Striking AcOH acceleration in direct intramolecular allylie amination reactions

Article abstract of DOI:10.1002/chem.200901946

An experiment was conducted to demonstrate that a strong accelerating effect occurs in direct intramolecular allylic amination when the reaction is conducted in AcOH rather than in CHCl or THF. A round bottom flask under air was charged with N-tosylcarbamate 3a-j, Pd(OAc)₂, bis-sulfoxide ligand LL, phenylbenzoquinone and AcOH. The reaction was allowed to stir at 45°C for 24 h. The reaction mixture was hydrolysed and extracted with AcOEt. The combined organic layers were dried over MgSO₄, filtered, and concentrated in vacuo. Cyclisation of 3a was repeated in CH₂Cl ₂ and in AcOH by using stoichiometric amounts of Pd(OAc) without benzoquinone. Stoichiometric tests and computational DFT analysis of the palladium reoxidation step provide an overview of the structural and energetic role of acetic acid in increasing the efficacy of the entire catalytic cycle.

Full text of DOI:10.1002/chem.200901946

DOI: 10.1002/chem.200901946
Striking AcOH Acceleration in Direct Intramolecular Allylic Amination
Reactions
Fady Nahra,[a] Frꢀdꢀric Liron,[a] Guillaume Prestat,[a] Carlo Mealli,[b]
Abdelatif Messaoudi,[b] and Giovanni Poli*[a]
Direct functionalisation of hydrocarbons has attracted
much interest for its great potential in organic synthesis.[1]
These processes, by avoiding the introduction of functional
groups, have atom- and step-economical advantages. In this
context, allylic functionalisation of olefins[2] is an active re-
search domain, as witnessed by recent papers on Rh-,[3]
Pd-[4] and Ru-catalysed[5] allylic acyloxylations and amina-
tions of olefins.
favours the formation of a (h3-allyl)palladium intermediate,
which is intramolecularly trapped according to a 5-exo pro-
cess, whereas phenylbenzoquinone (PhBQ) carries out Pd
re-oxidation. We selected but-3-enyl N-tosylcarbamate (1),
and pent-4-enyl N-tosylcarbamate (3a) as models[7] to test
the extension of Whiteꢀs protocol from oxazolidinones to
oxazinanones (Table 1). Under Whiteꢀs conditions [Pd-
Following our interest in the synthesis of nitrogen deriva-
tives through palladium catalysis,[6] we recently started a
project aiming at developing a new method for the synthesis
of b-amino acids and 1,3-amino alcohols. The target struc-
tures ensue from the cleavage of a 4-vinyl substituted 1,3-ox-
azinanone ring, itself accessible through direct intramolecu-
lar N-functionalisation of the allylic position of a pent-4-
enyl carbamate (Scheme 1).
Table 1. From oxazolidinones to oxazinanones as targets: a preliminary
screening.[a]
Substrate
Product
Solvent
T
[h]
Yield
[%][b]
1
2
3
THF
CH2Cl2
AcOH
72
72
24
73
79
90
4
5
6
THF
CH2Cl2
AcOH
72
72
24
77
95
70
ꢀ
Scheme 1. Allylic C H activation strategy toward b-aminoacids.
[a] 1 or 3a (0.5 mmol), Pd
ethane (LL) (15 mol%), PhBQ (1.1 equiv), 458C. [b] Isolated yields.
ACHTUNGRTENN(UNG OAc)2 (10 mol%), 1,2-bis(phenylsulfinyl)
The strategy is inspired by the recent results reported by
White on the Pd-catalysed allylic amination of but-3-enyl
carbamates.[4e] In this protocol, a disulfoxide chelating ligand
AHCTUNGRTEG(NNUN OAc)2 (10 mol%), 1,2-bis(phenylsulfinyl)ethane (LL; 15
[a] F. Nahra, Dr. F. Liron, Dr. G. Prestat, Prof. G. Poli
Institut Parisien de Chimie Molꢁculaire UMR CNRS 7201
UPMC Univ Paris 06, 4, Place Jussieu, case 183
75005, Paris (France)
mol%), (PhBQ; 1.1 equiv), THF, 458C], N-tosyl carbamate
1 was converted into oxazolidinone 2 in 73% yield after
72 h (entry 1). Use of CH2Cl2 as a solvent (entry 2) in-
creased the yield (79%). Similarly, the homologated N-to-
sylcarbamate 3a, under the same conditions as entry 1, af-
forded the expected oxazinanone 4a in 77% yield (entry 4).
Again, improvement of the yield (95%) was observed on
going from THF to CH2Cl2 (Table 1, entry 5).
Fax : (+33)1-44277567
E-mail: giovanni.poli@upmc.fr
[b] Dr. C. Mealli, Dr. A. Messaoudi
Istituto di Chimica dei Composti Organometallici
ICCOM-CNR, Via Madonna del Piano 10
50019 Sesto Fiorentino, Firenze (Italy)
Although the formal transition from oxazolidinones to ox-
azinones turned out to work uneventfully, we were intrigued
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901946.
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Chem. Eur. J. 2009, 15, 11078 – 11082

COMMUNICATION
by the sluggishness of these transformations and suspected
that a particularly recalcitrant step in the catalytic cycle was
responsible for the slow process. The catalytic cycle
(Scheme 2) starts with alkene coordination to the [Pd(LL)-
was analogously observed with the homologated substrate
3a, although in this case the yield of oxazinanone 4a was
not improved (entries 4 and 5 vs. 6).
The effect of the solvent in catalytic experiments was then
investigated in more detail. To this purpose, a set of bis(ho-
moallyl)-substituted carbamates (3b–f) and another of ho-
moallyl-substituted carbamates (3g–j) were synthesised[7]
and subjected to the direct allylic amination conditions in
CH2Cl2 for 72 h and AcOH for 24 h. The results are report-
ed in Table 2. Although the reactions proceeded in both sol-
vents, only AcOH permitted total consumption of the start-
ing substrates. Furthermore, in this solvent the reaction
Table 2. Oxazinanones 4b–j through direct allylic amination.[a]
3
4
Solvent
dr[b]
83:17
Yield
[%][c]
1
2
AcOH
CH2Cl2
65
12
ꢁ95:5
Scheme 2. Catalytic cycle of the direct intramolecular allylic amination.
3
4
AcOH
CH2Cl2
89:11
74:26
67
10
ACHTUNGTRENNUNG(OAc)2] to give I (step a) followed by abstraction of one al-
lylic hydrogen to afford the corresponding (h3-allyl)palladi-
5
6
AcOH
CH2Cl2
74:26
ꢁ95:5
62
8
um complex II (step b). Then, the process continues through
0
ꢀ
C N bond ring closure and formation of the Pd -ligated het-
erocycle III (step c). Alkene-to-benzoquinone[8,9] exchange
7
8
AcOH
CH2Cl2
85:15
60:40
69
87
ACTHNUTRGNEUNG
on Pd0 releases the final product and generates [Pd(LL)(h2-
BQ)] IV (step d), which is converted into dihydroquinone
(DHQ) and [Pd(LL)(OAc)2] (step e). Also, an equilibrium
AHCTUNGTRENNUNG
9
10
AcOH
CH2Cl2
87:13
87:13
64
36
between the (h3-allyl)palladium complex II and a transient
species such as the [Pd0(LL)
cannot be ruled out.
ACHTUNGTNER(NUNG olefin)] complex V (step f)
11
12
AcOH
CH2Cl2
67:33
50:50
54
22
We speculated that either the allylic activation (step b) or
the re-oxidation of Pd0 (step e) could slow down the reac-
tion. With reference to step e, Bꢃckvall provided convincing
evidence that protic activation of benzoquinone is needed
for such a redox process.[10,11] Furthermore, acetic acid has
been noted to dramatically enhance the reactivity of intra-
molecular alkene insertions into (h3-allyl)palladium com-
plexes.[12–14] Finally, AcOH as the solvent may affect the
equilibrium of the reversible oxidative addition of the Pd0
13
14
AcOH
CH2Cl2
60:40
60:40
15
15
15
16
AcOH
CH2Cl2
63:37
60:40
57
35
complexes III and/or IV to give [Pd(H)(LL)ACTHNUTRGNEUNG
(OAc)][15,16]
(steps g and h), thereby modifying the amount of the resting
state complex.
17
18
AcOH
CH2Cl2
71:29
60:40
37
30
All the above considerations suggest that acetic acid may
have a beneficial effect in the direct allylic amination pro-
vided that a switch toward an undesired allylic acetoxylation
of the substrate (step f) is avoided.[17,18] Indeed, submission
of 1 to the direct allylic amination in the presence of acetic
acid as the sole solvent (Table 1, entry 3) gave oxazolidinone
2 in 90% yield in only 24 h! A marked accelerating effect
[a] Reaction times: 24 h in AcOH, 72 h in CH2Cl2. [b] Diastereomeric
ratio determined by 1H NMR spectroscopy on the crude product. The
relative configuration of the major diastereoisomer was determined by
NOESY experiments as syn for entries 1–10, whereas it could not be de-
termined for entries 11–18. [c] Isolated yields.
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G. Poli et al.
times were much shorter, the yields were better, and, in the
case of the bishomoallylically substituted substrates, the dia-
stereoselectivities were enhanced. For example, 4c was ob-
tained in 10% yield in CH2Cl2 and in a more satisfactory
67% yield in AcOH (Table 2 entries 3 and 4) and 4e as a
60:40 mixture of diastereoisomers in CH2Cl2 and as an 85:15
mixture in AcOH (entries 7 and 8). The diastereoselectivity
of this allylic amination was found to be very dependent on
the substitution pattern when run in CH2Cl2, ranging from
ꢁ95:5 to 60:40. On the other hand, AcOH as solvent led to
diastereoselectivities ranging from 89:11 to 60:40. For homo-
allyl-substituted substrates, the solvent effect was less dra-
matic, although noticeable (entries 11–18). For example, ox-
azinanone 4g was obtained in 22% yield as a 50:50 mixture
of diastereoisomers in CH2Cl2, whereas a 67:33 diastereo-
meric ratio was obtained in 54% yield in AcOH (entries 11
and 12). Compound 4j was obtained as a 60:40 mixture of
diastereoisomers in CH2Cl2 and this ratio rose to 71:29 in
AcOH (entries 17 and 18).
These results suggest that in an aprotic solvent, such as
CH2Cl2, BQ coordination is necessary to allow the whole
process to occur,[19] and prove that AcOH can enable it,
even in the absence of BQ, presumably by assisting ionisa-
tion through protonation of the coordinated acetate ligands.
Steps d and e were then inspected by DFT calculations.[20]
As to the former step, the oxazinanone–benzoquinone sub-
stitution (III!IV) is found to be exothermic by
ꢀ8.7 kcalmolꢀ1. Then, the detailed events in step e have
ACHTUNGTRENNUNG
been monitored in terms of free energies. The [Pd0(LL)(h2-
BQ)] complex IV with two hydrogen-bonded AcOH mole-
cules (the zero-energy point A in Figure 1) undergoes a con-
jugate addition of Pd0 to BQ through an energy barrier
(TSA–B) as high as +17.9 kcalmolꢀ1, to give the “fleeting”[21]
intermediate B (+10.6 kcalmolꢀ1). The Pd atom in complex
B is coordinated by LL, AcOꢀ and BQ ligands. The last one
ꢀ
interacts with the metal through the C atom (Pd C=
2.19 ꢄ) which lies a to the carbonyl and bends off the C6
ring by about 108. In this step the persistent hydrogen-bond-
ing between a second AcOH molecule and BQ is crucial.
Indeed, without this interaction the barrier would rise up by
about 50%. Furthermore, the concerted incoming of the
acetate ligand is fundamental for metal oxidation, as without
it, the simply protonated BQ would stay in a likely unpro-
ductive dihapto coordination to Pd0.[22] The electronic flow
from A to B shows that the entering AcOꢀ ligand assists the
transfer of the originally back-donated metal electron pair
To gain further mechanistic insight, cyclisation of 3a was
repeated in CH2Cl2 and in AcOH by using stoichiometric
amounts of PdACHTUNGTRENNUNG(OAc)2 without benzoquinone. While the
former experiment led to complete recovery of the starting
material, the latter test afforded cleanly the expected oxazi-
nanone 4a in 72% yield after 22 h at 458C (Scheme 3).
to the carbon atom, which formally becomes a s-alkyl donor
8
ꢀ
toward the oxidised d metal. However, the rather long Pd
Scheme 3. Stoichiometric conversion of 3a into 4a.
Figure 1. Structural and free energy profile with global electron flow for step e in Scheme 2. The reported DG values in kcalmolꢀ1 are those of the PCM
Model in AcOH solution. All structural details are given in the Supporting Information.
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ꢀ
C H Activation
COMMUNICATION
ꢀ
C distance (2.19 ꢄ) and the sum of the bond angles at the
coordinated a-C atom (S=3508)[23] suggest for B an already
pronounced phenoxide nature with the known h1-coordina-
tion to PdII of its quasi-aromatic ring.[24]
Keywords: amination · C H activation · density functional
calculations · palladium · solvent effects
Then, the phenol character is fully attained in C, in which
the BQ carbonyl oxygen atom acts as the 2eꢀ donor. Such a
major structural rearrangement requires a significant barrier
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(TSB–C =+14.0 kcalmolꢀ1)
and
is
favoured
by
ꢀ2.4 kcalmolꢀ1 with respect to B. Again, the step is assisted
by strong hydrogen-bonding of the second AcOH molecule
(O···H is only 1.47 ꢄ in TSB–C), as without it the barrier in-
creases by 50%. The final step proceeds smoothly (TSC–D
+3.7 kcalmolꢀ1) and entails the second BQ protonation
with the concomitant coordination of another AcOꢀ ligand
to reach the global minimum D (ꢀ2.1 kcalmolꢀ1 lower than
A). This minimum consists of dihydroquinone and the re-
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generated catalyst [Pd(LL)ACHTNUTRGENNG(U OAc)2], which are held together
by residual hydrogen bonding. These results show that,
through the entire process, the two AcOH molecules do not
merely release protons and provide acetate ligands, but deci-
sively control the redox and coordination properties of the
metal.
In conclusion, we have demonstrated that a strong accel-
erating effect occurs in direct intramolecular allylic amina-
tion when the reaction is conducted in AcOH rather than in
CH2Cl2 or THF.[19] Under these conditions the yields are
usually much higher and diastereomeric ratios less depen-
dent on the substitution pattern of the substrate than under
neutral conditions. Stoichiometric tests and computational
DFT analysis of the palladium reoxidation step provide an
overview of the structural and energetic role of acetic acid
in increasing the efficacy of the entire catalytic cycle.[25]
[7] For the synthesis of the cyclisation precursors see the Supporting In-
formation.
[8] For sake of simplicity, benzoquinone, instead of phenylbenzoqui-
none, is shown in the mechanism. However, these reactions work
almost equally well with the former oxidizing agent.
[9] BQ–LL ligand exchange may take place prior (ref. [4c]) or after
(see below) the cyclisation step.
Experimental Section
[10] H. Grennberg, A. Gogoll, J.-E. Bꢃckvall, Organometallics 1993, 12,
1790–1793.
Typical procedure for the synthesis of oxazinan-2-ones (4a–j): A round
bottom flask under air was charged with N-tosylcarbamate 3a–j
(0.30 mmol, 1.0 equiv), PdACHTNURTGNEUNG(OAc)2 (6.7 mg, 0.03 mmol, 0.1 equiv), bis-sulf-
oxide ligand LL (12.5 mg, 0.045 mmol, 0.15 equiv), phenylbenzoquinone
(59.1 mg, 0.321 mmol, 1.07 equiv) and AcOH (0.8 mL). The reaction was
allowed to stir at 458C for 24 h. The reaction mixture was hydrolysed
and extracted with AcOEt. The combined organic layers were dried over
MgSO4, filtered, and concentrated in vacuo. Purification by column chro-
matography (Cyclohexane/AcOEt, 80/20) afforded oxazinan-2-ones (4a–
j).
[11] In the mechanism postulated in reference [10], Pd0 nucleophilic con-
jugate addition to benzoquinone is triggered through protonation by
acetic acid. Subsequent aromatisation through C-to-O palladium
shift, followed by ligand displacement releases PdACHTNUTRGNEUNG(OAc)2. .
Computational details: DFT calculations, at the B3LYP level,[26] were
performed with the Gaussian03 program.[27]The SDD[28] and 6–31G-
ACHTUNGTRENNUNG
(d,p)[29] basis sets applied to the Pd atom and the other elements, respec-
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evaluated by means of PCM single-point calculations on the gas-phase-
optimised structures.[30]
[13] To rationalise this fact, Negishi (ref. [14a]) proposed that acetic acid
may prevent decomposition of HPdOAc and the resulting degrada-
tion of the (h3-allyl)palladium complex intermediate. Alternatively,
Echavarren (ref. [14b]) suggested that this solvent may promote pro-
tonation of an acetate ligand bound to Pd, thereby facilitating the
formation of a reactive cationic complex.
Acknowledgements
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We gratefully acknowledge the support of UPMC, CNRS, CNR,
CINECA, CASPUR and COST D40.
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T
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widely known process. See reference [4a], and references therein.
[18] Concerning this potential problem, the huge effective molarity asso-
ciated with a five- or six-membered ring formation should override
alternative intermolecular processes. Should a complex of type V be
formed, this could re-enter the catalytic cycle through equilibration
with II.
[19] The non-reactivity of the stoichiometric experiment in CH2Cl2 sug-
gests that a further role of BQ in the catalytic experiments is activa-
tion of step a or b (e.g., by ionisation). BQ activation of step c
(ref. [4c]) without activation of steps a or b seems unlikely due to
the evident irreversibility of step b.
[20] MeSOCH2CH2SOMe (chiral isomer) was taken as a model of ligand
LL for the computational study. A DFT study of the entire catalytic
cycle will appear elsewhere.
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shown in reference [11]. The calculations show that the proton, con-
tested between the coordinated BQ molecule and acetate anion, in-
variably leads to AcOH.
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Received: July 14, 2009
[23] In a comparable complex, with an exocyclic CO function, the adja-
cent ring carbon atom is more pyramidal (ꢀ=3428) and more
Please note: Minor changes have been made to this publication in
strongly bound to PdII (Pd C=2.13 ꢄ); see: L. Xu, Q. Shi, X. Li, X.
Chemistry–A European Journal Early View. The Editor.
ꢀ
Jia, X. Huang, R. Wang, Z. Zhou, Z. Lin, A. S. C. Chan, Chem.
Commun. 2003, 1666–1667.
Published online: September 30, 2009
11082
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Chem. Eur. J. 2009, 15, 11078 – 11082