Versatile Homoallylic Boronates by Chemo-, SN2′-, Diastereo- and Enantioselective Catalytic Sequence of Cu?H Addition to Vinyl-B(pin)/Allylic Substitution

Article abstract of DOI:10.1002/anie.201611444

A highly chemo-, diastereo- and enantioselective catalytic method that efficiently combines a silyl hydride, vinyl-B(pin) (pin=pinacolato) and (E)-1,2-disubstituted allylic phosphates is introduced. Reactions, best promoted by a Cu-based complex with a chiral sulfonate-containing N-heterocyclic carbene, are broadly applicable. Aryl-, heteroaryl-, alkenyl-, alkynyl- and alkyl-substituted allylic phosphates may thus be converted to the corresponding homoallylic boronates and then alcohols (after C?B bond oxidation) in 46–91 % yield and in up to >98 % S_N2′:S_N2 ratio, 96:4 diastereomeric ratio and 98:2 enantiomeric ratio. The reasons why an NHC?Cu catalyst is uniquely effective (vs. the corresponding phosphine systems) and the basis for different trends in stereoselectivity are provided with the aid of DFT calculations.

Full text of DOI:10.1002/anie.201611444

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International Edition: DOI: 10.1002/anie.201611444
German Edition: DOI: 10.1002/ange.201611444
Multicomponent Catalysis
Versatile Homoallylic Boronates by Chemo-, SN2’-, Diastereo- and
À
Enantioselective Catalytic Sequence of Cu H Addition to Vinyl-
B(pin)/Allylic Substitution
Jaehee Lee, Sebastian Torker, and Amir H. Hoveyda*
Dedicated to Professor Samuel J. Danishefsky
Abstract: A highly chemo-, diastereo- and enantioselective
catalytic method that efficiently combines a silyl hydride, vinyl-
B(pin) (pin = pinacolato) and (E)-1,2-disubstituted allylic
phosphates is introduced. Reactions, best promoted by a Cu-
based complex with a chiral sulfonate-containing N-hetero-
cyclic carbene, are broadly applicable. Aryl-, heteroaryl-,
alkenyl-, alkynyl- and alkyl-substituted allylic phosphates may
thus be converted to the corresponding homoallylic boronates
À
and then alcohols (after C B bond oxidation) in 46–91% yield
and in up to > 98% SN2’:SN2 ratio, 96:4 diastereomeric ratio
À
and 98:2 enantiomeric ratio. The reasons why an NHC Cu
catalyst is uniquely effective (vs. the corresponding phosphine
systems) and the basis for different trends in stereoselectivity
are provided with the aid of DFT calculations.
C
atalytic strategies for enantioselective preparation of
organic molecules with a boron-substituted stereogenic
carbon center are much sought after in organic chemistry.[1]
Notable examples are enantioselective boron-hydride addi-
tions to alkenes with Rh- or Ir-based complexes[2] and related
diboryl additions with Pt-, Pd- or carbohydrate-derived
catalysts[3] (Scheme 1a). An alternative approach entails
À
site- and enantioselective Cu B(pin) (pin = pinacolato) addi-
tion to an olefin followed by in situ protonation (proto-boryl
addition)[4] or allylic substitution (boron-allyl addition)[5] of
À
the Cu C bond; these transformations are typically promoted
by a chiral Cu-based complex (Scheme 1a).
Scheme 1. Related previous work, the key objectives of this study and
the associated challenges. Abbreviations: pin=pinacolato; G, R=var-
ious functional groups; L=ligand.
One other disconnection would entail enantioselective
[6]
À
Cu H addition to commercially available vinyl-B(pin) and
an ensuing SN2’-selective allylic substitution involving a 1,2-
disubstituted alkene (Scheme 1b). Products bearing an allylic
C-substituted and a homoallylic (pin)B-substituted carbon
stereogenic center would be obtained. With G = Me simple
oxidation to a secondary homoallylic alcohol could result in
diastereo- and enantiomerically enrich product expected
from crotyl addition to acetaldehyde, a transformation that,
to the best of our knowledge, remains without a catalytic
enantioselective variant.[7] Nonetheless, the above plan might
present several complications. A chiral catalyst must promote
À
efficient, diastereo- and enantioselective Cu H addition
followed by allylic substitution in preference to two poten-
tially competing routes. One pathway could involve reaction
of Cu-alkoxide with vinyl-B(pin) (vs. a hydride reagent) to
yield a vinyl-Cu complex, which might then react with an
allylic phosphate;[8] alternatively, the Cu H might react
À
directly with the allyl electrophile.[9]
Here, we demonstrate that a sulfonate-containing chiral
NHC–Cu complex can efficiently promote the general trans-
formation in Scheme 1b with high chemo-, SN2’-, diastereo-
and enantioselectivity.[10] While this manuscript was being
prepared, reactions involving (E)-1,2-disubstituted alkenyl-
B(pin) substrates and allylphosphate with bis-phosphine-Cu
catalysts were disclosed (to generate products with a single
stereogenic center);[11] processes were highly enantioselective
[*] J. Lee, Dr. S. Torker, Prof. A. H. Hoveyda
Department of Chemistry, Merkert Chemistry Center, Boston College
Chestnut Hill, MA 02467 (USA)
E-mail: amir.hoveyda@bc.edu
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201611444.
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but the only reported case with an
(E)-1,2-disubstituted allylic phos-
phate furnished the desired prod-
uct in 78:22 diastereomeric ratio
(d.r.) and 73:27 enantiomeric ratio
(e.r.), hinting at the difficulty of
the proposed class of reactions.
We started by gauging the
effectiveness of various chiral bis-
phosphine ligands to promote the
process involving vinyl-B(pin),
(E)-1,2-disubstiuted allylic phos-
phate 1a with polymethylhydrosi-
loxane (PMHS).[12] In three cases,
the product from SN2 mode of
addition (3a) was the major com-
ponent with moderate selectivity
(phos-1, phos-2 and phos-5), and
in two instances it was formed
almost exclusively (phos-4 and
phos-6). With every bis-phosphine
ligand, 2a and 3a were generated
in low d.r. and e.r. This was some-
what surprising since chiral phos-
phines—and some of these exact
ligands—have been shown to be
optimal in several transformations
that begin with enantioselective
À
Cu H addition to an alkene
(phos-2,[13] phos-3,[10] and phos-
6).[6f–h] We took these findings as
an indication that the desired
sequence of reactions demands
a distinct set of catalysts.
Results were more encourag-
ing with N-heterocyclic carbene
(NHC) systems (Scheme 2). With
imid-1[14] as NHC precursor, 2a
was the major component of the
product mixture (2a:3a, 69:31)
but stereoselectivity remained
low. There was further improve-
ment with the NHC-Cu complex
derived from sulfonate-containing
imid-2,[15] which afforded 2a
exclusively and in appreciable
Scheme 2. Identification of an effective chiral catalyst. Conv. and d.r. determined by analysis of
unpurified product mixtures by 1H NMR analysis. Enantioselectivity determined by HPLC analysis.
Yields are for purified products. Abbreviations: phos=phosphine ligand; imid=imidazolinium salt;
nd=not determined. See the Supporting Information for details.
d.r. and/or e.r. [13:87 and 80:20 (for the major diastereomer),
respectively]. Another unexpected observation was that the
catalyst derived from imid-3,[16] where the Mes unit (2,4,6-
trimethylphenyl) is replaced by a 3,5-(2,4,6-triisopropoyl-
phenyl) group, high 2a:3a ratio persisted (94:6) with stereo-
selectivity improving greatly as well (94:6 d.r. and e.r. for
major diastereomer of 2a). Hence, there was a mechanistically
suggestive reversal in diastereoselectivity along with substan-
tially lower enantioselectivity with imid-2 (vs. imid-3; see
below for further discussion).
oxidation (Scheme 3). Reactions were performed at ambient
temperature with 5.5 mol% imid-3 and 5.0 mol% CuCl along
with three equivalents of inexpensive PMHS and with LiOtBu
as base (identified as optimal base with further screening);[17]
only a small excess of vinyl-B(pin) sufficed (1.1 equiv). g-
Addition products (SN2’ pathway) were uniformly high (2:3,
86:14 to > 98:2) as was the diastereo- and enantioselectivity
with which 2a–2l were formed (90:10–96:4 d.r., 94:6–98:2
e.r.). Pure 2a–2l were obtained in 60–84% yield after silica
gel chromatography. Transformations proceeded with simi-
larly high efficiency and selectivity regardless of whether the
aryl group within the allylic phosphate was sterically hindered
(cf., 2c–f), electron withdrawing (cf. 2k,l) or electron
Many aryl-substituted allylic phosphates could be con-
verted to isolable homoallylic boronates, which were con-
À
verted to the corresponding alcohols after C B bond
2
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Scheme 4. Reactions with heteroaryl-, alkenyl- and alkynyl-substituted
allylic phosphates. Same conditions as in Scheme 3, except 7.5 mol%
imid-3 and 7.0 mol% CuCl were used for 4 and 6. Conv. and d.r.
1
determined by analysis of unpurified product mixtures by H NMR
analysis (Æ2%). Enantioselectivity determined by HPLC analysis
(Æ1%). Yields are for purified products (Æ5%). See the Supporting
Information for details.
Scheme 3. Reactions with aryl-substituted allylic phosphates. Same
conditions as in Scheme 2, except that LiOtBu was used as base.
Conv. and d.r. determined by analysis of unpurified product mixtures
by 1H NMR analysis (Æ2%). Enantioselectivity determined by HPLC
analysis (Æ1%). Yields are for purified products (Æ5%). See the
Supporting Information for details.
donating (cf. 2d, 2h). The lower SN2’ selectivity with 2l may
be attributed to direct alkylation of the exceptionally electro-
philic p-nitrophenyl-substituted allylphosphate (vs. Cu-
alkene complexation and allyl transfer; see below for further
analysis).
Allyl electrophiles containing a heterocyclic substituent
such as a pyridyl (4, Scheme 4) or a benzothiophene group
can be used (5). However, SN2’:SN2 and diastereoselectivities
were somewhat lower in such instances and the final products
at times contained a small amount of impurity from the SN2
addition (cf. 4, Scheme 4). Similar results were obtained with
a dienylphosphate (cf. 6, Scheme 4). The transformation with
the corresponding enynylphosphate (cf. 7) was more SN2’-
(> 98% vs. 87% for 6) and enantioselective (97:3 vs. 92:8 e.r.
for 6). In the case of 6 none of the product from SN2’’ mode of
Scheme 5. Reactions with alkyl-substituted allylic phosphates. Same
conditions as in Scheme 3, except 7.5 mol% imid-3 and 7.0 mol%
CuCl was used for 9. Conv. and d.r. determined by analysis of
unpurified product mixtures by 1H NMR analysis (Æ2%). Enantioselec-
tivity determined by HPLC analysis (Æ1%). Yields are for purified
products (Æ5%). See the Supporting Information for details.
reaction was detected, and the lower yield for 7 (46%) might
[18]
À
be due to competitive Cu H addition to the alkynyl group.
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Alkyl-substituted, and particularly Me-substituted allylic
phosphates, are suitable substrates (Scheme 5). As high-
lighted by synthesis of 8 and 9 (Scheme 5), while somewhat
less enantioselective compared to when aryl-substituted
allylic phosphates are utilized (Scheme 2), reaction with the
larger cyclohexyl-substituted allylic phosphate was efficient
with 7.0 mol% catalyst loading. In both cases SN2’ selectivity
was exceptional (> 98%), diastereoselectivities were high
(92:8–93:7 d.r.) and pure products were obtained in 91% and
79% yield, respectively.
Of special value are the transformations involving Me-
substituted allylic phosphate 1b, which, when performed on
5 mmol scale and with 2.0 mol% catalyst loading, afforded 10
in 55% yield (volatile compound), > 98:2 SN2’:SN2, 92:8 d.r.
and 93:7 e.r. after purification (Scheme 5). This is a valuable
fragment that has been used in a total synthesis of a biolog-
ically active analog of natural product chondramide C.[19]
Previously, however, preparation of enantiomerically pure
10 entailed the use of Brownꢀs chiral auxiliary,[20] necessitating
somewhat forcing conditions along with the use of stoichio-
metric amounts of an exceptionally strong base (nBuLi/
KOtBu to give nBuK). Another functionalization procedure
entails conversion of the homoallylic boronate formed by the
reaction of allylic phosphate 1c to the corresponding 2-furyl
product [Eq. (1)].[21]
For insight regarding the unique effectiveness of imid-3-
derived catalyst and the selectivity differences with imid-2,
a series of DFT calculations were carried out. These studies
indicate[17] that the most favored mode of Cu H addition to
À
vinyl-B(pin) with imid-3 probably arises from coordination of
a pinacolato oxygen atom to the alkali metal counter-ion (I,
Figure 1a).[22] For a detailed computational investigation of
Figure 1. Regarding the origin of diastereo- and enantioselectivity. Computations have been performed at the MN12SX/Def2TZVPPthf(SMD) level
after geometry optimization performed with the ONIOM method M06L/Def2SVP:UFFthf(PCM); Ar=2,6-(iPr)2C6H3. See the Supporting Information
for details.
4
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Conflict of interest
The authors declare no conflict of interest.
Keywords: boron · copper · enantioselective catalysis ·
enantioselective synthesis · multicomponent reactions
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À
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Acknowledgements
Financial support was provided by the NIH (GM-47480) and
the National Science Foundation (CHE-1362763).
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À
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Manuscript received: November 22, 2016
Final Article published: && &&, &&&&
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Communications
Multicomponent Catalysis
J. Lee, S. Torker,
A. H. Hoveyda*
&&&& — &&&&
Versatile Homoallylic Boronates by
Chemo-, SN2’-, Diastereo- and
Enantioselective Catalytic Sequence of
A desirable combination: A silyl hydride
and vinyl-B(pin) (pin=pinacolato), both
commercially available, may be merged
with readily accessible (E)-1,2-disubsti-
tuted allylic phosphates to afford an
assortment of homoallylic boronates
efficiently and with selectivity. A sulfo-
nate-containing N-heterocyclic carbene
ligand is found to be optimal.
À
Cu H Addition to Vinyl-B(pin)/Allylic
Substitution
Angew. Chem. Int. Ed. 2016, 55, 1 – 7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!