Derivatives of phosphorous acid as a new class of ligands for Pd-catalyzed allylation

Article abstract of DOI:10.1023/A:1011302416744

Amido- and aminophosphites and hydrospirophosphoranes can be used as ligands in the Pd-catalyzed allylation of ethyl malonate with ethyl (3-phenylprop-2-enyl) carbonate. Bidentate ligands (RO)₂P - O(CH₂)_n - NR′₂

Full text of DOI:10.1023/A:1011302416744

554
Russian Chemical Bulletin, International Edition, Vol. 50, No. 3, pp. 554—556, March, 2001
Derivatives of phosphorous acid as a new class of ligands
for Pd-catalyzed allylation
S. K. Moiseev, R. V. Lebedev, A. I. Polosukhin, and V. N. Kalinin«
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 117813 Moscow, Russian Federation.
Fax: +7 (095) 135 6549. E-mail: vkalin@ineos.ac.ru
Amido- and aminophosphites and hydrospirophosphoranes can be used as ligands in the
Pd-catalyzed allylation of ethyl malonate with ethyl (3-phenylprop-2-enyl) carbonate. Bidentate
ligands (RO)2P—O(CH2)n—NR´2 (n = 2 and 3) were found to be the most effective ligands.
Key words: allylation, palladium, P,N-ligands, aminophosphites.
Scheme 1
Pd-catalyzed allylation of various organic compounds
is an important method for constructing carbon—carbon
and carbon—heteroatom bonds.1 This reaction proceeds
through the formation of cationic η3-allylpalladium(II)
intermediates that are attacked by a nucleophile. The
nature of palladium-coordinated ligands is essential in
this process. Reactions of allylic esters (allyl carbonates,
allyl acetates, etc.) with alkyl malonates are a standard
test for the efficiency of one or another ligand in cata-
lytic processes.2
The number of current papers dealing with phospho-
rous acid derivatives in catalytic processes increases
steadily. The reasons for this tendency are the following.
These compounds are easily prepared, contain no P—C
bond and are thus resistant to oxidation, and have
pronounced π-acceptor character, making a transition
metal atom more electrophilic.3
In this work, amido- and aminophosphites and
hydrospirophosphorane were used as ligands for the first
time. The goal of this work is to study how the catalytic
efficiency of these ligands depends on the character of
P- and N-donor centers (i.e., the type and bulkiness of
substituents and the length of a chain between the donor
atoms). This will enable one to synthesize ligands with
the desired properties. The mono- and bidentate com-
pounds 1—8 we studied4—8 are shown in Scheme 1.
The reaction of ethyl malonate with ethyl (3-phenyl-
prop-2-enyl) carbonate (9) was chosen as a model
(Scheme 2). It is known that allyl carbonates are of
special interest for Pd-catalyzed allylation, because in
many cases they can react in neutral media, i.e., no base
is needed.9 In addition, the reaction of ethyl malonate
with compound 9 yields one isomer of product 10, while
the use of the corresponding acetate can also result in
allylic rearrangement.10
Me
Me
Me
O PO
NMe2
2
3
PNEt2
2
O
1
PO
NMe2
O
2
4
O
O
O
PO
NMe2
N
O P
2
5
2
O
Me
O
O
N
O
H
P
O
P NEt2
P NEt2
O
O
Me
7
Me
6
8
complexing atom is very labile.11 Ligands 1—8 were
added to a dark red solution of Pd2(dba)3. A light yellow
color of the reaction mixture indicated the completion
of the exchange reaction. The ligand exchange rate can
be estimated from the time it takes for this change in
color.
The catalytic agent was generated in situ by ligand
exchange reaction. The precursor of a catalyst was a
palladium complex with dibenzylideneacetone (dba)
Pd2(dba)3, in which the bond between the olefin and the
Ligands 1, 3, and 4 change their color at room
temperature within 5—10 min, while compound 5 dis-
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 529—531, March, 2001.
1066-5285/01/5003-554 $25.00 © 2001 Plenum Publishing Corporation

New class of ligands for Pd-catalyzed allylation
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 3, March, 2001
555
Scheme 2
atoms, and even more slowly for ligand 2 with a bulkier
substituent at the oxygen atoms.
A special place among the ligands is occupied by
hydrophosphorane 6, whose structure appears unfit for
coordination with metal (see Scheme 1). However, this
compound was shown12 to function as a P-monodentate
phosphite ligand in complexation with Rh and Pd,
coordinating in the form of PIII tautomer 11 with open
chain (Scheme 3).
COOEt
COOEt
Pd2(dba)3, Ligand
Ph
OCO2Et
+
THF, 65 °C
9
COOEt
COOEt
Ph
10
Scheme 3
colors over a course of 30 min. It takes three hours for
ligand 2 to coordinate at 45 °C. Ligands 7 and 8 are not
coordinated by palladium under the chosen conditions.
The yields of product 10 and the reaction conditions
are given in Table 1.
O
HO
R1
Me
R2
O
P
O
P
O
O
H
O
O
Ligands 3 and 4 were found to be most effective in
the process under examination. They allow one to obtain
compound 10 in high yields in neutral media. In the
presence of 1, allylation under neutral conditions is low-
yielding. In the case of ligands 2, 5, and 6, a base is
required, and the product forms in moderate yields.
Amidophosphites 7 and 8 with the ring phosphorus
atom proved to be unsuitable ligands for this catalytic
process, as they cannot replace dba in the catalyst
precursor under the given conditions.
Me
Me
6
11
R1 = H, R2 = Me or R1 = Me, R2 = H
Thus,
aminophosphites
3
and
4
(RO)2P—O(CH2)n—NR´2 (n = 2 and 3) with the P and
N atoms in the acyclic fragments were found to be the
most effective ligands among the compounds studied for
Pd-catalyzed allylation.
Aminophosphite 1 containing the P atom in the side
chain rather than in the cyclic fragment is capable of
rapid ligand exchange with Pd2(dba)3, but its catalytic
power is low.
Ligands 2—5 differ in the length of a chain between
the P and N atoms (three-atom chains in compounds 2,
3, and 5 and a four-atom chain in 4). In addition, the P
and N atoms in their molecules are included either in
acyclic or in heterocyclic fragments. In the case of 3 and
4, ligand exchange with Pd2(dba)3 is rapid, despite
different P...N chain lengths. The process occurs more
slowly for ligand 5, which contains heterocyclic P and N
Experimental
All reactions were carried out in an atmosphere of dry
argon. Tetrahydrofuran was distilled over sodium benzophenone
ketyl before use. Pd2(dba)3 and carbonate 9 14 were prepared
13
according to the known procedures. Ethyl malonate was puri-
fied by vacuum distillation. The structure of product 10 was
confirmed by comparing its boiling point and 1H NMR and the
mass spectra obtained in this work with the literature data.15
Palladium-catalyzed allylation (general procedure). A. Neu-
tral conditions. A solution of Pd2(dba)3 (10 mg, 0.01 mmol) and
an equimolar amount of ligand (1—8) in 4 mL of THF was
stirred at 20 °C until the solution turned light yellow. Then,
ethyl malonate (240 mg, 1.5 mmol) in 2 mL of THF and ethyl
(3-phenylprop-2-enyl) carbonate (206 mg, 1 mmol) (9) in 2 mL
of THF were added. The reaction mixture was refluxed with
stirring until compound 9 disappeared. The yield of product 10
was determined by HPLC with ethylbenzene as the internal
standard.
Table 1. Pd-catalyzed allylation of ethyl malonate with ethyl
(3-phenylprop-2-enyl) carbonate (9) in THF at 65 °C
a
LigandBase
Timeb/h
Yieldc (%)
1
2
—
—
24
24
2
2
2
24
3
24
4
38
—
67
92
100
—
85
—
63
B. Basic conditions. The reactions were carried out as de-
scribed above, but in the presence of N,O-bis(trimethyl-
silyl)acetamide (BSA) (305 mg, 1.5 mmol) and anhydrous
KOAc (5 mg, 0.05 mmol).
BSA/KOAc
3
4
5
—
—
—
The authors are grateful to K. N. Gavrilov and
A. V. Korostylev for providing the organophosphorus
ligands.
BSA/KOAc
—
BSA/KOAc
6
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 99-03-
32899), the Federal Target Program "Integratsiya" (Grant
Nos. 234 and K0599), and the INTAS (Grant No.
IR-97-12060).
a BSA = N,O-bis(trimethylsilyl)acetamide.
b A point after which carbonate 9 is no longer detected by TLC.
c Determined by HPLC on a Varian 5000 LIQUID chromato-
graph with ethylbenzene as the internal standard (Separon
SGX, hexane/PriOH = 99.5/0.5).

556
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 3, March, 2001
Moiseev et al.
8. H. Germa, M. Sanchez, R. Burgada, and R. Wolf, Bull. Soc.
Chim. France, 1970, 612.
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Received June 28, 2000;
in revised form October 15, 2000