Tetrahedron Letters 50 (2009) 7093–7095
Tetrahedron Letters
Birch reduction of aryldialkylphosphine–boranes
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Marek Stankevic , K. Michał Pietrusiewicz
Department of Organic Chemistry, Faculty of Chemistry, Marie Curie-Sklodowska University, Gliniana 33, 20-614 Lublin, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Aryldialkylphosphine–boranes undergo facile Birch-type reduction to afford cyclohexadienyldialkylphos-
phine–boranes in high yields. Judicious choice of the metal and the reaction conditions allows for com-
plete elimination of the undesired P–Ph bond cleavage.
Received 24 August 2009
Revised 23 September 2009
Accepted 2 October 2009
Available online 8 October 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Apart from the established methods for the modification of
organophosphorus compounds via transformations at phosphorus
or through elaboration of their carbon chains,1 structural modifica-
tions of P-aryl groups in organophosphorus compounds are mark-
edly less developed and are effected mainly within the outer
sphere of the aryl substituent.2–6 The classical and synthetically
useful method for arene modification leading to 1,4-cyclohexadie-
nyl systems through a two-electron reduction by alkali metals, the
Birch reduction, is only scarcely represented in organophosphorus
chemistry. Examples include exploratory Birch reductions of some
electron-rich tertiary triarylphosphines7–9 and a more recent re-
lated dearomatisation of N-benzyl diarylphosphinamides through
anionic cyclisation in the presence of a lithium base.10,11 An effi-
cient conversion of a common P-phenyl or P-aryl substituent into
a non-aromatic 1,4-cyclohexadienyl group possessing an isolated
double bond functionality bonded to a phosphorus atom could
open new possibilities for the synthesis of many other structurally
diverse phosphorus compounds. Typically, however, treatment of
P-phenyl (or P-aryl) substituted organophosphorus compounds
with alkali metals results in cleavage of the P-phenyl (or P-aryl)
bond,12–14 hence modification of the reaction conditions is re-
quired to shift the reaction towards the formation of the Birch-type
products. Herein, we present our results on Birch reductions of
dialkylphenylphosphine–boranes which are expected to furnish
valuable electron-rich, protected trialkylphosphine- boranes.15
Four model dialkylphenylphosphine–boranes 1a–d were se-
lected for the test experiments (Fig. 1). They represent symmetri-
cally and non-symmetrically substituted cyclic and acyclic
systems and include one example possessing a cleavable benzyl
substituent.
BH
P
BH
P
BH
P
H B
3
3
3
3
P
Me
t-Bu
t-Bu
Ph
Me
1b
Figure 1. Model phosphine–boranes 1a–d.
Me
1a
1c
1d
reduction product 3a were both detected in the reaction mixture
along with some unreacted starting material (Scheme 1).
An increased amount of alkali metal and methanol in the reaction
mixture shifted the reaction completely towards formation of the
secondary phosphine–borane 2a and the presence of the Birch
reduction product 3a was not detected (Scheme 1). Although the
viability of the desired Birch reduction was in principle confirmed,
it became apparent that more favourable reaction conditions were
needed to secure the formation of the Birch–type product with high-
er selectivity. Thus, we screened the reactivity of the model phos-
phine–boranes 1a–d with different alkali metals under modified
Birch reduction conditions. The results are presented in Table 1.
Under standard conditions (À70 °C, 5 min) lithium was the least
selective among the alkali metals tested and showed the greatest
tendency to cause P–Ph bond cleavage (Table 1, entries 1 and 7).
The reason may lie in its high redox potential (À3.04 V) which
forces cleavage of the P–Ph bond. The tendency to favour P–Ph
bond cleavage as compared to Birch reduction decreased on chang-
ing the metal from lithium to potassium (Table 1, entries 3, 6, 9 and
14) and to sodium (Table 1, entries 2, 5, 8 and 11), with the latter
showing the highest selectivity towards the formation of 3-(1,4-
cyclohexadienyl)-substituted phosphine–boranes.
Preliminary experiments with 1a performed under classical
Birch conditions (2 equiv Na/MeOH, NH3 liq.) revealed that the
reductions were not selective and the secondary phosphine–bor-
ane 2a, a product of P–Ph bond cleavage, and the expected Birch
The reduction of benzyl-t-butyl-phenylphosphine–borane (1d)
was different from the three other boranes as even with sodium
as the reducing agent, secondary phosphine–borane 4d was ob-
tained as the main product as a result of benzyl group cleavage (Ta-
ble 1, entry 11). Shortening the reaction time had little or no
influence on the product ratio (Table 1, entry 15). It appeared, how-
ever, that the proton source added to the reaction mixture could
markedly influence both the product ratio and the reaction yields
* Corresponding author. Tel./fax: +48 81 524 2251.
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0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
Stankevi?, Marek
