PUSH-PULL CARBENES: ALKOXYCYANOCARBENES

Article abstract of DOI:10.1016/0040-4039(91)85003-N

Benzyloxycyanocarbene is generated thermally from the corresponding diazirine and readily adds to electron-rich alkenes in accord with theoretical expectations.

Full text of DOI:10.1016/0040-4039(91)85003-N

Tetrahedron Letters, Voi32, No.17, pp 1925d928, 1991
Printed in Great Britain
0040 4039/91 $3.00+ .00
Pergamon Press plc
P U S H - P U L L CARBENES: ALKOXYCYANOCARBENES
R o b e r t A. M o s s , * Tadeusz Zdrojewski, 1 K a r s t e n Krogh-Jespersen,*
M a r e k Wlostowski, I and Alexander M a t r o
Department of Chemistry, Rutgers, The State University of New J e r s e y
New Brunswick, New Jersey 08903
Benzyloxycyanoaarbene is generated thermally from the corresponding diazirine and
Summary.
readily adds to electron-rich alkenes in accord with theoretical expectations.
Simple resonance theory presents intriguing possibilities for carbenes that carry both
strongly donating and strongly withdrawing substituents; i.e,, push-pull carhenes.
Extensive resonance interactions might be anticipated for alkoxyeyanoearbenes, i, in w h i c h
the carbenie carbon atom is sp hybridized.
However, our geometry optimized HF/6-31G*//6
strongly bent (0 C-C angle 107.9°),
31G* calculationsz reveal MeO-C-CN (la) to be
a
-
ground state singlet (2s), that is 28.7 kcal/mol (MP3/6 31G*//6-31G*) more stable than the
(relaxed) triplet carbene (2t).
At the HF/4-31C//3C level, orbital energies calculated for la are ahoy0 (a)
£Lu~ (P) +1.41 eV. These values are similar to those calculated for phenylfluorocarbene
(-10.23 and +1.51 eV), 3 latent ambiphile~ with pronounced electrophilic reactivity toward
-
-11.12 and
-
a
common alkenes.5, 6 Simple frontier molecular orbital energy arguments5 predict ambiphilia
reactivity for carbene la toward alkenes, whereas the paired, interacting orbital
calculations recently reported for HO-C CN7 indicate that electrophil~c reactivity should
be exhibited in additions to electron-rich alkenes, with
a selectivity "between" those of
CCI 2 and CFCI, whose electrophilic behavior is well documented. 5
Finally, the carbene selectivity index, ~cxy, calculated for la by our empirical
correlation equationS, 8 using the aR+ and aI constants for the MeO and CN substituents,
affords
m
=
I.ii,
a
v a l u e b e t w e e n those of CCI2 (i.00) and CFCI (1.22). 5
This result
agrees with Fujimoto's conclusions, 7 and a g a i n predicts m a r k e d electropbilic reactivity for
carbene la,
+
--
+
+
--
1
(a, R - Me; b, R = PhCH2)
H3C
1.314
C
1 346
H3C
1.427~17 .
1277
~
C
1472
1420 117'7°
132"9° ""
169 2°
7
"
!
~ 172.5 °
0
2s
1 . 1 c 3 % N
2t
1 1 8 6 ~ N
1925

1926
Here we r e p o r t that an alkoxycyanocarbene, as r e p r e s e n t e d by b e n z y l o x y e y a n o c a r b e n e , ib,
a p p e a r s to he g e n e r a t e d from an a p p r o p r i a t e d i a z i r i n e p r e c u r s o r , and to add r e a d i l y to
e l e c t r o n - r i c h a l k e n e s in a c c o r d with expectations. Thus, mmol of h e n z y l i s o u r e a tosylate,
3, was o x i d i z e d~ to 3-chloro-3-benzyloxydiazirine, 4, with h y p o c h l o r i t e , as p r e v i o u s l y
d e s c r i b e d I° (Scheme i). DMF s o l u t i o n (25 ml) of was c o m b i n e d with -50 ml of c o n d e n s e d
isobutene, and then s o l u t i o n of i0 nunol of n-Bu4N+CN- (96%, Fluka) in 20 ml of DMF was
a d d e d over 15 min to the v i g o r o u s l y s t i r r e d d i a z i r i n e / a l k e n e / D M F s o l u t i o n at 65 ° to -70°C.
t r a n s i e n t red c o l o r developed, then f a d e d to y e l l o w a f t e r 30 min (see b e l o w ) . A f t e r
50°C, the r e a c t i o n m i x t u r e was w a r m e d to a m b i e n t temperature, the i s o b u t e n e was
5
A
4
a
A
4 h
at
<
a l l o w e d to escape, and the c r u d e p r o d u c t was i s o l a t e d by an ice w a t e r quench, f o l l o w e d by
p e n t a n e extraction.
R3-R~-H), that was f u r t h e r p u r i f i e d by GC (SE-30, 120°C).
i s o u r e a 3, thus r e p r e s e n t i n g the end r e s u l t of reactions.
e y c l o p r o p a n a t i o n step is p r o b a b l y -50%, b a s e d on our p r i o r e x p e r i e n c e with y i e l d s in the
G r a h a m o x i d a t i o n (3 4) 9 and d i a z i r i n e e x c h a n g e (4 -. 5) 11 reactions.
Tile p u r i f i e d p r o d u c t was c h a r a c t e r i z e d by m i e r o a n a l y s i s (C,H,N); IR s p e c t r o s c o p y (~CN
2220 cm film); and 200 MHz p r o t o n NMR s p e c t r o s c o p y [6cDc~3].I0 , 1.07, 1.01, 0.98, AB q,
6.0 Hz, 2H ( c y c l o p r o p y l H's); 1.32, 1.26, 2s, 3H each (Me's); 4,68, s, 2H (CH20) ; 7.34,
s, 5H (Ph).
The t r a n s i e n t red c o l o r o b s e r v e d d u r i n g the e x c h a n g e r e a c t i o n of c h l o r o d i a z i r i n e
c y a n i d e ion s u g g e s t s the f o r m a t i o n of b e n z y l o x y d i a z o a c e t o n l t r i ] e , PhCH20(NC)C NZ.
However, a d d i t i o n of the i s o b u t e n e s u b s t r a t e a f t e r the red c o l o r had faded, only m a r g i n a l l y
r e d u c e d the y i e l d of (to 12%), so that the diazoalkane, if f o r m e d does not a p p e a r to be
p r e c u r s o r of the p r o d u c t c y c l o p r o p a n e . It s e e m s most a p p r o p r i a t e to r e p r e s e n t the
d i a z i r i n e e x c h a n g e r e a c t i o n12 in S c h e m e as p r o c e e d i n g to b e n z y l o x y c y a n o d i a z i r i n e (5) from
w h i c h PhCH20 CN (ib) a r i s e s t h e r m o l y t i c a l l y at 50°C. 13-15
We o b t a i n e d 170 mg (15%) of 90% pure c y c ] o p r o p a n e
6
(RI-R2 Me,
The y i e l d of
6
is b a s e d on
3
The a c t u a l y i e l d of the
~
6
I
J
4
with
e
6
a
I
C
In the a b s e n c e of an a d d e d suhstrate, d i a z i r i n e
5 d e c o m p o s e s over the t e m p e r a t u r e r a n g e
-50 ° to
]5°C to n i t r i l e containing, orange, t a r r y sL~stances.
Importantly, only
a
low
S c h e m e 1
H2+
~
OCt"
PhCH20
Bu4N+ CN"
PhCH2OCNH2, OTs-
,
PcH2oi
C l a N
DMF, < 50°, 4 h
NC
3
4
5
R1
R3
OCH2Ph
CN
_N2
-50 ° to 10°
PhCH20\
R1 R2C=CR3R4
/C:
R
2
~
NC
lb
n4 6

1927
< ~ !
a, R1
=
R 2
=
H; b, R1
=
H, R 2
=
MeO
c, R1 = H, R2=CN; d, R1 = CN, R2= MeO
7
yield (<5%) of ~-phenylacetonitrile is formed.
to an ion pair [PhCH2+ OC CN ], and thence to PhCH2CN with the loss of CO.
fragmentation pathway is very facile for benzyloxychlorocarbene, PhCH20-C CI, w h i c h
quantitatively affords benzyl chloride. I° With lh, however, the strong sp2-sp C-C
between the earbenic and cyano c a r b o n atoms resists fragmentation. The strong
PhCllzO behaves similarly. 15
In reactions similar to that described for the cyclopropanation of isobutene,
PhCH20 C-CN was also added to tetramethylethylene (14%), trimethylethylene (22%), and
trans butene (8%). Again, the isolated yields are b a s e d on isourea 3; the cyclopropanation
steps must occur with considerably higher efficiencies. These additional cyclopropanes (6)
In the case of the trimethylethylene substrate, isomeric
i0:i distribution upon the addition of lb. When the
Thus, carbene ib does not readily fragment
An analogous
~
bond
C
F bond of
C
F
were fully characterized.
2
cyclopropanes were formed in
a
reactions of Scheme
trimethylethylene as the olefinic substrate, about 7% of an (impure) adduct of MeO C~CN
(la) could be isolated. However, the diazirine exchange procedure, and product isolation,
proved to be easier in the benzyloxy (ib) series.
The ready formation of eyclopropanes by additions of PhCH20-C-CN to common electron
rich alkenes is in accord with theoretical predictions of patent electrophillc reactivity
for alkoxyeyanocarbenes. 17 However, because we have not yet been able to prepare stable
solutions of cyanodiazirine 5, we cannot now document the ambiphi]icity anticipated for lb.
Thus, the cyanide ion, required for the diazirine exchange reaction, polymerizes
eiectrophilic alkenes (e.g. methyl acrylate or acrylonitrile) when these are present.
The reason for the instability of remains unclear. HF/3 21G//3 21G calculations2 on
diazirines 7a-7d reveal that the LUMO (N ~* orbital) energy decreases with cysno
substitution, and is lowest for 7d, tile analogue of 5. The calculated I~MO energies (eV)
are: 7a, 3.34; 7b, 2.93; 7c, 2.12; and 7d, 1.89. This suggests that the N-N ~* orbitals of
or 7d m i g h t be sufficiently good acceptors to he subject to
One can w r i t e mechanisms for cyanodiazirine to cyanodiazomethane
i were carried out starting with O-methylisourea tosylate, with
6
4
~ 5
5
N
alkoxycyanodiazirines
nucleophilic addition.
5
a
isomerlzation that is initiated by the addition of cyanide ion to the diazirine N-N bond. 18
The diazoalkane isomer of w o u l d likely be very unstable, readily decomposing to carbene
lb. On the other hand, our experiments provide no evidence for major role of diazoalkane
in the reaction sequence of Scheme i, We are continuing our studies of push pull carbenes.
5
a
Acknowledgments.
We are grateful to the National Science Foundation, the National
Science Foundation-Polish Academy of Sciences Cooperative Program (T.Z), and the Petroleum
Research Fund for financial support.

1928
References and Notes
(i)
Visiting Scientist on leave from the Technical University (Politeehnika), Warsaw
Poland.
(2)
All ab initio calculations made use of the GAUSSIAN 88 series of programs: M.J.
Frisch, M. Head-Gordon, H.B. Sehlegel, K. Raghavachari, J.S. Binkley, C. Gonzalez,
D.J. Defrees, D.J. Fox, R.A. Whiteside, R. Seeger, C.F. Melius, J. Baker, R.L.
Martin, L.R. Kahn, J.J.P. Stewart, E.M. Fluder, S. Topiol, and J.A. Pople, GAUSSIAN
88, Gaussian, Inc., Pittsburgh, Pa.
A detailed description of the theoretical
methods used in this work can be found in W.J. Hehre, L, Radom, J.A. Pople, and
P.v.R. Schleyer, "Ab Initio Molecular Orbital Theory," Wiley Interscience, New York,
1986.
(3)
N.G. Rondan, K.N. Houk, and R.A. Moss, J. Am. Chem. See., 102, 1770 (1980).
(4)
R.A. Moss, H. Fan, L.M. Hadel, S. Shen, J. W~ostowska, M. W~ostowski, and K. Krogh-
Jespersen, Tetrahedron Lett,, 28, 4779 (1987).
See also N. Soundararajan, M.S.
Platz, J.E. Jackson, M.P. Doyle, S-M. Oon, M.T.H. Liu, and S.M. Anand, J. Am. Chem.
See,, ii0, 7143 (1988).
(5)
(6)
R.A. Moss, Acc. Chem. Res., 22, 15 (1989); Ibid., 13, 58 (1980).
R.A. Moss and J.R. Przyhyla, Tetrahedron, 25, 647 (1969).
(7)
H. Fujimoto, S. Ohwaki, J. Endo, and K. Fukui, Gazz. Chim. Ital,, 120, 229 (1990).
R.A. Moss, C.B. Mallon, and C-T. He, J. Am. Chem. Soc., 99, 4105 (1977).
W.H. Graham, J. Am. Chem. Soc., 87, 4396 (1965).
(8)
(9)
(i0)
(ii)
R.A. Moss, B.K. Wilk, and L.M. Hadel, Tetrahedron Lett., 28, 1969 (1987).
R.A. Moss in "Chemistry of Diazirines~" Vol. I, M.T.H. Liu, ed., CRC Press, Boca
Raton, Florida, 1987, pp. 99ff.
(12)
In absence of CN-, chlorodiazirine 4 is quite stable at -50°C.
(13)
Red colors are also observed during alkene cyelopropanation reactions with
Ph-C-CN, derived from (similarly unstable) 3-cyano-3-phenyldiazirine; the latter
precursor is obtained by the reaction of Bu4N+CN with 3-bromo 3-phenyldiazirine i~
We find no (IR) evidence for the intermediacy of ~-phenyldiazoacetonitrile in this
reaction sequence, where the diazoslkane should be stable in solution at 0°C. 15 At
this time, we believe the red colors to be red herrings.
(14)
(15)
(16)
(17)
R.A. Moss, G. Kmiecik-bawrynowiez, and D.P. Cox, Synth. Commune, 14, 21 (1984).
R. Breslow and C. Yuan, J. Am. Chem, Soc~, 80, 5991 (1958).
R.A. Moss and T. Zdrojewski, J. Phys. 0rg. Chem., 3, 694 (1990).
The observed disparate solttbilities of the substrate olefins in cold DMF precludes
the performance of meaningful relative reactivity (competition) experiments betwe0n
the olefins.
(18)
The conversion of 3-bromo-3-phenyldiszirine to benzonitrile is initiated by the
addition of azide ion to the diazirine's N-N bond: K.E. Bainbridge and W.P. Dailey,
Tetrahedron Lett., 30, 4901 (1989); X. Creary and A.F. Sky, J. Am. Chem, Soc., 112,
368 (1990).
(Received in USA 29 January 1991)