Reduction of tertiary phosphine oxides to phosphine-boranes using Ti(Oi-Pr)4/BH3-THF

Article abstract of DOI:10.1016/j.tet.2021.132057

A new method for reduction of tertiary phosphine oxides leading to the formation of tertiary phosphine-boranes has been developed. The BH₃-THF/Ti(Oi-Pr)₄ reducing system enables conversion of triaryl, diarylalkyl and trialkylphosphine oxides directly to their borane analogues in good to high yields. In contrast to the previously reported protocols, the presence of activating groups in the structure of starting material is not necessary for the reaction to occur. The reaction is highly stereoselective and proceeds with predominant retention of configuration at the phosphorus atom. A plausible mechanism of reduction of the P[dbnd]O bond by BH₃-THF/Ti(Oi-Pr)₄ has been proposed.

Full text of DOI:10.1016/j.tet.2021.132057

Tetrahedron 86 (2021) 132057
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Tetrahedron
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Reduction of tertiary phosphine oxides to phosphine-boranes using
Ti(Oi-Pr)4/BH3-THF
Sylwia Sowa*, K. Michał Pietrusiewicz*
Department of Organic Chemistry, Faculty of Chemistry, Institute of Chemical Sciences, Marie Curie-Sklodowska University in Lublin, 33 Gliniana St., Lublin,
20-614, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
A new method for reduction of tertiary phosphine oxides leading to the formation of tertiary phosphine-
boranes has been developed. The BH3-THF/Ti(Oi-Pr)4 reducing system enables conversion of triaryl,
diarylalkyl and trialkylphosphine oxides directly to their borane analogues in good to high yields. In
contrast to the previously reported protocols, the presence of activating groups in the structure of
starting material is not necessary for the reaction to occur. The reaction is highly stereoselective and
proceeds with predominant retention of configuration at the phosphorus atom. A plausible mechanism
of reduction of the P]O bond by BH3-THF/Ti(Oi-Pr)4 has been proposed.
Received 3 January 2021
Received in revised form
17 February 2021
Accepted 23 February 2021
Available online 4 March 2021
Keywords:
Reduction
© 2021 Elsevier Ltd. All rights reserved.
Phosphine
Phosphine oxide
Lewis acid
Stereoselectivity
1. Introduction
same group has observed a partial racemization of phosphorus
center (Scheme 1, eq. B) [9].
The reduction of phosphine oxides to phosphines, especially in a
stereoselective way is still one of the most explored topics in the
current organophosphorus chemistry [1]. Till now many different
reducing agents were tested in transformation of phosphine oxides
to phosphines including hydrides [2], silanes [3], metal complexes
[4] and organoboron compounds [5]. Among them silanes are still
considered as the most efficient in deoxygenation of phosphine
oxides due to their chemo- and stereoselectivity [6]. In 1994,
Lawrence et al. developed the first reduction of P]O bond by si-
lanes catalysed by a Lewis acid [7]. They showed that the reduction
of tertiary phosphine oxides using PHMS occurs under much milder
conditions when run in the presence of stoichiometric amounts of
More recently, reduction methods based on the silane/catalyst
system have attracted even more attention. Not only Ti(Oi-Pr)4 but
also other Lewis acids [InBr3 [10], Cu(OTf)2 [11], B(C6F5)3 [12],
electrophilic fluorophosphonium cations (EPCs) [12]] as well as
Brønsted acids [bis(2-chlorophenyl)borinic acid [13], bis(p-nitro-
phenyl)phosphoric acid (BNPA) [14], TfOH [15], MsOH [15b]] have
been reported to promote silane P]O reductions, when added in
either catalytic or stoichiometric amounts. Among these protocols a
significant advance in this field was the one developed by Beller’s
group on the use of strong Brønsted acid along with a large excess
of diethoxymethylsilane [14a]. This new reducing mixture showed
unprecedented tolerance to a number of sensitive functional
groups present in the phosphine oxide structure, e.g., formyl, cyano,
allyl, ester, and amide, and trimethylsilyl group, and was shown to
proceed with inversion of configuration at phosphorus atom
(Scheme 1, eq. C). The other use of BNPA with DPDS, reported by
Aldrich, enabled also the reduction of (S,S)-DIPAMP(O)2 in a ster-
eoselective way also with inversion of configuration at the phos-
phorus atom (Scheme 1, eq. D) [14c].
Ti(Oi-Pr)4 and proceeds in
a predominantly stereoretentive
manner. At the same time, they also presented the first catalytic
reduction of P]O bond using triethoxysilane assisted by the same
Lewis acid and occurring with identical stereochemical outcome
(Scheme 1, eq. A). Later, Ti(Oi-Pr)4 was employed in another cata-
lytic reduction of P]O bond using 1,1,3,3-tetramethyldisiloxane
(TMDS) [8], but in case of the reduction of (S,S)-DIPAMP(O)2 the
In the meantime, methodologies offering direct transformations
of phosphine oxides into easily handled and storable borane-
protected phosphines without isolation of the intermediate free
phosphines have also been developed and start to become practical
* Corresponding authors.
E-mail addresses: sylwia.sowa@poczta.umcs.lublin.pl (S. Sowa), kazimierz.
pietrusiewicz@poczta.umcs.lublin.pl (K. M. Pietrusiewicz).
https://doi.org/10.1016/j.tet.2021.132057
0040-4020/© 2021 Elsevier Ltd. All rights reserved.

S. Sowa and K.M. Pietrusiewicz
Tetrahedron 86 (2021) 132057
BH3 complexes according to the existing protocols.
Herein, we wish to present the use of Ti(Oi-Pr)4/BH3-THF
reducing system which combines the beneficial presence of a Lewis
acid and reducing ability of BH3 and enables the direct trans-
formation of tertiary phosphine oxides to their P-borane de-
rivatives. To our best knowledge such process has never been
reported in the literature.
2. Results and discussion
At the outset, we have investigated the influence of an added
Lewis acid on BH3 reduction of 1-phenylphospholane 1-oxide (1a)
known already to undergo transformation into 2a using BH3 com-
plexes alone, but only after prolonged heating at elevated tem-
perature (60 ꢀC, 2 d) [18]. The results of our screening experiments
are listed in Table 1.
As the data collected in Table 1 indicate, this conversion can be
effectively carried out at room temperature and in short reaction
time when carried out in the presence of a Lewis acid. We focused
our attention mainly on the use of Ti(Oi-Pr)4 which was one of the
most effective additives in the reported reductions by silanes [7e9].
First, we subjected 1a to reduction by BH3-THF in the presence
of a catalytic amount of Ti(Oi-Pr)4 (15% mol, Table 1 entries 1e2).
These preliminary experiments revealed that conversion of starting
material cannot be increased by prolonging the reaction time but
that it was rather dependent on amount of added Lewis acid
(Table 1, entries 1e3). However, even stoichiometric amount of
Ti(Oi-Pr)4 was not enough to transform 1a into 2a within expected
time (Table 1, entry 4). It turned out that using 3-fold excess of
Lewis acid was most beneficial and allowed to achieve complete
conversion of 1a to 2a after 40 min (Table 1, entry 5). Briefly
attempted use of other Lewis acids turned out to be much less
effective (Table 1, entries 6e8). In case of B(OMe)3 only 12% of ex-
pected product was obtained, even when the reaction was carried
out at 60 ꢀC for 24 h (Table 1, entry 6). In turn, BF3*Et2O complex
under the same conditions has not provide 2a (Table 1, entry 7). A
catalytic amount of Yb(OTf)3 used as an additive did not result in
complete conversion of 1a after 48 h at rt, and lead to desired 2a in
only 22% yield.
Scheme 1. Examples of reduction of P]O bond with silanes in presence of Ti(Oi-Pr)4
proceeding in a stereoselective way.
alternatives to reductions stopping at the phosphine stage. Indeed,
the method (LiAlH4/NaBH4/CeCl3) [16] introduced by Imamoto and
a family of reducing systems reported successively by Gilheany’s
group [17] provided phosphine-boranes with excellent yields
directly from their phosphine oxide precursors. Two methods
introduced by Gilheany at al., i.e., reduction of phosphine oxides
using Meerwein salt/NaBH4 [17a] and reduction of diastereomeri-
cally enriched menthoxyphosphonium salts [17b] using LiBH4 or L-
selectride [17c] provide phosphine boranes in the stereoselective
manner.
After this brief optimisation of the reaction conditions we
turned then to check the possibility of similar reductions of
representative tertiary phosphine oxides 3 which were known to
be resistant to BH3-THF alone. The results are summarised in
Table 2.
Table 1
Optimisation of reaction of 1a in presence of different Lewis acids.
Among methods enabling conversion of P]O bond to PeBH3
moiety, those using BH3 complexes as reducing agents (pioneered
€
by Koster and Morita [5a], and continued by Keglevich [18] and our
group [19]) have gained recently considerable importance. In case
of secondary phosphine oxides [19] this transformation occur un-
der mild conditions and with high stereoselectivity. It has been
found especially successful in reductions of phosphine oxides
No.
Lewis acid (equiv.)
Conditions
Isolated yield of 2a (%)
possessing heteroatom activating groups in the
a and b positions as
1
2
3
4
5
6
7
8
Ti(Oi-Pr)4 (0.15)
Ti(Oi-Pr)4 (0.15)
Ti(Oi-Pr)4 (0.5)
Ti(Oi-Pr)4 (1.0)
Ti(Oi-Pr)4 (3.0)
B(OMe)3 (3.0)
BF3*Et2O (3.0)
Yb(OTf)3 (0.1)
rt, 55 min
rt, 4 h
37a
37a
50a
70a
98
ꢀ
demonstrated recently by Kiełbasinski [20], us [21], and others [22].
With a possible exception of five-membered ring phosphine oxides
[18], the major limitation of this strategy has been its inefficiency in
reductions of tertiary phosphine oxides which do not possess OH,
NH or SH moiety in proximity to the P centre.
rt, 55 min
rt, 55 min
rt, 40 min
60 ꢀC, 24 h
60 ꢀC, 24 h
rt, 48 h
12a
0a
22a
Therefore, we were interested in expanding the use of BH3 on
reduction of tertiary phosphine oxides which cannot be reduced by
a
Starting material present in the crude reaction mixture.
2

S. Sowa and K.M. Pietrusiewicz
Tetrahedron 86 (2021) 132057
In the case of triphenylphosphine oxide (3a) it was quickly
found that even with keeping the optimised stoichiometry of the
reducing mixture, it was necessary to increase the reaction tem-
perature to 80 ꢀC in order to obtain a significant level of the
reduction (Table 2, entry 1). Interestingly, in this case the product of
the reaction was not the expected phosphine-borane 5a but the
corresponding phosphine 4a which was isolated in 80% yield.
When 3a was subjected to a reaction with BH3-THF/Ti(Oi-Pr)4 in a 6
equiv./3 equiv. ratio, we were pleased to find that 3a was
completely transformed into the corresponding tertiary
phosphine-borane 5a (91% isolated yield) after 24 h (Table 2, entry
2). The same conditions used for reduction of diphenylmethyl-
phosphine oxide 3b resulted in the formation of a mixture of 4b and
5b (isolated in 40% and 35% yield, respectively) (Table 2, entry 3).
Lowering the amount of BH3 complex failed to provide uncom-
plexed phosphine 4b as the sole product, as it was observed in
reaction of 3a, but in this case we were able to isolate major free
phosphine 4b in 57% yield (Table 2, entry 4). For comparison, we
also tried to use BH3eSMe2 (4 equiv.) as a hydride source in re-
ductions of 3b. In this case, 3b was fully consumed and was con-
verted directly to the expected phosphine-borane 5b. No free
phosphine 4a was detected. However, 5b was isolated in only 44%
yield (Table 2, entry 5) due probably to the presence of another
Scheme 2. Reduction of tertiary phosphine oxides 3c-i with BH3-THF/Ti(Oi-Pr)4.
unidentified product detected in the crude reaction mixture by 31
P
NMR (dP 48.0 ppm, broad peak) which could not be isolated.
Table 2
Optimisation of reduction of tertiary phosphine oxides 3a-b with BH3-THF/Ti(Oi-Pr)4.
Entry Starting material
Conditions
Ratio of 4/5 after 24 h (31P NMR)
Ratio of 4/5 after
Product
Yield (%)a,b
48 h before addition of the second
part of BH3-THF (31P NMR)
1
BH3-THF (3 equiv.), 24 h
4a/5a 80/0c
-
80 (80)c,d
91 (100)d
2
3
4
5
6
BH3-THF (6 equiv.), 24 h
BH3-THF (6 equiv.), 24 h
BH3-THF (4 equiv.), 24 h
BH3eSMe2 (4 equiv.), 24 h
BH3-THF (3 þ 3 equiv.), 48 h
4a/5a 0/100
4b/5b 44/56
4b/5b 59/31
4b/5b 0/77
e
40 (44)d,e
45 (56)d,f
-
57 (59)c,d,e
20 (31)c,d,f
-
-
44 (77)d
83 (100)
4b/5b 33/63c
4b/5b 22/78
a
Isolated yields of product.
b
Numbers in parentheses indicate yields according to31P NMR after additional complexation of phosphine with BH3-THF.
c
d
e
f
Some amounts of the starting material were still present in the final reaction mixture.
Reaction was quenched without the additional boranation step.
The yield of 4b.
The yield of 5b.
3

S. Sowa and K.M. Pietrusiewicz
Tetrahedron 86 (2021) 132057
Therefore, to avoid this impediment, we focused our attention on
application of BH3-THF in the further study.
Because in case of 3b addition of 6 equivalents of BH3-THF at
once did not result in effective transformation of P]O bond to
PeBH3 (Table 2, entry 3), we decided to add reagents initially in a 3
equiv./3 equiv. ratio and, after 24 h, add another 3 equiv. of BH3-THF.
We have found that under these modified conditions the selectivity
of the reaction was markedly shifted towards the formation of 5b,
but still 22% of the produced phosphine 4b remained uncomplexed
in the reaction mixture even after 48 h (Table 2, entry 6). To com-
plete the formation of 5b additional portion of BH3-THF (3 equiv.)
was added and the reaction was stirred at room temperature until
disappearance of 4b. After this additional complexation, the reac-
tion cleanly yielded phosphine-borane 5b in 83% isolated yield.
Next, the developed conditions were applied in reductions of a
series of phosphine oxides of varied substitution pattern (Scheme
2). First we investigated another alkyldiarylphosphine oxide 3c
(PAMPO), possessing more crowded P]O centre. As expected, it
exhibited low susceptibility to reduction under the studied condi-
tions and even after 48 h the starting 3c was consumed only in
about 74%. Moreover, the phosphine 4c appeared apparently the
most resistant towards complexation by BH3 among all the inves-
tigated cases and phosphine-borane 5c was isolated in only mod-
erate yield (47%). On the other hand, low complexation level (about
60% of 4c and only 14% of 5c in the crude reaction mixture after
48 h, see SI: Table 1) implies a reasonable access to 4c under these
reaction conditions. In addition, different reactivity of 3b and 3c
points towards steric effects as an important factor influencing
reduction rate of tertiary phosphine oxides. Interestingly, replacing
BH3-THF with BH3eSMe2 as a hydride source in reduction of 3c
resulted in complete conversion of 3c into desired phosphine-
borane 5c which was finally isolated in 65% yield (Scheme 2, see
also SI, Table 1).
Scheme 3. Attempted reduction of phosphine oxide 6 with BH3eSMe2/Ti(Oi-Pr)4.
phosphine-borane 5i was the only product formed and it was
separated from the unconverted 3i in 33% yield. Interestingly, the
conversion of 3i could not be improved by changing the borane
reagent to BH3eSMe2, like it was observed for 3c (Scheme 2). With
this reagent the formation of only traces of 5i was observed (not
isolated).
We were also interested in checking whether phosphine oxides
possessing free OH group in the g-position (a position too distant to
promote reduction by BH3 according to our previously reported
protocol [21a]) can be reduced with BH3/Ti(Oi-Pr)4 reducing
mixture. However, the attempted reduction of
g-hydrox-
yphosphine oxide 6 under the both studied conditions failed
completely. In reaction of 6 with BH3eSMe2 only the starting oxide
6 was observed in the final reaction mixture (31P NMR) (Scheme 3).
In turn, use of BH3-THF resulted in the formation of a mixture of
phosphine oxides in which unreacted 6 was still the major
constituent.
In the last part of this study, we investigated stereochemical
outcome of the studied reduction. Toward this end, enantiomeri-
cally pure (SP)-PAMPO (Sp)-3c was subjected to our optimised
reduction protocol and gave 5c in 58% yield (Scheme 4, path a).
Then, the resulting 5c was subjected to P-deprotection in the
presence of DABCO at 40 ꢀC for 6 h followed by oxidation of the
resulting phosphine by H2O2 to afford optically active 3c (Scheme 4,
path b). Comparison of signs of the specific rotations of starting
phosphine oxide (Sp)-3c with the one obtained in the correlation
process revealed that both compounds possess the identical
configuration at phosphorus. Hence, the absolute configurations of
the newly formed oxide 3c was assigned as SP. These results indi-
cated that the studied reduction had to occur with retention of
configuration at phosphorus since the next two steps in the cor-
relation i.e., deboranation of phosphineꢁborane by DABCO [16,23]
Applying analogous reduction conditions to diary-
lalkylphosphine oxide 3d, possessing protected sulfanyl moiety,
resulted in nearly full consumption of 3d as well as in much better
selectivity towards formation of phosphine-borane and afforded 5d
in 65% isolated yield (Scheme 2).
Then, we investigated reductions of dialkylarylphoshine oxides
3e-h. In all the four cases, we observed complete conversions of 3e-
h as well as the preferential formations of phosphine-boranes 5e-h
as the decidedly major products in the crude reaction mixtures. In
the cases of 3e-g, additional complexation step secured complete
transformation of 4e-g to the desired phosphine-borane 5e-g what
was confirmed by 31P NMR. However, the isolated yields of 5e-g fell
in the range of 61e73%. Most probably, the applied water work-up
must have lowered markedly the isolated yields in these cases. The
highest amount of phosphine-borane 5 formed directly was
observed in the case of 3h where the ratio of phosphine 4h to
phosphine-borane 5h equaled to 6/94 (see SI, Table 1). In this case
additional complexation with BH3 was omitted but isolated yield of
5h was again lowered to 56%.
Finally, it is interesting to compare facility of reduction of cyclic
phosphine oxides 1a and 3g by BH3-THF/Ti(Oi-Pr)4. While at room
temperature the five-membered phospholane oxide 1a was swiftly
reduced within 40 min (Table 1, entry 5), reduction of the six-
membered ring phosphorinane oxide 3g under the same condi-
tions proceeded much more slowly to reach still incomplete 77%
conversion after 24 h. This observation stays in line with early
suggestion by Keglevich et al. that the five-membered ring strain
facilitates the reduction of P]O bond with BH3 [18].
In contrast to 3a-h, when tricyclohexylphosphine oxide (3i) was
treated with BH3-THF/Ti(Oi-Pr)4 system under the studied condi-
tions the reaction was much slower and reached only 58% con-
version in standard 48 h (see SI, Table 1). Nevertheless, the desired
Scheme 4. Reduction of optically active phosphine oxide (Sp)-3c to phosphine-borane
5c and its chemical correlation to (Sp)-3c.
4

S. Sowa and K.M. Pietrusiewicz
Tetrahedron 86 (2021) 132057
and oxidation of phosphine by H2O2 [24], were already known to
proceed with retention of configuration at the phosphorus atom. As
evidenced by the HPLC analysis of the two oxides on a chiral sta-
tionary phase, the reduction took place with some loss of enan-
tiomeric purity as the oxide obtained in the correlation process was
found to be of 86% enantiomeric purity. This result closely re-
sembles partial racemisations observed already by Lawrence [7]
and by Lemaire [9] in their reductions of phosphine oxides by si-
lanes promoted by Ti(Oi-Pr)4. Based on the early observation by
Mislow et al. [25] that metal hydrides cause racemization of
phosphines, it is tempting to assume that the involvement of the
titanium hydride in the studied reductions mediated by Ti(Oi-Pr)4
may be responsible for the observed loss of enantiomeric purity of
the produced phosphine.
A plausible mechanism of the studied reduction process which
bears some resemblance to the mechanistic proposal reported for
analogous reductions utilising (RO)3SiH/Ti(Oi-Pr)4 system [7] is
presented in Scheme 5. It commences with the formation of tita-
nium hydride (IV) from Ti(OR)4 (I) and BH3-THF (II) via TS III
(Scheme 5, eq. 1). The formation of titanium hydride IV is indicated
by the development of a black colour (typically associated with the
formation of titanium hydrides) which fades off when the reaction
progresses. Then, triisopropoxytitanium hydride (IV) attacks the
phosphoryl bond of VI with the formation of hydrophosphonium
salt VIII via four-membered TS VII. Upon action of the second
molecule of titanium hydride IV, assisted probably by triethyl
titanoate anion, the hydrophosphonium salt VIII liberates free
phosphine X and hydrogen, together with the formation of trie-
thyltitanoate anhydride IX. Finally, added further equivalents of
BH3-THF (II) secure full conversion of X to XI, which with 3 or even
6 eq. of II originally applied has not always been complete (Scheme
4, eq. 2). Apparently, before this last protection stage, the free
phosphine X, which according to this mechanism is formed with
retention of configuration at P, can undergo partial racemization as
it was observed in reduction of the optically active PAMPO (cf.
Scheme 4). The proposed mechanism is fully consistent with the
retention of configuration at phosphorus already evidenced in the
correlation presented in Scheme 4.
phosphine-borane 5, as confirmed in cases of 3a, 3b, and possibly
also 3c (cf. Table 2, entries 1e2, 3e4, and Scheme 2, respectively).
This has not been optimised within the present work but appar-
ently it may be used already to practical advantage at least in re-
ductions of triarylphosphine oxides.
3. Conclusions
In summary, we have presented an efficient method for the
direct one-pot conversion of tertiary phosphine oxides to the cor-
responding phosphine-borane derivatives using BH3-THF in com-
bination with Ti(Oi-Pr)4 as the reducing agents. The utility of the
reduction process has been validated for triaryl, diarylalkyl, dia-
lkylaryl and trialkylphosphine oxides. It is worth noting, that the
developed reduction method expands the scope of the use of BH3 as
reducing agent also on simple tertiary phosphine oxides and thus,
makes it complementary to previously reported reductions by this
reagent which have been limited so far to five-membered ring
phosphine oxides [18], secondary phosphine oxides [19], and ter-
tiary phosphine oxides bearing proximal -XH (X ¼ N, O, or S)
activating groups [20e22]. The stereochemical course of the
reduction process has been established by chemical correlation and
a plausible mechanism of the reduction of the P]O bond by BH3-
THF/Ti(Oi-Pr)4 has been proposed.
It is also important to stress that the proposed use of BH3 in
combination with Ti(Oi-Pr)4 allows to avoid toxicity and potential
hazard connected with the analogous use of silanes [7].
4. Experimental section
All reactions were performed under an argon atmosphere using
Schlenk techniques. Only dry solvents were used, and glassware
was heated under vacuum prior use. All chemicals were used as
received unless noted otherwise. Solvents for chromatography and
crystallization were distilled once before use and the solvents for
extraction were used as received. Toluene were distilled from so-
dium/benzophenone ketyl under argon.
1H NMR, 31P NMR and 13C NMR spectra were recorded on Bruker
Advance 500 spectrometer at ambient temperature in CDCl3 unless
otherwise noted. Chemical shifts (d) are reported in ppm from
Finally, the last equation (Scheme 5, eq. 3) presents the possible
regeneration of titanium hydride (IV) in reaction of IX with another
molecule of BH3. This would explain why use of Ti(Oi-Pr)4 in only
catalytic amounts could also be effective, and why it was necessary
to add extra 3 equiv. of BH3-THF to achieve complete boranation of
the formed phosphine. However, the enhanced consumption of BH3
in reductions of TieO bonds gives a possibility to obtain reduced
phosphorus compound in either form, as free phosphine 4 or as
tetramethylsilane with the solvent as an internal indicator (CDCl3
7.27 ppm for 1H and 77 ppm for 13C). Mass spectra were recorded
on Shimadzu GC-MS QP2010S in electron ionization (EI). Melting
points were determined on Büchi Melting Point M ꢁ 560 in a
capillary tube and were uncorrected. HPLC-HRMS was performed
on Shimazu HRMS ESI-IT-TOF using reverse phase stationary phase
with water/MeCN 65:35 as eluent, electrospray ionization (ESI),
and IT-TOF detector. Optical rotations were measured on Perkin
Elmer 341LC using a 1 mL cell with a 10 cm path length and are
reported as follows: [a] [25]D (c: g/100 mL, in solvent). Elementary
analyses were performed on PERKIN ELMER CHN 2400. Thin-layer
chromatography (TLC) was performed with precoated silica gel
plates and visualized by UV light or KMnO4 solution or iodide on
silica gel. The reaction mixtures were purified by column chroma-
tography over silica gel (60e240 mesh).
4.1. Procedures
A. Procedure for the reduction of phospholane oxide 1a using
Ti(Oi-Pr)4/BH3-THF (Table 1, entry 5)
In a Schlenk tube (25 mL) equipped with a magnetic stirrer and
an argon inlet tertiary phosphine oxide 1a (0.4 mmol) in anhydrous
toluene (4 mL) was placed. Then, Ti(Oi-Pr)4 (0.355 mL, 1.2 mmol)
was added followed by BH3-THF complex (1.2 mL, 1.2 mmol, 1 M
Scheme 5. Plausible mechanism of reduction of optically active phosphine oxide (Sp)-
3c with BH3-THF/Ti(Oi-Pr)4.
5

S. Sowa and K.M. Pietrusiewicz
Tetrahedron 86 (2021) 132057
solution in THF). After addition of BH3 complex the reaction
mixture was stirred at rt for 40 min. Reaction mixture was
quenched with addition of 5% NaHCO3 solution (5 mL). Then re-
action mixture was transferred to separation funnel and extracted
with CHCl3 (4 ꢂ 10 mL). The collected organic phases were dried
over Na2SO4, filtered off and evaporated to dryness. The residue
was purified by column chromatography using silica gel hexane/
AcOEt 6:1 as eluent.
procedure B to afford 5b (0.0355 g, 0.166 mmol, 83%) as a colourless
oil; Rf 0.53 (hexane/AcOEt 6:1). HRMS (ESI) found: 427.2076.
C
26H31B2P2 requires 2 Mþ, 427.2076). 1H NMR (500 MHz, CDCl3)
d
0.70e1.35 (bm, 3H), 1.88 (d, JH-P ¼ 10.09 Hz, 3H), 7.43e7.52 (m,
6H), 7.65e7.69 (m, 4H). 13C NMR (125 MHz, CDCl3)
d 11.8 (d, JP-
¼ 40.9 Hz, CH3), 128.8 (d, JP-C ¼ 10.0 Hz, CH), 130.4 (d, JP-
C
¼ 56.3 Hz, C), 131.1 (d, JP-C ¼ 1.8 Hz, CH), 131.7 (d, JP-C ¼ 10.0 Hz,
C
CH). 31P NMR (202 MHz, CDCl3)
d 9.75 (bm). These data are
1-Phenylphospholane-borane [26] (2a). (Table 1, entry 4). 1a
(0.072 g, 0.4 mmol) was reacted according to general procedure A
to afford 2a (0.0697 g, 0.392 mmol, 98%) as a colourless oil; Rf 0.70
consistent with those previously reported. [2g].
Diphenyl(methyl)phosphine [28] (4b). (Table 2, entry 4).
3b (0.0432 g, 0.2 mmol) was reacted with BH3-THF (0.8 mL,
0.8 mmol, 1 M solution in THF) according to modified general
procedure B to afford 4b (0.0228 g, 0.114 mmol, 57%) as a colourless
oil and 5b (0.0086 g, 0.04 mmol, 20%).
(hexane/AcOEt 2:1). 1H NMR (500 MHz, CDCl3)
d
0.50e1.20 (m, 3H),
2.00e2.29 (m, 8H), 7.46e7.53 (m, 3H), 7.72e7.75 (m, 2H). 13C NMR
(75 MHz, CDCl3)
26.7 (d, JP-C ¼ 37.1 Hz, CH2), 27.4 (s, CH2), 128.7 (d,
d
JP-C ¼ 9.8 Hz, CH), 130.9 (d, JP-C ¼ 2.3 Hz, CH), 131.3 (d, JP-C ¼ 46.6 Hz,
4b
C), 131.3 (d, JP-C ¼ 8.9 Hz, CH). 31P NMR (202 MHz, CDCl3)
d
28.60
Rf 0.85 (hexane/AcOEt 6:1). HRMS (ESI) found: 217.0779;
(bm). These data are consistent with those previously reported. [26].
C
13H13OP [M þ O þ H]þ requires: 217.0777. 1H NMR (500 MHz,
CDCl3)
d
1.65 (d, JH-P ¼ 3.47 Hz, 3H), 7.33e7.37 (m, 6H), 7.42e7.45
B. General Procedure for the reduction of tertiary phosphine ox-
(m, 4H). 13C NMR (125 MHz, CDCl3)
d
12.5 (d, JC-P ¼ 13.6 Hz, CH3),
ides 3 using Ti(Oi-Pr)4/BH3-THF (Table 2)
128.35 (d, JC-P ¼ 4.5 Hz, CH), 128.4 (d, JC-P ¼ 1.8 Hz, CH), 132.8 (d, JC-
¼ 18.7 Hz, CH), 140.1 (d, JC-P ¼ 11.8 Hz, C). 31P NMR (202 MHz,
P
In a Schlenk tube (25 mL) equipped with a magnetic stirrer and
an argon inlet, tertiary phosphine oxide 3 (0.2 mmol) in anhydrous
toluene (1 mL) was placed. Then, Ti(Oi-Pr)4 (0.177 mL, 0.6 mmol)
was added followed by BH3-THF complex (0.6 mL, 0.6 mmol, 1 M
solution in THF). After addition of BH3 the reaction mixture was
heated at 80 ꢀC for 24 h. Then, another portion of BH3-THF (0.6 mL,
0.6 mmol, 1 M solution in THF) was added and the mixture was left
at 80 ꢀC for 24 h to complete conversion of reaction and then re-
action was again checked using 31P NMR technique. Then, when
necessary another portion of BH3-THF (0.2 mL, 0.2 mmol, 1 M so-
lution in THF) was added to transform free phosphine to
phosphine-borane and reaction was left at rt until free phosphine
was consumed (3e10 h). After this time reaction was checked using
31P NMR technique. Then, the reaction mixture was quenched with
addition of 5% NaHCO3 solution (5 mL). Then reaction mixture was
transferred to separation funnel and extracted with CHCl3
(3 ꢂ 10 mL). The collected organic phases were dried over Na2SO4,
filtered off and evaporated to dryness. The residue was purified by
column chromatography using silica gel hexane/AcOEt 10:1 as
eluent.
CDCl3)
reported. [28]
d
ꢁ26.82 (s). These data are consistent with those previously
o-Anisyl(methyl)phenylphosphine-borane [29] (5c). (Table 2,
entry 7). 3c (0.0492 g, 0.2 mmol) was reacted according to general
procedure B to afford 5c (0.0216 g, 0.094 mmol, 47%) as a white
solid; Rf 0.40 (hexane/AcOEt 10:1). 1H NMR (500 MHz, CDCl3)
d
0.64e1.35 (m, 3H), 1.95 (d, JH-P ¼ 10.56 Hz, 3H), 3.70 (s, 3H),
6.88e6.90 (m, 1H), 7.06e7.09 (m, 1H), 7.38e7.42 (m, 3H), 7.49e7.51
(m, 1H), 7.61e7.65 (m, 2H), 7.86e7.91 (m, 1H). 13C NMR (125 MHz,
CDCl3)
d
161.3 (s, C), 135.6 (d, JP-C ¼ 14.5 Hz, CH), 133.7 (d, JP-
¼ 2.7 Hz, CH), 128.5 (d, JP-C ¼ 59.0 Hz, C), 131.0 (d, JP-C ¼ 10.0 Hz,
C
CH), 130.3 (d, JP-C ¼ 2.7 Hz, CH), 128.3 (d, JP-C ¼ 10.9 Hz, CH),121.1 (d,
JP-C ¼ 11.8 Hz, CH), 117.3 (d, JP-C ¼ 54.5 Hz, C), 111.1 (d, JP-C ¼ 4.5 Hz,
CH), 51.3 (s, 3H), 10.5 (d, JP-C ¼ 42.7 Hz, C). 31P NMR (202 MHz,
CDCl3)
d
8.17 (bm). MS (70 eV): m/z (%): 230 (59) [M-BH3]þ, 229
(44), 213 (13), 212 (10), 199 (62), 197 (37), 183 (51), 170 (11). These
data are consistent with those previously reported. [29].
Diphenyl(phenylthiomethyl)phosphine-borane
(5d).
(Table 2, entry 8). 3d (0.0648 g, 0.2 mmol) was reacted according to
general procedure B to afford 5d (0.0419 g, 0.13 mmol, 65%) as a
white solid; Rf 0.46 (hexane/AcOEt 10:1); m.p. ¼ 75.5e76.5 ꢀC;
HRMS (ESI) found: 325.0818; C19H17OPS [M-BH3þO þ H]þ requires:
Triphenylphosphine [15b] (4a). (Table 2, entry 1). 3a (0.0556 g,
0.2 mmol) was reacted with BH3-THF (0.6 mL, 0.6 mmol, 1 M so-
lution in THF) according to modified general procedure B to afford
4a (0.0419 g, 0.16 mmol, 80%) as a white solid; Rf 0.81 (hexane/
325.0810. 1H NMR (500 MHz, CDCl3)
d 0.76e1.48 (m, 3H), 3.72 (d, JH-
¼ 7.25 Hz, 2H), 7.20e7.29 (m, 5H), 7.45e7.56 (m, 6H), 7.73e7.77
P
AcOEt 6:1). 1H NMR (500 MHz, CDCl3)
NMR (125 MHz, CDCl3)
¼ 4.5 Hz, CH), 128.4 (d, JP-C ¼ 1.8 Hz, CH), 128.6 (d, JP-C ¼ 1.8 Hz,
d
7.43e7.61 (m, 15H). 13C
(m, 4H). 13C NMR (125 MHz, CDCl3)
d
33.8 (d, JP-C ¼ 31.8 Hz, CH2),
d
123.2 (d, JP-C ¼ 7.2 Hz, CH), 123.6 (d, JP-
126.9 (s, CH), 127.8 (d, JP-C ¼ 55.4 Hz, C), 128.8 (d, JP-C ¼ 10.0 Hz, CH),
129.9 (s, CH), 131.6 (d, JP-C ¼ 2.7 Hz, CH), 132.6 (d, JP-C ¼ 9.1 Hz, CH),
C
CH). 31P NMR (202 MHz, CDCl3)
d
ꢁ5.36 (s). MS (70 eV): m/z (%): 263
135.8 (d, JP-C ¼ 4.5 Hz, C). 31P NMR (202 MHz, CDCl3)
d 18.99 (bm).
(10), 262 (67) [M]þ, 261 (20), 184 (20), 183 (100). These data are
consistent with those previously reported. [15b].
MS (70 eV): m/z (%): 308 (15) [M-BH3]þ, 307 (7), 263 (13), 262 (50),
217 (14), 199 (54), 197 (10).
Triphenylphosphine-borane [27] (5a). (Table 2, entry 2). 3a
(0.0556 g, 0.2 mmol) was reacted with BH3-THF (1.2 mL, 1.2 mmol,
1 M solution in THF) according to modified general procedure B to
afford 5a (0.0503 g, 0.182 mmol, 91%) as a white solid; Rf 0.62
Dimethyl(phenyl)phosphine-borane [2g] (5e). (Table 2, entry
9). 3e (0.05 g, 0.324 mmol) was reacted according to general pro-
cedure B to afford 5e (0.03 g, 0.198 mmol, 61%) as a colourless oil; Rf
0.55 (hexane/AcOEt 6:1). 1H NMR (500 MHz, CDCl3)
d
0.44e1.12
(bm, 3H), 1.58 (d, JH-P ¼ 10.40 Hz, 6H), 7.46e7.51 (m, 3H), 7.71e7.77
(m, 2H). 13C NMR (125 MHz, CDCl3)
(hexane/AcOEt 6:1). 1H NMR (500 MHz, CDCl3)
d
0.70e1.40 (bm,
3H), 7.43e7.61 (m, 15H). 13C NMR (125 MHz, CDCl3)
d
128.2 (d, JP-
d
12.9 (d, JP-C ¼ 39.1 Hz, CH3),
¼ 10.0 Hz, CH), 129.2 (d, JP-C ¼ 58.1 Hz, C), 131.2 (d, JP-C ¼ 2.7 Hz,
128.8 (d, JP-C ¼ 10.0 Hz, CH), 130.8 (d, JP-C ¼ 10.0 Hz, CH), 130.9 (d, JP-
C
CH), 133.2 (d, JP-C ¼ 10.0 Hz, CH). 31P NMR (202 MHz, CDCl3)
d
20.59
¼ 54.5 Hz, C), 131.2 (d, JP-C ¼ 1.8 Hz, CH). 31P NMR (202 MHz,
C
(bm). MS (70 eV): m/z (%): 263 (12), 262 (67) [M-BH3]þ, 261 (21),
184 (21), 183 (100). These data are consistent with those previously
reported. [27].
CDCl3)
d
2.56 (bm). MS (70 eV): m/z (%): 151 (5) [M]þ, 150 (4), 149
(35), 148 (12), 139 (12), 138 (100) [M ꢁ BH3], 123 (39), 121 (37).
These data are consistent with those previously reported. [2g].
Diisopropyl(phenyl)phosphine-borane [30] (5f). (Table 2,
entry 10). 3f (0.042 g, 0.2 mmol) was reacted according to general
procedure B to afford 5f (0.0229 g, 0.144 mmol, 72%) as a white
Diphenyl(methyl)phosphine-borane [2g] (5b). (Table 2, entry
6).
3b (0.0432 g, 0.2 mmol) was reacted according to general
6

S. Sowa and K.M. Pietrusiewicz
Tetrahedron 86 (2021) 132057
solid; Rf 0.40 (hexane/AcOEt 10:1); m.p. ¼ 35.9e36.7 ꢀC. 1H NMR
the reaction mixture was transferred to separation funnel and
extracted with CHCl3 (4 ꢂ 10 mL). The collected organic phases
were dried over Na2SO4, filtered off and evaporated to dryness. The
residue was purified by column chromatography using silica gel
hexane/AcOEt (v/v ¼ 10:1) as eluent affording (RP)-5c as a white
(500 MHz, CDCl3)
d
0.26e0.93 (bm, 3H), 1.05 (dd, JH-H ¼ 6.94 Hz, JH-
¼ 13.87 Hz, 3H), 1.17 (dd, JH-H ¼ 6.94 Hz, JH-P ¼ 15.45 Hz, 3H),
P
2.32e2.43 (m, 2H), 7.44e7.53 (m, 3H), 7.69e7.73 (m, 2H). 13C NMR
(125 MHz, CDCl3)
d
16.6 (d, JC-P ¼ 1.8 Hz, CH3), 16.7 (s, CH3), 21.7 (d,
JC-P ¼ 34.5 Hz, CH), 125.2 (d, JC-P ¼ 48.1 Hz, C), 128.4 (d, JC-P ¼ 9.1 Hz,
solid (0.0338 g, 0.139 mmol, 58%). [
a
]
ꢁ26.0ꢀ (c 0.5, MeOH) (86%
D
CH), 131.1 (d, JC-P ¼ 2.7 Hz, CH), 133.3 (d, JC-P ¼ 7.3 Hz, CH). 31P NMR
ee). HPLC IA: tR ¼ 15.555 min (minor enantiomer), tR ¼ 16.259 min
(202 MHz, CDCl3)
d 33.71 (bm). MS (70 eV): m/z (%): 194 (39) [M-
(major enantiomer); 95:5 hexane/2-propanol; flow: 0.5 mL/min.
BH3]þ, 152 (44), 151 (13), 110 (48), 109 (100), 108 (37), 107 (33).
1-Phenylphosphorinane-borane [31] (5g). (Table 2, entry 11).
3g (0.078 g, 0.4 mmol) was reacted according to general procedure
B to afford 5g (0.056 g, 0.292 mmol, 73%) as a colorless oil; Rf 0.53
In
a
Schlenk tube (20 mL), (o-anisyl)(methyl)phenyl-
phosphineꢁborane (5c) (0.0275 g, 0.113 mmol) in anhydrous
toluene (2 mL) was dissolved. Then DABCO (0.021 g, 0.187 mmol)
was added, and the mixture was stirred at 40 ꢀC for 6 h. After
cooling, solvent was removed under reduced pressure and 2 mL of
DCM was added. Then 1 mL of H2O2 was added, and the mixture
was stirred at room temperature for 1.5 h. The mixture was
extracted with DCM (3 ꢂ 10 mL). The combined organic phases
were dried over anhydrous MgSO4, filtered, and evaporated to
dryness. The residue was purified by column chromatography
(hexane/AcOEt 6:1). 1H NMR (500 MHz, CDCl3)
d 0.40e1.06 (m, 3H),
1.40e1.71 (m, 1H), 1.70e1.77 (m, 1H), 1.93e1.94 (m, 2H), 1.95e2.05
(m, 8H), 7.45e7.50 (m, 3H), 7.68e7.72 (m, 2H). 13C NMR (125 MHz,
CDCl3)
d
21.8 (d, JC-P ¼ 4.5 Hz, CH2), 23.1 (d, JC-P ¼ 34.5 Hz, CH2), 26.7
(d, JC-P ¼ 5.5 Hz, CH2), 128.8 (d, JC-P ¼ 10.0 Hz, CH), 130.3 (d, JC-
¼ 52.7 Hz, C), 130.9 (d, JC-P ¼ 2.7 Hz, CH), 131.0 (d, JC-P ¼ 8.2 Hz,
P
CH). 31P NMR (202 MHz, CDCl3)
d
3.25 (bm). MS (70 eV): m/z (%):
(chloroform/ethyl acetate/methanol
¼
30:5:1) yielding 3c
179 (11), 178 (39) [M-BH3]þ, 150 (44), 149 (42), 134 (10), 124 (16),
122 (11). These data are consistent with those previously reported.
[31]
(0.0147 g, 0.06 mmol, 53%). [
a
]
D ꢁ21.4ꢀ (c 0.7, MeOH) (84% ee). HPLC
OD-H: tR ¼ 16.225 min (minor enantiomer), tR ¼ 18.228 min (major
enantiomer); 90:10 hexane/2-propanol; flow: 1.0 mL/min.
t-Butylphenyl(methoxymethyl)phosphine-borane [21a] (5h).
(Table 2, entry 12). 3h (0.0452 g, 0.2 mmol) according to general
procedure B to afford 5h (0.0251 g, 0.112 mmol, 56%) as a white
solid; Rf 0.40 (hexane/AcOEt 10:1). 1H NMR (500 MHz, CDCl3)
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
d
0.27e0.92 (bm, 3H), 1.20 (d, JH-P ¼ 14.19 Hz, 9H), 2.93 (s, 3H),
4.13e4.25 (m, 2H), 7.43e7.47 (m, 2H), 7.52e7.54 (m, 1H), 7.91e7.94
(m, 2H). 13C NMR (125 MHz, CDCl3)
d
26.2 (d, JP-C ¼ 1.8 Hz, CH3),
2
29.8 (d, JP-C ¼ 31.8 Hz, C), 61.7 (d, JP-C ¼ 9.1 Hz, OCH3), 68.6 (d, JP-
Acknowledgments
¼ 41.8 Hz, CH2O), 126.5 (d, JP-C ¼ 49.9 Hz, C), 128.2 (d, JP-C ¼ 9.1 Hz,
C
CH), 131.3 (d, JP-C ¼ 2.7 Hz, CH), 139.9 (d, JP-C ¼ 8.2 Hz, CH). 31P NMR
ꢁ
Sylwia Sowa wishes to thank Prof. Marek Stankevic for kindly
reading the manuscript. Authors want to thank Dr. Kamil Dziuba for
the offering of (SP)-PAMPO for our study.
(202 MHz, CDCl3)
d 28.90 (bm). MS (70 eV): m/z (%): 211 (2), 210
(12) [M-BH3]þ, 180 (6), 154 (20), 125 (7), 124 (72), 123 (14), 122 (26),
121 (27). These data are consistent with those previously reported.
[21a].
Appendix A. Supplementary data
Tricyclohexylphosphine-borane [32] (5j). (Table 2, entry 13).
3i (0.059 g, 0.2 mmol) according to general procedure B to afford 5i
(0.0194 g, 0.066 mmol, 33%) as a white solid; Rf 0.76 (hexane/AcOEt
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2021.132057.
10:1). 1H NMR (500 MHz, CDCl3)
d
ꢁ0.04 -0.64 (bm, 3H), 1.18e1.44
(m, 15H), 1.68e1.96 (m, 18H). 13C NMR (125 MHz, CDCl3)
d
26.1 (s,
References
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d 27.77 (bm). MS
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