Stereoselective addition reactions of diphenylphosphine to pyridyl and pyrimidylalkynes: Chiral 1,2-diheteroaryl-1,2-bis(diphenylphosphino)ethanes and their Group 6 metal carbonyl complexes

Article abstract of DOI:10.1016/S0022-328X(98)01078-X

The base-catalysed addition of diphenylphosphine to the diarylethynes RCCR′ (R=R′=2-pyridyl 1; R=R′=3-pyridyl 2; R=2-pyridyl, R′=3-pyridyl 3; R=phenyl, R′=2-pyridyl 4, 3-pyridyl 5, 2-pyrimidyl 6) yield diphosphines of general formula Ph₂PCH(R)CH(R′)PPh₂ together with alkene by-products Ph₂PC(R)=CHR′ and HC(R)=C(R′)PPh₂ in all cases except 1. Selected P,P′-coordinated M(CO)₄ complexes (M=Mo, W) of the diphosphines have been prepared and their ¹H-, ¹³C- and ³¹P-NMR data are presented. The pattern of ¹³CO-NMR signals for the tetracarbonyl complexes was used unambiguously to determine the stereochemistry of the parent diphosphine. At moderately elevated temperatures, nitrogen coordination of 2-pyridyl and 2-pyrimidyl groups occurred for tetracarbonyl complexes of meso- or erythro-stereochemistry, but not for complexes of rac- or threo-form, to yield corresponding fac-tricarbonyl complexes. At 162°C the complex cis-rac-(CO)₄W{P,P′-Ph₂PCH(R)CH(R)PPh₂} (R=R′=2-pyridyl) is converted quantitatively into fac-erythro-(CO)₃W{P,P′,N-Ph₂PCH(R)CH(R)PPh ₂} via an inversion/N-coordination pathway.

Full text of DOI:10.1016/S0022-328X(98)01078-X

Journal of Organometallic Chemistry 577 (1999) 305–315
Stereoselective addition reactions of diphenylphosphine to pyridyl and
pyrimidylalkynes: chiral
1,2-diheteroaryl-1,2-bis(diphenylphosphino)ethanes and their Group 6
metal carbonyl complexes
Jonathan L. Bookham *, Darren M. Smithies
Department of Chemical and Life Sciences, Uni6ersity of Northumbria at Newcastle, Newcastle upon Tyne, NE1 8ST, UK
Received 22 October 1998
Abstract
The base-catalysed addition of diphenylphosphine to the diarylethynes RCꢀCR% (R=R%=2-pyridyl 1; R=R%=3-pyridyl 2;
R=2-pyridyl, R%=3-pyridyl 3; R=phenyl, R%=2-pyridyl 4, 3-pyridyl 5, 2-pyrimidyl 6) yield diphosphines of general formula
Ph2PCH(R)CH(R%)PPh2 together with alkene by-products Ph2PC(R)ꢁCHR% and HC(R)ꢁC(R%)PPh2 in all cases except 1. Selected
1
P,P%-coordinated M(CO)4 complexes (M=Mo, W) of the diphosphines have been prepared and their H-, 13C- and 31P-NMR
data are presented. The pattern of 13CO-NMR signals for the tetracarbonyl complexes was used unambiguously to determine the
stereochemistry of the parent diphosphine. At moderately elevated temperatures, nitrogen coordination of 2-pyridyl and
2-pyrimidyl groups occurred for tetracarbonyl complexes of meso- or erythro-stereochemistry, but not for complexes of rac- or
threo-form, to yield corresponding fac-tricarbonyl complexes. At 162°C the complex cis-rac-(CO)4W{P,P%-
Ph2PCH(R)CH(R)PPh2}
(R=R%=2-pyridyl)
is
converted
quantitatively
into
fac-erythro-(CO)3W{P,P%,N-
Ph2PCH(R)CH(R)PPh2} via an inversion/N-coordination pathway. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Stereoselective addition; Diphenylphosphine; Group 6 metals; Carbonyl complexes
we have shown recently that these are also viable, as
shown in Scheme 1 [11,12], and give potentially valu-
able products with structures related to that of ‘chi-
raphos’ [2,3-bis(diphenylphosphino)butane].
As part of a continuing study into the addition
reactions of phosphines to multiple bonds we have now
extended Scheme 1 to include bis-heterocyclic alkynes
in order to prepare potentially important chiral phos-
phine ligands with additional donor atoms such as
oxygen, sulfur, and in this paper, nitrogen. Phosphorus/
1. Introduction
The addition of primary and secondary phosphines
to activated CꢁC double bonds is now an established
method for the preparation of polyphosphines that are
valuable as chelating ligands [1–5]. With appropriate
modifications this sort of addition reaction is also capa-
ble of yielding ambidentate phosphorus ligands con-
taining, for example, pendant arms with N or S donor
atoms by addition of N–H or S–H bonded species to a
CꢁC double bond bearing a phosphine residue [6–8], or
by addition of P–H bonded species to CꢁC double
bonds bearing heteroatomic atoms or groups [9,10].
Less use has been made of additions to triple bonds but
* Corresponding author. Fax: +44-191-2273519.
Scheme 1.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01078-X

306
J.L. Bookham, D.M. Smithies / Journal of Organometallic Chemistry 577 (1999) 305–315
2.2. Hydrophosphination reactions
nitrogen donor ligands have been the subject of re-
newed and refocused studies in recent years; an excel-
lent review of pyridyl phosphines and related species by
Newkome [13] describes the variety and versatility of
these ligands in their coordination chemistry. More
specifically, Brown and co-workers [14] have demon-
strated effective asymmetric hydroboration using a
rhodium complex of 1-(2-diphenylphosphino-1-naph-
thyl)isoquinoline, a ligand related structurally to the
more familiar ‘binap’ [2,2%-bis(diphenylphosphino)-1,1%-
binaphthyl] system. Their study is just one example of
recent interest in chiral P–N ligands for asymmetric
synthesis [15].
In a recent communication we have reported a pre-
liminary account of the unusual inversion behaviour of
rac-Ph2PCH(R)CH(R)PPh2 (R=2-pyridyl) [16]. In this
paper we report a broad range of reactions of
diphenylphosphine with diarylethynes containing com-
binations of 2-pyridyl, 3-pyridyl, 2-pyrimidyl, and
phenyl substituents to yield symmetrical and unsym-
In THF/triethylamine, and in the presence of cata-
lytic amounts of potassium tert-butoxide, di(2-
pyridyl)ethyne 1 reacted rapidly with two equivalents of
diphenylphosphine at room temperature (r.t.) to afford
meso-1,2-di(2-pyridyl)-1,2-bis(diphenylphosphino)eth-
ane 7 and rac-1,2-di(2-pyridyl)-1,2-bis(diphenylphos-
phino)ethane 8 each in ca. 40% isolated yield.
Corresponding reactions of diphenylphosphine with
2–6 occur as summarised in Scheme 2. Specific out-
comes and isolated yields are in shown in Table 1.
The isolated compounds of meso-stereochemistry 7
and 9 are essentially insoluble in all common organic
solvents including dichloromethane, chloroform,
methanol, ethanol, and importantly, the reaction sol-
vent THF, from which crude products precipitate as
they form. Compounds of the rac-form and the less
symmetrical erythro-, or threo-isomers are more soluble
in dichloromethane, chloroform and THF but not also
insoluble in methanol or ethanol. Separation of 7 and 8
was therefore readily achieved by fractional crystallisa-
tion but in the majority of other cases the comparable
solubilities of diastereomer pairs combined with the
formation of appreciable quantities of the correspond-
ing phosphinoalkene intermediates (i–iv) precluded the
separation and isolation of analytically pure products
although their NMR parameters could be obtained
satisfactorily from crude products. Elemental analyses
for the diphosphine ligands are therefore reported for
their metal carbonyl derivatives for which purification
was straightforward (Table 2).
metrical
chiral
diphosphines
of
the
type
Ph2PCH(R)CH(R%)PPh2. The preparation and reactions
of octahedral complexes of general formula M(CO)4L
and M(CO)3L [M=Mo, W; L=diphosphine ligand]
are also reported as well as multielement NMR charac-
terisation of all products.
2. Results and discussion
2.1. Alkyne synthesis
Di(2-pyridyl)ethyne 1 was prepared using a modifica-
tion of Rossi’s method [17] via a Pd(0) catalysed cou-
pling reaction of 2-methyl-3-butyn-2-ol with two
equivalents of 2-bromopyridine under phase transfer
conditions. Similar reactions yielded di(3-pyridyl)ethyne
2 from 3-bromopyridine, and the unsymmetrical alkyne
(2-pyridyl)(3-pyridyl)ethyne 3 from the appropriate
stepwise reaction. Each of these reactions afforded yel-
low or pale yellow solids in good yield following distil-
lation under reduced pressure. The products were found
to be essentially pure by GC/MS and NMR and en-
tirely suitable for subsequent reactions and were there-
fore used as obtained. Phenyl(2-pyridyl)ethyne 4,
All phosphines reported here are moderately sensitive
to aerial oxidation in the solid state, and rather more so
in solution. Their isomeric identities were deduced from
the 13CO-NMR spectra of their metal carbonyl deriva-
tives (see later).
In two previous studies [11,12], we have shown that
additions of diphenylphosphine to diphenyl- and di-
tolylethynes proceed via Z-alkene intermediates which
react further under the experimental conditions to give
meso- (or erythro-)diphosphines. The corresponding E-
isomers are inert to further addition and can occasion-
ally be isolated from the reaction mixture. Similarly
here, the addition of diphenylphosphine to 1–6 is likely
to initially yield one or more of the corresponding
isomeric alkene intermediates (i–iv), and any subse-
quent addition yields one or both diphosphine isomers.
Alkenes of E stereochemistry (i or ii) were identified in
the reaction mixtures from all ethynes except 1 using
31P-NMR spectroscopy (see Table 1) but were not
isolated pure. In the case of 5 both E-isomers were
detected. In contrast to our previous studies, rac- or
threo-diphosphines are formed directly in the majority
phenyl(3-pyridyl)ethyne
5
and
phenyl(2-pyrim-
idyl)ethyne 6 were prepared from phenylethyne and the
appropriate aryl bromide using a modification of the
method of Sonogashira [18]. Analytically pure speci-
mens of all alkynes were obtained following a further
vacuum sublimation/distillation and/or column chro-
matography and showed no significant differences in
their subsequent reactions to those compounds ob-
tained directly from the reaction mixtures.

J.L. Bookham, D.M. Smithies / Journal of Organometallic Chemistry 577 (1999) 305–315
307
Scheme 2.
of cases reported herein. In both previous studies re-
ferred to earlier only meso- or the geometrically related
erythro-isomers were obtained.
afforded 21, 23–27, and 29, respectively. In each of
these cases the diphosphine ligand adopts a P,P%-coor-
dination mode (Scheme 3). Occasionally, crystallisation
from the reaction mixture yielded an analytically pure
sample directly, in other cases these were obtained by
recrystallisation from methanol/chloroform. Similar re-
actions with W(CO)4(pip)2 in refluxing chloroform
yielded 20, 22, 28, and 30 from their corresponding free
ligands 7, 8, 17, and 18, respectively. All preceding
complexes were isolated in good yield as air-stable pale
yellow crystalline solids which are soluble in
dichloromethane and chloroform but insoluble in
alcohols.
On refluxing a solution in chloroform of the cis-
meso-{P,P%}molybdenum tetracarbonyl derivative 19
virtually quantitative conversion to its fac-erythro-
{P,P%,N}tricarbonyl counterpart 31 occurred via dis-
placement of a CO ligand by a pyridyl nitrogen.
Compounds 33–35 were prepared similarly from 24,
25, and 27, respectively. Reaction of the free ligands 7,
11, 13, and 17 with Mo(CO)4(pip)2 in refluxing chloro-
form yielded 31 and 33–35, directly. The formation of
General aspects of the stereochemistry of addition of
phosphines to alkynes have been described or discussed
in a number of published papers [19–23]. Previously,
we proposed that the formation a meso- (or erythro-
)diphosphine via a Z-alkene intermediate can only re-
sult effectively from successive anti- and syn-additions
to the CꢀC and CꢁC multiple bonds, respectively [12].
The formation of rac- and threo-isomers in several
instances reported here, presumably also via their Z-
alkene precursors, reveals that anti-/anti-addition is
also a viable pathway for the alkynes in this study. The
formation of tetraphenyl diphosphane in similar reac-
tions has also been observed previously [12].
2.3. Complexation reactions
Compound 7 reacted smoothly with Mo(CO)4(pip)2
(pip=piperidine) in dichloromethane at r.t. to give 19,
similar reactions with 8, 9, 11, 13, 14, 17, and 18

308
J.L. Bookham, D.M. Smithies / Journal of Organometallic Chemistry 577 (1999) 305–315
Table 1
conversion is essentially quantitative after 24 h at 110°C
and must involve an inversion at one of the chiral sp3
hybridised backbone carbon atoms. At 162°C (diglyme
[2,5,8-trioxanonane] at reflux) the same conversions, 20
to 32 and 22 to 32, are more rapid and are essentially
complete in 4 and 12 h, respectively. No other complex
of the rac or threo forms reported herein (21, 26, 29,
and 30) undergo inversion analogous to that of 22
under similar experimental conditions. Prolonged reflux
in diglyme of the molybdenum complexes 21, 26, 29
results in a very slow formation of insoluble brown
products, possibly polymeric in nature. Prolonged
reflux of a solution of 30 in diglyme effects no change.
All facially P,P%,N-coordinated tricarbonyl com-
plexes were obtained in high yield as air-stable orange
or red crystalline solids with significantly lower solubil-
ity than their tetracarbonyl precursors in all common
organic solvents.
Yields of products from the reactions of diphenylphosphine with
various alkynesa
Alkyne Major
productsb
1
2
3
4
5
6
7 [40%] 38% 8 [60%] 39% –
9 [38%] 34% 10 [0%]
11 [40%] 26% 12 [15%]
13 [32%] 25% 14 [6%] 5% i or ii [55%] Ph2PPPh2 [7%]
15 [36%] 16 [0%] i and ii [50%] Ph2PPPh2 [14%]
17 [39%] 22% 18 [11%] 8% i or ii [33%] Ph2PPPh2 [17%]
–
i (ꢀii) [29%] Ph2PPPh2 [31%]
i or ii [12%] Ph2PPPh2 [18%]
a Approximate molar proportions are calculated from 31P-NMR
integrals obtained directly from the reaction mixtures and are shown
in square brackets. Approximate isolated yields, where relevant, are
shown in italics.
b Compound labelling is shown in Scheme 2. Intermediate alkenes
are identified as of type (i)–(iv) for the relevant alkyne/reaction.
these complexes demonstrates the ability of 2-pyridyl
and 2-pyrimidyl groups on the ligand backbone to
undergo facile N-coordination. Attempts to force simi-
lar N-coordination of a 3-pyridyl group in 23 were
unsuccessful using these or more forcing conditions.
Analogous N-coordination reactions of the tungsten
complexes 20 and 28 were achieved by refluxing their
solutions in toluene at 110°C for 12 h and gave 32 and
36, respectively. The higher temperature required for
carbonyl substitution in tungsten derivatives in com-
parison to their molybdenum counterparts is well-docu-
mented [24]. Surprisingly, similar treatment of the
rac-{P,P%}tungsten complex 22 also yielded 32. This
The inability of the complexes of rac- or threo-stere-
ochemistry 21, 22, 26, 29, 30 to undergo simple N-coor-
dination with retention of stereochemistry can be
explained on steric grounds. In a previous study on
their C-phenyl analogues we have shown that in the
crystalline state the two C-phenyl groups adopt exclu-
sively equatorial positions with respect to the chelate
ring (Fig. 1(iii)) rather than the alternative arrangement
where they are both axial (Fig. 1(iv)) [11]. The former
arrangement minimises steric interactions between C-
phenyl and P-phenyl groups which would be significant
in the latter conformation. If the complexes reported
here predominantly adopt analogous conformations in
solution then N-coordination of a C-pyridyl or C-
pyrimidyl group in an available axial position forces the
other C-aryl substituent into an axial position with the
result that both groups then experience severe steric
interactions with the P-phenyl groups.
Table 2
Analytical data (%) for the new isolated species with calculated values
in parentheses
Compound
C
H
N
Conversely, in the case of meso stereochemistry, the
two substituents between them necessarily adopt one
axial and one equatorial position (Fig. 1(i/ii)). There-
fore, coordination of a C-(2-pyridyl) or C-(2-pyrimidyl)
group in a complex of meso or erythro stereochemistry
is likely to cause little additional steric disruption over
that already present in the tetracarbonyl derivative
itself. Support for this postulation is provided by
scrutiny of the NMR parameters (see later).
19a
20
21
22
23
24b
25c
26
27
28
29
31d
32
33
34
35
59.0 (58.3)
56.7 (56.6)
62.8 (63.2)
56.6 (56.6)
63.0 (63.2)
56.6 (56.0)
62.2 (62.2)
64.5 (64.8)
62.8 (63.2)
56.0 (56.6)
63.5 (63.2)
63.4 (63.2)
57.1 (57.1)
63.7 (63.9)
65.6 (65.7)
66.9 (67.0)
3.7 (3.8)
3.6 (3.6)
3.8 (4.0)
3.5 (3.6)
3.8 (4.0)
3.4 (3.5)
3.7 (4.0)
4.0 (4.1)
3.7 (4.0)
3.3 (3.5)
3.8 (4.0)
4.1 (4.3)
3.6 (3.7)
4.1 (4.1)
4.3 (4.3)
4.5 (4.6)
3.3 (3.3)
3.3 (3.3)
3.5 (3.7)
3.3 (3.3)
3.6 (3.7)
3.2 (3.2)
1.7 (1.7)
1.8 (1.8)
3.5 (3.7)
3.2 (3.3)
3.5 (3.7)
3.7 (3.7)
3.4 (3.4)
3.7 (3.8)
1.8 (1.9)
3.3 (3.4)
2.4. Deuterium exchange studies
In a refluxing mixture of diglyme/D2O (100:1 v/v,
b.p. ca. 135°C, 24 h) the conversion of 22 to 32 as
described results in ca. 70% deuterium incorporation at
each of the two ligand backbone positions (H1 and H2)
as determined from the 1H-NMR spectrum. Experi-
ments involving a shorter reflux period (ca. 6 h) allowed
the recovery of unreacted starting material which NMR
spectroscopy showed to be ca. 55% deuterated at the H1
a Isolated as a 1:1 dichloromethane solvate to which these figures
apply.
b Isolated as a 1:1 chloroform solvate to which these figures apply.
c Isolated as a 1:0.5 dichloromethane solvate to which these figures
apply.
d Isolated as a 1:0.5 methanol solvate to which these figures apply.

J.L. Bookham, D.M. Smithies / Journal of Organometallic Chemistry 577 (1999) 305–315
309
Scheme 3.
and H2 positions. Surprisingly also, the conversion of
20 to 32 shows 30% deuterium exchange under similar
conditions (24 h reflux) although no inversion is actu-
ally necessary for this transformation. Relevant 1H-
NMR spectra from these experiments have been
published in a preliminary communication [16].
The formation of 32 from 22 clearly involves inver-
sion at one of the backbone carbon atoms and the
results of the H/D exchange experiments suggest
strongly a mechanism involving deprotonation at a
backbone carbon atom to give a carbanion followed by
reprotonation. Inversion of carbon via a carbanion
intermediate during a chemical transformation has
many literature precedents [25] although this case is an
unusual example. In this instance inversion, in the
absence of added base, implies that the pyridyl groups
in 22 are themselves strong enough bases at the reaction
temperature to deprotonate a backbone CH group.
This process is probably also facilitated by a relatively
high acidity of the CH protons (in accord with the
known acidities of methyl protons in 2-methylpyridine
[26] and methylphosphine derivatives [27]) and en-
hanced further by the coordination of the phosphorus.
The inability of 21, 29, and 30 to undergo inversion
under similar conditions also suggests that both the
number and exact locations of the pyridyl nitrogens are
crucial to the deprotonation step, a requirement with
close parallels to that of certain proton-sponges [28]
and that the deprotonation step may be an intramolec-
ular rather than an intermolecular process. The recov-
ery of partially deuterated 22 combined with the
formation of deuterated 32 and the absence of 20 from
reaction mixtures show that (i) the deprotonation step
is reversible, (ii) the reprotonation step is non-
diastereoselective, and (iii) that the N-coordination step
is rapid at the reaction temperature for the meso-form.

310
J.L. Bookham, D.M. Smithies / Journal of Organometallic Chemistry 577 (1999) 305–315
2.5. NMR parameters
Patterns of resonances in the 13C spectra in the
carbonyl region for the tetracarbonyl complexes 19–30
serve unequivocally to identify the stereochemistry of
the parent ligands. The spectrum of 19 shows one
equatorial and two axial resonances consistent only
with meso stereochemistry. The spectrum of its isomer
21 shows one equatorial and one axial resonance con-
sistent with the rac form. Moreover the chemical shift
of the two equivalent axial carbonyl carbons (210.7
ppm) in 21 lies approximately at the mean position of
the two inequivalent axial signals in 19 which are
separated by 6.6 ppm (206.9 and 213.5 ppm). Corre-
sponding patterns are seen in other meso or rac com-
plexes 20, 22 and 23. The less symmetrical
tetracarbonyl complexes 24–30 each have two equato-
rial and two axial carbonyl signals in their 13C-NMR
spectra irrespective of ligand stereochemistry but close
inspection of the patterns allows a deduction of the
stereochemistry. As an example, 25 exhibits signals
separated by 5.4 ppm (213.3 and 207.9 ppm) which
from chemical shift and coupling data are clearly from
two chemically inequivalent axial carbonyls. In its iso-
mer 26 these resonances are at separated by 0.2 ppm
(211.0 and 211.2 ppm) and this reflects very similar but
not identical chemical environments. Comparison with
the chemical shift separations for the unambiguous
meso and rac patterns of 19 and 21 mentioned earlier
(6.6 and 0 ppm, respectively) leads to the clear assign-
ment of 25 and 26 as erythro and threo, respectively.
Corresponding patterns of axial resonances also iden-
tify the stereochemistry in the other tetracarbonyl
derivatives 27–30, the general pattern being separations
of 5.4–6.3 ppm for the erythro complexes and 0.2–1.1
ppm for the threo ones. Small chemical shift separations
between the inequivalent equatorial carbonyls are also
observed in 24–30. These are in the mutually exclusive
ranges 1.2–1.6 ppm and 0.4–0.6 ppm for erythro- and
threo-stereochemistries, respectively but these are too
close to each other to be used diagnostically with any
certainty. Representative 13CO spectra for the four
possible stereochemical forms are shown in Fig. 2(a)–
(d).
1
Selected 31P-, H- and 13C-NMR parameters for all
isolated species are given in Tables 3 and 4.
2.5.1. Free ligands
31P chemical shifts for the the meso and erythro free
ligands are typically 10–12 ppm higher than for their
rac and threo counterparts, this feature parallels that
found for their bis(C-phenyl) analogues previously re-
ported. Magnitudes of J(31P31P) for the unsymmetrical
3
species range from 11.0 to 30.0 Hz, although no obvi-
ous pattern is evident. The spectra of 7 and 8 showed
significant broadening of the 31P resonance which is
possibly indicative of restricted rotation within the
molecule. The unsymmetrical diphosphines 11, 13–15,
17 and 18 each show broadening of only one of the two
1
resonances. The H and 13C spectra for the free ligands
are highly complex and only parameters for the back-
bone CH groups were deducible. No clear distinction
between the various stereochemistries is apparent from
the available H- and 13C-NMR data for these species
1
and this is as expected based on the known difficulty in
rationalising values of J(31P1H) and J(31P13C) in
molecules containing PIII.
2.5.2. Tetracarbonyl complexes
l31P for the complexes 19–30 are in conformity with
those expected from considerations of size of chelate-
rings incorporating phosphorus for the relevant metal
[29]. In contrast to the H- and 13C-NMR data for the
1
free ligands, corresponding data for their tetracarbonyl
complexes reveal extensive information on stereochem-
istry and also, importantly, on chelate ring conforma-
tions.
The symmetrical tetracarbonyl derivatives 19–23
have resonances for H1 and H2 which can be analysed
as arising from the A portion of an AA%XX% spin
system [30] (A=H1/ H2, X=P1/P2). In this system no
single coupling constant can be determined from analy-
sis of the spectrum without making assumptions con-
cerning the magnitudes and signs of the other coupling
constants defining the spin system. In contrast, the
lower symmetry of complexes 24–30 permits a full
analysis and the diagnostically important parameter
3J(H1H2) can be determined. The magnitudes of
3J(H1H2) in complexes of erythro- and threo-stereo-
chemistry (4.0–4.6 and 13.7–14.1 Hz, respectively) in-
dicate H1–C1–C2–H2 dihedral angles of ca. 50 and
Fig. 1. Newman projections down the C2–C1 bond showing idealised
staggered conformations of meso-(R=R%), erythro-(R"R%), rac-
(R=R%), and threo-(R"R%) diphosphine complexes of formula
M(CO)4L. Carbonyl ligands and P-phenyl groups have been omitted
for clarity.

J.L. Bookham, D.M. Smithies / Journal of Organometallic Chemistry 577 (1999) 305–315
311
Table 3
Selected 31P- and 1H-NMR data for the new diphosphine ligands and their complexesa
Com-
l(31P)b,c
l(H1)j,k
l(H2)j,k
l(HR)l
l(HR% l
)
pound
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
7
8
9
+3.1
−9.5
−4.5
4.95
4.37
4.86
4.95
4.37
4.86
11
13
14
15
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
−1.3, +0.5 (15.7)
−1.6, −0.1 (15.9)
−10.9, −12.0 (11.0)
−4.7, −5.0 (20.7)
+1.1, +2.8 (14.6)
−7.7, −10.8 (30.0)
+68.2
4.74 (9.7, 6.0, 3.7)
4.65 (9.2, 6.1, 4.9)
4.30 (7.9, 5.9, 3.0)
4.20 (m)
4.87 (11.4, 7.5, 2.7)
4.53 (7.6, 5.3, 5.3)
4.82 [11.1]
4.82 [9.5]
4.82 [0]
4.84 [0]
4.56 [9.2]
4.49 (9.7, 7.9, 3.5)
4.54 (9.2, 9.2, 3.4 b)
3.90 (7.9, 5.9, 3.0)
4.20 (m)
4.79 (11.4, 4.7, 4.1)
4.11 (7.6, 5.0, 5.0)
4.82 [11.1]
4.82 [9.5]
4.82 [0]
4.84 [0]
4.56 [9.2]
8.38, 6.91, 6.87, 5.66
8.37, 6.93, 6.89, 5.69
8.02, 6.64, 6.92, 6.24
8.03, 6.66, 6.94, 6.27
8.03, 8.26, 6.75, 6.93
8.14, 6.94, 6.87, 6.04
8.09, 6.93, 6.85, 6.06
8.06,
8.10, 6.81
8.09, 6.83
8.38, 6.91, 6.87, 5.66
8.37, 6.93, 6.89, 5.69
8.02, 6.64, 6.92, 6.24
8.03, 6.66, 6.94, 6.27
8.03, 8.26, 6.75, 6.93
8.09, 8.30, 6.68, 6.14
6.44, 6.90, 7.06
6.48,
6.26, 6.90, 7.06
6.28, 6.90, 7.06
6.52, 6.73, 6.78
+52.9d
+63.8
+47.1e
+69.6
+67.3, +70.3 (11.0)
+65.9, +68.9 (11.0)
+63.4, +65.0 (17.1)
+66.2, +69.0 (11.0)
+51.4, +54.5 (B1)f
+63.9, +64.9 (15.9)
+47.1, +48.4 (6.1)g
+52.2, +65.5 (4.9)
+41.5, +58.3 (3.0)h
+50.5, +63.9 (3.6)
+49.2, +63.1 (4.9)
+50.0, +60.3 (7.3)
+ 40.9, +52.9 (B1)i
4.62 (4.0, 36.6, 11.3)
4.66 (4.3, 37.2, 11.6)
4.52 (m)
4.45 (4.0, 8.9, 4.6)
4.47 (4.3, 9.1, 5.2)
4.52 (m)
m
m m
,
m, 6.07
,
5.06 (4.4, 37.8, 11.0)
5.09 (4.6, 37.2, 11.6)
4.79 (14.1, 5.0, 4.3)
4.80 (13.7, 4.6, 3.5)
4.80 (3.0, 32.5, 8.6)
4.98 (3.0, 32.0, 8.5)
4.43 (3.0, 32.0, 8.0)
4.46 (3.0, 32.6, 8.5)
4.82 (3.0, 33.0, 8.2)
5.05 (3.0, 33.0, 8.2)
4.46 (4.4, 9.5, 4.8)
4.49 (4.6, 10.4, 4.6)
4.75 (14.1, 6.9, 4.9)
4.76 (13.7, 6.4, 4.0)
4.33 (3.0, 8.8, 6.6)
4.37 (3.0, 9.5, 6.4)
4.11 (3.0, 8.9, 5.5)
4.08 (3.0, 9.2, 6.1)
4.11 (3.0, 9.8, 6.6)
4.09 (3.0, 9.8, 7.0)
8.01, 6.55
8.02, 6.57
6.54, 6.73, 6.80
9.18, 7.03, 7.50, 7.29
9.30, 6.99, 7.53, 7.29
9.36, 7.16, 7.50, 6.86
9.33, 7.13, 7.46, 6.89
9.27, 7.05, 8.51
9.50, 7.01, 8.55
8.28, 6.85, 6.85, 5.27
8.27, 6.81, 6.84, 5.38
7.64, 8.20, 6.58, 5.65
5.86, 6.81, 6.95
5.90, 6.83, 6.95
5.93, 6.84, 6.97
a NMR labelling as in Scheme 3.
3
b In ppm (90.2ppm) to high frequency of external 85% H3PO4 (0.0 ppm), numbers in brackets are magnitudes of J(31P31P) where measurable
from the spectrum directly.
c Data for the free ligands (7–18) are from reaction mixtures in THF, data for the complexes (19–36) are from pure materials in CDCl3.
d 1J(183W31P)=239.5 Hz.
e 1J(183W31P)=230.9 Hz.
f 1J(183W31P)=238.1, 241.5 Hz.
g 1J(183W31P)=232.0, 231.4 Hz.
h 1J(183W31P)=212.4, 218.5 Hz.
i 1J(183W31P)=216.1, 216.1 Hz.
j In ppm (90.01) relative to internal TMS=0.0 ppm, entries in parentheses are coupling constants, values of J(1H1H) are underlined, values
2
of J(31P1H) follow in order of decreasing magnitude, entries in square brackets are values of ꢀ J(31P1H)+3J(31P1H)ꢀ in cases where the individual
components can not be determined by inspection due to molecular symmetry.
k Relative assignment of H1 and H2 is arbitrary and may be reversed.
l Quoted in order l(H3), l(H4), l(H5), etc., for each aromatic group.
m Not determined due to unfavourable signal overlap.
180°, respectively, on the basis of a Karplus relation-
ship [31] and this confirms the dominance in solution of
conformation (iii) of Fig. 1 for the rac and threo
isomers. Proton chemical shift separations between H1
and H2 in erythro isomers are significantly larger than
for the threo isomers and this reflects the spatially and
hence chemically dissimilar (axial and equatorial) envi-
ronments of H1 and H2 in the former case and the
similar (axial and axial) positions in the latter. Further
corroboration is evident in the striking differences be-
tween coupling parameters for H1 and H2 in erythro
complexes in comparison to their similarity in each
threo case. 13C parameters for C1 and C2 show similar
but less pronounced features.
In molecules of rac- or meso-stereochemistry the
aromatic groups fall into three chemically inequivalent
types whereas in complexes of erythro- or threo-stereo-
chemistry all six aromatic groups are inequivalent. As a
result the aromatic regions of the spectra are complex.
However in all species the C-aromatic substituents give
remarkably disperse signals which allows a near com-
plete analysis for these groups. Particularly striking are
the peculiarly low chemical shifts for protons ortho to
the carbon bonded to the ligand backbone. In 19 these

312
J.L. Bookham, D.M. Smithies / Journal of Organometallic Chemistry 577 (1999) 305–315
Table 4
Selected 13C-NMR data for the new diphosphine ligands and their complexesa
Com-
l(C1)b
l(C2)b
l(C3)b
l(C4)b
l(C5)b
l(C6)b
pound
c
c
7
–
–
–
–
8
9
48.2 [2.7]
48.2 [2.7]
–
–
–
–
–
–
–
–
c
c
11
13
14
15
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
45.8 (22.9, 20.2) 51.4 (23.8, 18.3)
48.5 (22.9, 18.3) 51.6 (d)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
45.7 (15.6, 15.6) 47.7 (d)
47.1 (16.1, 16.1) 49.3 (18.4, 15.6)
48.8 (24.8, 20.2) 52.8 (12.0, 12.0)
46.9 (18.3, 18.3) 49.4 (16.5, 12.8)
53.7 [30.1]
54.8 [32.9]
53.3 [36.6]
54.7 [39.0]
50.6 [31.7]
53.7 [30.1]
54.8 [32.9]
53.3 [36.6]
54.7 [39.0]
50.6 [31.7]
213.5 (10.0, 10.0)
205.7 (9.1, 9.1)
210.7 (8.0, 8.0)
203.1 (6.4, 6.4)
214.4 (8.5, 8.5)
213.2 (9.8, 9.8)
213.3 (9.8, 9.8)
211.2 (8.6, 8.6)
213.9 (9.8, 9.8)
205.9 (8.6, 8.6)
211.5 (8.5, 8.5)
203.7 (7.3, 7.3)
228.0 (8.1, 8.1)
220.2 (4.9, 4.9)
227.9 (7.4, 7.4)
228.3 (7.3, 7.3)
228.0 (7.6, 7.6)
219.8 (4.9, 4.9)
217.4 [18.9]
208.2 [19.5]
216.7 [17.1]
207.9 [18.3]
215.8 [17.1]
217.6 (26.9, 8.6)
218.0 (26.5, 8.5)
216.9 (24.4, 8.6)
217.8 (26.9, 8.5)
208.4 (25.7, 6.1)
217.0 (24.4, 8.5)
208.1 (23.2, 6.1)
222.1 (36.5, 10.0)
218.3 (35.4, 7.4)
223.0 (36.6, 9.8)
223.4 (36.6, 9.8)
220.7 (36.0, 9.8)
217.8 (34.8, 7.4)
217.4 [18.9]
208.2 [19.5]
216.7 [17.1]
207.9 [18.3]
215.8 [17.1]
216.1 (28.5, 8.6)
216.4 (27.5, 8.5)
216.5 (25.7, 8.6)
216.2 (27.5, 8.5)
207.2 (26.8, 6.1)
216.4 (25.7, 8.5)
207.5 (23.4, 6.1)
220.8 (36.5, 10.0)
215.5 (36.7, 7.4)
221.0 (36.6, 9.8)
221.3 (36.6, 9.8)
222.9 (35.4, 9.8)
215.0 (35.4, 7.4)
206.9 (7.6, 7.6)
199.3 (4.3, 4.3)
210.7 (8.0, 8.0)
203.1 (6.4, 6.4)
206.3 (6.7, 6.7)
207.5 (6.3, 6.3)
207.9, (6.7, 6.7)
211.0 (8.0, 8.0)
207.6 (7.0, 7.0)
200.2 (4.3, 4.3)
210.4 (8.0, 8.0)
202.8 (7.3, 4.9)
–
54.4 (12.7, 12.7) 48.8 (20.8, 9.8)
54.1 (14.8, 14.8) 51.8, (20.1, 11.0)
53.2 (23.2, 13.4) 51.8 (22.6, 15.3)
55.4 (15.0, 15.0) 52.3 (19.5, 9.8)
55.7 (18.3, 13.4) 54.0 (17.1, 15.9)
53.1 (e)
53.1 (e)
54.6 (20.7, 20.7) 53.7 (23.2, 15.9)
55.3 (17.2, 17.0) 45.9 (17.6, 7.4)
58.6 (17.7, 17.7) 46.3 (17.1, 11.0)
58.5 (19.5, 14.7) 40.9 (18.3, 8.5)
58.7 (19.2, 15.2) 43.8 (17.1, 8.5)
58.5 (17.6, 17.6) 44.4 (16.5, 8.5)
60.5 (17.7, 17.7) 44.0 (15.9, 12.8)
–
–
–
–
–
a NMR labelling as in Scheme 3, relative assignments of C1 and C2, of C3 and C6, and of C4 and C5 are arbitrary and each may be reversed.
n
b In ppm (90.1 ppm) relative to internal TMS=0.0 ppm, entries in parentheses are J(31P13C) placed in order of decreasing magnitude, entries
1
in square brackets are values of ꢀ J(31P13C)+2J(31P13C)ꢀ for cases where the individual components can not be determined by inspection due to
molecular symmetry.
c Not determined due to insolubility of the compound.
d Not resolved due to a broad signal.
e Not determined due to unfavourable signal overlap and/or molecular symmetry.
protons (H6) give a signal at 5.66 ppm which is ca. 1.5
ppm lower than would normally be anticipated. The
most likely cause of this low chemical shift is probably
anistropic (‘ring-current’) effects from the nearby aro-
matic groups. This feature is repeated in all tetracar-
bonyl derivatives reported herein (with the exception of
23). In the unsymmetrical species each C-aromatic
group exhibits this feature to some extent and the effect
is generally greater in complexes of meso or erythro
geometry than their rac or threo counterparts.
NMR parameters for the erythro-tricarbonyl complexes
and their erythro-tetracarbonyl precursors. A simple
N-coordination step for a tetracarbonyl complex results
in relatively small changes in the magnitudes of all five
couplings involving H1 and/or H2. These small changes
support the earlier proposal that in the erythro-te-
tracarbonyl derivatives an aromatic substituent occu-
pies a position close to that required for axial
coordination and hence causes little conformational
perturbation on doing so. Fig. 3 shows relevant por-
1
tions of selected H spectra to illustrate this proposal.
2.5.3. Tricarbonyl deri6ati6es
N-Coordination of one of the aromatic groups
causes ca. 1 and 0.5 ppm increases, respectively, in l for
the protons ortho and para to the coordinating nitro-
gen, the effects on the meta protons being less pro-
nounced. Interproton coupling constants involving
aromatic protons are unremarkable and are not re-
ported here.
1
The 31P, 13C and H parameters of the tricarbonyl
complexes 31–36 are consistent with the structures
shown in Scheme 3 but in no case is the ligand stereo-
chemistry itself simply apparent. However, consider-
ation of the NMR data supports that proposed in
1
Scheme 3, in particular the close parallel between H-

J.L. Bookham, D.M. Smithies / Journal of Organometallic Chemistry 577 (1999) 305–315
313
Fig. 2. (a)–(d) 13C-NMR spectra at 67.8 MHz of the carbonyl regions for 21, 19, 25, and 26 illustrating the diagnostic patterns for rac-, meso-,
erythro-, and threo-stereochemistries, respectively.
3. Experimental
mixture was then filtered through a course filter and the
product extracted with diethylether/water. After drying
and evaporation of the solvent under reduced pressure
the product was distilled at reduced pressure (240°C, 1
mmHg) to give the product as yellow oil (17.4 g, 77%)
which crystallised slowly on standing.
Solvents were dried and deaerated by standard proce-
dures prior to use and all manipulations were per-
formed under an atmosphere of dry N2. Diphenyl-
phosphine and benzyltriethylammonium chloride were
purchased from Strem Chemicals Inc. and Lancaster
Synthesis, respectively, and used without further purifi-
cation. Mo(CO)4(piperidine)2 and W(CO)4(piperidine)2
[24] and Pd(PPh3)4 [32] were prepared immediately
before use by previously published methods.
3.1.2. Di(3-pyridyl)ethyne 2
This was prepared and isolated using a procedure
analogous to that above using 3-bromopyridine in place
of 2-bromopyridine and gave a product of similar
appearance and in a similar yield to that described
above.
3.1. Alkyne synthesis
3.1.1. Di(2-pyridyl)ethyne 1
3.1.3. (2-Pyridyl)(3-pyridyl)ethyne 3
2-Bromopyridine (40 g, 0.25 mol) and 2-methyl-3-bu-
tyn-2-ol (10.5 g, 0.125 mol) were dissolved in toluene
(100 cm3) and added to a suspension of Pd(PPh3)4 (2 g,
catalytic amount), (2 g, catalytic amount), and benzyl-
triethylammonium chloride (2 g, catalytic amount), in
5.5 N aqueous sodium hydroxide (100 cm3). The two
phase mixture was stirred under nitrogen at 60–70°C
until the reaction, as monitored by GC/MS analysis of
the toluene layer, was complete (ca. 168 h). The cooled
Again, this was prepared and isolated similarly to the
above using a one-pot stepwise 1:1 reaction of 2-bro-
mopyridine with 2-methyl-3-butyn-2-ol at 40°C for 48 h
followed by addition of 3-bromopyridine and stirring at
60–70°C for a further 96 h. The first step of the
reaction was monitored by GC/MS to ensure the com-
plete conversion of 2-bromopyridine to 4-(2-pyridyl)-2-
methyl-3-butyn-2-ol before introduction of the second
reagent.

314
J.L. Bookham, D.M. Smithies / Journal of Organometallic Chemistry 577 (1999) 305–315
Fig. 3. Portions of the 1H-NMR spectra at 270 MHz of the H1 and H2 regions for 27, 35, and 29 [(a)–(c), respectively] showing the similar
patterns for a cis-erythro-tetracarbonyl complex 27 and its fac-erythro-tricarbonyl derivative 35; this is consistent in each case with chemically
dissimilar (axial and equatorial) protons H1 and H2. In contrast, the corresponding threo-isomer 29 gives a highly second order pattern consistent
with chemically similar H1 and H2 environments. The signal at l 5.27 ppm in (a) is due to dichloromethane as an impurity.
3.1.4. Phenyl(2-pyridyl)ethyne 4
to stand until crystallisation was complete. The resul-
tant solid was filtered, washed with methanol (50 cm3)
and dried in vacuo. The mother liquor and the
methanol washings were combined and refrigerated for
48 h to give a further crystalline product which was
isolated as before. Concentration of the mother liquor
followed by extended refrigeration yielded further crys-
talline products in some cases. In all instances the first
product to crystallise was the diphosphine with meso-
or erythro-stereochemistry. Commonly, the second
product was the diphosphine of rac- or threo-stereo-
chemistry (when formed—see Table 1) and this was
often contaminated with one or both of the correspond-
ing phosphinoalkenes of E-stereochemistry (see Scheme
2 and Table 1). Later crystallising fractions contained
greater proportions of these alkenes.
A mixture of phenylacetylene (10 g, 0.1 mol), 2-bro-
mopyridine (16 g, 0.1 mol), PdCl2(PPh3)2 (1 g, catalytic
amount), and CuI (1 g, catalytic amount) in diisopropy-
lamine (100 cm3) was stirred under an inert atmosphere
for 24 h. After the addition of diethylether (100 cm3)
the solution was filtered and the solvent was removed at
the pump to give a crude brown oil. The product was
obtained as a pale yellow liquid (17 g, 90%) after
distillation of the crude residue under reduced pressure
(ca. 200°C, 1 mmHg). Phenyl(3-pyridyl)ethyne 5 and
phenyl(2-pyrimidyl)ethyne 6 were obtained similarly us-
ing 3-bromopyridine and 2-bromopyrimidine in place
of 2-bromopyridine.
3.2. Hydrophosphination reactions
3.2.1. Typical procedure
3.3. Diphosphine complexation reactions
A solution of the ethyne (20 mmol), diphenylphos-
phine (40 mmol) and potassium tert-butoxide (0.5 g,
catalytic amount) in a mixture of dry THF (50 cm3)
and triethylamine (25 cm3) was stirred under an atmo-
sphere of dry nitrogen for 2 h. Methanol (25 cm3) was
then added to the mixture which was subsequently left
3.3.1. Tetracarbonyl molybdenum deri6ati6es
In a typical procedure mixture of the diphosphine (4
mmol) and Mo(CO)4(piperidine)2 (1.5 g, 4 mmol) in
dichloromethane (40 cm3) was stirred under nitrogen at
ambient temperature for 2 h. The resulting solution was

J.L. Bookham, D.M. Smithies / Journal of Organometallic Chemistry 577 (1999) 305–315
315
filtered and methanol (40 cm3) was added to the filtrate.
References
The solution was refrigerated until crystallisation ap-
peared complete at which point the product was
filtered, washed with methanol (20 cm3) and dried
under vacuum. Recrystallisation from dichloro-
methane/methanol when necessary yielded the product
as air stable pale-yellow crystals. Typical yields 80–
90%.
[1] L. Maier, in: G.M. Kosolapoff, L. Maier (Eds.), Organic Phos-
phorus Compounds, vol. 1, Wiley, New York, 1972.
[2] F.R. Hartley (Ed.), The Chemistry of Organophosphorus Com-
pounds, vol. 1, Wiley, New York, 1990.
[3] R.B. King, P.N. Kapoor, J. Am. Chem. Soc. 91 (1969) 5191.
[4] R.B. King, P.N. Kapoor, J. Am. Chem. Soc. 93 (1971) 4158.
[5] J.L. Bookham, W. McFarlane, I.J. Colquhoun, J. Chem. Soc.
Chem. Commun. (1986) 1041.
[6] G.R. Cooper, F. Hassan, B.L. Shaw, M. Tornton-Pett, J. Chem.
Soc. Chem. Commun. (1985) 614.
3.3.2. Tetracarbonyl tungsten complexes
[7] D.L. Dubois, W.H. Myers, D. Meek, J. Chem. Soc. Dalton
Trans. (1975) 1011.
[8] R. Uriarte, T.J. Mazanec, K.D. Tau, D. Meek, Inorg. Chem. 19
(1980) 79.
[9] D.L. Dubois, A. Miedaner, R.C. Haltiwanger, J. Am. Chem.
Soc. 113 (1991) 8753.
[10] R.B. King, W.F. Masler, J. Am. Chem. Soc. 99 (1977) 4001.
[11] J.L. Bookham, W. McFarlane, M. Thorton-Pett, S. Jones, J.
Chem. Soc. Dalton Trans. (1990) 3621.
In a typical procedure a mixture of the diphosphine
(4 mmol) and W(CO)4(piperidine)2 (1.8 g, 4 mmol) in
chloroform (40 cm3) was refluxed under nitrogen for 6
h. After cooling to r.t. and filtration the products were
isolated from the filtrate as pale yellow crystals in
exactly the same manner as described for the molybde-
num derivatives above. Typical yields 80–90%.
[12] J.L. Bookham, D.M. Smithies, A. Wright, M. Thorton-Pett, W.
McFarlane, J. Chem. Soc. Dalton Trans. (1998) 811.
[13] G.R. Newkome, Chem. Rev. 93 (1993) 2067.
[14] (a) N.W. Alcock, J.M. Brown, D.I. Hulmes, Tetrahedron Asym-
metry 4 (1993) 742. (b) J.M. Brown, D.I. Hulmes, T.P. Layzell,
J. Chem. Soc. Chem. Commun. (1993) 1673.
[15] For example, see: (a) J.M. Valk, T.D.W. Claridge, J.M. Brown,
D. Hibbs, M.B. Hursthouse, Tetrahedron Asymmetry 6 (1995)
2597. (b) J.M. Valk, G.A Whitlock, T.P. Layzell, J.M. Brown,
Tetrahedron Asymmetry 6 (1995) 2593. (c) H Doucet, J.M.
Brown, Tetrahedron Asymmetry 8 (1997) 3775. (d) A. Albinati,
F. Lianza, H. Berger, P.S. Pregosin, H. Ru¨egger, R.W. Kunz,
Inorg. Chem. 32 (1993) 478. (e) C.G. Arena, F. Nicolo`, D.
Drommi, G. Bruno, F. Faraone, J. Chem. Soc. Chem. Commun.
(1994) 2251.
3.3.3. Tricarbonyl molybdenum deri6ati6es
In a typical procedure a solution of the appropriate
tetracarbonyl precursor (2 mmol) in chloroform (25
cm3) was refluxed under nitrogen for 2 h. The cooled
solution was filtered and methanol (25 cm3) was added
to the filtrate. On standing orange/red crystals were
deposited which were collected by filtration, washed
with methanol, and dried in vacuo. Typical yields 80–
90%.
3.3.4. Tricarbonyl tungsten deri6ati6es
These were prepared in a similar manner to the
tricarbonyl molybdenum derivatives above using a 4 h
reflux in toluene in place of the 2 h reflux in chloro-
form. The isolation procedure was the same and yielded
orange/red crystalline products in similarly high yields
None of the metal carbonyl complexes 19–36 exhib-
ited clean melting behaviour; typically, decomposition
without melting occurred on heating to temperatures of
250°C or above.
[16] J.L. Bookham, Inorg. Chem. Commun. 1 (1998) 309.
[17] A. Carpita, A. Lessi, R. Rossi, Synth. Commun. (1984) 571.
[18] K. Sonogashira, Y. Tohda, N. Hagiara, Tetrahedron Lett. (1975)
4467.
[19] A.M. Aguiar, T.G. Archibald, Tetrahedron Lett. (1966) 5541,
[20] H. Hoffmann, H.J. Diehr, Chem. Ber. 98 (1965) 363.
[21] K. Maitra, V.J. Catalano, J.H. Nelson, Bull. Soc. Chim. Fr. 134
(1997) 471.
[22] W.L. Wilson, N.W. Alcock, E.C. Alyea, S. Song, J.H. Nelson,
Bull. Soc. Chim. Fr. 130 (1993) 673.
[23] K. Maitra, V.J. Catalano, J.H. Nelson, J. Organomet. Chem.
529 (1997) 409.
[24] D.J. Darensbourg, R.L. Kump, Inorg. Chem. 17 (1978) 2680.
[25] For example, see: R.S. Atkinson, Stereoselective Synthesis, Wi-
ley, Chichester, UK, 1995, and references therein.
[26] R.R. Fraser, T.S. Mansour, S. Savard, J. Org. Chem. 50 (1985)
3232.
[27] H.H. Karsch, Z. Naturforsch. Teil B 37 (1982) 284.
[28] T. Saupe, C. Krieger, H.A. Staab, Angew. Chem. Int. Ed. Engl.
27 (1988) 865.
NMR spectra of pure products were recorded using
CDCl3 solutions contained in 5 mm outside diameter
tubes on a JEOL EX270 spectrometer operating at
1
270.1, 67.8 and 109.4 MHz for H, 13C and 31P, respec-
tively. 31P-NMR spectra of reaction mixtures were ob-
tained following the addition of ca. 5% C6D6 to provide
an internal locking signal.
[29] P.E. Garrou, Inorg. Chem. 14 (1975) 1435.
[30] J.W. Emsley, J. Feeney, L.H. Sutcliffe, High Resolution NMR
Spectroscopy, vol. 1, Pergamon, Oxford, 1965.
Acknowledgements
[31] (a) M. Karplus, J. Am. Chem. Soc. 85 (1963) 2870. (b) C.J.
Hawkins, J.A. Palmer, Coord. Chem. Rev. 44 (1982) 1.
[32] D.R. Clouson, Inorg. Synth. 13 (1971) 121.
The authors would like to thank the EPSRC for their
support.
.