TY - JOUR
T1 - Diastereotopic Group-Selective Reactions at π-Arene Chromium Derivatives
T2 - Deprotonation and Nucleophilic Addition Reactions of Substrates Bearing Benzylic Chiral Centers
AU - Heppert, Joseph A.
AU - Aubé, Jeffrey
AU - Thomas-Miller, M. Elizabeth
AU - Milligan, Michael L.
AU - Takusagawa, Fusao
PY - 1990
Y1 - 1990
N2 - (η6-(MeHXC)C6H5)Cr(CO)3 (X = NMe2 or OR) substrates undergo deprotonation by alkyllithium bases to produce, after quenching with electrophiles (E+), high diastereomeric excesses (de) of (η6-o-E- (MeHXC)C6H4)Cr(CO)3 products. The N,N-dimethylamino substrate shows excellent chemoselectivity for ortho metalation, in addition to having ≥96% de in the formation of the product isomers. The propensity for ether derivatives to undergo benzylic deprotonation interferes with chemoselectivity in these substrates, although the selectivity for ortho substitution can be improved by employing chelating ether substituents and reduced reaction temperatures. The stereochemistry of deprotonation is evidently controlled by the tendency of the heteroatom side chain to adopt a conformation that minimizes steric interactions with the Cr(CO)3 moiety and promotes the delivery of the ligated base to a specific ortho hydrogen. NOEDS studies indicate that the conformation of the benzylic side chain in the solutions containing only (η6- (MeHXC)C6H5)Cr(CO)3 probably closely mirrors those involved in the delivery of the alkyllithium reagent. Reactions between (η6-(MeH(RO)C)C6H5)Cr(CO)3 and tert-butyllithium in diethyl ether produced significant quantities of (η4,η2-2-tert-butylcyclohexadienylidene-exo-etliane)Cr(CO)3 after the mixture is quenched with a proton donor. The reaction displayed a peculiar temperature dependence, with an approximate 1:1 ratio of the S*,S*,R* and S*,S*,S* diastereomers being produced if the quench is performed below -40 °C and a 10:1 ratio of the S*,S*,R* and S*,S*,S* isomers, respectively, being produced by quenching the reaction at an ambient temperature. An X-ray crystallographic study unambiguously identified the molecule formed in predominance at elevated temperatures as the S*,S*,R* exo-ethane Z isomer. Crystal data for (η6-(R*,R*)-o-(Ph2P)(MeH(Me2N)C)C6H4)Cr(CO)3 at 20 °C: a = 14.056 (4) Å,b = 9.730 (3) Å, c = 17.736 (5) Å, α = 105.78 (2)°, Z = 4, Dcalc = 1.30, space group P21/c and R(F) = 0.044, RW(F) = 0.051 for 1434 reflections. Crystal data for (η4,η2-S*,S*,R*-2-tert-butylcyclohexadienylidene-exo-ethane)Cr(CO)3 at 23 °C: a = 13.457 (4) Å, b = 8.505 (2) Å, c = 14.350 (2) Å, α = 112.92 (2)°, Z = 4, Dcalc = 1.31, space group P2x1/c and R(F) = 0.038, Rw(F) = 0.050 for 1496 reflections.
AB - (η6-(MeHXC)C6H5)Cr(CO)3 (X = NMe2 or OR) substrates undergo deprotonation by alkyllithium bases to produce, after quenching with electrophiles (E+), high diastereomeric excesses (de) of (η6-o-E- (MeHXC)C6H4)Cr(CO)3 products. The N,N-dimethylamino substrate shows excellent chemoselectivity for ortho metalation, in addition to having ≥96% de in the formation of the product isomers. The propensity for ether derivatives to undergo benzylic deprotonation interferes with chemoselectivity in these substrates, although the selectivity for ortho substitution can be improved by employing chelating ether substituents and reduced reaction temperatures. The stereochemistry of deprotonation is evidently controlled by the tendency of the heteroatom side chain to adopt a conformation that minimizes steric interactions with the Cr(CO)3 moiety and promotes the delivery of the ligated base to a specific ortho hydrogen. NOEDS studies indicate that the conformation of the benzylic side chain in the solutions containing only (η6- (MeHXC)C6H5)Cr(CO)3 probably closely mirrors those involved in the delivery of the alkyllithium reagent. Reactions between (η6-(MeH(RO)C)C6H5)Cr(CO)3 and tert-butyllithium in diethyl ether produced significant quantities of (η4,η2-2-tert-butylcyclohexadienylidene-exo-etliane)Cr(CO)3 after the mixture is quenched with a proton donor. The reaction displayed a peculiar temperature dependence, with an approximate 1:1 ratio of the S*,S*,R* and S*,S*,S* diastereomers being produced if the quench is performed below -40 °C and a 10:1 ratio of the S*,S*,R* and S*,S*,S* isomers, respectively, being produced by quenching the reaction at an ambient temperature. An X-ray crystallographic study unambiguously identified the molecule formed in predominance at elevated temperatures as the S*,S*,R* exo-ethane Z isomer. Crystal data for (η6-(R*,R*)-o-(Ph2P)(MeH(Me2N)C)C6H4)Cr(CO)3 at 20 °C: a = 14.056 (4) Å,b = 9.730 (3) Å, c = 17.736 (5) Å, α = 105.78 (2)°, Z = 4, Dcalc = 1.30, space group P21/c and R(F) = 0.044, RW(F) = 0.051 for 1434 reflections. Crystal data for (η4,η2-S*,S*,R*-2-tert-butylcyclohexadienylidene-exo-ethane)Cr(CO)3 at 23 °C: a = 13.457 (4) Å, b = 8.505 (2) Å, c = 14.350 (2) Å, α = 112.92 (2)°, Z = 4, Dcalc = 1.31, space group P2x1/c and R(F) = 0.038, Rw(F) = 0.050 for 1496 reflections.
UR - http://www.scopus.com/inward/record.url?scp=0025402542&partnerID=8YFLogxK
U2 - 10.1021/om00117a031
DO - 10.1021/om00117a031
M3 - Article
AN - SCOPUS:0025402542
VL - 9
SP - 727
EP - 739
JO - Organometallics
JF - Organometallics
SN - 0276-7333
IS - 3
ER -