The diastereomeric methyl rhenium complex [CpRe(NO){P(Me)(Ph)(2-C6H4NMe2)}(CH3)] was prepared in two steps from chiral racemic [CpRe(NO)(CO)(NCMe)]BF4 and the chiral racemic phosphine P(Me)(Ph)(2-C6H4NMe2). The unlike diastereomer reacts preferentially with MeSO3H to give the ring-closed ionic complex unlike-[CpRe(NO){P(Me)(Ph)(2-C6H4NMe2)}]MeSO3 along with unreacted like-[CpRe(NO){P(Me)(Ph)(2-C6H4NMe2)}(CH3)], which is easily separated and converted to like-[CpRe(NO){P(Me)(Ph)(2-C6H4NMe2)}]MeSO3. Starting from (R)-P(Me)(Ph)(2-C6H4NMe2), the diastereomerically and enantiomerically pure complexes (RRe,SP)-[CpRe(NO){P(Me)(Ph)(2-C6H4NMe2)}]MeSO3 and (SRe,SP)-[CpRe(NO){P(Me)(Ph)(2-C6H4NMe2)}]MeSO3 were obtained. Thus, this reaction sequence demonstrates a highly diastereoselective proton transfer from a functionalized chiral phosphine to a transition metal. Furthermore, it provides efficient access to enantiomerically pure half-sandwich rhenium complexes.
The chiral N,P ligand P(Me)(Ph)[8‐(2‐methylquinolinyl)] (3) was synthesized and separated into its enantiomers via diastereomeric palladium complexes. The reactions of 3 and (RP)‐3 with [CpRe(CO)(NO)(NCMe)]BF4 (7) gave thediastereomeric complexes [CpRe(CO)(NO){P(Me)(Ph)(C10H8N)}]BF4 [8 and (RRe,SP/SRe,SP)‐8], which, upon borohydride reduction, yielded the corresponding methyl complexes [CpRe(NO){P(Me)(Ph)(C10H8N)}(CH3)] [9 and (RRe,SP/SRe,SP)‐9]. Treatment of 9 with HBF4 under carefully controlled conditions gave the diastereomerically pure chelates [CpRe(NO){P(Me)(Ph)(C10H8N)}]BF4 [(RRe,SP/SRe,RP)‐10, (RRe,RP/SRe,SP)‐10 and (RRe,SP)‐10]. The chelate ring was opened with NaSH to produce the hydrosulfido complexes [CpRe(NO){P(Me)(Ph)(C10H8N)}(SH)] [(RRe,SP/SRe,RP)‐11, (RRe,RP/SRe,SP)‐11 and (RRe,SP)‐11]. Each step in this sequence proceeded with retention of configuration at rhenium. Complex 11 underwent acid‐promoted condensation with aldehydes to give thioaldehyde complexes [CpRe(NO){P(Me)(Ph)(C10H8N)}(S=CHR)]BF4 (12a–d, R = Ph, Me, 4‐C6H4OMe, C6F5). The addition of nucleophiles X– to 12a gave rhenium‐coordinated α‐chiral thiolate complexes [CpRe(NO){P(Me)(Ph)(C10H8N)}{SC(H)(Ph)(X)}] (13a–e, X = acac, PhCH2S, EtS, tBuS, CN) with 42–89 % de. The thiolate can readily be cleaved from the rhenium complex by a methylation/chelate ring‐closure strategy. The stereochemistry of the entire reaction sequence was corroborated for each step by X‐ray crystallography.
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