SYNTHESIS OF STERICALLY HINDERED SECONDARY
215
of methyl halocarboxylates. After 10 h at 130 C, the
conversion of silylphosphine Ia in reaction (1) was
about 80% (31P NMR data) and no longer increased
over the course of 7 h of heating (further heating gave
rise to palladium black formation). Distillation of the
reaction mixture gave phosphine IVc mixture with
the closely boiling starting iodide IIId. The mixture
was separated by a procedure we described previously
[1], namely, by shaking with moist ether. The easiest
hydrolyzed trimethylsilyl 3-iodobenzoate (IIId) pre-
cipitates as 3-iodobenzoic acid and could be filtered
off from unchanged phosphine IVc whose yield after
all operations was about 40%.
arylated phosphine Ve (R = i-Pr, Y = 2-COOSiMe3)
in the mixture was no more than 10% ( 18.2 ppm),
and the major reaction product occurePd to be se-
condary phosphine VIb (R = i-Pr, Y = 2-COOSiMe3).
Summarizing the results of the synthesis of 2- and
3-phosphine-substituted benzoic esters IV and V (Y =
COOCH3, COOSiMe3) we can conclude that reac-
tions (1) and (2) are hardly suitable in this case. By-
processes induced by the evolving bromo(iodo)tri-
methylsilanes practically exclude the possibility of
synthesis of tertiary phosphines IV with Y = COOR.
The situation with trimethylsilyl (dialkylphosphino)-
benzoates is only slightly better. Hence, if meta-sub-
stituted phosphine IVa can still be prepared in a
moderate yield, but ortho-products are scarcely
formed by steric reasons.
Attempted synthesis of bisarylated phosphines Vc
and Vd [R = i-Pr, Y = 2-COOCH3 (c); R = tert-Bu,
Y = 2-COOCH3 (d)] by reaction (2) by heating of the
reaction mixtures of bis-silylphosphines IIa and IIb
with methyl 2-bromobenzoate IIIb (12 and 30 h at
120 C, respectively) gave, instead of phosphines V,
secondary phosphines VIa and VIb whose fraction in
mixture with other phosphorus-containing unidentified
products was no higher than 40 50%. Evidently, the
monoarylated silylphosphine formed in the first stage
of cross coupling undergoes protodesilylation under
the action of a proton-donor agent generated from
methyl ester IIIb or its phosphorylated derivative
under the action of bromotrimethylsilane under rigid
conditions. Note that bromoarene IIIa (Y = 3-CF3)
was considerably more stable in this respect and
permitted to prepare not only bisarylated products Va
and Vb, but also silylphosphine IVa. When trimethyl-
silyl 2-iodobenzoate (IIIe) was used instead of methyl
ester IIIb, reaction (2) with phosphine IIa occurred
faster (5 h, 124 C), but the fraction of tertiary di-
Cross coupling (1) proved unsuccessful even for
the synthesis of o-substituted phosphines IV (Y =
2-OCH3). Hence, upon heating of silylphosphine Ib
with 2-haloanizoles IIIf and IIIg [Hlg = Br (f), J (g)]
for 20 h at 135 C the conversion did not exceed, by
31P NMR data, 30 40%, and in both cases, according
1
to H NMR data, the major product was phosphine
IVd (Y = 2-OSiMe3), rather than compound IVe
(Y = 2-OCH3).
Individual phosphine IVe was prepared in high
yield by another method [7] involving fast exchange
of halogen for lithium in o-haloanizoles IIIf and IIIg
under the action of phenyllithium at 20 C. Further
phosphorylation of 2-methoxyphenyllithium with di-
organylchlorophosphine allows o-alkoxy-substituted
phosphines IV to be prepared under mild conditions.
OCH3
I
OCH3
OCH3
PhLi, Et2O
t-Bu(Ph)PCl, Et2O
(6)
20 C
50 C
20 C, 10 min
Li
P(Ph)Bu-t
IIg
IVe
We extended this synthetic procedure to derivatives
of o-diorganylphosphino-substituted benzoic acids and
obtained phosphine IVf (R1 = R2 = i-Pr, Y =
2-COOSiMe3) in high yield. At present we are study-
ing the possibility of synthesis of other o- and m-
derivatives of phosphine-substituted benzoic acids by
a more convenient and economical method [compared
to cross coupling (1)] analogous to reaction (6).
TMS (1H) and external trichlorofluoromethane (19F)
and 85% phosphoric acid (31P). All manipulations
were carried out under dry argon and in dry solvents
purified by standard procedures.
General procedure of cross coupling. Synthesis
of phosphines IVa, IVb, V, and VI. Silylphosphine
I or II and aryl halide III in 1.0:0.9 or 1.0:1.8 molar
ratios, and 3 5 mol% of PdCl2(CH3CN)2 or PdCl2
(PPh3)2 were placed in an ampule preliminarily eva-
cuated and then filled with argon. The ampule was
sealed and thermostated with stirring under the condi-
EXPERIMENTAL
The H, 19F, and 31P NMR spectra were obtained
1
on a Varian VXR-400 spectrometer against internal
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 2 2005
Veits
