The Newman-Kwart rearrangement of O-aryl thiocarbamates: Substantial reduction in reaction temperatures through palladium catalysis

Full text of DOI:10.1002/anie.200903908

Communications
DOI: 10.1002/anie.200903908
Palladium Catalysis
The Newman–Kwart Rearrangement of O-Aryl Thiocarbamates:
Substantial Reduction in Reaction Temperatures through Palladium
Catalysis**
Jeremy N. Harvey, Jesffls Jover, Guy C. Lloyd-Jones,* Jonathan D. Moseley, Paul Murray, and
Joseph S. Renny
Thermally induced OAr!SAr migration in aryl thiocarbamates
(1!2, Scheme 1)[1] is commonly referred to[2] as the
Newman–Kwart rearrangement (“NKR”)[3] and belongs to
a group of rearrangements that generate Ar–S/N compounds
from phenols.[4] Of these, only the NKR has been extensively
utilized,[1] with applications as broad-ranging as medicinal
chemistry,[5] chiral ligand synthesis,[6] supramolecular chemis-
try,[7] molecular switches,[8] molecular rods,[9] dendrimers,[10]
organocatalysts[11] and helicenes.[12] The NKR has also been
applied industrially,[1,5] more recently by applying micro-
wave[13,14a] and flow-reactor technologies.[14]
Scheme 2. Strategies for catalysis (B, C) of the thermal NKR (A).
reaction conditions.[17] Clearly any significant reduction in
reaction temperature would be advantageous. Herein we
report on the development of the first catalyst[18,19] for the
NKR 1!2, facilitating rearrangement at substantially lower
temperatures.
Scheme 1. Classic thermal Newman–Kwart rearrangement (NKR).[1, 2]
We considered two distinct approaches to facilitate
catalysis of the reaction. In the first (B, Scheme 2) a p-acid
“M” could be used to increase the electrophilicity of the aryl
ring, thus lowering the barrier to 1,3-oxathietane genera-
tion.[16,18] Choosing one of the most active substrates for NKR,
There are many favorable aspects to NKR,[1] including:
1) the facile generation of 1 from the corresponding phenol, a
moiety that is readily accessible and is often commercially
available;[15] 2) simple hydrolysis of 2 liberates the thiol,
ArSH; 3) the thiocarbamate group provides both the Ar–O
activation and the source of sulfur—no additional reagents
are required; and 4) 1 and 2 are usually highly crystalline,[13c]
aiding handling and purification.
The one major drawback to NKR is that high temper-
atures (200–3008C) are required to access the strained 1,3-
oxathietane transition state (see mechanism A,[1,16]
Scheme 2). Not only do such high temperatures present
issues in terms of practicality and safety, but they can also
induce “charring” and other undesired side reactions, mean-
ing that fragile substrates are not amenable to the harsh
X = p-NO2 (1a) which undergoes thermal NKR at 1808C,[3b]
a
range of cationic and neutral Lewis acids were explored in this
regard and whilst some success was obtained with Ni and Mg
complexes, the catalytic effects were specific to 1a[20] suggest-
ing that activation is by complexation to the NO2 group,[21,22]
not to the aromatic ring p-system.
We then explored an alternative strategy[18b] (C) in which
ꢀ
the Ar O bond is cleaved by insertion of a low-valent metal
“M”, with the thiocarbamate tautomer facilitating reductive
ꢀ
elimination of M as the new Ar S bond is formed. Given the
known oxidative addition of Pd0 complexes to arylsulfo-
nates,[23] we tested a range of simple phosphine ligands in
combination with 10 mol% Pd0 in N,N-dimethylacetamide at
708C (P/Pd = 2). Of these ligands, tBu3P was uniquely
effective, giving rise to 47% 2a after 24 h. The effect of
solvent was then explored and from N,N-dimethylacetamide,
MeCN, CH2Cl2, THF, PhCF3 and toluene, the latter was found
to be by far the most effective, allowing quantitative
conversion of 1a at 1008C in 2.5 h with just 2 mol% Pd
(Scheme 3, and Table 1, entries 1–4). In the absence of
catalyst, there was no detectable rearrangement.
[*] Prof. Dr. J. N. Harvey, Dr. J. Jover, Prof. Dr. G. C. Lloyd-Jones,
J. S. Renny
School of Chemistry, University of Bristol
Cantock’s Close, Bristol, BS81TS (UK)
Fax: (+44)117-929-8611
E-mail: guy.lloyd-jones@bris.ac.uk
Dr. J. D. Moseley, Dr. P. Murray
AstraZeneca, Avlon Works, Severn Road, Hallen, Bristol (UK)
[**] We thank AstraZeneca Global PR&D for generous funding and Dr.
Ross T. Burn (AZ) for assistance with MS analysis. G.C.L.-J. is a
Royal Society Wolfson Research Merit awardee.
The [Pd(tBu3P)2] catalyst was tested with a small range of
simple aryl thiocarbamates (1a–h, 1008C, toluene, Table 1,
entries 5–11). In all cases rearrangement was catalyzed,
allowing NKR at a substantially lower temperature than
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903908.
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Scheme 3. Palladium-catalyzed NKR of thiocarbamate 1a. Conversions
determined by HPLC; limits of detection 0.05%.
Table 1: Pd-catalyzed NKR versus standard thermal conditions.[3b,13a]
Entry
Ar=
Catalyzed[a]
t [h]
Thermal[b]
T
Conv.,[c]
T
1[d]
2[d]
3[d]
4[d]
5[d]
6[d]
7[f]
p-NO2-C6H4 (1a)[e]
p-NO2-C6H4 (1a)[e]
p-NO2-C6H4 (1a)[e]
p-NO2-C6H4 (1a)[e]
p-CN-C6H4 (1b)[e]
p-CF3-C6H4 (1c)[e]
p-F-C6H4 (1d)[g]
24
24
24
2.5
4
3%, 218C
15%, 508C
66%, 708C
1808C
1808C
1808C
1808C
2208C
2608C
>2958C
2408C
>2958C
>2958C
2858C
>99%, 1008C
>99%, 1008C
>99%, 1008C
98%, 1008C
>99%, 1008C
90%, 1008C
92%, 1008C
>99%, 1008C
4
21
12
14
14
4
8[d]
9[h]
10[i]
11[d]
p-CO2Me-C6H4 (1e)[g]
p-Me-C6H4 (1 f)[g]
p-MeO-C6H4 (1g)[g]
2-C10H7 (1h)[e]
Figure 1. Effect of [Pd] and [tBu3P] on the extent of maximum cross-
over [%] as a function of conversion (x-axis) of [18O1]-1a and [2H2]-1a.
The lines through data points are solely an aid to the eye.
[a] Catalyst [Pd(tBu3P)2], 1008C, 1a–h in toluene. [b] Ref. [3b,13a].
[c] Conversion determined by HPLC analysis. [d] 2 mol% Pd.
[e] 0.111m. [f] 4 mol% Pd. [g] 0.221m. [h] 6 mol% Pd. [i] 8 mol% Pd.
aggregation, for example, dimerization, of the catalyst resting
state, which is attenuated by 1a and by tBu3P.
Analysis of the [Pd(tBu3P)2]-catalyzed NKR of 1a by DFT
(B3LYP/6-31G*/lacv3p)[24] suggests that catalyst activation
proceeds through associative substitution (I, Scheme 4) of
tBu3P by substrate 1a (a dissociative mechanism may also be
possible).[27] Coordination of Pd to the thiocarbonyl sulfur of
1a in the resulting Pd0 species [Pd(1a)(tBu3P)] was found to
that required for the standard thermal conditions. Electron-
rich substrates, which are less reactive in the standard
NKR,[3b] were also less susceptible to catalysis. Nonetheless,
in no cases were any side-products detected, and the reaction
was readily scaled up. For example, 2 g of naphthyl substrate
1h (1.8m in toluene, 2 mol% [Pd(tBu3P)2], 1008C, 4 h,
> 99%) gave analytically pure 2h in 90% isolated yield. In
comparison, the uncatalyzed rearrangement requires heating
to 2858C and affords 80% 2h.[3b]
ꢀ
facilitate oxidative insertion (II) into the Ar O bond through
=
a five-membered (-S-Pd-CAr-O-C( O)-) transition state (3a).
Extensive attempts to locate a three-membered (Pd-CAr-O)
transition state for a conventional oxidative addition pro-
cess[28] were unsuccessful; this TS may exist but would
The mechanism of the [Pd(tBu3P)2]-catalyzed reaction of
1a was investigated. Reaction rates displayed a predomi-
nantly first-order dependency on both [1a] and [Pd] with a
slight acceleration, not inhibition, on addition of excess tBu3P
ligand.[24] Reaction of a mixture of [2H2]-1a and [18O1]-1a,
produced not only the expected isotopologues [2H2]-2a and
[18O1]-2a, but also generated unlabelled 2a and double-
labelled [2H2,18O1]-2a (Figure 1) through cross-over of the
thiocarbamoyl moiety with the aromatic ring.[24]
The cross-over is only partial (30–50% of maximum),
increasing throughout reaction and becoming substantial at
ꢀ
high conversions. This eliminates sole turnover through [Pd
ꢀ
S(CO)NMe2] or [Pd Ar] carriers, which would lead to full
(100%) cross-over throughout the reaction. Control experi-
ments, including labelled products, e.g. [2H6,34S]-2a, showed
that there is no exchange in the substrate (1a), and very little
exchange in the product (2a), during the main phase of
reaction. However, as 1a becomes depleted, scrambling in 2a
begins to contribute extensively, leading to the steep upward
curvature in Figure 1.[26] The extent of cross-over during
productive turnover (1a!2a) increases with Pd concentra-
tion and decreases when excess tBu3P ligand is added. In
conjunction with the reaction kinetics, these factors suggest
Scheme 4. Working model for Pd-catalyzed NKR of 1a to 2a. Numbers
in parenthesis are DFT-derived ZPE corrected energies in kcalmolꢀ1
.
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certainly lie at higher energy. The predicted on-cycle resting
state is a k2-S(CO)NMe2 intermediate (4a), which can
undergo reversible reductive elimination (III) to give a
[Pd(2a)(tBu3P)] complex through
a more conventional
three-membered TS. Substitution of 2a by 1a leads, in the
predicted turnover-limiting step (IV), to product (2a) with the
regeneration of [Pd(1a)(tBu3P)]. Intermediate 4a can also
cross-over (V) through the corresponding m-O,S-bound dimer
(4)2. The equilibration of the resting state between (4a)2 and
[Pd(2a)(tBu3P)] accounts for the extent of cross-over increas-
ing with both conversion (decreasing [1a]) and catalyst
loading (increasing [4]). The TS for substitution of product
(2a) in [Pd(2a)(tBu3P)] by tBu3P (VI) is only 4 kcalmolꢀ1
higher in energy than that involving 1a (IV). The extent of
cross-over, particularly at high conversions of 1a, can thus be
attenuated by added tBu3P.
In summary, we report the first catalyst for the Newman–
Kwart rearrangement (1!2),[1] an efficient reaction for
generation of Ar–S compounds from phenols. This reaction
normally requires high temperatures (200–3008C), but in the
presence of [Pd(tBu3P)2] proceeds smoothly at 1008C with a
range of substrates (Table 1, entries 4–11). Substrates such as
1a which bear activating electron-withdrawing substituents,
rearrange, albeit inefficiently, at temperatures as low as 218C
(entry 1). Preliminary investigations suggest that a thio-
coordinated monophosphine–palladium complex, [Pd(1)-
(tBu3P)], engages in an oxidative addition/tautomerisation/
reductive elimination sequence,[18,29] and that the latter parts
of the cycle are readily accessible through re-insertion of Pd
into the CAr S bond of 2a.[26] The steric bulk in tBu3P appears
ꢀ
to favour loss of one phosphine from [Pd(tBu3P)2]; nonethe-
less other ligands, and indeed metals, may be more effective.
The thiocarbamate moiety, although readily installed and
inexpensive,[15] is not the ideal partner for the catalytic events;
other Ar-OC(X)-R species may undergo catalyzed rearrange-
ment at much lower temperatures.[30] We are currently
exploring these aspects in detail.
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Received: July 16, 2009
Published online: September 10, 2009
[11] O. Hara, T. Nakamura, K. Makino, Y. Hamada, Heterocycles
2006, 68, 101.
Keywords: homogeneous catalysis · palladium ·
reaction mechanism · rearrangement · sulfur compounds
.
´
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[3] a) H. Kwart, E. R. Evans, J. Org. Chem. 1966, 31, 410; b) M. S.
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[4] Smiles rearrangement: a) A. A. Levy, H. C. Rains, S. Smiles, J.
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[16] The NKR has been subject to detailed mechanistic studies, most
of which support reaction proceeding through A, see ref. [3b]
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[24] See Supporting Information for full details.
[25] An empirical rate equation ꢀd[1a]/dt = kobs[1a] satisfied most
of the data (kobs = k{[Pd(tBu3P)2]ꢀn}; k = 0.53mꢀ1 sꢀ1
, n =
1.1 mm), with slight upward curvature in plots of kobs vs
[Pd(tBu3P)2] and in ln([1a]0/[1a]t) vs t, particularly with excess
tBu3P ligand.
[26] Control experiments with a 1:1 mixture of [2H2]-2a/[18O1]-2a
(0.111m in toluene) confirmed that [Pd(tBu3P)2] (2 mol%)
catalyzed exchange, with equilibrium (equimolar [2H2]-2a/
[18O1]-2a/2a/[18O1,2H2]-2a) being attained in less than 600 s at
1008C. Cross-over or post-turnover exchange is degenerate in
the absence of isotopic labelling and should not affect synthetic
[17] See, for example: V. V. Kane, A. Gerdes, W. Grahn, L. Ernst, I.
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ꢀ
[18] a) Cross-coupling of Ar X with RSH has been developed
extensively, see: E. Alvaro, J. F. Hartwig, J. Am. Chem. Soc.
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thiocarbamate rearrangements: H.-J. Gais, A. Bꢀhme, J. Org.
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ꢀ
utility unless mixtures of dialkyl thiocarbamates (R2NC(S) OAr
ꢀ
+ R’2NC(S) OAr) are employed. Under such circumstances,
reactions run to ca. 90% conversion, using low catalyst loadings,
with a large excess of tBu3P ligand, would minimise cross-over.
[27] Z. Li, Y. Fu, Q.-X. Guo, L. Liu, Organometallics 2008, 27, 4043.
[28] M. Ahlquist, P.-O. Norrby, Organometallics 2007, 26, 550.
[29] The preliminary mechanistic study (kinetics, computation and
labelling) reported herein focused on the p-NO2-bearing sub-
strate 1a. As the identity of X is changed, the rate of turnover is
affected substantially (Table 1). Whether this arises from a
change in resting state, or in mechanism, and whether this affects
the extent of cross-over, has not yet been investigated. The
energy profile determined by DFT analysis in an identical
manner to 1a, but with X = H, showed a similar pathway but
with the following differences in transition state barriers:
ꢀ1.6 kcalmolꢀ1 for step I; + 4.0 kcalmolꢀ1 for step II; + 0.7 kcal
molꢀ1 for step III; ꢀ3.3 kcalmolꢀ1 for step IV; + 1.2 kcalmolꢀ1
for step VI. The key change is thus the oxidative addition step
(II), consistent with lower reactivity displayed by less electron-
withdrawing substituents (Table 1).
[19] a) Catalytic BF3 reduces the rearrangement temperature of
dimethyl-O-(3-pyridyl)thiocarbamate from 2508C to 1908C, see
ref. [3b]; b) see also: S. Narayan, V. V. Folkin, K. B. Sharpless in
Organic Reactions in Water: Principles, Strategies and Applica-
tions (Ed.: U. Lindstrom), Blackwell, Oxford, 2007, pp. 350 –
365.
[20] 5 mol% of Ni0 colloid (generated by in situ reduction of NiCl2
with TolMgCl, or by decomposition of [Ni(cod)2] (cod = cyclo-
octadiene)) or 100 mol% MgBr2, in refluxing toluene induced
complete rearrangement of o-, m- and p-1a. Substrates 1b, 1c,
1g and 1h failed to rearrange under these conditions.
[21] W. L. Driessen, L. M. Van Geldrop, W. L. Groeneveld, Recl.
Trav. Chim. Pays-Bas 1970, 89, 1271.
[22] [18O-1a]/[2H2-1a] rearranged without cross-over.
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[30] The hindered N,N-dimethylthiocarbamates derived from 2,2’-
dihydroxy-1,1’-binaphthalene (D. Fabbri, G. Delogu, O. De Luc-
chi, J. Org. Chem. 1993, 58, 1748) and hydroxy cyclophane
(ref. [17]) have, so far, failed to undergo catalyzed rearrange-
ment. However, it is anticipated that more efficient catalysts will
be developed in due course.
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