Mechanistic aspects of the titanocene dichloride-isopropyl Grignard reduction of alkyl bromides

Abstract

The stereodiemiatiy of the reductions of cis- and trans-4-methylcyclohyl-1-d bromide with the ayitem isopropybruigiiesiui bromidp-5% dicyclopendidyltitauium dichloride (argon) and of the unhbeled bromides with liaopropylmasnemunbronude icyc by 2H NMR analysis the resulting cis- and trans 4-melhyIcyclhsxane-I-d. Complet steraoequilibration occurs in some step and predominantly equatorial hydropcn (or deuterium) bslraclion by tbe pmumed 4-methylcyclubeiyl radical iindicated Radical Involvftmait . confirmed by the identification of substantial amounts of cydid hydrocarbnnn in the rduction the 6-bromo-hex-1-eneimd 6-bromo-hept-1-ene, whereas complet cydizaflun and bydrugen abslmclion ro provide 3-methyltethydrofuran charncterLwo tht reduction of 2-(allylcoxy)ethyl broinidfr Preiminary cipcrimentc that titanocene dichloride only feebly coulyxes the reduction of secondary bromides by todiura borohydride under aerobic conditions in N,N-di-methylformunide, iuch reductiooi Dpven hele procptdin with predominant inversion of confdialiun. Cyclized hydrocirbons from suitable bromo precursors (above) were not observed, indicating minor role for free radical in this system. A report that dicydoptntatlienyltilinium diohlorid exprience a dimethyl sulfoxide induced → σ rearrangement of both cyclopentidimtyl gmupi ii shown to be incorrect.