Triazacyclohexane Chromium Triflate Complexes as Precursors for the Catalytic Selective Olefin Trimerisation and its Investigation by Mass Spectrometry

A novel set of linear α-olefin trimerisation pre-catalysts is presented. The R3TACCr(OTf)3 (R3TAC = 1,3,5-trialkyl-1,3,5-triazacyclohexane) complexes produce an active system after facile activation with less than 10 equivalents of trialkylaluminium. Isomerisation was observed in many cases, which is proposed to occur via the π-allyl complex mechanism, though this was minimised by optimisation of the reaction conditions. The pre-catalysts can be synthesised from R3TACCrCl3 in neat TfOH or by addition of R3TAC to Cr(OTf)3 or better Cr(OTf)3(THF)3, the synthesis and structure of which is presented here. The use of this highly defined system allowed the identification of 2-methyl-1-hexene as a side product of activation with AlMe3, in agreement with the proposed metallacyclic mechanism. Isomer distribution of the trimer product is similar to R3TACCrCl3/MAO activated systems and depends mostly on the ligand substituent R. ESI mass spectrometry of an ortho-difluorobenzene solution of the activated catalyst was analysed at different stages of reaction. A series of signals was observed that matched those expected for cationic chromium species predicted by the metallacyclic mechanism. In particular, [R3TACCrMe2]+ was observed to form immediately after alkylation with AlMe3, while [R3TACCr(olefin)n]+ (n = 0, 1, 2, 3, 4 and olefin = 1-hexene or 1-octene) formed after addition of 1-hexene. Absence of any detected tetramer and observation of [R3TACCr(olefin)4]+ leads to the conclusion that a metallacycloheptane-olefin complex may lead to β-H abstraction rather than insertion and may even be required to avoid formation of [R3TACCr(olefin)1]+ which is observed in only very small amounts indicating a low stability. Chromium(I) complexes [R3TACCr(arene)]+ with the arene solvent can also be observed and their signal intensity indicate a relative stability of o-C6H4F2 < cymene < benzene < PhF, toluene < p-xylene

Dearomatized BIAN Alkaline-Earth Alkyl Catalysts for the Intramolecular Hydroamination of Hindered Aminoalkenes

Reaction of a sterically encumbered bis­(imino)­acenapthene (dipp-BIAN) with either potassium alkyl or the heavier alkaline-earth dialkyl [Ae­{CH­(SiMe₃)₂}₂(THF)₂] (Ae = Mg, Ca, Sr) reagents results in dearomatization of the aromatic ligand. The heteroleptic alkaline-earth alkyl species show enhanced stability toward Schlenk-type redistribution but undergo solution exchange when the bis­(trimethylsilyl)­methyl substituent is replaced by an anionic ligand of lower overall steric demands. In contrast, analogous reactions performed with [Ba­{CH­(SiMe₃)₂}₂(THF)₂] evidenced facile solution redistribution and resulted in an unusual C–C coupling reaction which is suggested to result from a sterically induced reductive process. An assessment of the Mg, Ca, and Sr alkyl compounds as precatalysts for the intramolecular hydroamination of aminoalkenes evidenced enhanced reactivity, which is ascribed to the greater solution stability of the catalytically active species. Most notably the calcium species may even be applied to the high-yielding cyclization of substrates bearing alkyl substitution at either of the alkenyl positions

Stoichiometric and Catalytic Reactivity of <i>tert</i>-Butylamine–Borane with Calcium Silylamides

The primary amine–borane <i>t</i>-BuNH₂·BH₃ reacts with a β-diketiminate-supported silylamido calcium complex with elimination of HN­(SiMe₃)₂ and formation of the corresponding primary amidoborane complex, in which the deprotonated amine–borane is attached to the alkaline-earth center via its nitrogen atom and anagostic interactions with the boron-bound hydrides. Catalytic dehydrocoupling reactions employed with this β-diketiminate precatalyst are found to be slow and complicated by protonation of the supporting ligand and the formation of a number of boron-containing products, all of which have been positively identified. In common with previous studies of group 2 catalyzed secondary amine–borane dehydrogenation, the first formed major product of the catalysis is identified by solution NMR and solid-state single-crystal X-ray studies to be a cyclic diborazane, [H^tBuN–BH₂]₂, the formation of which is accompanied by variable proportions of diamidoborane and aminoborane products. The active calcium species is also observed to be depleted during the catalysis due to the formation of hydrocarbon-insoluble [Ca­(BH₄)₂·THF]_∞, which has also been structurally characterized. Continued heating of these reaction mixtures results in the formation of cyclic trimeric 1,3,5-tri-<i>tert</i>-butylborazine, which is proposed to form through the intermediacy of [H^tBuN–BH₂]₂ by an, as yet, undefined sequence of borazane dehydrogenation and ring expansion reactions

Dearomatized BIAN Alkaline-Earth Alkyl Catalysts for the Intramolecular Hydroamination of Hindered Aminoalkenes

Reaction of a sterically encumbered bis­(imino)­acenapthene (dipp-BIAN) with either potassium alkyl or the heavier alkaline-earth dialkyl [Ae­{CH­(SiMe₃)₂}₂(THF)₂] (Ae = Mg, Ca, Sr) reagents results in dearomatization of the aromatic ligand. The heteroleptic alkaline-earth alkyl species show enhanced stability toward Schlenk-type redistribution but undergo solution exchange when the bis­(trimethylsilyl)­methyl substituent is replaced by an anionic ligand of lower overall steric demands. In contrast, analogous reactions performed with [Ba­{CH­(SiMe₃)₂}₂(THF)₂] evidenced facile solution redistribution and resulted in an unusual C–C coupling reaction which is suggested to result from a sterically induced reductive process. An assessment of the Mg, Ca, and Sr alkyl compounds as precatalysts for the intramolecular hydroamination of aminoalkenes evidenced enhanced reactivity, which is ascribed to the greater solution stability of the catalytically active species. Most notably the calcium species may even be applied to the high-yielding cyclization of substrates bearing alkyl substitution at either of the alkenyl positions

Total Synthesis of (+)-Grandifloracin by Iron Complexation of a Microbial Arene Oxidation Product

(+)-Grandifloracin was synthesized from sodium benzoate by means of a dearomatizing dihydroxylation that proceeds with unusual regioselectivity. Iron diene complexes formed from the arene oxidation product permit the use of otherwise inaccessible transformations. The synthetic material was shown to be antipodal to the natural product, thus determining the absolute configuration of grandifloracin for the first time

Total Synthesis of (+)-Grandifloracin by Iron Complexation of a Microbial Arene Oxidation Product

(+)-Grandifloracin was synthesized from sodium benzoate by means of a dearomatizing dihydroxylation that proceeds with unusual regioselectivity. Iron diene complexes formed from the arene oxidation product permit the use of otherwise inaccessible transformations. The synthetic material was shown to be antipodal to the natural product, thus determining the absolute configuration of grandifloracin for the first time

Radical Ring Opening Polymerization of Cyclic Ketene Acetals Derived From d‑Glucal

A cyclic ketene acetal (CKA) derived from d-glucal was synthesized, and its polymerization using free radicals has been investigated. NMR analysis of the resulting polymers revealed the formation of polyacetal–polyester copolymers, with up to 78% of ester linkages formed by radical ring-opening polymerization (rROP). Conversely, the polymerization of the monomer-saturated analogue only produced acetal linkages, demonstrating that the alkene functionality within the d-glucal pyranose ring is essential to promote ring-opening and ester formation, likely via the stabilization of an allyl radical. The thermal properties of the polymers were linked to the ratio of the ester and acetal linkages. Copolymerization with methyl methacrylate (MMA) afforded statistically PMMA-rich copolymers (66–98%) with linkages prone to hydrolytic degradation and decreased glass-transition temperatures. The retention of the pseudoglucal alkene function offers opportunities to functionalize further these bioderived (co)polymers