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Incarcerated Carbenes.

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Highlights
Carbene Chemistry
Incarcerated Carbenes
Wolfgang Kirmse*
Keywords:
carbenes · cyclodextrins · hemicarcerands · host–guest
systems · supramolecular chemistry
Carbenes (divalent derivatives of carbon) are, for the most part, fleeting
intermediates.[1] The search for persistent carbenes has only recently been
successful. Intrinsic stabilization by amino groups (Arduengo, Alder)[2] and
phosphanyl groups (Bertrand)[3] provided “bottleable” singlet carbenes such as
1 and 2 (Scheme 1). Steric protection,
Scheme 1. Persistent carbenes.
together with electron delocalization,
led to long-lived triplet carbenes (Tomioka).[4] The success of this approach is
illustrated by carbene 3, which currently
holds the record in lifetime (14 days at
25 8C).[5]
The isolation of carbenes in a rigid
matrix under cryogenic conditions is an
efficient method of extrinsic stabilization.[6] At 8–15 K the thermal energy is
[*] Prof. Dr. W. Kirmse
Fakultt fr Chemie
Ruhr-Universitt Bochum
Universittsstrasse 150
44780 Bochum (Germany)
Fax: (+ 49) 234-3214-353
E-mail:
wolfgang.kirmse@ruhr-uni-bochum.de
2476
insufficient to induce carbene rear- Brinker and his associates.[7] Cyclodexrangements (with the exception of H trins (CyDs, 4) are cyclic oligomers of ashifts which proceed by quantum me- d-glucose (Scheme 2). The dimensions
chanical tunneling). Diffusion of the of their toroidal-shaped cavities depend
carbenes to give dimers is suppressed on the ring size. Zeolites are microby the rigid medium, although intermo- porous aluminosilicate materials whose
lecular processes can be studied by free volume is influenced by the Si/Al
“doping” the matrix with a reactant. ratio and the counterion. Both types of
UV/Vis, ESR, and IR spectroscopy have hosts have been widely employed in
been employed to identify the embed- supramolecular chemistry. Aziadamanded carbenes. However, the most power- tane (6) and a-cyclodextrin were found
ful tool for structural elucidation, NMR to give the complex (6-CyD)2·6, for
spectroscopy, is not applicable.
which structure 7 was proposed on the
Encapsulation can serve to immobi- basis of induced circular dichroism
lize carbenes at ambient temperatures. (ICD) analyses.[7b]
To that end, the carbene precursor
The photochemistry of 7 is dominat(most often a diazirine) is introduced ed by innermolecular reactions of adainto a suitable host (see below), and the mantylidene.[7c,f] Insertion into the 2’host–guest complex is then irradiated. and 3’-OH groups of the host gave the 6The carbenes thus generated
are protected from dimerization, from reaction with diazirine (formation of azine),
and from reaction with bulkphase molecules that are too
large to enter the inner
phase. However, reactive
carbenes will attack XH
bonds, C=C bonds, and heteroatoms of the host. Host–
guest reactions are termed
innermolecular to distinguish
them from intermolecular reactions of the guest with outer-phase molecules and intramolecular reactions that
involve only the guest. Rearrangements of encapsulated carbenes do occur, but
they may be affected by restrictions in space. Processes
with small volumes of activation should be favored.
The enclosure of diazirines in cyclodextrins and/or
zeolites has been studied by Scheme 2. Chemistry of cyclodextrin-entrapped carbenes.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200500546
Angew. Chem. Int. Ed. 2005, 44, 2476 –2479
Angewandte
Chemie
CyD derivatives 11, which account for
ca. 50 % of the entrapped diazirine 6.
These products are analogues of the
ethers 8 that arise from adamantylidene
in alcohol solutions. The 2’- and 3’-OH
groups are located on or close to the
wider aperture of 6-CyD. Insertion into
the 6’-OH groups, located on the opposite face, was not observed. A second
major product was adamantane (10, ca.
30 %), formed by reduction of 6 and
concomitant oxidation of the host. The
intramolecular CH insertion leading to
9 was not significantly enhanced relative
to that observed in photolyses of 6 in
solution. While 7 did not produce the
azine 12, substantial amounts of 12 were
formed from 7-CyD·6 and 8-CyD·6.[7f]
These data indicate that 6 is effectively
isolated in the 1:2 complex with 6-CyD
but rather poorly in the 1:1 complexes
with 7-CyD and 8-CyD. Complexes of 3chloro-3-phenyldiazirine (5) with CyDs
gave similar results although the structure of the innermolecular products was
not fully elucidated.[7c]
The study of zeolite-entrapped carbenes is hampered by the inevitable
presence of water. Thus 6 produced
large amounts of adamantan-2-ol,[7a]
and benzaldehyde was the major product obtained from 5.[8] Recently, chloro(phenyl)carbene was directly observed,
by means of nanosecond diffuse reflectance spectroscopy, within the cavities
of Y zeolites.[8] The decay rates
(k 105 s1) were comparable to those
observed in isooctane or acetonitrile.
The formation of host–guest complexes of cyclodextrins and other “prefabricated” hosts is a reversible process.
To avoid dissociation, the complexes
must be processed as solids, with a
limited choice of photochemical and
spectroscopic methods. These drawbacks can be overcome by assembling
the host in the presence of the guest.
Hemicarcerands, a class of host molecules developed by Crams group in the
early 1990s,[9] are made by connecting
two bowl-shaped cavitands (resorcin[4]arenes, 13) with appropriate linkers
(Scheme 3). Short linkers provide rigid
carcerands, which incarcerate (small)
guest molecules permanently. With increasing length of the linker, the flexibility and permeability of the host is
enhanced. Hemicarcerands incarcerate
and liberate guests at elevated temperAngew. Chem. Int. Ed. 2005, 44, 2476 –2479
Scheme 3. Formation of hemicarcerands and hemicarceplexes. Ms = methanesulfonyl.
atures; the hemicarciplexes thus produced are stable at ambient temperature. However, this route to 15·guest is
not applicable to thermally labile diazirines. The decay of some diazirines
was even found to be accelerated by
incarceration.[10] Fortunately, 14 can be
isolated as an intermediate en route to
15. Introduction of the guest at this
stage, followed by closing of the “open
door”, provides 15·guest under mild
conditions. The solubility of the complexes can be adjusted by the choice of
the substituents R.
Hemicarcerands have served to stabilize a series of elusive intermediates.[11]
The success story started with Crams
report on incarcerated cyclobutadiene.[12] Somewhat later, Warmuths
group added o-benzyne,[13] cycloheptatetraene (18 a),[14] bridgehead alkenes
(19, 20),[15] and 1-azacyclohepta-1,2,4,6tetraene (21)[16] to the list of guests
(Scheme 4). Compounds 18–20 were
made by way of carbene rearrangements.
The case of phenylcarbene (17) is
particularly instructive.[14] When incarcerated phenyldiazirine (15·16) was
photolyzed at ambient temperature, 17
was found to insert into the CH bonds
and aryl units of 15. The undesired
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innermolecular reactions were suppressed by deuteration of the CH2
groups of 15, lower temperatures, and
triplet excitation transfer through the
walls of the hemicarcerand. The incarcerated cycloheptatetraenes (15·18) thus
obtained in up to 67 % yield are stable at
25 8C in the absence of oxygen but
regenerate 17 at 70–100 8C. NMR studies of 15·18 confirmed the twisted, nonplanar structure of 18. The enantiomerization barrier of 18 b was estimated as
82 1 kJ mol1.[14c] (To that end, a chiral
analogue of 15 was prepared from 14
and (S,S)-1,4-di-O-tosyl-2,3-O-isopropylene-l-threitol).
Most recently, the incarceration of a
singlet carbene has been accomplished.
Fluoro(phenoxy)carbene (23) is a fleeting though relatively selective species.
Aside from addition reactions with alkenes,[17] only dimerization has been
observed. Therefore, 23 is unlikely to
attack the wall of hemicarcerands. The
photolysis of 15·22 in degassed CH2Cl2
at 77 K afforded almost quantitatively a
new hemicarceplex, 15·23, which persisted for days at 25 8C.[18] The 13C
chemical shift of the divalent carbon of
15·23 is d = 285.7 ppm, intermediate to
those of diaminocarbenes (d = 235–
255 ppm) and amino(aryl)carbenes
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2477
Highlights
Incarceration holds much promise
for the investigation of other carbenes.
The method is particularly suited for
singlet carbenes that do not persist in
solution but are reluctant to undergo
intramolecular and/or innermolecular
reactions. Incarceration could also grant
longevity to triplet carbenes that decay
mainly by dimerization. Taking carbene
incarceration to its limits will be a
fascinating endeavor.
Scheme 4. Reactive intermediates generated
inside hemicarcerands.
(d = 300–310 ppm). The 13C and
19
F NMR data point to strong electron
donation from fluorine and oxygen to
the divalent carbon. NOE experiments
indicate that the incarcerated carbene
prefers the cis conformation 23 b. For
the free carbene, the trans conformation
23 a should be more favorable according
to DFT calculations. The only chemical
transformation of 15·23 observed so far
was a slow, acid-catalyzed reaction with
water [Eq. (1)]. (Both H2O and HF
molecules can enter the inner phase of
15.)
2 15 23 þ H2 O
! 15 PhOCHO þ 15 PhOCHF2
2478
ð1Þ
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