William J. Kerr

Magnesium Bisamides as Reagents in Synthesis

As part of the continued requirement for more selective reagents in organic synthesis, magnesium bisamides are becoming established as a class of organometallic bases with considerable potential. Their relatively mild reactivity, combined with their high degree of steric congestion, leads to a distinct class of reagents with significantly different chemo-, regio-, stereo- and enantioselectivities when compared with existing species and protocols.

Development of modified Pauson-Khand reactions with ethylene and utilisation in the total synthesis of (+)-taylorione

Two complementary Pauson-Khand annulation protocols for use with the gaseous alkene, ethylene, are described. These N-oxide promoted reactions (10 examples) are shown to proceed under both mild autoclave conditions or, more conveniently, at atmospheric pressure. The developed methodology has been utilised as the key transformation in the total synthesis of the sesquiterpene (+)-taylorione which has been realised in a good overall yield from readily available (+)-2-carene.

Promotion of the chromium carbene Dötz annulation reaction under sonochemical and dry state adsorption conditions

Abstract The chromium carbene Dotz annulation reaction proceeds very rapidly in the presence of ultrasound giving good yields of quinone product on oxidative workup. Moreover, the yield of annulated product has been improved by adsorbing the reagents onto silica and heating at moderate temperatures. Both these modifications have proved to be successful for a variety of complexes and alkynes.

Conditions for deuterium exchange mediated by iridium complexes formed in situ

Abstract A series of iridium-based complexes formed in situ, containing pyridine, phosphines, triphenylarsine, triphenylstibine, and triphenylamine as ligands, has been screened for ability to mediate ortho-exchange of hydrogen in a series of model substrates. Improved incorporation into a number of substrate classes has been achieved. The electronic properties and number of ligands at the metal centre are instrumental in determining which catalysts are best suited to exchange in any given substrate.

The brucine N-oxide-promoted asymmetric Pauson–Khand reaction

The brucine N-oxide-mediated asymmetric Pauson–Khand reaction has been further investigated. It was found that the best levels of enantioselection were obtained with substituted propargylic alcohol complexes. The studies also revealed that when acetone or 1,2-dimethoxyethane is used as the reaction solvent, enhanced levels of enantioselectivity, up to an enantiomeric ratio of 11:89, are achieved.

Deuterium exchange mediated by an iridium–phosphine complex formed in situ

Bis(triorganophosphine)(cyclooctadiene)iridium(I) tetrafluoroborates, 2, are readily generated in situ and utilised for the exchange of deuterium into a variety of aromatic substrates. The efficiencies of deuterium exchange using 2 formed in this way are comparable to those observed in exchange processes where the isolated pre-catalysts were used.

Elucidating the mechanism of the photochemical Pauson–Khand reaction: matrix photochemistry of phenylacetylenehexacarbonyldicobalt

Abstract Short wavelength UV irradiation of (phenylacetylene)Co 2 (CO) 6 ( 3 ) in argon matrices at 12 K results in loss of CO to form (phenylacetylene)Co 2 (CO) 5 ( 4 ). Reversal of this reaction occurs on longer wavelength irradiation. When the reaction is instead carried out in mixed argon/nitrogen matrices (phenylacetylene)Co 2 (CO) 5 (N 2 ) ( 5 ) is formed as an additional product. Irradiation at λ =390 nm allows selective reversal of photoproduct 4 to the parent species, leaving photoproduct 5 unaffected. When 3 is irradiated in pure nitrogen matrices photoproduct 4 is not formed, but two photoproducts containing coordinated dinitrogen are observed. One of these may confidently be assigned as 5 , but the identity of the second cannot at present be specified with certainty. The results gained are discussed in the light of the much more widely studied and better understood thermal chemistry of such complexes.

Promotion of the chromium carbene dötz annulation reaction under dry state adsorption conditions

Abstract The chromium carbene annulation has been carried out in the solid-state by adsorption of the reagents onto silica, neutral alumina and anhydrous magnesium sulphate. The reaction rate and product yield are examined.

The Khand reaction of some trisubstituted alkenes. Uses of the catalytic version of the reaction

Abstract Results are presented which suggest that in the absence of strong polar influences, 4,4,5-substituted cyclopentenones predominate over their 4,5,5-isomers when trisubstituted alkenes are employed in the Khand reaction. Double bonds shifts prior to annellation are a feature of two of the examples studied. Efficient use of these relatively unreactive alkenes is made possible by employing an acetylene/carbon monoxide atmosphere to allow recycling of the cobalt carbonyl fragment.

The utility of vinyl ethers and vinyl esters in the Khand reaction. The value of vinyl esters as ethylene equivalents and a modified synthesis of (+)-taylorione as an example

Abstract The behaviour of various oxygenated alkenes in the Khand cyclisation reaction has been studied. Although several vinyl ethers reacted to give the expected oxygenated cyclopentenone products, usually with good levels of regioselectivity, the use of vinyl esters was found to afford, as the major products, reduced cyclopentenones in which the carbon–oxygen bond had been cleaved. This unexpected reaction was developed into an alternative procedure to using ethylene gas in the Khand reaction and was found to be applicable with a variety of alkyne substrates. The method was then extended to form the key step in the synthesis of the natural product (+)- taylorione (and (+)- nortaylorione ).