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Title: Efficient & novel approaches to relay chirality in the synthesis of planar chiral metallocenes
Author: Amin, Jahangir
Awarding Body: University of East Anglia
Current Institution: University of East Anglia
Date of Award: 2010
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Intermolecular and intramolecular dynamic kinetic resolution were detected for organocobalt complexed diynes prepared from the complexation of an alkyne to dicobalt hexacarbonyl, Co2(CO)6 and subsequent manipulation of the carbocationic charge generated at the carbon α– to the alkyne moiety via to the prior treatment with a Co2(CO)6– stabilised enyne, propargylic alcohol and alkynyl radical in a interesting departure from the Nicholas reaction. Diastereomeric ratios of up to 1:0 in favour of the anti diastereoisomer and yields of up to 67% were observed. Ether and ester linked diynes were synthesised from secondary terminal propargylic alcohols. On heating with (η5– cyclopentadienyl)bis(triphenylphosphine)cobalt(I), new chiral transition metal complexes were formed with high diastereomeric ratios in greater than 70% yield displaying in some cases four elements of chirality; central carbon chirality, chiral–at–metal chirality, conformational propeller phosphine chirality and conformational atropisomerism. Using mainly two representative examples the first a 7,5–membered cyclic lactone and the second a 5,5–membered cyclic ether complex the origins of chirality were explored which indicated the central carbon chirality relayed its stereochemical information to the stereogenic transition metal center thus resulting in a opposite configuration to the remote central carbon element of chirality i.e. (SC,RCo). Due to the unique properties of the 7,5–membered cyclic lactone complex a relatively stable major conformation propeller isomer of triphenylphosphine was found to be locked in space e.g. (SC,RCo,P). NMR analysis showed the presence of a dynamic equilibrium as three minor peaks of approximate equal intensity were observed which could be conformational atropisomers arising from hindered rotation between two interacting aromatics as a result of conformational flexibility inherent of the lactone architecture e.g. (P,aR), (R,aS), (M,aR), (M,aS). With respect to the 5,5–membered cyclic ether complex using a chiral ether diyne in a oxidative cyclisation reaction was found to result in chirality being relayed from the carbon stereogenic center to the chiral–at–metal stereocenter. As with the lactone complexes, an opposite configuration was observed. However, with respect to the conformation of triphenylphosphine it appeared to be non selective upon initial complexation. Recrystallisation gave an enriched (M) propeller conformational isomer which allowed the configurational stability of the chiral metal center to be measured which suggested that a mechanism did exist for epimerisation at metal but occurred only at an ambient temperature over several hours. Chiral metallocyclopentadiene complexes were treated with a selection of isocyanides and found to give novel planar chiral (η5–cyclopentadienyl)(η4–iminocyclopentadiene)cobalt metallocenes in good yields >60% yield and in some cases a diastereomeric ratio of up to 5:1. The isocyanide insertion reaction was in some cases complementary to the results observed for the isocyanides isoelectronic equivalent carbon monoxide which gave similar planar chiral (η5– cyclopentadienone)(η5–cyclopentadienyl)cobalt metallocenes. Preliminary results suggests because the isocyanide is smaller, in the diastereomeric transition state it is less discriminating compared to the chiral–at–metal complexes which are highly diastereoselective with respect to the configuration at metal. The methodology was extended to a non racemic example and found which gave a diastereomeric ratio of 2:1 and 92% yield.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available