Copper-catalyzed γ-selective and stereospecific substitution reaction of allylic carbonates with diboron: Efficient route to chiral allylboron compounds

Article abstract of DOI:10.1021/ja056099x

A copper-catalyzed γ-selective and stereospecific substitution reaction of allylic carbonates with a diboron reagent affording allylboron compounds is described. Boryl group was selectively introduced at the γ-position of the leaving carbonate group. Func

Full text of DOI:10.1021/ja056099x

Published on Web 10/28/2005
Copper-Catalyzed γ-Selective and Stereospecific Substitution Reaction of
Allylic Carbonates with Diboron: Efficient Route to Chiral Allylboron
Compounds
Hajime Ito,* Chika Kawakami, and Masaya Sawamura*
DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, and PRESTO, JST,
Sapporo 060-0810, Japan
Received September 5, 2005; E-mail: hajito@sci.hokudai.ac.jp; sawamura@sci.hokudai.ac.jp
Table 1. Reaction of Allylic Carbonates with Diboron 2a
Allylboron compounds are useful reagents for the stereoselective
synthesis of homoallylic alcohols.1 A desirable but unprecedented
method for the preparation of the boron reagent would be the
γ-selectiVe and stereospecific substitution reaction of an allyl
alcohol derivative with a boron reagent.2-6 This would allow easy
access to functionalized7 or optically active8-10 allylboron com-
pounds. Herein, we report that a Cu(I) complex coordinated with
the Xantphos11 ligand can catalyze the γ-selective substitution
Selectivityc
reaction of allylic carbonates with bis(pinacolato)diboron to afford
the corresponding allylboronates. The reaction of the optically active
allylic carbonates with an R-stereogenic center underwent R-to-γ
chirality transfer with anti-stereochemistry, leading to chiral allyl-
boronates with a boron-substituted stereogenic center.
entry
catalyst
solvent
yieldb (%)
3a/3b
E/Z (3a)d
1
2
3
4
Cu(O-t-Bu)-Xantphos THF
100
11
44
37
2
0
0
96
99:1
>99:<1
>99:<1
>99:<1
nd
nd
nd
57:43
97:3
62:38
97:3
96:4
nd
nd
nd
98:2
Cu(O-t-Bu)-dppe
Cu(O-t-Bu)-dppp
Cu(O-t-Bu)-dppf
Cu(O-t-Bu)-PPh3
CuOTf-PBu3
THF
THF
THF
THF
DMF
DMF
DMSO
5
During our continued investigations of group 11 metal-catalyzed
reactions, we observed that a chelating phosphine (Xantphos)
featuring a large natural bite-angle produced a remarkable rate-
acceleration for copper-catalyzed hydrosilane alcoholysis.12 This
outcome suggested that the large bite-angle ligand would activate
Cu(I)-OR for σ-bond metathesis with an H-Si bond of the
hydrosilane to form a monomeric, highly active copper hydride,
Cu(I)-H. Drawing an analogy between H-Si and B-B, it was
hoped that Xantphos-ligated Cu-OR would react with a diboron
reagent to form a Cu-B species useful as a “formal boryl
nucleophile”.5,13 Formal SN2′ attack of the Cu-B species on an
allylic carbonate would allow γ-selective formation of an allylboron
compound along with a copper carbonate that would undergo
decarboxylation to regenerate the Cu-OR. The reaction of allylic
6
7e,f
8f
CuCl-KOAc-LiCl
Pd(dba)2
a Conditions: catalyst (5 mol %), allylic carbonates 1a (0.25 mmol),
diboron 2 (0.5 mmol), and solvent (0.25 mL). b Yield of all allylboronates
was determined by GC. c Ratio of regio- and E/Z-isomers was determined
by GC. d The Z-isomer of 3b was not detected. e Catalyst (110 mol %) was
used. f Compound 2 (0.28 mmol) and 1.5 mL of the solvent were used.
(PPh3) or other diphosphines (dppe, dppp, dppf), however, resulted
in decreased catalytic activities (entries 2-6). In the case of dppe
(entry 2), the product was obtained in a low yield with poor E/Z
selectivity. Although the dppp (entry 3) and dppf (entry 4)
complexes possessed higher activities and exhibited high regio- and
E/Z-selectivities, their activities were still significantly lower than
Scheme 1. Expected Catalytic Cycle
that of the Xantphos complex. The PPh3 complex (entry 5) was
nearly inactive. Furthermore, CuOTf-PBu3
6,13
(entry 6) and the
stoichiometric CuCl-KOAc-LiCl system5,6 (entry 7), which have
been reported to catalyze the borylation of R,â-unsaturated com-
pounds and allyl chloride, were ineffective for the reaction of 1a.
Low-valent palladium complex Pd(dba)2 (entry 8) catalyzed the
conversion, but afforded a mixture of 3a and 3b in a ratio of
57:43; the low regioselectivity is attributable to the loss of
regiochemical information of substrate 1a upon formation of a
(π-allyl)palladium(II) complex intermediate.2
A series of allylboronates was synthesized from allylic carbonates
using the Cu(I)-Xantphos catalyst (Table 2). The Cu(I)-catalyzed
reaction converted allylic carbonates 1a (Table 1, entry 1) and 1b
(Table 2, entry 1) into 3a and 3b, respectively, as major products.
As shown in entries 2 and 3, high γ-selectivity was retained even
when the allylic systems are located at the terminal of the carbon
chain. Accordingly, carbonates 1c and 1d were converted into the
branched (3c) and linear (3d) boronates, respectively. The reaction
was applicable to cyclic carbonate 1e (entry 4). Allylboronates
containing acetal, ester, and isolated alkene were also obtained in
high yields (entries 5-7, respectively).
carbonate 1a and bis(pinacolato)diboron 2 in the presence of
Cu(O-t-Bu)-Xantphos catalyst (5 mol %) proceeded at room
temperature to give the corresponding allylboronate in a quantitative
yield with high regio- (3a:3b ) 99:1) and E/Z- (E-3a:Z-3a )
97:3) selectivities (Table 1, entry 1). The use of a monophosphine
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J. AM. CHEM. SOC. 2005, 127, 16034-16035
10.1021/ja056099x CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Cu(I)-Xantphos-Catalyzed Reaction of Allylic
Carbonates with Diboron 2a
upon displacements of prochiral allyl alcohol derivatives in the
presence of an optically active copper catalyst are currently
underway.
Supporting Information Available: Experimental procedures and
compound characterization data (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
References
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a Conditions: catalyst (5 mol %), allylic carbonates 1a (0.50 mmol),
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The reaction also offers an expeditious and general route to
optically active allylboronates that have a stereogenic carbon with
a boryl substituent.14 The reaction of (S)-(E)-1i (98% ee) with 2 in
the presence of Cu(I)-Xantphos catalyst (10 mol %) at 0 °C
underwent an R-to-γ chirality transfer accompanied by only a slight
loss of enantiomeric purity to afford (S)-(E)-3i (96% ee) along with
a small amount of the R-substituted E-isomer and γ-Z-isomer (eq
1). On the other hand, the reaction of (S)-(Z)-1i (97% ee), the
Z-isomer with the same configuration at the chiral center as (S)-
(E)-1i, afforded (R)-(E)-3i (97% ee), the antipode of the product
derived from (S)-(E)-1i, with complete chirality transfer and
γ-selectivity (eq 2). The observed stereochemical outcome can be
explained by the anti-attack of the borylcopper to an allyl carbonate
in a conformation that avoids an allylic 1,3-strain.15
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In conclusion, the γ-selective and stereospecific copper-catalyzed
substitution of allylic carbonates with a diboron reagent offers a
convenient and powerful method for the synthesis of allylboronates.
Investigations into the asymmetric synthesis of allylboronates based
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