Chemistry - A European Journal
10.1002/chem.201602818
COMMUNICATION
alkynes such as phenylacetylene, but in contrast benzonitrile
reacts with two equivalents of HBpin to give the N,N-diborated
benzylamine (Table 1, Entry 14).
Returning to the aliphatic substrates reported in Figure 1, use of
1b also allows for an improvement in reactivity compared to that
obtained with 1a (Table 2). This enhanced reactivity is acutely
observed for 2,3-butadiene, valencene and β-pinene (Entries 1,
In summary, we have developed a new catalytic system for the
HB of alkenes using HBpin and HBcat that does not need
exogenous reducing agents or activators and can be undertaken
with a strict 1:1 ratio of reagents. The chemistry has been
extended beyond classical substrates and includes natural
products, vinyl arenes and alkynes. RPKA demonstrates that the
reaction is first order in substrates and catalyst, and
3, 4) where the reactions are between five and eight times faster. stoichiometric studies provide evidence for a catalytic cycle that
Remarkably, although α-pinene shows no reactivity with 1a, 80%
conversion is observed after 16 h at 90 °C with 1b, however, this
occurs with concomitant isomerization to form 2n, a reaction
previously only reported by Chirik using cobalt catalysis.[14d]
is likely to proceed via an iron hydride. Reactivity of styrenes
demonstrates that a subtle change in ligand structure can lead
to a vast change in regioselectivity, whilst alkynes show much
improved reactivity with this change in pre-catalyst. Double HB
of diphenylacetylene is possible and gives the geminal
dipinacolborane product.
Table 2: Improved reaction conditions and yields of aliphatic substrates are
also obtained when 1b is employed as a pre-catalyst.
Entry Product Conditions to obtain 100% spectroscopic yield
Acknowledgements
1a (5 mol%)
RT, 36 h
1b (5 mol%)
RT, 7 h
We thank the EPSRC UK National Mass Spectrometry Facility at
Swansea University for MS analysis and the EPSRC for funding
(EP/M019810/1).
1
2
3
4
5
2e
2g
2l
60 °C, 7 h
60 °C, 16 h
60 °C, 16 h
RT, 16 h
60 °C, 2.5 h
60 °C, 2 h
60 °C, 2.5 h
RT, 16 h
Keywords: iron • homogeneous catalysis • hydroboration •
alkenes • alkynes
2n
2p
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Conditions: 5 mol% 1a or 1b, 0.4 mL C6D6. Spectroscopic yield obtained by
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as the acidic proton of phenylacetylene results in loss of
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1
by H NMR, but this is not inconceivable; three coordinate iron
hydrides are incredibly reactive and their isolation or even
detection is not trivial.[20] Although we have already shown that
catalyst initiation for such transformations is likely to be radical
mediated,[7b] addition of radical trap (iodomethyl)cyclopropane,
to the reaction of 1-hexene, HBpin and 5 mol% 1a after 30
minutes (16% product formed) results in reaction quenching. To
support our proposed reaction mechanism Reaction Progress
Kinetic Analysis studies were undertaken.[10] No catalyst
deactivation or product inhibition is detected and the reaction is
determined to be first order in 1a, HBpin and 1-hexene,[13] thus
supporting our postulated mechanism.
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