Selective additions to unsaturated carbon-carbon bonds by the use of N-heterocyclic carbene-copper(I) catalysts

The development of easily-accessible and densely-functionalized synthetic intermediates is a longstanding goal of the organic chemistry community. The use of transition metal catalysts has allowed many synthetic targets to be produced with minimal waste. Copper(I) has distinct advantages to its use as a catalyst for organic transformations, in that it is relatively cheap and non-toxic by comparison to other metals which see frequent use. Hydroboration chemistry, in particular, has seen some of these benefits, as copper(I) catalyst systems for hydroboration have been the source of much research in the past 15 years, enabling the production of a wide variety of complex boron-containing synthetic intermediates via addition reactions to unsaturated bonds. The research contained herein describes our efforts to improve the methods for functionalization of unsaturated carbon-carbon bonds using N-heterocyclic carbene-copper(I) catalysts.;The formation of chiral allylboronates via allylic substitution is an important class of reactions owing to the versatile products that result, which can be used subsequently to produce allylic alcohols, homoallylic alcohols as well as to form C-C bonds via coupling reactions. Previously, only a few examples of chiral allylboronates have been described by the asymmetric addition of boron, therefore this was designated a good starting point for our research into this area. Through the use of a copper(I) catalyst with a 6-membered N-heterocyclic carbene (6-NHC) ligand, we have demonstrated the ability to produce chiral allylboronates with a high degree of selectivity. Additionally, this system was found to be stereoconvergant, meaning that both the E and Z-isomeric starting materials resulted in products with the same absolute stereochemistry.;In addition to polarized allylic alkenes, we also investigated the hydroboration of alkenes which are strained but otherwise unactivated, a class of substrates understudied for copper-catalyzed hydroboration. Through the use of a 5-NHC ligand pioneered by Glorius, we were able to achieve the selective hydroboration of unfunctionalized strained alkenes in the presence of non-strained alkenes for a variety of norbornene and styrenic substrates.;In addition to the hydroboration of C-C double bonds, we then became interested in the addition of boron to alkynyl species to produce functionalized vinylboronates. Regioselective hydroborations utilizing copper(I) catalysis have been reported for terminal alkynes and for some internal alkyne systems, however propargylic substrates have been understudied and the ability to access any desired regioiosmer remains elusive. Our efforts have shown that boron addition to propargylic alcohols can be controlled by the careful selection of catalyst and O-protecting group. Specifically, we have demonstrated that the use of a 6-NHC-Cu(I) complex with a m-nitrophenyl propargyl ether results in selectivity for the alpha position with a high degree of selectivity. The use of a 5-NHC-Cu(I) complex to an unprotected propargylic alcohol produced the desired beta-borylated material, also with good selectivity. In this way we have been able to produce a variety of functionalized vinylboronates to which we then turned our attention.;In an effort to showcase the utility of these vinylboronate species, we have also developed an ate-mediated allylic substitution, or AMAS, which can produce tertiary allylboronates by the simple addition of organomagnesium reagent. A variety of Grignard reagents were employed, and it was demonstrated that the AMAS procedure is compatible with most functional groups that are stable under Grignard conditions. To showcase the synthetic viability of this procedure we performed a total synthesis of the sugar molecule methyl axenoside from entirely achiral starting materials. This natural product was synthesized in 9 steps with an overall yield of 10%.