Allenylidenes: their multifaceted chemistry at rhodium
Abstract
Rhodium allenylidenes, which represent a class of ‘flat’
metallabutatrienes, are useful tools for the generation of metal-bound
species that are hardly accessible (sometimes not accessible at all) by
other synthetic routes. In contrast to various octahedral
allenylidene metal complexes, e.g. of Cr
0
,
Mn
I
or Ru
II
, the square-planar rhodium compounds
trans-
[RhX(![[double bond, length as m-dash]](https://www.rsc.org/images/entities/char_e001.gif)
CCCRR′)(PPr
i
3
)
2
] are attacked by both nucleophiles and electrophiles.
Moreover, non-polar substrates such as H
2
or Cl
2
react cleanly with the title complexes to give products in which the
allenylidene unit is preserved as part of a newly formed ligand.
Attempts
to prepare a square-planar cationic metallaheptahexaene,
trans-[Rh(CCCRR′)
2
(PP
r
i
3
)
2
]
+
, unexpectedly led to
the formation of two isomeric hexapentaenerhodium(I)
complexes
of which the isomer having the polyene coordinated in an unsymmetrical
fashion is the thermodynamically more stable. C–C coupling
reactions
also occur on treatment of
trans-[RhCl(CCCPh
2
)(PPr
i
3
)
2
] with vinyl Grignard reagents or
phenylacetylene leading to highly unsaturated RhC
5
and
RhC
5
P frameworks from which vinylallenes and
phosphacumuleneylides are obtained.
CCCRR′)(PPr
i
3
)
2
] are attacked by both nucleophiles and electrophiles.
Moreover, non-polar substrates such as H
2
or Cl
2
react cleanly with the title complexes to give products in which the
allenylidene unit is preserved as part of a newly formed ligand.
Attempts
to prepare a square-planar cationic metallaheptahexaene,
trans-[Rh(CCCRR′)
2
(PP
r
i
3
)
2
]
+
, unexpectedly led to
the formation of two isomeric hexapentaenerhodium(I)
complexes
of which the isomer having the polyene coordinated in an unsymmetrical
fashion is the thermodynamically more stable. C–C coupling
reactions
also occur on treatment of
trans-[RhCl(CCCPh
2
)(PPr
i
3
)
2
] with vinyl Grignard reagents or
phenylacetylene leading to highly unsaturated RhC
5
and
RhC
5
P frameworks from which vinylallenes and
phosphacumuleneylides are obtained.
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