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An Extremely Long-Lived Triplet Carbene; Reactivity Optical Absorption Spectrum and Kinetics of Highly Congested Diarylcarbenes.

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M. M. Bradford, A n d . Biuchrm.1974,13,3221. Theenzyme used wasextracted
from porcine kidney (type Ill-CP, Sigma).
This comparison is not ideal because the layers that we have built by sequential
dddition d o not exhibit as high a level of helicity as those prepared by polymerization. We rationalize this difference as resulting from low levels of partial
racemization during the multistep operations, where a single amino acid of the
opposite hand inhibits formation of the helical secondary structure for several
residues to both sides of the defect.
A. W. Adamsoii, Ph,v.sica/ ChernrArrj- of Surfuces, 5th ed. Wiley Intersuence,
New York. 1990. pp. 332-36X. R. H. Dettre, R. E. Johnson, Jr.. J. PIijs. Chrm.
1965.69. 1507.
S. K . Burle), P. R. David. A. Taylor. W. N . Lipscomb. Pioc. Nurl. Acad. Sci.
1.S4 1990. h7. 6878 This crystal structure wai obtained on peptiddse obtained
from bovine leiise-that used in our experiments was obtained from porcine
kidney. It has been suggested that these enzymes are very similar, if not identiCL~I
See. F Jurnak. A. Rich. L. van Loon-Klaassen, H. Bloemendal, A. Taylor,
F H . Carpenter J Mo/. Bid. 1977, If?, 149 and ref. [14].
W. N. Lipscomb. H. Kim, Biuchrmtsfrj 1993. 32. 8465; W N . Lipscomb.
personal communication.
An Extremely Long-Lived Triplet Carbene;
Reactivity, Optical Absorption Spectrum, and
Kinetics of Highly Congested Diarylcarbenes **
Hideo Tomioka,* Hidetsumu Okada,
Tetsuya Watanabe, and Katsuyuki Hirai
Table 1. Conditions and product distribution for the decomposition of diazo compounds 1 [a].
In contrast to the long history of persistent radicals,[’] which
started with the work of Moses Gomberg in 1890, that of persistent carbenes is much shorter. Only a few years ago “bottleable” carbenes were first isolated.12s31 These carbenes are stabilized not only by bulky protecting groups but also by
heteroatom substituents directly connected to the carbenic carbon atom. Owing to these substituents, these carbenes have
singlet ground states. But their electronic configurations are still
a topic of debate.‘41The next challenge is the stabilization of a
carbene hopefully having a triplet ground state by all-carbon
substituents. The most persistent triplet carbene known so far is
k
R
1
L
R
k
R
R
3
\4
a : R = H. R’= Me
[“I
b:R=Me.R’=H
c:R=R’=Me
Prof. Dr. H. Tomioka, H. Okada. T. Wdtanabe. K. Hirdi
Chemistry Department for Materials, Faculty of Engineering
Mic Univcrsity
Tsu. Mie 514 (Japan)
Telefax: Int. code (592)31-9471
This work w’as supported by the Japanese Ministry of Education, Science. and
Culture.
+
[**I
Angew. Chmi. I n ! . Ed. Engl. 1994. 33. Xu. H
?i:
dimesitylcarbene (2 a), which was first generated by Zimmerman and P a s k o ~ i c h ~in~1964.
l
The congestion at the carbene
center in 2a is reflected in its unique behavior. for example its
proclivity to dimerize and reluctance to attack secondary carbon-hydrogen bonds. Carbene 2a was not stable enough to be
isolated, and systematic kinetic studies were also not conducted
for further characterization. If four additional methyl groups
are introduced at the mrta positions, the carbenic center would
be more crowded, because of the “buttressing effect” on the four
ortho methyl groups.[61In view of the scarcity of the data in this
field despite recent growing interest in triplet carbenes as potential organic ferr~magnets,~’]
we became interested in designing
and generating persistent triplet carbenes. Thus, we have generated a series of polymethylated diphenylcarbenes 2a-c and investigated their reactivities by flash photolysis techniques. We
have found that they are exceptionally long-lived for diarylcarbenes with lifetimes on the order of seconds.
The new precursor diazomethanes 1 b, c were prepared by
essentially the same procedure”. ‘1 employed for dimesityldiazomethane. The polymethylated diphenylcarbenes were then generated either by thermolysis or by photolysis of the diazomethanes, and the products were analyzed by conventional
methods (Table 1). The reactions observed with didurylcarbene
Yield [ ‘ X ] [h]
Diazo cmpd
Conditions
3
4
la
lb
lc
la
Ib
lc
C,H6/110 C
C,H,/140 C
C,H,/140 C
hv/C,H,jlO ’C
hv!C,H,/10 C
A ~ , ~ C , H , ,‘c
.IO
14
0
0
61
29
5
63
96
90
0
41
79
[a] Irradiations were carried out on a 20 mM solution of I in degassed benzene with
a 300 W Hg lamp and a Pyrex filter. Thermolyses were performed in degassed
henzene and in a sealed Pyrex tube. [b] Determined by gas chromatography and
N M R spectroscopy.
(2 b) and decamethyldiphenylcarbene (2 c) were analogous to
those observed with dimesitylcarbene (2 a). These carbenes did
not react with the diazomethane precursors to give ketazines.
Instead they reacted either by dimerization to form tetraarylethylene 3 or by attack at an ortho methyl group to give
1,2-dihydrocyclobutabenzene (“benzocyclobutene”) 4. A key
difference is found, however, in the formation of 4. In the case
of 221, 4a was produced only by thermolysis at 140“C, and 2a
underwent mainly dimerization when photolyzed at room temperature. On the other hand, in the case of 2b, benzocyclobutene 4 b was produced not only by thermolysis but also by
photolysis along with the dimer. Compound 2c produced 4 c as
major product at the expense of 3a even in the photolytic run.
Irradiation (2 > 300 nm) of 1 a in a 2-methyltetrahydrofuran
(MTHF) glass at 4 K gave a paramagnetic species readily characterized from its EPR spectrum as triplet didurylcarbene 2 a.
The EPR signals from 2 were stable for at least several hours at
this temperature and survived even at 110 K . These observations established the triplet as the ground state of this carbene.[’]
Optical spectroscopy in the frozen medium gave analogous
but more intriguing results. Figure 1 b shows the absorption
spectrum obtained after irradiation of 1 b in a MTHF glass at
77 K. The spectrum displays two distinct features: two sharp,
intense UV bands with maxima at 321 and 335 nm. and in the
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visible portion of the spectrum two weak, broad, overlapping
bands with apparent maxima at 466 and 495 nm. The glassy
solution did not exhibit any changes in its absorption spectrum
for several hours when kept at 7 7 K . However, when it was
allowed to warm to room temperature and then cooled to 77 K,
the characteristic absorption bands disappeared (vide infra) ,
The absorption spectra of several aryl carbenes in frozen media
have been reported.[' O1 Typically, they display an intense UV
band and a weak visible transition. These features are present in
the spectrum obtained in the photolysis of 1b. Moreover, the
product analysis of the reaction solution showed the presence of
dimer 3b as well as benzocyclobutene 4b. Thus, the absorption
spectrum can be attributed to didurylcarbene (2 b) generated by
the photodissociation of 1 b.
gas matrices at low temperature both upon photolysis and upon
thawing the matrix.["] Moreover, several o-xylylenes have been
reported to exhibit a broad absorption band centered at around
350 to 460 nm depending on the substituents.["1
Flash photolysis of 1 b (1.0 x
M) in a degassed benzene
solution at room temperature with a xenon flash lamp (pulse
width of 10 ps) produced a transient species absorbing at 310350 nm; the apparent maximum at around 335 nm was coincident with the xenon pulse (Fig. 2). The decay kinetics of the
0 03s
370nrn
t
I
0 D 000s
11
'1
-
I
Rw
321nm
T
Ill I
300
350
A
450
400
Ainm
500
__t
Fig. 2. Absorption spectra of the intermediates formed by irradiation of I b in
degassed benzene. recorded 50 gs after the photolysis pulse. The inset in the upper
right shows the time course of the absorptions at 335 and 370 nm (oscillogram
traces). 0 . D = optical density.
n
466nm 495,111
300
500
400
600
Ainm
Fig. 1. a) UVjVIS spectrum of I a in MTHF at 77 K. b) Same sample after 6 min
irradiation (i> 300 nm). c) Same sample after the matrix was warmed to 110 K.
The spectral changes of the matrix were carefully monitored
as a function of temperature as the matrix thawed. At higher
temperatures a new broad absorption at 375 nm appeared and
increased as carbene absorption bands decreased (Fig. 1 c). The
species responsible for this new absorption showed appreciable
thermal stability; even at room temperature it persisted a few
seconds before decaying. These observations suggest that the
absorption at 375 nm must be due to an intermediate formed
from the initially generated triplet carbene that leads to the final
products.
A plausible candidate for the intermediate, which also explains the formation of benzocyclobutene, is o-xylylene Sb, formed
by intramolecular 1,4 H shift from
R'
an ovtho-methyl group to the carbenic center of 2b. Coincidently, the
R
R'R
formation of o-xylylene from o-tolylmethylene and its subsequent cycIization
was demonstrated by maR
trix isolation spectroscopy in inert
5
@
874
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transients indicates that the absorption at 335 nm decays within
1 s and a new species with an absorption maximum at around
370 nm arises, which is too long-lived to be monitored by our
system. Product analysis of the spent solution showed the presence of 3 b and 4 b. Thus, on the basis of the low-temperature
spectrum coupled with chemical analysis, we assign the initially
formed transient with a maximum a t 335 nm to carbene 2 b and
the second to o-quinodimethane 5 b. This interpretation is supported by the trapping experiments using oxygen. In the flash
photolysis of an oxygen-saturated benzene solution of 1 b a
broad absorption band with a maximum at 390 nm appeared at
the expense of the absorption due to the carbene. The reaction
mixture was found to contain octamethylbenzophenone as the
main product. It is well documented that diarylcarbenes with
triplet ground states are readily trapped by oxygen to generate
the corresponding diarylketone oxides." 31 Thus, our observation can be interpreted as indicating that the triplet carbene 2 b
is trapped by oxygen to form the carbonyl oxide, which confirms that the transient absorption quenched by oxygen is due to
2b. The rate constant for the quenching of 2 b by oxygen was
determined to be 7.1 x l o 7 M - ' s - ' . The inset in Figure 2 shows
the decay of 2 b and the formation of 5 b over time and indicates
that the decay of 2 b is kinetically correlated with the growth of
Sb. The decay rate constant was determined to be 2.1 & 0.1 s-',
while the growth rate constant was 1.8 k 0.2 s-'. Since the
product analysis shows that carbene 2 b decays not only by
intramolecular H abstraction but also by dimerization, the difference in the rate constants can be ascribed to the dimerizdtion
rate constant (k,) under the assumption that the rate constant
for the formation of 5b is identical with that for H abstraction
( k J . Since the difference in the rate constants, 0.3 s C 1 ,is equal
by
to k,[2b], k , is determined to be 6.4 k 0.7 *lo4 M - I s - '
0570-0(333,'94'OXOK-OX74$ iO.OO+ .25,'0
A n g m . Chrm. Int. Ed. Enngl. 1994, 33, No. 8
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using [2b] obtained from the extinction coefficient of 2 b and
absorbance after the pulse. A half-life ( t l , J of roughly 310 ms
was determined for 2 b from the decay curve, while the lifetime
based on k, was estimated to be 555 -t 62 ms. Similar measurements for the carbenes 2 a and 2c gave values of k,, k , , and t i j z
(z) for 2 a of 7.3 +1.5x l o 5 M - ~ s - ' , 3.1 0 . 4 s - ' , and 210
(323 42) ms, respectively, while 2 c decayed unimolecularly
with k i = 6.9 & 0.5 s - l (z = I 4 5 + 1 0 m s ) .
These results indicate that when four methyl groups are introduced at the meta positions of the ortho-methylated diphenylcarbene, the carbenic center is more effectively blocked by the
four ortho methyl groups presumably owing to the buttressing
effect exerted by the meta substituents. Didurylcarbene is found
to be some five orders of magnitude longer lived than the parent
diphenylcarbene (2 ps in cyclohexane) . [ l 4 I However, when two
more methyl groups are introduced at the para positions, the
carbenic center is forced to react with the orlho methyl groups,
which are brought much closer to the carbene, and the carbene
is shorter lived.
Support is lent to this explanation by semiempirical calculations. Optimized geometries calculated for a series of polymethylated phenylcarbenes with the programm PM3-ROHF/
CI (4 x 4)" 'I indicate that the distances between carbenic
carbon and o-methyl carbon atom for mesityl-. duryl-, and pentamethylphenylcarbenes are 2.8461, 2.7942, and 2.7778 pm, respectively.
The present study reveals that polymethylated diphenylcarbenes are exceptionally long-lived for diarylcarbenes, although
they are not persistent enough to be isolable. The buttressing
effects effectively strengthen the ortho effect that protects the
reactive center. This strategy may prove successful in the preparation of a "bottleable" triplet carbene.
Received: October 27,1993 [Z 6455 IE]
German version: Angew Chem. 1994, 106, 944
[ l ] Reviews: A. R. Forrester. J. M. Hay, R. H. Thomson, Organic Chcmistrj of
SruhlL, RudiruO. Academic Press, London, 1968: D. Griller, K. U. Ingold, Arc.
C h ~ t ~Rrs.
i . 1976. 9, 13: M. Ballester, hid. 1985, 18, 380.
[2] A. 1 ~ a uH.
. Grdtrmacher, A. Baceiredo, G. Bertrand, J. A m . Chem. Soc. 1988,
110. 6463; A. Igau, A. Baceiredo, G . Trinquier, G Bertrand, Angew. Chem.
1989. lU/, 617: Anpew. Chem. Int. Engl. 1989, 28, 621; G . R. Gilette, A. Baceiredo. G. Bertrand. ibid. 1990, 102, 1486 and 1990. 2Y. 1429; D. A. Dixon,
K . D . Dobbs. A . J. Arduengo, 111, G. Bertrand. J. Am. Chem. Soc. 1991, 1/3,
8782
[3] A. J. Arduengo. 111, R. L. Harlow. M. Kline, J. Am. Chern. Soc. 1991, 113,361;
A. J Arduengo. 111, M. Kline, J. C. Calabrese, F, Davidson, ibid. 1991, 113,
9704: A. J. Arduengo. 111, H. V. Rasika Dias, R. L. Harlow, M. Kline, ibid.
1992. 114. 5530
[4] M. Regitz, Angew. Chem. 1991, 103,691; Angew. Chem. In/. Ed. Engl. 1991,30,
674; R. Dagani. Chem. Eng. News 1991, 69. No. 4, p. 19.
[5] H. E Zimmerman, D. H. Paskovich, J Am. Chem. So<. 1964.86. 2149.
[6] a) See for instance, L. N. Ferguson, The Modern Srructural Theory of Organic
Chemi.vrr. Prentice-Hall, Englewood-Cliffs, NJ, USA, 1963, Chapters 3 and 5 ;
b) for the buttressing effect in carbene chemistry see: H. Tomioka, K. Kimoto.
H. Murata. Y. Izawa, J. Chem. Soc. Perkin Trans. 1 1991, 471, and references
therein.
[7] For a review see: H. Iwamura, Pure Appl. Cliem. 1986.58.187; Adv. P h u . Org.
C'hrm. 1990. 26. 179.
[XI Diazomethanes 2b, c were prepared by the treatment of diarylketimine with
N,O, folloued by the reduction of the corresponding N-nitrosoketimine with
LiAIH,. They could be purified by gel permeation chromatography. 2b: Orangecrystals,m.p. 168.0-169.O"C: 'HNMR(CC1,) 6 =1.98(s, 12H),2.10(s,
12H). 6.7X (br. s. 2 H ) ; I R (KBr): v = 2038 c m - ' ; 2c: orange crystals, m.p.
141.2-144.0"C: ' H N M R (CCI,): 6 = 2.10 (s, 12H). 2.22 (s, 12H), 2.26 (s,
6 H ) . IR (KBr): v = 2040cm-l.
[9] H. Tomioka, T. Takui. K. Itoh. unpublished.
[lo] See for example A. Trozzolo. Acc. Chem. Res. 1968, I, 329.
[ i l l R. J. McMahon, 0. L. Chapman, J. Am. Chem. Suc. 1987. 109, 683; 0. L.
Chapman, J. W Johnson, R. J. McMahon. P. R. West, ibid. 1988, I / 0 ,
501
[12] a ) K . K. de Fonseka, J. J. McCullough, A. J. Yarwood, J. Am. Chem. SOC.1979,
101. 3277, b) G Quinkert. Angew. Chem. 1975.87, 790; Angew. Chem. I n f . Ed.
. Ed. EngI. 1994. 33. No. 8
Angcw. ( ' h ~ mInt.
Engl. 1975, 14, 851; c) V. Wintgens. J. C. Netto-Ferreira. H. L. C a d J. C
Scaiano, J. Am. Chem. S O C . 1990. 112. 2363. and references therein: d) J. C.
Netto-Ferreira. V. Wintgens, J. C. Scaiano. Terrahedron L e f t . 1989.30. 6851 ; c)
H. G. Korth, K . U. Ingold, R. Sustman, H. de Groct. H. Sies. Angeit. Chem.
1992, 104, 915; .4npe4r. Chem. Inr. E d Engl. 1992, 31. 891
[13] For a review see' W. Sander, Angew. Clienr. 1990. 102, 362; A n g w . Chcn?.In1
Ed. Engl. 1990, 29, 344.
[14] L. M. Hadel, V. M. Maloney, M. S. Platz. W. G . McGimpsey. J. C. Smiano, J.
P h w Chcm. 1986, YO. 2488.
[15] J. J. P. Stewart, J. Compt. Chem. 1989, 10, 209, 221; MOPAC version 6 01
(JCPE#P044). was used.
Ether-Solvated Sodium Ions in Salts Containing
n-Hydrocarbon Anions: Crystallization, Structures, and Semiempirical Solvation Energies **
Hans Bock,* Christian Nather, Zdenek Havlas,
Andreas John, and Claudia Arad
Dedicated to Professor Heinz Noth
on the occasion of'his 65th birthday
In numerous reactions in solution, the solvation of cations[']
can exert influence on the multidimensional network of interrelated equilibria of electron transfer, ion pair formation, o r aggregation"'] and, therefore, control processes from a geochemical["] to a biochemical['g1nature. The selection of the Na@ion
is advantageous for the investigation of the complex interdependencies, because, due to its small ionic radius and high solvation
enthalpy"] together with the relatively low reduction potential
of sodium metal,[*]this cation forms countless
and compounds"', 3bJ of impressive structural diversity. Using Na@ as
example, we report on a method to specifically prepare optimally ether-solvated metal cations by one-electron reduction of unsaturated hydrocarbons with alkali metals in ether solution and
to crystallize the solvent-separated n-hydrocarbanion salts
(Scheme 1).
Under aprotic reduction conditions the n-hydrocarbons
(Scheme 1) selectively form their radical anions, which, due to
the extensive delocalization of the negative charge in their rather
large n-systems, no longer possess negatively charged centers[leXg1suited to form contact ion pairs A, and therefore crystallize as solvent-separated ion pairs BL4].One of the many possible variations is demonstrated by fluorene (Scheme I ) , the
sodium reduction of which leads to H, formation.[41
[Meta{RO(CH2CH,0),-,
[Meta{RO(CH,CH,O),-
),, C, Hke] A
l},,][C,H~~]B
[*I Prof. Dr. H . Bock. DipLChem. C. Nither, Dipl.-Chem. A. John,
Dipl.-Chem. C. Arad
Chemisches Institut der Universitit
Marie-Curie-Strasse 11. D-60439 Frankfurt am Main ( F R G )
Telefax: Int. code (069)5800-9188
+
[**I
Dr. Z. Havlas
Institute of Organic Chemistry and Biochemistry of the Czech Academy of
Sciences, Prague (Czech Republic).
Structures of Charge-Perturbed and Sterically Overcrowded Molecules as well
as Interactions in Crystals, Part 31. The investigations have been supported by
the A. Messer Foundation, the Bundesland Hessen, the Deutsche Forschungsgemeinschaft. and the Fonds der Chemischen Industrie. - Part 30: H. Bock. I .
Gobel, C . Nither, Z. Havlas, A. Gavezzotti, G . Filippini. AnKen. Chrm. 1993,
105. 1823: Angew. Chem. Int. Ed. Engl. 1993. 32. 1755.
<C VCH V~rlugsgeselhchafimbH. 0-69451 M'rvnheim, fYY4
0570-0833;Y4:080X-ON75 B 10.00
+ ZS$l
875
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