From olefins to ketones via a 2-rhodaoxetane complex

Article abstract of DOI:10.1002/anie.200705802

(Figure Presented) Quantitative oxygenation of 1,5-cyclooctadiene to 4-cyclooctenone with molecular oxygen by a rhodium complex proceeds via a 2-rhoda(III)oxetane intermediate (see scheme), which undergoes a facile β-elimination reaction to give the unsaturated ketone selectively instead of the epoxide.

Full text of DOI:10.1002/anie.200705802

Communications
DOI: 10.1002/anie.200705802
Olefin Oxygenation
From Olefins to Ketones via a 2-Rhodaoxetane Complex**
M. Pilar del Río, Miguel A. Ciriano, and Cristina Tejel*
Oxidation reactions yielding valuable products from organic
raw materials are one of the most problematic transforma-
tions although they constitute industrial core technologies.
Moreover, the present main challenge and goal in oxidation is
to perform these reactions with no waste and by using
environmentally friendly oxidants such as oxygen[1] and
hydrogen peroxide.[2] Significantly, epoxides, the typical
product of the oxygenation of olefins catalyzed by complexes
of the middle transition metals,[3] have rarely been evoked as
products in rhodium chemistry. Oxidations using rhodium
compounds are characterized by excellent selectivity for
formation of methyl ketones from terminal olefins.[4] From
the recently reviewed[4a] stoichiometric rhodium oxygenation
reactions, it appears that rhodaoxetanes could be considered
as viable intermediates in catalytic reactions. However, very
few compounds with a 2-rhoda(III)oxetane ring have been
reported to date. Moreover, the next step in a catalytic cycle
to yield an oxygenated organic compound has a sole reported
Scheme 2. Characterized compounds in the stoichiometric oxygenation
example, the formation of acetaldehyde and acetone from a 2-
rhoda(III)oxetane derived from ethene or propene, respec-
tively (Scheme 1).[5] The key reductive elimination step,
ofcod to 4-cyclooctenone.
from dioxygen were incorporated with 100% atom efficiency.
On treating a suspension of 1 with trimethylphosphane in
[D6]benzene at 358C, quantitative elimination of the organic
fragment as 4-cyclooctenone (2) and formation of pentacoor-
dinate RhI complex [Rh(PhN3Ph)(PMe3)3] (3) was observed
(Scheme 2). This result provides new insight into the oxygen-
ation of olefins with dioxygen, because it shows for the first
time formation of a ketone from an isolated 2-rhoda-
(III)oxetane complex resulting from an internal olefin.
Another interesting feature of the reductive elimination of
4-cyclooctenone from 1 is the absence of the phosphane
oxide, typically produced in related reactions involving
olefins, dioxygen, and phosphanes.[7]
The organic product 4-cyclooctenone was identified by
GC-MSand NMR spectroscopy, while complex 3 was
characterized by comparison of its spectroscopic data with
those of pure samples prepared by alternative methods. Thus,
3 was straightforwardly prepared by reaction of [Rh-
(PhN3Ph)(C8H12)] with PMe3. The most relevant spectroscop-
ic data of 3 are the two signals (in 2:1 ratio) corresponding to
an AB2X (X = 103Rh) pattern in the 31P{1H} NMR spectrum.
Monitoring the reaction of 1 with PMe3 at 208C by NMR
spectroscopy revealed that the coordination of two phos-
phane ligands occurs initially to give observable intermediate
[Rh(PhN3Ph)(OC8H12)(PMe3)2] (4). Complex 4 then under-
goes reductive elimination to give 4-cyclooctenone sponta-
neously (see the Supporting Information). One additional
molar equivalent of PMe3 is required to recover the triaze-
nerhodium(I) fragment as [Rh(PhN3Ph)(PMe3)3] (3); other-
wise, formation of 3 is accompanied by formation of another
unknown species.
Scheme 1. Oxygenation and reductive elimination steps in the forma-
tion of carbonyl compounds from olefins.
fundamental for possible catalytic cycles in oxygenation of
olefins, is poorly understood. Herein we address this reaction
and report on the stoichiometric elimination of a ketone from
a 2-rhoda(III)oxetane complex on addition of P-donor
ligands.
We have previously described[6] facile oxygenation of
[Rh(PhN3Ph)(C8H12)] (C8H12 = 1,5-cyclooctadiene = cod,
PhN3Ph = 1,3-diphenyltriazenide) with dioxygen to give
dinuclear 2-rhoda(III)oxetane complex [{Rh(PhN3Ph)-
(OC8H12)}2] (1, Scheme 2), in which both oxygen atoms
[*] M. P. del Río, Prof. M. A. Ciriano, Dr. C. Tejel
Departamento de Química Inorgµnica
Instituto de Ciencia de Materiales de Aragón
C.S.I.C.-Universidad de Zaragoza, Pedro Cerbuna, 12
50009-Zaragoza (Spain)
Fax: (+34)976-761-187
E-mail: ctejel@unizar.es
[**] The generous financial support from MEC/FEDER (Project
CTQ2005-06807) and DGA (PIP 019/2005) is gratefully acknowl-
edged. M.P.R. thanks DGA (Diputación General de Aragón) for a
fellowship
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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The trans disposition of the PMe3 ligands in 4 was evident
from the ABX pattern observed in the 31P{1H} NMR spectrum
=
(JP, P = 493 Hz). The carbon atoms of the uncoordinated C C
double bond give rise to singlets at d = 132.8 and 130.8 ppm
in the 13C{1H} NMR spectrum. Finally, the presence of the 2-
rhoda(III)oxetane moiety in 4 was deduced from the apt-
13C{1H} NMR spectrum, which showed signals for two HC
groups at d = 11.0 (JC,Rh = 17 Hz, CHRh) and 94.7 ppm
(2JC,Rh = 4 Hz, CHORh).
Once we identified key species 4, we could propose a
plausible mechanism leading to 4-cyclooctenone (Scheme 3).
Scheme 4. Reactions competing with ketone elimination.
after heating in the presence of trimethylphosphane in
toluene at 1008C for 3 h.
Complex 5 was found to be a mixture of stereoisomers
(5a/5b 3:1; Scheme 4), while the expected isomer having the
PMe3 ligand trans to C1 was not observed. Both isomers were
1
easily identified in the H,1H NOESY spectrum, since the
methyl groups from the PMe3 ligands gave strong nOe cross-
peaks only with protons H6 and H4, respectively (see the
Supporting Information).
In order to close the hypothetical catalytic cycle, the
reaction of [Rh(PhN3Ph)(PMe3)3] (3) with dioxygen was also
investigated. On treatment of a solution of 3 in [D6]benzene
with neat O2, complete transformation of all trimethylphos-
phane ligands into the oxide OPMe3 was the only process
identified. However, if the oxygenation reaction of 3 is carried
out in the presence of 1,5-cyclooctadiene, clean transforma-
tion of 1 into [Rh(PhN3Ph)(cod)(PMe3)] (7) and two molar
equivalents of OPMe3 occurs. The formation of 7 most
probably involves the sequence of reactions shown in
Scheme 5. It is reasonable to assume dissociation of one
PMe3 ligand in 3, which is followed by oxidative addition of
O2 to give dioxygen complex C. In this complex, intra-
molecular oxygen transfer to the phosphorus atoms would
give the phosphane oxide, which is easily replaced by free
trimethylphosphane and cod to form 7. Unfortunately, com-
plex 7 does not react further with oxygen.
Scheme 3. Possible mechanism for ketone elimination from 4.
The first step is bridge splitting of 1 and replacement of the
=
coordinated C C bond by a second equivalent of PMe3 to
form octahedral complex 4. At this stage, the coordinative
vacancy required for b-hydrogen elimination could be
provided by hemilabile behavior of the triazenide ligand.
À
Thus, the Rh N bond trans to the alkyl group could be easily
cleaved to give the 16-electron species A, which would
undergo b-hydrogen elimination to give hydride enolate
complex B. From B, reductive elimination accomplished by
coordination of PMe3 would give cycloocta-1,5-dien-1-ol and
complex 3, as observed. Note that the alternative direct
reductive elimination from 4 to give the epoxide as product
does not occur, and that ketone elimination requires the
coordination of an additional ligand.
The role of temperature in the quantitative formation of
4-cyclooctenone is critical. At higher temperatures, the rate of
the elimination reaction increases markedly, but the yield of
the ketone decreases (66% at 658C for 1 h). Under these
conditions the rhodium-containing products were a mixture
of 3 (66%) and [Rh(PhN3Ph)(HOC8H11)(PMe3)] (5, 33%;
Scheme 4). The latter was found to be the result of a side
reaction involving isomerisation of 1 to the hydroxyallyl
derivative 6 with coordination of trimethylphosphane.
We previously observed[6] that 1 is kinetically unstable and
forms [{Rh(PhN3Ph)(HOC8H11)}n] (6) on heating in
[D6]benzene at 658C. Accordingly, complex 5 results quanti-
tatively from the reaction of 6 with PMe3, as observed in a
separate experiment. Complex 5 is inert towards reductive
elimination reactions, and thus it is recovered unaltered even
Scheme 5. Possible mechanism for the formation of 7.
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Communications
cold pentane (2 ” 5 mL) and vacuum-dried. Yield: 90.5 mg (60%).
C,H,N analysis calcd (%) for C23H31N3PORh: C 55.32, H 6.26, N 8.41;
found: C 55.43, H 6.16, N 8.57; 1H NMR (300 MHz, [D6]benzene,
258C, TMS): 5a: d = 7.62 (d, 3JH,H = 7.8 Hz, 2H; Ho’), 7.301 (t, 3JH,H
= 7.8 Hz, 2H; Hm’), 7.21 (t, 3JH,H = 7.8 Hz, 2H; Hm’’), 7.15 (2H; Ho’’),
7.00 (m, 1H; Hp’), 6.93 (t, 3JH,H = 8.0 Hz, 1H; Hp’’), 5.53 (m, 1H; H6),
Previous work on olefin oxygenation reactions by rho-
dium complexes in the presence of phosphanes were system-
atically associated with formation of phosphane oxide, and
from these observations the assumption that these processes
require a sacrificial reducing agent to consume one oxygen
atom was established. However, this study unequivocally
shows that formation of 4-cyclooctenone and OPMe3 are two
independent processes, each of which involves one molecule
of dioxygen and occurs with 100% atom efficiency. Moreover,
4-cyclooctenone is the sole elimination product from the
isolated 2-rhoda(III)oxetane compound.
In conclusion, the present work clearly shows that
rhodaoxetanes can be intermediates in the synthesis of
carbonyl compounds from olefins in rhodium chemistry, and
they are the origin of the excellent selectivity in the oxidation
of olefins to ketones. This process also occurs with internal
olefins, for which the reductive elimination step of a ketone
from a 2-rhoda(III)oxetane compound on addition of a
phosphane ligand has been documented here. Developing
these stoichiometric reactions into catalytic process in the
absence of phosphanes is a challenge for the future.
4.81 (t, 3JH,H = 8.1 Hz, 1H; H5), 3.92 (m, 1H; H4), 3.06 (brd, 3JH,H
=
4.1 Hz, 1H; H2), 2.59 (vt, 1H; H1), 2.32 (m, 1H; H8’), 1.59 (m, 1H;
2
H7’), 1.53 (m, 2H; H3’ + H3’’), 1.23 (d, JH,P = 9.7 Hz, 9H; PMe3),
1.18 (m, 1H; H8’’), 0.95 (m, 1H; H7’’), 0.81 (d, 3JH,H = 12.9 Hz, 1H;
3
3
OH); 5b: d = 7.82 (d, JH,H = 8.0 Hz, 2H; Ho’), 7.305 (m, JH,H
=
8.0 Hz, 2H; Hm’), 7.20 (t, JH,H = 8.0 Hz, 2H; Hm’’), 7.05 (d, JH,H
=
3
3
8.0 Hz, 2H; Ho’’), 6.98 (m, 1H; Hp’), 6.91 (t, 3JH,H = 8.0 Hz, 1H; Hp’’),
5.46 (m, 1H; H4), 4.76 (t, 3JH,H = 8.1 Hz, 1H; H5), 3.85 (m, 1H; H6),
3
3
3.46 (brd, JH,H = 12.0 Hz, 1H; H2), 2.98 (d, JH,H = 12.1 Hz, 1H;
OH), 2.68 (vt, 1H; H1), 1.90 (m, 1H; H3’), 1.58 (m, 1H; H7’), 1.56 (m,
1H; H8’), 1.33 (m, 1H; H3’’), 1.27 (m, 1H; H7’’), 0.97 (m, 1H; H8’’),
0.88 ppm (d, JH,P = 9.3 Hz, 9H; PMe3). 31P{1H} NMR (121 MHz,
2
[D6]benzene, 258C): 5a: d = À5.9 ppm (d, JP, Rh = 164 Hz); 5b: d =
6.6 (d, JP, R h = 163 Hz). 13C{1H} NMR (75 MHz, [D6]benzene, 258C,
TMS): 5a: d = 155.6 (Ci’+i’’), 129.5 (Cm’’), 129.2 (Cm’), 123.0 (Cp’), 123.0
2
(Cp’’), 117.9 (Co’), 116.9 (Co’’), 101.7 (dd, JC,Rh = 4.1 Hz, JC,P
=
2.0 Hz; C5), 90.4 (dd, JC,Rh = 30 Hz, 2JC,P = 5 Hz; C6), 89.3 (d, JC,P
3
= 2.0 Hz; C2), 60.3 (d, JC,Rh = 12.6 Hz; C4), 49.9 (dd, JC,Rh
=
2
3
2
21.7 Hz, JC,P = 6.5 Hz; C1), 44.2 (dd, JC,P = 10.3 Hz, JC,Rh
=
3
1.4 Hz; C8), 37.9 (C3), 22.3 (d, JC,P = 4.1 Hz; C7), 17.5 ppm (dd,
2
JC,P = 27.4 Hz, JC,Rh = 1.2 Hz; PMe3); 5b: d = 152.2 (Ci’), 149.7
(Ci’’), 129.5 (Cm’), 129.1 (Cm’’), 123.6 (Cp’), 123.4 (Cp’’), 119.1 (Co’), 117.0
Experimental Section
3
(Co’’), 101.2 (dd, JC,Rh = 4.1 Hz, 2JC,P = 2.4 Hz; C5), 90.3 (d, JC,P
=
3: Heating a red suspension of [{Rh(PhN3Ph)(C8H12)}2] (100.0 mg,
0.12 mmol) in toluene (5 mL) and PMe3 (130.3 mL, 1.47 mmol) at
658C produced a red solution in 2 h. Concentration of this solution to
ca. 3 mL and addition of pentane (8 mL) led to crystallization of the
complex. The red solid was collected by filtration, washed with
2
8.4 Hz; C2), 86.6 (dd, JC,Rh = 29.7 Hz, JC,P = 5.1 Hz; C4), 63.9 (d,
JC,Rh = 12.9 Hz; C6), 49.7 (dd, JC,Rh = 21.3 Hz, 2JC,P = 5.7 Hz; C1),
3
3
40.5 (d, JC,P = 5.1 Hz; C8), 33.4 (d, JC,P = 4.0 Hz; C3), 24.6 (C7),
16.3 ppm (dd, JC,P = 27.0 Hz, 2JC,Rh = 1.2 Hz; PMe3).
pentane (2 ” 5 mL), and vacuum-dried. Yield: 106.8 mg (82%).
7:
A suspension of [{Rh(PhNNNPh)(C8H12)}2] (100.0 mg,
3
1H NMR (300 MHz, CDCl3, 258C, TMS): d = 8.32 (dd, JH,H
=
0.12 mmol) in toluene (5 mL) was heated for 5 min at 808C to
achieve full dissolution of the complex. Then neat PMe3 (21.7 mL,
0.25 mmol) was added dropwise to give an orange solution, which was
evaporated to dryness. The residue was dissolved in pentane and the
extract filtered over kieselguhr. The filtrate was concentrated to 3 mL
and left overnight at À308C to afford orange crystals, which were
separated by decantation and vacuum-dried. Yield: 88.1 mg (74.1%).
8.5 Hz, JH,H = 1.2 Hz, 4H; Ho), 7.42 (dd, JH,H = 8.5, 7.3 Hz, 4H;
4
3
Hm), 7.09 (tt, JH,H = 7.3 Hz, JH,H = 1.2 Hz, 2H; Hp), 1.12 (m, 9H;
3
4
CH3), 1.07 ppm (m, 18H; CH3); 31P{1H} NMR (121 MHz, CDCl3,
2
258C): d = À6.6 (dt, JP, Rh = 155 Hz, JP, P = 47 Hz, 1P), À15.3 (dd,
2
JP, Rh
=
137, JP, P
=
47 Hz, 2P); C,H,N analysis calcd (%) for
C21H37N3P3Rh: C 47.83, H 7.07, N 7.97; found: C 47.79, H 7.38, N 7.61.
4: Neat PMe3 (10.5 mL, 0.12 mmol) was added to a suspension of
[{Rh(PhN3Ph)(OC8H12)}2] (1, 16.1 mg, 0.02 mmol) in [D6]benzene
(0.5 mL) and the reaction was monitored by NMR spectroscopy.
After 6 h at 208C the main complex was found to be 4. 1H NMR
1H NMR (300 MHz, [D6]benzene, 258C, TMS): d = 7.82 (d, 3JH,H
=
3
3
8.4 Hz, 4H; Ho), 7.32 (t, JH,H = 7.8 Hz, 4H; Hm), 6.97 (t, JH,H
=
p
=
7.3 Hz, 2H; H ), 3.85(br, 4H; HC ), 2.34 (m, 4H; CH2), 1.86 (m, 4H;
2
CH2), 0.65 ppm (d, JH,P
=
9.3 Hz, 9H; PMe3); 31P{1H} NMR
3
(300 MHz, [D6]benzene, 258C, TMS): d = 7.57 (d, JH,H = 7.5 Hz,
(121 MHz, [D6]benzene, 258C):
d
=
À6.59 ppm (d, JP, Rh
=
3
3
2H; Ho’), 7.38 (d, JH,H = 7.5 Hz, 2H, Ho’’), 7.23 (t, JH,H = 7.5 Hz,
141.5 Hz; PMe3); 13C{1H} NMR (75 MHz, [D6]benzene, 258C,
3
3
2H; Hm’), 7.17 (t, JH,H = 7.5 Hz, 2H; Hm’’), 6.91 (t, JH,H = 7.3 Hz,
1H; Hp’), 6.90 (t, 3JH,H = 7.3 Hz, 1H; Hp’’), 5.95 (m, 1H; H5), 5.84 (m,
1H; H6), 4.85 (m, 1H; H2), 2.69 (m, 1H; H1), 2.54 (m, 1H; H7a), 2.37
(m, 1H; H8a), 2.19 (m, 1H; H7b), 2.09 (m, 1H; H4a), 2.04 (m, 1H;
H8b), 1.96 (m, 1H; H3a), 1.94 (m, 1H; H4b), 1.80 (m, 1H; H3b), 1.13
TMS): d = 151.2 (Ci), 129.3 (Cm), 122.5 (Cp), 118.2 (Co), 80.2 (d,
=
JC,Rh = 12.5 Hz; HC ), 32.2 (CH2), 14.3 ppm (dd, JC,P = 28.9 Hz,
JC,Rh = 0.95 Hz; PMe3); C,H,N analysis calcd (%) for C23H31N3PRh:
C 57.15, H 6.46, N 8.69; found: C 57.21, H 6.35, N 8.89.
2
2
(d, JH,P = 6.6 Hz, 9H), 1.11 ppm (d, JH,P = 6.3 Hz, 9H; PMe3);
Received: December 12, 2007
Published online: February 22, 2008
31P{1H} NMR (121 MHz, [D6]benzene, 258C): d = À8.76 (dA, JP, Rh
=
116 Hz), À11.8 ppm (dB, JP, Rh = 120 Hz, JA,B = 493 Hz); 13C{1H}
NMR (75 MHz, [D6]benzene, 258C, TMS): d = 150.5 (Ci’), 150.3 (d,
2JC,Rh = 3 Hz; Ci’’), 132.8 (C5), 130.8 (C6), 129.5 (Cm’), 129.1 (Cm’’),
123.6 (Cp’), 123.4 (Cp’’), 117.9 (Co’), 117.2 (Co’’), 94.7 (d, 2JC,Rh = 4 Hz;
C2), 40.5 (C3), 38.7 (m, C8), 28.8 (t, 4JC,P = 3 Hz; C7), 22.4 (C4), 15.3
2
Keywords: ketones · oxetanes · oxidation · oxygenation ·
.
rhodium
2
and 13.4 (2 ” vq; PMe3), 11.0 ppm (dt, JC,Rh = 17 Hz, JC,P = 3 Hz;
C1).
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Chemie
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