Fundamental organometallic chemistry under bimetallic influence: Driving β-hydride elimination and diverting migratory insertion at Cu and Ni

Article abstract of DOI:10.1039/c6dt04533b

Bimetallic effects on stoichiometric β-hydride elimination and migratory insertion reactions were examined. Bimetallic reaction conditions drove β-hydride elimination at Cu, while bimetallic C-B elimination occurred in the absence of β-hydrogens. The inhe

Full text of DOI:10.1039/c6dt04533b

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DOI: 10.1039/C6DT04533B
Journal Name
COMMUNICATION
Fundamental organometallic chemistry under bimetallic
influence: driving -hydride elimination and diverting migratory
insertion at Cu and Ni
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Thomas J. Mazzacano, Noel J. Leon, Greyson W. Waldhart and Neal P. Mankad*
www.rsc.org/
Bimetallic effects on stoichiometric -hydride elimination and reactivity and selectivity of
-hydride elimination and
migratory insertion reactions were examined. Bimetallic reaction migratory insertion pathways at Cu(NHC) and Ni(pincer) sites,
conditions drove -hydride elimination at Cu, while bimetallic C-B and also report a novel bimetallic C-B elimination reaction. The
elimination occurred in the absence of -hydrogens. The inherent continued identification and mechanistic understanding of
migratory insertion chemistry of alkynes at Ni was diverted under such bimetallic reaction pathways is crucial because it will
bimetallic reaction conditions to favor C-H deprotonation.
inform future catalyst development with these and related
heterobimetallic systems.
A typical approach to catalysis with organometallic systems is
to identify metals (e.g. Ru, Rh, Pd) that are effective as single-
site catalysts, and then to utilize ligand design to tailor the
kinetics and selectivity of the fundamental organometallic
reaction pathways (e.g. oxidative addition, reductive
Table 1. Alkylcopper -hydride elimination driven by bimetallic cooperation.
elimination, migratory insertion,

-hydride elimination) that
1
need to be managed during catalysis. Detailed understanding
of those fundamental organometallic reactions has
contributed to the unrivalled success of such single-site
2
precious metal catalysis. An alternative and underutilized
approach is multimetallic catalysis,3 wherein two or more
metal sites either cooperate in mediating fundamental bond-
breaking and bond-forming reactions or otherwise serve to
modulate the kinetics and/or selectivity of key steps during
Entry
[MCO
none
FeCp(CO)
Mn(CO)
RuCp*(CO)
RuCp*(CO)
WCp*(CO)
]
E
% 2a
17
60
61
30
% 3a
32
10
11
27
1
2
3
4
5
6
none
Bpin
Bpin
Bpin
H
2
5
4
catalysis. This and related innovative mechanistic paradigms
2
2
stand to either expand the range of chemical elements capable
of catalysing key transformations or lead to emergent catalytic
86
12
11
83
3
H
5
-14
reactivity that complements single-site catalyst systems.
In this context, our group has developed heterobimetallic
a
Determined by quantitative GC analysis.
1
5
16
(
NHC)M-[Mco] catalysts for C-H borylation, alkyne semi-
A common factor influencing single-site catalysis involving
metal-alkyl intermediates is the relative rate of reductive
elimination compared to -hydride elimination. For
heterobimetallic catalysis involving two separated metal sites,
unimolecular -hydride elimination is expected to outcompete
bimolecular reductive elimination in most cases. However, we
have observed previously that bimetallic C-H reductive
elimination from (NHC)CuR complexes reacting with [MCO]H
1
7
18
hydrogenation,
and CO
2
deoxygenation
(NHC = N-
-
heterocyclic carbene, M = Cu or Ag, [MCO] = metal carbonyl

-
anion e.g. [FeCp(CO)
2
] ). The discoveries of these catalytic
transformations have relied on our efforts to understand the
fundamental nature of cooperative bond-breaking and bond-
forming reactions employed by the relevant heterobimetallic

1
9-24
systems.
As part of these ongoing studies, in this
communication we examine the influences of [MCO] sites on
partners can outcompete single-site
-hydride elimination in
some circumstances, even when R is an alkyl group with
1
7,23
Department of Chemistry, University of Illinois at Chicago, 845 W. Taylor St.,
Chicago, IL 60642, USA.
Electronic Supplementary Information (ESI) available: experimental procedures,
spectral data. CCDC deposition number 1518635. See DOI: 10.1039/x0xx00000x
multiple

-hydrogens.
To examine this issue further, we
-boroalkylcopper
chose to examine the reactivity of a
complex, of which the (IPr)CuCH(Ph)CHBpin derivative is

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Journal Name
isolable but is known to undergo reversible
elimination under mild conditions (IPr
diisopropylphenyl)imidazol-2-ylidene, pin = pinacolate). To by steric hindrance. On the other hand, Rp*H was the most
avoid complications from side reactions involving thermal effective additive at driving -hydride elimination (Table 1,
-hydride elimination driven by bimetallic
reactions with the 6Pr ligand that is known to give a thermally H-H elimination is clearly faster than bimetallic C-H elimination
robust copper-hydride species (6Pr 1,3-bis(2,6- to form 3 by direct reaction of 1 and Rp*H. On the other hand,
diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-

-hydride RuCp*(CO)
2
View Article Online
) was less effective at driving -hydride elimination
DOI: 10.1039/C6DT04533B
=
1,3-bis(2,6- (Table 1, Entry 4), possibly due to B-H elimination being slowed
2
5

26
decomposition of unstable [(IPr)CuH]
2
,
we chose to examine Entry 5). Here, the 
=
bimetallic C-H elimination product 3 was favoured over
-boroalkyl complex 1 was synthesized by the hydride elimination product 2 when Wp*H (Wp* = WCp*(CO)

3
-
)
2
7
ylidene). The

reaction of (6Pr)CuBpin with styrene, in analogy to Sadighi’s was used in place of Rp*H (Table 1, Entry 6). In this case, the
2
5
synthesis of the IPr derivative.
Heating 1 to 70°C in benzene-d
small quantities of the -hydride elimination product 2, along apparently serves to accelerate bimetallic C-H elimination.
formation of decomposition product 3 (Table 1, Entry 1). The Collectively, these results show that -hydride elimination
mass balance in this experiment consisted of unreacted 1. This from an (NHC)CuR complex can be driven by the presence of
behaviour is distinct from the IPr-supported derivative that [MCO]E complexes, and that the relative rates of single-site
quantitatively isomerizes to the -boroalkyl isomer under hydride elimination compared to bimetallic reductive
similar conditions. Exposing 1 to the same conditions in the elimination can be tailored by tuning both [MCO] and E.
presence of FpBpin (Fp = FeCp(CO) , pin = pinacolate) Although bimetallic reductive elimination can compete with
increased the yield of 2 and suppressed formation of 3 (Table hydride elimination when E = H, no evidence for bimetallic C-B
, Entry 2). NMR analysis of the product mixture indicated reductive elimination from 1 was observed when E = Bpin. To
stoichiometric formation of HBpin and (6Pr)CuFp, the latter of examine the possibility of bimetallic C-B elimination in the
which was confirmed by independent synthesis from (6Pr)CuCl absence of -hydride elimination pathways, we studied the
and KFp. The presence of FpBpin apparently drives the reactivity of alkenylcopper complex 4 derived from 5-decyne
hydride elimination equilibrium involving 1 by trapping insertion into [(IPr)CuH] (Figure 1b). Reaction of 4 and McBpin
observed behaviour is likely impacted by the estimated ~7 pK
a
for 48 h resulted only unit increase in acidity of Wp*H compared to Rp*H,29 which
6



-

2
5
2
-
1


-
2
[
(6Pr)CuH]
2
through bimetallic B-H elimination (Figure 1a).
at room temperature for 16 h resulted in quantitative
formation of the C-B elimination product 5. To our knowledge,
no previous examples of bimetallic C-B bond forming reactions
have been reported despite the prevalence of C-B bond-
3
0,31
forming catalysis in organic synthesis.
(
a)
O
PR2
Ni
H
O
O
PR2
Ni
C6H6, r.t., 12 h
R = iPr or tBu
O
+
PR2
O C WCp(CO)2
H2 (1 atm)
C6D6, r.t., 24 h
R = tBu
PR2
HWCp(CO)3
(
b)
W
Ni
Figure 1. (a) Hypothetical mechanism for bimetallic cooperation driving an alkylcopper
-hydride elimination equilibrium. (b) Observation of a bimetallic C-B elimination
reaction.

Figure 2. (a) Cooperative hydrogenation/dehydrogenation chemistry with Ni. (b)
tBu
Solid-state structure of [( POCOP)Ni][Wp] shown as thermal ellipsoids (50%
probability, H atoms omitted). Selected data for the bridging CO ligand (distances
in Å, angles in °): Ni-O, 1.911(4); O-C, 1.225(7); C-W, 1.874(5); Ni-O-C, 161.1(5);
O-C-W, 175.3(5).
No measurable impact on the yields of 2 and 3 were
observed when McBpin (Mc = Mn(CO) ) was used in place of
5
To complement these observations involving organocopper
FpBpin (Table 1, Entry 3). Although McBpin is expected to be complexes, we have observed bimetallic effects relevant to the
more reactive than FpBpin on the basis of the superior leaving reverse reaction of
-hydride elimination, i.e. migratory
group ability of [Mc] , it is possible that a counterbalancing insertion, by examining Ni systems. Levina et al. reported
effect is the instability of (6Pr)CuMc, which we were unable to [(PCP)Ni][MCO
bimetallic complexes that form with

-
28
]
identify in the product mixtures or synthesize independently. isocarbonyl bridging rather than with direct Ni-M bonds (PCP =
The more sterically hindered complex Rp*Bpin (Rp* = 2,6-C H (CH PtBu ) , M = Mp or Wp, Mp = MoCp(CO) , Wp =
6
3
2
2 2
CO
3
2
| J. Name., 2012, 00, 1-3
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WCp(CO)
3
).32 For comparison to known migratory insertion species but rather formed a 3:1 mixture (7) of 2,1- and 1,2-
View Article Online
DOI: 10.1039/C6DT04533B
R
chemistry (see below), we prepared [( POCOP)Ni][MCO
analogues ( POCOP = 2,6-C H (OPtBu ) ;
6 3 2 2
iPrPOCOP = 2,6-
]
insertion products (Figure 3a).
tBu
6 3 2 2
C H (OPiPr ) ). In analogy to the established dehydrogenative
procedure for the PCP derivatives, POCOP Ni/W derivatives
R
were prepared by reacting ( POCOP)NiH with WpH in pentanes
R
at room temperature for 12 h to produce [( POCOP)Ni][Wp]
through loss of
H
2
(Figure 2a). Analysis of the
tBu
[
( POCOP)Ni][Wp] derivative by X-ray crystallography
revealed the expected isocarbonyl bridging, with structural
parameters quite similar to those observed by Levina et al
3
2
(
Figure 2b). This derivative could also be rehydrogenated by Figure 4. Hypothetical mechanistic scheme to account for observed selectivity.
exposure to H
2
(1 atm) at room temperature for 24 h to
R
produce the corresponding ( POCOP)NiH and WpH, albeit with
only 56% conversion (Figure 2a).
Rather than changing the metal between Ni and Pd to
switch selectivity between C-H activation versus migratory
insertion, we have found that the presence of a [MCO] metal
site can act as a selectivity switch instead. Exposing
iPr
(
POCOP)NiH to phenylacetylene in the presence of WpH
produced acetylide complex 8 as the major product, and only
produced small quantities of 7 as minor products (Figure 3b).
Thus, the selectivity between C-H activation and migratory
insertion is inverted by the presence or absence, respectively,
of a [MCO]H additive. We assume that the minor products 7
form from direct reaction of phenylacetylene with
(
POCOP)NiH, while the major product 8 forms by initial
iPr
dehydrogenative formation of [( POCOP)Ni][Wp] followed by
subsequent reaction of the bimetallic complex with
phenylacetylene (Figure 4). Consistent with this proposal,
iPr
exposing the bimetallic complex [( POCOP)Ni][Wp] to
phenylacetylene produced 8 quantitatively with no detectable
formation of any components of 7 (Figure 3c). Ozerov has used
iPr
the ( POCOP)Pd system to catalyse dehydrogenative
3
4
borylation of terminal alkynes, thus demonstrating the utility
of having a system that is highly selective for activation of the
terminal C-H bond of an alkyne in preference to engaging with
the π-bond in migratory insertion reactions. The bimetallic
selectivity effect identified here could, in principle, be used
similarly under catalytic conditions to favour functionalization
of alkyne C-H bonds over hydrofunctionaliztion of the π-bond
without require use of a noble metal like Pd.
In conclusion, late transition metal systems capable of
insertion/elimination chemistry with unsaturated organic
substrates are impacted dramatically under bimetallic reaction
conditions, particularly when the second metal additive is
capable of acting as a proton/hydride shuttle that drives or
repositions relevant equilibria. The identification and
understanding of these effects will inform bimetallic catalyst
development by our group and others.
Figure 3. (a) Metal-dependent selectivity observed by Guan. (b) Bimetallic switch
in selectivity. (c) Bimetallic C-H activation of diphenylacetylene.
Guan recently studied the reaction chemistry of
(RPOCOP)MH complexes with phenylacetylene (M = Ni or Pd).
33
Acknowledgements
iPr
The ( POCOP)PdH complex was shown to favour
dehydrogenative C-H activation to form the acetylide complex
Funding was provided by the NSF (CHE-1362294). N.P.M. is an
Alfred P. Sloan Research Fellow, and T.J.M. is a Paaren
Graduate Fellow (UIC). Prof. Justin Mohr and his group
provided access to a GC.
6
over migratory insertion to form an alkenyl complex by a
iPr
factor of 13:1. The ( POCOP)NiH derivative, on the other
hand, did not form any detectable amount of the acetylide
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