Nucleophilic Character of a Charge Neutral, High Oxidation State d 0 Zirconium Trimethylenemethane Complex

Article abstract of DOI:10.1021/ja0493897

The nucleophilic character of a charge neutral, high oxidation d⁰ zirconium trimethylenemethane (TMM) class of compound of general structure Cp*Zr(TMM)[N(R¹)C(Me)N(R²)], 1a (R¹ = R² = i-Pr) and 1b (R¹ = t-Bu, R² = Et), is presented through documentation of its reactivity with a range of alkyl and silyl halides and triflates, including unactivated ones such as ethyl triflate. These results should contribute to efforts directed toward expanding the synthetic chemist's toolbox of synthetic methods for the construction of complex organic molecules. Copyright

Full text of DOI:10.1021/ja0493897

Published on Web 04/21/2004
Nucleophilic Character of a Charge Neutral, High Oxidation State d0
Zirconium Trimethylenemethane Complex
Denis A. Kissounko and Lawrence R. Sita*
Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742
Received February 3, 2004; E-mail: lsita@umd.edu
Scheme 1
Organozirconium chemistry, as applied to organic synthesis, has
become synonymous with bis(cyclopentadienyl)zirconium(IV) and
-zirconium(II) complexes (also known as zirconocenes), and after
nearly 50 years of development,1 its value to the synthetic chemist
continues to grow at a constant pace.2 With respect to carbon-
carbon bond-forming reactions, however, organozirconocene de-
rivatives are still principally limited to insertion of XdY or XtY
species (e.g., alkenes, alkynes, carbon monoxide, and isonitriles)
into preexisting Zr-C bonds, and as recently pointed out by Negishi
and Huo,3 this state of affairs represents “a stiflingly severe
restriction, which must nevertheless be dealt with by synthetic
low ∆Gcq value of 10.1 kcal mol-1 (Tc ) 273 K) for the barrier to
chemists.” One strategy by which to achieve this goal is to look
for new patterns of reactivity among nonzirconocene complexes
that might then expand the range of synthetic possiblities. Herein,
we present the nucleophilic behavior of charge neutral, high
oxidation state d0 zirconium trimethylenemethane (TMM) com-
plexes of the general formula Cp*Zr(TMM)[N(R1)C(Me)N(R2)]
(Cp* ) η5-C5Me5) (1) that can engage in carbon-carbon bond-
forming reactions with organic electrophiles, including unactivated
ones. While knowledge of this reactivity may now lead to the
development of new classes of bifunctional TMM reagents for the
construction of complex organic molecules,4 perhaps the more
important extension of these findings is that the monocyclopenta-
dienyl/amidinate ligand combination upon which 1 is based may
prove complementary to zirconocenes for developing a range of
new synthetic methodologies.5
To date, only four early transition-metal TMM complexes have
been reported, but within this group, three strongly differing bonding
schemes between the metal and the TMM fragment, which is a
formal-2charged,six-electrondonorligand,havebeendescribed.6-10
More specifically, the η4-TMM bonding mode (A) has been invoked
for neutral Cp*Ta(TMM)Me26 and anionic [Cp*Zr(TMM)(µ-Cl)2Li-
(TMEDA)]7 (TMEDA ) N,N,N′,N′-tetramethylethylenediamine),
whereas the zirconocene complex, Cp*2Zr(TMM),8 clearly adopts
the σ2-TMM structure (B) in both solution and the solid state.
rotation of the TMM fragment in 1a, coupled with a facile reaction
between 1b and electrophilic B(C6F5)3 to produce the charge-neutral
zwitterion, {Cp*Zr[η3-CH2C(CH2)CH2B(C6F5)3][N(t-Bu)C(Me)N-
(Et)]}9b, led us to speculate that the TMM fragment in 1 might
possess significant zwitterionic ground state character due to a
contribution from a resonance structure in which the TMM fragment
can be represented as a resonance-delocalized coordinated allyl
anion as in structure D. Preliminary theoretical support for this
hypothesis was provided by DFT methods, as implemented by the
Jaguar 3.5 computational package, which not only provided an
optimized equilibrium structure for 1a that closely mirrored that
of the experimentally determined solid-state structure, but which
further revealed that the electrostatic potential of this complex as
mapped onto the overall density surface possesses a high concentra-
tion of negative charge on the TMM fragment that gives rise to an
associated molecular dipole of 3.5 D.11 This latter finding then led
to a further hypothesis that compounds such as 1a and 1b ought to
be reactive toward various electrophiles and, perhaps, even unac-
tivated ones at that. To probe this idea, the nucleophilic character
of 1a was tested with a wide range of organic and silyl electrophiles,
and as Scheme 1 reveals, it was quickly discovered that apparent
nucleophilic attack by the TMM moiety proceeded with alkyl
triflates, benzyl bromide, trimethylsilylchloride and -triflate to
produce the corresponding substituted allyl complexes 2a-e in
nearly quantitative yields as determined by 1H NMR spectroscopy
(50-98% isolated yields).11 To the best that we can establish, these
reactions represent the first examples of the uncatalyzed direct
nucleophilic substitution of organohalides or organotriflates by an
early transition-metal organometallic complex, and they are more
significant given both the d0 high oxidation state and formal electron
deficiency of the metal center in 1, which suggests that, similar to
organozirconocenes, these complexes should possess very little, if
any, intrinsic nucleophilicity.12 It is further important to distinguish
these nucleophilic substitution reactions from the well-known
insertion of C-X multiple bonds (X ) O and N) into early
transition-metal alkyl and metal allyl bonds, including TMM
complexes,8,10b as these most likely proceed through a mechanisti-
cally different path involving prior coordination of the X atom to
the metal center.2,3 In fact, compound 1a, not surprisingly, was
found to readily react with benzaldehyde and benzophenone to
On the basis of crystallographic information, we previously reported
that derivatives of 1 where R1 ) R2 ) i-Pr (1a) and R1 ) t-Bu, R2
) Et (1b), are best represented as supporting the σ2,π-TMM
interaction (C) in the solid state because of the large observed
difference in Zr-C distances between two sets of this geometric
parameter involving the metal center and the three carbon termini
of the TMM fragment (cf. Zr-C: 2.3774(17) and 2.3888(17) Å
(bonded) and 2.642 Å (nonbonded) in 1a). As depicted, all of these
structures, A-C, conform to generally accepted conventions, and
none are particularly suggestive of any unusual physical property
or reactivity. However, subsequent observation of a significantly
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J. AM. CHEM. SOC. 2004, 126, 5946-5947
10.1021/ja0493897 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
“one-pot” fashion. Further studies are currently under investigation
to explore the utility of 2 for the palladium-catalyzed zirconium-
Negishi coupling with electrophiles.12
In summary, this report presents the nucleophilic behavior of a
charge neutral, high oxidation state zirconium TMM complex that,
based upon standard conventions, would not have been readily
expected. However, with the availability of computational methods
that includes new means by which to analyze the nature of metal-
ligand bonding interactions,14 the precise origin of this novel
behavior should be forthcoming. Additional investigations of unique
reaction paths involving 1 and other Zr(IV) and Z(II) complexes
based on the monocyclopentadienyl/amidinate ligand combination
are also currently in progress.
Figure 1. Molecular structure (30% thermal ellipsoids) of 2a. Hydrogen
atoms have been removed for the sake of clarity.
Scheme 2
Acknowledgment. Funding for this work was provided by the
NSF (CHE-0092493) for which we are grateful.
Supporting Information Available: Experimental details (PDF,
CIF). This material is available free of charge via the Internet at http://
pubs.acs.org.
provide the heterocyclic insertion products 3a and 3b, respectively,
shown in Scheme 2.
As proof of structure, crystallographic analyses of 2a and 3b
were conducted, which revealed a molecular structure for the former
compound in which the new C5H9 moiety can best be described as
a distorted σ,π-allyl ligand (cf. Zr(1)-C(22) 2.371(3) Å vs Zr(1)-
C(20) 2.619(3) Å and Zr(1)-C(21) 2.559(3) Å) (see Figure 1).11,13
It is possible, however, that this deviation from a symmetric η3-
allyl ligation might be enforced more by nonbonded steric interac-
tions, as suggested by a space-filling representation of the molecular
structure, rather than a consequence of any electronic factors.
Finally, in the solid-state structure of 3b (not shown), the allyl
moiety is clearly bound in an η1-fashion with the metal center.11
To further probe the nucleophilic behavior of this class of TMM
complex, the reaction of 1a with benzyl bromide was studied in
more detail. To begin, this reaction is clearly facilitated by more
polar solvents (cf. kobs ) 0.0026(3) s-1 (C6D6) vs 0.0075(3) s-1
(THF-d8) in formation of 2c under pseudo-first-order conditions at
30 °C using a 10-fold excess of benzyl bromide).11 Further, a
detailed kinetic analysis of the formation of 2c in THF-d8 over the
temperature range of 10-40 °C (five points) yielded the thermo-
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