Abstraction of β-hydrogen vs. alkyl groups in reactions of dialkylzinc compounds and bis(oxazolinyl)borane

Article abstract of DOI:10.1039/c1cc16113j

An ambiphilic bis(oxazolinyl)borane proligand and zinc dialkyls react via alkyl group transfer or β-hydrogen abstraction. The latter process is favored by formation of a bis(oxazolinyl)borane-zinc adduct that positions a β-hydrogen in the proximity of the Lewis acid center.

Full text of DOI:10.1039/c1cc16113j

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COMMUNICATION
Abstraction of b-hydrogen vs. alkyl groups in reactions of dialkylzinc
compounds and bis(oxazolinyl)boranew
Jing Zhu, Debabrata Mukherjee and Aaron D. Sadow*
Received 30th September 2011, Accepted 22nd October 2011
DOI: 10.1039/c1cc16113j
An ambiphilic bis(oxazolinyl)borane proligand and zinc dialkyls
react via alkyl group transfer or b-hydrogen abstraction. The latter
process is favored by formation of a bis(oxazolinyl)borane-zinc
adduct that positions a b-hydrogen in the proximity of the Lewis
acid center.
a main group alkoxide to the electrophilic carbon of a coordinated
carbonyl.5
Interactions of borane-containing ligands with organo-
metallics are interesting in this context, and recent studies of
ambiphilic ligands in organometallic chemistry have described
M-B bond formations as well as alkyl group abstractions.6 In
the chemistry of oxazolinylboranes, PhB(OxMe2)2 (OxMe2
=
Reactions of hydrocarbyl metal compounds and Lewis acids such
as tris(perfluorophenyl)borane typically proceed by anionic group
abstraction to form cationic or zwitterionic metal complexes.1
These reactions have been studied primarily with b-hydrogen-free
groups (H, Me, CH2Ph), perhaps because transition-metal alkyls
containing b-H often undergo elimination. In contrast, main
group organometallics including Et2Zn undergo b-elimination
only under unusual photolytic or strongly reducing conditions.2
Therefore, the interaction between a Lewis acid and a main group
metal hydrocarbyl could provide insight into the nucleophilic sites
of metal alkyls. These reactions could also clarify pathways
concealed under catalytic conditions. For example, olefin
polymerization systems contain Lewis acids, main group organo-
metallics (e.g., Et2Zn), and metal-alkyl chains containing
b-hydrogen.3 While unsaturated polymer end groups are typically
associated with b-elimination, b-abstraction also provides olefinic
by-products and could be important in polymerization processes.
In fact, B(C6F5)3 or [Ph3C][B(C6F5)4] react with main group
organometallic compounds containing b-hydrogen through
at least two pathways.4 Et2Zn and B(C6F5)3 react to give
EtZn(m-Et)B(C6F5)3, but treatment with [Ph3C][B(C6F5)4]
gives Ph3CH, C2H4, and [EtZn][B(C6F5)4].4a Sterics may affect
b-hydrogen vs. alkyl group abstraction; thus (k2-bpzmp)AlEt2
(bpzmp = bis(pyrazolyl)methane) and B(C6F5)3 react by ethyl-
group abstraction while the isobutyl aluminum compound gives
isobutene and k2-(bpzmp)Al(i-Bu)HB(C6F5)3.4b The b-abstrac-
tions are not limited to alkyls, and a zinc amide and B(C6F5)3
react via b-hydrogen abstraction.4d Furthermore, the proposed
mechanisms of Meerwein-Pondorff-Verley reductions and
Oppenhauer oxidations involve transfer of a b-hydrogen from
bis(4,4-dimethyl-2-oxazolinyl)) and AlMe3 react to form
methide-abstracted {k2-PhMeB(OxMe2)2}AlMe2,7 containing
a bis(oxazolinyl)borate ligand.8 Here we report contrasting
reactions of organozinc compounds and PhB(OxMe2
) that
2
involve alkyl group or b-hydrogen abstraction and strategies
to control the relative rates of the two pathways.
Reaction of b-hydrogen-free ZnMe2 and PhB(OxMe2)2 gives
{k2-PhMeB(OxMe2)2}ZnMe [1; eqn (1)]. The 1H NMR spectrum
of Cs-symmetric 1 contained a ZnMe singlet at ꢀ0.18 ppm and a
broad BMe signal at 0.94 ppm. The latter resonance was
unambiguously assigned by a crosspeak in a 1H–11B HMBC experi-
ment that correlated with a 11B NMR resonance at ꢀ16.7 ppm.
The C2v-symmetric structural isomer Me2B(OxMe2)2ZnPh is
not observed nor is ZnMe/BMe exchange detected in an
EXSY experiment, suggesting that B-Me bond formation is
irreversible. The spectroscopic data is consistent with 1 as a
monomeric species containing a three-coordinate zinc center
because dimeric [{PhMeB(OxMe2)2}Zn(m-Me)]2 would likely
be a mixture of diastereomers.
ð1Þ
This assignment is further supported by DOSY experiments.9
Similar diffusion coefficients of 1 (8.27 ꢁ 10ꢀ10 m2 sꢀ1, 296 K)
and crystallographically characterized monomeric ToMZnMe10
(7.45 ꢁ 10ꢀ10 m2
s
ꢀ1, 296 K) suggest similar hydrodynamic
radii for the two compounds and that 1 is monomeric.
Reactions of PhB(OxMe2
)
and Ph2Zn or Bn2Zn
(Bn = CH2Ph) in benzene provide the expected products 2
and 3 [eqn (1)]. However, the reaction of PhB(OxMe2
and
2
Department of Chemistry, Iowa State University, Ames, IA 50011,
USA. E-mail: sadow@iastate.edu; Fax: +1 515 294 0105;
Tel: +1 515 294 8069
w Electronic supplementary information (ESI) available: Synthesis
and characterization of oxazolinylboratozinc compounds 1–3, 5, 7,
9, 10, and Et2Zn(DPE) and details for DOSY experiments. See DOI:
10.1039/c1cc16113j
)
2
Et2Zn in benzene-d6 gives a mixture of ethylborate and
hydridoborate compounds after 1 day (Fig. 1). The 11B
NMR spectrum of the reaction mixture contained a singlet
at ꢀ15.1 ppm and a doublet at ꢀ19.1 ppm (1JBH = 88 Hz).
c
464 Chem. Commun., 2012, 48, 464–466
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Table 1 Reactant concentration and alkyl : hydridoborate product ratioa
Reactant concentration (mM)
[4] : [5]b
10.1
5.3
2.7
1.8
4.2 : 1
3.3 : 1
2.9 : 1
2.1 : 1
a
Effect of concentration on the ratio of [Ph(Et)B(OxMe2)2ZnEt]
(4) : [Ph(H)B(OxMe2)2ZnEt] (5) formed in the reaction of PhB(OxMe2
)
2
b
and Et2Zn in benzene-d6. The ratio was measured by integration of
1
the H NMR spectrum of the reaction mixture.
group abstraction or b-hydrogen abstraction. First, we verified
that alkyl or hydride group abstraction is irreversible. Dissolution
of a solid mixture of 4 and 5 (formed in benzene-d6) in THF-d8
gives the same mixture in the same ratio as obtained in benzene.
Heating this THF-d8 solution to 80 1C does not change the
ratio of hydridoborate to ethylborate zinc. Additionally, 5 is
unchanged in benzene-d6 at 80 1C for at least 2 days. Although
we do not know which product is thermodynamically favored,
the irreversibility of the reactions suggest that the products
form under kinetic control.
Fig. 1 Effects of benzene versus THF as solvent in reactions of
b-hydrogen-containing dialkylzinc compounds and PhB(OxMe2)2.
Ethylene (5.25 ppm) and two sets of zinc ethyl resonances were
detected in the 1H NMR spectrum, and these data suggested
ethyl-abstracted {PhEtB(OxMe2)2}ZnEt (4) and hydridoborate
{PhHB(OxMe2)2}ZnEt (5) as the products. The former product
is suggested by the pathway described by eqn (1), while the
latter species is independently synthesized (see below) and
apparently forms by transfer of a b-hydrogen from the ethyl
ligand to boron. Mixtures of hydridoboratozinc and alkyl-
borato organozinc compounds are also formed upon treat-
ment of PhB(OxMe2)2 with n-Pr2Zn or i-Bu2Zn (Fig. 1).
In contrast, these reactions are selective for hydridoborate
formation in THF. For example, PhB(OxMe2)2 and Et2Zn give
5 in THF or THF-d8 as the only product detected over 1 day
or isolated (Fig. 1). The 1H NMR spectrum of isolated 5,
recorded in benzene-d6, contained only one set of ethyl
resonances at 0.62 (2 H) and 1.51 ppm (3 H). In a 1H–11B
HMQC experiment, the borohydride resonance at 3.53 ppm
correlated with the 11B NMR doublet at ꢀ19.1 (1JBH = 88 Hz).
These spectra confirm that 5 is one of the two species
generated in benzene. Similar results are obtained upon
The reactants’ concentrations affect the ratio of alkylborate
to hydridoborate products. Table 1 shows that as the concen-
tration of the reactants increases, the relative amount of alkyl
group transfer increases from 2 : 1 to 4 : 1. Qualitative kinetics
experiments show that the rate of reaction also increases as
concentration increases. The change in product ratio indicates
that the alkyl group abstraction rate increases at the expense
of b-hydrogen abstraction. We suggest, then, that alkyl group
abstraction may involve a bi-molecular interaction of PhB(OxMe2
)
2
and Et2Zn whereas hydride abstraction occurs primarily through
an intramolecular rate-determining step that is less affected by
concentration.
Selective hydride abstraction in THF can be rationalized in
the context of this competition between intramolecular and
intermolecular reaction pathways that favors the former.
Coordination of THF to PhB(OxMe2)2 to give (THF)BPh(OxMe2
)
2
inhibits alkyl group transfer during bi-molecular collisions
by coordinating to the Lewis acid site.11 However, coordination
treatment of PhB(OxMe2
)
with n-Pr2Zn and i-Bu2Zn in
2
THF to form {k2-PhHB(OxMe2)2}Zn(n-Pr) (7) and
{k2-PhHB(OxMe2)2}Zn(i-Bu) (9). Thus, zinc dialkyls containing
b-CH groups react with PhB(OxMe2)2 in THF via b-hydrogen
abstraction rather than alkyl group abstraction, whereas
both alkyl group abstraction and b-hydrogen abstraction are
observed in benzene.
of (THF)BPh(OxMe2
) to Et2Zn is not blocked by formation
2
of Et2Zn(THF)2 but instead gives the intermediate
(THF)BPh(OxMe2)2ZnEt2 [bis(oxazolines are better ligands for
Zn(II) than THF]. Upon dissociation of THF to form
BPh(OxMe2)2ZnEt2, intramolecular b-hydrogen abstraction
provides 5 (Fig. 2).
Given these results and b-H abstraction in an aluminum
An alternative pathway, in which a Et2ZnL2 species under-
goes b-hydrogen elimination to give Et(H)ZnL2 followed by
isobutyl,4a the interactions of PhB(OxMe2
) and aluminum
2
alkyls were revisited. However, AlEt3 and PhB(OxMe2
) in
2
benzene or THF provide only the alkyl-abstracted product
{k2-PhEtB(OxMe2)2}AlEt2 [10, eqn (2)].
(2)
Next, these reactions were more closely investigated to
identify the factors that influence the pathway to favor alkyl
Fig. 2 Proposed mechanism for hydridoborate formation.
Chem. Commun., 2012, 48, 464–466 465
c
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Table 2 Effect of diamine ligands on b-hydrogen vs. alkyl group
zinc alkyl group to electrophiles when the electrophile and
alkyl ligand are appropriately positioned. These observations
may provide strategies for controlling hydride and alkyl
transfer processes in synthetic applications.
abstractiona
Reactant
Concentration (mM)
[4] : [5]b
Et2Zn
Et2Zn(TMEDA)
Et2Zn(DPE)
5.3
5.1
5.1
3.3 : 1
4.5 : 1
5.7 : 1
We are grateful to the National Science Foundation (CHE-
0955635) and (CRIF-0946687) for financial support of this
work. We thank KaKing Yan and Richard R. Thompson for
preparation of starting materials and useful discussions. A.D.
S. is an Alfred P. Sloan Fellow.
a
b
Conditions: room temperature in benzene-d6. The ratio was measured
1
by integration of the H NMR spectrum of the reaction mixture.
zinc hydride abstraction, is unlikely for several reasons. First,
diethylzinc itself does not readily b-H eliminate,2 and the higher
coordinate, 18-electron Et2Zn(THF)2 and PhB(OxMe2)2ZnEt2
species that lack open orbitals are even less likely to undergo
b-elimination. Furthermore, the b-elimination intermediate,
Et(H)ZnL2, would likely transfer both hydride and ethyl groups
to boron, whereas only hydrogen abstraction is observed in
THF. Comparison of these mechanism also reveals that olefinic
(or more generally, unsaturated) by-products do not provide
sufficient evidence to distinguish b-hydrogen elimination from
abstraction.
Notes and references
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R. Centore and A. Tuzi, Organometallics, 2006, 25, 266–274;
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M. L. Hlavinka, J. R. Hagadorn and E. Y. X. Chen, J. Am. Chem.
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1961, 26, 290–292; (b) B.-T. Ko, C.-C. Wu and C.-C. Lin, Organo-
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D. Bourissou, Angew. Chem., Int. Ed., 2006, 45, 1611–1614;
(b) M.-H. Thibault, J. Boudreau, S. Mathiotte, F. Drouin,
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The proposed mechanism was further tested by treatment of
PhB(OxMe2)2 with Et2Zn(TMEDA) or Et2Zn(DPE) (TMEDA =
tetramethylethylenediamine; DPE = dipyrrolidine ethane),12
where the diamine zinc starting materials might inhibit formation
of PhB(OxMe2)2ZnEt2. In fact, increased alkyl group transfer
relative to b-H abstraction occurs in the presence of TMEDA
and DPE (Table 2) further supporting the notion that alkyl
borate formation occurs through a bimolecular rate-determining
step. Despite the effect by TMEDA and DPE on the reaction
pathway, [Ph(R)B(OxMe2)2]ꢀ (R = H, Et) are superior ligands for
zinc, and the final zinc products are 4 and 5.
Upon addition of two equiv. of TMEDA or DPE (relative
to Et2Zn), reaction times exceed 2 days in both benzene and
THF. Presumably, the diamines coordinate to PhB(OxMe2
)
2
and Et2Zn. However, the effect of TMEDA is sufficient to give
a trace amount of alkylborate product 4 in THF. Furthermore,
with 2 equiv. of TMEDA in benzene, the hydride abstraction
pathway is greatly suppressed yielding 4 as the major product.
Finally, selective ethyl group transfer from AlEt3 by
PhB(OxMe2
) is consistent with alkyl abstraction following
2
an intermolecular mechanism. Formation of the requisite
{k2-PhB(OxMe2)2}AlEt3 for b-H abstraction is unlikely based
on the smaller ionic radius of four-coordiante Al(III) (0.39 A) vs.
Zn(II) (0.6 A)13 which gives apparent coordinative saturation at
four ligands in these systems.7
7 J. F. Dunne, K. Manna, J. W. Wiench, A. Ellern, M. Pruski and
A. D. Sadow, Dalton Trans., 2010, 39, 641–653.
8 C. Mazet, V. Kohler and A. Pfaltz, Angew. Chem., Int. Ed., 2005,
44, 4888–4891.
´
9 (a) P. S. Pregosin, E. Martınez-Viviente and P. G. A. Kumar,
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and H. Ruegger, Organometallics, 2000, 19, 2551–2555.
10 D. Mukherjee, R. R. Thompson, A. Ellern and A. D. Sadow,
ACS Catal., 2011, 1, 698–702.
Our results show that the b-hydrogen in zinc alkyls have
significant nucleophilicity. These results contrast the lack of
alkylzinc-based b-agostic structures and b-hydrogen elimination
reactions that are typically associated with activated C-H’s.
Furthermore, we show that the selectivity between b-hydrogen
and alkyl group abstraction is not only governed by sterics, but
also by the trajectory by which the electrophilic center encounters
the alkyl ligand. Thus, b-hydrogen is readily transferred from an
11 The 11B NMR spectrum of oligomeric PhB(OxMe2
) in toluene is
2
featureless at room temperature (see ref. 7), but in THF a signal is
observed at ꢀ8.77 ppm for a monomeric THF-borane adduct.
12 (a) J. G. Noltes and J. Boersma, J. Organomet. Chem., 1967, 9, 1–4;
(b) F. F. Blicke and E.-P. Tsao, J. Am. Chem. Soc., 1953, 75, 3999–4002.
13 R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr.,
Theor. Gen. Crystallogr., 1976, 32, 751.
c
466 Chem. Commun., 2012, 48, 464–466
This journal is The Royal Society of Chemistry 2012