Alessandro Bismuto

The reaction of digallane (dpp-bian)Ga−Ga(dpp-bian) (2) (dpp-bian=1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) with allyl chloride (AllCl) proceeded by a two-electron oxidative addition to afford paramagnetic complexes (dpp-bian)Ga(η¹-All)Cl (3) and (dpp-bian)(Cl)Ga−Ga(Cl)(dpp-bian) (4). Treatment of complex 4 with pyridine induced an intramolecular redox process, which resulted in the diamagnetic complex (dpp-bian)Ga(Py)Cl (5). In reaction with allyl bromide, complex 2 gave metal- and ligand-centered addition products (dpp-bian)Ga(η¹-All)Br (6) and (dpp-bian-All)(Br)Ga−Ga(Br)(dpp-bian-All) (7). The reaction of digallane 2 with Ph₃SnNCO afforded (dpp-bian)Ga(SnPh₃)₂ (8) and (dpp-bian)(NCO)Ga−Ga(NCO)(dpp-bian) (9). Treatment of GaCl₃ with (dpp-bian)Na in diethyl ether resulted in the formation of (dpp-bian)GaCl₂ (10). Diorganylgallium derivatives (dpp-bian)GaR₂ (R=Ph, 11; tBu, 14; Me, 15; Bn, 16) and (dpp-bian)Ga(η¹-All)R (R=nBu, 12; Cp, 13) were synthesized from complexes 3, 10, Bn₂GaCl, or tBu₂GaCl by salt metathesis. The salt elimination reaction between (dpp-bian)GaI₂ (17) and tBuLi was accompanied by reduction of both the metal and the dpp-bian ligand, which resulted in digallane 2 as the final product. Similarly, the reaction of complex 10 with MentMgCl (Ment=menthyl) proceeded with reduction of the dpp-bian ligand to give the diamagnetic complex [(dpp-bian)GaCl₂][Mg₂Cl₃(THF)₆] (18). Compounds 11, 12, 13, 15, and 16 were thermally robust, whereas compound 14 decomposed when heated at reflux in toluene to give complex (dpp-bian-tBu)GatBu₂ (19). Both complexes 7 and 19 contain R-substituted dpp-bian ligand: in the former compound the allyl group was attached to the imino-carbon atom, whereas in complex 19, the tBu group was situated on the naphthalene ring. Crystal structures of complexes 3, 8, 9, 10, 13, 14, 18, and 19 were determined by single-crystal X-ray analysis. The presence of dpp-bian radical anions in 3, 6, 8, and 10–16 was determined by ESR spectroscopy.

Gallium complexes: Redox-active catalysts can substantially expand the reactivity of metal complexes, which can enable possible applications into catalysis. They are commonly incorporated into transition-metal complexes; however, this paper reports the oxidative addition of organic substrates to a gallium complex of redox-active 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (see scheme).

The Cover Feature displays possible alkali cation/anionic base pairs across the periodic table. In their Full Paper, F. Matthias Bickelhaupt et al. discuss the results of an extensive exploration and detailed analyses of alkali metal cation affinities (AMCA) of archetypal anionic bases with electrophilic centers across the periodic table. AMCAs are significantly weaker and show a number of different trends if compared to the corresponding proton affinities (PAs). Bonding analyses trace the differences to the more diffuse nature and higher energy of the alkali cation ns LUMO as compared to the proton 1s LUMO. More information can be found in the Full Paper by F. Matthias Bickelhaupt et al. on page 2604 in Issue 19, 2017 (DOI:10.1002/asia.201700956).

The enantiospecific coupling of secondary and tertiary boronic esters to aromatics has been investigated. Using p-lithiated phenylacetylenes and a range of boronic esters coupling has been achieved by the addition of N-bromosuccinimide (NBS). The alkyne functionality of the intermediate boronate complex reacts with NBS triggering the 1,2-migration of the group on boron to carbon giving a dearomatized bromoallene intermediate. At this point elimination and rearomatization occurs with neopentyl boronic esters, giving the coupled products. However, using pinacol boronic esters, the boron moiety migrates to the adjacent carbon resulting in formation of ortho boron-incorporated coupled products. The synthetic utility of the boron incorporated product has been demonstrated by orthogonal transformation of both the alkyne and boronic ester functionalities.

Find your way: The enantiospecific coupling of secondary and tertiary boronic esters to p-arylacetylenes has been achieved by the addition of N-bromosuccinimide. By tuning the steric environment around boron, the coupled product with or without boron can be targeted. The boron containing product is highly versatile as each functional group can be transformed chemoselectively.

Deprotonation of the doubly arylene-bridged diborane(6) derivative 1H₂ with (Me₃Si)₃CLi or (Me₃Si)₂NK gives the B−B σ-bonded species M[1H] in essentially quantitative yields (THF, room temperature; M=Li, K, arylene=4,4′-di-tert-butyl-2,2′-biphenylylene). With nBuLi as the base, the yield of Li[1H] drops to 20 % and the 1,1-bis(9-borafluorenyl)butane Li[2H] is formed as a side product (30 %). In addition to the 1,1-butanediyl fragment, the two boron atoms of Li[2H] are linked by a μ-H bridge. In the closely related molecule Li[3H], the corresponding μ-H atom can be abstracted with (Me₃Si)₃CLi to afford the B−B-bonded conjugated base Li₂[3] (THF, 150 °C; 15 %). Li[1H] and Li[2H] were characterized by NMR spectroscopy and X-ray crystallography.

Fluid identity: A B−B bond was formed through the deprotonation of a doubly arylene-bridged diborane(6) derivative. The reaction shows that organoboranes are not necessarily hydridic and paves the way for new access routes to electron-precise diboranes.

Activation of the sp³ C−F bond in 2-trifluoromethyl-1-alkenes was accomplished through treatment with a Lewis acid. In the presence of an equimolar amount of EtAlCl₂, the (trifluoromethyl)alkenes readily underwent an S_N1′-type reaction with arenes through a Friedel–Crafts-type mechanism via elimination of a fluoride ion to afford 3,3-difluoroallylated arenes in good yields. This selective activation of one C−F bond of the CF₃ group provides a synthetic method for accessing biologically and synthetically important 1,1-difluoro-1-alkenes.

A clean break: C−F bond activation in 2-trifluoromethyl-1-alkenes was accomplished through treatment with a Lewis acid. In the presence of EtAlCl₂, the (trifluoromethyl)alkenes readily undergo elimination of a F⁻ ion and S_N1′-type reaction with arenes through a Friedel–Crafts-type mechanism to give 3,3-difluoroallylated arenes in good yields. This selective activation of just one sp³ C−F bond of the CF₃ group provides facile access to 1,1-difluoro-1-alkenes.

B(C₆F₅)₃ has been found to be an effective catalyst for reduction of pyridines and other electron-deficient N-heteroarenes with hydrosilanes (or hydroboranes) and amines as the reducing reagents. The success of this development hinges upon the realization of a cascade process of dearomative hydrosilylation (or hydroboration) and transfer hydrogenation. The broad functional-group tolerance (e.g. ketone, ester, unactivated olefins, nitro, nitrile, heterocycles, etc.) implies high practical utility.

Reduction cascade: An operationally simple B(C₆F₅)₃-catalyzed pyridine reduction method has been developed. The reaction occurs by a cascade process of dearomative hydrosilylation (or hydroboration) and transfer hydrogenation. The reduction features very broad functional-group tolerance.