The lithiation and acyl transfer reactions of phosphine oxides, sulfides and boranes in the synthesis of cyclopropanes

Article abstract of DOI:10.1039/b817436a

Phosphine oxides are lithiated much faster than phosphine sulfides and phosphine boranes. Phosphine sulfides are in turn lithiated much more readily than phosphine boranes. It was possible to trap a phosphine sulfide THF in one case which upon treatment with t-BuOK gave cyclopropane, showing that phosphine sulfides readily undergo both phosphinoyl transfer and cyclopropane ring closure just like their phosphine oxide counterparts. The obtained data show that phosphine oxides are easily lithiated and undergo phosphoryl transfer much more readily and faster than phosphine sulfides and phosphine boranes. The observations suggest that it would be possible to perform reactions involving phosphine oxides in the presence of phosphine boranes or phosphine sulfides, potentially allowing regioselective alkylation of phosphine oxides in the presence of phosphine boranes or phosphine sulfides.

Full text of DOI:10.1039/b817436a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
The lithiation and acyl transfer reactions of phosphine oxides, sulfides and
boranes in the synthesis of cyclopropanes†‡
Celia Clarke,a David J. Fox,*b Daniel Sejer Pedersenc and Stuart Warrena
Received 13th October 2008, Accepted 15th January 2009
First published as an Advance Article on the web 23rd February 2009
DOI: 10.1039/b817436a
Phosphine oxides are lithiated much faster than phosphine sulfides and phosphine boranes. Phosphine
sulfides are in turn lithiated much more readily than phosphine boranes. It was possible to trap a
phosphine sulfide THF in one case which upon treatment with t-BuOK gave cyclopropane, showing that
phosphine sulfides readily undergo both phosphinoyl transfer and cyclopropane ring closure just like
their phosphine oxide counterparts. The obtained data show that phosphine oxides are easily lithiated
and undergo phosphoryl transfer much more readily and faster than phosphine sulfides and phosphine
boranes. The observations suggest that it would be possible to perform reactions involving phosphine
oxides in the presence of phosphine boranes or phosphine sulfides, potentially allowing regioselective
alkylation of phosphine oxides in the presence of phosphine boranes or phosphine sulfides.
We have previously reported the asymmetric diphenylphosphine
oxide mediated cyclopropanation cascade reaction incorporating
both acyl and phosphoryl transfers as key steps (Scheme 1, eqn (1),
X = O) starting from a,g-1,2 and b,g- and g-substituted3,4 diphenyl-
phosphine oxides. More recently we reported the asymmetric
cyclopropanation cascade reaction starting from b-substituted
diphenylphosphine oxides 7 (Eq. 2) using an Evans oxazolidinone
auxiliary to achieve asymmetric induction.5 During the investiga-
tion of b-substituted cyclopropanation substrates we discovered
that the diphenylphosphinoyl substituent was not compatible with
the conditions used for the asymmetric alkylation (Step ii) resulting
in loss of the auxiliary.5 We successfully circumvented this problem
by using diphenylphosphine borane 6 that could be converted to
phosphine oxide 7 at a later stage (Step iv).
It was a natural extension of this work to investigate if
phosphine boranes (1, X = BH3, Scheme 1) could be converted to
cyclopropanes 8 directly, eliminating the need for prior conversion
to the phosphine oxide. Moreover, we decided to investigate
the properties of diphenylphosphine sulfides (1, X = S) in this
context. The key requirements for the cyclopropanation cascade
reaction to occur are: i) the substrate is lithiated by a base (e.g.
LDA), ii) the lithiated species 1 undergoes acyl transfer, 2, iii) the
phosphine derivative 3 is sufficiently electrophilic to be attacked
Scheme 1 X = O, BH3 or S. Reagents and conditions: i) LDA, THF, -78
by an internal oxygen nucleophile, and iv) enolate 4 can cyclise to
to 0 ◦C; ii) LHMDS, THF, RX, -78 ◦C to room temp.; iii) NaBH4, LiCl,
EtOH; iv) a. DABCO, PhMe, 40 ◦C, b. H2O2, c. PhCOCl, Et3N, DMAP,
CH2Cl2.
give cyclopropane 5.
aCambridge University, University Chemical Laboratory, Lensfield Road,
Cambridge, CB2 1EW, U.K.
Lithiated phosphine sulfides have been employed in reactions
with aldehydes,6 ketones7 and carbonates,8 suggesting that an
intramolecular acyl transfer is plausible. Moreover, Imamoto has
used LDA as the base in an intermolecular acyl transfer reaction
between methyl diphenylphosphine borane 10 and ethyl benzoate.9
Because phosphine boranes and sulfides are not expected to
chelate lithium as well as phosphine oxides, this may be reflected
in a lower or even a reverse diastereoselectivity in the formation
of the 5-membered hemi-ketal 2. With phosphine oxides the
final cyclisation step is highly selective for the trans-cyclopropane
product as this minimises unfavourable steric interactions in the
bDepartment of Chemistry, University of Warwick, Gibbet Hill Road,
Coventry, CV4 7AL, U.K. E-mail: d.j.fox@warwick.ac.uk
cUniversity of Copenhagen, Department of Medicinal Chemistry, Univer-
sitetsparken 2, 2100, Copenhagen, Denmark
† We dedicate this paper to Professor A. B. Holmes in recognition of Andy’s
distinguished contributions to organic chemistry and many years working
together at Cambridge.
‡ Electronic supplementary information (ESI) available: Experimental
and analytical details for all compounds, and coordinates and energies
of calculated structures. CCDC reference number 720729. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b817436a
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transition state. Hence, we expect to observe the same selectivity
for the trans-cyclopropane product with the similarly bulky
phosphine boranes and sulfides.
phosphines 25 to 27 were prepared (Scheme 4). Phosphine
derivatives 25 and 26 were synthesised using conventional
methods. Phosphine sulfide 27 was prepared in one-pot by a
modification of Pellon’s method12 in the presence of elemental
sulfur.
To best be able to monitor the reaction, we decided to study the
acyl and phosphoryl transfers separately. To this end we would
use Hutton’s method of trapping the tetrahedral intermediate 2
(Scheme 1), as the corresponding trimethylsilyl ether.10 For our
initial investigation of the acyl transfer reaction we decided to use
g-substituted substrates 15 to 17 (Scheme 2). All three substrates
were easily accessible by opening styrene oxide with lithiated
phosphines 9 to 11 followed by benzoylation of alcohols 12 to
14 by a stepwise or a one-pot route.
Scheme 4 Reagents and conditions: i) LiBH4, ether, 0 ◦C, 99%; ii) DABCO,
sulfur, toluene, 40 ◦C, 64%; iii) n-BuLi, THF, PhCOCl, 0 ◦C. 25, 76%; 26,
83%; 27, 83%.
Scheme
2
Compounds 10 and 11 were prepared from diphenyl-
As expected, treatment of phosphine oxide 25 with LDA in
the presence of chlorotrimethylsilane gave THF 28 (Scheme 5).
However, a significant amount of dihydrofuran 29 was also
isolated. Dihydrofuran 29 is probably the result of lithiation of
the desired THF product 28, followed by elimination to give
methylphosphine by mixing with BH3·THF or elemental sulfur respec-
tively. Reagents and conditions: i) n-BuLi, then styrene oxide, THF, 0 ◦C to
room temp. 12, 65%; 13, 81%; 14, 61%; ii) PhCOCl, DMAP, Et3N, CH2Cl2.
15, 86%; 16, 71%; 17, 92%; iii) a. n-BuLi, then styrene oxide, THF, 0 ◦C to
room temp., b. PhCOCl. 15, 91%; 16, 81%; 17, 64%.
Treatment of phosphine oxide 15 with LDA and
chlorotrimethylsilane, under the conventional cyclopropanation
conditions,11 gave the expected THF product 18 in good
yield (Scheme 3). However, using the same conditions both
phosphine borane 16 and phosphine sulfide 17 failed to give
any THF product. The C-silylated esters 19 and 20 were
isolated as significant components in both cases. Moreover,
small amounts of free alcohols 13 and 14 and the corresponding
O-silylated compounds, were identified by 1H NMR. This suggests
that g-deprotonation is competitive with a-deprotonation for
compounds 16 and 17 and that phosphine boranes and sulfides are
deprotonated much more slowly at the a-position when compared
to phosphine oxides. In order to prevent g-deprotonation we
decided to remove the anion-stabilising phenyl g-substituent
and use a non-substituted carbon chain. Hence g-benzoyloxy
Scheme 3 Reagents and conditions: i) LDA, THF, TMSCl, -78 to 0 ◦C.
18, 83%; 19, 27%; 20, 26%.
Scheme 5 Reagents and conditions: i) LDA, THF, TMSCl, -78 to 0 ◦C.
28, 37%; 29, 19%; 22, 83%; 31, 34%; 32, 10%.
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dihydrofuran 34, followed by further deprotonation to give 35
and quenching with chlorotrimethylsilane (Scheme 6).
cleavage and g-deprotonation are hindered by the gem-dimethyl
substituents but so too is a-deprotonation.
Whereas the above experiments demonstrate that acyl transfer
is possible, if slow, for phosphine sulfides, phosphine boranes
failed to yield any THF product. It appears that the success of
the intermolecular acyl transfer reaction of phosphine boranes
developed by Imamoto9 is an anomaly. In the case of the substrate
which most closely resembles our compounds, ethyl benzoate,
the yield was rather low, possibly reflecting similar difficulties
as are found for our substrates. That the reaction goes at all
may be attributed to the proton being removed from a primary
alkyl carbon so that the rate of deprotonation is competitive with
nucleophilic attack of LDA on the electrophile; the substitution
of an alkyl group for an a-hydrogen atom has been sufficient
to inhibit metalation in less acidic phosphines.13 It is telling that
Imamoto obtains a better yield when dimethyl carbonate is used
since the rate of attack of LDA on this electrophile will be reduced
with respect to that on the ester while the rate of deprotonation
remains constant.
Given that the unsubstituted phosphine sulfide 27 had under-
gone ring-closure to give THF 31, a sample was treated with
potassium tert-butoxide as with the corresponding phosphine
oxides11 to give cyclopropane 41 in good yield (Scheme 8).
Phosphine sulfides might be useful in a more general synthesis
of cyclopropanes where phosphine oxides are known to fail due
to Lewis basicity.5
Scheme 6 Reagents and conditions: i) LDA, THF, TMSCl, -78 to 0 ◦C.
Phosphine borane 26 failed to give any THF product.
The major products in this case were alcohol 22 and N,N-
diisopropylbenzamide formed by nucleophilic cleavage of the
carboxylic ester by LDA. The corresponding reaction with
phosphine sulfide 27 did give the desired THF 31, as well as the
bis-silylated phosphine sulfide 32 and alcohol 23. THF 31 was
obtained as a single diastereoisomer and the stereochemistry was
confirmed by X-ray crystallography (Scheme 5). Just as observed
for phosphine oxides, the phosphorus-containing group lies syn
to the silyl ether group, suggesting an interaction between the
phosphine substituent and the O-coordinated lithium atom.10
Although g-deprotonation is no longer a competing reaction,
the absence of g-substituents has made the carboxylic ester more
susceptible to cleavage by LDA. The cleavage was only observed
for phosphine borane 26 and phosphine sulfide 27, presumably
because lithiation at the a-position is much slower than for the
corresponding phosphine oxide 25, and the rate of ester cleavage
becomes significant. We reasoned that double substitution at the g-
position (37 to 39, Scheme 7) would prevent g-deprotonation and
inhibit ester cleavage, encouraging the formation of THF products.
The three cyclisation substrates were prepared by alkylating phos-
phines 9 to 11 with 2,2-dimethyloxirane followed by benzoylation
in one pot. On treatment with LDA and chlorotrimethylsilane,
phosphine oxide 37 underwent clean conversion to THF 40.
However, under the same conditions phosphine borane 38 gave
only starting material. In the case of phosphine sulfide 39, a trace
of the desired THF was detected by 1H NMR of the crude reaction
mixture but the remainder was starting material. Evidently ester
Scheme 8 Reagents and conditions: i) t-BuOK, t-BuOH, 35 ◦C, 77%.
The relative rates of lithiation of phosphine oxide, boranes
and sulfides were examined in more detail. Initially, we lithi-
ated mixtures of two of the three phosphine derivatives with
LDA in the presence of cyclobutanone as an internal trapping
agent (Scheme 9).14 Cyclobutanones enolise very slowly and we
envisaged employing this method to study the relative rates of
lithiation via the relative ratio of the addition products. By keeping
the level of LDA sufficiently low, the reaction kinetics could be
approximated to pseudo-first-order in base.
Scheme 9 X is not equal to Y. X or Y = O, BH3, S. Reagents and conditions:
i) LDA, THF, -78 to 0 ◦C.
Ratios of products would be monitored by NMR of crude reac-
tion mixtures. To allow accurate determination of alkylation ratios,
authentic samples of all three cyclobutanone addition products 45
Scheme 7 Reagents and conditions: i) a. n-BuLi, then 2,2-dimethyloxirane,
THF, 0 ◦C to room temp.; b. PhCOCl, DMAP, Et3N, CH2Cl2. 37, 76%;
38, 75%; 39, 84%; ii) LDA, THF, TMSCl, -78 to 0 ◦C, 55%.
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to 47 were required (Scheme 10). These were easily accessible via
lithiation with LDA and alkylation with cyclobutanone.
As before, the level of phosphine oxide product was considerably
higher than those of both the phosphine borane and the phosphine
sulfide. The experiment also clearly showed that phosphine sulfides
are more acidic than phosphine boranes, mirroring the acyl
transfer experiments above (Scheme 5).
The success in the formation of acyl transfer products from
the series of benzoylated phosphine derivatives is reflected in the
lithiation competition experiments, i.e. phosphine oxides lithiate
faster than the sulfides, which in turn lithiate faster than the
boranes. This experimental trend in rates of lithiation was also
investigated by DFT calculations at the B3LYP/6-31G(d) level15–20
using PC-GAMESS.21 Initially, two sets of reaction pathways were
calculated. One set of structures involves a lithium amide base, and
the other an alkyl lithium, to represent the two sets of experiments.
The lithium-amide-containing calculations were based on the X-
ray crystal structure of the stable complex22 48 (Fig. 1). This struc-
ture, derived from lithium bis(trimethylsilyl)amide (LHMDS) and
diphenylmethylphosphine oxide, does not react to give a lithiated
phosphine oxide. LHMDS does not lithiate simple phosphine
oxides at synthetically useful rates. Lithium dialkylamides are
more reactive however, and in the calculations LiNMe2 is used
as a model for LDA, and dimethyl ether as a model for THF.23
In addition, trimethylphosphine derivatives 49–51 (Scheme 11)
are used to represent diphenylmethylphosphine derivatives 42–44
(Scheme 10). Both kinetic measurements and calculations indicate
that LDA can react as a monomer23,24 or a dimer,25,26 often with
coordination of the substrate to one of the lithium atoms through a
Lewis-basic atom such as oxygen. Structure energy minimisations
were performed for phosphine oxide 49, sulfide 50 and borane
51. Lithiation transition structures 55–57 were then found (with
one imaginary vibration), based on the structure of complex
48 but including only one phosphine and a dimethyl ether as
a THF substitute. Ground states (52–54 and 58–60) on either
side of the lithiation transition state were found by initial IRC
calculation and minimisation. Pre-lithiation structures 52–54 are
similar to complex 48, whereas post-lithiation structures 58–60
involve carbon–lithium bonds, as observed in the X-ray structures
of lithiated phosphine oxides.27
Scheme 10 Reagents and conditions: i) LDA, THF, then cyclobutanone,
-78 ◦C. 45, 67%; 46, 37%; 47, 66%.
Finally, mixtures of the three combinations of two of diphenyl-
methylphosphines 42 to 44 (X π Y, Scheme 9) were prepared
and treated with 0.1 equivalents of LDA in the presence of
cyclobutanone. However, to our surprise only the phosphine oxide
gave the expected addition product 45. The sulfide and boranes
adducts 46 and 47 were not formed, even in the reaction without
the phosphine oxide. Seemingly, deprotonation of phosphine
boranes and phosphine sulfides is slower than formation of the
enolate of cyclobutanone.
Next, an alternative method of measuring the relative rates of
lithiation, by treating the three combinations of phosphines 42 to
44 with n-butyllithium followed by a MeOD quench, was tested
(Table 1). If equilibration of the lithiated derivatives were fast,
the absence of an internal quench would mean that the method
could not be used to measure relative deprotonation rates. Still, it
would provide valuable information on ratios at equilibrium. In
order to assess which ratio was being measured by the method,
two different experiments were carried out. In one experiment the
mixture was quenched with MeOD at -78 ◦C after 30 min and
in the other, the lithiated species were allowed to equilibrate at
0 ◦C for 18 h followed by quenching with MeOD. The ratios of the
derivatives were identical (Table 1), implying either that the kinetic
and thermodynamic ratios are identical or that thermodynamic
equilibration is rapid.
Table 1 Reagents and conditions: i) a. n-BuLi, THF. b. MeOD. n = 1–3
Starting mixture
Amount of each compound deuterated (%)a
X
Y
42
43
44
42
42
43
43
44
44
45 (93)
95 (>99)
—
<1 (<1)
—
<1 (<1)
—
<1 (<1)
88 (78)
Fig. 1
The energies of these starting materials and complexes can
be compared relative to dimethyl ether 62 and its complex 61
(Scheme 11). For each phosphine reaction complex (both ground
states and transition state), theoretical reaction energies were
a Initial numbers refer to experiment with quenching at -78 ◦C after 30 min.
Numbers in brackets refer to ratios obtained in equilibration experiment
(0 ◦C for 18 h) before deuteration. Determined by 1H NMR.
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Scheme 11
calculated for the displacement of dimethyl ether 62 from complex
61. Overall, this process results in a relative energy scale so that
each reaction complex 52–60 can be compared to its own starting
material 49–51. Table 2 shows the relative energies (after ZPE
correction) for the initial complexes, the lithiation transition states,
and the product complexes. Zero-point energies are calculated
using unscaled vibrational frequencies.
The results show that the phosphine oxide pre-lithiation com-
plex 52 is more stable (by 41 kJ mol-1) than the related sulfide 53,
which in turn is more stable (by 5 kJ mol-1) than the borane 54.
This relative stability is reflected in the transition state energies
(55 is more stable than 56, which is more stable than 57), and
the products 58–60. While the initial lithiations to give complexes
58–60 are rather endothermic, these structures can rearrange to
give the less unfavourable lithiated dimers 63–65 and regenerate
half of a lithium amide dimer 61 and dimethyl amine. The
oxide dimer is a thermodynamically favoured product, but the
sulfide and borane dimers are slightly unfavoured. In reality these
lithiations do happen as demonstrated above (e.g. Scheme 10)
and by others,6–9 but it may be that the most stable of the many
possible product solution aggregates and solvates have not been
modelled. The reaction of the boranes and sulfides are however
sluggish in comparison with the oxides, even though the reactions
are predicted to occur by comparing the pKa of a secondary amine
(ca. 44 in DMSO)28 with that of a phosphine oxide (ca. 31 in
DMSO) or sulfide (ca. 30 in DMSO).29
The transition state energies (relative to uncomplexed start-
ing materials) are consistent with the experimentally observed
Table 2
Initial complex ground states
Relative energya/
Lithiation transition states
Relative energya/
Lithiated product ground states
Relative energya/
Lithiated product dimers
Relative energyb/
X
Structure
kJ mol-1
Structure
kJ mol-1
Structure
kJ mol-1
Structure
kJ mol-1
O
S
BH3
52
53
54
-24.7
+16.7
+22.2
55
56
57
+35.8
+74.4
+88.6
58
59
60
+19.4
+41.4
+57.7
63
64
65
-32.6
+11.1
+22.7
a Energies (after ZPE correction) are calculated for the reaction A in Scheme 11 to make each complex (and dimethyl ether 62) from the respective starting
material (49–51) and complex 61. b Energies (after ZPE correction) are calculated for the reaction B in Scheme 11 to make half of each complex (63–65)
(and dimethyl amine) from the respective starting material (49–51), half of dimer complex 61 and an additional free dimethyl ether 62.
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Table 3
Initial complex ground states
Relative energya/
Lithiation transition states
Relative energya/
Lithiated product ground states
Relative energya/
Lithiated product tetramers
Relative energya/
kJ mol-1
X
Structure
kJ mol-1
Structure
kJ mol-1
Structure
kJ mol-1
Structure
O
S
BH3
66
67
68
+2.1
+38.5
+49.6
69
70
71
+48.0
+81.5
+99.0
72
73
74
-56.8b
-41.2b
-19.5b
76
77
78
-115.9c
-55.8c
-25.4c
a Energies (after ZPE correction) are calculated for the reaction in Scheme 12 to make each complex from the respective starting material (49–51) and a
quarter of complex 75. b Energy of methane is included. c Energies of methane and dimethyl ether 62 are included.
Scheme 12
lithiation of phosphine oxides being much faster than for the
related sulfides and boranes. However, if the transition states
are compared to the respective pre-lithiation complexes the
activation energies do not follow the previous pattern, and the
phosphine oxide should lithiate slower than the sulfide (52 to 55
+61 kJ mol-1, 53 to 56 +58 kJ mol-1, 54 to 57 +67 kJ mol-1).
These comparisons imply that the total activation energy from
uncomplexed phosphine derivative (49–51) must be considered
when rationalising the observed rates. In turn this implies that the
complexes 53 and 54 are not heavily populated (even if complex
52 is heavily populated) and that the majority of the phosphine
derivatives 50 and 51 are uncomplexed in the bulk of the reaction.
A similar set of calculations was performed to model the
lithiations with butyllithium (Scheme 12). In this case CH3–Li and
trimethylphosphine derivatives were used as substitutes. Unlike for
other lithiation systems using alkyllithiums and where experiment
and calculation often point to the reaction of alkyllithium
monomers30–34 or dimers,35 little is known about the stoichiometry
of pre-reaction complexes of phosphine oxides and alkyllithiums.
For simplicity a one-to-one model complex was chosen, with a
single chelating solvent molecule. The energies of the complexes
66–71 are, as above, derived from the theoretical reaction of
each starting material (49–51) with dimethyl ether–methyllithium
tetramer36 complex 75. The energy of the by-product methane is
included in the energies shown for complexes 72–74 (Table 3). As
above the lithiated phosphine derivatives can aggregate to give for
example tetramers27 76–78, but in this case the increased basicity
of methyllithium is quite enough to ensure complete lithiation
without this additional driving force. The solvated dimers 63–65
are more stable, per phosphorus, than the tetramers 76–78 and
two unsolvated ethers 62, and these solvated dimers can of course
be the reaction product with either base.
For complexes 66–74 the calculated energies show the same
trends as those involving the lithium amide dimers above. The
phosphine oxide forms the most stable complex with the alkyl-
lithium, and the energetically favourable interaction results in the
lithiation transition states and product energies also being more
stable. In each case the sulfide is less stable and the borane the least
stable. Again it is interesting to note that the activation energy from
pre-lithiation complex to transition state is lower for the sulfide
than the oxide (66 to 69 +46 kJ mol-1, 67 to 70 +43 kJ mol-1, 68 to
71 +50 kJ mol-1). This reduced rearrangement energy may result
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Table 4
Warren group methodology with different phosphine derivatives,
requires preparation via the phosphine oxide and interconversion
of derivatives as a necessary step. These observations do suggest
that it would be possible to perform reactions involving phosphine
oxides in the presence of a phosphine borane or a phosphine
sulfide. This could potentially find applications in regioselective
alkylation of phosphine oxides in the presence of phosphine
boranes or phosphine sulfides, for example in the Horner–Wittig
olefination,38 the phosphine oxide mediated cyclopropanation
cascade reaction11,39 or the synthesis of non-symmetric chiral bis-
phosphine ligands for metal catalysis.40
Uncoordinated ground states
Relative energya/
Coordinated ground states
Relative energya/
X
Structure
kJ mol-1
Structure
kJ mol-1
O
S
79
80
0
-4.5
+1.4
82
83
84
-120.6
-104.6
-82.7
BH3 81
a Energies are all relative to the energy of formation of complex 79 from
oxide 49.
from a reduced build-up of strain in the transition state of the
sulfide due to a longer P–S bond length, but it is known that the
thio-carbonyl compounds can be more acidic than their oxygen
counterparts.37
Acknowledgements
We thank Dr John Davies for crystallography and the EPSRC for
financial assistance towards the purchase of the Nonius CCD
diffractometer. DSP thanks Torben & Alice Frimodts Fond,
Brødrene Hartmanns Fond and Direktør Ib Henriksens Fond for
financial support.
A final set of calculations highlights how significant the
coordination of lithium in the three types of phosphines changes
the overall stability of the alkyllithium products (Fig. 2). In each
case the energy (relative to the respective starting material 49–51)
of the simple unsolvated alkyllithium was calculated for two con-
formations. The two conformations are ground-state structures
involving either close contact of the lithium to the oxygen, sulfur
or borane, or no close contact in an anti-arrangement around
the phosphorus–carbon bond. Table 4 shows that the energies of
lithiation to give the uncoordinated compounds 79–81 are similar.
However once the C–P bond is rotated and the lithium coordinates
to the oxygen, sulfur or borane (82–84), the oxide is significantly
more stable than the sulfide, which is in turn more stable than the
borane. These results are similar to those calculated above, and
again emphasise the large contribution that the oxygen–lithium
interaction makes to the chemistry of phosphine oxides.5
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