|| Transition – Metal Allenylidene Complexes
Transition – metal allenylidene complexes can become tools for organic synthesis more important than the carbene compounds of type l n m=cr 2
María L. Buil, Miguel A. Esteruelas, Ana M. López and Enrique Oñate
The hydroamination of alkynes in the presence of transition
metal complexes is an attractive route to prepare numerous classes of organonitrogen
molecules . Two basic approaches have been employed to effect aminations
and involve either alkyne or amine activation routes. Alkyne activation is generally
accomplished with late transition-metals, which render p -alkyne intermediates.
The coordination of the alkyne to the metallic center enhances its electrophilic
character, making the alkyne susceptible of undergoing the direct nucleophilic
addition of the amine .
The evidence that vinylidene-metal intermediates, L n M=C=CHR,
are easily formed from terminal alkynes and transition metals, brings along
an alternative alkyne activation route. The reactivity of the vinylidene-metal
moiety is dominated by the electrophilicity and nucleophilicity of the C a and
C b atoms, respectively . As a result, one of the N-H bonds of primary amines
adds across the highly polarized C a -C b double bond of the vinylidene to afford
"aminocarbene derivatives" .
Allenylidene compounds L n M=C=C=CR 2
, which belong to the series of unsaturated carbene derivatives L n
M=C(=C) m CR 2 (m>0), has been much less studied than
the vinylidene complexes. EHT-MO calculations indicate that the carbon atoms
of the unsaturated chain of the allenylidene are alternatively electron-poor
and electron-rich, starting from the metal center . Hence, electrophilic
centers are located at the C a and the C g atoms, while
the C b atom is a nucleophilic site.
The coordination of the p -acidic carbonyl group to the metallic
fragment containing the allenylidene ligand enhances the reactivity associated
with the allenylidene spine. Thus, we have recently shown that the allenylidene
ligand of the complex [Ru( h 5 -C 5 H 5 )(=C=C=CPh
2 )(CO)-(P i Pr 3 )]BF 4 ( 1
) reacts with propargylamines and diallylamine to afford heterocyclic
compounds, which are the result of the addition of the nitrogen atom of the
amine to the C a atom of the allenylidene and one or two C-C couplings.
Similarly to the vinylidene complexes, the C a -C
b double bond of the allenylidene ligand of 1 undergoes
the addition of one of the N-H bonds of propargylamine, to give the secondary
azoniabutadienyl derivative 2 (eq. 1). Treatment of the latter
with KOH in methanol produces its deprotonation and the formation of the bicycle
complex 3 as a result of a double C-C coupling, the C a and
C g atoms of the allenylidene of 1 and the C b atom of the
allenylidene with the C b atom of the propargylamine.
The formation of the bicycle from the unsaturated h 1 -carbon
donor ligand of 2 is a three-elemental-step reaction involving:
(i) deprotonation of the nitrogen atom, (ii) propargyl to allene isomerization
catalyzed by the solvent (methanol), and (iii) double intramolecular C-C coupling.
The three membered ring is a result of a novel intramolecular cyclopropanation,
which is induced by the allenic unit and it involves the initial nucleophilic
attack of the central carbon of the unsaturated Ru-C 3 chain at the
central carbon atom of the allenic unit.
The propargyl-allenic isomerization involves a 1,3-hydrogen
shift within the substituent of the amine. To block this process, we have carried
out the reaction of 1 with 1,1-diethylpropargylamine, which
does not contain hydrogen atoms in a -position with regard to the nitrogen atom.
In dichloromethane, the reaction leads to the dihydropyridiniumyl derivative
4 (eq. 2). Its formation involves the selective N,C g
addition of the amine to the C a -C b double bond
of the allenylidene ligand of 1 .
In contrast to 1,1-diethylpropargylamine but in agreement
with propargylamine, the N-H bond of N-methylpropargylamine is added to the
C a -C b double bond of the allenylidene ligand of 1
, to afford the tertiary azoniabutadienyl derivative 5 ,
which was isolated as a mixture of the isomers 5a and 5b
(eq. 3). Treatment at –78ºC of tetrahydrofuran solutions of the
isomeric mixture with sodium methoxide gives a 1:1 mixture of the dihydronaphtopyrrolyl
diastereomers 6a and 6b .
The new polycycle is the result of two carbon-carbon couplings
in 5 , the C b atom of the original C 3
chain with the central carbon atom of the propargyl unit and, at the same time,
an ortho carbon atom of one of the phenyl groups with the terminal (C(sp)) atom.
These couplings can be rationalized as an intramolecular Diels-Alder reaction
in an allenyl-amino-diphenyallenyl intermediate, where the C b -C
g double bond and one of the two phenyl groups of the diphenylallenyl
fragment act as an inner-outer ring diene and the C=CH 2 double bond
of the another allenyl fragment acts as a dienophile. The formation of two diastereomers
in the reaction is the consequence of the chirality of the ruthenium and the
two possible approaches of the dienophile to the diene.
Diallylamine, which contains C-C double bonds instead of a
C-C triple bond, reacts in a similar manner to propargylamine and N-methylpropargylamine.
The addition of this organic substrate to 1 leads to the tertiary
N-allyl-4-azonia-1,3,6-heptatrienyl derivative 7 (eq.4), as
a result of the addition of the N-H bond of the amine to the C a
-C b double bond of the allenylidene. In the presence of bases, there
are marked differences in behavior between the previously described azonia derivatives
and 7 . In contrast to the general trend, the deprotonation
of 7 does not occur at the CH=CPh 2 group but at
one of the NCH 2 -carbon atoms. Although the allyl units are inequivalent,
the deprotonation of both moieties is equally favored. As a result, the treatment
of 7 with sodium methoxide in tetrahydrofuran affords a 1:1
isomeric mixture of the ruthenapyrrolinone complex 8 and the
pyrrolinyl compound 9 .
The formation of 8 involves the deprotonation
of the allyl group disposed trans to the CH=CPh 2 unit, followed by
the intramolecular attack of the deprotonated NCH-carbon atom to the carbonyl
ligand. Complex 9 is the result of the deprotonation of the
allyl group trans disposed to the metallic center and the subsequent
intramolecular nucleophilic attack of the deprotonated NCH-carbon atom to the
CPh 2 -carbon atom of the CH=CPh 2 moiety. Both the formation
of 8 and 9 can be rationalized as dipolar
The high degree of stereocontrol in the formation of 8
and 9 should be pointed out. As a result of the
prochirality of the NCH 2 carbon atoms, two pairs of enantiomers
of each isomer could be formed during the reaction. However, only one pair of
8 and one pair of 9 are obtained. This suggests
that the configuration of the ruthenium atom of 1 , and therefore
of 7 , determines the configuration of the asymmetric NCH-carbon
atom of the heterocyles of 8 and 9 .
Complex 9 reacts with tetrafluoroboric acid
to give the cationic derivative 10 (eq. 5). The latter, which
is an isomer of 7 is the result of the addition of the proton
of the acid to the C b atom of 9 . In the solid state, complex
10 is stable at room temperature. However in dichloromethane
as solvent, it evolves to afford 11 . The process involves
the opening of the five-membered hetero-ring, formally as a consequence of the
split of the CPh 2 -CH(vinyl) bond, along with a proton transfer
from C b to the terminal carbon atom of the vinyl substituent.
Today is rare to find a complete total synthesis that does
not use a transition-metal-based reaction. In this respect, carbene complexes
are one of the most useful tools. The reactions shown here indicate that the
potentiality of the allenylidene compounds can become greater than that of carbene
complexes of type L n M=CR 2 . The preparation of transition-metal
allenylidene derivatives is very easy , and the presence of three
reactive centers (unsaturated C 3 chain) or more (unsaturated chain
plus substituents) in the h 1 -carbon ligand allows one to build,
in one or two steps, organic skeletons, which require multistep procedures in
conventional organic synthesis.
M. L. Buil, M. A. Esteruelas, A. M. López, E. Oñate,
Organometallics 2003, 22 , 162; ibid. 2003, 22
We are grateful for financial support from the MCYT of Spain
(Projects BQU2002-00606 and PPQ2000-0488-P4-02) M.L.B. thanks the Ministerio
de Ciencia y Tecnología (CICYT) of Spain for a Ramón y Cajal project.
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