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Strecker Synthesis - Organic Chemistry Portal
Two different types of NHC–Cu complexes catalyze protoborations of terminal allenes to afford valuable 1,1- or trisubstituted vinylboron species with high site- and stereoselectivity. The scope of the method, application to natural product synthesis and mechanistic basis for the observed selectivity trends are presented.
After a brief optimisation of the reaction conditions (see ESI), we found the best conditions for the synthesis of [Cu(Cl)(IPr)] are the ones used for the synthesis of [Au(Cl)(IPr)]. In order to evaluate the versatility of this protocol we carried out the reaction using different NHC ligands and copper salts under the optimised conditions. The results are summarised in . It should be noted that, although the reaction time can be significantly decreased by adding a large excess of base, all reactions were carried out with 2 equivalents in order to minimise waste.
Mechanism of the Strecker Synthesis
The utility of the method, particularly in relation to that of alkyl-substituted allenes, is demonstrated by synthesis of the C1–C10 fragment of the macrolide elansolid A, a recently identified antibiotic natural product (). Efficient preparation of 31 is illustrative of the ease with which mono-substituted allenes are accessed. NHC–Cu-catalyzed protoboration with hydroxyl-containing 31 with 4e, followed by Pd- catalyzed cross-coupling (without isolation/ purification of the vinylboron intermediate) affords triene 32 with >98% site selectivity, in 72% overall yield and as only the E stereoisomer (>98%). A total synthesis of elansolid A has not been reported.
A one-pot procedure for the synthesis of [Cu(X)(NHC)] (X = Cl, Br, I) is reported. The reaction is applicable to a wide range of saturated and unsaturated NHC ligands, is scalable and proceeds under mild conditions using technical grade solvents in air.
Lactone synthesis - Organic Chemistry Portal
Regarding the use of these species in organometallic synthesis, homo- and heteroleptic bis-NHC-Cu(I) complexes were employed as carbene transfer reagents to other transition metals.
The observed selectivity trends, dictated by catalyst structure, can be rationalized by the pathways outlined in , as supported by DFT calculations. With either catalyst type (derived from 3 or 4e), Cu–B addition places the NHC–Cu initially at the less hindered site of the mono-substituted allene (→i). Subsequent γ-protonation via ii, the favorability of which is indicated by calculations, causes preferential formation of the 1,1-disubstituted vinylboron product. The latter part of the above route, however, pertains mainly to catalysts with the larger NHC ligand (i.e., 3). With the smaller catalyst derived from 4e, conversion of the complex i to isomeric iii, bearing a secondary Cu–C bond, becomes sufficiently favored; theoretical studies reveal that allylcopper iii is higher in energy and can more swiftly undergo protonation via iv (vs ii) to afford trisubstituted B(pin)-substituted alkenes (Curtin–Hammett kinetics). The greater reactivity of iii appear to be partially the result of the higher-energy HOMO of the more substituted Cu–C bond; moreover, since the trisubstituted olefin is energetically favored, the activation barrier to protonation that furnishes such entities would be lower (Hammond’s postulate). Transition structure iv, engendering high stereoselectivity, allows for minimization of steric repulsion between the allene substituent (G) and the B(pin) and NHC–Cu units; there is little 1,3-diaxial repulsion to discourage formation of iv. Based on the above scenario, with the larger NHC ligand 3, protonation of the kinetically-generated allylcopper species is faster than equilibration between i and iii and is therefore product-determining (non-Curtin-Hammett); with smaller catalysts, it is the more facile protonation of the higher energy allylcopper (iii) that determines the identity of the major product.
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N-Heterocycle synthesis - Organic chemistry
In 2011, we demonstrated that by altering the structure of the NHC ligand of a Cu-based catalyst, terminal alkynes may be efficiently converted to α- or β-vinylboron entities (). Reactions involve site-selective addition of an NHC–Cu–B(pin) complex followed by protonation of the vinylcopper intermediate by MeOH (net protoboration). More recently, as shown in , in conjunction with studies regarding NHC–Cu-catalyzed enantioselective allylic substitution reactions, we showed that the resulting allenes undergo Cu-catalyzed allene protoboration to afford 1,1-disubstituted vinylboron products with ~90:10 selectivity., Considering the value of either isomeric form and the limited number of protocols for their preparation,- we set out to establish the scope of the catalytic process (), as well as determine whether the trisubstituted vinylboron compounds can be synthesized selectively with a different set of NHC ligands.
Lactone synthesis - Organic chemistry
AB - Less than five years ago we reported the NHC-catalysed (4+2) annulation of dienol ethers and unsaturated acyl fluorides. From a mechanistic perspective, this reaction likely involves a vinylogous Michael addition followed by an aldol/β-lactonisation cascade. In this account, the discovery of this reaction and ensuing studies into its mechanism and utility in multistep synthesis will be examined. The subsequent development of chiral catalysts designed for this reaction and the achievement of a first-generation and later second-generation approach to an enantioselective variant of this reaction will be discussed. Finally, related redox isomerisation cascades leading to benzaldehydes will be introduced, as will reactions in the field of NHC catalysis that exploit similar reaction cascades. 1 Introduction 2 Reaction Design and Discovery 3 Mechanistic Studies and β-Lactone Interception 4 Enantioselective Cyclohexenyl β-Lactone Synthesis 5 Enantioselective Cyclohexadiene Synthesis 6 Redox Isomerisation 7 Related NHC Catalysis 8 Conclusions.
Thieme E-Journals - Synthesis / Full Text
The proposal in offers an explanation for other observations. The lower selectivity with the less hindered allenes in (8-10) might be because the intermediacy of iii () is slightly competitive. In contrast, with 4e (), all transformations proceed with high selectivity, since it is the rate of allylcopper protonation that is critical (vs which is generated favorably). The inefficiency of syntheses of silyl ethers 25-26 () is likely due to facile Cu–alkoxide elimination of intermediates such as 36 in (perhaps by syn elimination), resulting in the eventual generation of 38 (among other products). In reactions with 3, the Cu–C bond in 39, remote from the silyl ether, undergoes γ-protonation, as isomerization is relatively disfavored. Vinylsilane 27 is formed mainly as a 1,1-disubstituted olefin likely since stabilization of electron density at the adjacent Cu–C site by the low-lying C–Si σ* orbitals in iv more effectively retards γ-protonation rate (vs ii) to disfavor the Curtin-Hammett pathway.
Suzuki Coupling - Organic Chemistry Portal
Moreover, one molecule of this complex could generate up to 230 molecules of ammonia, the highest catalytic activity ever.
The newly designed complex is an important achievement towards the development of energy-saving next-generation nitrogen fixation systems, which carry a lot of potential.
"Our research group including Aya Eizawa, a graduate student, produced ammonia from nitrogen gas efficiently under mild reaction conditions using a novel molybdenum-dinitrogen complex designed for high catalytic performance for ammonia synthesis," says Nishibayashi.
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