ca01 only detailed ToC 1..24

122 Science of Synthesis 2.6 Complexes of Cr, Mo, and W without CO Ligands
N
Me2 Cr
Cl
Cl
ClMg
THF, −30 o C
49%
Butane-1,4-diyl(ç 5 -pentamethylcyclopentadienyl)(trimethylphosphine)chromium(III);
Typical Procedure: [137]
[CrCp*Cl 2(THF)] (100 mLof a 0.085 M THF soln, 8.5 mmol) was treated with Me 3P(1mL,
9.88 mmol) and cooled to –508C. 1,4-Dilithiobutane (35 mLof a 0.277 M soln in Et 2O) was
added slowly and the mixture was stirred at –358C. The mixture was evaporated to dryness
and the residue was extracted with pentane (2 ” 150 mL) at –208C. The extract was
filtered, cooled to –788C, and the resulting blood-red crystals isolated and dried under
high vacuum; yield: 2.06 g (76%).
2.6.5.2 Method 2:
By Reductive Coupling of Alkenes
This method is specific for metallacyclopentanes. The alkene-coupling process is favored
by metal reduction. A typical synthetic strategy is the in situ reduction of a metal halide
precursor in the presence of the alkene; see, for example, the synthesis of 79 in Scheme
34. [137] An alkylidene precursor may also lead to a metallacycle with elimination of the carbene
ligand as in the synthesis of 81, representing a deactivation pathway for alkene metathesis
catalysts. [138] The two alkenes may be generated in situ in the coordination sphere
by rearrangement processes, such as intramolecular hydrogen transfer from an alkyl–vinyl
precursor. [139]
MgCl
N
Me 2
Scheme 34 Metallacyclopentanes by Reductive Coupling of Alkenes [137,138]
N
Me2 TMS
N
Cr
Cl
Cl
Ph
N
W
CH2But CH2But N
TMS
THF, −30 o Mg, H2C CH2 (excess)
C
90%
H2C CH2
(excess), 80 o C
TMS
N
N
Me 2
Cr
79
Ph
N
W CH2 N
TMS
Cr
80
TMS
N
Ph
N
W
N
TMS
81
Butane-1,4-diyl(ç 5 -pentamethylcyclopentadienyl)(trimethylphosphine)chromium(III);
Typical Procedure: [137]
The compound described in the experimental procedure in Section 2.6.5.1 can also be prepared
in 68% yield by reacting [CrCp*Cl 2(THF)] with active Mg and Me 3PinanEt 2O soln
saturated with ethene at –78 to –108C.