1. 9
Carbenes, Carbenoides
and Nitrenes

2. Introduction Carbenes are molecules containing divalent carbon atoms,
but carbenes are neutral molecules.
It is possible to have several divalent carbons in a single
molecules (hexacarbene)

3. Introduction Carbenoids are molecules in which all the carbons are
tetravalent, but which have properties resembling those of
carbenes. Typically, carbenoides have carbon atoms that
are simultaneously bonded both to metal atoms and to
halogen atoms.
Nitrenes are compounds such as ::N-C6H5, that contain
monovalent nitrogen atoms.

4. Formation of Carbenes and Nitrenes
The thermal decomposition are obtained either from organo-
diazo compounds or by a-elimination reactions of organo-
halides.
The decomposition of organodiazo compounds are
frequently catalyzed by salts of heavy metals such as
copper, palladium and rhodium – intermediates is
carbenoides

5. Formation of Carbenes and Nitrenes
Preparation of diazo compounds
(1) Lithium or sodium salt of p-toluenesulfonylhydrazone
(2) Base-catalyzed decomposition of N-nitrosoamides

6. Formation of Carbenes and Nitrenes
Preparation of diazo compounds
(3) Oxidation of hydrazones of carbonyl compounds
(4) Thermal rearrangements of diazirines

7. Formation of Carbenes and Nitrenes
Preparation of halocarbenes
Reactions of strong bases with organic polyhalides that
lack hydrogens on b-carbons. Instead, the bases abstract
protons from the polyhalogenated carbons.

8. Formation of Carbenes and Nitrenes
Preparation of halocarbenes
Reactions of strong bases with organic polyhalides that
lack hydrogens on b-carbons. Instead, the bases abstract
protons from the polyhalogenated carbons.
The a-halolithium compounds act as carbenoids rather
than dissociating to form free carbenes

9. Formation of Carbenes and Nitrenes
Carbenoids formed from reactions of dihalides with heavy
metals are relatively stable at room temperature, and are
more selective in their reactions than most carbenes.
Simmons-Smith reagent
Nitrenes are most frequently formed by thermolysis or
pyrolysis of organic azides
a-Elimination reactions

10. Singlet and Triplet Carbenes
Singlet state – two unshared electrons have paired spins
and are located in a single orbital, leaving a second
orbital vacant. (carbocation and carbanion type)
Triplet state – two electrons have unpaired spins, and one
electron is located in each nonbonded orbital. (diradical
type)
According to Hund’s rule, triplet forms of carbenes should
be more stable than singlet forms. In the case of CH2:, about
9 kcal/mol is different between two states. However,
substituents with hetero atom (B, N, X) stabilze singlet
structures more strongly than triplet structures.
CH2: more stable in triplet state
CCl2: more stable in singlet state

11. Singlet and Triplet Carbenes
Several heterocyclic carbenes are indefinitely stable at
room temperature and can be melted at high temperature
and resolidified without decomposition.
Theoretical calculation indicate that these molecules do
possess a good deal of aromatic stabilization.

12. Singlet and Triplet Carbenes
In carbenes with relatively small singlet-triplet gaps,
intersystem crossing between the singlet and triplet forms
is faster than other reactions of the triplets, in which
reactions may proceed via the singlet forms even though
only small amounts of those forms are in equilibrium with
the triplet forms. The ratio of reactions attributable to
singlet vs triplet forms may be increased in more polar
solvents.
Singlet and triplet forms of carbenes differ in their
geometries.

13. Singlet and Triplet Carbenes
Carbenes having relatively linear structures are more likely
to have triplet structures, while relatively nonlinear carbenes
are more likely to be singlets.
n = 9 or 10 : singlet
n = 11 or 12 : triplet

14. Addition to Double Bonds
Cyclopropane Formation
Addition to double bonds and triple bonds can yield
cyclopropanes and cyclopropenes.

15. Addition to Double Bonds
Cyclopropane Formation
Intramolecular addition to double bonds has proved to be
useful procedure for the formation of polycyclic molecules.

16. Addition to Double Bonds
Cyclopropane Formation
The reactions of carbethoxycarbene with benzene provided
bicyclic intermediates which rapidly undergo electrocyclic
reactions to form cycloheptatriene derivatives. However,
reactions of carbenes with polycyclic aromatic molecules
yield stable cyclopropane structures.

17. Addition to Double Bonds
Cyclopropane Formation
The reactions of carbethoxycarbene with benzene provided
bicyclic intermediates which rapidly undergo electrocyclic
reactions to form cycloheptatriene derivatives. However,
reactions of carbenes with polycyclic aromatic molecules
yield stable cyclopropane structures.

18. Addition to Double Bonds
1,4- and 1,6-Addition Reactions
Carbenes add to double bonds of conjugated dienes to form
cyclopropanes in 1,2-cycloaddition processes. 1,4-Additions
to form five-membered rings proceed in the s-cis conformers.

19. Addition to Double Bonds
1,4- and 1,6-Addition Reactions
Only intramolecular 1,4-addition is observed in the reactions
of carbene 4.
Retro 1,4-addition reactions of carbenes with dienes can
proceed quite easily.

20. Addition to Double Bonds
1,4- and 1,6-Addition Reactions
Several 1,6-addition reactions have been reported, but their
mechanism are unclear. The reaction proceed via triplet
state implying that the addition is not concerted.

21. Addition to Double Bonds
Stereochemistry of Additions to Double Bonds
Triplet carbenes initially add to double bond to form triplet
diradicals in whch rotation around single bonds are usually faster than intersystem crossing to singlet to result in the
nonstereospecific reactions.

22. Addition to Double Bonds
Stereochemistry of Additions to Double Bonds
Singlet carbenes usually add stereospecificcally to alkenes,
forming cyclopropanes that retain the geometries of the
alkenes.

23. Addition to Double Bonds
Stereochemistry of Additions to Double Bonds
Simmons-Smith reactions of alkenes also provided cyclo-
propnaes stereospecifically with retention of the alkene
geometry.
Nitrenes react in a manner similar to carbenes

24. Addition to Double Bonds
Reactivities in Addition Reactions
Most singlet carbenes add to alkenes substituted with EDG
in preference to those substituted with EWG.
Carbenes substituted with halogens or other EWG show the
greatest preference for reaction with electron-rich double
bond.
The preference pi-bond of an alkene initially interacts with the empty p orbital of a carbene (electrophilic stage) and unshared electron pair on the original carbenic carbon can participate in forming the cyclopropane ring (nuclephilic stage), in which two-stages reactions appear to be single-step reactions.

25. Addition to Double Bonds
Reactivities in Addition Reactions
Carbenes substituted with strongly EDG such as methoxy or
dialkylamino groups, have been found to be nucleophilic
reagents which will add rapidly to electron-poor double bonds.

26. Addition to Double Bonds
Reactivities in Addition Reactions
p-Tolylchlorocarbene add to both electron-rich and electron-
poor double bonds in preference to typical alkene double
bonds. (ambiphilic carbenes)
Stereochemistry of cyclopropane formation from
nucleophilic carbenes showed inversion which involves
the formation of a zwitterionic intermediate.

27. Addition to Double Bonds
Reactivities in Addition Reactions
Triplet forms of carbenes are less affected by electron-
donating or electron-withdrawing substitutents on double
bonds than are single forms. However, triplet carbenes react very rapidly with conjugated double bonds in which
reactions can proceed via resonance stabilized diradical
intermediates.

28. Insertion Reactions Singlet Carbenes
Singlet methylene can react with alkanes and cycloalkanes
by insertion into carbon-hydrogen bonds with non-selectivity
primary C-H
tertiary C-H
vinylic C-H
allylic C-H

29. Insertion Reactions Singlet Carbenes
Other singlet carbenes are more selective than methylene. Dichlorocarbene can insert into allylic or benzylic C-H bonds but does not react with nonallylic primary or secondary
bonds. It will insert into some tertiary C-H bonds.

30. Insertion Reactions Singlet Carbenes
Insertion of singlet carbenes into C-H bonds proceed with
retention of configuration.

31. Insertion Reactions Singlet Carbenes
Carbenes do not insert into unstrained C-C bonds, but
insertions into very strained C-C bonds take place.
Remarkably, even addition to two single bonds has been
observed.

32. Insertion Reactions Triplet Carbenes
Triplet carbenes can also insert into C-H bonds, but
insertions appear to proceed by multistep processes.
Initially, triplet carbenes abstract hydrogen atoms from C-H
bonds to form radical pairs, which combine to form the
insertion products.

33. Rearrangements Hydrogen and Alkyl Group Migrations
1,2-Migration of hydrogens occur extremely easily in singlet carbenes.
Hydrogen migrations in three-membered ring carbenes
would yield highly strained double bonds

34. Rearrangements Hydrogen and Alkyl Group Migrations
Alkyl groups in carbenes tend to migrate much more slowly
than hydrogens, and their migrations are usually barely
detectable.
Alkyl migration are common if
they result in the expansion of
strained rings.

35. Rearrangements Hydrogen and Alkyl Group Migrations
Alkyl migration also predominates when a hydroxy group
can stabilizes formation of an empty orbital at the migration origins.
Alkyl group migrations in nitrenes are more competitive
with hydrogen migrations than in carbenes, but hydrogen
Migrations still predominate

36. Rearrangements The Nature of the Rearrangement Process
1,2-Migrations of hydrogens and alkyl groups in carbenes are often described as insertions of the divalent carbons
into neighboring C-H and C-C bonds. However, it seems
more useful to regard these reactions as analogous to
carbocation rearrangements.

37. Rearrangements Rearrangements of Carbenoides Formed from Vinyl Halides
Formation of Alkynes
Rearrangement of carbenoids, in which the bonds between the vinyl
carbons and halogen ions are at least partially broken before
rearrangement take place.
The carbenoide intermediate can be intercepted by the addition of alkenes,
Yielding cyclopropane derivatives.

38. Rearrangements Rearrangements of Carbenoides Formed from Vinyl Halides
Formation of Alkynes
Carbenoide intermediates can be demonstrated by the use
of isotopically labeled starting materials.
Rearrangements are stereospecific, and
that it is invariably the groups trans to the
halogens that migrate.

39. Rearrangements Rearrangements of Carbenoides Formed from Vinyl Halides
Formation of 1-Halocyclopentenes
Base-induced rearrangements of halomethylenecyclobutanes
principally yield 1-halocyclopentenes along with only small
amounts of cyclopentenyl ethers forming from the reactions
of halomethylenecyclobutanes with alkoxide.
Double migration is increased when the halides
Ion is a good leaving group (I->Br->Cl->>F-)

40. Rearrangements Rearrangements of Carbenoides Formed from Vinyl Halides
Formation of 1-Halocyclopentenes
Proposed mechanism for the double migration
Double migration appear to be closely related to rearrangements leading
to acetylenes, except that the departing halide ions attack the sites from
which the ring carbons are migrating.