Organometallics
Article
(DPDS),36 and (EtO)2MeSiH/(RO)2P(O)OH;37 aluminum
hydrides such as LiAlH4,38,39 LiAlH4/CeCl3,40 AlH3,41 and
HAl(i-Bu)2;42 low-valent metals such as SmI2/HMPA
(hexamethylphosphoramide)43 or Cp2TiCl2/Mg;44 hydrocar-
this, a simple procedure for the conversion of 4 to 5 would be a
great advantage. Such a process might also permit access to
other challenging phosphine(III) and metal catalysts as well as
permitting the recovery of the valuable phosphine(III) ligands:
“closing the phosphorus cycle” is of increasing importance due
to environmental and availability concerns.82−84 Herein, we
report a new activation/deprotection of phosphine(V) oxides
without the use of harsh reaction conditions, metals, or
sacrificial phosphanes. Intermediate CPSs are directly con-
verted to desired phosphines by reaction with hexachlor-
odisilane. Mechanistic details have been elucidated by
experimentation and supported by computation. The “one-
pot” procedure affords excellent yields of pure phosphine(III)
ligands that can be telescoped into formation of transition
metal catalysts without the prior need for silica gel
chromatography.
bon/activated carbon;45 and electrochemical reduction.46−48
A
mild iodine-catalyzed reduction of phosphine(V) oxides
employing a sacrificial electron-rich phosphine was developed
by Laven and Kullberg,49 while Li et al.50 employed less
expensive phosphite, although in both cases P(V)O-
containing contaminants must be removed from the final
products. Thus, disadvantages of these procedures include
harsh reaction conditions, toxic and/or highly reactive,
potentially explosive reducing agents, narrow scope or
undesirable side reactions, e.g., C−P,51,52 C−O,52 or P−
N53−56 bond cleavage, and laborious column chromatography
to purify the desired phosphine(III).
Reduction of Activated Chlorophosphonium Salts.
The inherent stability of the P(V)O has compelled others to
explore sequential activation reduction methods, i.e., the
conversion of the phosphine(V) oxide to more reactive
chlorophosphonium salts (CPS) and subsequent reduction
(Scheme 1b,c). Horner, Hoffmann, and Beck first published
the reduction of chlorotriphenylphosphonium chloride
(Ph3PCl2) in 1958,57 with both LiAlH4 and sodium. The
following year a sequential activation and deprotection was
published, converting triphenylphosphine(V) oxide (Ph3PO)
first to activated CPS, Ph3PCl2, before it was reduced to
triphenylphosphine (Ph3P) with sodium metal.58 Being readily
afforded via inexpensive chlorinating reagents,59 CPSs have
also been reduced with aluminum/metal salts,60 alkali
metals,57,58 LiAlH4,57,61,62 thiols/Et3N,63 activated carbon,45
Hantzsch ester/Et3N,64 electrochemically,46−48,65,66 elemental
aluminum67,68 or silicon,69 and hydrogenolysis,70 which may
be catalyzed by frustrated Lewis pairs (FLPs).71,72 Harsh metal
bases and Grignard reagents have even been used to deprotect
RESULTS AND DISCUSSION
■
Reduction of Activated CPSs with Disilane. In 1996,
BASF reported the generation of tetrachlorosilane (SiCl4)
when the CPS, Ph3PCl2 (2), was heated with elemental silicon
at 185 °C.69 Not wanting to expose our ligand precursor to
such harsh reaction conditions, we hypothesized that
hexachlorodisilane might serve as a suitable surrogate for
elemental silicon and similarly generate 2 equiv of SiCl4 on
reactions with a CPS. The abundant industrial byproduct
Ph3PO (1) appeared to be the ideal test substrate,3,4 and was
easily converted to activated Ph3PCl2 (2) with inexpensive
oxalyl chloride.59 Gratifyingly on reaction with 1.1 equiv of
hexachlorodisilane (Si2Cl6) at room temperature, both 1H
NMR and 31P NMR indicated the immediate, clean, and
complete formation of Ph3P (3), with 29Si NMR showing only
the formation of tetrachlorosilane, SiCl4 (δ = −18.8 ppm).
Motivated by the ability of Si2Cl6 to reduce 2, we chose to
explore other disilanes (Table 1, entries 2−10): 1,1,2,2-
tetrachloro-1,2-dimethyldisilane (Si2Me2Cl4), hexamethyl-
disilane (Si2Me6), and hexaphenyldisilane (Si2Ph6), which
might generate the corresponding attractive byproducts
certain CPSs.73 Alternatively, CPS can be converted to
74,75
phosphine−boranes by either NaBH4
or LiBH4,76−79
although ultimately the borane “protecting group” itself
requires removal (Scheme 1b,d,e).
Motivation to Develop a New Facile Reduction of
Phosphine(V) Oxides. Our interest in phosphine(V) oxides
reduction originates from our desire to explore bulky N-
phosphinomethyl-functionalized N-heterocyclic carbene li-
gands (NHCPs)80,81 as potential ligands for new olefin
metathesis catalyst (Scheme 2).19 Progress has been severely
Table 1. Reaction of Phosphonium Salts with Disilanes
a
a
Scheme 2. Problematic Reduction of NHCP Precursor
entry CPS 2a−c, X =
disilane
equiv
time
conv to 3 [%]
1
2
3
4
5
6
7
8
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
OTf
OTf
OTf
BArCl
Si2Cl6
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.0
1.0
1.1
1.1
1.1
4
5 min
5 min
1 day
2 days
3 days
4 days
5 days
6 days
1 day
1 day
10 min
1 day
2 days
2 days
100
0
Si2Me2Cl4
Si2Me2Cl4
Si2Me2Cl4
Si2Me2Cl4
Si2Me2Cl4
Si2Me2Cl4
Si2Me2Cl4
Si2Me6
Si2Ph6
Si2Cl6
Si2Cl6
Si2Cl6
28
55
72
78
83
100
0
0
7
80
100
0
a
Synthesis of NHCP 6 via the challenging reduction of phosphine(V)
oxide in azolium salt 4 to phosphine(III) 5.
hampered due to difficulties accessing azolium salt 5, with the
problematic reduction of 4 being achievable only with a large
excess of trichlorosilane (27.0 equiv) in anhydrous degassed
chlorobenzene at elevated temperature over 2 days.19 As well
as the lengthy reaction time, we experienced some reprodu-
cibility issues, with the unsuccessful reduction being
accompanied by the decomposition of the precious azolinium
4, previously obtained via a multistep synthesis.19 In light of
9
10
11
12
13
14
Si2Cl6
a
Conversion judged by 31P NMR of 2a−c relative to 3.
694
Organometallics 2021, 40, 693−701