Catalytic hydroboration by an imido-hydrido complex of Mo(iv)

Article abstract of DOI:10.1039/c1cc14508h

The imido-hydrido complex (ArN)Mo(H)(Cl)(PMe₃)₃ catalyses a variety of hydroboration reactions, including the first example of catalytic addition of HBCat to nitriles to form the bis(borylated) amines RCH₂N(BCat)₂

Full text of DOI:10.1039/c1cc14508h

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Catalytic hydroboration by an imido-hydrido complex of Mo(IV)w
Andrey Y. Khalimon,a Philip Farha,a Lyudmila G. Kuzminab and Georgii I. Nikonov*a
Received 24th July 2011, Accepted 25th October 2011
DOI: 10.1039/c1cc14508h
The imido-hydrido complex (ArN)Mo(H)(Cl)(PMe3)3 catalyses
a variety of hydroboration reactions, including the first example
of catalytic addition of HBCat to nitriles to form the
bis(borylated) amines RCH2N(BCat)2. The latter species easily
(styrene, 3-hexyne, and phenylacetylene) in the presence of 1
(5 mol%) affords the boro-substituted alkanes and alkenes,
respectively (Table 1, entries 5–7); albeit the 1-catalysed reaction
with styrene also gives large amounts of trans-PhCHQCHB(Cat)
and ethylbenzene. In contrast, 1 showed reduced or no cata-
lytic activity in the hydroboration of 1-hexene, cyclohexene,
a-methylstyrene, 1-octyne and PhCRCHCH3. The last but
not the least, the hydroboration of nitriles (MeCN and PhCN)
catalysed by 1 (5 mol%) leads to products of double addition
of HBCat across the CRN bond, RCH2N(BCat)2 (Table 1,
entries 8 and 9).
undergoes chemoselective coupling with aldehydes R0C(O)H to
0
Q
yield imines RCH2N C(H)R .
Hydroboration of unsaturated substrates is a reaction of
immense importance for organic chemistry.1,2 Whereas, alkyl
and aryl boranes add to multiple C–C, C–O, and C–N bonds
easily, the hydroboration by deactivated boranes, such as
HBCat (Cat = catechol), calls for the application of transition
metal catalysis, which to date is mostly done by late transition
metals (Rh, Ir etc).2,3 The latter are notorious for their high
cost and toxicity. While early metals are usually both cheaper
and more environmentally benign, only a few catalytic examples
are known, and those are mostly limited to Ti and Zr.4
Stoichiometric reactions of HBCat with olefins have been
shown for Nb and Ta metallocene complexes.5 Much less is
known about catalytic hydroboration of carbonyl derivatives,6,7
with no reported examples for esters and nitriles.8
Since nitriles react faster than ketones and alkynes we also
tried polyfunctional compounds. Hydroboration of acrylonitrile,
3-(2-oxocyclohexyl)propanenitrile, 4-acetyl-benzonitrile, and
a mixture of PhCN/Ph2C(O) (1 : 1) was not chemoselective.11
In contrast, addition of HBCat to 5-hexyne-nitrile occurs
selectively on the alkyne moiety, leaving the nitrile group
unreacted.11
Importantly, the products of nitrile hydroboration,
RCH2N(Bcat)2, easily react with aldehydes to give imines
RCH2NQCHR0. Taken together, these novel hydroboration
and coupling reactions constitute a useful synthetic transfor-
mation of nitriles to imines.12 This reaction is remarkable in that
it proceeds chemoselectively with aldehydes but not with ketones.
In order to elucidate the mechanism of nitrile hydroboration,
we studied the stoichiometric reactivity of 1.13 Addition of
nitriles to 1 results in the methylenamide derivatives trans-
(ArN)Mo(Cl)(NQCHR)(PMe3)2 (2–5; Scheme 1). It is note-
worthy that unlike catalytic reactions (vide supra) insertion of
the CRN bond into the Mo–H bond is chemoselective,
tolerating ketone and non-conjugated alkene functionalities.
We have recently reported catalytic and mechanistic studies
on hydrosilylation reactions mediated by the imido-hydrido
complex (ArN)Mo(H)(Cl)(PMe3)3 (1; Ar = 2,6-iPr2C6H3),
including one of the first examples of nitrile hydrosilylation.9
Believing that HBCat may exhibit even superior reactivity
patterns due to its enhanced Lewis acidity, we elected to study
the hydroboration catalysed by 1. Here we report the results of
these efforts, including an insight into the mechanism and
observation of unusual B–Hꢀ ꢀ ꢀM agostic and borylimino
intermediates.
Complex 1 shows catalytic activity in a diversity of hydro-
boration processes (Table 1). Thus, ketones (iPr2C(O),
Ph2C(O), PhC(O)Me10), and esters, MeC(O)OEt, are easily
converted to the corresponding boryl ethers (Table 1, entries
1–4). Addition of HBCat to alkenes4b–g,5 and alkynes4a,h–i
Table 1 Catalytic hydroboration with HBCat mediated by 1a
Entry
Substrate
Product(s)
t, h
Yield, %b
1
2
3
4
5
iPr2C(O)
Ph2C(O)
PhC(O)Me
MeC(O)OEt
PhCHQCH2
(iPr)2CH(OBCat)
Ph2CH(OBCat)
PhCH(OBCat)Me
EtOBCat
PhCH2CH2BCat
PhCHQCHBCat
PhCH2CH3
24
24
24
24
20
91
100
99
100
32
a Chemistry Department, Brock University, 500 Glenridge Ave., St.
Catharines, L2S 3A1, ON, Canada. E-mail: gnikonov@brocku.ca;
Tel: +1 905 6885550, ext 3350
53
15
b Kurnakov Institute of General and Inorganic Chemistry RAS,
Leninskii Prosp. 31, 119991, Moscow, Russia.
E-mail: kuzmina@igic.ras.ru
6
7
8
9
3-hexyne
PhCRCH
MeCN
EtCHQC(Et)BCat
PhCHQCHBCat
EtN(BCat)2
24
20
12
12
94
99
100
100
PhCN
PhCH2N(BCat)2
w Electronic supplementary information (ESI) available: Experimental
details, full table for catalytic hydroboration reactions. CCDC 836951.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c1cc14508h
a
Conditions: 5 mol% of 1, 22 1C, C6D6, substrate/HBCat = 1 : 1
(1 : 2 ratio for entries 4, 8 and 9), Csubst = 0.4 M. NMR yields.
b
c
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Chem. Commun., 2012, 48, 455–457 455

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stoichiometric reactions of 1 with 4-acetylbenzonitrile and
4-formylbenzonitrile to give 5 and 7, respectively.16
The reaction of 3 with HBCat was followed by NMR
spectroscopy at low temperature. At ꢁ30 1C, the formation
of a mixture of two bis(phosphine) compounds was observed.
One of the products has a Cs symmetric NMR structure
with two equivalent PMe3 ligands giving rise to a singlet at
Scheme 1 Preparation of methylenamide complexes of Mo.
1
ꢁ0.9 ppm in 31P NMR. H NMR revealed a downfield imine
Control reactions of 1 with 1 : 1 mixtures of PhCN and ketones
(acetone, acetophenone, and cyclohexanone) result in the
exclusive formation of 3.14 In contrast, reactions of 1 with
acrylonitrile and 4-formylbenzonitrile afford complexes 6 and
7, respectively.
signal at 8.92 ppm, coupled in 1H–13C HSQC to the 13C NMR
signal at 172.0 ppm. Also, 11B NMR showed the presence of
a 4-coordinate boron centre exhibiting a doublet at 2.2 ppm
(vs. 29 ppm for HBCat) with reduced B–H coupling (JB–H
E
55 Hz).17 These spectroscopic features indicate an agostic
borane structure tentatively formulated as the amido-borane
adduct (ArNQ)Mo(Cl){k3-N(QCHPh)(CatB–Hꢀ ꢀ ꢀ)}(PMe3)2
(9; Scheme 2).18
The second product (10) in the mixture is produced from 9
upon gentle increase of temperature. However, all attempts to
find a temperature regime for the full conversion of 9 were
unsuccessful. The 1H NMR spectrum of 10 at ꢁ50 1C shows a
downfield imine signal at 8.31 ppm (s, coupled in 1H–13C
HSQC NMR to the 13C NMR signal at 154.2 ppm) and a
broad upfield hydride resonance at ꢁ2.94 ppm. The two non-
equivalent PMe3 groups give rise to two mutually coupled
doublets at ꢁ1.4 ppm and ꢁ13.0 ppm in the 31P NMR
Compounds 2–7 were characterized by spectroscopic methods
(IR, NMR) and X-ray diffraction for 3 (Fig. 1). Complex 3 has
a distorted trigonal bipyramidal structure, with two trans
PMe3 ligands occupying the apical positions. The
Mo1–N2–C13 bond angle is almost linear (172.3(4)1) suggest-
ing that the [NQCHPh] fragment acts as a 4e donor15
stabilizing the 18e valence shell, assuming that the linear imide
ArN2ꢁ (Mo1–N1–C1 175.0(3)1) also donates 6e. Compounds
2–5 and 7 give rise to diagnostic imine proton (7.09–7.43 ppm)
and carbon signals (145.4–153.5 ppm) in their 1H and 13C
NMR spectra, respectively.
2
spectrum, with the large JP–P = 212.0 Hz suggesting trans-
arrangement. The 11B NMR spectrum revealed the presence
of an essentially 3-coordinate boron centre, which gives rise
to a broad signal at 10.2 ppm, not coupled to the hydride
at ꢁ2.94 ppm. All together these features are consistent
with the formation of a k1-(N-boryl)imine derivative (ArN)-
Mo(H)(Cl){k1-N(BCat)QCHPh}(PMe3)2 (10; Scheme 2). The
non-equivalency of phosphines is then explained by the
restricted rotation around the Mo–N bond at ꢁ50 1C.
Interestingly, the addition of PhCN to the methyl derivative
2 leads to a slow (24 h at RT) release of acetonitrile to
form complex 3, indicating a-CH bond activation in the
methylenamide ligand. To the best of our knowledge, such
a reversible nitrile insertion into an early metal–hydride
bond has been previously observed only for complex
Cp*2Sc(NQCHR).15c The possibility of a-CH activation in
the methylenamide ligand was further confirmed by the
reaction of 3 with benzaldehyde, which in the presence of
PMe3 leads to exclusive formation of the benzoxy derivative
(ArN)Mo(Cl)(OBn)(PMe3)3 (8).9 However, no transfer hydro-
genation was observed in reactions of 3 with acetone or
acetophenone even upon heating up to 60 1C. Such a difference
in the reactivity of 3 towards aldehydes and ketones allows
us to explain the difference in chemoselectivity of the
Heating to 25 1C leads to disappearance of 9 and 10 and
formation of (ArN)Mo(H)(Cl){Z2-CatBNQCHPh}(PMe3)2
(11; Scheme 2). The 31P NMR spectrum of this species shows
two mutually coupled doublets at 2.4 and ꢁ5.2 ppm (2JP–P
=
88.5 Hz). The 1H NMR spectrum of 11 revealed an upfield
imine proton at 5.00 ppm (dd, 3JH–P = 3.1 Hz), diagnostic for
the Z2-NQCHPh moiety, which is further supported by a
significant upfield shift of the 13C NMR resonance for the
imine carbon (62.9 ppm, found by 1H–13C HSQC NMR). The
MoH signal, in contrast, is shifted downfield to 7.06 ppm
1
2
(found by H–31P HSQC NMR; JH–P = 45.0 and 50.9 Hz),
suggesting the cis disposition of the hydride and imido ligands,
Fig. 1 Molecular structure of 3 (bond lengths in A, angles in 1).
Hydrogen atoms except H13a are omitted. Mo1–N1 1.761(4),
Mo1–N2 1.843(4), N2–C13 1.279(6), C1–N1–Mo1 175.0(3),
C13–N2–Mo1 172.3(4), P1–Mo1–P2 164.60(5), Cl1–Mo1–N2
122.42(13), N1–Mo1–N2 116.45(17).
Scheme 2 Suggested mechanism for the hydroboration of PhCN.
c
456 Chem. Commun., 2012, 48, 455–457
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as in the parent complex 1.9 The downfield 11B NMR signal at
10.1 ppm indicates a 3-coordinate boron.
R. Booque and F. Maseras, J. Am. Chem. Soc., 1996, 118, 10936;
(i) D. H. Motry, A. G. Brazil and M. R. Smith III, J. Am. Chem.
Soc., 1997, 119, 2743; (j) J. F. Hartwig and C. N. Muhoro,
Organometallics, 2000, 19, 30; (k) Y. D. Wang, G. Kimball,
A. S. Prashad and Y. Wang, Tetrahedron Lett., 2005, 46, 8777.
5 (a) D. R. Lantero, D. L. Ward and M. R. Smith III, J. Am. Chem.
Soc., 1997, 119, 9699; (b) D. R. Lantero, S. L. Miller, J.-Y. Cho,
D. L. Ward and M. R. Smith III, Organometallics, 1999, 18, 235.
6 Ketones: (a) D. A. Evans and A. H. Hoveyda, J. Org. Chem., 1990,
55, 519; (b) C. W. Lindsley and M. DiMare, Tetrahedron Lett.,
1994, 35, 5141; (c) G. Giffels, C. Dreisbach, U. Kragl,
M. Weiderding, H. Waldmann and C. Wandrey, Angew. Chem.,
Int. Ed. Engl., 1995, 34, 2005; (d) A. J. Blake, A. Cunningham,
A. Ford, S. J. Teat and S. Woodward, Chem.–Eur. J., 2000,
6, 3586; (e) I. Sarvary, F. Almqvist and T. Frejd, Chem.–Eur. J.,
2001, 7, 2158; (f) S.-G. Roh, Y.-C. Park, T.-J. Kim and J. H. Jeong,
Polyhedron, 2001, 20, 1961; (g) P. Hegarty, R. Lau and
W. B. Motherwell, Tetrahedron Lett., 2003, 44, 1851;
(h) M. Locatelli and P. G. Cozzi, Angew. Chem., Int. Ed., 2003,
42, 4928; (i) S. G. Roh, J. U. Yoon and J. H. Jeong, Polyhedron,
2004, 23, 2063; (j) L. Koren-Selfridge, H. N. Londino,
J. K. Vellucci, B. J. Simmons, C. P. Casey and T. B. Clark,
Organometallics, 2009, 28, 2085.
Addition of another equiv. of HBCat to 11 does not allow
for the observation of any further intermediates. Only the
release of PhCH2N(BCat)2 and formation of a mixture of 1,
(ArN)MoCl2(PMe3)319 and unknown decomposition products
was observed. How the borylimine part of 11 is reduced into
amine still remains unclear. But it is clear that this last step of a
possible catalytic cycle (Scheme 2) is assisted by HBCat.
On the other hand, 1 reacts with HBCat very sluggishly:
after 24 h at room temperature only B20% conversion of 1 to
19
a mixture of (ArN)MoCl2(PMe3)3 and a highly fluxional
dihydride complex (ArN)MoH2(PMe3)3 (12) was observed by
NMR. No oxidative addition of borane to Mo and formation
of a Mo boryl complex, such as (ArN)Mo(Cl)(BCat)(PMe3)x
(x = 2, 3),20 takes place.
A similar mechanism can be also suggested for the hydro-
boration of carbonyl compounds. Indeed, we found that the
9
reaction of HBCat with (ArN)Mo(Cl)(OBn)(PMe3)3 (8),
7 Imines: R. T. Baker, J. C. Calabrese and S. A. Westcott,
J. Organomet. Chem., 1995, 498, 109.
formed upon the reaction of 1 with PhC(O)H, immediately
regenerates complex 1. For nitriles bearing carbonyl substituents,
the insertion of the CQO and CRN moieties into the Mo–H
bond of 1 becomes competitive in the presence of large excess
of HBCat21 resulting in the loss of chemoselectivity of hydro-
boration under catalytic conditions.
8 For uncatalysed addition of active boranes to nitriles, see:
(a) Y. Chujo, I. Tomita and T. Saegusa, Macromolecules, 1994,
27, 6714; (b) K. Wade, M. G. Davidson, M. A. Fox, W. R. Gill,
T. G. Hibbert and J. A. H. Maceride, Phosphorus, Sulfur Silicon
Relat. Elem., 1997, 124–125, 73; (c) D. Jaganyi and A. Mzinyati,
Polyhedron, 2006, 25, 2730.
9 E. Peterson, A. Y. Khalimon, R. Simionescu, L. G. Kuzmina,
J. A. K. Howard and G. I. Nikonov, J. Am. Chem. Soc., 2009,
131, 908.
In conclusion, complex 1 was found to catalyse a variety of
hydroboration reactions, including the so far unknown cata-
lytic addition of HBCat to nitriles to form bis(boryl) amines.
The latter compounds can be easily converted to imines by the
reaction with aldehydes. The hydroboration of nitriles pro-
ceeds via a series of novel agostic borylamido and borylimino
complexes.
10 Uncatalysed reaction gives only 50% conversion after 2 days.
11 See the Supporting Information.
12 Similar transformations are known for bis(silyl) amines. However,
the reactions require either harsh conditions or the presence of a
catalyst: (a) N. Duffaut and J. P. Dupin, Bull. Soc. Chim. Fr., 1966,
10, 3205; (b) R. J. P. Corriu, V. Huynh, J. J. E. Moreau and
M. Pataud-Sat, J. Organomet. Chem., 1983, 225, 359;
(c) T. Morimoto and M. Sekiya, Chem. Lett., 1985, 1371.
13 For the stoichiometric reactivity of 1 with carbonyls, see ref. 9.
14 No ketone insertion into the Mo–H bond of 1 was observed under
these conditions. However, treatment of 1 with PhCN/cyclohexanone
(1 : 1) leads to a 5 : 1 mixture of 3 and (ArN)Mo(Cl)(OCy)(PMe3)3.
15 (a) H. M. M. Shearer and J. D. Sowerby, J. Chem. Soc., Dalton
Trans., 1973, 2629; (b) G. Erker, W. Fromberg, J. L. Atwood and
W. E. Hunter, Angew. Chem., 1984, 96, 72; (c) J. E. Bercaw,
D. L. Davies and P. T. Wolczanski, Organometallics, 1986, 5, 443;
(d) M. F. C. Guedes da Silva, J. J. R. Frausto da Silva and
A. J. L. Pombeiro, Inorg. Chem., 2002, 41, 219; (e) Y. Tanabe,
H. Seino, Y. Ishii and M. Hidai, J. Am. Chem. Soc., 2000, 122, 1690.
16 The observed chemoselectivity towards the formation of alkoxides
in the reaction of 1 with aldonitriles can be explained in terms of
transfer hydrogenation from 3 because nitriles react faster with 1
than aldehydes (2 h for PhCN vs. 5–6 h for PhC(O)H).
This work was supported by NSERC (DG grant to G.I.N)
and RFBR (grant to L.G.K.). A.Y.K. thanks the OGS for a
student PhD scholarship. G.I.N. further thanks the CFI/OIT
for a generous equipment grant.
Notes and references
1 H. C. Brown, Hydroboration, Wiley, NY, 1962.
2 Selected reviews: (a) L. Tonks and J. M. Williams, Contemp. Org.
Synth., 1997, 353; (b) I. Beletskaya and A. Pelter, Tetrahedron,
1997, 53, 4957; (c) T. Hayashi, in Comprehensive Asymmetric catalysis,
ed. E. N. Jacobsen, A. Pfaltz and H. Yamamoto, Springer, NY, 1999,
vol. 1, p. 349; (d) H. Braunschweig and M. Colling, Coord. Chem.
Rev., 2001, 223, 1; (e) C. M. Crudden and D. Edwards, Eur. J. Org.
Chem., 2003, 4695; (f) W. Carruthers and I. Coldham, Modern
Methods of Organic Synthesis, Cambridge University Press,
Cambridge, 4th edn, 2004, pp. 315–331; (g) C. Pubill-Ulldemolins,
A. Bonet, C. Bo, H. Gulyas and E. Fernandez, Org. Biomol. Chem.,
2010, 8, 2667.
17 11B NMR spectra for the reaction of 3 with HBCat were taken at
ꢁ30 1C, which results in the significant broadening of 11B NMR
resonances thus preventing the extraction of accurate values of JB–H
.
18 For related agostic borylamides, see: T. D. Forster,
H. M. Tuononen, M. Parvez and R. Roesler, J. Am. Chem. Soc.,
2009, 131, 6689.
19 S. K. Ignatov, A. Y. Khalimon, N. H. Rees, A. G. Razuvaev,
P. Mountford and G. I. Nikonov, Inorg. Chem., 2009, 48, 9605.
20 Formation of boryl complexes is proposed in the hydroboration
reactions catalysed by late transition metal complexes. For example,
see ref. 3.
21 Addition of aldehydes to 1 starts with the formation of adduct
trans-(ArN)Mo(H)(Cl)(Z2-OQCRH)(PMe3)2 which in the
presence of large excess of RHC(O) rearranges slowly ( Z 5 h)
into an alkoxy complex via PMe3 dissociation (see ref. 9). Addition
of a large excess of borane could significantly accelerate this
process making it competitive with (or even faster than) the
formation of methylenamide derivatives (B2 h).
3 (a) D. Mannig and H. Noth, Angew. Chem., Int. Ed. Engl., 1985,
¨
¨
24, 878; (b) K. Burgess and M. J. Ohlmeyer, Chem. Rev., 1991,
91, 1179; (c) C. M. Vogels and S. A. Westcott, Curr. Org. Chem.,
2005, 9, 687.
4 For examples of alkene and alkyne hydroborations, see:
(a) D. A. Evans, A. R. Muci and R. Sturmer, J. Org. Chem.,
1993, 58, 5307; (b) K. Burgess and W. A. van der Donk,
Organometallics, 1994, 13, 3616; (c) K. Burgess and W. A. van
der Donk, J. Am. Chem. Soc., 1994, 116, 6561; (d) E. A. Bijpost,
R. Duchateau and J. H. Teuben, J. Mol. Catal. A: Chem., 1995,
95, 121; (e) S. Pereira and M. Srebnik, Organometallics, 1995,
14, 3127; (f) S. Pereira and M. Srebnik, Tetrahedron Lett., 1996,
37, 3283; (g) X. He and J. F. Hartwig, J. Am. Chem. Soc., 1996,
118, 1696; (h) J. F. Hartwig, C. N. Muhoro, X. He, O. Eisenstein,
c
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Chem. Commun., 2012, 48, 455–457 457