Reactions of Osmium-Pinacolboryl Complexes: Preparation of the
First Vinylideneboronate Esters
Miguel A. Esteruelas,* Ana M. Ló pez,* Malka Mora, and Enrique Oñ ate
Departamento de Química Inorga ́ nica-Instituto de Síntesis Química y Cata ́ lisis Homoge ́ nea (ISQCH), Universidad de
Zaragoza-CSIC, 50009 Zaragoza, Spain
* S Supporting Information
ABSTRACT: The reactions of OsHCl(CO)(PR
3
)
2
(PR
3
=P
i
Pr
3
(1a), PCy
3
(1b)) with pinacolborane (HBpin) afford the
boryl-dihydrogen derivatives Os(Bpin)Cl(η
2
-H
2
)(CO)(PR
3
)
2
(PR
3
=P
i
Pr
3
(2a), PCy
3
(2b)), which lose the H
2
ligand to give
the five-coordinate boryl complexes Os(Bpin)Cl(CO)(PR
3
)
2
(PR
3
=P
i
Pr
3
(3a), PCy
3
(3b)). Treatment of 3a,b with LiCCPh
leads to the five-coordinate alkynyl-boryl derivatives Os(Bpin)(CCPh)(CO)(PR
3
)
2
(PR
3
=P
i
Pr
3
(4a), PCy
3
(4b)), which
under a carbon monoxide atmosphere evolve into the vinylideneboronate esters Os{CC(Bpin)Ph}(CO)
2
(PR
3
)
2
(PR
3
=
P
i
Pr
3
(5a), PCy
3
(5b)). Complex 5a is also prepared by stirring of a toluene solution of 3a under CO and subsequent reaction of
the resulting cis-dicarbonyl compound Os(Bpin)Cl(CO)
2
(P
i
Pr
3
)
2
(6) with LiCCPh. The X-ray structures of 3a, 4a, and 5b are
also reported.
■
INTRODUCTION
Organoboronate esters are useful synthetic intermediates, due
to the myriad reactions that can be performed with these
compounds.
1
They can be prepared by transition-metal-
promoted boron additions to organic fragments.
2
Among the
complexes with metal-boron bonds, boryl derivatives exhibit
the strongest potential for this aim.
3
The use of pinacolborane
has given access to a large number of handy species for organic
chemists.
4
Transition-metal-vinylidene complexes
5
are also intermedi-
ates in a number of synthetically important transformations.
6
Borylvinylidenes are a promising new type of transition-metal
compound that should join the advantages of both organo-
boronate reagents and vinylidene complexes. The first
derivatives of this type were reported a few months ago.
7
They are diaminoboryl- and fluoroaminoborylvinylidene
species. The key boryl ligands are generated on the metal
center coordination sphere, by the reactions of an osmium-
tetrahydroborate complex with anilines. Now, we have
developed an easy procedure to prepare vinylideneboronate
esters, starting from the commercially available pinacolborane.
■
RESULTS AND DISCUSSION
An efficient method to prepare boryl complexes is the addition
of hydroboranes to transition metals in low oxidation states.
8
In
agreement with this, at room temperature, the monohydride
complexes OsHCl(CO)(PR
3
)
2
(PR
3
=P
i
Pr
3
(1a), PCy
3
(1b))
react with pinacolborane to give the boryl -dihydrogen
derivatives Os(Bpin)Cl(η
2
-H
2
)(CO)(PR
3
)
2
(PR
3
=P
i
Pr
3
(2a), PCy
3
(2b)), which lose molecular hydrogen. The five-
coordinate boryl complexes Os(Bpin)Cl(CO)(P
i
Pr
3
)
2
(PR
3
=
P
i
Pr
3
(3a), PCy
3
(3b)) are isolated as yellow solids in high
yields, 93% (3a) and 68% (3b), according to Scheme 1. The
related triphenylphosphine compound Os(Bcat)Cl(CO)-
(PPh
3
)
2
has been previously prepared by reaction of the aryl
complex Os(Ph)Cl(CO)(PPh
3
)
2
with cathecolborane.
9
The formation of the dihydrogen compounds 2a,b probably
involves the oxidative addition of the H-B bond of the borane
to the osmium atom of 1a,b and the subsequent dihydride-
dihydrogen tautomerization of the resulting dihydride. The
nonclassical character of the OsH
2
units is supported by the
1
H
NMR resonances in toluene-d
8
, which at 298 K have a 300
MHz T
1
(min) value of 13 ± 1 ms. This value corresponds to a
Received: June 7, 2011
Published: November 22, 2011
Scheme 1
Article
pubs.acs.org/Organometallics
© 2011 American Chemical Society 2965 dx.doi.org/10.1021/om200485k | Organometallics 2012, 31, 2965-2970