Room Temperature Iron-Catalyzed Transfer Hydrogenation and Regioselective Deuteration of Carbon-Carbon Double Bonds

Article abstract of DOI:10.1021/jacs.8b11553

An iron catalyst has been developed for the transfer hydrogenation of carbon-carbon multiple bonds. Using a well-defined β-diketiminate iron(II) precatalyst, a sacrificial amine and a borane, even simple, unactivated alkenes such as 1-hexene undergo hydrogenation within 1 h at room temperature. Tuning the reagent stoichiometry allows for semi- and complete hydrogenation of terminal alkynes. It is also possible to hydrogenate aminoalkenes and aminoalkynes without poisoning the catalyst through competitive amine ligation. Furthermore, by exploiting the separate protic and hydridic nature of the reagents, it is possible to regioselectively prepare monoisotopically labeled products. DFT calculations define a mechanism for the transfer hydrogenation of propene with ⁿBuNH₂ and HBpin that involves the initial formation of an iron(II)-hydride active species, 1,2-insertion of propene, and rate-limiting protonolysis of the resultant alkyl by the amine N-H bond. This mechanism is fully consistent with the selective deuteration studies, although the calculations also highlight alkene hydroboration and amine-borane dehydrocoupling as competitive processes. This was resolved by reassessing the nature of the active transfer hydrogenation agent: experimentally, a gel is observed in catalysis, and calculations suggest this can be formulated as an oligomeric species comprising H-bonded amine-borane adducts. Gel formation serves to reduce the effective concentrations of free HBpin and ⁿBuNH₂ and so disfavors both hydroboration and dehydrocoupling while allowing alkene migratory insertion (and hence transfer hydrogenation) to dominate.

Full text of DOI:10.1021/jacs.8b11553

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Article
Room temperature iron-catalyzed transfer hydrogenation and
regioselective deuteration of carbon-carbon double bonds
Maialen Espinal-Viguri, Samuel Neale, Nathan T. Coles, Stuart A. Macgregor, and Ruth L Webster
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11553 • Publication Date (Web): 05 Dec 2018
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Room temperature iron-catalyzed transfer hydrogenation and
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Maialen Espinal-Viguri, Samuel E. Neale, Nathan T. Coles, Stuart A. Macgregor*† and Ruth L.
Webster
§‡
†
§‡
*
§
§
†
Department of Chemistry, University of Bath, Claverton Down, Bath, United Kingdom, BA2 7AY.
Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, United Kingdom, EH14 4AS.
ABSTRACT: An iron catalyst has been developed for the transfer hydrogenation of carbon-carbon multiple bonds. Using a well-
defined -diketiminate iron(II) pre-catalyst, a sacrificial amine and a borane, even simple, unactivated alkenes such as 1-hexene
undergo hydrogenation within 1 hour at room temperature. Tuning the reagent stoichiometry allows for semi- and complete
hydrogenation of terminal alkynes. It is also possible to hydrogenate amino-alkenes and amino-alkynes without poisoning the catalyst
through competitive amine ligation. Furthermore, by exploiting the separate protic and hydridic nature of the reagents it is possible
to regioselectively prepare mono-isotopically labeled products. DFT calculations define a mechanism for the transfer hydrogenation
n
of propene with BuNH
2
and HBpin that involves the initial formation of an iron(II)-hydride active species, 1,2-insertion of propene
and rate-limiting protonolysis of the resultant alkyl by the amine N-H bond. This mechanism is fully consistent with the selective
deuteration studies, although the calculations also highlight alkene hydroboration and amine-borane dehydrocoupling as competitive
processes. This was resolved by re-assessing the nature of the active transfer hydrogenation agent: experimentally a gel is observed
in catalysis and calculations suggest this can be formulated as an oligomeric species comprising H-bonded amine-borane adducts.
n
Gel formation serves to reduce the effective concentrations of free HBpin and BuNH
2
and so disfavors both hydroboration and
dehydrocoupling while allowing alkene migratory insertion (and hence transfer hydrogenation) to dominate.
1
. Introduction
Westcott’s report using 5 mol% Wilkinson’s catalyst, 1.1
equivalent HBcat and 4-vinylaniline, where the double bond is
reduced and the N,N-diborylated product is obtained. Luo and
Transfer hydrogenation is well explored using metal catalysis;
classic examples include the Meerwein-Ponndorf-Verley reduction
using an aluminium alkoxide and use of Shvo’s Ru-hydride
complex or the chiral Noyori Ru-based catalyst for
enantioselective transfer hydrogenation. Very often these
reactions reduce a carbonyl compound using a simple sacrificial
alcohol. In contrast, using an amine and a borane as the transfer
hydrogenation agents is less common. In this regard, most reactions
9
1
Liu’s Co-catalyzed Z- and E-selective semi-hydrogenation of
1
0
2
3 3
alkynes uses NH ·BH as the hydride source but in this case
3
MeOH or EtOH act as the proton source. More recently El-Sepelgy
reported an Mn-pincer complex for the semi-hydrogenation of
alkynes with a computed catalytic cycle that implies separate
1
1
proton and hydride transfer events, whilst Driess used an Mn-
12
silylene system for a similar series of transformations.
2
rely on dehydrogenation of the amine and borane to release H ,
which then undertakes the reduction. Hydrogenations of this type
with homogeneous Zr, Cr, Re, Ru, Co, Rh, Ni, Pd and Cu catalysts,
Frustrated Lewis Pairs and borate organocatalysts are known for
the reduction of alkenes, alkynes, imines, nitriles, nitro groups,
Catalysis that proceeds via σ-bond metathesis offers the ideal
opportunity to exploit the combined protic/hydridic nature of
amines and boranes for the reduction of unactivated, non-polarized
double bonds and to regioselectively install deuterium. Unlike
standard transfer hydrogenation reactions which often rely on an
alcohol as the sole proton and hydride source, this alternative
method allows for more facile discrimination between positions on
the double bond, particularly when employing simple alkene
substrates.
4
aldehydes and ketones.
It is important to note that the reduction of unsaturated bonds by an
amine and borane that makes direct use of their separate protic and
hydridic natures is not common. Berke and co-workers have
5
exploited the highly polarized nature of vinyl malononitriles or
6
+
−
imines to transfer H and H from ammonia-borane. They are able
We have previously demonstrated the ability of the iron β-
δ+
δ−
13
to deuterate C selectively when employing NH
3
·BD
3
and N/C
diketiminate moiety to effect catalytic alkene hydroboration,
14
when employing ND ·BH . Although catalyst-free, this reaction is
3
3
hydrophosphination
and amine-borane dehydrocoupling
Here we hypothesized that if we could access a
1
5
limited to highly polarized unsaturated systems and deuteration has
reactions.
only been explored with one imine (N-benzylidene aniline) and one
catalytic cycle that exploited iron β-diketiminate’s innate ability to
facilitate both hydride transfer and protonation reactions via σ-bond
metathesis (Scheme 1), we should be able to hydrogenate
unactivated alkenes using an amine and a borane: an unprecedented
transformation in iron catalyzed hydrogenation chemistry. It is
worth noting that several examples of iron-catalyzed hydrogenation
malononitrile
substrate
(2-cyclohexylidenemalononitrile).
Building on the work of Berke, Yang and Du used a chiral
phosphoric acid in the presence of ammonia-borane to undertake
7
enantioselective imine transfer hydrogenation and Braunschweig
has undertaken transfer hydrogenation of iminoboranes using
8
NH
3
·BD
3
and ND
3
·BH
3
.
Metal-catalyzed examples include
2
using H have been reported, including the reduction of carbonyls,
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alkenes, alkynes, nitro groups and α,β-unsaturated systems with H
2
challenging substrates, e.g. a primary amino-alkene or hydroxy-
1
6
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4
5
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5
5
6
pressures ranging from 1 to 30 bar. Important to note is Wolf and
von Wangelin’s use of an Fe(I) β-diketiminate dimer, which fails
to show any activity for the hydrogenation of styrene using 2 bar
alkene, have been hydrogenated, although both were in poor yield
(20% and 10% respectively). There are also only limited examples
of iron-catalyzed transfer hydrogenations that tackle simple C-C
sp or sp bonds : most examples focus on carbonyls or α,β-
2
0
1
7
2
21
22
H
2
;
indicating that we are able to access a different reaction
2
3
mechanism from that of low oxidation state Fe catalysis. To the
best of our knowledge only two examples exist where more
unsaturated systems.
1
1
Hydrophosphination/
Hydroamination
Hydroboration
HBpin
YH2
pinBCH2Si(CH3)3
Si(CH3)4
iPr
iPr N
iPr
iPr
iPr
N
N
N
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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9
0
1
2
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fe
iPr
Fe i
iPr
Pr
H
Bpin
HY
H
H
HY
R
R
HBpin
YH2
protonolysis
insertion
iPr
iPr
iPr
iPr
N
N
N
N
iPr Fe iPr
Fe i
iPr
Pr
H
HY
R
1
=
Transfer hydrogenation
iPr
iPr
N
N
(
this work)
Fe iPr
iPr
selective mono
deuteration by
D-amine
1
TMS
H2N
Si(CH3)4
selective mono
deuteration by
DBpin
H
N
N
H
Bpin
H
Ar
Ar
Fe
R
HN
BpinHN
H2N
protonolysis
N
R
N
N
R
N
Ar
Ar
Ar
Ar
Fe
Fe
H
H
insertion
13
Scheme 1. A hypothetical transfer hydrogenation process based on mechanistic understanding of hydroboration and
14a
24
hydrophosphination /hydroamination.
2
. Substrate Scope
complete. We started by testing transfer hydrogenation across a
range of different unsaturated systems (Table 1). The majority of
reactions are complete within minutes at room temperature using
10 mol% 1 but were left for 1 hour to ensure maximum conversion
Following a short optimization procedure, we found that a primary
amine is the optimum source of protons; to avoid any steric issues
n
we decided to use BuNH
2
as an inexpensive, sacrificial proton
25
for all substrates. There is no reaction in the absence of 1 whilst
source. Reactions using secondary amines proceed but result in a
significantly poorer yield of product. HBpin is the best source of
hydride. Reaction progress can be monitored visually:
instantaneous gelation occurs when the final reagent (HBpin) is
added to the mixture (see ESI). This dissipates as the reactions
proceed and the solution returns to its original viscosity when
an excess of HBpin leads to competitive hydroboration and amine-
borane dehydrocoupling, supporting our computational studies (see
Section 3 and ESI).
n
Table 1. Transfer hydrogenation substrate scope using BuNH
2
and HBpin.
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1
(10 mol%)
1
2
3
n
BuNH (1 eq)
2
Me
Pr
Me
HBpin (1 eq)
i
iPr
N
N
R
R
iPrFe iPr
C D , 1 h, RT
6
6
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
.25 mmol
n
TMS
1
(
- pinBNH Bu)
Entry
Starting Material
Product
Yield (%)a
91
1
2
2a
2a
(28)
(48)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
2a
4
5
6
2b 88
2c 91
2d 81
O
O
Ph
Ph
7b
8c
2e
2f
(46)
80
R
R
9d
R =
H
2g 45
2h 62
1
1
1
0
1
2
4-Me
4-OMe
2i
2j
77
65
4-CF
4-Cl
4-Br
3-Br
4-Ph
3
13
2k (65)
2l (38)
1
1
4
5
2m (80)
16
2n 100
17e
2d 99
2o 98
1
8f
Ph
Ph
Ph
1
2
9
0
2p 97
Ph
2q 95
1g
Bpin
2r
57
2
2
Ph
2h
Ph
2p 68
Ph
Ph
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2
2
3i
4j
Ph
Ph
2s
46
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Ph
Ph
2g 68
2
5i
2t
45
tBu
tBu
n
Reaction conditions: all reactions performed in a J-Young NMR tube. 0.25 mmol alkene/alkyne, 0.25 mmol BuNH
2
, 0.25 mmol HBpin,
, 1 h, RT. All reactions give 100% conversion to product with the exception of results in parentheses (also see Ref
5). Conversion to product is based on uptake of starting material. Yields in bold are isolated yield. Other yields are spectroscopic in the
: loss of product occurred during isolation (vacuum
a
1
2
6 6
0 mol% 1, 0.5 mL C D
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
6 6
presence of an internal standard whereby the product was not separated from C D
b
n
c
n
distillation of product and solvent). 18 h, 60 °C, 0.25 mmol aniline (no H
6.5 h, RT. 16 h, RT. 4 h, 90 °C. 0.5 mmol HBpin, 0.25 mmol BuNH , 4 h, 90 °C. 0.5 mmol HBpin, 0.5 mmol BuNH , 16 h, 90 °C.
2
Bu used). 18 h, 90 °C, 0.25 mmol aniline (no BuNH
2 2
2
used).
d
i
e
f
g
n
h
n
n
j
n
0.125 mmol H
2
N Bu, 0.125 mmol HBpin: maximum yield of product is 50%. 0.5 mmol H
2
N Bu, 0.5 mmol HBpin, 2 h.
The reaction tolerates aliphatic alkenes and functional groups such
as epoxides (2c, Entry 5) along with electron donating (2i, Entry
11), electron withdrawing (2j, Entry 12) and halo-substituted
(Entries 13, 14, 15) styrenes. More challenging terpene natural
products can also be hydrogenated. β-Pinene is reduced at 60 °C
without any evidence for isomerization to α-pinene (Entry 7). The
exo-double bond of valencene is selectively reduced over the endo-
double bond (Entry 8). Other internal alkene substrates such as
trans-β-methylstyrene (Entry 17) proceed more slowly requiring
16 h at RT to go to completion, while α-methylstyrene (Entry 18)
requires heating to 90 °C for 4 h for complete conversion. 2,3-
Dimethylbuta-1,3-diene can be mono-hydrogenated selectively to
form 2,3-dimethylbut-1-ene (2q, Entry 20) or an extra equivalent
of HBpin can be added for dual functionalization to give (2,3-
dimethylbutyl)pinacolborane (2r, Entry 21). Diphenylacetylene
does not stop cleanly at stilbene and therefore two equivalents of
amine and HBpin are needed to generate 1,2-diphenylethane (2p,
mmol allyl benzene with the catalyst loading scaled down to 0.5
mol% and aniline as the proton source.
Based on our hypothetical catalytic cycle (Scheme 1), it should be
possible to hydrogenate aminoalkenes and aminoalkynes without
the presence of sacrificial BuNH . This would also demonstrate a
2
much greater level of functional group tolerance than that reported
using other iron-catalyzed hydrogenation methodologies,
where competing coordination of nucleophilic substituents is often
detrimental to catalysis. This is indeed the case and a wide range of
amine-containing alkenes and alkynes are tolerated (Table 2). Once
again, hydrogenation is complete within minutes at room
temperature and the nitrogen-boron bond that forms is readily
cleaved during work-up, allowing the isolation of the primary
aliphatic amine product in high yield.
n
9
18-19
Entry 22). Terminal acetylenes yield the styrene product when
using HBpin and BuNH
3 and 25), or two equivalents of amine and borane yield
ethylbenzene (Entry 24). The reaction is also amenable to scale-up
n
2
as limiting reagents (2s and 2t, Entries
2
with 82% 2d obtained after 8 h at RT from the hydrogenation of 5
Table 2. Transfer hydrogenation of aminoalkenes and aminoalkynes.
1
(10 mol%)
HBpin (1 eq)
work-up
Y
Y
Y
NHBpin
NH2
NH2
C D , 30 min, RT
6
6
0
.25 mmol
100% Spectroscopic Yield
Isolated Yield (%)
Entry Starting Material
Product
Isolated Yield (%)
NH2
NH2
1
2
3a 92
Ph Ph
Ph Ph
NH2
NH2
3b 89
Ph Ph
Ph Ph
NH2
NH2
3
4
5
3c 80
3b 95
3d 71
NH2
NH2
Ph Ph
Ph Ph
NH2
NH2
6
NH2
NH2
3e 94
Ph Ph
Ph Ph
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7a
8
3f
-
NH2
NH2
1
2
3
4
5
6
7
8
9
NH2
NH2
3g 86
3h 88
3i 82
3j 89
NH2
NH2
9
NH2
NH2
10
11
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
NH2
NH2
NH2
NH2
1
1
2b
3i 50
3k 74
NH2
NH2
3b
NH2
NH2
14c
3l 90
4a 82
TMS
TMS
OH
OH
OH
OH
1
1
5
6a
4b
-
Reaction conditions: all reactions performed in a J-Young NMR tube. 0.25 mmol alkene, 0.25 mmol HBpin, 10 mol% 1, 0.5 mL C
min, RT. All spectroscopic yields are 100%. Not isolated due to volatility, 100% spectroscopic yield based on formation of N–B or O–B
product (see ESI). 0.5 mmol HBpin. 4 : 1 ratio of trans : cis products.
6
D
6
, 30
a
b
c
The system operates for internal aliphatic alkenes (Entries 4-6),
with no observable reduction in rate of reaction, and for volatile
substrates such as allyl amine (3f, Entry 7). Reduction of 2- and 4-
ethynylaniline does not stop cleanly at the vinylaniline: two
equivalents of HBpin are necessary to form the ethylaniline (3i and
3k, Entries 12 and 13). Alcohols can also be used as the proton
source, with 4-allylphenol (4a, Entry 15) and propargyl alcohol
steady decrease in anti-Markovnikov selectivity moving from
electron rich 4-OMe styrene (5d), through to styrene (5e) and
finally almost complete erosion of selectivity when 4-CF styrene
3
(5f) is employed. A similar trend is observed when DBpin is used
(Scheme 4b): aliphatic substrates (6a to 6c) give good
3
Markovnikov-selective mono-deuteration whereas 4-CF styrene
(6f) gives almost a 1:1 mixture of Markovnikov and anti-
Markovnikov product. With the exception of 5h and 6h, substrates
that have the potential to undergo double bond isomerization (e.g.
5a to 5c, 5g, 6a to 6c and 6g), deuterium incorporation at other
positions in the molecule is not observed. The fact that deuteration
of the α, β and γ carbon is observed for 5h and 6h suggests that a
different mechanism, that involves isomerization, is at play. This is
a facet of reactivity that we are currently investigating.
(
4b, Entry 16) readily undergoing reduction at room temperature.
Propargyl alcohol selectively generates allyl alcohol as the product;
this is an intuitive result since there is only one proton per molecule
of substrate. When used as a reagent, allyl alcohol cannot be
hydrogenated to form propanol, presumably the slightly higher pK
of this substrate prevents reactivity.
a
Isotopic labelling has also been carried out (Scheme 2). Based on
our proposed catalytic cycle, an N,N-d -amine should lead to
selective mono-deuteration at the terminal (anti-Markovnikov)
position and DBpin should result in deuteration at the internal
Markovnikov) position of the double bond. If HD is released and
undertakes the reduction there should be no regioselectivity. We
can report that mono-deuteration of aliphatic alkenes is possible
2
(
2
using N,N-d -aniline (Scheme 4a). Double deuteration is not
observed. This is a rare example of selective catalytic mono-
deuteration of a carbon-carbon double bond,26 with most other
examples in the literature furnishing deuteration across the double
2
bond, simply because D is necessary. For aliphatic substrates (5a
to 5c), discrimination between the positions on the double bond is
largely dictated by sterics, unfortunately when styrenes are
employed electronics begin to compete and there is a drop-off in
selectivity for the terminal position (5d to 5f). In fact there is a
Scheme 2. a) Deuterium labelling occurs with good selectivity for
functionalization of the terminal carbon when N,N-d -aniline is
2
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Page 6 of 19
employed. b) DBpin results in a good level of selectivity for
deuteration at the internal position of the double bond.
3. Computational studies
DFT studies were undertaken to model the transfer hydrogenation
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
n
a)
1 (10 mol%)
of propene as a simple model substrate using BuNH
2
and HBpin
HBpin (1 eq)
and the full Fe β-diketiminate precursor, LFeSiMe , 1 (denoted I
3
D
R
R
in the computational study). Calculations employed the BP86
functional with an SDD pseudopotential and basis set on Fe and a
standard double- + polarization basis set on other atoms (BS1) for
initial optimizations and free energy calculations. Energies were
then recomputed with the B3PW91 functional including the effects
of dispersion (D3 parameter set with Becke-Johnson damping),
D NC H (1 eq)
2
6 5
0.25 mmol
C D , RT, 16 h
6
6
8% D
7% D
13% D
O
Ph
5a
93% D
5b
87% D
5c
6 6
C H solvent (PCM approach) and an extended def2tzvp basis set.
92% D
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
This protocol resulted from benchmarking studies on β-H transfer
i
in LFe– Bu for which experimental data have been reported by
83% D
OMe
76% D
63% D
CF3
27
Holland and co-workers (see ESI). Calculations also routinely
assessed stationary points in both the quintet and the triplet spin-
5
states, with the quintet usually being more stable: for example, I
5f
(the quintet form of I) lies 19.4 kcal/mol below I (the triplet form).
3
5d
5e
17% D
24% D
Me
37% D
Selected test calculations on singlet spin-states showed such
species to be even higher in energy (see ESI). As reported
2
8
D2-nH N
HO
previously by Holland and Cundari, the triplet spin-state becomes
energetically relevant for the alkene addition and migratory
insertion steps.
n
91% D
52% D
Me
9
% D 5ga
8% D 5ha
3.1 Defining the nature for the transfer hydrogenation agent.
40% D
Our initial mechanistic hypothesis for transfer hydrogenation is set
out in Scheme 1 and highlights the central role of an LFe–hydride
species and its reaction with alkenes to form an LFe–alkyl. Both
species can engage in a number of potentially competing processes,
in particular, for LFe–alkyl protonolysis with amine is the proposed
alkane release step in transfer hydrogenation, while reaction with
HBpin would lead to hydroboration. Indeed, 1 has been shown to
b)
1 (10 mol%)
DBpin (1 eq)
D
R
R
H NC H (1 eq)
2
6 5
0.25 mmol
C D , RT, 16 h
6 6
93% D
97% D
91% D
O
1
3
Ph
promote both the catalytic hydroboration of alkenes, and the
1
5
6a
3% D
6b
9% D
6c
dehydrocoupling of amine-boranes. An initial assessment of these
7% D
3
n
processes was therefore undertaken at the LFe– Pr primary alkyl
intermediate implicated in our model system, along with the related
n
3% D
OMe
25% D
44% D
CF3
dehydrocoupling of BuH NBHpin that would also result in
2
alkane formation along with H
details).
2
release (see Scheme 3 and ESI for
6d
6e
6f
Bpin
67% D
75% D
Me
56% D
LFe +
H
H
HBpin
H2N
HO
hydroboration
4% D
50% D
G‡ = +14.2 kcal/mol
G = -8.9 kcal/mol
H
nBuH2N-BHpin
LFe
Me
H
LFe
+
9
6% D 6gb
6% D 6hb
- nBuHN=Bpin
H
44% D
H
H
Reaction conditions: all reactions performed in a J-Young NMR
tube. Results are an average of two runs. Left for 16 h to ensure
dehydrocoupling
G‡ = +16.2 kcal/mol
nBuNH
H
LFe +

2
complete conversion. 0.25 mmol alkene, 0.25 mmol amine, 0.25
G = -26.9 kcal/mol
n
a
NH Bu
mmol borane, 10 mol% 1, 0.5 mL C
6
D
6
, 16 h, RT. N,N-d
2
- or O-
Me
Pr
Me i
i
b
Pr
protonolysis
d
1
-substrate (no aniline employed). No aniline employed.
LFe =
N
N
G‡ = +21.2 kcal/mol
G = -21.9 kcal/mol
iPrFeiPr
Scheme 3. Computed energetics (kcal/mol, B3PW91(D3-
BJ,C )/def2tzvp//BP86/BS1 level) for different competing
reactions at LFe– Pr. Where several steps are involved ΔG
6
H
6
n
‡
n
represents the total energy span, relative to LFe Pr and the
appropriate reactants.
The results in Scheme 3 show that, while all three processes are
n
significantly exergonic, hydroboration to form Pr–Bpin and LFe–
H is most accessible kinetically with a barrier of 14.2 kcal/mol, 7.0
n
kcal/mol lower than that for protonolysis with BuNH
2
. This is
13
consistent with alkene hydroboration reported in earlier studies;
however in the present context this result suggests that free HBpin
cannot be present in solution. Instead the implication is that HBpin
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Journal of the American Chemical Society
is sequestered in the presence of amine, for example by formation
tetrameric H-bonded adducts AB2 and AB4 was found to be
increasingly favored, with ΔG = +0.1 kcal/mol and -3.5 kcal/mol
respectively. These values also include a very large, negative,
n
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
of an amine-borane adduct, BuH
2
NBHpin. Dehydrocoupling of
n
n
31
BuH
2
NBHpin at LFe– Pr is also computed to be accessible, with
-1
a barrier of 16.2 kcal/mol .
translational entropy which is known to be over-estimated in free
3
2
energy calculations based on isolated molecules. This indicates
that the formation of oligomeric species, ABn, will be even more
favorable than the computed values imply and so would be
consistent with the presence of the gel seen experimentally. We
therefore propose that oligomeric ABn is the active species in
transfer hydrogenation in this case, and in the following
mechanistic study we will model such a species with dimeric AB2.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 4. Computed energetics (kcal/mol, B3PW91(D3-
BJ,C H )/def2tzvp//BP86/BS1 level) for competing amine-
6 6
borane dehydrocoupling and alkene insertion processes at LFe–
‡
H. Where several steps are involved ΔG represents the total
energy span, relative to LFeH and the appropriate reactants.
Migratory insertion involves a spin-crossover mechanism
between the quintet and triplet surfaces - see text for details.
Having inferred the involvement of amine-borane adducts in the
transfer hydrogenation process, we also found the dehydrocoupling
n
of BuH
2
NBHpin at LFe–H to be very facile (Scheme 4). This is
consistent with the room temperature dehydrocoupling of amine-
boranes reported elsewhere.15 However, dehydrocoupling was also
computed to be more accessible than propene migratory insertion,
implying that the former process should dominate to give amino-
borane and H
hydrogenation, but this would be inconsistent with the observed
selectivity of transfer hydrogenation that clearly indicates free H
2 2
. In principle H could then effect the alkene
2
2
9
is not involved.
Figure 1. Computed structures and free energies of formation for
To resolve these discrepancies we returned to a more detailed
n
n
amine-borane adducts [ BuH NBHpin] (n = 1, 2 and 4). Selected
assessment of the nature of the “ BuH
2
NBHpin” adduct that is
2
n
distances (in Å) are provided for AB2 and these are representative
of the metrics seen in the other adducts. For AB4 free N-H protons
on the edge of the tetramer are highlighted with a tick whereas those
involved in intramolecular H-bonding are marked with a cross.
being formed in solution. As mentioned above, addition of HBpin
to the reaction mixture results in the immediate formation of a
thick, immiscible gel which only slowly dissolves as catalysis
proceeds. A study by NMR spectroscopy gives some evidence for
11
Lewis acid-base adduct formation. The B NMR of 0.11 mmol
HBpin in 0.5 mL THF displays a doublet at 28.4 ppm (J = 178.1
3
.2 Mechanism of catalytic transfer hydrogenation.
n
Hz). Addition of 0.1 mmol H
2
N Bu (no catalyst added) results in
Computed profiles for key steps in the transfer hydrogenation of
instantaneous gelation, complete loss of the HBpin signal and
formation of a new doublet at 20.9 ppm (J = 160.5 Hz), indicating
increased shielding of the boron nucleus by nitrogen, but without
loss of B-H. This is consistent with Lewis acid-base adduct
formation e.g. AB1, AB2, etc., see below. The gel resulted in
5
propene with AB2 mediated by catalyst I are shown in Figure 2.
5
Activation of I is initiated by coordination of AB2 via a BH σ-
interaction (FeH = 1.92 Å, B–H = 1.29 Å) to give the 4-
5
coordinate adduct II (+14.7 kcal/mol). Protonolysis of the
5
1
Fealkyl bond then proceeds via TS(II-III) at +22.4 kcal/mol to
significant line broadening of the H NMR spectrum and therefore
5
form 3-coordinate LFeH, III, with concomitant dehydrocoupling
DOSY analysis to investigate molecular weight was not successful.
After 24 h at room temperature, the gel disappears, the adduct
signal is lost and a singlet associated with the dehydrocoupled
of one amine-borane unit within AB2 to give AB2DHC which
5
features one amino- and one amine-borane moiety. III is the active
n
species in catalysis and incorporates a hydride ligand that is derived
from HBpin. This initiation process has an overall barrier of 22.4
kcal/mol and is exergonic by 22.1 kcal/mol.
product BuHNBpin is observed at 24.7 ppm (Bertrand has shown
that certain amines and boranes can dehydrocouple under catalyst-
3
0
free conditions ) along with return of the HBpin doublet at 28.4
ppm (added in excess).
For the catalytic cycle we focus on the process based on 1,2-
n
insertion to form the primary alkyl intermediate LFe- Pr. The issue
Given these observations, the accessibility of various
n
of the regioselectivity of alkene insertion will be considered below.
[
BuH
(Figure 1). While formation of BuH
to be endergonic by 3.1 kcal/mol, formation of dimeric and
2 n
NBHpin] adducts was also assessed computationally
n
2
NBHpin (AB1) is computed
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Page 8 of 19
N
N
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(
a)
(c)
N
N
N
N
Fe 1.92
2
.06
N
N
H 1.29
Fe 1.76
Fe 1.70
2.24
1.87
Me Si
3
H 1.36
H 1.75
Fe 1.97
Me Si
1.66
H
0.87
B
3
H 1.31
H
B
H
B
A
.03
H
1.33
H
1.59
1
A
H
A
B
A
A
H
A
B
5TS(II-III)
H
A
5TS(VI-III)H
H
1.05
B
A
B
H
+
22.4
-9.6
H
B
H
H
H
+14.7
- SiMe4
H
+
AB2
.0
5
+ AB2
- H2
-
II
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
H
-
20.0
VIH
B
A
B
5
B
A
B
0
-22.1
-
5
5
I
III
A
A
H
H
AB2DHC
AB2DHC
N
N
N
N
Fe
2
Fe
1.61
H
.00
-22.1
-39.2
5
5
III
III
Me Si
3
N
N
N
N
Fe
1.61
Fe
1
.61
(b)
H
H
N
N
N
N
N
N
Fe 1
.65
2.09
1.45
Fe 1.76
2
.30
Fe
N
N
H
H
1.59
1.38
.96 Fe 1.50
1.60
1
N
N
1.37
H
H
H
H
1.45
B
5TS(IV-V)1Pr 1.98
Fe1.78
3III
1.41
A
3IV1Pr
8.7
-5.7
H
-0.4
A
1.50 1.19
H
-
B
5TS(VI-VII)1Pr
+
-
9.3
3V
-18.2
1Pr
N
N
3
-16.2
TS(IV-V)1Pr
H
5IV1Pr
+
N
N
Fe 1.61
3.15
-
14.8
N
N
H
A
H
Fe 1.68
-
22.1
2
.04 Fe 1.52
-
H
2.21
5
-27.5
VI1Pr
B
III
N
N
H
.70
A
H
B
5
H
2.06
1.43
-33.5
1
5
B
A
Fe
V1Pr
H
B
N
N
2.05
1.63
N
N
Fe
H
A
H
1.40
N
N
1
.61
Fe 1.99
5TS(VIII-III)
H
H
H
Fe
2.04
H
H
B
A
A
-44.8
2
.01
H
1.28
5TS(VII-VIII)
-
H
B
B
-54.2
H
A
.03
1
A
5VIII
-55.2
=
N
N
H
N
N
-
59.2
VII
Ar
Ar
B
5
-61.9
III
Ar = 2,6-iPr C H
2
6
3
5
H
N
N
N
N
H
H
H
Fe 1.65
.28
N
N
Fe
1.61
2
B
A
A
B
Fe 1.84
2.06
AB2 =
A
H
A
H
H
B
1.36
H
B
H
H
B
B
A
A
H
H
5
Figure 2. Computed free energy profiles (kcal/mol) for (a) the activation of pre-catalyst I with AB2, (b) catalytic transfer hydrogenation of
5
5
propene at III with AB2, and (c) competing dehydrocoupling of AB2 at III. Data presented are for the quintet spin state except for alkene
migratory insertion, where relevant triplet structures are also shown in grey. Selected distances are provided in Å.
Addition of propene to 5III forms adduct 5IV1Pr from which
migratory insertion can proceed on the quintet surface via TS(IV-
V)1Pr at -5.7 kcal/mol. In this case, however, the equivalent triplet
transition state, TS(IV-V)1Pr, is found to be more stable at -9.3
kcal/mol. Beyond this transition state the subsequent LFe- Pr
intermediate reverts to the quintet state ( V1Pr, = -33.5 kcal/mol). A
dual spin-crossover mechanism33 is therefore in operation for the
migratory insertion, which proceeds (assuming facile spin-
crossover ) with an overall barrier, relative to III, of 12.8
kcal/mol and ΔG = -11.4 kcal/mol. These findings are consistent
with Holland and Cundari’s study of similar alkene insertion
5
3
4
5
3
n
5
28
5
processes. Protonolysis of V1Pr initially proceeds in a similar
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5
5IVR 3/5TS(IV-V)R
+7.3 +12.8 / +16.4
+5.2 +15.7 / +18.1
+4.7 +12.6 / +15.8
+8.4 +14.5 / +11.7
5VR
way to the activation of I, with coordination of AB2 to form adduct
Substrate / Selectivity
Propene (R = 1Pr) / 1,2
Propene (R = 2Pr) / 2,1
Styrene (R = 1St) / 1,2
Styrene (R = 2St) / 2,1
5
5
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
VI1Pr (-27.5 kcal/mol) and proton transfer via TS(VI-VII)1Pr at -
6.2 kcal/mol to release propane. In contrast to I, however, an
amidoborate adduct, VII, is formed in this case (Fe–N = 2.06 Å )
featuring a β-agostic interaction (FeH = 1.84 Å, B–H = 1.36 Å).
β-H transfer via TS(VII-VIII) then yields VIII, in which
AB2DHC is bound through the amide nitrogen (Fe–N = 2.28 Å).
-11.4
-10.2
-11.4
-13.9
5
1
5
5
5
5
5
35
Dissociation of AB2DHC via TS(VIII-III) then reforms III.
For styrene the experimental labelling studies indicate reaction via
the primary metal alkyl (i.e. LFe–CH CH Ph, V1St) is still favored,
The three major processes along the catalytic cycle, migratory
insertion, protonolysis and β-H elimination, have computed energy
spans of 12.8 kcal/mol, 17.3 kcal/mol and 14.4 kcal/mol and this,
along with the exergonicity of each step means that propane release
via protonolysis of VI1Pr through TS(VI-VII) is turnover-
limiting. The modest barrier of 17.3 kcal/mol is consistent with
efficient catalysis proceeding at room temperature. 1,2-insertion
would also be consistent with the experimental labelling studies in
2
2
albeit with a reduced selectivity of around 75% (Scheme 2). This is
3
reflected in a reduced difference of 1.9 kcal/mol between TS(IV-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
5
5
V)1St and TS(IV-V)2St. However, for the 2,1-insertion the quintet
5
transition state, TS(IV-V)2St, at 11.7 kcal/mol is now computed to
3
be 0.9 kcal/mol more stable than TS(IV-V)1St. This implies
reaction via the secondary alkyl and hence a change in selectivity.
Experimentally the precise selectivity is sensitive to electronic
effects with, for example, the reaction of DBpin with 4-CF styrene
3
giving a 56:44 internal:terminal ratio. Moreover, the relative
energies of these triplet and quintet transition states are very
sensitive to the functional employed, and in particular the
percentage of exact exchange. Reproducing precise selectivities is
therefore challenging; however, based on our overall proposed
mechanism, the observations that DBpin labels the internal
that the hydride derived from HBpin adds to the internal carbon,
n
while the proton derived from BuNH
2
adds to the terminal carbon.
5
As noted previously, amine-borane dehydrocoupling at III can
compete with alkene migratory insertion and so this process was
also assessed with AB2 (see Figure 2c). After formation of a BH
5

-bound adduct, VI
H
(-20.0 kcal/mol), dehydrocoupling involves
a concerted protonolysis of the Fe–H bond coupled to hydride
transfer to Fe via TS(VI-III)
5
H 2
(-9.6 kcal/mol) to afford H ,
position, and PhND
these reactions proceed via the primary alkyl.
. Conclusions
2
labels the terminal position clearly indicate
5
AB2DHC and III. The overall barrier for this process is 12.5
27,36,37
kcal/mol, close to that for the migratory insertion of propene (ΔG‡
4
=
12.8 kcal/mol). However, an additional consequence of the
In summary we have developed a remarkably simple yet rapid and
mild method for the hydrogenation of alkenes using an amine and
HBpin as the hydrogen sources. The chemistry tolerates a range of
functional groups, is operational for both alkenes and alkynes,
whilst ‘internal’ proton sources, for example aminoalkenes, can be
employed. Hydroxyalkenes and alkynes can also undergo
hydrogenation if the alcohol functionality is acidic. Selective
mono-deuteration can be undertaken either at the terminal or
internal position simply by varying the deuterating agent.
amine-borane oligomer formation is that the number of N-H
protons available for reaction will be significantly reduced, with the
majority being involved in intramolecular H-bonding. This is
illustrated in Figure 1 for AB4 where only 2 of the 8 protons are
available due to their location on the edge of the aggregate
(highlighted with a tick). With larger oligomers the proportion of
free N-H bonds will drop further. Thus not only does adduct and
oligomer formation remove free HBpin from solution (and so avoid
hydroboration) it also has the effect of reducing the effective
concentration of the amine, allowing the alkene migratory insertion
Experimental observations and DFT calculations suggest that
n
transfer hydrogenation reactions using BuNH
2
and HBpin involve
(and thus transfer hydrogenation) to dominate.
an oligomeric gel comprised of amine-borane adducts linked via
intermolecular H-bonding. Calculations using an amine-borane
dimer to model the transfer hydrogenation reagent for propene
define a mechanism involving formation of an Fe-hydride active
species, 1,2-insertion and rate-limiting protonolysis of the resultant
primary alkyl. This mechanism is consistent with the observed
3
.3 Regioselectivity studies
The computed catalytic cycle in Figure 2b indicates that alkene
5
insertion to form V1Pr is irreversible, as the onward reaction via
5
TS(VI-VII)1Pr, is more accessible (by 6.9 kcal/mol) than the
5
reverse β-H transfer to reform IV1Pr. Similar patterns were
computed for the alternative pathway involving the 2,1-migratory
insertion of propene and the reaction of the resultant secondary
2
deuteration regioselectivities with DBpin and PhND . The
formation of the oligomeric gel is important in the chemoselectivity
of this system as it serves to reduce the concentrations of both free
HBpin and active N-H bonds, disfavoring the competing
hydroboration and amine-borane dehydrocoupling reactions. It is
also clear that this transfer hydrogenation system presents
opportunities for expansion of the substrate scope, for both
hydrogenation and deuteration, along with enantiocontrol and these
are avenues we are currently investigating.
i
5
metal-alkyl, LFe– Pr ( V2Pr), as well as for the analogous reactions
with styrene (see ESI for full details). The regioselectivity of
transfer hydrogenation can therefore be understood by considering
the energies of the various migratory insertion transition states
TS(IV-V).
Computed free energies for the stationary points for the migratory
5
insertions of propene and styrene with III are provided in Table 3.
For propene a clear kinetic preference for 1,2-insertion to form
n
3
LFe– Pr is seen, with TS(IV-V)1Pr at 12.8 kcal/mol computed to
ASSOCIATED CONTENT
Supporting Information
3
be 2.9 kcal/mol more stable than TS(IV-V)2Pr. Both quintet
transition states are less accessible. This result is consistent with
highly selective (> 90%) deuteration studies with a range of
Analysis data and NMR spectra for all products is provided in the
supporting information (PDF file). A separate computational
supporting information (PDF file) is also provided which contains
all data relevant to the computational studies reported here,
including methodology, benchmarking studies, energetics and
coordinates of characterized stationary points. In addition a
separate file (.xyz file) contains the coordinates of all the species
discussed in this publication, readable by various molecular viewer
n
aliphatic alkenes in which reactions with DBpin and BuND
2
label
the internal and terminal positons respectively (cf. Scheme 2).
Table 3. Computed free energies (kcal/mol) of stationary points
associated with the 1,2- and 2,1-migratory insertion of propene and
5
5
styrene at LFeH, III. Data are quoted relative to III and the free
alkene.
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Journal of the American Chemical Society
Page 10 of 19
programs. The Supporting Information is available free of charge
Tandem
hydrogenation mediated by [Pd(NHC)(PR
heterocyclic carbene) catalysts. Chem. Commun. 2013, 49,
005; i) Erickson, K. A.; Stelmach, J. P. W.; Mucha, N. T.;
Waterman, R., Zirconium-Catalyzed Amine Borane
Dehydrocoupling and Transfer Hydrogenation.
Organometallics 2015, 34, 4693; j) Pagano, J. K.; Stelmach,
J. P. W.; Waterman, R., Cobalt-catalyzed ammonia borane
dehydrocoupling and transfer hydrogenation under aerobic
conditions. Dalton Trans. 2015, 44, 12074; k) Shao, Z.; Fu,
S.; Wei, M.; Zhou, S.; Liu, Q., Mild and Selective Cobalt-
Catalyzed Chemodivergent Transfer Hydrogenation of
Nitriles. Angew. Chem. Int. Ed. 2016, 55, 14653; l) Zhou, Q.;
Zhang, L.; Meng, W.; Feng, X.; Yang, J.; Du, H., Borane-
Catalyzed Transfer Hydrogenations of Pyridines with
Ammonia Borane. Org. Lett. 2016, 18, 5189; m)
Korytiakova, E.; Thiel, N. O.; Pape, F.; Teichert, J. F.,
Copper(i)-catalysed transfer hydrogenations with ammonia
borane. Chem. Commun. 2017, 53, 732.
ammonia
borane
dehydrogenation/alkene
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
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on the ACS Publications website.
AUTHOR INFORMATION
Corresponding Author
3
)] (NHC = N-
1
*
*
S.A.Macgregor@hw.ac.uk
R.L.Webster@bath.ac.uk
Author Contributions
‡ MEV and NTC contributed equally.
Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENT
The EPSRC is thanked for funding and the EPSRC UK National
Mass Spectrometry Facility at Swansea University is thanked for
mass spectrometry analyses.
(
5) Yang, X.; Fox, T.; Berke, H., Facile metal free regioselective
transfer hydrogenation of polarized olefins with ammonia
borane. Chem. Commun. 2011, 47, 2053.
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Complexes
with
Chiral
Bidentate
Phosphines.
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bar H
2 3 4
with a FeCl -LiAlH catalyst. Green Chem. 2015, 17,
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015, 2, 692; h) Xu, R.; Chakraborty, S.; Bellows, S. M.;
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1
1
(25) B NMR of the crude reaction mixture shows a small amount
of competing hydroboration. For example, for the
hydrogenation of allylbenzene using HBpin and nBuNH
2
gives ~6% Ph(CH
Ph(CH Bpin and PhND
(also see Scheme 2).
2
)
3
Bpin, using PhNH
2
instead gives ~12%
CHDCH Bpin
2
)
3
2
gives ~8% PhCH
2
2
operationally simple formal hydrogenation using Fe(OTf)
and NaBH . Org. Biomol. Chem. 2014, 12, 5082.
20) Morris, R. H., Mechanisms of the H - and transfer
3
4
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(
2
hydrogenation of polar bonds catalyzed by iron group
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21) a) Nishiguchi, T.; Fukuzumi, K., Homogeneous transfer-
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hydrogenolysis.
Cyclooctadienes
I.
Transfer-hydrogenation
Catalyzed
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(28) Bellows, S. M.; Cundari, T. R.; Holland, P. L., Spin
a new layer of reactive amine-borane N-H bonds to engage
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Crossover during β-Hydride Elimination in High-Spin
Iron(II)– and Cobalt(II)–Alkyl Complexes. Organometallics
in catalysis, should be accessible.
(36) Dias, R. P.; Prates, M. S. L.; De Almeida, W. B.; Rocha, W.
R., DFT Study of the Ligand Effects on the Regioselectivity
of the Insertion Reaction of Olefins in the Complexes
2
013, 32, 4741.
n
(
29) Moreover, hydrogenolysis of the putative LFe– Pr
intermediate to form LFe–H and propane is computed to have
a barrier of 28.3 kcal/mol.
2 3
HRh(CO) (PR )(L) (R = H, F, Et, Ph, OEt, OPh, and L =
Propene, Styrene). Int. J. Quantum Chem 2011, 111, 1280.
(37) We also examined the mechanism of transfer hydrogen of
alkenes with aniline and HBpin. In this case no evidence for
an amine-borane adduct was seen experimentally and no
minimum corresponding to such a structure could be located
computationally. Transfer hydrogenation is therefore likely
to involve the free aniline and HBpin in this case.
Protonolysis of the key LFe-propyl intermediate by aniline
was computed to involve a transition state at +17.2 kcal/mol.
(
30) Romero, E. A.; Peltier, J. L.; Jazzar, R.; Bertrand, G.,
Catalyst-free dehydrocoupling of amines, alcohols, and thiols
with pinacol borane and 9-borabicyclononane (9-BBN).
Chem. Commun. 2016, 52, 10563.
31) The structures of AB2 and AB4 reflect the importance of
intramolecular hydrogen bonding involving the Bpin moiety.
Calculations to locate an alternative form of AB2 based on
dihydrogen bonding suggest that such structures are not local
minima.
(32) Grimme, S.; Schreiner, P. R., Computational Chemistry: The
Fate of Current Methods and Future Challenges. Angew.
Chem. Int. Ed. 2018, 57, 4170.
33) Sun, Y.; Tang, H.; Chen, K.; Hu, L.; Yao, J.; Shaik, S.; Chen,
H., Two-State Reactivity in Low-Valent Iron-Mediated C–H
Activation and the Implications for Other First-Row
Transition Metals. J. Am. Chem. Soc. 2016, 138, 3715.
(
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This is significantly lower than the equivalent reaction with
n
2
BuNH , (+21.2 kcal/mol, see Scheme 3) but remains less
accessible than hydroboration with HBpin (+14.2 kcal/mol).
Hydroboration is observed as a side reaction in the
aniline/HBpin transfer hydrogen system (see Ref 25),
however, the accessibility of this process is clearly over-
estimated in the current model. This overall conclusion was
not significantly affected by the choice of function (see ESI).
(
(34) Harvey, J. N., Spin-forbidden reactions: computational
insight into mechanisms and kinetics. Wiley Interdisciplinary
Reviews: Computational Molecular Science 2013, 4, 1.
(
35) Release of amino-borane from AB2DHC has a computed ΔG
of +0.6 kcal/mol, suggesting that this process, that will reveal
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via
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from DBpin
N
N
Fe i
R
iPr
Deuterium
from D-amine
Pr
TMS
H
R
H
R
HBpin
+
H2NR



Mild RT
Rapid 1 hour
Broad substrate scope >40 examples
Alkenes, Alkynes, Aminoalkenes, Aminoalkynes
Selective Mono-deuteration
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Mechanistic understanding
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Scheme 1. A hypothetical transfer hydrogenation process based on mechanistic understanding of
hydroboration13 and hydrophosphination14a/hydroamination.24
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Scheme 2. a) Deuterium labelling occurs with good selectivity for functionalization of the terminal carbon
when N,N-d -aniline is employed. b) DBpin results in a good level of selectivity for deuter-ation at the
2
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Scheme 3. Computed energetics (kcal/mol, B3PW91(D3-BJ,C H )/def2tzvp//BP86/BS1 level) for different
6
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competing reactions at LFe– Pr. Where several steps are involved ΔG represents the total energy span,
n
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Scheme 4. Computed energetics (kcal/mol, B3PW91(D3-BJ,C6H6)/def2tzvp//BP86/BS1 level) for competing
amine-borane dehydrocoupling and alkene insertion processes at LFe–H. Where several steps are involved
ΔG‡ represents the total energy span, relative to LFe–H and the appropriate reactants. Migratory insertion
involves a spin-crossover mechanism between the quintet and triplet surfaces - see text for details.
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n
Figure 1. Computed structures and free energies of formation for amine-borane adducts [ BuH NBHpin] (n
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n
= 1, 2 and 4). Selected distances (in Å) are provided for AB2 and these are representative of the metrics
seen in the other adducts. For AB4 free N-H protons on the edge of the tetramer are highlighted with a tick
whereas those involved in intramolecular H-bonding are marked with a cross.
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Figure 2. Computed free energy profiles (kcal/mol) for (a) the activation of pre-catalyst 5I with AB2, (b)
catalytic transfer hydrogenation of propene at 5III with AB2, and (c) competing dehydrocoupling of AB2 at
5III. Data presented are for the quintet spin state except for alkene migratory insertion, where relevant
triplet structures are also shown in grey. Selected distances are provided in Å.
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