Synthesis of Z-(pinacolato)allylboron and Z-(pinacolato)alkenylboron compounds through stereoselective catalytic cross-metathesis

Article abstract of DOI:10.1021/ja403188t

The first examples of catalytic cross-metathesis (CM) reactions that furnish Z-(pinacolato)allylboron and Z-(pinacolato)alkenylboron compounds are disclosed. Products are generated with high Z selectivity by the use of a W-based monoaryloxide pyrrolide (MAP) complex (up to 91% yield and >98:2 Z:E). The more sterically demanding Z-alkenylboron species are obtained in the presence of Mo-based MAP complexes in up to 93% yield and 97% Z selectivity. Z-selective CM with 1,3-dienes and aryl olefins are reported for the first time. The utility of the approach, in combination with catalytic cross coupling, is demonstrated by a concise and stereoselective synthesis of anticancer agent combretastatin A-4.

Full text of DOI:10.1021/ja403188t

Communication
pubs.acs.org/JACS
Synthesis of Z‑(Pinacolato)allylboron and Z‑(Pinacolato)alkenylboron
Compounds through Stereoselective Catalytic Cross-Metathesis
Elizabeth T. Kiesewetter,† Robert V. O’Brien,† Elsie C. Yu,† Simon J. Meek,† Richard R. Schrock,‡
and Amir H. Hoveyda*,†
†Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, United States
‡Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
Scheme 1
ABSTRACT: The first examples of catalytic cross-meta-
thesis (CM) reactions that furnish Z-(pinacolato)-
allylboron and Z-(pinacolato)alkenylboron compounds
are disclosed. Products are generated with high Z
selectivity by the use of a W-based monoaryloxide
pyrrolide (MAP) complex (up to 91% yield and >98:2
selectivity.13 Transformations are promoted by W or Mo
Z:E). The more sterically demanding Z-alkenylboron
species are obtained in the presence of Mo-based MAP
complexes in up to 93% yield and 97% Z selectivity. Z-
selective CM with 1,3-dienes and aryl olefins are reported
for the first time. The utility of the approach, in
combination with catalytic cross coupling, is demonstrated
by a concise and stereoselective synthesis of anticancer
agent combretastatin A-4.
complexes, offering a generalizable, and at times unique, entry
to stereoselective synthesis of a range of valuable organoboron
reagents and intermediates.
We began by examining reactions with allyl-B(pin) 4. A
challenge in promoting this class of transformations does not lie
as much in attaining high efficiency but rather in whether erosion
of kinetic stereoselectivity due to post-CM isomerization can be
avoided as the reaction progresses toward high conversion.13a
Thus, although we had determined earlier that Z-selective
homocoupling of allyl-B(pin) can be promoted efficiently with
W-based monoaryloxide pyrrolide (MAP) complexes,14 access
to the less hindered disubstituted alkenes would require
identifying a catalyst that strikes the desired balance between
reactivity and selectivity. As the screening data in entries 1−4 of
Table 1 indicate, when Mo-based MAP complexes 1a,b or 2a,b
are used, CM of allyl-B(pin) 4 and 1-decene proceeds readily to
50−75% conversion, but Z selectivity is moderate (69:31−87:13
Z:E). Control experiments indicate stereoselectivity is dimin-
ished as the reactions proceed further; this is especially true when
the more reactive adamantylimido alkylidenes derived from 1a
are involved (63% conv and ∼1:1 Z:E after 24 h), although such
complexes likely deliver high kinetic Z selectivity consistent with
the previously suggested stereochemical model (large size
difference between the adamantyl and aryloxy ligands).13a,c
With the more hindered Mo arylimido systems, there is less post-
CM isomerization (vs 1a,b), but kinetic selectivity is probably
also lower on the basis of the above rationale. In the presence of
the less active but more stereodifferentiating W alkylidene 3
(increased size differential between imido and aryloxide
groups),13a,c 6a is isolated with 95:5 Z:E selectivity (entry 5,
Table 1);15 oxidative workup delivers Z-allylic alcohol 6a in 65%
overall yield. To the best of our knowledge, this is the first report
of a W-catalyzed Z-selective CM. Reactions with commonly used
and moderately E-selective Ru-based carbenes require 12 h at 40
°C (e.g., Grubbs second-generation: 18% yield of 6a, 73% E, in
rganoboron compounds are vital to chemical synthesis,
O
and among them, (pinacolato)allylboron [allyl-B(pin)]
and (pinacolato)alkenylboron [alkenyl-B(pin)] reagents hold a
prominent position.1 The former set is used in countless
stereoselective additions,2 and the latter are partners in a myriad
of catalytic cross couplings.3 Since the stereochemical outcome
of transformations with such entities is contingent on whether a
Z- or an E-organoboron is employed,4,5 of considerable value are
methods for facile and efficient access to stereodefined acyclic
allyl-B(pin) and alkenyl-B(pin) compounds. Contrary to the E
isomers, however, protocols for selective synthesis of higher-
energy Z-allyl- or Z-alkenylboron species are uncommon, and
especially scarce are related processes that are catalytic. Ni-
catalyzed hydroboration of dienes has been shown to afford Z-
allyl-B(pin) products;6 Ir-, Rh-7 and, most recently, Ru-
catalyzed8 methods for trans-boron-hydride additions to
terminal alkynes have been devised to furnish Z-alkenyl-B(pin)
compounds. Such protocols were introduced as alternatives to
noncatalytic procedures, which, while highly stereoselective, are
multistep and require strong base (e.g., n-BuK9a or n-BuLi9b) or
acid (e.g., glacial acetic acid).10 Stereoselective cross-metathesis
(CM)11 of commercially available allyl- or alkenyl-B(pin) with
terminal alkenes offers an attractive general strategy for synthesis
of Z-allyl- or Z-alkenylboron entities (Scheme 1), one that is
entirely distinct from catalytic alkyne hydroborations mentioned
above; nonetheless, all existing CM protocols deliver the
corresponding E isomers predominantly.12 Herein, we demon-
strate that catalytic CM can be used to access Z-allyl- or Z-
alkenylboron compounds efficiently and with high stereo-
Received: March 30, 2013
© XXXX American Chemical Society
A
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Communication
a
Table 1. Z-Selective CM with Allyl-B(pin) 4: Catalyst
Screening
Scheme 2. Z-Selective Catalytic CM with Allyl-B(pin) 4
a
b
c
b
entry complex; mol % time (h) conv (%)
yield of 6a (%)
Z:E
1
2
3
4
5
1a; 3.0
1b; 3.0
2a; 5.0
2b; 5.0
3; 5.0
4.0
24
50
66
75
52
78
nd
nd
nd
nd
65
87:13
81:19
78:22
69:31
95:5
5.0
4.0
2.0
a
Reactions were performed in C6H6 at 22 °C with 5.0 equiv of cross
partner. All yields are overall for two steps. Synthesis of 8e involved
the use of Mo complex 2a under otherwise identical conditions, and
formation of 8f involved treatment with NaOH. See the Supporting
Information for details.
a
Reactions performed in C6H6 under N2 atm with 5.0 equiv of 1-
b
decene. Oxidative workup: H2O2, NaOH, 22 °C, 2.0 h. Determined
by analysis of 1H NMR spectra of unpurified mixtures (of 5a for conv
and of 6a for Z:E) and refer to consumption of the limiting substrate
( 2%). See the Supporting Information for details. Yield of isolated
and purified 6a (isomeric mixture). nd = not determined.
c
alkylidene carbon to diminish catalyst activity. Furthermore,
vinylboron 11 is more sterically demanding than the cross
partners examined thus far (including allyl-B(pin) 4).13a
2.0 h at 22 °C);12a when Mo(CHCMe2Ph)(N(2,6-(i-Pr)2C6H3)-
(OCMe(CF3)2)2 (7) is used, 6a is obtained in 54% yield and with
moderate E selectivity (76:24 E:Z).16
Catalyst screening was performed with 11 and β-branched
terminal alkene 12 (Table 2); there is appreciable conversion to
13a (50−98% conv) and ≥86:14 Z:E selectivity under
conditions where, unlike reactions with 4, vinyl-B(pin) 11 is
present in excess. In additional contrast to syntheses of the more
exposed alkenes of Z-allyl-B(pin)s 5a−g (Scheme 2), it is Mo
complexes 2a,b that provide the highest efficiency and
stereoselectivity: 13a is obtained in ∼68% yield and 93:7 Z
selectivity (entries 3 and 4, Table 2). CM is highly Z-selective
with the less sterically demanding 1a, but CM proceeds to only
Various terminal alkenes undergo Z-selective CM with allyl-
B(pin) 4 within two hours at ambient temperature (Scheme 2).
The Z-disubstituted allyl-B(pin) compounds, sensitive to
purification and isolation, were directly treated with benzalde-
hyde to afford the derived homoallylic alcohols; the diastereo-
selectivity levels with which 8a−8f and 10 are obtained serve as
an indicator of the isomeric purity of the corresponding allyl-
B(pin) intermediate.12 Disubstituted allylboron compounds are
formed in 91:9 to >98:2 Z:E selectivity (based on dr), and the
unsaturated alcohols are isolated in 68−91% yield. In one
instance (8e), the use of Mo complex 2a is required, since the
sizable silyl ether likely causes the W alkylidenes derived from 3
to be less efficient (30% conv, 23% yield). Furthermore, the
disubstituted alkene in 5e is relatively hindered, and Z-to-E
interconversion by post-CM isomerization with the more active
Mo catalyst is less of a factor (vs a more accessible alkene such as
5a). Another notable finding in Scheme 2 relates to the first
instance of a Z-selective CM reaction with a 1,3-diene,17
affording 10 in 72% yield and 95:5 dr; such products were not
reported in connection to the Ni-catalyzed protocol.6
Table 2. Z-Selective CM with Vinyl-B(pin) 11: Catalyst
Screening
a
b
c
b
entry
complex; mol %
conv (%)
yield (%)
Z:E
1
2
3
4
5
1a; 3.0
1b; 3.0
2a; 5.0
2b; 5.0
3; 5.0
56
50
95
98
66
nd
nd
68
69
nd
96:4
93:7
93:7
93:7
86:14
Next, we probed the possibility of Z-selective CM with vinyl-
B(pin) 11. This class of transformations, Mo- or W-catalyzed
variants of which are unknown, poses several distinct challenges.
In contrast to formerly reported processes13a as well as those
mentioned above, the p orbital at the boron center of a B(pin)-
substituted alkylidene can stabilize electron density at the
a
b
Performed under N2 atm with 5.0 equiv of 11. Determined by
analysis of 1H NMR spectra of unpurified mixtures and refer to
consumption of the limiting substrate ( 2%). See the Supporting
Information for details. Yield of isolated and purified 13a (isomeric
mixture). nd = not determined.
c
B
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Journal of the American Chemical Society
Communication
56% conversion (entry 1, Table 2); this is likely due to lower
stability of the derived alkylidenes, including the methylidene
generated from ethylene byproduct (ambient pressure used).
Reaction with W-based 3 proceeds less readily than with most
Mo variants, but surprisingly, and for reasons that are unclear, it is
less stereoselective (entry 5; 86:14 Z:E). We elected to use
complex 2a for further studies (vs 2b) for its relative ease of
preparation.
formed in 93:7 Z:E selectivity and isolated as the pure Z isomer
(>98%) in 80% yield. Allyl-B(pin) 13i, obtained in 60% yield22
and 97:3 Z:E, belongs to a class of reagents with demonstrated
utility in stereoselective synthesis;23 the CM strategy can allow
access to related entities that bear differentiated boryl units. Z-
Alkenyl-B(pin) entities represented by 13g−i were not prepared
by Ir-, Rh-, or Ru-catalyzed alkyne hydroborations.7,8
We then turned to reactions involving the most sterically
demanding combination of substrates thus far, CM between
vinyl-B(pin) and styrenes; in the context of Z-selective
transformations, aryl olefins have only been used in ring-
opening/CM processes.13c CM of 11 and styrene proceeds with
maximum Z selectivity when the more stereodifferentiating
adamantylimido Mo MAP complex 1a is employed; 14a is
isolated in 66% yield and 95:5 Z:E ratio.24 Reactions with
electron-rich methoxy-substituted styrenes proceed with sim-
ilarly high Z selectivity (14b,c in 69% yield and 93:7−95:5 Z:E).
CM with p-(trifluoromethyl)styrene is highly efficient, proceed-
ing to 97% conversion to afford 14d in 93% yield and 95:5 Z:E. In
general, higher efficiencies are observed with excess styrene (vs
11); this might be attributed to the more reactive benzylidenes
being better capable of undergoing CM with 11 vs an
electronically stabilized B(pin)-substituted alkylidene (from
initial reaction with 11) reacting with an aryl olefin. Furthermore,
electron-rich styrenes (cf. 14c and 14e) are less prone to
homodimerization (reaction with another electron-rich olefin)
than CM with electron deficient 11 and can therefore be
employed in relatively slight excess (1.5 equiv). Finally, in
contrast to transformations with alkyl olefins (Scheme 3),
reduced pressure (100 Torr) leads to improved selectivity
probably because the methylidene derived from adamantylimido
1a is more reactive and readily subject to decomposition (e.g.,
14a formed in 60:40 Z:E at ambient pressure).13a
Two additional points regarding CM with 11 merit note: (1)
Unlike reactions with excess amounts of unhindered terminal
alkenes, including allyl-B(pin) 4, stereoselectivities do not
improve under a vacuum. This might arise from competitive
formation of the relatively stable B(pin)-substituted alkylidene,
preventing the availability of methylidene complexes, which are
more reactive and thus capable of promoting isomerization.18
Moreover, use of excess vinyl-B(pin) 11 reduces the
concentration of alkyl-substituted alkylidenes, species that are
better capable of causing loss of stereoselectivity (vs B(pin)-
substituted alkylidenes).19 (2) Reactions with 5.0 mol % second-
generation Grubbs catalyst or the corresponding phosphine-free
variant are efficient but afford 13a in ∼90:10 E:Z selectivity
(∼70% yield).16 When Mo bis-alkoxide 7 is used, there is only
33% conversion of 12 after 10 min (no further conversion after
24 h), and 13a is isolated in 15% yield and 68:32 Z:E.16,20 The
latter inefficiency might be attributed to the low stability of the
highly Lewis acidic bis(hexafluoro)-alkoxide Mo alkylidene,
rendered more electron-deficient by its B(pin) substituent. The
significantly higher conversion values furnished by the more
robust MAP complexes (Table 2) are therefore noteworthy.21
An assortment of Z-alkenyl-B(pin) compounds can be
obtained in 51−92% yield after purification, in contrast to
allylboron compounds, and 90:10−97:3 Z:E (Scheme 3).
Reactions with a sizable cyclohexyl (13d) or silyl ether (13e)
substituent or an electron-deficient allylic amide (13f) proceed
to 78−94% conversion, affording the Z-alkenyl-B(pin) products
in 51−71% yield and 93:7−96:4 Z:E ratio. Stereoselective
synthesis of Z,E-diene 13g (72% yield, 93% Z) provides another
example of CM involving an acyclic 1,3-diene. Enol ether 13h is
It is significant that the catalytic protocol is effective for
synthesis of trimethoxyaryl-substituted vinyl-B(pin) 14e (73%
yield, 96:4 Z:E; Scheme 4); Pd-catalyzed cross coupling with the
appropriate aryl bromide affords combretastatin A-4, a member
of a class of antitumor agents. Stereoselective synthesis of the
medicinal agent, 10 000 times more active than its E isomer,25
demonstrates the power of combining catalytic CM and Suzuki−
Scheme 3. Z-Selective Catalytic CM with Vinyl-B(pin) 11:
a
Generality
Scheme 4. Z-Selective CM with Vinyl-B(pin) 11 and
a
Styrenes
a
a
Reactions were performed in C6H6 at 22 °C with 5.0 equiv of 11;
Reactions were performed in C6H6 at 22 °C with 5.0 equiv of styrene
yields are of Z/E mixtures, except for 13h (>98% Z). See the
Supporting Information for details.
under N2 atm, except for 14c and 14e where 1.5 equiv was used; yields
are of Z/E mixtures. See the Supporting Information for details.
C
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Journal of the American Chemical Society
Communication
(9) (a) Roush, W. R.; Ando, K.; Powers, D. B.; Palkowitz, A. D.;
Halterman, R. L. J. Am. Chem. Soc. 1990, 112, 6339. (b) Ramachandran,
P. V.; Pratihar, D.; Biswas, D. Chem. Commun. 2005, 1988.
(10) Molander, G. A.; Ellis, N. M. J. Org. Chem. 2008, 73, 6841.
(11) Overview of catalytic olefin metathesis: (a) Hoveyda, A. H.;
Zhugralin, A. R. Nature 2007, 450, 243. Recent review on catalytic CM:
(b) Prunet, J.; Grimaud, L. In Metathesis in Natural Product Synthesis:
Strategies, Substrates and Catalysts; Cossy, J., Arseniyadis, S., Meyer, C.,
Eds; VCH-Wiley: Weinheim, 2010; p 287.
Miyaura-type processes to obtain congested Z-alkenes by an
approach that is more concise than those adopted formerly.24,26
The studies described above put forth several key additions to
a limited repertoire of efficient Z-selective CM reactions.13,27
A
W- and two Mo-based alkylidenes have emerged as optimal
catalysts, underscoring the importance of the structural and
chemical diversity of high oxidation-state alkylidenes to the
development of methods that bear substantial scope. Equally
notable is the facility with which the reactions of sterically
demanding substrates proceed; this is in contrast to the activity
levels exhibited thus far by Z-selective Ru-based carbenes.26
(12) (a) Goldberg, S. D.; Grubbs, R. H. Angew. Chem., Int. Ed. 2002, 41,
807. (b) Morrill, C.; Grubbs, R. H. J. Org. Chem. 2003, 68, 6031. See SI
for data with commonly used Ru and Mo complexes.
(13) Z-selective Mo-catalyzed CM involving enol ethers and allylic
amides: (a) Meek, S. J.; O’Brien, R. V.; Llaveria, J.; Schrock, R. R.;
Hoveyda, A. H. Nature 2011, 471, 461. Related studies: (b) Ibrahem, I.;
Yu, M.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131,
3844. (c) Yu, M.; Ibrahem, I.; Hasegawa, M.; Schrock, R. R.; Hoveyda, A.
H. J. Am. Chem. Soc. 2012, 134, 2788.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(14) Jiang, A. J.; Zhao, Y.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem.
Soc. 2009, 131, 16630.
AUTHOR INFORMATION
■
(15) W-based MAP complexes promote RCM with lower post-
metathesis isomerization vs Mo alkylidenes: Yu, M.; Wang, C.; Kyle, A.
F.; Jakubec, P.; Dixon, D. J.; Schrock, R. R.; Hoveyda, A. H. Nature 2011,
479, 88.
Corresponding Author
amir.hoveyda@bc.edu
Notes
(16) See the Supporting Information for details.
The authors declare the following competing financial
interest(s): A.H.H. and R.R.S. are founders of a company that
utilizes the reported approach.
(17) Catalytic Z-selective homocoupling of 1,3-dienes: Townsend, E.
M.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2012, 134, 11334.
(18) Lower conversion is observed with excess alkene (vs 11), likely
because the alkyl-substituted alkylidene is shorter living than one with a
B(pin) unit.
(19) Vinyl-B(pin) 11 undergoes relatively facile, and likely not readily
reversible, homocoupling to generate ethylene, providing another
rationale for the higher conversion observed when it is used in excess.
(20) The low Z:E ratio is congruent with the ability of this commonly
used complex to afford moderate Z selectivity, which might be
diminished through post-CM isomerization. Wang, C.; Yu, M.; Kyle,
A. F.; Jakubec, P.; Dixon, D. J.; Schrock, R. R.; Hoveyda, A. H. Chem.
Eur. J. 2013, 19, 2726.
ACKNOWLEDGMENTS
■
Financial support was provided by the NIH (GM-59426 and
GM-47480).
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