Copper-Catalyzed Borylation of Cyclic Sulfamidates: Access to Enantiomerically Pure (β-and γ-Amino-alkyl)boronic Esters

Article abstract of DOI:10.1002/ejoc.201501492

Cyclic sulfamidates undergo borylation under copper-catalyzed conditions using B₂pin₂to give enantiomerically (and diasteromerically) defined (aminoalkyl)boronic esters. External iodide is essential, but the intermediacy of simple al

Full text of DOI:10.1002/ejoc.201501492

DOI: 10.1002/ejoc.201501492
Communication
Boronic Esters
Copper-Catalyzed Borylation of Cyclic Sulfamidates: Access to
Enantiomerically Pure (β-and γ-Aminoalkyl)boronic Esters
Nina Ursinyova,[a] Robin B. Bedford,[a] and Timothy Gallagher*[a]
Abstract: Cyclic sulfamidates undergo borylation under cop- iodides has been excluded; N-sulfated intermediates are key in
per-catalyzed conditions using B2pin2 to give enantiomerically the borylation sequence. Based on stereochemical studies and
(and diasteromerically) defined (aminoalkyl)boronic esters. Ex- trapping experiments, the involvement of carbon-centered radi-
ternal iodide is essential, but the intermediacy of simple alkyl cals under these copper-catalyzed conditions appears likely.
Introduction
terms, however, cyclic sulfamidates, which can be viewed as
alkyl pseudohalides, are differentiated from more conventional
substrates, such as alkyl halides or tosylates, under Cu-mediated
borylation conditions.
Alkyl (i.e. sp3) boronic esters, together with the corresponding
boronic acids and trifluoroborates, represent a versatile class of
cross-coupling agents that have extended significantly the util-
ity and scope of Suzuki–Miyaura-type cross-coupling reactions
to encompass sp3–sp2 and sp3–sp3 processes.[1–3] Alkyl halides
(as well as pseudohalides such as tosylates) provide access to
the requisite sp3-based boron reagents by Pd- or Ni-,[4a–4c]
Zn-,[4d] Fe-[4e,4f] or (and perhaps of most versatility) the Cu-cata-
lyzed borylation processes[5] described first by Marder and
Lin.[5d,5f]
Results and Discussion
We evaluated initially a series of metals (Pd, Ni, Zn, Fe) to pro-
mote borylation, but did not achieve good turnover and yields.
Copper, which is also attractive for reasons of scalability, cost
and low toxicity, however, proved effective with cyclic sulfami-
dates.[9] The (S)-phenylalanine-derived substrate 1a[6b] was
screened against Marder's efficient room-temperature Cu-cata-
lyzed conditions for borylation of alkyl halides (Scheme 2).[5d]
The target boronate ester 2a was isolated by using these condi-
tions but in poor yield (5 %) and only after an extended reac-
tion time. Following an extensive evaluation of reaction condi-
tions, we identified two necessary modifications: the optimal
base (LiOtBu) and – more critically – the presence of an external
iodide source (e.g. Bu4NI). Use of these modified conditions pro-
vided boronic ester 2a[10] in 56 % yield at room temperature
after a reaction time of 2 h.
Our interest in this area has centered on the use of stable
and readily available 1,2- and 1,3-cyclic sulfamidates 1, which
have already found extensive utility as electrophiles to con-
struct C–C, C–N and C–O bonds within heterocyclic synthesis.[6]
However, their utility in either direct or indirect sp3-based cross-
coupling reactions is essentially unexplored.[7]
Here we report on the application of cyclic sulfamidates 1 as
substrates for Cu-catalyzed borylation to provide access to a
range of stereochemically defined and enantiomerically pure
(aminoalkyl)boronic esters 2[8] (Scheme 1). In mechanistic
Scheme 1.
[a] School of Chemistry, University of Bristol
Bristol BS8 1TS, United Kingdom
E-mail: t.gallagher@bristol.ac.uk
http://www.bris.ac.uk/chemistry/people/tim-c-gallagher/overview.html
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201501492.
© 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. ·
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in
any medium, provided the original work is properly cited.
Scheme 2. Cu-catalyzed borylation of 1a under (a) Marder's conditions for
alkyl halides and (b) with key modifications including an external iodide
source to provide boronic ester 2a. [a] Enantiomeric purity of 2a was deter-
mined by using ( )-2a from ( )-1a as a standard, and no erosion of stereo-
chemical integrity was detected by chiral HPLC.
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Replacing CuI with either Pd2dba3 or NiI2 gave no trace of react, and subjecting the 5-benzyl sulfamidate 1t to the opti-
2a, and the combination of CuI and PPh3 was crucial for conver- mized borylation conditions led only to allylic amine 3[12] by
sion; 2a was not observed in the absence of either of these competing elimination (Scheme 3).
reagents. Addition of Bu4NI, which had been applied success-
fully[5d] (but also without PPh3) to the borylation of alkyl tos-
ylates, improved significantly the yield of 2a, and the presence
of this external iodide source was critical.[11]
Using the iodide-mediated conditions shown in Scheme 2,
we have assessed the scope of this reaction with respect to 1,2-
and 1,3-cyclic sulfamidates. A range of substrates across 1b–p
function to provide (β- and γ-aminoalkyl)boronic esters 2b–p
(Scheme 2, Table 1). These include substrates with a substituent
at the reacting center (and disubstituted variants) as an entry
to secondary boronic esters (see also Scheme 4).
Scheme 3. Substrates that failed to undergo Cu-catalyzed borylation. [a] Gen-
eral Cu-catalyzed borylation conditions as in Table 1.
Table 1. Synthetic scope of cyclic sulfamidates for (β- and γ-aminoalkyl)-
boronic esters 2b–p.
Disubstituted cyclic sulfamidates 1u–w provide stereochemi-
cally defined products but, to date, are less efficient in terms
of conversion and yield (Scheme 4). In these cases, the major
byproducts observed, 5 and 6, resulted from C–O reduction.[13]
Nevertheless, these substrates also reflect on the nature of the
mechanism of Cu-catalyzed borylation, which is discussed be-
low.
[a] The precursor cyclic sulfamidates 1b–p are not shown explicitly but corre-
spond directly to the products 2b–p seen here. [b] The feasibility of a sulfon-
amide-based substrate depends on ring size (compare 2e and 2m). [c] Prod-
uct 2h was racemic (see text), and 2i was assumed to be racemic.
Scheme 4. Stereochemical outcomes for disubstituted 1,2- and 1,3-cyclic sulf-
amidates. [a] General Cu-catalyzed borylation conditions as in Table 1.
[b] Stereochemistry of 2u–w was determined by oxidation (NaOH, H2O, H2O2,
0 °C) to give the corresponding secondary alcohol as illustrated by the con-
version of 2u to 4. Analogous transformations (leading to either the same
A number of constraints and limitations were also apparent.
Electron-withdrawing carbonyl-based N-protecting groups (Boc,
Cbz) were highly preferred and essential within 1,2-cyclic sulf-
amidates for effective conversion. In these cases, the corre-
sponding N-benzyl, N-H and N-sulfonyl variants failed to give
the desired boronic esters 2c–e. Conventionally, 1,2-cyclic sulf-
amidates are more reactive (e.g., towards ring-opening with nu-
cleophiles[6]) than the 1,3-homologues; therefore, it was inter-
esting that here the 1,3-series 1 (n = 1) proved in general to be
somewhat better substrates for Cu-catalyzed borylation (com-
pare yields of 2a and 2l; 2f and 2n). Certain benzylic substrates,
such as 1q and 1r, underwent rapid decomposition (likely oxid-
ation or elimination), but the location of the benzyl moiety is
amino alcohol used to prepare the starting cyclic sulfamidate or its diastereo-
mer) secured the stereochemistry of boronic esters 2v and 2w (see Support-
ing Information).
In the cases of the product boronic esters 2u–w, the stereo-
chemistry of the borylation process was determined by oxid-
ation of the boronic esters to the corresponding (and known)
secondary alcohols. This process proceeds with retention of
stereochemistry at the reacting center and is illustrated here for
2u.
Given our initial assumptions as to the alkyl pseudohalide
important (compare 1o to give 2o; Table 1). Sterically demand- character of cyclic sulfamidates but their failure, for example, to
ing substrates, such as the 4,4-dimethyl variant 1s, failed to undergo borylation under otherwise well-established condi-
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tions, we have carried out a series of experiments to gain some ate and, additionally, that the presence of the N-sulfate moiety
insight into the mechanism of Cu-catalyzed borylation of 2. The favorably influences the subsequent Cu-catalyzed borylation,
role for an external iodide source was explored, prompting the based on the comparison to a simple primary alkyl iodide.
obvious explanation that facile nucleophilic opening of the cy-
clic sulfamidate provides directly a simple alkyl iodide that re-
acts as expected on the basis of Marder's earlier work.[5d]
The mechanism of Cu-catalyzed borylation of aryl and alkyl
halides has been probed,[15] and there is some but not defini-
tive evidence for the intermediacy of alkyl radicals. Our studies
Two substrates were investigated by directly comparing the support the likely involvement of radical species, at least with
reactivity of the cyclic sulfamidate and the equivalent alkyl respect to the carbon center. Using 1 equiv. of TEMPO as a
iodide (Scheme 5). As discussed above, 1,2-cyclic sulfamidate radical trap with 1f as the cyclic sulfamidate substrate, we iso-
1a underwent Cu-mediated borylation to give 2a in 56 % yield lated adduct 10 in 37 % yield and observed no borylation (i.e.
at room temperature after 2 h [Scheme 5 (a)]. The correspond- ester 2f) product [Scheme 6 (a)]. Cyclohexa-1,4-diene also com-
ing primary iodide 7 failed to react appreciably at room temper- pletely suppressed boronic ester formation from 1f, although
ature; however, boronic ester 2a was obtained in essentially the we were unable to isolate an adduct in this case.[13]
same yield following heating at 80 °C for 18 h.
Scheme 6. (a) Use of TEMPO as a C-radical trap and (b) loss of enantiomeric
integrity for a 5-methyl-1,2-cyclic sulfamidate on borylation.
Further support for a radical mechanism comes from use of
enantiomerically pure 5-substituted 1,2-cyclic sulfamidate 1h
[Scheme 6 (b)]. Copper-catalyzed borylation of (R)-1h under
standard conditions gave a 56 % isolated yield of racemic sec-
ondary boronic ester 2g, a result that was confirmed by using
racemic 1h as a control to validate chiral HPLC analysis of the
products.[14] It is also pertinent to reflect on the stereochemical
results shown in Scheme 4 for disubstituted 1,2- and 1,3-cyclic
sulfamidates. Borylation of trans-1u gave boronic ester 2u in
45 % yield, the stereochemistry of which was established by
stereospecific oxidation to give the known alcohol 4. In this
case, although stereochemically clean, the original configura-
tion at the C-5 stereocenter was lost. Similarly, the cis-config-
ured 1,3-cyclic sulfamidates 1v and 1w gave retention and in-
version, respectively, at the borylating center (C-6), where the
outcome at the intermediate carbon radical is subject to a high
level of internal diasteromeric control. In these latter cases, al-
though yields were moderate (and competitive reduction was
seen), no trace of the diastereomeric boronic esters was de-
tected.
Scheme 5. Cyclic sulfamidate vs. alkyl iodide; comparison of relative reaction
rates and yields. (a) Differences in time/reaction temperature to achieve the
same conversion. (b) Differential yields at room temperature after a fixed time
(2 h). (c) Generation, isolation of N-sulfate 9 and conversion to boronic ester
2k.
The 1,3-variant 1k provided an alternative perspective of this
differential reactivity. While 1k gave boronic ester 2k in 77 %
yield at room temperature after 2 h, the corresponding alkyl
iodide 8 led to 40 % of 2k under the same conditions but with
added iodide [Scheme 5 (b)]. These results both suggest that a
simple alkyl iodide (7 or 8) is not an intermediate in the conver-
sion of cyclic sulfamidates to the corresponding boronic esters.
Further insight into this process was gleaned from closer
study of the initial reaction of the cyclic sulfamidate with iodide.
Exposure of 1k to NaI (in the presence of Et3N, which was es-
sential to retain the N-sulfate unit) in anhydrous acetone gave
the hydrolytically sensitive N-sulfate 9 as a colorless solid
[Scheme 5 (c)]. Subsequent exposure of isolated 9[14] to the
Conclusions
standard Cu-mediated borylation procedure gave 2k in 77 % We have developed a variant of copper-catalyzed borylation
yield, which parallels the observation in Scheme 5 (b). From this that is suited to the exploitation of readily available cyclic sulf-
we conclude that the N-sulfated iodide 9 is the key intermedi- amidates and extends the scope of the copper-based proce-
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[4]
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dures to synthesize stereochemically defined alkylboronic
esters. Cyclic sulfamidates are readily available and offer access
to enantiomerically (or diastereomerically) pure (aminoalkyl)-
boronic esters suitable for further exploitation in cross-coupling
processes. While our initial premise was that cyclic sulfamidates
would perform as pseudohalides (cf., tosylates) and undergo
borylation under well-established conditions, that proved not
to be the case. There are substrate-specific features, such as the
intermediacy of an N-sulfated iodide that clearly imbue cyclic
sulfamidates with major advantages over the corresponding
(i.e., simple amino-based) alkyl halides. Another important as-
pect is the key requirement of an N-acylated cyclic sulfamidate;
N-alkyl/benzyl or N-sulfonyl analogues are not efficient sub-
strates in our hands. Further studies are required to probe the
role of the N-acyl moiety, but this may, like the N-sulfate, serve
to activate the reacting C–O bond and/or provide a ligand for
any intermediate copper species. The involvement of radical
intermediates in this chemistry can lead to loss of stereo-
chemistry at the reacting C–O center, but the outcome is sub-
strate-dependent and can provide good overall levels of stereo-
control.
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CCDC 1433121 (for 2a) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre.
[8]
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Acknowledgments
We thank Dr. M. Haddow for X-ray crystallographic analysis of 2a.
N. U. thanks the Bristol Chemical Synthesis Centre for Doctoral
Training for a studentship funded by EPSRC (EP/G036764/1).
Keywords: Cyclic sulfamidates · Boron · Copper · Radical
reactions · (Aminoalkyl)boronic esters
[9]
By using 1a, Pd-catalyzed borylation was extensively screened. Despite
evaluating a wide range of catalyst/ligand combinations and reaction
conditions, we were able to obtain 2a (from 1a) but in (at best) 28 %
yield. This result was achieved by using Biscoe's conditions:[4a] Pd2dba3
(0.5 mol-%), tBu2MePHBF4 (3 mol-%), K3PO4·H2O (2 equiv.), B2pin2
(1.2 equiv.), H2O (15 equiv.), tBuOH, 60 °C, 18 h.
X-ray crystallography also served to confirm the structure of 2a; see
Supporting Information.
Cu-catalyzed borylation of 1a also works with Bu4NBr, but the yield of
2a dropped to 29 %. All optimization details (Cu, halide sources, solvents
etc.) are available in the Supporting Information.
Cyclic sulfamidate 1t (and other N-substituted analogues) is sensitive to
base-mediated elimination and are therefore useful to employ[6b] as test
substrates to help to define the limits of a methodology under study.
Preliminary efforts to suppress the formation of reduction products such
as 5 and 6 centered on the choice of solvent, and DMF proved to be
the most effective; see Supporting Information.
Molecular ion peak for N-sulfate intermediate 9 was observed by nega-
tive-ion ESI-MS. A change in the C=O stretching frequency of the Boc
group was also seen by IR spectroscopy. Bu4NI did not affect the conver-
sion of iodide 9 into boronic ester 2k.
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The mechanism of Cu-catalyzed borylation has been studied by
Marder,[5d,5f] Chung[5b] and Ito,[5e] and various options have been recog-
nised. Chung observed ring opening of cyclopropylmethyl bromide [to
give the corresponding (3-butenyl)boronate]. Marder has reported that
borylation of 6-bromohex-1-ene leads to cyclopentylmethyl boronate
(via a cyclopentylmethyl radical?), but attempts to scavenge a radical
[15]
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intermediate by using cyclohexa-1,4-diene failed. We have been able to
suppress borylation using two radical traps (TEMPO and cyclohexa-1,4-
diene), and the consequences adjacent to the reacting C–O bond – 2u
and 2w both show a change in the stereochemistry at the reacting cen-
ter; Schemes 4 and 6 (b) – support the participation of a carbon-cen-
tered radical under these Cu-catalyzed conditions. Under Pd-mediated
borylation conditions, (R)-1h gave a poor (18 %) yield of 2h but as a
single enantiomer by HPLC, although the absolute configuration of this
product has not yet been determined.
Received: November 27, 2015
Published Online: December 23, 2015
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Boronic Esters
Copper-catalyzed borylation of 1,2-
and 1,3-cyclic sulfamidates provides
stereochemically defined (β- and γ-
aminoalkyl)boronate esters; iodide is
essential but borylation is accelerated
by the presence of the N-sulfate
moiety.
N. Ursinyova, R. B. Bedford,
T. Gallagher* .............................. 673–677
Copper-Catalyzed Borylation of Cy-
clic Sulfamidates: Access to Enantio-
merically Pure (β-and γ-Amino-
alkyl)boronic Esters
DOI: 10.1002/ejoc.201501492
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