DIASTEREOTOPIC GROUP-SELECTIVE REACTIONS AT PI-ARENE CHROMIUM DERIVATIVES - DEPROTONATION AND NUCLEOPHILIC-ADDITION REACTIONS OF SUBSTRATES BEARING BENZYLIC CHIRAL CENTERS

(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. © 1990, American Chemical Society. All rights reserved.