Silicon-directed rhenium-catalyzed allylic carbaminations and oxidative fragmentations of γ-silyl allylic alcohols

Article abstract of DOI:10.1002/chem.201400104

A highly regioselective allylic substitution of β-silyl allylic alcohols has been achieved that provides the branched isomer as a single product. This high level of regiocontrol is achieved through the use of a vinyl silane group that can perform a Hiyama coupling providing 1,3-disubstituted allylic amines. An unusual oxidative fragmentation product was also observed at elevated temperature that appears to proceed by a Fleming-Tamao-type oxidation-elimination pathway.

Full text of DOI:10.1002/chem.201400104

DOI: 10.1002/chem.201400104
Communication
&
Organic Synthesis
Silicon-Directed Rhenium-Catalyzed Allylic Carbaminations and
Oxidative Fragmentations of g-Silyl Allylic Alcohols
Sanjay W. Chavhan and Matthew J. Cook*[a]
and most widely available rhenium source and is significantly
Abstract: A highly regioselective allylic substitution of b-
less costly than many Ru, Rh, Ir, Pt, and Pd catalysts.[15] Ghorai
silyl allylic alcohols has been achieved that provides the
demonstrated the use of Re2O7 as a catalyst for the carbamina-
branched isomer as a single product. This high level of re-
tion and amidation of 1,3-disubstituted allylic alcohols includ-
giocontrol is achieved through the use of a vinyl silane
ing one example of an unsymmetrical substrate.[14a] This single
group that can perform a Hiyama coupling providing 1,3-
example provided the conjugated product as a single isomer
disubstituted allylic amines. An unusual oxidative frag-
[Eq. (1)]. We hypothesized that we could reverse this selectivity
mentation product was also observed at elevated temper-
through the use of a silicon-directing group thus producing
ature that appears to proceed by a Fleming–Tamao-type
the benzylic carbamate.[16] The corresponding vinylsilane could
oxidation–elimination pathway.
be elaborated further through reactions such as the Flem-
ming–Tamao[17] oxidation or Hiyama coupling.[18] Herein, we
report a Re2O7-catalyzed carbamination and amidation reaction
Allylic amines are versatile reagents that have seen many appli-
cations in synthesis including amino acid synthesis and hetero-
cycle formation.[1] The two most commonly used pathways to
install these functional groups are by nucleophilic attack of an
a,b-unsaturated imine or through allylic substitution reactions.
Although the nucleophilic attack of an organometallic (or hy-
dride) can provide the requisite allylic amine, the nature of the
nucleophile can lead to issues with functional-group toler-
ance.[2] Transition-metal-catalyzed allylic aminations, which pro-
ceed by p-allyl intermediates, have become a powerful tool
and operate under mild reaction conditions.[3] The regiochem-
istry of these reactions is generally not determined by the leav-
ing-group position but by the catalyst used and the substitu-
ents that are present on the allylic group. In general, palladium
catalysts provide linear products,[4] whereas most other metals
(Ru, Rh, Ir, Mo) provide the regioisomeric branched product.[5]
In particular, iridium(I) catalysts have been shown by several
groups to provide the branched amine products with high
regio- and stereoselectivity.[6] The use of unsymmetrical disub-
stituted allylic systems has not been reported to the same
level.
with complete nonconjugated selectivity alongside an unpre-
cedented oxidative fragmentation to form cinnamaldehydes
[Eq. (2)].
Firstly we had to establish the formation of an allylic cation
that was proposed by Ghorai but was not proved conclusively.
All of their examples had substitution occurring on the same
carbon atom as the alcohol with no allylic transposition report-
ed, although they did note the reaction was not stereospecific,
which suggests the intermediacy of a planar cation. To corrob-
orate the proposal of an allylic cation intermediate, we used al-
lylic alcohol 1 in the reaction and found that the major carba-
mation product was indeed the linear product 2 with no ob-
servable traces of branched product. The remainder of the
mass balance was made up of dimeric ether product 3 result-
ing from the reaction of alcohol 1 with the allylic cation
[Eq. (3)].
Rhenium has emerged recently as a competent homogene-
ous catalyst for many reactions including: glycosidations,[7] the
Meyer–Schuster rearrangement,[8] reductive aminations and
imine reductions,[9] macrolactonizations,[10] allyic rearrange-
ments,[11] cyclizations of hydroxylpolyenes,[12] annulations reac-
tions,[13] and allylic substitutions.[14] Re2O7 is the least expensive
[a] Dr. S. W. Chavhan, Dr. M. J. Cook
School of Chemistry and Chemical Engineering
Queen’s University Belfast
Belfast. BT9 5AG (Northern Ireland)
Fax: (+44)28-9097-6524
E-mail: m.cook@qub.ac.uk
Following the confirmation of an allylic cationic intermedi-
ate, we examined whether we could use a silyl group to direct
addition to the benzylic, less conjugated terminus. Indeed, this
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400104.
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Table 1. Substrate scope.
Table 2. Silane scope.
Entry
1
Product
Entry
4
Product
Entry Product
Entry Product
1
7
2
3
5
6
2
3
8
9
[a] Isolated yield of a pure single isomer.
4
10
5
6
11
12
Scheme 1. Hiyama couplings of 7b. dba=dibenzylideneacetone; TBAF=te-
tra-n-butylammonium fluoride.
diaryl allylic carbamates 8a–c. A sp2–sp3 coupling was also ach-
ieved with benzyl bromide to afford 8d. This allowed us
a rapid and completely regioselective entry into 1,3-disubsti-
tuted allylic carbamate compounds.
[a] Isolated yield of a pure single isomer. [b] Reaction times were general-
ly 3 h; for exact timings of each reaction see the Supporting Information.
The use of different aminating agents was then explored.
We found that all primary carbamate groups worked in similar
yields to the carbobenzyloxy (Cbz) group allowing for the in-
stallation of tert-butoxycarbonyl (Boc) 9a and methyl carba-
mate groups 9b (Table 3). Amides were also examined and
was the case and the scope of this reaction was examined
(Table 1). We found the reaction to be tolerant of a host of
functionality including ethers, aryl halides, free phenols 5a–h.
In general, the more electron-rich the aromatic ring the faster
the reaction with trimethoxybenzene substrate 5j affording
full conversion in just 5 min. Of particular note is the use of
the bis(allylic) alcohol 4k that provides the corresponding bis-
(allylic) carbamate as a single regioisomer in good yield. When
strongly electron-withdrawing substituents are used, the reac-
tion slows significantly with 4-nitrophenyl substrate 5l under-
going the transformation in a much reduced yield. When alkyl-
substituted compounds are used, the amination reaction does
not proceed, with several unidentifiable byproducts detected
alongside unreacted starting alcohol. In all cases only one re-
gioisomer was ever observed from this reaction.
Table 3. Carbamate/amide scope.
Entry
1
Product
Entry
4
Product
5
6
7
We next examined whether other silanes could be incorpo-
rated to further increase the synthetic utility of the products
(Table 2). We found that the reaction was tolerant of a wide
range of alkyl, aryl, allyl, and benzyl substituents with little
effect on reactivity, with all providing similar results to the
parent phenyldimethylsilane as a single regioisomer. In particu-
lar, benzyldimethylsilane 7b was of interest due to its uses in
Hiyama coupling reactions (Scheme 1).[19]
2
3
8
Indeed, the benzyldimethylsilyl substrate 7b was capable of
undergoing Hiyama couplings. A range of sp2–sp2 coupling
could be achieved with a variety of aryl iodides to form 1,3-
[a] Isolated yield of a pure single isomer. [b] Reaction performed at 458C.
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both primary and secondary amides coupled in a similar
manner to the carbamates albeit in slightly reduced yields
9c,d. Finally we examined cyclic carbamates and in particular
oxazolidinones to examine whether any stereoselectivity could
be imparted from these chiral groups. These did couple in
good yield and with moderate selectivity with a 2:1 diastereo-
meric ratio being observed in all cases when using the iPr, Ph,
Bn, and indane groups on the oxazolidinone 9e–h.
Table 4. Scope of the oxidative rearrangement.
Entry
1
Product
Entry
5
Product
During the course of our studies we discovered an interest-
ing byproduct when the reaction was performed at elevated
temperatures. Instead of the carbamate product 5a being pro-
duced, cinnamaldehyde 10a was formed in 58% yield
[Eq. (4)].[20] To investigate this further, we performed a series of
control experiments. Firstly, in the absence of benzyl carba-
mate, cinnamaldehyde was produced in almost identical yield
to that shown in Equation (4), thus ruling out any role of
benzyl carbamate in the formation of 10a [Eq. (5)].
2
6
3
4
7
8
[a] Isolated yield.
use of enantioenriched allylic alcohol (S)-4a was found to lead
to a racemic mixture of product 10a [Eq. (7)]. We also incorpo-
rated deuterium at the benzylic position and found no erosion
of deuterium content with the corresponding deuterated prod-
ucts 12 and 13 being isolated with complete deuterium
incorporation.
Kinetic isotope experiments were performed and a kH/kD
value of 1.15 for the carbamation reaction was found [Eq. (8)],
which suggested that the formation of a sp2 carbocation at
the benzylic position was rate limiting. For the oxidative re-
arrangement reaction, a kH/kD value of 1.04 was obtained,
which indicated proton transfer or change of geometry was
unlikely to be the rate-limiting step.
We also examined whether the addition of additional H2O
would increase the formation of 10a. When between 1 and
10 equivalents of H2O were used there was no observable in-
crease in the yield of 10a, thus suggesting that a pathway
other than an exogenous nucleophilic attack in operation
[Eq. (6)].
A brief investigation of the substrate scope was carried out,
which revealed that this oxidative rearrangement occurs in
several other aromatic substituted substrates 10b–g in moder-
ate to good yields (Table 4). Alkyl substituents do not perform
this rearrangement with quantitative recovery of starting mate-
rial. Interestingly, many of products 10 are formed as a mixture
of E/Z isomers, with the major isomer being the E in all cases.
To help elucidate both mechanistic pathways in operation
(Scheme 2), we performed several key experiments. Firstly, the
Re2O7 has poor solubility in anhydrous methylene chloride
and the reactions are performed by using undried commercial
solvent and open to the atmosphere. These factors, combined
with the formation of water during the reaction, allow for the
possibility that perrhenic acid is actually the active cata-
Scheme 2. Control experiments.
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lyst.[21–23] To test this, we investigated the effect of basic addi-
tives on the reaction. Initially we examined an amine base and
found that a catalytic amount of Et3N completely inhibits both
reaction pathways. As amine bases can also coordinate with
the Re catalyst we tested the heterogeneous, inorganic base
K2CO3. Once again both reactions are inhibited and no prod-
ucts are formed, which suggests that the active catalyst is
indeed perrhenic acid or a derivative (Scheme 3).
Tamao reaction with air as the terminal oxidant. Once the enol
ether has been formed, a very rapid acid-mediated elimination
reaction can occur to form cinnamaldehyde. The formation of
E/Z isomers also suggests that this type of elimination is occur-
ing, as Re-catalysed allylic transposition reactions, such as the
Meyer–Schuster rearrangement, generally occur with complete
E control.
In conclusion, we have applied Ghorai’s method to provide
a completely regioselective allylic amination reaction with
complete nonconjugated selectivity. We have used the result-
ing vinyl silane in both sp2 and sp3 Hiyama couplings and ex-
plored the scope of aminating agent and silane. We have also
discovered what we believe to be a completely new oxidative
rearrangement that provides cinnamaldehydes. Mechanistic in-
vestigations have demonstrated the pathways involved for
both reactions that are in accordance with the data obtained.
Experimental Section
Scheme 3. Possible mechanistic pathways.
Allylic carbamination reaction
Re2O7 (1.5 mol%) was added to a stirred solution of allylic alcohol
(1 equiv) and amine (1.2 equiv) in dichloromethane (0.2m) and the
solution was stirred at room temperature (or at 458C, as men-
tioned in the table) in a flask open to the atmosphere. After the
completion of the reaction, as determined by TLC analysis, saturat-
ed ammonium chloride (0.5 mL per 1 mL of dichloromethane) was
added and the solution was extracted with dichloromethane (3ꢁ
10 mL). The combined organic extract was dried over anhydrous
sodium sulfate, filtered, concentrated, and then chromatographed
(typically 9:1 hexane/ethyl acetate) to afford requisite (E)-allylic
carbamate.
We also performed the reaction under strictly anhydrous
and anaerobic conditions using freshly distilled CH2Cl2 and
under an argon atmosphere, respectively. When anhydrous
conditions were used, no reaction was observed in either path-
way, which indicated that perrhenic acid was the active cata-
lyst. When oxygen was excluded from the reaction, amination
still proceeded albeit in a slightly reduced yield; however, the
formation of cinnamaldehyde was completely inhibited, which
suggested molecular oxygen was participating in the reaction
(Scheme 3).
Oxidative fragmentation reaction
The control experiments, combined with the kinetic isotope
effect data allows us to postulate mechanisms for both path-
ways. The carbamation reaction appears to be catalyzed by
perrhenic acid instead of Re2O7, as in the absence of water no
reaction is observed. This acidic catalyst mediates the rate-lim-
iting formation of an allylic cation that can undergo outer-
sphere attack by the nucleophile. When basic additives or nu-
cleophiles are used, the reaction is retarded, thus suggesting
acidic catalysis. Alternative acid catalysts were tried with a simi-
lar pKa to [Re2O7(H2O)2] (À1.25) and no reaction was observed,
which suggested that while perrhenic acid was the catalyst it
operated in a more complex manner than simple acid catalysis.
The mechanistic pathway for the formation of cinnamaldehyde
10a proceeds by a very different pathway. The very small ki-
netic isotope effect suggests there is no formation of an allylic
cation prior to the rate-limiting step and instead CÀSi oxida-
tion takes place in a Flemming–Tamao-type reaction. This
could occur either through oxidation of the Si atom followed
by C-migration, as in the Flemming–Tamao, or by an epoxyda-
tion of the vinyl silane followed by rearrangement.[24] The
former is much more likely as no intermediates were ever de-
tected and a very clean reaction occured with the remainder
of the mass balance containing starting allylic alcohol. Modu-
lating the silane used to less oxidizable groups led to poor
conversions, which indicates a catalytic oxidative Flemming–
Re2O7 (1.5 mol%) was added to a stirred solution of allylic alcohol
(1 equiv) and amine (1.2 equiv) in dichloromethane (0.2m) and the
solution was heated at 458C overnight in a round-bottomed flask
fitted with a condenser open to the atmosphere. After the comple-
tion of the reaction, as determined by TLC analysis, saturated am-
monium chloride (0.5 mL per 1 mL of dichloromethane) was added
and the solution was extracted with dichloromethane (3ꢁ10 mL).
The combined organic extract was dried over anhydrous sodium
sulfate, filtered, concentrated and then chromatographed (typically
9:1 hexane/ethyl acetate) to afford requisite cinnamaldehyde.
Acknowledgements
This work was generously supported by the EU through
a Marie-Curie International Incoming Fellowship (S.C.).
Keywords: allylic substitution
coupling · rhenium · silicon
· carbamination · Hiyama
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Received: January 9, 2014
Published online on March 27, 2014
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