AbstractThe work comprising this thesis involved the investigation of a wide variety of chemistry, the unifying thread of which was the use of a peculiarly effective borane called pinacolborane (HBpin). While this borane is not a particularly novel reagent, having seen wide use in hydroborations and in C-C cross-coupling reactions, during the course of this work it demonstrated a particularly broad and unusual diversity of reactivity when combined with substrates beyond its usual purview.
The simplest HBpin mode of reactivity investigated in this thesis was its ability to reduce P(V) phosphine oxides to P(III) phosphines under mild conditions without the addition of a catalyst. This reduction is not in itself an unusual transformation for a borane to perform; however, HBpin achieves it to produce the free phosphine without any subsequent formation of P-B adduct species, a trait unique among borane reducing agents. While it could be argued the P-B adducts obtained by the reduction of phosphine oxides using other boranes such as THF.BH3 constitute usefully protected species given the reactivity of free phosphines, this unique property of HBpin allows the use of the P(V) phosphine oxide substrate itself as the protected species, instead of the adduct. This potentially eliminates the requirement for an additional deprotection step in the synthesis of phosphines and represents an improvement on the current state of the art in this area.
The other interesting properties demonstrated by HBpin elaborated on in this thesis were in combination with a cheap and simple to make dimeric iron pre-catalyst; [Fe(salen)]2-µ-oxo. The first of these involved the direct interaction between HBpin and this iron complex; HBpin is theorised firstly to reduce the two iron centres from their initial +III oxidation state to form Fe(II) monomeric complexes. Another molecule of HBpin is then theorised to react with the resultant Fe(II) complex to form an Fe-H species, breaking an Fe-O bond, with the leftover Bpin residue associating with the salen ligand to stabilise the resultant phenolate. The novel Fe-HBpin adduct species theorised to be produced under these conditions was highly active in the cyclotrimerisation of terminal alkynes to form substituted benzenes, and this useful reaction proceeded using catalytic quantities of iron and HBpin under mild and economically relevant conditions.
The final mode of reactivity investigated was also in combination with the dimeric iron pre-catalyst, but this time it was not only the catalyst receiving the reductive attentions of the HBpin. A multitude of carbonyl containing substrates were subjected to HBpin in combination with the [Fe(salen)]2-µ-oxo pre-catalyst, and these conditions proved competent for the reduction of a wide variety of aldehydes, ketones and certain esters to their corresponding alcohols via boronate esters. These catalytic reductions also pointed the way towards some unique reactivity which allowed HBpin in combination with [Fe(salen)]2-µ-oxo to reduce organic nitro compounds, which was investigated and shown to result in the formation of an array of imine/amine species. Paradoxically the most interesting substrate for these nitro reduction reactions also turned out to be the simplest; nitromethane. Instead of producing the more obvious methylamine product from nitromethane, the HBpin/iron system formed a derivative of highly unstable methanimine, previously isolated only at cryogenic temperatures, which may allow its first use as a C-N building block for organic synthesis. In addition, methanimine is found in the interstellar medium and is implicated in the prebiotic formation of the simplest amino acid glycine, and any mild, ambient temperature/pressure access to it would therefore be very useful to research in these areas.
|Date of Award
|24 Jun 2020
|Ruth Webster (Supervisor) & Emma Emanuelsson Patterson (Supervisor)