as tris(p-tolyl)phosphine and tris(3,5)xylylphosphines, but at
a slower rate (entries 7 and 8). With tris(p-fluorophe-
nyl)phosphine (entry 9), even a modest excess of
Na2K-SG(I) and longer reaction time yielded only deflu-
orination products, and an 18-fold excess led to diphenyl
phosphide, as expected from plain triphenylphosphine cleav-
age. No reaction was seen with tributylphosphine (entry 10).
Quenching of the diphenyl phosphide with butyl or
homoallyl bromides achieved monofunctionalization in good
yield (entries 1 and 11), but activated electrophiles such as
benzyl and allyl bromides formed quaternary salts.
Next, we looked at cleavage reactions of monoalkyl-
diarylphosphines to see if we could further functionalize
them. The targets would be phosphines bearing three different
substituents and, hence, chiral (albeit racemic). Interestingly,
alkyldiarylphosphines underwent cleavage losing an aryl ring
to afford the corresponding alkyl aryl phosphide solutions.
However, dialkylarylphosphines resist cleavage completely.
Understanding the factors controlling selectivity in the
cleavage of alkyldiarylphosphines may allow design and
control of stereochemistry and redox potentials of organo-
metallic complexes.42 Such reactions have been studied43
but mostly in the context of P-arylated bisphosphines with
varying spacers.44-49 For cleavage of the simpler butyl-
diphenylphosphine, we find nearly exclusive dearylation to
form butyl phenyl phosphide (Scheme 2). This result seems
as a radical, diaryl phosphide formation might be expected.
This latter process may explain the previously reported
selective cleavage26 of trimethylsilyldiphenylphosphine to
form diaryl phosphide, a result we have confirmed as well.
Scheme 2 above summarizes results of cleavage with
subsequent alkylation to form nonsymmetric phosphines.
These reactions could be carried out via two sequential
dearylation/alkylation cycles in one pot without the need for
isolation of the intermediate monoalkylated phosphines. As
long as sufficient M-SG was added in the second cycle, no
difficulties arose due to the presence of slight excesses of
alkyl halide quenchers such as 1-bromobutane and bromocy-
clohexane. Overall, product yields were between 68-89%.
This approach did not, however, extend to a third cycle; like
their trialkyl congeners, the dialkylarylphosphines (e.g.,
Et2PPh) did not undergo any detectable cleavage reactions
with M-SG reagents.
Finally, as an illustration of its synthetic utility, this new
method was used to make the chiral ligand DIOP [2,2-
dimethyl-4,5-bis(diphenylphosphinomethyl)dioxolane] (Scheme
3). DIOP is used widely as a ligand for asymmetric versions
Scheme 3. Preparation of DIOP from Diaryl Phosphide Solution
Scheme 2
.
Reductive Cleavage of Diarylalkylphosphine to
Tertiary Phosphines
of hydroformylation,51 hydrogenation,52-59 allylic alkyla-
tion,60 radical addition, and polymerization61 reactions. This
is an attractive ligand as it can be directly accessed from
tartaric acid via acetal formation, esterification, reduction
with LiAlH4 and activation by tosylate formation. The final
step involves a nucleophilic displacement of -OTs by -PPh2.
K2Na alloy has been used for this purpose in synthesis.62-64
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assuming they are formed directly in the cleavage. However,
since alkyl-H bond dissociation energies are typically 5-15
kcal/mol lower than aryl-H, if the severed group departed
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