Forming Stereogenic Centers in Acyclic Systems from Alkynes

Article abstract of DOI:10.1002/anie.201504756

The combined carbometalation/zinc homologation followed by reactions with α-heterosubstituted aldehydes and imines proceed through a chair-like transition structure with the substituent of the incoming aldehyde residue preferentially occupying a pseudo-axial position to avoid the two gauche interactions. The heteroatom in the axial position produces a chelated intermediate (and not a Cornforth-Evans transition structure for α-chloro aldehydes and imines) leading to a face differentiation in the allylation reaction. This method provides access to functionalized products in which three new carbon-carbon bonds and two to three stereogenic centers, including a quaternary one, were created in acyclic systems in a single-pot operation from simple alkynes. All-carbon quaternary stereocenter: The combined carbometalation/zinc homologation of alkynes followed by reactions with α-heterosubstituted aldehydes and imines provides access to functionalized acyclic adducts. These adducts obtained in a single-pot reaction have three new carbon-carbon bonds and two to three stereogenic centers, including a quaternary carbon stereocenter.

Full text of DOI:10.1002/anie.201504756

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International Edition: DOI: 10.1002/anie.201504756
German Edition: DOI: 10.1002/ange.201504756
Acyclic Stereoselection Very Important Paper
Forming Stereogenic Centers in Acyclic Systems from Alkynes**
Roxane Vabre, Biana Island, Claudia J. Diehl, Peter R. Schreiner, and Ilan Marek*
Abstract: The combined carbometalation/zinc homologation
followed by reactions with a-heterosubstituted aldehydes and
imines proceed through a chair-like transition structure with
the substituent of the incoming aldehyde residue preferentially
occupying a pseudo-axial position to avoid the two gauche
interactions. The heteroatom in the axial position produces
a chelated intermediate (and not a Cornforth–Evans transition
structure for a-chloro aldehydes and imines) leading to a face
differentiation in the allylation reaction. This method provides
Scheme 1. Diastereoselectivity in the g,g’-allylation reaction of carbonyl
groups.
access to functionalized products in which three new carbon–
carbon bonds and two to three stereogenic centers, including
a quaternary one, were created in acyclic systems in a single-pot
operation from simple alkynes.
man–Traxler chair-like transition structure in which the
I
n the past few decades, we have witnessed tremendous
incoming aldehyde aryl (alkyl) group occupies a pseudo-
equatorial position to avoid potential 1,3-diaxial steric
interactions with A as described for 1 (Scheme 1, A = sulfox-
ide, alkyl, oxazolidinone). However, the diastereoselectivity
resulting from the reaction of g,g’-disubstituted allylzinc (A =
H) with the same aldehydes lead to the opposite diastereo-
isomer as the same incoming aldehyde group now prefers to
occupy a pseudo-axial position to avoid the two gauche
interactions with the R1 and R2 groups (2, Scheme 1).[7] This
axial preference in the Zimmerman–Traxler transition struc-
ture for the allylation reactions has been successfully
extended to the allylation of ketones,[8a] a-alkoxyallylation
of aldehydes and ketones.[8b,c]
Beside this interesting mechanistic stereochemical out-
come, one potential advantage of having the substituent of the
carbonyl group occupying a pseudo-axial position would be
the ability to stereocontrol the allylation reactions of a-
heterosubstituted aldehydes. Indeed, in the double diaster-
eodifferentiation between chiral aldehydes and achiral allyl-
metal reagents, the a-heterosubstituted aldehydes tend to
display different facial selectivity with either (Z)- or (E)-
substituted allylmetal species.[9] In most cases where the
incoming aldehyde residue possessing a stereogenic center
occupies a pseudo-equatorial position, the stereochemistry of
the final products originates from minimum steric interactions
in the transition structure between the allylic partners and the
chiral aldehydes.[10] These observations show that the config-
uration of the allylmetal species and the aldehyde are
interdependent in determining the final stereochemical out-
come of the reaction and moderate to good diastereoselec-
tivities are obtained (Scheme 2, path A).[11] However, as
intramolecular chelation between zinc and heteroatoms (O,
S, N)[12] has been successfully used to control the stereochem-
ical outcome of various transformations,[13] and as the
incoming aldehyde group occupies a pseudo-axial position,
then this a-heteroatom should lead to a chelated transition
structure. Such chelation should then promote a face differ-
entiation in the g,g’-allylation reaction of prochiral carbonyl
achievements in the area of acyclic stereocontrol for the
construction of carbon–carbon as well as carbon–heteroatom
bonds in a stereoselective manner.[1] However, when struc-
tural complexity increases, the difficulty in reaching the
desired products usually leads to a single carbon–carbon
bond-forming event per chemical step between two compo-
nents.[2] One element of structure that invariably increases the
difficulty of a chemical synthesis is the presence of a quater-
nary carbon stereogenic center in the target molecule.[3,4] In
this context, polyfunctional intermediates[5] such as carbe-
noids species (Scheme 1)[6] could be used as efficient tools to
create consecutively the same number of carbon–carbon
bonds as in a multi-step process, including quaternary carbon
stereogenic centers, but in a single-pot operation. Particularly
interesting is the difference of stereochemical outcome for the
reaction of g,g’,b-trisubstituted allylzinc (Scheme 1, A = sulf-
oxide, alkyl, oxazolidinone) and g,g’-disubstituted allylzinc
(Scheme 1, A = H) with aldehydes. In the former case, the
diastereoselectivity of the reaction results from a Zimmer-
[*] Dr. R. Vabre,[+] M. Sc. B. Island,[+] Prof. Dr. I. Marek
The Mallat Family Laboratory of Organic Chemistry
Schulich Faculty of Chemistry and
Lise Meitner-Minerva Center for
Computational Quantum Chemistry
Technion-Israel Institute of Technology
Technion City, Haifa 32000 (Israel)
E-mail: chilanm@tx.technion.ac.il
B. Sc. C. J. Diehl, Prof. Dr. P. R. Schreiner
Institute of Organic Chemistry, Justus-Liebig University
Heinrich-Buff-Ring 58, 35392 Giessen (Germany)
[+] These authors contributed equally to this work.
[**] This research was supported by grants from the German-Israeli
project Cooperation (grant number DIP-597/19-1) and by the Israel
Science Foundation administrated by the Israel Academy of
Sciences and Humanities (140/12).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504756.
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Table 1: Diastereoselective formation of homoallylic alcohols 5a–g.
Entry
R
R1
R2 R3 R4 R5
d.r.[a]
Yield [%][b]
1
2
3
4
5
6
7
CH2I
CH2I
CH2I
CH2I
CH2I
Et (1a)
Bu Ph
H
H
H
H
Me 96:4:0:0 45 (5a)
Me 96:4:0:0 45 (5b)
TBS 96:4:0:0 42 (5c)
TBS 96:4:0:0 43 (5d)
Bu (1b) Et Ph
Bu (1b) Et Ph
Bu (1b) Et Et
Hex (1c) Et Ph Ph Me 97:3:0:0 53 (5e)
Me 94:6:0:0 58 (5 f)
OCOCF3 Hex (1c) Et Ph Ph Me 95:5:0:0 65 (5g)
OCOCF3 Hex (1c) Et Ph
H
[a] Determined by 1H NMR analysis of crude reaction mixture. [b] Yields
of the major diastereoisomer determined after purification by column
chromatography on silica gel.
Scheme 2. Known work (path A) and synthetic strategy presented
herein (path B).
derivatives as one face is shielded by the alkyl group R3 as
described in Scheme 2 (path B: this work). The proposed
Scheme is appealing but the success of this single-pot strategy
leading to complex molecular fragments with high diastereo-
isomeric ratio requires complete control of all parameters
summarized in Scheme 2, path B. We started our study by
performing a carbocupration reaction[14] on terminal alkyne
1a leading to vinyl copper species 2a. A zinc homologation/
allylation reaction was performed by successive addition of
Et2Zn and CH2I2 [that leads to the in situ formation of the
Simmons–Smith–Furukawa zinc carbenoid 3a Zn(CH2I)2][5,15]
and of the model 2-methoxy-2-phenylacetaldehyde 4a (R3 =
Ph, R4 = H, R5 = Me).
occupies a pseudo-axial position in the chair-like transition
structure to avoid the two gauche interactions with the g,g’-
disubstituted allylzinc species. As expected, the alkoxy group
chelates intramolecularly the zinc center inducing a differ-
entiation between the two prochiral faces of the carbonyl
group as described in the scheme above Table 1.[16] However,
despite the excellent diastereoselectivities, yields need to be
improved. Therefore, various carbenoid species were tested
(i.e. samarium and aluminium carbenoids) in the homologa-
tion reaction of vinyl copper and the use of the Shi carbenoid,
easily prepared by mixing equimolar amount of Et2Zn, CH2I2,
and trifluoroacetic acid (TFA),[17] was found to be the best
compromise for good diastereoselectivity and yield. Hence,
the addition of the Shi carbenoid ICH2ZnOCOCF3 to the
vinyl copper species 2 in the presence of either a-substituted
aldehyde or ketone led to the homoallylic alcohols 5 f,g in
better yields (based on three consecutive chemical steps in
one-pot reaction) and diastereoselectivities (Table 1, see
entries 6 and 7, respectively).
We were pleased to observe the formation of the expected
homoallylic alcohol 5a possessing three stereogenic centers,
including the formation of a quaternary carbon stereogenic
center, in outstanding diastereoselectivity (d.r. > 96:4:0:0)
but in moderate 45% yield, although based on three
consecutive chemical steps in
a single-pot operation
(Table 1, entry 1). To have a better mechanistic insight, we
then permuted the nature of the alkyl groups R1 and R2
present on the alkyne and on the organocopper, respectively,
and performed the same combined carbometalation/zinc
homologation/allylation reaction. The opposite diastereoiso-
mer 5b was obtained in identical diastereoisomeric ratio and
yield suggesting that the reaction proceeds through a cyclic
transition structure (Table 1, entry 2). To make the method-
ology synthetically more attractive, the methyl group of the
ether (R5 = Me, Table 1, entries 1 and 2) was replaced by an
easily removable silyl ether (R5 = SiMe2tBu) and similar
diastereoselectivities were observed (Table 1, entry 3). The
same type of diastereoselectivity was observed if the a-
substituent of the aldehyde was an alkyl group (R3 = Et,
Table 1, entry 4) or if a ketone was used (R4 = Ph, Table 1,
entry 5). The configuration has been determined by X-ray
analysis on 5e (see the Supporting Information) and suggests
that the biggest substituent of the carbonyl derivative
Having in hand an approach for acyclic stereocontrol of
several stereogenic centers including the formation of qua-
ternary carbon stereogenic centers such as 5a–g, we turned
our attention to the formation of products that could be easily
functionalized by standard chemical reactions. In this context,
we were particularly interested to investigate the behavior of
a-chloro aldehydes in our reaction sequence. Indeed, the
allylation of a-halo carbonyl derivatives is well-known to
proceed through a Cornforth–Evans transition structure
(non-chelation pathway where dipole-dipole interactions are
=
responsible for the anti-periplanar orientation of the C O
and C-X),[18] as illustrated by the addition of cinnamylzinc
chloride to 3-chlorobutan-2-one at low temperature.[19] On
the other hand, pioneering studies of Walsh showed that the
diastereoselectivity could be reversed through coordination
[20]
À
of the C Cl bond to metals with a-chloro sulfonyl imines.
As the incoming carbonyl alkyl group occupies a pseudo-axial
position in the allylation reaction of g,g’-disubstituted allyl-
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zinc, the addition of a-chloro carbonyl species should lead to
a chelated chair-like transition structure providing good
diastereoselectivity. We were indeed pleased to see that
when the combined sequence of carbocupration reaction on
terminal alkynes followed by the in situ zinc homologation
with the Shi carbenoid ICH2ZnOCOCF3 and allylation
reaction with a-chloro aldehydes 6 and a-chloro imines 7
was performed, the corresponding homoallylic alcohols and
amines 8a–g and 9a–c were obtained in good overall isolated
yields (Table 2). When R3 was a primary alkyl group (Table 2,
cupration/zinc homologation/allylation reactions proceed
smoothly with the expected stereochemical outcome, good
to excellent diastereoselectivities were restricted to sterically
encumbered R3 groups. To meet this requirement while
keeping in mind the goal of preparing several stereogenic
centers in acyclic systems, a-chloro aldehydes and imines
10a–c and 11a–b, possessing an additional stereogenic center
to mimic the required branched alkyl groups, were prepared
from known procedures in excellent diastereo and enantio-
meric ratios (Figure S1, see the Supporting Information).
When the combined carbometalation/zinc homologation
and addition of a-chloro aldehyde 10a was performed on 1c
(R1 = Hex), the product 12a was obtained in a good overall
yield as two diastereomers in a ratio of 89:11 (Scheme 3),
Table 2: Diastereoselective formation of homoallylic alcohols and
amines 8 and 9.
Entry R1
R2
R3
X
d.r.[a]
Yield [%][b]
1
2
3
4
5
6
7
8
9
10
Et (1a)
Bu (1b)
Hex (1c) Et
Et (1a)
Hex (1c) Et
Bu (1b)
Pr (1d)
Pr (1d)
Hex (1c) Et
Pr (1d) Et
Et
Bu
Et
iPr
iPr
O (6a)
O (6b)
O (6b)
O (6b)
65:35
87:13
88:12:0:0 64 (8c)
83:17:0:0 60 (8d)
100:0:0:0 53 (8e)
85:15:0:0 59 (8 f)
80:20:0:0 60 (8g)
61 (8a)
53 (8b)
Hex iPr
tBu O (6c)
iPr
iPr
tBu NTs (7a) 100:0:0:0 70 (9a)
iPr
iPr
Et
Et
Et
O (6b)
O (6b)
NTs (7b) 90:10:0:0 63 (9b)
NTs (7b) 85:15:0:0 60 (9c)
[a] Determined by 1H NMR analysis of crude reaction mixture.
[b] Determined after purification by column chromatography on silica
gel.
entry 1, R3 = Et), the diastereoselectivity of the allylation
reaction on the a-chloro aldehyde was rather poor. By
increasing the size of the substituent R3, good to excellent
diastereomeric ratios were obtained (Table 2, compare
entries 1, 2 and 5). When the two alkyl groups present on
the alkyne and on the organocopper are different, the same
diastereoisomeric ratio was obtained suggesting that the
stereochemistry at the newly formed quaternary all-carbon
stereocenter is fully controlled (Table 2, entry 3). When the
nature of these two alkyl groups was permuted, similar
diastereomeric ratios were obtained implying that the reac-
tion indeed proceeds through a closed transition state
(Table 2, compare entries 3 and 4). The same trend was
observed for the reactions of a-chloro imines 7 as homoallyl
amines 9a–c were obtained in moderate to good yields as two
diastereoisomers ranging from 85:15:0:0 to 100:0:0:0 ratios
(Table 2, entries 8–10).
The relative configuration of the products has been
determined by X-ray analysis of 9a (see the Supporting
Information) and clearly suggests a transition structure in
which the substituent of the aldehyde (or imine) indeed
occupies a pseudo-axial position in a chair-like Zimmerman–
Traxler transition structure in which the chloride atom
chelates intramolecularly the zinc center of the g,g’-disub-
stituted allylzinc species. Although these combined carbo-
Scheme 3. Diastereoselective reaction with a-chloro aldehydes 10 and
imines 11.
which is very similar to the ratio previously obtained when a-
chloro aldehyde 6b was used (compare Table 2, entry 3 with
12a in Scheme 3).
Therefore, the additional stereogenic center present in the
a-chloro aldehyde does not have a detrimental effect on the
stereochemical outcome and indeed mimic an iPr group. The
opposite diastereoisomer at the quaternary carbon center 12a
can easily be prepared by permuting the nature of the
substituent R1 and R2 and this method can be extended to
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V. K. Aggarwal, Nature 2014, 513, 183; k) C. G. Watson, A.
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Interestingly, when the combined reaction was performed on
the opposite diastereoisomer 10b or on differently substituted
chiral a-chloro aldehyde (10c), similar diastereoselectivity
are obtained (formation of 12d–f). Finally, when chiral a-
chloro imines were used (11a,b), only two out of the eight
possible diastereoisomers were obtained in an approximately
90:10 ratio whatever are the absolute configuration of the
starting imines (11a or 11b). In conclusion, the combined
carbometalation/zinc homologation and reactions with a-
heterosubstituted aldehydes and imines proceed through
chair-like transition structures with the substituent of the
incoming aldehyde residue preferentially occupying
a pseudo-axial position to avoid the two gauche interactions.
Moreover, the heteroatom present in the axial position
produces a chelated intermediate (and not a Cornforth–
Evans transition structure for a-chloro aldehydes and imines)
leading to face differentiation in the allylzincation reaction.
This method provides access to functionalized adducts in
which three new carbon-carbon bonds and two to three
stereogenic centers, including a quaternary carbon stereo-
genic center, were created in acyclic systems in a single-pot
operation from simple alkynes.
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Received: May 26, 2015
Published online: June 30, 2015
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