Germylene-induced hydrogenation of benzophenone

Article abstract of DOI:10.1021/om034025t

Ge[CH(SiMe₃)₂]₂ reacts with benzophenone to form a conjugated triene. Wilkinson's catalyst, Rh(PPh₃)₃Cl, is an effective precatalyst for the isomerization of one double bond and hydrogenation of a second double bond of this triene. This reaction demonstrates that germylene activation of the phenone moiety yields reactive double bonds amenable to catalytic transformation, even with bulky catalysts such as the presumed active form of Wilkinson's catalyst, Rh(PPh₃)₂Cl.

Full text of DOI:10.1021/om034025t

Organometallics 2003, 22, 4613-4615
4613
Ger m ylen e-In d u ced Hyd r ogen a tion of Ben zop h en on e
Ryan D. Sweeder, Zuzanna T. Cygan, Mark M. Banaszak Holl,* and
J eff W. Kampf
Chemistry Department, University of Michigan, Ann Arbor, Michigan 48109-1055
Received J uly 8, 2003
Sch em e 1. Activa tion of Ben zop h en on e to F or m a
Con ju ga ted Tr ien e F ollow ed by Ca ta lytic
Hyd r ogen a tion of On e of th e Dou ble Bon d s
Summary: Ge[CH(SiMe3)2]2 reacts with benzophenone to
form a conjugated triene. Wilkinson’s catalyst, Rh-
(PPh3)3Cl, is an effective precatalyst for the isomerization
of one double bond and hydrogenation of a second double
bond of this triene. This reaction demonstrates that
germylene activation of the phenone moiety yields reac-
tive double bonds amenable to catalytic transformation,
even with bulky catalysts such as the presumed active
form of Wilkinson’s catalyst, Rh(PPh3)2Cl.
In tr od u ction
Aromatic double bonds exhibit a greater degree of
stability than standard double bonds, even when two
or three such bonds are in conjugation. Thus, a number
of interesting organic reactions either are more difficult
when applied to an aromatic system (for example,
hydrogenation) or have met with only limited success
(such as the Diels-Alder reaction).1-3 Activation of
aromatic rings for a wider range of reactivity could take
a number of forms, including (1) allowing the reaction
of interest to occur, (2) controlling the number and/or
location of the reactive double bonds on the ring, and
(3) differentiation of multiple aromatic rings.
germylene fading within 2 min to generate a yellow color
indicative of the quantitative formation of 2. Addition
of 8 mol % of Wilkinson’s catalyst, Rh(PPh3)3Cl (3), and
3 atm of hydrogen gas results in a partial hydrogenation
to give 4 (Scheme 1). The 1H NMR and 13C NMR spectra
of 4 suggest hydrogenation of a single double bond of
the activated phenyl ring along with an internal isomer-
ization. A single-crystal X-ray diffraction study supports
this assignment (Figure 1.) The bond lengths of the
activated ring indicate single bonds of 1.503-1.538 Å
along with two remaining double bonds of 1.338(2) Å
(C1-C2) and 1.356(2) Å (C6-C7). Further hydrogena-
tion of the two remaining double bonds is likely pre-
vented by steric constraints of the germylene. The
unactivated phenyl group is not hydrogenated, as 3 is
incapable of directly hydrogenating aromatic rings.
Unlike 2, compound 4 is not in equilibrium with 1 and
the corresponding ketone.
This hydrogenation reaction succeeds by having a
substantially faster rate of reaction than the equilibrium
liberating free 1. In the absence of benzophenone, 1 is
catalytically hydrogenated by 3 to the germane H2Ge-
[CH(SiMe3)2]2. In addition, the catalyst itself can react
directly with 1 by undergoing ligand exchange to lose a
phosphine, as reported by Lappert.6 Neither of these
other products is observed.
We have recently reported the activation of a variety
of phenones using Ge[CH(SiMe3)2]2 (1) to give the
conjugated triene 2 (Scheme 1).4 This activated phenone
is structurally prepared to undergo a variety of standard
organic reactions. However, the presence of an equilib-
rium between 2, free benzophenone, and 1 complicates
reaction design.4,5 Although the equilibrium heavily
1
favors 2 (it is the only species observed in the H NMR
and UV/vis spectra), reaction with free germylene occurs
over several hours upon the addition of typical ger-
mylene traps (benzil, dienes, alkynes) to a solution of
2. For example, addition of 1.05 equiv of benzil to a 0.04
M solution of 2 in benzene resulted in quantitative
formation of benzophenone and benzil-trapped ger-
mylene in 4 h. Therefore, a successful reaction design
must include reagents that are inert toward 1 or
undergo reaction with one or more double bonds of 2
more quickly than formation of free 1 from the equilib-
rium shown in Scheme 1.
Resu lts a n d Discu ssion
Addition of 1 equiv of benzophenone to a solution of
1 in benzene at 20 °C results in the orange color of the
Catalytic deuteration of 2 with 3 atm of D2 leads to
incorporation of three deuterium atoms. One D is
located on each of the newly formed methylene groups
on the hydrogenated ring (C3-C5), as determined by
both 1H and 13C NMR spectroscopy. The peaks in the
(1) Chordia, M. D.; Smith, P. L.; Meiere, S. H.; Sabat, M.; Harman,
W. D. J . Am. Chem. Soc. 2001, 123, 10756-10757.
(2) Cossu, S.; Battaggia, S.; DeLucchi, O. J . Org. Chem. 1997, 62,
4162-4163.
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(4) Sweeder, R. D.; Miller, K. A.; Banaszak Holl, M. M.; Kampf, J .
W. Organometallics 2002, 21, 457-459.
(5) Sweeder, R. D.; Gdula, R. L.; Ludwig, B. J .; Banaszak Holl, M.
M.; Kampf, J . W. Organometallics 2003, 22, 3222-3229.
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10.1021/om034025t CCC: $25.00 © 2003 American Chemical Society
Publication on Web 09/23/2003

4614 Organometallics, Vol. 22, No. 22, 2003
Notes
3, the double bond would usually be subsequently
hydrogenated. In the case of 4, further hydrogenation
is not observed, possibly inhibited by the steric bulk of
the bis(trimethylsilyl)methyl groups on the germanium.
The partial hydrogenation, catalyzed by Wilkinson’s
catalyst, of a triene generated from benzophenone is
novel. However, the partial hydrogenation of aromatic
and substituted aromatic compounds is well-docu-
mented. Hydrogenation of aromatic rings has been
successfully accomplished,10,11 even in the presence of
a variety of functional groups.12-14 Benzene can also be
selectively hydrogenated to cyclohexene using a variety
of rhodium catalysts with minimal functional group
tolerance.15 Selective hydrogenation of specific double
bonds or the hydrogenation of only one aromatic ring
when several are present has been far less successful.
Hydrogenation of only one ring of multicyclic aromatics
and selective hydrogenation of a single double bond of
perinaphthylene or perianthracene have been accom-
plished using a rhodium cluster catalyst.16,17
F igu r e 1. ORTEP structure of 4-d 3 with Si(CH3)3 omitted
for clarity. Selected bond lengths (Å) and angles (deg):
Ge1-O1 ) 1.8441(10), Ge1-C1 ) 1.9400(15), C1-C2 )
1.338(2), C2-C3 ) 1.510(2), C4-C5 ) 1.506(2), C1-C6 )
1.467(2), C6-C7 ) 1.356(2); O1-Ge10-C1 ) 89.76(6), C2-
C1-C6 ) 122.25(14). Thermal ellipsoids at 50% prob-
ability.
Sch em e 2. P r op osed Step w ise Mech a n ism for
Ad d ition of a Th ir d Deu ter iu m Atom Com bin ed
w ith Dou ble-Bon d Isom er iza tion
Exp er im en ta l Section
Gen er a l P r oced u r es. 1 was synthesized via literature
procedures.18 All reactions were carried out using standard dry
benzene under an argon atmosphere.
1,1-Bis(b is(t r im e t h ylsilyl)m e t h yl)-3-p h e n yl-1,4,5,6-
tetr a h yd r oben zo[c][1,2]oxa ger m ole (4). A glass bomb was
charged with 1 (100 mg, 0.256 mmol) and benzophenone (46
mg, 0.25 mmol) in benzene (4 mL). Within 2 min the orange
color of 1 faded, leaving a yellow color indicative of triene
formation. Chlorotris(triphenylphosphine)rhodium (20 mg,
0.022 mmol, 8 mol %) was added to the solution. The solution
was frozen at -78 °C and the headspace in the bomb evacu-
ated. The bomb was back-filled with 3 atm of H2. The solution
was thawed and stirred at room temperature for 16 h. The
volatile components were removed in vacuo, leaving a red
residue that was dissolved in hexane and filtered through
Celite to remove the catalyst. The solvent was removed to yield
98 mg of crude hydrogenated material (67% crude yield). The
crude material (35 mg) was recrystallized from propionitrile
(1.5 mL) to give 10.6 mg of analytically pure 4 (30% yield on
recrystallization). 1H NMR (400 MHz, C6D6): δ 7.80 (d, 3J (H,H)
1H NMR spectrum of the deuterated materials show a
50% intensity decrease for each of the methylene
carbons. Additionally, an increase in the complexity of
the splitting pattern is observed due to H-D coupling.
The proton-decoupled 13C NMR spectrum shows C-D
coupling of 17-20 Hz for the three CH2 peaks present.
The IR spectrum indicated D incorporation with bands
observed at 2170 and 2037 cm-1 correlating to ν(C-D).
Literature precedence suggests the isomerization step
occurs via a migratory insertion followed by â-hydride
elimination (Scheme 2).7,8 This mechanism of isomer-
ization ensures that deuterium is incorporated from the
added D2 gas. Equilibrium geometry calculations (B3LYP
DFT, 6-31G*)9 indicate a decrease in energy of 27 kJ /
mol for such an isomerization. Scheme 2 illustrates the
addition of the third deuterium and the isomerization
after the double-bond hydrogenation step. However, it
is also possible that the isomerization precedes the
hydrogenation step. Although â-hydride elimination
may occur during typical hydrogenations catalyzed by
3
) 8.0 Hz, 2 H; Ar H), 7.23 (t, J (H,H) ) 8.0 Hz, 2 H; Ar H),
7.10 (t, 3J (H,H) ) 8.0 Hz, 1 H; Ar H), 6.03 (t, 3J (H,H) ) 4.4
Hz, 1 H; CdCH), 2.57 (t, 3J (H,H) ) 6.0 Hz, 2 H; CH2), 2.09
(td, 3J (H,H) ) 6.0 Hz, 3J (H,H) ) 4.4 Hz, 2 H; CH2), 1.56
3
(pseudo quintet, J (H,H) ) 6.0 Hz, 2 H; CH2), 0.30 (s, 18 H;
SiMe3), 0.23 (s, 18 H; SiMe3), 0.16 (s, 2 H; GeCHSi). 13C NMR
(100.5 MHz, C6D6): δ 154.37, 142.83, 137.20, 131.44, 128.53,
128.13, 127.67, and 114.71 (Ar CdC), 27.44, 26.44, and 22.47
(CH2), 12.33 (GeCHSi), 3.46 and 3.33 (SiMe3). IR (neat): ν 1661
cm-1 (CdC). Anal. Calcd for C27H50GeOSi4: C, 56.34; H, 8.75.
Found: C, 56.02; H, 8.83.
(10) Lin, S. D.; Vannice, M. A. J . Catal. 1993, 143, 539-553.
(11) Muetterties, E. L.; Bleeke, J . R. Acc. Chem. Res. 1979, 12, 324-
331.
(12) J ourdant, A.; Gonzalez-Zamora, E.; Zhu, J . P. J . Org. Chem.
2002, 67, 3163-3164.
(13) Hara, Y.; Inagaki, H. Chem. Lett. 2002, 1116-1117.
(14) Boxwell, C. J .; Dyson, P. J .; Ellis, D. J .; Welton, T. J . Am. Chem.
Soc. 2002, 124, 9334-9335.
(15) Kluson, P.; Cerveny, L. Appl. Catal. A - Gen. 1995, 128, 13-
31.
(7) Harrod, J . F.; Chalk, A. J . J . Am. Chem. Soc. 1966, 88, 3491-
(16) Grey, R. A.; Pez, G. P.; Wallo, A. J . Am. Chem. Soc. 1980, 102,
5948-5949.
3497.
(8) Osborn, J . A.; J ardine, F. H.; Young, J . F.; Wilkinson, G. J . Chem.
(17) Nagashima, H.; Suzuki, A.; Nobata, M.; Itoh, K. J . Am. Chem.
Soc. 1996, 118, 687-688.
Soc. A 1966, 1711-1732.
(9) Spartan ‘02, version 1.0.2 ed.; Wavefunction Inc., Irvine, CA,
2002.
(18) Fjeldberg, T.; Haaland, A.; Schilling, B. E. R.; Lappert, M. F.;
Thorne, A. J . J . Chem. Soc., Dalton Trans. 1986, 1551-1556.

Notes
1,1-Bis(b is(t r im e t h ylsilyl)m e t h yl)-3-p h e n yl-1,4,5,6-
Organometallics, Vol. 22, No. 22, 2003 4615
and normal-focus Mo-target X-ray tube operated at 2000 W
power (50 kV, 40 mA). The detector was placed at a distance
4.950 cm from the crystal. Analysis of the data showed
negligible decay during data collection; the data were processed
with SADABS19 and corrected for absorption. The structure
was solved and refined with the Bruker SHELXTL20 software
package by full-matrix least-squares procedures on F2 for all
reflections. All non-hydrogen atoms were refined anisotropi-
cally with the hydrogen atoms placed in idealized positions.
The file CCDC-208103 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB21EZ, U.K.; fax (+44) 1223-336-033;
email deposit@ccdc.cam.ac.uk).
tetr a h yd r oben zo[c][1,2]oxa ger m ole-4,5,6-d 3 (4-d 3). The
synthesis was identical with that above, except D2 was used
in place of H2. 1H NMR (400 MHz, C6D6): δ 7.80 (d, 3J (H,H) )
3
8.0 Hz, 2 H; Ar H), 7.23 (t, J (H,H) ) 8.0 Hz, 2 H; Ar H), 7.10
3
3
(t, J (H,H) ) 8.0 Hz, 1 H; Ar H), 6.03 (d, J (H,H) ) 4.4 Hz, 1
H; CdCH), 2.52 (m, 1 H; CH2), 2.06 (m, 1 H; CH2), 1.53 (m, 1
H; CH2), 0.30 (s, 18 H; SiMe3), 0.23 (s, 18 H; SiMe3), 0.16 (s, 2
H; GeCHSi). 13C NMR (100.5 MHz, C6D6): δ 154.37, 142.90,
137.26, 131.43, 128.33, 128.13, 127.65, and 114.70 (Ar CdC),
27.28 (t, 1J (C,D) ) 18 Hz), 25.95 (t, 1J (C,D) ) 17 Hz), and 21.87
(t, 1J (C,D) ) 20 Hz) (CH2), 12.36 (GeCHSi), 3.40 and 3.29
(SiMe3). IR (neat): ν 2170, 2037 cm-1 (C-D), 1662 cm-1 (Cd
C). EI/MS: [M]+ 579.333 au. Anal. Calcd for C27H47D3GeOSi4:
C, 56.05; H/D, 8.71. Found: C, 56.25; H/D, 8.76.
Str u ctu r a l Deter m in a tion of 4. Colorless blocks of 4-d 3
were grown from a propionitrile solution at 22 °C. Crystal data
for 4-d 3 (C27H47D3GeOSi4): Mr ) 578.64, T ) 118(2) K, triclinic,
space group P1h (No. 2), a ) 9.3590(16) Å, b ) 11.6647(19) Å,
c ) 15.660(3) Å, R ) 100.674(3)°, â ) 103.803(3)°, γ ) 91.311-
(3)°, V ) 1627.5(5) Å3, Z ) 2, d(calcd) ) 1.181 g cm-3, µ )
1.105 mm-1, λ ) 0.710 73 Å, 2θmax ) 56.68°, 16 189 measured
reflections, 7941 independent reflections, 7174 reflections
greater than 2σ(I), 310 refined parameters, GOF ) 1.031, R1
) 0.0281, wR2 ) 0.0720 (I > 2σ(I)), R1 ) 0.0329, wR2 ) 0.0748
(for all data), largest difference peak and hole 0.607 and
-0.292 e Å-3. A crystal of dimensions 0.22 × 0.22 × 0.16 mm
was mounted on a standard Bruker SMART CCD-based X-ray
diffractometer equipped with an LT-2 low-temperature device
Ack n ow led gm en t. We thank the Petroleum Re-
search Fund for financial support of this project.
Su p p or tin g In for m a tion Ava ila ble: Tables giving X-ray
crystallographic data for compound 4; these data are also
available as CIF files. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM034025T
(19) SADABS, Program for Empirical Absorption Correction of Area
Detector Data; University of Gottingen: Gottingen, Germany, 1996.
(20) SHELXTL, v. 5.10; Bruker Analytical X-Ray: Madison, WI,
1997.