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N-Lithiomethyl-N NN N-tetramethyldiethylene-triamine The First Alkyllithium Compound which is Monomeric in Hydrocarbons.

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2:ArnixtureofSc(1.31 g,3.8 mmol)inDMSO(30mL)andAc,O(20mL)was
stirred for 3.5 h at room temperature. The mixture was partitioned between
200 mL of cold 20% NaOH and 300 mL of Et,O under ice cooling, and the
aqueous phase was extracted several times with Et,O. The combined organic
solutions were washed with brine and dried over Na,SO,. Flash chromatography with hexane/CH,Cl,/EtOAc (10: 10: 1 to 5:5:1) afforded 2 (1.01 g, 78 %)as
an orange solid. M.p. 176-178"C, decomp.; [a]EO -818, c = 0.111 in CHCI,;
IR(KBr): i. [cm-'1 = 1959(s), 1896(s), 1869(s), 1682(s), 1437(m), 1287(s);
N-Lithiomethyl-N,N"",N"-tetramethy ldiethylenetriamine: The First Alkyllithium Compound which
is Monomeric in Hydrocarbons
By Gerhard u! Klumpp,* Hendrikus Luitjes, Marius Schakel,
Franciscus J. J de Kanter, Robert R Schmitz
and Nicolaas J. R . van Eikema Hommes
'HNMR(270MHz,CDC1,):6=6.00(s,1H),5.11(s,1H),3.85(s,3H),3.80
(s, 3H), 2.91 (m, l H ) , 2.69 (m, 2H), 2.36 (m, IH), 2.11 (m, 2H); '-'CNMR
(62.9 MHz, CDCI,, additional DEPT): 6 = 21.9(CH2),28.0(CH2), 37.3(CHz),
56.7(CH3), 57.3(CH3), 74.3(CH), 75.5(CH), 87.8, 109.9, 130.5, 135.5, 196.0,
231.3.
Received: December 4, 1991 [Z 5055 IE]
German version: Angew. Chem. 1992, 104, 640
[I] a) J, P. Collman, L. S. Hegedus, J. R. Norton, R. G. Finke, Principles and
Applications of Organotransition Metal Chemistry, University Science
Books, Mill Valley, CA, USA, 1987, Chap. 20; b) V. N. Kalinin, Russ.
Chem. Rev. 1987,56,682-700; c) M. Uemura, H. Nishimura, T. Minami,
Y Hayashi, J. Am. Chem. Soc. 1991, 113, 5402-5410.
[2] a ) S. A. Look, W. Fenical, R. S. Jacobs, J. Clardy, Proc. Natl. Acad. Sci.
USA 1986,83,6238-6240; b) S. A. Look, W. Fenical, G. K. Matsumoto,
J. Clardy, J. Org. Chem. 1986, 51, 5140-5145; c) V. Roussis, 2. Wu, W.
Fenical, S. A. Strobel, G. D. Van Duyne, J. Clardy, ibid. 1990, 55,49164922.
[3] a) A. Solladie-Cavallo in Advances in M e l d o r g a n i c Chemistry, Vol. 1
(Ed.: L. S. Liebeskind), JAI Press, Greenwich, CT, USA, 1989, p. 99133 and references cited therein.; b) K. Schlogel in OrganomelaNics in
Organic Synthesis 2 (Eds.: H. Werner, G. Erker), Springer, Berlin, 1989,
p. 63; c) S. G. Davies, C. L. Goodfellow, Synlett 1989, 59-61; J. Chem.
Soc. Perkin Trans. I . 1989.192-193; d) S. Top, G. Jaouen, C. Baldoli, P.
Del Buttero, S. Maiorana, J Organomet. Chem. 1991,413, 125-135, and
references cited therein.
[4] a) G. Jaouen. A. Meyer, J. Am. Chem. SOC.1975,97,4667-4672; b) S. G.
Davies, C. L. Goodfellow,J. Organornet. Chem. 1988,340,195-201; c) B.
Ohlson, C. Ullenius, S. Jagner, C. Grivet, E. Wenger, E. P. Kundig, ibid.
1989. 365, 243 -267, and references cited therein.; J. Brocard, L. Pelinski,
J. Lebibi, M. Mahmoudi, L. Maciejewski, Tetrahedron, 1989,45,709-720,
and references cited therein.
[5] a) E. J. Corey, R. K. Bakshi, S. Shibata, J. Am. Chem. SOC.1987, f09,
5551 -5553; b) E. J. Corey, R. K. Bakshi, S. Shibata, C. P. Chen., V. K.
Singh, ibid. 1987, 109, 7925-7926; c)T. K. Jones, J. J. Mohan, L. C.
Xavier, T. J. Blacklock, D. J. Mathre, P. Sohar, E. T. Turner Jones, R. A.
Reamer, F. E. Roberts, E. J. J. Grabowski, J. Org. Chem. 1991, 56, 763769. and references cited therein.
[6] All compounds were characterized by the usual spectroscopic methods and
gave correct elemental analyses.
[7] E. P. Kundig, C. Perret, S. Spichiger, G. Bernardinelli, J. Organomet.
Chem. 1985,286, 183-200.
[El C. A. L. Mahaffy, P. L. Pauson, Inorg. Synth. 1979, 19, 154.
[9] A time-resolved study of the reactions by means of HPLC showed that (in
contrast to the complexation of I-indanol [c]) the synlanti ratio (5:6)
decreases continuously during the conversions. This loss of selectivity,
which is attributable to a change of the Cr(CO), group to the opposite n
face. is more pronounced (faster), the more methoxy substituents are
present on the arene ring. In our laboratory the complete diastereoselection described by Davies et al. in [4b] for the complexation of ruc4a under
similar conditions (19 h reaction time, 36% yield) could not be observed.
[lo] S. G. Levine. B. Gopalaknshnan, Tetrahedron Lett. 1982, 23, 12391240.
[I I] X-ray crystal structure analysis of 10: Enraf-Nonius CAD4 diffractometer, Cu,, radiation, 2 0 = 120"; empirical absorption correction based on
scans; structure determination with direct methods (SHELXS 86); the
positions of the C and 0 atoms were determined by difference syntheses,
and the hydrogen atoms in calculated positions were not refined.
C,,H,,CrO, . 0.5EtOAc, trigonal, space group P 3,21, a = b =
11.5377(7). c = 32.438(2) A, V = 3739.6(8) A',
2 = 6, pCalrd
=
1.379 gcm-'; 2159 independent reflections, ofwhich 2104 with I > 0 were
used, R = 0.057, R, = 0.055. The residual density was less than
0.54 eA - 3 . The absolute configuration was deduced from anomalous scattering of the Cr atom (R = 0.095, R, = 0.092 for the enantiomorphic
structure in the space group P 3,21). Further details of the crystal structure
investigation may be obtained from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlich-techniscbe Information mbH. DW-7514 Eggenstein-Leopoldshafen2 (FRG) on quoting the depository
number CSD-55874, the names of the authors, and the journal citation.
[12] Preparation of 10 (m.p. 101 "C; [a:' +160, c = 0.51 in CHCI,): H . G .
Schmalz, B. Milks, unpublished.
+
Angew. Chem. int. Ed. Engl. 3f (1992) No. 5
0 VCH
In hydrocarbon solution alkyllithium compounds usually
exist as tetramers or hexamers.['. Addition of suitable
Lewis bases B (ethers, tertiary amines) converts the hexamers
into Lewis-base complexes with a tetrameric alkyllithium
unit [R4Li4]. n B (n = 1-4)[']. Further dissociation into
dimer complexes [R,Li,] . 4 B is observed only at very low
temperatures in THF, and even trace amounts of hydrocarbons suffice to prevent it.[31By contrast, the presence of
polydentate Lewis bases such as N,N,N',N'-tetramethylethylenediamine (TMEDA), which corresponds to two
bases B,[31or N ,N,N',N",N"-pentamethyldiethylenetriamine
(PMDTA), which corresponds to three bases B,l4] is advantageous. If the organic groups contained in the aggregates
are bulky (R = tert-butyl, neopentyl), the aforementioned
favorable conditions even allow the formation of complexed
monomers [RLi] . 3 B, and dimers [R,Li,] . 4 B are formed
in ether.14].
Due to the chelate effect, intramolecular Lewis-base
groups lead to particularly stable complexes. Hence, we have
synthesized secondary alkyllithium compounds containing
two Lewis-base units. In hydrocarbon solution at room temperature, they exist exclusively as dimers.['] We have now
observed that N-lithiomethyl-N,N',N",N"-tetramethyldiethylenetriamine (l),recently prepared by usL6]and containing three Lewis-base groups, is monomeric in hydrocarbon solutions at about 5 "C. As far as we know, this is unprecedented for an alkyllithium compound. Cryoscopy of
solutions of crystals of 1['] in benzene yielded a value of 1.06
for the degree of aggregation. At 195 K, 6Li NMR spectra of
solutions of crystalline 1 (6Li-labeled) in pentane exhibit signals at 6 = 1.75 (species A) and 1.35 (species B)[**I in an
intensity ratio of 1.27: I which coalesce, regardless of the
concentration of 1, at 227.7 K (36.80 MHz) and 232.2 K
(58.88 MHz), respectively.[*I 1 :1 :I-Triplets at 6 = 51 .O and
52.9 ['J('3C,6Li) = 13.9 Hz] could be recognized in the
100.61 MHz I3C NMR spectrum at 225 K, proving that A
and B are monomers whose CI carbon atoms bind to a single
lithium atom.r9] At low temperatures, other signals in the
C,H-decoupled 13C NMR spectra['" and in the 'H NMR
spectra" 'I of pentane solutions of 1, also indicate the presence of two species in the above ratio. At higher temperatures the signal patterns attributable to (stereo)isomers and
diastereotopicity coalesced, and finally the spectra corre[*] Prof. Dr. F. W. Klumpp, Drs.-Ing. H. Luitjes, Dr. M. Schakel, Dr. F. J. J.
de Kanter, R. F. Schmitz
Scheikundig Laboratorium, Vrije Universiteit
De Boelelaan 1083, NL-1081 HV Amsterdam (The Netherlands)
Dr. N. J. R. van Eikema Hommes
Institut fur Organische Chemie der Universitit Erlangen-Nurnberg
[**I In the following text the experimentally observed species A and B are
distinguished from the calculated structures 1 a and 1b.
Verlagsgesellschaft mbH. W-6940 Weinheim, 1992
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sponding to the averaged structure 1 were obtained. At
275 K 13C-6Licoupling had vanished. A/B interconversion
as well as exchange processes, known for ordinaryrza1and for
intramolecularly coordinated[51 alkyllithium compounds,
are possible causes of the temperature-dependence. The existence of 1 as a monomer is presumably due to steric hindrance of aggregation and to intramolecular Li-N coordination.
MNDO calculations1' I' yielded structures 1 a(A) and
1 b(B) as the most stable states of 1. Remarkably, in 1 a, b N2
and N3 coordinate to lithium as expected (dNZIN31-Li
= 2.22
[2.22] A), while N1 interacts only weakly, at most (dN1- L i =
2.67 A). Despite the tricoordinate nature of lithium implied
by this result, the enthalpies of formation obtained for 1 a, b
are lower than those calculated for potential dimers of 1
containing coordinatively saturated tetracoordinated lithiUnfavorable steric effects in the dimers seem to be
1
la
Ere, = 0 kJ mol
lb
'
E,,,
=
4 kJ mol.'
responsible for their higher enthalpies of formation. The
enthalpy of activation calculated for the transformation
la(A) 4 1 b(B), essentially corresponding to an inversion of
N1, amounts to 40 kJrnol-'.[''] This is in satisfactory agreement with the experimental value, AHA++B
= AH:-, =
43 _+ 3 kJmol-'
= -28 + I 3 Jmol-'K-', AS:-,
=
- 26 f13 J mol - 'K- '), derived from the coalescence temperatures of the 6Li resonance signals and of two pairs of
13C signals of A/B (100.61 MHz; NCH,, 6 = 56.60
56.32, T, = 235.5 K ; (NCH,)', 6 = 63.24 + 61.36, T, =
255.2 K).r'41
Two coalescence phenomena were observed for the signals
of the CH,Li protons: Due to fast A/B interconversion the
original four resonance signals of the diastereotopic protons
Ha and H, in A ( l a) and B(l b), respectively, fuse at about
258 K to form two very broad
(Haand H,, respectively, in the averaged structure), which for their part
coalesce into a broad singlet[161at approximately 275 K
[AG* (275 K) ca. 53 kJmol-'I. This indicates fast exchange
and, as a consequence, equivalence of the two diastereotopic
protons of the averaged structure. Equivalence of Ha and H,
in A(l a) and B(1 b) is not induced by 1 a/l b interconversion,
and, according to theoretical studies, inversion of C-, another possible cause, should only occur in aggregates with comparable ea~e.1"~Do 1 a, b(A, B), monomeric derivatives of
a-aminomethyllithium containing two additional Lewisbase centers, invert easier and in a different way than monomeric methyllithium? The loss of 13C-6Licoupling at higher
temperatures suggests Li-C bond breaking in the monomer
(allowing for inversion of C - ) or intermolecular Li-C exchange involving dimers of higher energy. Inversion of N2
(MNDO enthalpy of activation 51 kJmol-')r121 could also
lead to HJH, exchange. Clarification of the actual mode
requires additional studies.
Stable intramolecular organolithium complexes like 1 a, b
and other members of this class of compounds prepared by
+
634
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W-6940 Wemheim, 1992
usL5]are models for labile intermolecular Lewis-base complexes [RLi] ' 3 B, [R,Li,] . 4 B, and [R,Li,] ' 4 B, and to
some degree their properties (structure, dynamics, reactivity)
should be exemplary for those of these solvent- and temperature-dependent components of organolithium reagents,
which are of such importance in synthesis.[5,l 9 I
Received: September 23,1991 [Z 4924 IE]
German version: Angew. Chem. 1992,104,624
CAS Registry number: 1, 129537-50-4.
(11 H. L. Lewis, T. L. Brown, 1 Am. Chem. Soc. 1970,92,4664-4670.
[2] a) Recently, octamers and nonamers were also proven to be present in
cyclopentane solutions of primary alkyllithium compounds (G. Fraenkel,
M. Henrichs, J. M. Hewitt, B. M. Su, M. J. Geckle, 1 Am. Chem. SOC.1980,
102,3345-3350; R. D. Thomas, R. M. Jensen, T. C. Young, Organometal/ics 1987, 4, 565-571); b) Menthyllithium is dimeric in benzene (W. H.
Glaze, C. H. Freeman, 1 Am. Chem. Soc. 1969, 91, 7198-7199.
[3] D. Seebach, R. Hassig, J. Gabriel, Helv. Chim. Acra. 1983.66, 308-337; W.
Bauer, D. Seebach, ibid. 1984,67,1972-1988.
[4] W. Bauer, W. R. Winchester, P. von Rague Schleyer, OrganometaNics1987,
4, 2371 -2379; G. Fraenkel, A. Chow, W. R. Winchester, J. Am. Chem.
Soc. 1987, 112, 6190-6198.
[5] W. Moene, M. Vos, F. J. J. de Kanter, G. W. Klumpp, A. L. Spek, J. Am.
Chem. Sue. 1989, 111, 3463-3465; F. J. J. de Kanter, G. W. Klumpp, K.
Kolthof, W Moene, M. Vos, A. L. Spek, unpublished results; M. Vos,
F. J. J. de Kanter, M. Schakel, N. J. R. van Eikema Hommes, G. W.
Klumpp, 1 Am. Chem. Soc. 1987, 109,2187-2188.
[6] M. Schakel, M. P. Aarnts, G. W Klumpp, Reel. Trav. Chim. Pays-Bas
1990, 109, 305-306.
[7] Crystals of 1 are stable indefinitely at - 80 "C. In pentane 1 (ca. 0.2 M)
decomposes at room temperature with a half-life of about 18 h (2
1+ Me,NLi + C,H, + Li(NMe)C,H,NMe, + PMDTA). During the
preparation of the cryoscopy experiment, solutions of 1 in benzene
(0.083 M) had to be kept a t room temperature for about 20 min. Solutions
for NMR studies, which were subjected to the same conditions, contained
no detectable decomposition products. We are still trying to obtain an
X-ray structure analysis of 1.
[8] Two solutions, differing in concentration by a factor of 10, were examined.
Chemical shifts are based o n 1 M LiBr in THF as external standard [S (50 %
LiBr in H,O): - 1.04,6 (24ithiobutane in C,D,): 0.771 and are not corrected for volume magnetic susceptibility.
[9] H. Gunther, D. Moskau, P. Bast, D. Schmalz, Angew. Chem. 1987, 99,
1242-1250.
(101 13C NMR (62.89 MHz, pentane, 195 K [265 K], TMS): 6 = 43.58
(N(3)Me2, A or B), 43.80 (N(3)Me,, B or A), 48.73 (N(3)Me,, B), 49.57
(N(3)Me,, A) [45.96], 44.79 (N(l)Me, A), 45.42 (N(l)Me, B) [45.10], 52.48
(N(2)Me, B), 54.41 (N(2)Me, A) [53.18], 51.34 (NCH,, B), 52.26 (NCH,,
A), 56.32 (NCH,, B), 56.60 (NCH,, A), 57.62 (NCH,, A + B), 61.36
(NCH,, B) 63.24 (NCH,, A) [52.78, 56.83, 57.88, 62.181; (100.61 MHz,
225 K) d = 51.0 (between 225 and 235 K also visible in 62.89 MHz spectrum) +52.9 (CH,Li, 'J(I3C, 6Li) =13.9 Hi!) [51.74].
[I 11 'H NMR (250 MHz, [D,,]pentane, 195 K [300 K], TMS): 6 = 2.40 + 2.10
[2.28] (s + s, ca. 3 H + ca. 3 H ; N(3)Me,, A B), 2.33 (s, ca. 1.5H;
N(2)Me, A), 2.35 (s, ca. 1.5H; N(2)Me, B) [2.37], 2.31 [2.35] (s, 3 H ;
N(l)Me, A + B), ca. 2 (overlapping signals; NCH,), 1.63 + 1.19 (d + d,
,J(H, H) =10.4 Hz, 1H; CH,Li, B + A, coalescence at ca. 258 K),
0.67 + 0.30 (d + d, 'J(H, H) = 10.4 Hz, 1 H; CH,Li, A + B, coalescence
at ca. 258 K) [1.03].
(121 In order to correct for known deficienciesof MNDO calculations (overestimation of C-Li bond strength and nonbonding repulsions) 60 kJmo1-I
was added to thecalculated energy of (1)2,[131
and 21 kJmol-' was added
to all calculated activation enthalpies (N. J. R. van Eikema Hommes, P.
von Ragu6 Schleyer, unpublished results).
[I31 E. Kaufmann, K. Raghavachari, A. E. Reed, P. von Rague Schleyer,
Orgunomelullics 1988, 7, 1597- 1607.
[I41 Method: H. Shanan-Atidi, K. H. Bar-Eli, J. Phys. Chem. 1970, 74, 961963. Although upon increase of temperature all other A/B signal pairs
recognizable at low temperature also coalesced, partial overlap with other
temperature-dependent signals restricted exact determination of coalescence temperatures to the four cases given. Rate constants evaluated from
the estimated coalescence temperatures [241 K (100.61 MHz) and 255 K
(62.89 MHz)] of the N(I)"CH3 and N(2)"CHCH,A/B signal pairs correspond to those of the A/B exchange as calculated for the respective temperatures from the activation parameters. The linewidths of the pairs of
"C signals [N(3)Me], + [N(3)Me], and [N(3)Me'], + p(3)Me'], are considerably larger than those of the other signals, and coalescence took place
between the measurements at 215 and 225 K (62.89 MHz) without prior
0570-0833/92/050S-O634$3.50+ .25/0
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Angew. Chem. In!. Ed. Engl. 31 (1992) No. 5
loss of identity of the A and B component in the (better resolved) low-field
pair. This suggests that the sequence, Li-N(3) decoordination, inversion of
N(3), and Li-N(3) recoordination, causes fast Me/Me' exchange, while
A/B exchange is still slow on the I3C NMR time scale.
~ 70 Hz. The low-field resonances were obscured by the signals of
[IS] A V , ,ca.
the residual protons of the solvent [D,,]pentane.
[I61 AG* =19.13 cj9.97 + log(cAv-')], Av ca. 230 Hz.
[I71 The value of AG* (281 K) for stereomutation of C- in aggregated alkyllithium compounds is ca. 60 kJrnol-'[l8'. Calculations for methyllithium
indicated the barrier for inversion of C- in the monomer to be ca.
55 kJmol-' higher than in the aggregate^"^'. On this basis the value of
AG* (275 K ) for inversion of C - in la, b is expected to be ca.
110 kJmo1-I.
[I81 G . Fraenkel, W E. Beckenbaugh, P. P. Yang, J. Am. Chem. SOC.1976,98,
6878- 6885.
[19] B. 0. T. Kammermeier, G. W. Klumpp, K. Kolthof, M. Vos, Tetrahedron
Letters 1991, 32, 3111-3114.
Addition of Benzyl Radicals to Alkenes: The Role
of Radical Deformation in the Transition State**
By Karoly HPberger, Manfred Walbiner,
and Hams Fixher*
Rate constants for the addition of alkyl radicals to alkenes
are influenced strongly by polar and steric effects from substituents of radicals and alkenes. Since the reactions are
exothermic (AH = - 50 to - 150 kJ mol-'), the overall reaction enthalphies and radical stabilizations play a minor role,
and the transition states are early. This also allows the rationalization of polar effects in terms of orbital interactions of
the unperturbed reactants.['] Calculated transition-state
structures vary little with changes in either radical or substrate. As indicated by Figure 1, the forming bond is rather
long (215-240 pm), yet the angle of attack is tightly prescribed (107-1 loo). The alkene center attacked is markedly
though not strongly deformed; the other center is hardly
affected. Most calculations yield pyramidalized radical centers.[']
50 kJmol-',[41 and little is known about addition rates of
such radicals. The addition of benzyl is estimated to be
exothermic, but less so than that of other alkyl radicals.[51A
comparison with rate constants for the addition of other
radicals, for instance tert-b~tyl,[~~z
b1 should reveal the influence of the reaction enthalpy. Furthermore, the SOMO
(SOMO = Singly Occupied Molecular Orbital) energy of
benzyl (IP =7.2 eV) is close to that of tert-butyl (IP = 6.7
eV).r6l The latter has clearly nucleophilic behavior,[3a.b1
hence benzyl should also be a nucleophilic radical. On parasubstitution of benzyl, its SOMO energy varies from about
- 7.9 to - 6.3 eV,16]whereas the stabilization energy is nearly unaffected.r4b1Previously, for tert-butyl, a strong dependence of the rate constants on the LUMO energies of the
alkenes (- 2.48 < EA 2 -0.21 eV) had been ob~erved.[~".~l
It might be expected that a similar variation of the benzyl
SOMO energy by substitution would cause a similarly strong
polar effect."]
Figure 2 shows rate constants for the addition of benzyl
and, for comparison, tert-butylr'] as a function of the alkene
LUMO energy. The linear correlation[3]with the alkene electron affinities for benzyl is significant (lg k,,, [M-'s-'] =
3.36 + 1.14 EA, 24 measurements, correlation coefficient
r = 0.87) but weaker than that for tert-butyl (lg k,,,
[M-'s-'] = 5.95 + 1.61 EA, 21 measurements, r = 0.89).
As expected, benzyl is less nucleophilic. The large scatter of
the data is not due to experimental error.
1
5
COOMe
1
.Ae.Ph
COOMe
.
CH&N
2
1
€A lev1
-
D
Fig. 2. Rate constants for the addition of benzyl ( 0 0 ) and terr-butyl ( 7 ) to
alkenes H2C = CY1Y2( 0 : EA estimated) in toluene at (23 iz 1) "C. The labels
next to the symbols for the benzyl and tert-butyl radicals indicate the substituents Y' and Y2 of the alkene. If only one suhtituent is specified, then
Y' = H (see also Table 1). Im = imidazolyl, 2-Py = 2-pyridyl, 4-Py = 4pyridyl.
-rc"
__---
Fig. 1. Transition-state structure for the addition of an alkyl radical to an
alkene. Calculated angles for CH, + C,H,: pI = 160", 'pz = loo", versus
cpl = 180". 'p2 = 90" in the initial state and pI = 12Y, p2= 110" in the product
state [2d].
In continuation of our previous work on the kinetics of
radical additions,13]we have now determined rate constants
for the reaction of benzyl radicals and some para-substituted
derivatives with a series of monosubstituted and 1,l-disubstituted alkenes. Benzyl is resonance-stabilized by about
[*I Prof. Dr. H. Fischer, Dr. K. Hebergerl'], DipLChem. M. Walhiner
Physikalisch-Chemisches Institut der Universitat
Winterthurerstrasse 190, CH-8057 Zurich (Switzerland)
['I Permanent address: Institute for Chemistry
Hungarian Academy of Sciences, Budapest
Postfach 17, H-1524 (Hungary)
[**I This work was supported by the Schweizerischen Nationalfonds zur
Forderung der wissenschaftlichen Forschung.
Angew. Chem. In!. Ed. Engl. 31 (1992) No. 5
0 VCH
In general, benzyl radicals react much more slowly than
tert-butyl with all alkenes, and it is tempting to attribute this
to the smaller exothermicity of the addition. However, this is
contradicted by the findings that the increased exothermicity
in the formation of benzyl-type radicals from styrenes does
not increase the rates correspondingly. Moreover, the measured difference in activation energies for the additions of
benzyl and lert-butyl is on the average 20 kJ mol- ; this is
too large to be accommodated by the enthalpy difference of
only 38 kJmol-'. Table 1 presents rate constants for addition of the different para-substituted benzyl radicals to selected alkenes. To a first approximation all the radicals show
the same weakly nucleophilic behavior. There are differences
in detail, but there is no obvious correlation with the radical
SOMO energiesr6] nor with o-scales for benzyl substituents.l8I Thus, polar orbital interactions appear to be asym-
firlagsgesellschaji mbH, W-6940 Weinheim, 1992
0570-0833/92l0505-0635$3.50+.25/0
635
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lithiomethyl, compounds, monomeric, tetramethyldiethylene, first, hydrocarbonic, triamino, alkyllithium
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