Addition of boranes to iminophosphines: Synthesis and reactivity of a new bulky hydroboration reagent

Article abstract of DOI:10.1016/j.jorganchem.2013.02.003

Two sterically diverse iminophosphines have been prepared from the addition of 2-Ph₂PC(O)H and 4-H₂NC₆H₄OMe (L1) and 2,6-H₂NC₆H₄(i-Pr)₂ (L2), and the addition of boranes to these substrates has been examined in an effort to reduce the imine CN bond. Reactions using borane-dimethylsulfide with the smaller imine L1 afforded adducts arising from initial coordination to the phosphorus atom and then the imine nitrogen. Addition of excess borane-dimethylsulfide eventually led to reduction of the imine and generated an active H₂BNRR′ unit, which went on to make several aminophosphine products. The sterically hindered imine L2 was used to retard the reactivity in the resulting aminoborane. Hydroboration studies with a new borane (7) showed good selectivity in reactions with 1-octene and vinyl arenes. Reactions of iminophosphines with alkyl and dialkylborane also gave adducts, making reduction of the imine increasingly difficult. Addition of the dialkoxyboranes, catecholborane and pinacolborane, were promising for the less hindered imine, but harsh reaction conditions (metal catalysts, microwave reactor and elevated temperatures and pressures) were required in reductions of the sterically-hindered imine, giving rise to complicated product distributions.

Full text of DOI:10.1016/j.jorganchem.2013.02.003

Journal of Organometallic Chemistry 731 (2013) 1e9
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Addition of boranes to iminophosphines: Synthesis and reactivity of a new bulky
hydroboration reagent
,
Richard J. Burford a, Michael J. Geier a, Christopher M. Vogels a, Andreas Decken b, Stephen A. Westcott a
*
a Department of Chemistry and Biochemistry, Mount Allison University, Sackville, New Brunswick, Canada E4L 1G8
b University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3
a r t i c l e i n f o
a b s t r a c t
Article history:
Two sterically diverse iminophosphines have been prepared from the addition of 2-Ph2PC(O)H and 4-
H2NC6H4OMe (L1) and 2,6-H2NC6H4(i-Pr)2 (L2), and the addition of boranes to these substrates has
been examined in an effort to reduce the imine C]N bond. Reactions using borane-dimethylsulfide with
the smaller imine L1 afforded adducts arising from initial coordination to the phosphorus atom and then
the imine nitrogen. Addition of excess borane-dimethylsulfide eventually led to reduction of the imine
and generated an active H2BNRR0 unit, which went on to make several aminophosphine products. The
sterically hindered imine L2 was used to retard the reactivity in the resulting aminoborane. Hydro-
boration studies with a new borane (7) showed good selectivity in reactions with 1-octene and vinyl
arenes. Reactions of iminophosphines with alkyl and dialkylborane also gave adducts, making reduction
of the imine increasingly difficult. Addition of the dialkoxyboranes, catecholborane and pinacolborane,
were promising for the less hindered imine, but harsh reaction conditions (metal catalysts, microwave
reactor and elevated temperatures and pressures) were required in reductions of the sterically-hindered
imine, giving rise to complicated product distributions.
Received 1 January 2013
Received in revised form
4 February 2013
Accepted 5 February 2013
Keywords:
Aminophosphines
Boranes
Catalysis
Hydroboration
Reduction
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
for generating amines from imines invokes the use of hydride
reducing agents, especially boron hydrides such as NaBH4.
Amines are a remarkably important class of compounds that
have widespread applications in industry and are key in-
termediates in many organic transformations [1]. A common and
efficient method of generating substituted amines is via the
reductive amination of aldehydes or ketones, which generally
proceeds by initial condensation with a primary or secondary
amine to give a carbinolamine intermediate and subsequently de-
hydrates to afford the imine or iminium ion (Scheme 1). Reduction
of the imine then generates the substituted amine, where the
choice of reducing agent is critical to the success of the reaction.
Although catalytic hydrogenation is usually an effective way of
reducing simple imines, these additions can sometimes give mix-
tures of products. For instance, with substrates containing both an
alkene and an imine functionality, competing reduction of both
unsaturated groups often results in low yields of the desired
product. Another limitation to this method involves reducing sub-
strates containing divalent sulfur, which may inhibit and eventually
deactivate the catalyst system. An alternate and selective method
Although a plethora of borohydride derivatives have subsequently
been designed for this purpose, reactions can suffer from low yields
and poor selectivities [2e6], especially in the case of sterically-
hindered imine substrates, as well as toxic side products, as
observed in cases utilizing NaBH3CN. As a result, a considerable
amount of research has concentrated on the development of less
toxic and more selective boron hydride reagents for these impor-
tant reactions.
Our interest in studying the addition of BeH bonds to unsatu-
rated organic molecules, naturally lead us to the reduction of im-
ines. In this study we have investigated the addition of
electronically- and sterically-diverse boranes to iminophosphines
derived from the addition of aniline derivatives to 2-
diphenylphosphinobenzaldehyde. There has been recent consid-
erable interest in the use of unsymmetrical bidentate ligands
containing both a nitrogen (amine) and phosphorus donor atom,
referred to as (P,N)-ligands, for their potential applications in co-
ordination chemistry and in homogenous catalysis [7e11]. Ligands
of this sort are of interest as they provide transition metals with a
‘hard’ amine and ‘soft’ phosphine donor, allowing them to stabilize
a variety of oxidation states and induce attractive transformations.
* Corresponding author. Tel.: þ1 506 364 2372; fax: þ1 506 364 2313.
E-mail address: swestcott@mta.ca (S.A. Westcott).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.02.003

2
R.J. Burford et al. / Journal of Organometallic Chemistry 731 (2013) 1e9
Scheme 1. General reductive amination pathways.
2. Experimental
collected by suction filtration to afford 2. Yield: 160 mg (38%).
Spectroscopic NMR data (in CDCl3): 1H
: 8.87 (s, 1H, C(H)]N), 8.23
d
2.1. General methods
(dd, JHH ¼ 7.6, 3.7 Hz, 1H, Ar), 7.74e7.44 (ov m, 12H, Ar), 7.05 (dd,
JHH ¼ 12.1, 7.6 Hz, 1H, Ar), 6.77 (m, 4H, Ar), 3.77 (s, 3H, OCH3), 2.11
Reagents and solvents were purchased from Aldrich Chemicals
and used as received. Rh(acac)(dppb) [12], dimesitylborane [13],
and L1 and L2 [14] were synthesized by previously reported
methods. NMR spectra were recorded on a JEOL JNM-GSX270 FT
NMR (1H 270 MHz; 11B 87 MHz; 13C 68 MHz; and 31P 109 MHz)
(br s, 3H, BH3), 1.51 (br s, 3H, BH3); 11
B
d
: ꢁ15 (br), ꢁ38 (br); 13C{1H}
d: 166.1 (d, JCP ¼ 5.1 Hz), 159.6, 144.2, 144.1 (d, JCP ¼ 8.7 Hz), 133.7 (d,
JCP ¼ 9.7 Hz), 133.5 (d, JCP ¼ 9.2 Hz), 133.1 (d, JCP ¼ 8.2 Hz), 132.2,
131.7, 130.4, 129.4 (d, JCP ¼ 10.2 Hz), 129.2 (d, JCP ¼ 8.2 Hz), 128.3,
123.4, 113.9, 55.6; 31P{1H}
d: 21.1. Anal. calcd for C26H28NB2OP
spectrometer. Chemical shifts (
residual solvent peaks (1H and 13C) or external BF3$OEt2
H3PO4
31P)] and coupling constants (J) in Hz. Multiplicities are
d
) are reported in ppm [relative to
(423.17): C 73.79, H 6.68, N 3.31; found: C 74.11, H 6.42, N 3.09.
(
11B) or
(
2.2.3. Reaction of L1 with 4 equivalents of H3B$SMe2
reported as singlet (s), doublet (d), triplet (t), quartet (q), septet
(sept), multiplet (m), broad (br), and overlapping (ov). Microwave
reactions were performed using a CEM Discover SP system in
standard closed vessels with the reaction temperature monitored
by the internal IR pyrometer. GCeMS analyses were conducted
using a Varian Saturn 2000 MS, coupled to a CP-3800 GC. The GC
was equipped with the 1177 injection port with a CP-8410 liquid
autoinjector connected to an SPB-1 (Supelco) fused-silica column
To a stirred toluene (5 mL) solution of L1 (100 mg, 0.25 mmol)
was added a toluene solution of H3B$SMe2 (0.5 mL of a 2.0 M solu-
tion, 1.0 mmol). The reaction was allowed to proceed for 18 h at
which point the solvent was removed under vacuum and the reac-
tion analyzed by multinuclear NMR spectroscopy. Selected spec-
troscopic NMR data (in CDCl3): 1H
d: 8.89 (s, C(H)]N), 8.74 (s,
C(H)]N), 8.23 (dd, JHH ¼ 7.7, 3.7 Hz, Ar), 7.92e7.36 (ov m, Ar), 7.24e
7.04 (ov m, Ar), 6.95e6.28 (ov m, Ar), 4.98 (s), 4.73 (m), 4.65 (s), 3.77
(s, OCH3), 3.72 (s, OCH3), 3.69 (s, OCH3), 3.62 (s, OCH3),1.77 (br s, BH3),
(30 m ꢀ 0.25 mm i.d. ꢀ 0.25
mm) attached to a 50 cm transfer line.
Elemental analyses were performed by Guelph Chemical Labora-
tories (Guelph, ON). All reactions were performed under an atmo-
sphere of nitrogen.
1.36 (br s, BH3); 11
B
d
: 42 (br), ꢁ15 (br), ꢁ38 (br); 31P{1H}
d: 22.2, 19.8.
2.2.4. Synthesis of 3
To a stirred toluene (5 mL) solution of L1 (800 mg, 2.02 mmol) was
added a toluene (2 mL) solution of catecholborane (250 mg,
2.08 mmol). The reaction was allowed to proceed for 18 h at which
point solvent was removed under vacuum. The resulting pale yellow
oil was triturated with Et2O (3 ꢀ 1 mL) to afford 3 as a white solid.
2.2. Synthesis and reactivity
2.2.1. Synthesis of 1
To a stirred toluene (5 mL) solution of L1 (395 mg, 1.00 mmol)
was added a toluene solution of H3B$SMe2 (0.5 mL of a 2.0 M so-
lution, 1.0 mmol). The reaction was allowed to proceed for 18 h at
which point solvent was removed under vacuum. The resulting
yellow solid was dissolved in hot Et2O (8 mL) and the solution
stored at ꢁ30 ꢂC. After 3 days, a precipitate was collected by suction
filtration to afford 1 as a yellow solid. Yield: 260 mg (64%). Spec-
Yield: 911 mg (88%). Spectroscopic NMR data (in CDCl3): 1H
d: 7.44e
7.23 (ov m, 13H, Ar), 7.15e7.05 (ov m, 4H, Ar), 7.20e6.96 (ov m, 2H,
Ar), 6.87 (m,1H, Ar), 6.76 (d, JHH ¼ 8.9 Hz, 2H, Ar), 5.08 (d, JHP ¼ 2.2 Hz,
2H, CH2), 3.76 (s, 3H, OCH3); 11 : 26(br); 13C{1H}
B d d: 155.9,148.7,142.9
(d, JCP ¼ 22.0 Hz),137.0,136.1 (d, JCP ¼ 10.2 Hz), 135.2 (d, JCP ¼ 14.8 Hz),
134.1 (d, JCP ¼ 19.5 Hz), 133.5, 129.1, 128.9 (d, JCP ¼ 18.9Hz),128.8,127.2,
127.0 (d, JCP ¼ 4.6 Hz), 123.9, 122.0, 114.2, 111.8, 55.5, 50.3 (d,
troscopic NMR data (in CDCl3): 1H
d: 8.83 (s, 1H, C(H)]N), 8.40
JCP ¼ 27.1 Hz); 31P{1H}
: ꢁ15.5. Anal. calcd for C32H27NBO3P (515.41):
d
(ddd, JHH ¼ 7.6, 3.7, 1.0 Hz, 1H, Ar), 7.72e7.34 (ov m, 12H, Ar), 7.05
C 74.57, H 5.29, N 2.72; found: C 74.32, H 5.12, N 2.88.
(dd, JHH ¼ 12.1, 7.6 Hz, 1H, Ar), 6.77 (m, 4H, Ar), 3.76 (s, 3H, OCH3),
1.66 (br s, 3H, BH3); 11B
d
: ꢁ36 (br); 13C{1H}
d: 158.6, 156.2 (d,
2.2.5. Synthesis of 4
JCP ¼ 6.7 Hz), 144.0, 139.7 (d, JCP ¼ 7.7 Hz), 134.1 (d, JCP ¼ 7.2 Hz),
133.5 (d, JCP ¼ 9.2 Hz), 131.7, 131.6, 130.3 (d, JCP ¼ 9.7 Hz), 129.5 (d,
JCP ¼ 8.2 Hz), 129.2 (d, JCP ¼ 9.7 Hz), 128.7, 128.4 (d, JCP ¼ 7.7 Hz),
To a stirred MeOH (5 mL) suspension of Rh(acac)(dppb)
(400 mg, 0.64 mmol) was added a MeOH (5 mL) solution of 3
(328 mg, 0.64 mmol). The reaction mixture was allowed to proceed
for 18 h at which point the clear red solution was concentrated to
3 mL under vacuum. Upon standing for 18 h at RT an orange pre-
cipitate formed and was collected by suction filtration to afford 4.
122.7, 114.2, 55.5; 31P{1H}
d: 20.6. Anal. calcd for C26H25NBOP
(409.33): C 76.29, H 6.17, N 3.42; found: C 76.03, H 5.99, N 3.15.
2.2.2. Synthesis of 2
Yield: 249 mg (34%). Spectroscopic NMR data (in THF-d8): 1H
d:
To a stirred toluene (5 mL) solution of L1 (395 mg, 1.00 mmol)
was added a toluene solution of H3B$SMe2 (1.0 mL of a 2.0 M so-
lution, 2.0 mmol). The reaction was allowed to proceed for 18 h at
which point the solvent was removed under vacuum to afford a
yellow solid. The solid was dissolved in Et2O (10 mL) and the so-
lution stored at ꢁ30 ꢂC. After several days a yellow precipitate was
7.58e7.05 (ov m, 29H, Ar), 6.94e6.71 (ov m, 6H, Ar), 6.64 (d,
JHH ¼ 8.4 Hz, 2H, Ar), 6.48 (t, JHH ¼ 8.2 Hz, 1H, Ar), 6.28e6.18 (ov m,
8H, Ar), 4.69 (br s, 1H, CHHNH), 4.11 (br s, 1H, CHHNH), 3.76 (s, 3H,
OCH3), 3.48 (t, JHH ¼ 11.0 Hz, 1H, CH2NH), 2.30 (br m, 2H, CH2), 2.19
(br m, 2H, CH2), 1.95 (br m, 2H, CH2), 1.59 (br m, 2H, CH2); 11
B d: 14.4

R.J. Burford et al. / Journal of Organometallic Chemistry 731 (2013) 1e9
3
(sharp); 13C{1H}
d
: 158.6, 153.1, 140.5 (m), 139.3 (d, JCP ¼ 14.5 Hz),
solution, 3.0 mmol). The reaction was allowed to proceed for 18 h at
which point solvent was removed under vacuum. The resulting
solid was dissolved in hot Et2O (5 mL) and the solution stored at
ꢁ30 ꢂC. A precipitate was collected by suction filtration to afford 7
as a white solid. Yield: 391 mg (82%). Spectroscopic NMR data (in
137.5, 136.7 (d, JCP ¼ 4.2 Hz), 136.1, 134.2 (m), 134.0, 133.0 (m), 132.2,
130.7, 130.5 (m), 129.6, 128.8 (m), 128.4, 128.0, 125.1, 122.2, 116.2,
114.0, 107.3, 58.2 (d, JCP ¼ 15.1 Hz), 55.1, 27.6 (d, JCP ¼ 21.8 Hz), 26.2
(d, JCP ¼ 27.5 Hz), 23.3, 20.6; 31P{1H}
: 45.6 (ddd, JPRh ¼ 160 Hz,
d
JPP ¼ 51, 37 Hz), 25.6 (ddd, JPP ¼ 291 Hz, JPRh ¼ 160 Hz, JPP ¼ 37 Hz),
10.5 (ddd, JPP ¼ 291 Hz, JPRh ¼ 155 Hz, JPP ¼ 50 Hz). Anal. calcd for
C66H60NBO5P3Rh (1153.98): C 68.69, H 5.25, N 1.21; found: C 69.32,
H 5.45, N 1.31.
CDCl3): 1H
d: 7.62e7.33 (ov m, 12H, Ar), 7.22e7.12 (ov m, 2H, Ar),
7.01e6.98 (ov m, 2H, Ar), 6.89 (ddd, JHH ¼ 11.9, 7.9, 1.2 Hz, 1H, Ar),
4.62 (s, 2H, CH2), 4.37 (br s, 2H, NBH2), 2.86 (sept, JHH ¼ 6.8 Hz, 2H,
CH(CH3)2), 1.51 (br s, 3H, BH3), 0.97 (d, JHH ¼ 6.8 Hz, 6H, CH(CH3)2),
0.91 (d, JHH ¼ 6.8 Hz, 6H, CH(CH3)2); 11
B
d
: 39 (br), ꢁ37 (br); 13C{1H}
2.2.6. Synthesis of 5
d
: 145.0, 144.3, 142.8 (d, JCP ¼ 10.7 Hz), 134.5 (d, JCP ¼ 7.2 Hz), 133.3
To a stirred THF (2 mL) solution of L1 (300 mg, 0.76 mmol) and
Rh(acac)(dppb) (10 mg, 0.02 mmol) was added a THF (2 mL) solu-
tion of pinacolborane (100 mg, 0.78 mmol). The reaction mixture
was heated in a CEM microwave reactor at 125 ꢂC for 0.5 h. Removal
of solvent under vacuum afforded a yellow oil which was dissolved
in hot hexane (2 mL) and the solution stored at ꢁ30 ꢂC. After 3 days
a pale yellow precipitate was collected by suction filtration to afford
(d, JCP ¼ 9.7 Hz), 131.4, 131.1, 129.6, 128.9 (d, JCP ¼ 10.2 Hz), 128.8,
128.0, 127.5 (d, JCP ¼ 8.7 Hz), 127.0, 123.8, 59.6 (d, JCP ¼ 5.6 Hz), 27.9,
25.1, 23.4; 31P{1H}
d: 21.2. Anal. calcd for C31H38NB2P (477.32): C
78.00, H 8.04, N 2.94; found: C 77.74, H 7.76, N 2.90.
2.2.10. Reaction of L2 with catecholborane
To a stirred THF (1 mL) solution of L2 (200 mg, 0.44 mmol) was
added a THF (1 mL) solution of catecholborane (53 mg, 0.44 mmol).
The reaction was allowed to proceed for 18 h at which point solvent
was removed under vacuum to give an orange-colored oil which
was analyzed by multinuclear NMR spectroscopy. Selected spec-
5. Yield: 286 mg (72%). Spectroscopic NMR data (in CDCl3): 1H
d:
7.67 (m, 1H, Ar), 7.37e7.28 (ov m, 6H, Ar), 7.12e7.01 (ov m, 8H, Ar),
6.89 (t, J ¼ 7.4 Hz, 1H, Ar), 6.70 (d, JHH ¼ 8.2 Hz, 2H, Ar), 5.26 (s, 2H,
CH2), 3.26 (s, 3H, OCH3), 1.06 (s, 12H, pin); 11 : 24 (br); 13C{1H}
B d d:
154.9,144.5 (d, JCP ¼ 21.5 Hz), 139.4, 136.5 (d, JCP ¼ 10.7 Hz), 135.1 (d,
JCP ¼ 15.4 Hz), 134.3 (d, JCP ¼ 19.5 Hz), 133.2, 129.0, 128.7 (d,
JCP ¼ 6.7 Hz), 128.1, 126.8, 126.2 (d, JCP ¼ 5.1 Hz), 122.0, 114.0, 82.6,
troscopic NMR data (in CDCl3): 1H
d: 8.17 (m, Ar), 7.55e6.86 (ov m,
Ar), 5.70 (d, JHH ¼ 12.1 Hz), 4.91 (d, JHP ¼ 2.2 Hz), 3.91 (sept,
JHH ¼ 6.8 Hz, CH(CH3)2), 3.11 (sept, JHH ¼ 6.8 Hz, CH(CH3)2), 2.83
(sept, JHH ¼ 6.8 Hz, CH(CH3)2), 1.54 (d, JHH ¼ 6.8 Hz, CH(CH3)2), 1.30
(d, JHH ¼ 6.8 Hz, CH(CH3)2), 1.07 (d, JHH ¼ 6.8 Hz, CH(CH3)2), 1.03 (d,
JHH ¼ 6.8 Hz, CH(CH3)2), 0.84 (d, JHH ¼ 6.8 Hz, CH(CH3)2), 0.18 (d,
54.6, 50.0 (d, JCP ¼ 28.2 Hz), 24.4; 31P{1H}
: ꢁ15.1. Anal. calcd for
d
C32H35NBO3P (523.49): C 73.41, H 6.75, N 2.68; found: C 73.66, H
7.02, N 2.33.
JHH ¼ 6.8 Hz, CH(CH3)2); 11
B
d
: 22 (br); 31P{1H}
d: ꢁ16.2, ꢁ17.3.
2.2.7. Synthesis of 6
To a stirred toluene (5 mL) solution of L2 (450 mg, 1.00 mmol)
was added a toluene solution of H3B$SMe2 (0.5 mL of a 2.0 M so-
lution, 1.0 mmol). The reaction was allowed to proceed for 18 h at
which point solvent was removed under vacuum. The resulting
solid was dissolved in hot Et2O (3 mL) and the solution stored at
ꢁ30 ꢂC. A yellow precipitate was collected by suction filtration to
afford 6 as a bright yellow solid. Yield: 357 mg (77%). Spectroscopic
2.2.11. Synthesis of 11
To a stirred THF (2 mL) solution of L2 (300 mg, 0.67 mmol) and
Rh(acac)(dppb) (3 mg, 0.005 mmol) was added a THF (2 mL) so-
lution of pinacolborane (171 mg, 1.34 mmol). The reaction mixture
was heated in a CEM microwave reactor at 125 ꢂC for 0.5 h. Removal
of solvent under vacuum gave an orange oil which was dissolved in
hot hexane (2 mL) and stored at ꢁ30 ꢂC. A pale orange solid
precipitated after several days and was collected by suction filtra-
tion to afford 11. Yield: 94 mg (24%). Spectroscopic NMR data (in
NMR data (in CDCl3): 1H
d: 8.66 (s, 1H, C(H)]N), 8.62 (ddd,
JHH ¼ 7.9, 3.9, 1.2 Hz, 1H, Ar), 7.67e7.38 (ov m, 12H, Ar), 7.07e7.01
(ov m, 4H, Ar), 2.61 (sept, JHH ¼ 6.8 Hz, 2H, CH(CH3)2), 1.56 (br s,
CDCl3): 1H
d: 7.45 (m, 1H, Ar), 7.29e7.22 (ov m, 9H, Ar), 7.12e6.96
3H, BH3), 0.96 (d, JHH ¼ 6.8 Hz, 12H, CH(CH3)2); 11
B
d
: ꢁ36 (br); 13
C
(ov m, 6H, Ar), 6.78 (m, 1H, Ar), 4.64 (d, JHP ¼ 2.7 Hz, 2H, CH2),
{1H}
d
: 159.2 (d, JCP ¼ 6.7 Hz), 148.6, 139.1 (d, JCP ¼ 8.2 Hz), 137.0,
3.11 (sept, JHH ¼ 6.9 Hz, 2H, CH(CH3)2), 1.29 (s, 12H, pin), 1.11 (d,
134.3 (d, JCP ¼ 6.1 Hz), 133.4 (d, JCP ¼ 9.2 Hz), 131.7, 130.8 (d,
JHH ¼ 6.9 Hz, 6H, CH(CH3)2), 0.80 (d, JHH ¼ 6.9 Hz, 6H, CH(CH3)2); 11
B
JCP ¼ 9.2 Hz), 130.0, 129.5, 129.2 (d, JCP ¼ 9.7 Hz), 128.7, 128.6 (d,
d
: 20 (br); 13C{1H}
d
: 147.3, 144.6 (d, JCP ¼ 27.0 Hz), 138.9, 137.6 (d,
JCP ¼ 8.2 Hz), 124.1, 122.7, 27.8, 23.2; 31P{1H}
d
: 20.8. Anal. calcd for
JCP ¼ 13.0 Hz), 136.6 (d, JCP ¼ 14.5 Hz), 133.7 (d, JCP ¼ 20.2 Hz), 133.6,
131.3 (d, JCP ¼ 4.7 Hz), 128.8, 128.4 (d, JCP ¼ 6.5 Hz), 128.1, 127.3,
126.7, 123.5, 82.8, 52.0 (d, JCP ¼ 23.4 Hz), 27.0, 25.6, 24.7, 23.1; 31P
C31H35NBP (463.48): C 80.33, H 7.63, N 3.02; found: C 80.12, H 7.82,
N 3.16.
{1H}
d
: ꢁ16.6. Anal. calcd for C37H45NBO2P (577.64): C 76.93, H 7.87,
2.2.8. Reaction of L2 with 2 equivalents of Me2S$BH3
N 2.43; found: C 77.14, H 8.22, N 2.64.
To a stirred toluene (5 mL) solution of L2 (360 mg, 0.80 mmol)
was added a toluene solution of H3B$SMe2 (0.8 mL of a 2.0 M so-
lution, 1.6 mmol). The reaction was allowed to proceed for 18 h at
which point solvent was removed under vacuum and the reaction
analyzed by multinuclear NMR spectroscopy. Selected spectro-
2.3. General method for hydroboration reactions
To a stirred C6D6 (0.5 mL) solution of 7 (89 mg, 0.19 mmol) was
added a C6D6 (0.5 mL) solution of the appropriate substrate
(0.19 mmol). The reaction mixture was heated at reflux for 4 h at
which point the reaction was analyzed by multinuclear NMR
spectroscopy. Products were confirmed by GCeMS following a
basic, oxidative workup [14].
scopic NMR data (in CDCl3): 1H
d: 9.35 (s, C(H)]N), 9.20 (s,
C(H)]N), 8.70 (m, Ar), 7.78e7.39 (ov m, Ar), 7.24e6.96 (ov m, Ar),
6.81 (m, Ar), 4.61 (s, CH2), 2.67 (sept, JHH ¼ 6.6 Hz, CH(CH3)2), 2.48
(sept, JHH ¼ 6.6 Hz, CH(CH3)2), 2.16 (br s, BH3), 1.19 (d, JHH ¼ 6.6 Hz,
CH(CH3)2), 1.17 (d, JHH ¼ 6.6 Hz, CH(CH3)2), 0.85 (d, JHH ¼ 6.6 Hz,
CH(CH3)2), 0.75 (d, JHH ¼ 6.6 Hz, CH(CH3)2); 11
B
d: 42 (br), ꢁ14 (br),
ꢁ37 (br); 31P{1H}
d
: 23.8, 22.6, 21.2.
2.3.1. Hydroboration of 1-octene with 7
Spectroscopic NMR data (in C6D6): 1H
d: 7.57 (m, Ar), 7.11e6.85
2.2.9. Synthesis of 7
To a stirred toluene (5 mL) solution of L2 (450 mg, 1.00 mmol)
was added a toluene solution of H3B$SMe2 (1.5 mL of a 2.0 M
(ov m, Ar), 6.75 (t, JHH ¼ 7.6 Hz, Ar), 4.91 (s, NCH2), 4.70 (br s, BH),
2.98 (sept, JHH ¼ 6.8 Hz, CH(CH3)2), 2.10 (br s, BH3), 1.45 (br m,
BCH2), 1.32e1.18 (ov m, eCH2e), 0.98 (d, JHH ¼ 6.8 Hz, CH(CH3)2),

4
R.J. Burford et al. / Journal of Organometallic Chemistry 731 (2013) 1e9
0.89 (d, JHH ¼ 6.8 Hz, CH(CH3)2), 0.87 (t, JHH ¼ 6.6 Hz, CH3); 11
B d: 38
(br), ꢁ37 (br); 31P{1H}
d: 22.7.
2.3.2. Hydroboration of 4-vinylanisole with 7
Selected spectroscopic NMR data (in C6D6): 1H
d: 7.62 (m, Ar),
7.30 (m, Ar), 7.20e6.93 (ov m, Ar), 6.84 (d, JHH ¼ 8.6 Hz, Ar), 4.96 (s,
CH2), 4.77 (s, CH2), 4.65 (br s), 3.37 (s, OCH3), 3.36 (s, OCH3), 3.35 (s,
OCH3), 3.03 (sept, JHH ¼ 6.9 Hz, CH(CH3)2), 2.89 (q, JHH ¼ 7.4 Hz,
CH(B)CH3), 2.65 (t, JHH ¼ 7.9 Hz, CH2CH2B), 2.10 (br s, BH3), 1.53 (t,
JHH ¼ 7.9 Hz, CH2CH2B), 1.24 (d, JHH ¼ 7.4 Hz, CH(B)CH3), 1.08 (d,
JHH ¼ 6.9 Hz, CH(CH3)2), 1.02 (d, JHH ¼ 6.9 Hz, CH(CH3)2), 1.01 (d,
Scheme 3. Addition of boranes to iminophosphine L1.
JHH ¼ 6.9 Hz, CH(CH3)2), 0.93 (d, JHH ¼ 6.9 Hz, CH(CH3)2); 11
B d: 39
2, along with a number of other products where the imine func-
tionality has been. Addition of excess borane afforded a mixture of
products where the imine double bond has been reduced and
newly generated RR0NBH2 species has either further reduced the
starting material and/or adducts 1 and 2. Addition of borane to
isolated 2 once again lead to a mixture of products.
We then decided to look at other boron hydrides in an effort to
affect this reduction more effectively by reducing the excessive
amount of borane required to facilitate this reaction. Unfortunately,
attempts to use thexylborane [18], 9-borabicyclo[3.3.1]-nonane
[19] and dimesitylborane [12] gave numerous boron-containing
products, as observed by 11B NMR spectroscopy, presumably
arising once again from competing coordination of these alkyl and
dialkylboranes to the Lewis basic P and N atoms. Not surprisingly,
reduction of the imine double bond became increasingly prob-
lematic with the increased size of the borane adducts.
(br), 29 (br), ꢁ36 (br); 31P{1H}
: 22.1, ꢁ16.6.
d
2.3.3. Hydroboration of 4-fluorostyrene with 7
Selected spectroscopic NMR data (in C6D6): 1H
d: 7.56 (m, Ar),
7.25 (m, Ar), 7.10e6.55 (ov m, Ar), 6.84 (d, JHH ¼ 8.6 Hz, Ar), 4.90 (s,
CH2), 4.73 (s, CH2), 4.56 (br s), 3.25 (sept, JHH ¼ 6.9 Hz, CH(CH3)2),
2.97 (q, JHH ¼ 7.2 Hz, CH(B)CH3), 2.41 (t, JHH ¼ 7.9 Hz, CH2CH2B), 2.04
(br s, BH3), 1.30 (t, JHH ¼ 7.9 Hz, CH2CH2B), 1.08e0.98 (ov m), 0.97e
0.94 (ov m), 0.89e0.86 (ov m); 11B
d
: 39 (br), 30 (br), ꢁ36 (br); 31
P
{1H}
d
: 22.8, ꢁ16.4.
2.4. X-ray crystallography
Crystals of 3 and 4 were grown from saturated diethyl ether
solutions at 25 ꢂC. Crystals of 7 were grown from a saturated THF
solution stored at ꢁ30 ꢂC. Single crystals were coated with Para-
tone-N oil, mounted using a polyimide MicroMount and frozen in
the cold nitrogen stream of the goniometer. A hemisphere of data
was collected on a Bruker AXS P4/SMART 1000 diffractometer using
u
and q
scans with a scan width of 0.3ꢂ and 10 s (3 and 7) and 30 s
(4) exposure times. The detector distance was 5 cm. The data were
reduced [15] and corrected for absorption [16]. The structures were
solved by direct methods and refined by full-matrix least squares
on F2 [17]. All non-hydrogen atoms were refined using anisotropic
displacement parameters. Hydrogen atoms were found in Fourier
difference maps and refined using isotropic displacement
parameters.
3. Results and discussion
The reduction of iminophosphines is a practical and efficient
method of generating a wide variety of aminophosphine com-
pounds. In this study, we initially investigated reductions of the
sterically-accessible iminophosphine L1 with a number of different
boron hydride reagents. We have found that addition of one
equivalent of H3B$SMe2 at room temperature gave the phosphine
borane adduct 1 as the only new boron-containing product
(Scheme 2). The 11B NMR spectrum for 1 shows a broad peak at
Fig. 1. The molecular structure of 3 with ellipsoids drawn at the 50% probability level
and hydrogen atoms omitted for clarity. Selected bond distances (A): PeC(7)
1.8329(17), PeC(13) 1.8350(18), PeC(1) 1.8431(16), C(19)eN 1.472(2), NeB 1.410(3), Be
O(2) 1.399(2), BeO(1) 1.400(3). Selected bond angles (ꢂ): BeNeC(26) 126.85(15), Be
NeC(19) 116.03(15), C(26)eNeC(19) 117.08(14), O(2)eBeO(1) 111.39(16), O(2)eBeN
127.91(18), O(1)eBeN 120.70(16).
ꢀ
d
ꢁ36 ppm for the coordinated borane along with a broad peak at
d
21 ppm in the 31P NMR spectrum. Addition of a second equivalent
of borane lead to moderate yields (ca. 35%) of the bis-adduct species
Scheme 2. Addition of borane-dimethylsulfide to iminophosphine L1.

Table 1
Crystallographic data-collection parameters.
Complex
3
4
7
Formula
C32H27BNO3P
515.33
Monoclinic
P2(1)/n
10.8594(16)
13.6857(19)
17.851(3)
90
90.127(2)
90
C70H67BNO6P3Rh
1224.88
Triclinic
C31H38B2NP
477.21
Monoclinic
P2(1)/c
12.4812(8)
9.2948(6)
24.2540(16)
90
95.9980(10)
90
Molecular weight
Crystal system
Space group
P-1
ꢀ
a/A
12.311(6)
14.083(6)
18.736(8)
77.990(6)
73.175(6)
77.456(7)
2998(2)
ꢀ
b/A
ꢀ
c/A
ꢂ
ꢂ
a
/
b
/
ꢂ
g
/
3
ꢀ
V/A
2653.0(7)
4
1.290
2798.3(3)
4
1.133
Z
2
rcalc./Mg mꢁ3
Crystal size/mm3
Temp/K
1.357
0.40 ꢀ 0.30 ꢀ 0.20
0.20 ꢀ 0.20 ꢀ 0.05
0.65 ꢀ 0.40 ꢀ 0.20
198(1)
173(1)
198(1)
ꢀ
ꢀ
ꢀ
Radiation
Mo-Ka
0.138
18,157
5942
451
(
l
¼ 0.71073 A)
Mo-Ka
0.419
20,707
12,988
740
(
l
¼ 0.71073 A)
Mo-Ka
0.118
18,914
6247
468
(
l
¼ 0.71073 A)
m
/mmꢁ1
Total reflections
Total unique reflections
No. of variables
q
Range/ꢂ
1.87e27.49
0.273 and ꢁ0.233
1.061
0.0380
0.1011
1.50e27.50
0.870 and ꢁ0.612
1.020
0.0385
0.1024
1.64e27.50
0.468 and ꢁ0.245
1.054
0.0397
0.1143
ꢀꢁ3
Largest difference peak/hole/e A
S (GoF) on F2
R1a (I > 2
s
(I))
wR2b (All data)
P
P
a
R1
¼
jjFoj ꢁ jFcjj= jFoj:
P
P
wR2 ¼ ð ½wðFo2 ꢁ Fc2Þ2ꢃ= ½wFo4ꢃÞ1=2
;
where w ¼ 1=½s2ðFo2Þ þ ð0:0339,PÞ2 þ ð0:7218,PÞꢃ ð3Þ; 1=½s2ðFo2Þ þ ð0:048,PÞ2 þ ð0:2178,PÞꢃ ð4Þ; 1=½s2ðFo2Þ þ ð0:0549,PÞ2 þ ð1:0288,PÞꢃ ð7Þ; where P ¼ ðmaxðFo2; 0Þ þ 2,Fc2Þ=3:
b

6
R.J. Burford et al. / Journal of Organometallic Chemistry 731 (2013) 1e9
Scheme 4. Generation of rhodium complex 4 from addition of HBcat to 3.
Dialkoxyboron compounds are a considerably less Lewis acidic
source of boron hydrides [20] and were examined next in an effort
to avoid unwanted coordination with the Lewis basic phosphine
and imine fragments (Scheme 3). As hoped, addition of cat-
echolborane (HBcat, cat ¼ 1,2-O2C6H4) proceeded smoothly at room
temperature over 18 h to give 3 as the major product along with a
minor amount of decomposition product B2cat3 [21]. The 11B NMR
carbonenitrogen bond is now reduced with a distance of C(19)eN
ꢀ
ꢀ
1.472(2) A, and the aminoboron bond is NeB 1.410(3) A, both of
which are consistent with analogous compounds reported previ-
ously [22]. The boron oxygen bonds of BeO(2) 1.399(2) and BeO(1)
ꢀ
1.400(3) A are typical for Bcat fragments. As expected, the chelating
boron oxygen angles (O(2)eBeO(1) 111.39(16)ꢂ) are considerably
shorter than the BN angles at O(2)eBeN 127.91(18)ꢂ and O(1)eBeN
120.70(16)ꢂ. There are no considerable PeB interactions in the solid
state.
spectrum for 3 shows a broad peak at
borane along with a broad peak at
spectrum for the uncoordinated phosphorus atom (c.f.
d 26 ppm for the amino-
d
ꢁ15.5 ppm in the 31P NMR
d
ꢁ14.3 ppm
As relatively long reaction times (12e18 h) were required to
facilitate the addition of HBcat to L1, we then decided to investigate
the possibility of using a transition metal catalyst to accelerate this
reaction. The transition metal catalyzed hydroboration of alkenes
and alkynes is an important reaction in organic synthesis and
considerable research has been conducted in this area [23].
Although a wide range of rhodium complexes can be used to
catalyze this reaction, we have examined two of the most active
and selective precatalysts in this study and found that catalytic
amounts of RhCl(PPh3)3 could indeed be used to facilitate this
addition, with only negligible amounts of decomposition products.
for the starting iminophosphine L1). The disappearance of the
imine N]CH bond in the 1H NMR spectra is followed by the
appearance of a doublet at
d
5.08 ppm (JHP ¼ 2.2 Hz) corresponding
to the reduced NCH2 group in 3. A similar disappearance for the
imine carbon is observed in the 13C NMR spectra along with the
formation of a peak at
d
50.3 (d, JCP ¼ 27.1 Hz) ppm, observed for the
reduced NCH2 group. Compound 3 was also characterized by a
single crystal X-ray diffraction study to confirm that the imine
functionality was indeed reduced. The molecular structure of 3 is
shown in Fig. 1 and crystallographic data provided in Table 1. The
ꢀ
Fig. 2. The molecular structure of 4 with ellipsoids drawn at the 50% probability level and hydrogen atoms omitted for clarity. Selected bond distances (A): RheN 2.205(2), RheP(2)
2.2356(9), RheP(1) 2.2894(9), RheP(3) 2.3393(10), BeO(1) 1.479(4), BeO(4) 1.479(4), BeO(3) 1.483(4), BeO(2) 1.493(4). Selected bond angles (ꢂ): NeRheP(2) 159.66(6), NeRheP(1)
88.45(6), P(2)eRheP(1) 99.95(4), NeRheP(3) 92.19(6), P(2)eRheP(3) 90.57(4), P(1)eRheP(3) 147.32(3), O(1)eBeO(4) 111.8(2), O(1)eBeO(3) 113.4(2), O(4)eBeO(3) 104.5(2), O(1)e
BeO(2) 105.0(2), O(4)eBeO(2) 111.0(2), O(3)eBeO(2) 111.4(2), C(20)eNeRh 96.86(14), C(19)eNeRh 121.62(14), C(20)eNeC(19) 110.03(19).

R.J. Burford et al. / Journal of Organometallic Chemistry 731 (2013) 1e9
7
ꢀ
2.2894(9) A. Similar bond distances have been observed in related
P,N rhodium systems [24e30]. Indeed, the rhodium amine bond
distances in an analogous aminophosphinite bis-chelate complex
derived from an N-substituted ephedrine group, developed by
ꢀ
Alper and co-workers [29], were 2.201(3) and 2.182(3) A, where the
amine ligands also adopted a configuration that was trans to the
softer phosphorous atoms. The relatively compressed P(1)eRhe
P(3) angle of 147.32(3) presumably results from steric crowding
arising from the bulky secondary aryl amine group. The arylspir-
oborate anion in 4 shows no appreciable bonding to the metal
Scheme 5. Addition of borane-dimethylsulfide to iminophosphine L2.
ꢀ
center and the BeO lengths, ranging from 1.479(4) to 1.493(4) A, are
Reactions could also be catalyzed using 2e5 mol% of Rh(a-
typical for those found in related systems [31], yet considerably
longer than those in 3, where the boron atom is three coordinate.
In comparison, reactions of pinacolborane (HBpin, pin ¼ 1,2-
O2C2Me4) and L1 at room temperature or at elevated tempera-
tures (THF at reflux), even in the presence of a rhodium catalyst, failed
to generate any significant amounts of the reduction product 5, as
ascertained by multinuclear NMR spectroscopy. Indeed, we have
found that 5 could only be produced using a catalytic amount
(5 mol%) of rhodium using a microwave reactor at elevated tem-
peratures and pressures [32].
We then decided to look at addition of boranes to the imino-
phosphine species L2 to see what affect increasing the steric bulk
on the imine group had on activities and selectivities. Not sur-
prisingly, addition of one equivalent of H3B$SMe2 to L2 gave the
corresponding phosphineeborane adduct 6 in good yield (77%).
Although addition of two equivalents of H3B$SMe2 to L2 gave a
number of products, reactions using three equivalents of borane
generated 7 cleanly, where the imine has been reduced fully to
afford a new RR0NBH2 species (Scheme 5). The 31P NMR spectra of 7
cac)(dppb)
(acac
¼
acetylacetonato,
dppb
¼
1,4-
a
bis(diphenylphosphino)-butane). This rhodium complex is
remarkably active and selective catalyst precursor for the hydro-
boration of a wide range of vinyl arenes using HBcat [11]. The active
catalyst is believe to be derived from the zwitterionic species
Rh(
reaction during catalysis [11]. In this study, we have found that
small amounts of [Rh(
2-o-Ph2PC6H4CH2NHAr)(dppb)][Bcat2] (4,
h
6-catBcat)(dppb), which arises from a boron-redistribution
k
Ar ¼ 4-C6H4OMe) crystallized out of solution upon completion of
reactions using Rh(acac)(dppb). We hypothesized that compound 4
arises from the addition of the reduced product 3 with excess HBcat
and adventitious water. Indeed, we were able to generate 4 in re-
actions of 3 with Rh(acac)(dppb) and excess HBcat in methanol,
along with a number of redistribution and degradation products
(Scheme 4). Once again, the formation of the [Bcat2]ꢁ anion arises
from a redistribution reaction. The 11B NMR spectrum for 4 shows
the expected sharp singlet at ca.
d 14 ppm, which is diagnostic for
the [Bcat2]ꢁ anion [11], and the 31P NMR spectrum shows that all
the phosphorus atoms in 4 are magnetically inequivalent. As such,
each peak appears as a doublet of doublet of doublets, with
coupling to rhodium and the two other neighboring phosphorus
atoms. Compound 4 was also confirmed by a single crystal X-ray
diffraction study and the molecular structure of which is shown in
showed a broad peak at
contained two resonances, one at
the other at
d
21.2 ppm, while the 11B NMR spectra
39 ppm for the new NBH2 boron
d
d
ꢁ37 ppm for the four coordinate adduct. To confirm
the formation of 7 in these reactions, a single crystal X-ray
diffraction study was preformed and the molecular structure of the
resulting study is shown in Fig. 3. The P(1)eB(2) bond of
ꢀ
Fig. 2. The metal amine bond distance is RheN 2.205(2) A and the
ꢀ
phosphine trans to this hemilabile ligand has a short RheP(2) bond
1.9232(17) A is similar to that seen in related systems [33,34]. For
ꢀ
length of 2.2356(9) A. Although the other phosphine atom in dppb
example, the analogous bond in a related borane adduct of a P,N-
ꢀ
ꢀ
shows a slightly elongated RheP distance of RheP(3) 2.3393(10) A,
ferrocenyl iminophosphine species is 1.918(4) A [34]. The N(1)e
ꢀ
the P,N ligand also has a considerably short RheP(1) bond at
B(1) bond distance of 1.380(2) A is somewhat shorter than that
ꢀ
Fig. 3. The molecular structure of 7 with ellipsoids drawn at the 50% probability level and hydrogen atoms omitted for clarity. Selected bond distances (A): P(1)eB(2) 1.9232(17),
N(1)eB(1) 1.380(2), N(1)eC(20) 1.4559(17), N(1)eC(19) 1.4763(18). Selected bond angles (ꢂ): C(13)eP(1)eB(2) 114.65(7), C(7)eP(1)eB(2) 111.53(7), C(1)eP(1)eB(2) 113.10(7), B(1)e
N(1)eC(20) 120.56(12), B(1)eN(1)eC(19) 125.93(12), C(20)eN(1)eC(19) 113.22(11).

8
R.J. Burford et al. / Journal of Organometallic Chemistry 731 (2013) 1e9
Scheme 6. Addition of borane 7 to 1-octene and vinyl arenes.
observed in C]NR$BH3 adducts. For comparison, the NeB distance
from initial coordination to the phosphorus atom and then the
imine nitrogen. Addition of excess borane-dimethylsulfide even-
tually led to reduction of the imine and generated an active
RR0NBH2 unit, which went on to make additional aminophosphine
products. Sterically hindered imines are required to stabilize the
newly formed RR0NBH2 units. Initial hydroboration with the new
borane 7 showed good selectivity for the linear addition products
with both 1-octene and vinyl arenes. Reactions of iminophosphines
with alkyl and dialkylborane were also complicated by adduct
formation, making reduction of the imine increasingly difficult
with increased steric bulk of the borane. Addition of dialkoxybor-
anes, catecholborane and pinacolborane, were promising for the
less hindered imine, but harsh reaction conditions and complicated
product distributions were observed in reactions with the
sterically-hindered imine. Further work in this area is currently
underway in an effort to expand the scope of these reductions, the
results of which will be published in due course.
in the borane adduct of 1,4,6-triazabicyclo[3.3.0]oct-4-ene, which
ꢀ
retains the imine functionality, is 1.552(2) A [35,36]. The two ni-
trogen carbon distances in 7 are similar at N(1)eC(20) 1.4559(17)
ꢀ
and N(1)eC(19) 1.4763(18) A, as expected for the reduced product.
What is remarkable about 7 is that we have isolated a novel
monosubstituted borane where the steric bulk of the amido group,
unlike in the case with L1, prevents the BeH bond from reacting
further with the bulky imine L2 and/or 6.
To examine the potential use of 7 as a hydroborating reagent, we
have carried out some initial studies on simple aliphatic and aro-
matic alkenes. Indeed, we have found that reactions of 7 with 1-
octene proceeded slowly (>48 h) at room temperature to give the
corresponding reduced organoborane 8 as the major boron-
containing species (Scheme 6). Using elevated temperatures
greatly reduced reaction times as the addition was completed
within 4 h at 80 ꢂC. Borane 7 could also be used effectively in the
reduction of vinyl arenes, with the major product being the ex-
pected terminal addition products 9 and 10. Interestingly, reactions
also gave minor amounts of the internal addition product where
the boron fragment has added to the carbon alpha to the arene ring.
The electronic nature of the vinyl arene group did not appear to
have a significant effect on hydroboration selectivities. In the case
of 4-vinylanisole, 4% of the branched product was produced, where
the steric bulk of the borane was presumably insufficient to affect
this reaction with complete selectivity. Similar selectivities have
been observed with related monoalkyl- [37,38] and arylboranes
[39]. Borane 7 was added to two equivalents of vinyl arene to see if
the second BeH could be used to reduce another alkene group.
Although longer reaction times were required (12 h), compound 7
could indeed be used to reduce two equivalents of alkene.
Addition of alkyl and dialkylboranes, such as thexylborane and
9-BBN, were also investigated but all afforded a complicated
mixture of products. Finally, we examined the addition of HBcat
and HBpin to the bulkier iminophosphine L2. Although addition
of HBcat to L2 proceeded at room temperature to generate some
of the corresponding reduced imine product, reactions were
complicated by borane degradation and complicated product
distributions. The use of metal complexes to catalyze this addition
did not significantly improve yields, and in some cases, more of
the degradation products were observed [11]. Additions with
HBpin were remarkably sluggish, but considerably cleaner, and
the reduced amine product 11 could be isolated, albeit in low
yields (24%).
Acknowledgments
Thanks are gratefully extended to Mount Allison University, the
University of New Brunswick, NSERC, and the Canada Research
Chair Program for financial support. We also thank Dan Chilliwack
Durant for his expert technical assistance and anonymous re-
viewers for the very helpful suggestions.
Appendix A. Supplementary material
CCDC 905415, 905422 and 905441 contain the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
http://ccdc.cam.ac.uk/data_request/cif.
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