f Silyl=SiMe2tBu
2e
2 f
1:0
1:0
77
72
7
1g
2g
1:0
72
[a] Conditions: diisopropylazodicarboxylate (1.2 equiv), CH2Cl2, 288C, 16–
24 h. [b] Isolated yields after purification.
[a] V. R. Sabbasani, Prof. D. Lee
Department of Chemistry, University of Illinois at Chicago
845 West Taylor Street, Chicago, IL, 60607 (USA)
E-mail: dsunglee@uic.edu
diisopropyl azodicarboxylate, CH2Cl2, 288C, 16–24 h), silylal-
lenes 1a and 1b provided 38 and 28 propargylic hydrazodicar-
boxylates 2a and 2b in 76 and 63% yields, respectively, with-
out allylic ene products 3a and 3b (Table 1, entries 1 and 2).[9]
The ene reaction of 1c and 1d, with a tethered prenyl group,
provided 2c and 2d in 68 and 55% yields without the ene re-
action occurring at the prenyl group (entries 3 and 4). Different
silyl groups other than trimethylsilyl (SiMe3) in allene[6k,10] 1e
[b] Prof. Y. Xia
College of Chemistry and Materials Engineering, Wenzhou University
Wenzhou, Zhejiang Province 325035 (P.R. China)
[c] Dr. G. Huang
Department of Chemistry, School of Science
Tianjin University, Tianjin 300072 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503243.
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(SiEt3) and 1 f (SiMe2tBu) did not affect the reaction profile, and
thus products 2e and 2 f were obtained in 77 and 72% yields,
respectively (entries 5 and 6). Similarly, silylallene 1g contain-
ing a cyclopropyl group afforded 2g in 72% yield (entry 7).
The contrasting reactivity of alkyl-substituted allenes involv-
ing an allylic C(sp3)ÀH bond participation was demonstrated
with allenes 1h and 1i (Scheme 1). Under the identical condi-
Scheme 1. Alder-ene reactions of alkyl-substituted allenes with diisopropyl
azodicarboxylate (DIAD).
tions, tert-butyl- and n-butyl-substituted allenes 1h and 1i pro-
vided 3h–l (linear) and 3h–b (branched) (2:1) in a 93% yield,
and 3i–l and 3i–b (2:1) in a 91% yield, respectively.[11] Ene re-
action products 2h and 2i involving the allenic C(sp2)ÀH
bonds were not observed. This opposite ene preference of 1h
and 1i compared to that of silylallenes implies a strong activat-
ing role of the silyl group for the Alder-ene reactions of allenic
C(sp2)ÀH bonds.
Figure 1. Calculated energies and geometries for the ene reaction of tBu-
and Me3Si-substituted allenes with azodicarboxylate.
The substituent effect on allene reactivity was further dem-
onstrated by a deuterium-labeled allene [D]1a. Compared to
1a, generating exclusively 2a with the participation of an al-
lenic C(sp2)ÀH bond within 16 h (entry 1 in Table 1), the corre-
sponding monodeuterated allene [D]1a provided a mixture of
[D]2a and [D]3a in a 1:3 ratio after a much longer time (48 h).
This contrasting reactivity between 1a and [D]1a clearly indi-
cates that the allenic C(sp2)ÀD bond lowers the rate of the ene
reaction of silyl–allene [D]1a compared to that of 1a.
To understand the substituent-dependent ene reactivity and
selectivity of allenic C(sp2)ÀH and allylic C(sp3)ÀH bonds, DFT
calculations for the reactions of 1a and 1h were carried out
with a slightly simplified azodicarboxylate (CO2Me instead of
CO2iPr) (Figure 1). Optimization and frequency calculations
were performed in CH2Cl2 at the (SMD)M06-2X/6-311++
G(2d,p) level of theory.[12,13] The calculated free energy values
(DG) are in good agreement with the experimental results for
which the silyl-substituted transition-state a-TS-Hb is preferred
to a-TS-Ha by 2.3 kcalmolÀ1 for 1a, leading favorably to 2a
(Figure 1a). On the contrary, the reaction of tert-butyl-substitut-
ed allene 1h proceeds via h-TS-Ha, which is 6.1 kcalmolÀ1
lower in energy than h-TS-Hb, thus only 3h was obtained.
Although both Alder-ene reactions involving the allenic
C(sp2)ÀH and allylic C(sp3)ÀH bonds are concerted according
to IRC calculations, different geometric structures were calcu-
lated. As exemplified by the silyl-substituted TS in Figure 1b, it
was found that in a-TS-Ha the incipient N2ÀC2 bond (1.93 Š) is
much shorter than the corresponding N2ÀC3 bond in a-TS-Hb
(2.37 Š), and the C4ÀHa bond being broken in a-TS-Ha is only
slightly elongated (1.13 Š), while the corresponding C1ÀHb
bond in a-TS-Hb is much longer (1.29 Š). Similar geometries
were calculated for h-TS-Ha and h-TS-Hb, as shown in Fig-
ure S1, Supporting Information.
Based on these geometric structures, we surmised that the
activation energy for the reaction with an allylic C(sp3)ÀH bond
via TS-Ha should be determined predominately by the interac-
tion between N2 and C2, and thus the electron-rich allene
should be more reactive. This is further supported by the fact
that the relative energy for h-TS-Ha is 1.2 kcalmolÀ1 lower than
that for a-TS-Ha, as natural population analysis (NPA) showed
the C2 atom is less positively charged in 1h than in 1a (Fig-
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Communication
ure 1c). On the other hand, the energy required for allenic
C(sp2)ÀH activation should be more relevant to the strength of
the C1ÀHb bond, which is substantially being broken in TS-Hb.
Theoretical support for this supposition was obtained by the
calculated bond dissociation energies in Figure 1c, for which
the D0 (C1ÀHb) is quite close to the D0 (C4ÀHa) for tert-butyl-
substituted allene 1h, and only marginal variation in D0 (C4À
Ha) is predicted upon changing tert-butyl to SiMe3 for 1a. On
the other hand, lowering as much as 5 kcalmolÀ1 in D0 (C1ÀHb)
was calculated for 1a, indicating dramatic activation of the a-
C(sp2)ÀH bond by the silyl group.[14] The relatively low bond
dissociation energy of the a-C(sp2)ÀH bond in silylallenes is be-
lieved to be a major factor rendering different reactivity and
selectivity for silylallenes in their Alder-ene reaction, otherwise
TS-Hb would be disfavored due to the higher energy required
for distortion of the silylated allene moiety and its hybridiza-
tion pattern change.[13]
Table 2. Ene reaction selectivity of cycloalkyl-based silylallenes.
Entry Silylallene
Ratio 2/3
Yield
[%][b]
1
76
92
90
2
3
Next, we examined the reactivity of cyclic silylallenes 1j–t
(Table 2), and the results of typical substrates were subse-
quently validated by DFT calculations (Table 3). Under the typi-
cal conditions, cyclobutanone-derived allene 1j afforded a mix-
ture of 2j and 3j (4.6:1) in 76% yield (Table 2, entry 1), but cy-
clopentane-based allene 1k provided only 3k in 92% yield
(entry 2). The calculated energy difference between TS-Ha and
TS-Hb for 1k was found to be 1.6 kcalmolÀ1, favoring 3k over
2k, which is in qualitative agreement with the observed ratio
of these products. The reaction of allene 1l provided 2l and 3l
in 90% combined yield with a 1.8:1 ratio (entry 3). The low se-
lectivity in this reaction is consistent with the marginal energy
4
5
73
91
75
94
78
difference between l-TS-Hb (28.5 kcalmolÀ1
) and l-TS-Ha
(29.1 kcalmolÀ1). The reaction of more substituted cyclohex-
ane-based silylallenes 1m and 1n showed much higher selec-
tivity but in opposite preference, generating 2m and 3n in
good yields (entries 4 and 5). These contrasting outcomes
might be the consequence of the steric effect of the substitu-
ents; the isopropyl group in 1m hinders the approach of azo-
dicarboxylate to the allylic hydrogen atom, whereas the cyclo-
propyl moiety in 1n would hinder the formation of an incipi-
ent CÀN bond at its vicinity. Cycloheptyl-containing silylallene
1o afforded 2o and 3o in 75% combined yield with a 1.5:1
ratio (entry 6), yet the corresponding tert-butyl- and n-butyl-
substituted allenes 1p and 1q afforded 3p and 3q exclusively
(entries 7 and 8). These examples further bolster the conclu-
sion that the silyl group promotes the ene reaction with the al-
lenic C(sp2)ÀH bond, which is further demonstrated by the
contrasting reactivity between cyclooctane-based allenes 1r
and 1s (entries 9 and 10). The reaction of silyl group-contain-
ing allene 1r engaged an allenic C(sp2)ÀH bond, affording
product 2r in 72% yield (entry 9), while that of tert-butyl-con-
taining allene 1s afforded an alternative ene product 3s (80%
yield) involving an allylic C(sp3)ÀH bond (entry 10). The con-
trasting behaviors of 1r and 1s were reproduced by computa-
tions, in which the activation energy for the allenic C(sp2)ÀH
bond of 1r is 1.2 kcalmolÀ1 lower than that with the allylic
6
7
8
72
80
9
10
11
84
[a] Reaction conditions: DIAD (1.2 equiv), CH2Cl2, 288C, 16–24 h. [b] Isolat-
ed yield for 2 and 3 after purification. [c] Diastereomeric mixture (1:1).
Table 3. Computational results for selected allenes (in kcalmolÀ1).
Allene
DG(TS-Ha)
DG(TS-Hb)
DDG[a]
2/3
0:1
1.8:1
1:0
1k
1l
1r
1s
25.2
29.1
26.9
26.0
26.8
28.5
25.7
31.9
À1.6
0.6
1.2
À5.9
0:1
[a] DDG=DG(TS-Ha)ÀDG(TS-Hb).
C(sp3)ÀH bond. On the other hand, tert-butyl-substituted al- uct. Silylallene 1t containing an extra double bond provided
lenes 1s showed the opposite preference by 5.9 kcalmolÀ1, 2t in 84% yield without the activation of its allylic CÀH bonds
which is consistent with the formation of 3s as the sole prod- (entry 11).
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The concerted nature of the Alder-ene reaction of silylallenes
with azodicarboxylate was illustrated by the reaction of chiral
silylallene 1u (optically pure) (Scheme 2). After the conversion
nificantly affect the selectivity trend, yet the corresponding
nonsilylated allenes engage allylic C(sp3)ÀH bonds favorably re-
gardless of their structural features. DFT calculations provide
further insight into the selectivity trend, and the dramatic
weakening of the a-allenic C(sp2)ÀH bond by the silyl group
was uncovered as the major factor for the different reactivity
of silyl- and alkyl-substituted allenes.
Acknowledgements
We thank UIC (LAS AFS, D.L.), NSF (CHE-0955972, D.L.),
TACOMA Technology (D.L.), the Zhejiang Provincial NSF
(LY13B020007, Y.X.), and the NNSF of China (21372178, Y.X.) for
financial support. All computations were carried out on the
High Performance Computation Platform of Wenzhou Universi-
ty.
Scheme 2. Application to the construction of a chiral pyrazoline containing
an amino-tertiary centre.
of 2u (the enantiomeric ratio was not determined at this
stage) to pyrazoline[15] 4u, its enantiomeric ratio was deter-
mined to be 92:8, which suggests that the ene reaction pro-
ceeded mainly in a concerted manner.[10,16] The result not only
validates the DFT predictions for the concerted mechanism
but also bodes well for potential utility of the process to con-
struct various chiral molecules containing an amino-tertiary
center.[17]
Keywords: alder-ene reaction
azodicarboxylates · bond dissociation energy · silylallenes
·
allenic CÀH bonds
·
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Received: August 17, 2015
Published online on October 20, 2015
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