ca01 only detailed ToC 1..24

2.6.7 Complexes with Doubly Bonded Heteroelement Ligands 129
Chlorodioxo(ç 5 -pentamethylcyclopentadienyl)molybdenum(VI): [162]
In a drybox, a 100-mLglass ampule (Teflon stopcock) was charged with a soln of
[MoCp*Cl(CO) 3] (0.980 g, 2.79 mmol) in toluene (50 mL). O 2 was bubbled through the soln
via a 24-gauge syringe needle at a rate of approximately 2 bubbles •s –1 while the soln was
irradiated with a 450-W medium-pressure Hg immersion lamp at 58C. The reaction was
monitored by IR spectroscopy and was stopped after 90 min when the absorbances due
to the carbonyl ligands of the starting material at 1700–2000 cm –1 had disappeared.
During the reaction the soln turned from a red-orange to an amber shade with some
blue solids precipitated on the walls of the reaction vessel. The toluene was removed under
vacuum, leaving a brown solid. The solid was taken up in toluene in the drybox and
the resulting soln filtered through a medium-porosity sintered-glass frit to remove insoluble
blue impurities. Recrystallization (toluene/hexane) afforded [MoCp*Cl(=O) 2] as a yellow
solid; yield: 0.508 g (61%); IR (benzene-d 6) í~ max: (Mo=O) 920 (s), 890 (s) cm –1 ; 1 HNMR
(benzene-d 6, ä): 1.63 (s, C 5Me 5).
ç 5 -Cyclopentadienyldioxo[(trimethylsilyl)methyl]tungsten (91,R 1 =CH 2TMS; M = W);
Typical Procedure: [163]
To a stirred, purple soln of [W(CH 2TMS) 2Cp(NO)] (1.20 g, 2.65 mmol) in Et 2O (50 mL) was
added a 30% by weight aq soln of H 2O 2 (0.22 mL, 2.8 mmol of H 2O 2). The initial purple color
of the mixture faded over the course of 1 h to pale yellow. An IR spectrum of the final yellow
soln was devoid of absorptions due to the nitrosyl reactant. Volatiles were removed
from the final mixture under reduced pressure to obtain a sticky yellow solid which was
dried at 208C/0.005 Torr for 2 h. Recrystallization of the resulting pale yellow solid from
Et 2O/hexanes (1:1) at –208C afforded [W(CH 2TMS)Cp(=O) 2] as a white microcrystalline solid;
yield: 0.50 g (51%); IR (benzene-d 6) í~ max: (W=O) 948 (s), 907 (s) cm –1 ; 1 H NMR (benzened
6, ä): 5.69 (s, 5H, Cp), 0.88 (s, 2H, CH 2), 0.28 (s, 9H, TMS).
Applications of Product Subclass 7 in Organic Synthesis
2.6.7.5 Method 5:
Catalytic Epoxidation of Alkenes
High oxidation state molybdenum oxo complexes are well-established catalysts for the
epoxidation of alkenes by alkyl hydroperoxides, such as in the production of 2-methyloxirane
(Halcon process). Chlorodioxo(ç 5 -pentamethylcyclopentadienyl)molybdenum(VI)
provides an organometallic example of a catalytically active system. The epoxidation reaction
is stereoselective, as shown by the selective formation of trans- and cis-1,2-diphenyloxirane
from the respective E- and Z-alkenes, and can be applied to highly substituted
alkenes; see Scheme 42. [162] Studies on this system have shown that the degradation of the
catalyst involves oxidative poisoning to an unreactive peroxo complex. [162]
Scheme 42 Catalytic Epoxidation of Alkenes [162]
+
Bu t OOH
MCpO2Cl (cat.)
benzene, 25
72%
oC O O
1,2-Epoxycyclooctane; Typical Procedure: [162]
In a drybox, a soln of [MoCp*ClO 2] (0.101 g, 0.338 mmol) in benzene (5 mL) was placed into
a 50-mLbomb. Cyclooctene (1.77 mL, 13.6 mmol) was added via a syringe. Outside the drybox,
3 M t-BuOOH in 2,2,4-trimethylpentane (11.3 mL, 34 mmol) was added. The bomb
was heated to 608C for 3 h. The soln was then diluted to a total volume of 150 mLwith
benzene. The organic layer was washed with H 2O (6 ” 100 mL) to remove excess hydroper-
for references see p 135