“…Our synthetic approach to a rotationally restricted rhenium carbene complex involved initial attachment of a 2-lithioethyl side chain to the cyclopentadiene ring of CpRe(CO) 3 followed by intramolecular attack of the lithium reagent on a carbonyl group (Scheme ). Reaction of CpRe(CO) 3 with n -BuLi in THF at −78 °C generated the known ring-metalated complex (η 5 -C 5 H 4 Li)Re(CO) 3 which reacted with ethylene oxide and then tosyl chloride to give the tosylate (η 5 -C 5 H 4 CH 2 CH 2 OTs)Re(CO) 3 ( 5 ) as a white solid in 89% isolated yield. 1 …”
The acyl anion complex
[(CO)2ReC(O)CH2CH2(η5-C5H4)]-Li+
(7) in which a two-carbon tether links
the cyclopentadienyl ring to the acyl carbon was synthesized by
attachment of a 2-lithioethyl side chain to the
cyclopentadiene ring of CpRe(CO)3 followed by
intramolecular attack of the lithium reagent on a carbonyl
group.
Alkylation of 7 with
(CH3)3O+BF4
-
occurred at oxygen to give the methoxycarbene complex
(CO)2ReC(OCH3)CH2CH2(η5-C5H4)(9), which was shown by X-ray
crystallography to have significant strain associated with the
tethered
ring. Protonation of 7 gave a mixture of hydroxycarbene
complex
(CO)2ReC(OH)CH2CH2(η5-C5H4)
(2) and the
metal acyl hydride complex
trans-(CO)2HReC(O)CH2CH2(η5-C5H4)
(3). The unusual observation of the metal
acyl hydride is attributed to the two-carbon tether introducing strain
into the three-legged piano stool geometry of
2 but leaving the four-legged piano stool geometry of
3 relatively unstrained. In agreement with this
hypothesis, no
strain was apparent in the X-ray structure of the three-carbon-tethered
methoxycarbene complex (CO)2ReC(OCH3)CH2CH2CH2(η5-C5H4)
(17) and only the hydroxycarbene tautomer
(CO)2ReC(OH)CH2CH2CH2(η5-C5H4)
(18) was observed at equilibrium. A two-carbon tether
did not introduce sufficient strain into the aminocarbene
complex
(CO)2ReC[NH(CH3)]CH2CH2(η5-C5H4)
(19) to allow observation of its iminoacyl hydride
tautomer.
“…Our synthetic approach to a rotationally restricted rhenium carbene complex involved initial attachment of a 2-lithioethyl side chain to the cyclopentadiene ring of CpRe(CO) 3 followed by intramolecular attack of the lithium reagent on a carbonyl group (Scheme ). Reaction of CpRe(CO) 3 with n -BuLi in THF at −78 °C generated the known ring-metalated complex (η 5 -C 5 H 4 Li)Re(CO) 3 which reacted with ethylene oxide and then tosyl chloride to give the tosylate (η 5 -C 5 H 4 CH 2 CH 2 OTs)Re(CO) 3 ( 5 ) as a white solid in 89% isolated yield. 1 …”
The acyl anion complex
[(CO)2ReC(O)CH2CH2(η5-C5H4)]-Li+
(7) in which a two-carbon tether links
the cyclopentadienyl ring to the acyl carbon was synthesized by
attachment of a 2-lithioethyl side chain to the
cyclopentadiene ring of CpRe(CO)3 followed by
intramolecular attack of the lithium reagent on a carbonyl
group.
Alkylation of 7 with
(CH3)3O+BF4
-
occurred at oxygen to give the methoxycarbene complex
(CO)2ReC(OCH3)CH2CH2(η5-C5H4)(9), which was shown by X-ray
crystallography to have significant strain associated with the
tethered
ring. Protonation of 7 gave a mixture of hydroxycarbene
complex
(CO)2ReC(OH)CH2CH2(η5-C5H4)
(2) and the
metal acyl hydride complex
trans-(CO)2HReC(O)CH2CH2(η5-C5H4)
(3). The unusual observation of the metal
acyl hydride is attributed to the two-carbon tether introducing strain
into the three-legged piano stool geometry of
2 but leaving the four-legged piano stool geometry of
3 relatively unstrained. In agreement with this
hypothesis, no
strain was apparent in the X-ray structure of the three-carbon-tethered
methoxycarbene complex (CO)2ReC(OCH3)CH2CH2CH2(η5-C5H4)
(17) and only the hydroxycarbene tautomer
(CO)2ReC(OH)CH2CH2CH2(η5-C5H4)
(18) was observed at equilibrium. A two-carbon tether
did not introduce sufficient strain into the aminocarbene
complex
(CO)2ReC[NH(CH3)]CH2CH2(η5-C5H4)
(19) to allow observation of its iminoacyl hydride
tautomer.
“…Fischer reported that n -BuLi in Et 2 O at room temperature adds to a carbonyl ligand of 2 to give the acyl anion [(η 5 -C 5 H 5 )(CO) 2 Re(CO)CH 2 CH 2 CH 2 CH 3 ] - Li + ( 8 ), which was then converted to (η 5 -C 5 H 5 )(CO) 2 ReC(OCH 3 )CH 2 CH 2 CH 2 CH 3 ( 9 ) in 8% overall yield . In contrast, Nesmeyanov reported that n -BuLi in THF at low temperature ring-metalates 2 to give 6 , which reacts with CO 2 to give ((η 5 -C 5 H 4 )CO 2 H)Re(CO) 3 in 93% overall yield after hydrolysis . To determine whether the change in chemose-lectivity is a solvent or temperature effect, we have studied the reactions of n -BuLi with 2 in more detail.…”
The ring-metalated complex
(η5-C5H4Li)Re(CO)3
generated by reaction of
(η5-C5H5)Re(CO)3
with n-BuLi in THF at −78 °C reacted with additional
(η5-C5H5)Re(CO)3
to produce the
dirhenium acyl anion
(η5-C5H5)(CO)2Re(CO)[(η5-C5H4)Re(CO)3]-Li+.
Protonation of this
acyl anion gave the dirhenium hydroxycarbene complex
(η5-C5H5)(CO)2ReC(OH)[(η5-C5H4)Re(CO)3], while methylation gave the methoxycarbene
complex
(η5-C5H5)(CO)2ReC(OCH3)[(η5-C5H4)Re(CO)3],
which was characterized by X-ray crystallography. The
methoxycarbene
complex reacted with n-BuLi to produce the butylcarbene
complex
(η5-C5H5)(CO)2ReC(CH2CH2CH2CH3)[(η5-C5H4)Re(CO)3].
“…381 ' 382 Through appropriate subsequent reactions the lithiated intermediate can be converted to a variety of ring-substituted products as listed in Table 9. [338][339][340]343,344 Significantly, with change in the solvent to diethyl ether, nucleophile is directed to carbon of coordinated CO; with [Re(CO) 3 Table 10 except for the vinylidene complex which is presented in Table 14 below and the dinitrogen complexes which are further discussed in Chapter 60. The dimer (62), noted as a minor product in Scheme 21, is observed in greater yield during photolysis of [Re(CO)3(r;-Cp)] in hexane; the dimer yields equimolar quantities of the parent and the phosphine-substituted complex upon reaction with PPI13 (equation 43).…”
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