Radical hydrometalation of functional ethylenic compounds: Radical autoinhibition changes the regioselectivity

Article abstract of DOI:10.1021/om049525g

Hydrometalation of carbon-carbon double bonds by group 14 hydrides is inhibited by carbonyl compounds-mainly by α,β-unsaturated carbonyl groups-as efficiently as by classical radical trapping compounds, such as galvinoxyl and hydroquinone. This phenomenon

Full text of DOI:10.1021/om049525g

446
Organometallics 2005, 24, 446-454
Radical Hydrometalation of Functional Ethylenic
Compounds: Radical Autoinhibition Changes the
Regioselectivity
Abdelkrim El Kadib,†,‡ Amal Feddouli,§ Pierre Rivie`re,*,† Fabien Delpech,†
Monique Rivie`re-Baudet,† Annie Castel,† Mohamed Ahra,‡ Aissa Hasnaoui,§
Francisco Burgos,| Juan M. Manriquez,| and Ivone Chavez|
Laboratoire d’He´te´rochimie Fondamentale et Applique´e, UMR 5069, Universite´ Paul Sabatier,
118 route de Narbonne, 31062 Toulouse Cedex 4, France, Laboratoire de Chimie Organique et
Organome´tallique, Faculte´ des Sciences, Universite´ Ibn Zohr, BP 6S, Agadir, Maroc,
Laboratoire des Substances Naturelles et des He´te´rocycles, Faculte´ des Sciences Semlalia, BP
2390, Marrakech 40000, Maroc, and Departamento de Quı´mica Inorga´nica, Facultad de
Quı´mica, Pontificia Universidad Cato´lica de Chile, Santiago, Casilla 306, Chile
Received June 30, 2004
Hydrometalation of carbon-carbon double bonds by group 14 hydrides is inhibited by
carbonyl compoundssmainly by R,â-unsaturated carbonyl groupssas efficiently as by
classical radical trapping compounds, such as galvinoxyl and hydroquinone. This phenom-
enon, in the case of polyfunctional CdC- and CdO-containing compounds, such as carvone,
leads to an autoinhibition of the exocyclic alkenylic hydrometalation under normal reaction
conditions, while under drastic conditions, the O-metalation of the R,â-unsaturated carbonyl
group occurs.
Introduction
tional compounds containing both CHdCH2 and CdO
unsaturation, the vinyl group is more reactive than the
carbonyl (eq 1),11,12 but a steric effect at the carbon-
carbon double bond can reverse the regioselectivity,
leading to 1,2-addition to the carbonyl group (eq 2)12 or
1,4-addition in a conjugated system (eq 3).12
In the particular field of group 14 chemistry and
especially in the case of silicon and germanium chem-
istry, hydrometalations of alkenes, of carbonyl com-
pounds, and of R,â-unsaturated carbonyl compounds
have been extensively studied.1-6 From these studies
emerged the catalytic effect of transition-metal com-
plexes (Pt, Ru, Rh, etc.),9,10 as well as an alternative
and useful radical-initiated reaction.6-8 In polyfunc-
* To whom correspondence should be addressed. E-mail:
riviere@chimie.ups-tlse.fr.
† Universite´ Paul Sabatier.
‡ Universite´ Ibn Zohr.
§ Faculte´ des Sciences Semlalia.
| Universidad Cato´lica de Chile.
(1) Armitage, D. A. Organosilanes. In Comprehensive Organome-
tallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;
Pergamon Press: New York, 1982; Vol. 2, pp 83-108.
(2) Armitage, D. A. Organosilanes. In Comprehensive Organome-
tallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;
Pergamon Press: New York, 1995; Vol. 2, pp 1-39.
(3) Birkofer, L.; Stuhl, O. General synthetic pathways to organo-
silicon compounds. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, U.K., 1989; pp 661-
663.
(4) Kendrick, T. C.; Parbhoo. C.; White J. W. Siloxane polymers and
copolymers. In The Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, Z., Eds.; Wiley: Chichester, U.K., 1989, pp 1342-1344.
(5) Ojima, I. The hydrosilylation reaction. In The Chemistry of
Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley:
Chichester, U.K., 1989; pp 1480-1525.
In these hydrometalations initiated by radical species,
halosilanes and halogermanes are more reactive than
their aryl or alkyl analogues (eq 4),12 and this sometimes
leads to a change in regioselectivity.
(6) Rivie`re, P.; Rivie`re-Baudet, M.; Satge´, J. Germanium. In Com-
prehensive Organometallic Chemistry; Pergamon Press: Oxford, U.K.,
1982; Vol. 2, pp 399-518 (COMC I). Rivie`re, P.; Rivie`re-Baudet, M.;
Satge´, J. Germanium. In Comprehensive Organometallic Chemistry;
Pergamon Press: Oxford, U.K., 1995; Vol. 2, pp 137-216 (COMC II),
(7) Chatgilialoglu, C. Organosilanes In Radical Chemistry: Prin-
ciples, Methods and Applications; Wiley: Chichester, U.K., 2004.
(8) Chatgilialoglu, C. Chem. Rev. 1995, 95, 1229.
(9) Speier, J. L. Homogeneous catalysis of hydrosilylation by transi-
tion metals. In Advances in Organometallic Chemistry; Stone, F. G.
A., West, R., Eds.; Academic Press: New York, 1979; Vol. 17, pp 407-
447.
(10) Skoda-Foldes, R.; Kollar, L.; Heil, B. J. Organomet. Chem. 1989,
366, 275.
In the case of conjugated carbonyl compounds, such
as benzaldehyde, benzophenone, mesityl oxide, o-nitro-
(11) Komarov, N. V.; Roman, V. K. Zh. Obshch. Khim. 1965, 35,
2017; Chem. Abstr. 1966, 64, 6675g.
10.1021/om049525g CCC: $30.25 © 2005 American Chemical Society
Publication on Web 12/22/2004

Radical Hydrometalation of Ethylenic Compounds
Organometallics, Vol. 24, No. 3, 2005 447
Table 1. Hydrometalation of Limonene and Carveol as a Function of Experimental Conditions
M-H
Y
T (°C)
100
130
100
130
20
130
130
130
130
20
time (h)
initiator or catalysta
inhibitora
reacn (%)
adduct
Ph3GeH
H
24
24
24
24
7
24
24
24
24
7
none
none
AIBN
RIS
none
none
none
none
none
0
70
72
∼100
65
0
1
UV
none
none
none
none
UV
galvinoxyl
benzophenone
nitrocinnamaldehyde
carvone
0
0
5
nitrocinnamaldehyde
0
Ph3GeH
OH
H
100
100
100
100
24
24
24
24
none
AIBN
none
none
none
70
95
0
2
3
none
nitrocinnamaldehyde
carvone
0
Ph2ClGeH
100
100
100
100
24
24
24
24
none
AIBN
none
none
none
none
69
97
0
galvinoxyl
nitrocinnamaldehyde
6
Ph3SiH
Cl3SiH
H
H
150
36
36
36
none
AIBN
RIS
none
none
none
0
0
0
3
80-120c
150
120
150
36
36
36
36
36
36
36
24
none
none
AIBN
none
none
none
none
Pt0
none
0
4b
none
25
80-120c
none
85
150
150
150
150
100
galvinoxyl
nitrocinnamaldehyde
dihydrocarvone
carvone
0
2
3
0
none
∼100
a Cf. General Comments in the Experimental Section. b References 17, 18, and 21. c Range of temperature and time explored to have
a convenient comparison with the other experiments.
cinnamaldehyde, etc., hydrometalation (M ) Ge) be-
comes difficult and is observed only with radical initia-
tion at high temperature (180-200 °C).12 Thus, a low
radical activity was ascribed to a high polarization of
the conjugated system. A more recent study of radical
hydrosilylation of unsaturated fatty acid methyl esters13
showed a “poisoning” of the radical chain reaction by
the esters, which could be avoided by a radical initiation
sequence (RIS)13 using a catalytic mixture of AIBN,
PhCO2O-t-Bu, and t-Bu2O2 and increasing the temper-
ature slowly from 20 °C up to 150 °C. At the same time,
studies of the hydrometalation of carvone (M ) Si)
showed an unexpected and complete deactivation of the
exocyclic carbon-carbon double bond.14-16
Therefore, in this work our purpose is to contribute
to the answers to the following questions.
(1) Why, in radical hydrometalation reactions, are
halosilanes or halogermanes more reactive than trior-
ganosilanes or triorganogermanes, which follows the
same scale of reactivity found in electrophilic hydro-
metalations?
(2) Why, in the case of nonconjugated unsaturated
carbonyl compounds, is specific radical-induced hydro-
metalation of the CdC bond usually observed?
(3) Why can a conjugated carbonyl group inhibit the
hydrometalation by group 14 hydrides of a nearby but
not conjugated CdC function?
With these purposes in mind, we developed a com-
parative study of the hydrometalation (Si, Ge) of li-
monene, carveol, dihydrocarvone, and carvone.
Results and Discussion
Limonene and Carveol. Thermally induced hydro-
metalation of limonene (Ge, Si) or carveol (Ge) is highly
favored by radical initiation but is inhibited by well-
known inhibitors, such as galvinoxyl.6,17-20 We observed
similar inhibition using various conjugated carbonyl
compounds (cf. Table 1). The regioselectivity in these
reactions corresponds to an anti-Markovnikov CdC
addition6-8 (Scheme 1). We observed differences in
reactivity of the organometallic compounds in accord
with the literature.1-9 Germanes are more reactive than
(12) Satge´, J.; Rivie`re, P. J. Organomet. Chem. 1969, 16, 71.
(13) Delpech, F.; Asgatay, S.; Castel, A.; Rivie`re, P.; Rivie`re-Baudet,
M.; Amine-Alami, A.; Manriquez, J. M. Appl. Organomet. Chem. 2001,
15, 626.
(14) Kogure, T.; Ojima, I. J. Organomet. Chem. 1982, 234, 249.
(15) Keinan, E.; Greenspoon, N. J. Am. Chem. Soc. 1986, 108, 7314.
(16) Schmidt, T. Tetrahedron Lett. 1994, 35, 3513.
(17) Valade, J.; Calas, R. Bull. Soc. Chim. Fr. 1958, 473.
(18) Skoda-Foldes, R.; Kollar, L.; Heil, B. J. Organomet. Chem. 1991,
408, 297.
(19) Vorenkov, M. V.; Yarosh, O. G.; Shchukina, L. V.; Alanov, A.
I.; Rudakov, G. A. Zh. Obshch. Khim. 1996, 66, 1941.
(20) Tanaka, R.; Iyoda, J.; Shiihara, I. Kogyo Kagaku Zasshi 1968,
71, 923 (CODEN, KGKZA7; ISSN, 0368-5462).

448 Organometallics, Vol. 24, No. 3, 2005
El Kadib et al.
Scheme 1
Table 2. DFT Parameters for Cl3Si• and Ph3Si• a
the corresponding silanes (cf. Ph3GeH/Ph3SiH), depend-
ing on the M-H bond dissociation energy (BDE) (ESi-H
) 397 kJ mol-1/EGe-H ) 341 kJ mol-1 for Me3MH).9,22-27
Within an organometallic series (Si or Ge), halo deriva-
tives RnCl3-n MH are more reactive than triorganometal
hydrides R3MH. However, as ∆BDE apparently is very
low in this case (<4 kJ mol-1),23,26 this result may be
interpreted in terms of the spin localization character-
izing the radical intermediates M• 8 (and also A) (Scheme
1). According to Bent’s rule28 and within the RnCl3-nM•
series, halogen substituents (compared to organic sub-
stituents) induce an increase in the p character contri-
bution of the metal in the metal-halogen bonds and
consequently an increase of the s character for the single
electron. This was established in (HnX)3Si• (X ) H, CH3,
Cl) by theoretical studies,29 which were confirmed by
ESR.8 We obtained similar results within our studied
PhnCl3-nSi• series by Hyperchem PM3 computation
(Figure 1), confirmed more accurately by theoretical
density functional theory (DFT) calculations (Table 2).
Within this series, Cl3Si• has the highest molecular
electronegativity (øM ) 0.1480) and also the highest
molecular hardness (ηM ) 0.1178) (Table 2) and will,
therefore, provide the greatest initial electron trans-
fer for reaction and the largest lowering of the activa-
tion energy.30 This results in the highest reactivity,
the metal charge being almost the same along the
RnCl3-nSi• series (Table 2). Then, the electronic struc-
ture of the intermediate radical can influence its addi-
tion to the substrate (b, Scheme 1), and also the
abstraction of hydrogen (c, Scheme 1).31,32
theor
exptl
eV hartree eVb hartreeb TSD MC
EC
Cl3Si•
Ph3Si•
I
7.24 0.2659 7.92 0.2910 0.736 0.728 %s: 56.94
A
ø
η
0.82 0.0302 2.50 0.0919
4.03 0.1480 5.21 0.1911
3.21 0.1178 2.70 0.0992
%p: 39.75
%d: 3.31
I
4.70 0.1726
0.73 0.0270
2.72 0.0998
1.98 0.0728
0.842 0.760 % s: 44.49
% p: 53.15
A
ø
η
% d: 2.36
a Legend: theor, theoretical; exptl, experimental; TSD, total spin
density; MC, metal charge; EC, electronic character; I ) -ꢀHOMO
A ) -ꢀLUMO
b Values taken from ref 30.
;
.
(21) Dindi, H.; Gregorovich, B.; Hazan, I.; Milligan, S.; U.S. Patent
5 527 936, 1996.
(22) Purcell, K. F.; Kotz, J. C. Inorganic Chemistry; W. B. Saun-
ders: London, 1977.
(23) McKean, D. C.; Torto, I.; Morrisson, A. R. J. Phys. Chem. 1982,
86, 307.
(24) Kanabus-Kaminska, J. M.; Hawari, J. A.; Griller, D.; Chatgil-
ialoglu, C. J. Am. Chem. Soc. 1987, 109, 5267.
(25) Wu, Y.-D.; Wong, C.-L. J. Org. Chem. 1995, 60, 821.
(26) Clark, K. B.; Griller, D. Organometallics 1991, 10, 746.
(27) Chatgilialoglu, C.; Newcomb, M. Adv. Organomet. Chem. 1999,
44, 67.
Figure 1. Spin localization density in RnCl3-n Si• (Hyper-
chem PM3 calculations).
(28) Bent, H. A. Chem. Rev. 1961, 61, 290.
(29) Guerra, M. J. Am. Chem. Soc. 1993, 115, 11926.
(30) Pearson, R. G. In Chemical Hardness; Wiley-VCH: Weinheim,
Germany, 1997.
Dihydrocarvone. Dihydrocarvone was almost as re-
active as limonene toward hydrogermylation. The reac-
tion led to similar regioselectivity (Scheme 2, Table 3).
(31) Drozdova, T. I.; Denisov, E. T. Kinet. Catal. 2002, 43, 10.

Radical Hydrometalation of Ethylenic Compounds
Organometallics, Vol. 24, No. 3, 2005 449
Scheme 2
Figure 2. Two possible radical intermediates in the
addition of Ph3GeH to dihydrocarvone. ECa > ECb; ∆E )
38.22 kJ mol-1 for the 2S,5R isomer as an example.
Scheme 3
Table 3. Hydrometalation of Dihydrocarvone
According to Experimental Conditions
mol-1. The differences in stabilization energies of Et/i-
Pr/t-Bu, for example, are in the range 2-7 kJ mol-1 32-34
.
In the case of silanes, hydrosilylation of alkenes was
more difficult and was achieved only by means of Pt0
catalysis (cf. general comments) for the most reactive
Cl3SiH (leading to 7) (Scheme 3, path a) (Table 3). In
the case of an alkylarylsilane (Ph2MeSiH), silylation of
the carbonyl group was obtained selectively using CuH‚
PPh3 as catalyst (eq 5).35
initiator
time
or
reacn
M-H
T (°C)
100
120
100
120
120
(h) catalyst
inhibitor
none
(%) adduct
Ph3GeH
24
24
24
24
24
none
none
0
12
5
6
6
none
AIBN none
∼100
none
none
galvinoxyl
nitrocinnam-
aldehyde
0
0
Ph2ClGeH 100
24
48
24
24
24
none
none
none
none
8
48
120
100
120
120
AIBN none
∼100
none
none
galvinoxyl
nitrocinnam-
aldehyde
7
5
Ph3SiH
150
24
none
none
0
0
0
80-120a 24
AIBN none
150
24
RIS
none
In the case of Cl3SiH under radical initiation, we also
observed a regioselective silylation of the carbonyl group
in accord with the relative stabilities of the two possible
radical intermediates Da and Db, which could be
implicated in the reaction (Scheme 3, path b) (Figure
3), the former undergoing a well-known hydrogen
elimination (Scheme 3, path c).36,37
Thus, in the case of dihydrocarvone one can speak of
radical-initiated autoinhibition, since the carbonyl group
of the molecule acts as an inhibitor for the hydrosilyl-
ation of the exocyclic carbon-carbon double bond in the
same molecule. This result is confirmed by the fact that
hydrosilylation of limonene, observed under mild ther-
Cl3SiH
Cl3SiH
150
100
36
36
none
Pt0
none
none
0
7
8
∼100
100
150
24
36
AIBN none
RIS none
a Range of temperature explored.
54
61
The regiospecific radical hydrometalation of the exo-
cyclic CdC bond can be explained by the relative
accessibility of the HOMO electrons in dihydrocarvone
(HOMO, ECdC ) -10.09 eV > ECdO ) -13.38 eV) to the
metal-centered radical (EPh Ge•
: HOMO, -8.66 eV;
3
LUMO, -2.27 eV) and by the relative stabilities of the
two possible radical intermediates (Ca and Cb) which
could be implicated in the reaction (Figure 2) (DFT
computations).
Although ∆E is not very large, it is larger than the
thermodynamic parameters which account for differ-
ences between the reactivities of primary, secondary,
or tertiary carbon-centered radicals in the propagation
step of the radical chain mechanism, whose energies of
activation for hydrogen abstraction are about 15 kJ
(32) Carey, F. A.; Sundberg, R. J. In Advanced Organic Chemistry,
3rd ed.; Plenum Press: London, 1990; Chapter 12, p 651.
(33) Huyser, E. S. In Free Radical Chain Reactions; Wiley-Inter-
science: London, 1970; p 91.
(34) Leroy, G.; Peeters, D.; Wilante, C. THEOCHEM 1982, 5, 217.
(35) Lipshutz, B. H.; Chrisman, W.; Noson, K. J. Organomet. Chem.
2001, 624, 367.
(36) Brook, A. G. Acylsilane Photochemistry. In The Chemistry of
Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley:
Chichester, U.K., 1989; p 984.

450 Organometallics, Vol. 24, No. 3, 2005
El Kadib et al.
Scheme 4
Figure 3. Two possible radical intermediates Da and Db
in the radical addition of Cl3SiH to dihydrocarvone. EDa
<
EDb; ∆E ) 45.33 kJ mol-1 for (2S,5R)-dihydrocarvone as
an example.
Table 4. Hydrometalation of Carvone
initiator or
catalyst
M-H
T (°C)
time (h)
reacn (%)
adduct
Ph3GeH
130
120
150
24
24
36
none
AIBN
RIS
0
26
85
9
the result of an easier trapping of metal-centered
radicals by the conjugated carbonyl group. In the case
of carvone, the relative scale of HOMO energy levels
explains the easier trapping of the metal-centered
radical by the conjugated carbonyl group (carvone,
HOMO ECdCCdO ) -9.7 eV > ECdC(exo) ) -10.17 eV;
Ph3Ge•, HOMO -8.66 eV, LUMO -2.27 eV, PM3
computations). Therefore, the reaction goes through the
most delocalized and stabilized32 intermediate Eb (Fig-
ure 4), whose formation was detected by ESR38 (Scheme
4).
This highly delocalized radical (Eb) is too “soft”
and requires too high an activation energy to be
able to initiate, under mild conditions, an abstraction
of hydrogen from M-H bonds31sas observed with
radical Cb in Scheme 2, path csand, therefore,
undergoes a break in the chain process. This can
explain why 1,4-addition was observed only under
drastic conditions (Scheme 4, path d). This hypoth-
esis is supported by the result of the second hydro-
germylation of carvone, which was observed under the
same mild experimental conditionsused for the hydro-
germylation of limonene or carveol (eq 7).
Ph2ClGeH
Cl3SiH
130
120
150
24
36
36
none
AIBN
RIS
17
55
∼100
0
16
41
10
8
150
120
150
24
24
36
none
AIBN
RIS
Pt0
Cl3SiH
100
24
∼100
12
mal conditions, is completely inhibited by catalytic
amounts of dihydrocarvone (Table 1).
Hydrometalation of Carvone. By comparison with
hydrocarvone, limonene, and carveol, carvone was less
reactive toward hydrometalation. In the case of tri-
phenylgermane, no thermally induced reaction was
observed in the range 80-150 °C. Radical initiation (by
AIBN, 80-100 °C) was ineffective. Initiation by radicals
at 130 °C (using PhCO2Ot-Bu) led to hydrogermylation
(9, 41%), which was completed only under RIS condi-
tions (see Table 4). The reaction was regiospecific and
corresponded to 1,4-addition to the conjugated R,â-
unsaturated carbonyl function (Scheme 4).
The 1,4-adduct 9 was isolated, characterized, and
chemically identified through the derivatives formed in
its acid hydrolysis (eq 6).
Thus, in carvone, a radical autoinhibition process
occurs, the intramolecular conjugated carbonyl imped-
ing the hydrogermylation of the exocyclic CdC bond.
The observed inhibition is similar to the intermolecular
This comparative study of hydrometalation of li-
monene, carveol, hydrocarvone and carvone shows
that the reactivity of the conjugated carbonyl function
is greater than that of the exocyclic CdCH2 group,
which usually is the more reactive. This fact could be
(37) Brook, A. G.; Duff, J. M. Can. J. Chem. 1973, 51, 352.
(38) Feddouli, A.; El Kadib, A.; Rivie`re, P.; Delpech, F.; Rivie`re-
Baudet, M.; Castel, A.; Manriquez, J. M.; Chavez, I.; Ait Itto, M. Y.;
Ahbala, M. Appl. Organomet. Chem. 2004, 18, 233.

Radical Hydrometalation of Ethylenic Compounds
Organometallics, Vol. 24, No. 3, 2005 451
Scheme 5
Figure 4. Possible radical intermediates in the reaction
of Ph3GeH with carvone. EEa > EEb; ∆E ) 10.93 kJ mol-1
calculated for (R)-(-)-carvone.
anion.39 The complex pattern of the spectra was very
satisfactorily simulated (Figure 5). This radical-induced
autoinhibition affects both alkenylic hydrogermylations
(Ph3GeH, PhCl2GeH) and hydrosilylations (Ph3SiH, Cl3-
SiH). Silanes showed a very low activity toward carvone,
Ph3SiH was inactive, and Cl3SiH gave under radical
initiation only a small percentage of 1,4-addition (Scheme
5, path a), while the alkenylic addition was obtained
quantitatively on Pt0 (Scheme 5, path b). In this case a
change from transition-metal catalysis to radical initia-
tion drastically changes the regioselectivity (Scheme 5).
In conclusion, the answers to the initial questions are
as follows.
(1) Halosilanes and halogermanes are the most reac-
tive in radical hydrometalation reactions, because the
halogen substituents on the metal enhance the molec-
ular electronegativity and the molecular hardness of the
metal-centered radical, favoring the initial electron
transfer.
(2) In the case of nonconjugated carbonyl containing
olefinic compounds, specific radical hydrometalation of
the CdC function (or of the carbonyl) is mainly depend-
ent on the energy parameters characterizing the differ-
ent possible intermediate radicals.
Figure 5. ESR study of reactions: (1) Ph3GeH + limon-
ene (toluene, hν) + o-NO2C6H4CHdCHCHO; (2) Ph3-
GeH + t-Bu2O2 (toluene, hν) + o-NO2C6H4CHdCHCHO;
(3) simulation using pure Gaussian functions of 1.6 G
width.
(3) R,â-unsaturated carbonyl functions act as very
efficient radical inhibitors for the radical-initiated hy-
drometalation of CdC bonds. In this last case, metal
catalysis (Pt, Pd, etc.) is recommended to obtain the
hydrometalation of the carbon-carbon double bond. For
the metalation of the carbonyl bond, recourse to RIS is
essential.
By a change in the initiation step of the reaction
(radical initiation or transition-metal catalysis) it is
possible to obtain a dramatic change in the orientation
of these hydrometalation reactions and to perform
especially difficult hydrometalation of industrial inter-
est.
radical inhibition reaction in the hydrometalation of
limonene, carveol, and hydrocarvone by carbonyl com-
pounds (Table 1). It can be explained by the trapping
of the metal-centered radicals having conjugated car-
bonyl groups undergoing a “poisoning” of the radical
chain (Scheme 1, path d). This hypothesis is supported
by the fact that R,â-unsaturated carbonyl compounds,
such as o-nitrocinnamaldehyde and carvone, are as good
inhibitors as galvinoxyl itself in these hydrometalation
reactions (Table 1). They act as a spin trap for group
14 element centered radicals. This was verified by an
ESR study of the reaction of Ph3GeH with limonene, in
the presence of o-nitrocinnamaldehyde, which showed
the formation of radical B (Scheme 1, Figure 5). This
radical was reproduced independently (eq 8) and ana-
Experimental Section
General Comments. All reactions were performed under
nitrogen using standard Schlenk tube techniques, Carius
tubes, and dry solvents. NMR spectra were recorded on Bruker
AC 80 (1H, 80 MHz) and AC 200 (13C in the sequence Jmod,
50.3 MHz) spectrometers and with a Bruker 300 MHz instru-
ment (29Si). Gas chromatography (GC) was undertaken on a
Hewlett-Packard HP 6890 instrument, and mass spectra were
recorded with a Hewlett-Packard HP5989 instrument in the
electron impact mode (EI, 70 eV) or with a Rybermag R10-10
(39) Rivie`re, P.; Castel, A.; Rivie`re-Baudet, M. Main Group Met.
Chem. 1994, 17, 679.
lyzed by following data of the corresponding radical

452 Organometallics, Vol. 24, No. 3, 2005
El Kadib et al.
spectrometer operating in the EI mode or by chemical desorp-
tion (DCI/CH4). Infrared spectra were recorded on a Perkin-
Elmer 1600FT spectrometer. ESR spectra were recorded on a
Bruker ER200 instrument with an EIP frequency meter, in
toluene solutions at 243 K. Elemental analyses were done by
the Centre de Microanalyses de l’Ecole Nationale Supe´rieure
de Chimie de Toulouse. Theoretical calculations were per-
formed at the Hyperchem PM3 level or with the Gaussian 98
package.40 Density functional theory (DFT) was employed with
the three-parameter hybrid exchange functional of Becke41 and
the Lee, Yang, and Parr correlation functional42 (B3LYP). The
basis set was the standard 6-311G included in Gaussian. The
geometries were obtained by full-geometry optimization at the
B3LYP level and checked by frequency calculations. Graphical
pictures were obtained with the MOLEKEL program.43
Karstedt’s catalyst Pt0 (platinum(0)-1,3-divinyl-1,1,3,3-tet-
ramethyldisiloxane complex, 0.100 M in poly(dimethylsilox-
ane)), initiators and inhibitors, were used in a 1% concentra-
tion relative to organic reagents. All the tests of hydrometalation
of limonene, carveol, and carvone with different catalysts or
radical initiators (Tables 1, 3, and 4) were performed on similar
samples of M-H (1 mmol) and reagent (1.1 mmol). All organic
reagents ((S)-(-)-limonene, (-)-carveol, (+)-dihydrocarvone,
and (R)-(-)-carvone) were obtained from Aldrich. The organ-
ogermanium compounds were prepared according to the
literature procedure.6 Triphenylsilane was purchased from
Aldrich.
Preparation of 2.
A mixture of (-)-carveol (0.80 g, 5.26 mmol), Ph3GeH (1.50 g,
4.92 mmol), and AIBN was heated at 100 °C in a Carius tube
for 24 h. The reaction was almost quantitative (95% by 1H
NMR analysis). The mixture was then distilled under reduced
pressure, giving 1.38 g of 2. Yield: 46%. Bp: 200 °C/0.1 mmHg.
IR (CDCl3, cm-1): ν 3602 (OH), 1662 (CdC). 1H NMR (CDCl3,
3
ppm): δ 0.90 (d, 3H, CH3 in 9, JCH-CH ) 6 Hz), 1.60 (m, 9H
3
in 4, 5, 6, 7, 8, and OH), 1.80 (s, 3H, CH3 on 10), 4.04 (m, 1H,
CH in 1), 5.50 (m, 1H, CH in 3), 7.42 (m, 9H, Ph), 7.55 (m,
6H, Ph). 13C NMR (CDCl3, ppm): δ 18.96 and 19.51 (C9), 19.07,
19.27, 19.31, and 19.82 (C8), 21.02 and 21.05 (C10), 27.59,
28.00, 29.08, and 29.27 (C4), 33.62, 33.95, 34.84, and 34.85
(C5), 37.35, 35.57, and 35.83 (C6), 40.35, 40.83, and 43.09 (C7),
68.84, 68.95, 71.32, and 71.39 (C1), 124.26, 125.68, and 125.73
(C3), 128.28 (C′3), 128.93 (C′4), 134.60 and 134.64 (C′1), 135.06
(C′2), 136.44, 137.30, 137.67 and 137.73 (C2). MS (EI; m/z
(%)): [M•+] 458 (25); [M•+ - PhH] 380 (100). Anal. Calcd for
C28H32GeO: C, 73.57; H, 7.06. Found: C, 72.97; H, 6.93
Preparation of 3.
Preparation of 1.
A mixture of (S)-(-)-limonene (0.80 g, 5.87 mmol) and Ph3-
GeH (1.50 g, 4.92 mmol) was heated at 130 °C in a Carius
tube under RIS conditions13 for 24 h. The reaction was almost
quantitative (1H NMR analysis). Distillation under reduced
pressure gave 1.57 g of 1. Yield: 72%. Bp: 180 °C/5 × 10-2
mmHg. IR (CDCl3, cm-1): ν 1644 (CdC). 1H NMR (CDCl3,
ppm): δ 0.82 (d, 3H, CH3 in 9, 3JCH-CH3 ) 6.5 Hz), 1.55 (m, 6H
in 4, 5, 7, 8), 1.62 (broad s, 3H, CH3 on 10), 1.80 (m, 4H, CH2
in 3, 6), 5.35 (m, 1H, CH in 2), 7.40 (m, 15H, Ph). 13C NMR
(CDCl3, ppm): δ 19.33 and 19.61 (C9), 19.69 and 19.87 (C8),
23.87 (C10), 25.35 and 27.07 (C5), 28.04 and 29.24 (C3), 31.30
and 31.06 (C6), 34.22 and 34.53 (C4), 41.03 and 41.55 (C7),
121.25 (C2), 128.45 (C′3), 129.07 and 130.18 (C′4), 134.15 (C1),
135.29 (C′2), 138.10 (C′1). MS (EI): m/z (%): [M•+] 442 (3%);
[M•+ - PhH] 364 (33%). Anal. Calcd for C28H32Ge: C, 76.23;
H, 7.31 Found: C, 76.11; H, 7.12.
A mixture of (S)-(-)-limonene (0.48 g, 3.54 mmol), Ph2ClGeH
(0.80 g, 3.04 mmol), and AIBN was heated in a Carius tube
for 24 h at 100 °C. The reaction was almost quantitative (97%
by 1H NMR analysis). Distillation under reduced pressure gave
0.92 g of 3. Yield: 76%. Bp: 164 °C/5 × 10-2 mmHg. IR (CDCl3,
1
cm-1): ν 1646 (CdC). H NMR (CDCl3, ppm): δ 0.98 (d, 3H,
CH3 in 9, 3JCH-CH3 ) 6.24 Hz), 1.33-1.56 (m, 6H in 4, 5, 7, 8),
1.71 (broad s, 3H, CH3 on 10), 1.82-2.20 (m, 4H, CH2 in 3, 6),
5.43 (m, 1H, CH in 2), 7.49-7.70 (m, 10H, Ph). 13C NMR
(CDCl3, ppm): δ 19.12 (C9), 23.60 (C10), 23.61 and 23.66 (C8),
27.93 and 28.09 (C5), 29.01 (C3), 30.81 and 30.99 (C6), 33.71
and 33.93 (C4), 40.61 and 40.92 (C7), 120.82 (C2), 128.67 (C′3),
130.25 (C′4), 133.55 (C′2), 134.01 (C1), 136.90 (C′1). MS (EI;
m/z (%)): [M•+] 400 (10); [M•+ - PhH] 322 (50), [M•+ - 2PhH]
322 (8), [Ph2GeCl] (100). Anal. Calcd for C22H27ClGe: C, 66.14;
H, 6.81; Cl, 8.87. Found: C, 66.37; H, 6.72; Cl, 8.74.
Preparation of 4.
(40) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J. A. Gaussian 98, revision A.7; Gaussian, Inc.: Pittsburgh, PA,
1998.
(41) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Becke, A. D. J.
Chem. Phys. 1993, 98, 1372. (c) Becke, A. D. J. Chem. Phys. 1993, 98,
5648.
(42) Lee, C.; Yang, W.; Parr, R. Phys. Rev. B 1988, 37, 785.
(43) Flukiger, P.; Luthi, H. P.; Portmann, S.; Weber, J. MOLEKEL
4.1; Swiss Center for Scientific Computing, Manno, Switzerland, 2000-
2001.
A mixture of (S)-(-)-limonene (1 g, 7.33 mmol), Cl3SiH (1 g,
7.46 mmol), and Pt0 was heated in a Carius tube for 24 h at
100 °C. The reaction was quasi-quantitative (1H NMR analy-
sis). Distillation under reduced pressure gave 1.30 g of 4.
Yield: 65%. Bp: 80 °C/5 × 10-2 mmHg. IR (CDCl3, cm-1): ν
1614(CdC) (conforms to literature17,19-21). 1H NMR (CDCl3,
ppm): δ 1.06 (d, 3H, CH3 in 9, 3JCH-CH3 ) 6.66 Hz), 1.17-1.37

Radical Hydrometalation of Ethylenic Compounds
Organometallics, Vol. 24, No. 3, 2005 453
(m, 6H in 4, 5, 7, 8), 1.65 (broad s, 3H, CH3 on 10), 1.92 (m,
4H, CH2 in 3, 6), 5.39 (m, 1H, CH in 2). 13C NMR (CDCl3,
ppm): δ 18.44 (C9), 21.68 and 21.81 (C10), 23.08 (C8), 26.66
(C5), 29.27 and 29.56 (C3), 32.57 (C6), 33.57 (C4), 43.75 (C7),
121.39 and 121.70 (C2), 138.06 (C1). 29Si NMR (CDCl3, ppm):
δ 13.44. MS (EI; m/z (%)): [M•+] 270 (20); [M•+ - Cl] 235 (11);
[M•+ - SiCl3] 137 (100); [M•+ - CH3CHCH2SiCl3] 95 (35).
Preparation of 5.
quantitative (1H NMR analysis). Distillation led to 1.74 g of
7. Yield: 61%. Bp: 114 °C/0.1 mmHg. IR (CDCl3, cm-1): ν 1707
(CdO). 1H NMR (CDCl3, ppm): δ 0.98 (d, 3H, CH3 in 10,
3
3JCH-CH ) 6.4 Hz), 1.06 (d, 3H, CH3 in 9, JCH-CH ) 6.4 Hz),
3
1.15-1.394 (m, 8H in 3, 4, 5, 7, 8), 1.96-2.36 (m, 3H in 2, 6).
13C NMR (CDCl3, ppm): δ 14.35 (C10), 18.46 (C9), 27.39 (C8),
28.80 (C4), 33.33 (C5), 34.50 (C3), 43.60 (C6), 45.01 (C2), 46.43
and 46.69 (C7), 212.28 (C1). 29Si NMR (CDCl3, ppm): δ 12.58.
MS (EI; m/z (%)): [M•+] 286 (10); [M•+ - CH3CHCH2SiCl3] 111
(100); [CH3CHCH2SiCl3] 175 (12). Anal. Calcd for C10H17Cl3-
SiO: C, 41.75; H, 5.96; Cl, 36.97. Found: C, 41.57; H, 6.17;
Cl, 36.42.
Preparation of 8.
A mixture of (+)-dihydrocarvone (0.70 g, 4.60 mmol) and Ph3-
GeH (1.22 g, 4 mmol) was heated in the presence of AIBN at
100 °C for 24 h. The reaction was almost quantitative (1H NMR
analysis). Distillation led to 1.12 g of pure 5. Yield: 62%. Bp:
212 °C/0.3 mmHg. IR (CDCl3, cm-1): ν 1706 (CdO). 1H NMR
(CDCl3, ppm): δ 0.91 (d, 3H, CH3 in 10, 3JCH-CH3 ) 6 Hz), 1.06
5, 7, 8), 2.45-2.60 (m, 3H3in 2, 6), 7.52 (m, 15H, Ph). 13C NMR
(CDCl3, ppm): δ 14.57 (C10), 18.92 (C9), 19.36 (C8), 29.19 (C4),
34.49 (C5), 34.95 (C3), 45.02 (C7), 45.68 (C6), 47.68 (C2),
128.37 (C′3), 129.00 (C′4), 134.60 and 137.43 (C′1), 135.02 (C′2),
213.5 (C1). MS (EI; m/z (%)): [M•+] 458 (15); [M•+ -Ph] 381
(30); [M•+ - 2Ph] 304 (100); [M•+ - 3Ph] 227 (40). Anal. Calcd
for C28H32GeO: C, 73.57; H, 7.06. Found: C, 73.27; H, 7.27.
Preparation of 6.
A mixture of (+)-dihydrocarvone (1.53 g, 10 mmol) and Cl3-
SiH (1.08 g, 8 mmol) was heated in the presence of AIBN in a
Carius tube at 100 °C for 24 h. The mixture was then distilled
under reduced pressure, leading to 1.53 g of 8. Yield: 54%.
Bp: 120 °C/0.1 mmHg. IR (CDCl3, cm-1): ν 1630, 1659 (Cd
C). 1H NMR (CDCl3, ppm): δ 1.62 (b s, 3H, CH3 in 9), 1.72
(broad s, 3H, CH3 in 10), 1.55-2.22 (m, 7H in 3, 4, 5, 6), 4.72
(m, 2H in 8). 13C NMR (CDCl3, ppm): δ 16.24 (C10), 20.80 (C9),
27.46 (C4), 30.60 (C3), 34.31 (C6), 42.14 (C5), 109.25 (C8),
115.92 (C2), 140.95 (C7), 148.26 (C1). MS (CI CH4; m/z (%)):
[M + 1] 285 (100); [M + 29] 313; [M + 41] 325. Anal. Calcd for
C10H15Cl3SiO: C, 42.04; H, 5.29; Cl, 37.23. Found: C, 42.37;
H, 5.12; Cl, 36.95.
3
(d, 3H, CH3 in 9, JCH-CH ) 6 Hz), 1.10-2.45 (m, 8H in 3, 4,
Preparation of 9.
A mixture of (+)-dihydrocarvone (0.60 g, 3.94 mmol) and Ph2-
ClGeH (0.80 g, 3.04 mmol) was heated in a Carius tube in the
presence of AIBN at 100 °C for 24 h. The reaction was almost
quantitative (1H NMR analysis). Distillation gave 0.74 g of 6.
Yield: 59%. Bp: 172 °C/6 × 10-2 mmHg. IR (CDCl3, cm-1): ν
1700 (CdO). 1H NMR (CDCl3, ppm): δ 0.91 (d, 3H, CH3 in 10,
A mixture of (R)-(-)-carvone (0.80 g, 5.33 mmol) and Ph3GeH
(1.50 g, 4.92 mmol) was heated in a Carius tube under RIS
1
conditions for 36 h. H NMR analysis showed 85% of 9. The
mixture was then distilled under reduced pressure, giving 1.64
g of pure 9. Yield: 69%. Bp: 175 °C/2 × 10-2 mmHg (in
conformity with ref 38). IR (CDCl3, cm-1): ν 1645, 1654 (Cd
3
3JCH-CH ) 6.6 Hz), 1.00 (d, 3H, CH3 in 9, JCH-CH ) 6.3 Hz),
3
1.08-1.392 (m, 8H in 3, 4, 5, 7, 8), 2.03-2.40 (m, 3H in 2, 6),
7.35-7.60 (m, 10H, Ph). 13C NMR (CDCl3, ppm): δ 14.46 (C10),
18.55 (C9), 24.30 and 24.46 (C8), 29.12 (C4), 33.99 (C5), 34.76
and 34.87 (C3), 44.94 (C2), 45.46 (C6), 46.85 (C7), 128.65 and
128.74 (C′3), 130.39 (C′4), 133.43 and 133.46 (C′2), 136.35 (C′1),
212.98 (C1). MS (EI; m/z (%)): [M•+] 416 (10); [M•+ - Ph] 339
(15); [M•+ - 2Ph] 262 (100%); [M•+ - 2Ph - Cl] 227 (12); [M•+
- Ph2GeCl] 153 (20). Anal. Calcd for C22H27ClGeO: C, 63.60;
H, 6.55; Cl, 8.53. Found: C, 63.28; H, 6.37; Cl, 8.15.
Preparation of 7.
1
C). H NMR (CDCl3, ppm): δ 1.30-2.50 (m, 7H, in 3, 4, 5, 6),
1.62 (broad s, 6H, CH3 in 9 and 10), 4.62 (m, 2H, CH2 in 8),
7.53 (m, 15H, Ph). 13C NMR (CDCl3, ppm): δ 16.55 (C10), 20.70
(C9), 27.94 (C4), 30.50 (C3), 36.71 (C6), 42.55 (C5), 108.49 (C8),
112.05 (C2), 128.45 (C′3), 129.07 (C′4), 134.66 (C′2), 135.01
(C7), 145.66 (C′1), 149.58 (C1). MS (EI; m/z (%)): [M•+] 456
(25); [M•+ - Ph] 379 (35); [M•+ -2Ph] 302 (100); [M•+ - 3Ph]
225 (75). Anal. Calcd for C28H30GeO: C, 73.89; H, 6.64.
Found: C, 74.32; H, 6.71.
A sample of 9 (0.12 g, 0.26 mmol) in THF solution was
treated with an excess of 6 M HCl and analyzed by GC. The
formation of Ph3GeCl and dihydrocarvone was almost quan-
titative.
Preparation of 10.
A mixture of (R)-(-)carvone (0.51 g, 3.40 mmol) and Ph2ClGeH
(0.86 g, 3.26 mmol) was heated in a Carius tube under RIS
conditions for 36 h. The reaction was almost quantitative (1H
NMR analysis). Distillation under reduced pressure gave 0.94
g of 10. Yield: 70%. Bp: 163 °C/5 × 10-2 mmHg. IR (CDCl3,
A mixture of (+)-dihydrocarvone (1.52 g, 10 mmol) and Cl3-
SiH (1.35 g, 10 mmol) was heated in the presence of Pt0 in a
Carius tube at 100 °C for 36 h. The reaction was almost
1
cm-1): ν 1670 (broad) (CdC). H NMR (CDCl3, ppm): δ 1.36

454 Organometallics, Vol. 24, No. 3, 2005
El Kadib et al.
and 135.25 (C′2), 137.40 and 137.90 (C′1), 145.96 and 146.07
(C1). MS (EI; m/z (%)): [M•+] 760 (22); [M•+ - Ph] 683 (10);
[M•+ - 2Ph] 606 (15); [M•+ - Ph3Ge] 457 (100). Anal. Calcd
for C46H46Ge2O: C, 72.68; H, 6.10. Found: C, 72.42; H, 6.06.
(b) From 9 and Ph3GeH. A mixture of 9 (0.23 g, 0.50
mmol) and Ph3GeH (0.15 g, 0.49 mmol) was heated in a Carius
tube for 24 h at 80 °C in the presence of AIBN. NMR analysis
showed quantitative formation of 11.38
Preparation of 12.
(broad s, 6H, CH3 in 9 and 10), 1.50-2.70 (m, 7H, in 3, 4, 5,
6), 4.75 (m, 2H, CH2 in 8), 7.50 (m, 10H, Ph). 13C NMR (CDCl3,
ppm): δ 19.82 (C10), 23.20 (C9), 26.91 (C6), 28.70 (C4), 34.01
(C3), 42.54 (C5), 109.83 (C8), 112.00 (C2), 128.24 (C′3), 128.59
(C′4), 133.23 (C′2), 135.39 (C7), 144.64 (C1), 146.71 (C′1). MS
(EI; m/z (%)): [M•+] 414 (10); [M•+ - Cl] 379 (16); [M•+ - Ph2-
ClGe] 151 (46); [Ph2ClGe] 263 (100). Anal. Calcd for C22H25-
ClGeO: C, 63.91; H, 6.09; Cl, 8.57. Found: C, 63.48; H, 5.87;
Cl, 8.73.
A mixture of (R)-(-)-carvone (1.50 g, 10 mmol) and Cl3SiH
(1.35 g, 10 mmol) was heated in a Carius tube in the presence
of Pt0 at 100 °C for 36 h. The reaction was quantitative (1H
NMR analysis). Distillation under reduced pressure led to 2.30
g of 12. Yield: 81%. Bp: 118 °C/0.2 mmHg. IR (CDCl3, cm-1):
ν 1711 (CdO), 1660 (CdC). 1H NMR (CDCl3, ppm): δ 1.05 (d,
3H, CH3 in 9, 3JCH-CH3 ) 6.5 Hz), 1.20-1.34 (m, 1H) and 1.46-
1.55 (m, 1H) (2H in 5,7), 1.57 (s, 3H, CH3 in 10), 1.72-2.48
(m, 6H, CH2 in 4, 6, 8), 6.69 (m, 1H, H in 3). 13C NMR (CDCl3,
ppm): δ 15.66 (C10), 18.23 (C9), 28.46 and 29.61 (C8), 32.54
(C5), 40.50 (C4), 41.66 (C6), 44.06 (C7), 135.47 (C2), 144.51
(C3), 199.47 (C1);. 29Si NMR (CDCl3, ppm): δ 12.37. MS (EI;
m/z (%)): [M•+] 284 (20); [M•+ - 3Cl] 179 (11); [M•+ - CH3-
CHCH2SiCl3] 109 (100). Anal. Calcd for C10H15Cl3SiO: C,
42.04; H, 5.29. Found: C, 41.83; H, 5.12.
Preparation of 11.
(a) From Carvone and Ph3GeH. A mixture of (R)-(-)-
carvone (0.30 g, 2 mmol) and Ph3GeH (1.20 g, 3.94 mmol) was
heated in a Carius tube under RIS conditions for 48 h. The
reaction mixture was distilled, leading to 0.91 g of 11. Yield:
66%. Bp: 255 °C/0.3 mmHg. IR (CDCl3, cm-1): ν 1653 (CdC).
1H NMR (CDCl3, ppm): δ 0.58-2.21 (m, 10H, CH2 in 3, 4, 6,
8 and CH in 5, 7), 1.07 (d, 3H, CH3 in 9, 3JCH-CH3 ) 6 Hz), 1.55
(s, 3H, CH3 in 10), 7.50 (m, 30H, Ph). 13C NMR (CDCl3, ppm):
δ 16.35 (C10), 18.70 and 19.01 (C9), 19.01 and 19.35 (C8), 27.52
and 29.21 (C4), 30.57 and 30.74 (C3), 32.80 and 34.50 (C5),
36.71 and 35.06 (C6), 43.07 and 45.05 (C7), 111.97 and 112.18
(C2), 128.87 and 129.09 (C′3), 129.71 and 130.18 (C′4), 134.68
The same reaction conducted with AIBN in a Carius tube
for 24 h at 120 °C yielded only 16% of 8, while the reaction
under RIS conditions (for 36 h) gave 41% of 8 (Table 4).
Acknowledgment. We thank the “Conseil Re´gional
Midi Pyre´ne´es” (AF grant), the Universite´ Paul Sa-
batier, Action Inte´gre´e France-Maroc (Grant No. 216/
SM/00), and the program ECOS CONICYT C01E06 and
C04E05 for partial financial support.
OM049525G