Transcript Carbenes and Nitrenes

Carbenes and Nitrenes
 Carbenes
are uncharged, electron
deficient molecular species that
contain a divalent carbon atom
surrounded by a sextet of electrons.
 Nitrenes are uncharged, electron
deficient molecular species that
contain a monovalent nitrogen atom
surrounded by a sextet of electrons.
States of Carbenes and Nitrenes
Singlet state: carbocation-like in nature,
trigonal planar geometry, electrophilic
character
 Triplet state: diradical-like in nature,
linear geometry

Formation reactions of carbenes
a)
_
R2 C N N
b)
R2 C C O
c)
RCHX 2 + base
+
d)
e)
f)
h
or

h
or

R C R + N2
(singlet )
R C R + CO
(singlet )
_ _ _
X
R CX 2
RCHX 2 + R'Li
(where X = I & Br)
R'X
RCHX 2 + R'Li
(where X = F & Cl)
R'H
CH 2 I2
Zn-Cu
ether
R C X
H
R C Li
X
X
R C Li
X
R C H + LiX
(carbenoid)
R C X + LiX
(carbenoid)
I CH 2 Zn I (Simmons-Sm ith reagent)
a carbenoid species t hat reacts stereospecifically
wit h alkenes t o give cyclopropanes but does not
insert int o alkane C-H bonds
Typical Carbene Reactions
a) In s e rtion in to a-bon d:
C H + CH2
b) Addi tion to a -bon d:
C C
C CH2 H
C C
+ CH2
C
_
c) Yli de type rxn :
R2 C + X Y
Y
+
R2 C X
R2 C
Y
X
an ylide
d) In tram ol e cu l ar in s e rtion :
H3 C
H
C
H3 C
H
H3 C
C
C CH3
H3 C
CH3
H3 C
C
+
CH CH2
H3 C
+
C CH3
H
(5%)
1,2-insertion
C H
e ) C rack i n g:
+
f) D ime riz ation :R2 C N N + CR2
(90%)
1,2-insertion
CH3
(5%)
1,3-insertion
+ H C C H
+
+
R2 C N N CR2
R2 C CR2 + N2
Mechanistic Aspects of Carbene Chemistry
Observations
A. Gas Ph ase Expe ri me n t at ve ry l ow pre s s u re :
CH3
_
+
CH3 (CH2 )4 CH3 + CH3 CH CH2 CH2 CH3
CH2 N N + CH3 CH2 CH2 CH2 CH3 h
(II)
(I)
~49%
~34%
CH3
+ CH3 CH2 CH CH2 CH3
(III)
~17%
P oint of Information: As the pressure of t he syst em is increased (nit rogen or argon added),
t he yields of (II) and (III) increase at t he expense of (I).
B. Liqu i d ph as e e xpe ri me n t:
_ H3 C
CH3
+
CH2 N N +
C C
H
H
h
cis-1,2-dimethylcyclopropane
(only organic product)
P oint of Information: When an inert diluent such as perfluoropropane is added to t he st art ing
materials, a mixture of cis- and trans-1,2-dimethylcyclopropanes is obtained.
Mechanistic Aspects of Carbene Chemistry
Expl an ati on for Gas Ph as e Expe rim en t:
At very low pressure, react ive, singlet carbene inserts in the alkane C-H bonds in a completely
random ,statist icalm anner. However, upon addit ion of nit rogen or argon gas t o t he react ion,
singlet carbene reverts t o less react ive (m ore select ive) t riplet carbene via non-product ive collisions
wit h t he inert diluent .T riplet carbene shows a preference for at tacking tertiary and secondary
hydrogen atoms rather t han prim ary hydrogen atoms. Hence, form at ion of more tert iary and
secondary abst raction product s at t he expense of prim ary abstract ion product .
Expl an ati on for Li qu i d Ph ase Expe rime n t:
C3 F8
CH2
CH2
singlet
+
C C
H3 C
CH3 H3 C
C C
H3 C
H
H
CH2
H
H
t riplet
+
H
H
CH2
diradical
CH3
C3 F8
H
spin
inversion H3 C
C C
cis only
+
t rans product
CH3
CH2
H
cis product
H
CH3
C C
rot ation
CH3
ring
closure
H
H3 C
H
C C
H3 C
ring closure
CH3
H
Nitrene Formation Reactions
_
R N N N
+
a)
h
or
R = alkyl,aryl, H 
_
N N N
+
b)
R SO2
R = alkyl, aryl
_
RO C N N N
O
R = alkyl, aryl
+
c)
d)
RO C NH O SO2
O
base
_
RO C N O SO2
O
R N + N2
h
or

h
or

R SO2 N + N2
RO C N + N2
O
NO2
NO2
_
RO C N + O SO2
O
(nosylate)
NO2
Mechanistic Aspects of Nitrene Chemistry
Cycloaddition - Observations
Liqu i d ph as e e xperi me n t
_ H3 C
CH3
R N N N +
C C
H
H
+
R
R
N
N
h
+
H3 C
H
CH3
H
H3 C
H
H
CH3
cis
trans
(predominant aziridine product ) (minor aziridine product )
P oint of Information: W hen an inert solvent is added to t he react ion mixture, more t ransproduct is obtained at the expense of t he cis-product.
Mechanistic Aspects of Nitrene
Chemistry
Cycloaddition
Expl an ati on for Ni tre n e C ycloaddi ti on :
R N
R N
singlet
+
C C
H3 C
CH3 H3 C
H
H
C C
H3 C
H
H
R
N
N
t riplet
+
H
H
R
diradical
spin
CH3 inversion H3 C
C C
CH3
rot ation
C C
CH3
ring
closure
cis product
R
N
H
H3 C
cis
+
t rans product
H
CH3
H
H
R
N
H
C C
H3 C
ring closure
CH3
H
Mechanistic Aspects of Nitrene
Chemistry
Insertion - Observations
H
_
+
R N N N + CH3 CH2 C CH2 CH2 CH3
R=
C OC2 H5
CH3
(+) S-3-methylhexane
H
N R
CH3 CH2 C CH2 CH2 CH3
CH2 Cl2
CH3
(+) R-product
O
Me th od of G en e rati on C on c. of Al k ane
R N3
R N3
R N3
R N3

h
h
h
O bs . rot. of prod.
o
100%
+1.69
1.2%
+1.71
26.8%
+1.69
100%
+1.68
o
o
o
Re te n tion of C on fig
99%
100%
99%
98%
Explanation of Nitrene Insertion



Observed rotation is independent of chiral alkane
concentration.
At less than 100% chiral alkane concentration (more
inert solvent present), more triplet nitrene is formed at
the expense of initially produced singlet nitrene..
The fact that higher concentration of triplet nitrene does
not affect the observed rotation of chiral product is
indicative of the inability of the triplet species to insert
into alkyl C-H bonds.
Evidence of Singlet Nitrene C-H Insertion
Selectivity
R N + alk an e
(si n gl e t)
alk an e i n se rti on produ cts
Re lative re activitie s
Al k an e
CH3 CH3
H3 C C
H
C CH3
H
R N
CH3 CH3
H3 C C
H
CH3 CH3
C CH3 + H3 C C
NH R
H
C CH2 NH
H
R
67.0 : 1.0
H H
H3 C C C CH3
H H
R N
H H
H H
H3 C C C CH 3 + H3 C C C CH2 NH
H H
R
H NH R
9.0 : 1.0
S in gl e t n i tre n e C -H i n s ertion s e le cti vity:t ertiary C-H > secondary C-H > primary C
Summary of Carbene/Nitrene Chemistry
Singlet carbenes and singlet nitrenes add to C=C bonds
in a one-step, stereospecific manner.
 Triplet carbenes and triplet nitrenes add to C=C bonds
in a two-step, non-stereospecific manner.
 Singlet carbenes insert into alkyl C-H bonds randomly,
whereas singlet nitrenes do so selectively. Both singlet
species insert into alkyl C-H bonds with retention of
configuration.
 Triplet carbenes insert into alkyl C-H bonds selectively,
but not stereospecifically.
 Triplet nitrenes do not insert into alkyl C-H bonds.
