Cyclopropanation of C60 via a fischer carbene complex

Tetrahedron Letters, Vol. 35. No. 51. pp. 9529-9532. 1994 Elsevia Science Ltd printed in GreatBritain (lo4&4039/94 $7.ooto.00

Pergamon 0040-4039(94)02208-9

Cyclopropanation of Cfjo Via a Fischer Carbene Complex Craig A. Merlic* and Holly D. Bendorf Department of Chemistry and Biochemistry University of California, Los Angeles, California 90024-1569

Absbvact: A new method for the preparation of methanqfUlerenes is demonstrated by the formation of I ,2-methyl(methoxymethano)fullerene-i&3

by the thermal reaction of C60 with [methyl(methoxy-

met~le~)]~~~~e)lpentacarbonylchromium.

In the quest to prepare and study derivatives of C&n, the methanofullerenes have become the most studied class of &-derived

compounds. t The methanofullerenes are quite stable and have shown promise in

applications in fields as diverse as material science and medicine. Wudl reported the synthesis of precursors to ‘pearl-necklace” polymers2 and Rubin recently synthesized ethynyl-substituted methanofullerenes for use in the preparation of macmc yclic and polymeric carbon allotropes3 Interest in the potential medical applications of fullemnes has spurred research into the preparation of water soluble methanofullerene derivatives. A water soluble methanofullerene prepared by Wudl and coworkers has been shown to inhibit HIV-l protease.

There

are several methods for the preparation of methanofullerenes. The most common is via a [3+2] cycloaddition of diazo compounds with f&u to yield intermediate pyrazolines which thermally extrude nitrogen to produce the methanofullerenes5 and has been used to synthesize a series of aryl- and ester-substituted derivatives.2>6 Strategies

involving

generation

of free carbenes

have also been employed

in the synthesis

of

methanofullerenes. Thermal decomposition of diazirines in the presence of C!+johas been utilized to prepare a glucose-derived methanofullerene7 and a chloro-substituted methanoful1erene.a Dichlommethanofulleren~ and dimethoxymethanofuRerenelo have also been prepamd by the addition of free carbenes. A “carbene-free’ methodology for the preparation of methanofullerenes involves the addition of bmmomalonate anions to Cijo followed by intramolecular substitution of the bromide, but this method is limited in scope.11 We envisioned a new route to methanofullerenes

using the well-documented

chemistry of Fischer carbene complexes. 12 Cyclopropanation of electron-deficient

cyclopropanation

olefins by chromium

carbenes is sensitive to the steric environment about the alkene and is generally only effective for monosubstituted substrates. However, the steric effects of the tetrasubstituted double bonds in C&Jwere expected

9529

9530

to be countred by the pyramidalized hence x-acidity,

of i&~.t3

nature of the double bonds combined with the high electrophilicity,

We report herein successful cyc~~ropa~tion

complex which provides another route to methanofullerenes Much of the reported c~mium these cornFunds sot react with &

were tested for @

~clop~anatio~

cyclopropanation,

so

t4J5 While the phenyl methoxy carbene complex did the more reactive methyl methoxy carbine UIXI@SC by flash chromatography

gave the desired 1,2-dihydn>-1~-m~thyl(methoxymethano)-full~n~~ junction.

has used fphenyl-

Rewtion d two quivaients of methyl methoxy carlme complex with QQ in vigorously

reacted successfully.

refluxing benzene for 72 h with subsequent purification meth~ofulle~ne

chemistry

and [~yl(me~~~~yiene)]~nt~~~nylchromium,

under a variety of themal conditions,

and

carbene

which excludes free carbene intermediates.

cone-~diat~

~methoxymethylene)lpentaearbonyichmm~u~

of C&J by a chromium

(98:2 hexane/ethyl

acetate}

as a brown solid (equation).

product was obtained as a single isomer resulting f&m cyclopropanatibn

The

of a 6,6 ring

‘Ihe presence of 26 signals (3 of greater intensity) in the fullerene region of the 13C NMR confirmed

the Cs symmetry of the pmduct (see Figure). 16 The ~5o_sp3 carbon resonance is at 83 ppm, the mcthanobridge carbon is obsezwzd at 73 ppm and the methyl and methoxy casbons are observed at 44 and 55 ppm, respectiwely. Two singlets‘ in the IH NMR at 3.65 and 2.28 ppm correspond protons. ritzy

The methanofullcrene

product

cBn be consistently

to the methoxy and methyl

obtained in 20% yield, however,thk

yield is not

and yields atrove 50% were achieved on multiple occasions.

Benzene C66

+

rMlJX

M

8

Carbeae derivatives of Cm can exist as isumeric ~t~anofuller~ne fuller&i

(methanoannulene)

fuller&d Structure is predhed confirm

this prediction.t3

methanofullerene

5,6 ring junction

6,6 ring junction

adducts. 17 For the unsubstituted

fo be the more stable isomer by ,0.34 kCd&kd,6a

derivative,

structure is the more stable isomer.

C61H2, the

and equilibration

for substituted derivatives, PM3 ealcuiations

However,

adducts and

These findings are in excellent

studies

predict

that the

agreement

with the

observation that the substitbted fulleroid isomers can he converted to methanofukrene isomers under thermal conditi~s.~,6b~t7

The ~than~ful~~ne

shifts of the carbons methanofullerene

dimctiy

and the fulleroid isomers are easily distinguish

attached

to the methano-bridge.

6b The sp3 hybridized

by the 1% NhSR carbons of the

isomers are observed between 70 and 90 ppm while the sp2 carbons attached to the methane-

bridge in the fulleroid isomers ~TGobserved between 130 and 150 ppm The observation of a signal at 83 ppm in the 1% NMR assigned~oo the ~-sp3 as a methanofullerene.

carbons definitively

This result is in accord with the experimentally

derivatives to exist as the methanofullerene isomer is not obmecl,

identifies the merhyl(me~ox~thano)fullemne

isomer rather than the full&d

observed preference for substituted isomer.wb

Although the fuller&d

it may have been formed during the course of the reaction since the reaction conditions

a~ sufficient K, induce its isomerization to the more stable methanofullerene

isomer.

9531

Successful compared

cyclopropanati~n

to the conditions

unencumbered it becomes

high electron-af%nity

for the reaction

of chromium

3-6 h)t4 and with more substituted,

of the cyclopmpanation

alkenes

co&ination

of &

reflux

carbenes

for 3 to 4 days. ,When with activated,

sterically

activated alkenes (100-l lO.C, 10-+I h),lS nature of the double bonds as welt as the

is believed

ruaction

to bc analogous

to that proposed

for

loss of a CO with free carbenes .*2.19 Thermally-induced This step is sensitive to the steric chromium ccmpkx.

and does not involve

of Qo generates an 7$-&t

nature of the alhenc sWatrate, but Q~J is an cxcelknt

org~ometallic

benzene

of C& which accounts for the modest yield in the C&u cyc~

The mechanism

coordination

chemistry, 20 and this electronic

cyclopropanation reaction. Formal [2+2] cycloaddition elimination

a vigorous

is only partially countered by the pyramidal&d

double bonds of m

and tkctronic

required

that C!eo is a sluggjsh substrate. The low reactivity of the unactivated tetrasubstitttted

apparent

subsequent

necessary

alkencs (WC,

electron-deficient

of Qo

%-acceptor, as is well-documented’by

effect is potentkly

its

crucial to the success of the

produces a metaUacyclobutane

from which reductive

yields the methanofirlkrene.

In conclusion, as a potentially

the cyclopropa~tion

of C!eu by a Fischer chromium

general route to mcthanofullerenes.

Overall, the oxygen

ca&ne

complex has been realii

stabilized

carbcne complexes

are

hkely to be of specific interest due to their ease of preparation and the pmence of the ether functionality which provides

a handle for further functionalixation

of the methanofulkrenes.

such as the more reactive molybdcnntn and non-hetematom readily with C&2’

stabii

are expected tc react

Studies on thcsc fronts are in progress.

i40

Figure. Fulkrene

Other Fischer carbene complexes

carbene complexes

138

region of the 13C Nh4R of 1,2-met.byl(methoxymethano)fullemne_CM

136

9532

Acknowledgement: research. References ::

Z:

2: 7. 8. 9. 10. 11. 12.

13. 14_ 15. 16.

17. 18. 19. 20. 21.

We thank the National

Science

Foundation

(CHE-9257081)

for support

of this

and Notes:

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(Received in USA 6 October 1994; accepted 3 November 1994)