Nucleophilic borylation of fluorobenzenes with reduced arylboranes.

Two challenging but rewarding topics in chemical synthesis are C-F-bond activation and the development of B-centered nucleophiles. We have now tackled both subjects simultaneously by forming aryl-B bonds via SNAr-type reactions on fluorobenzenes under mild conditions using Na2[FluBBFlu], Li2[HBFlu], and Li2[Me2DBA] (BFlu = 9-borafluorenyl, Me2DBA = 9,10-dimethyl-9,10-dihydro-9,10-diboraanthracene).

A Chemiluminescent Tetraaryl Diborane(4) Tetraanion.

Two subvalent, redox-active diborane(4) anions, [3]4- and [3]2- , carrying exceptionally high negative charge densities are reported: Reduction of 9-methoxy-9-borafluorene with Li granules without stirring leads to the crystallization of the B(sp3 )-B(sp2 ) diborane(5) anion salt Li[5]. [5]- contains a 2,2'-biphenyldiyl-bridged B-B core, a chelating 2,2'-biphenyldiyl moiety, and a MeO substituent. Reduction of Li[5] with Na metal gives the Na+ salt of the tetraanion [3]4- in which two doubly reduced 9-borafluorenyl fragments are linked via a B-B single bond. Comproportionation of Li[5] and Na4 [3] quantitatively furnishes the diborane(4) dianion salt Na2 [3], the doubly boron-doped congener of 9,9'-bis(fluorenylidene). Under acid catalysis, Na2 [3] undergoes a formal Stone-Wales rearrangement to yield a dibenzo[g,p]chrysene derivative with B=B core. Na2 [3] shows boron-centered nucleophilicity toward n-butyl chloride. Na4 [3] produces bright blue chemiluminescence when exposed to air.

The 9H-9-Borafluorene Dianion: A Surrogate for Elusive Diarylboryl Anion Nucleophiles.

Double reduction of the THF adduct of 9H-9-borafluorene (1⋅THF) with excess alkali metal affords the dianion salts M2 [1] in essentially quantitative yields (M=Li-K). Even though the added charge is stabilized through π delocalization, [1]2- acts as a formal boron nucleophile toward organoboron (1⋅THF) and tetrel halide electrophiles (MeCl, Et3 SiCl, Me3 SnCl) to form B-B/C/Si/Sn bonds. The substrate dependence of open-shell versus closed-shell pathways has been investigated.

A redox-active diborane platform performs C(sp3)-H activation and nucleophilic substitution reactions.

Organoboranes are among the most versatile and widely used reagents in synthetic chemistry. A significant further expansion of their application spectrum would be achievable if boron-containing reactive intermediates capable of inserting into C-H bonds or performing nucleophilic substitution reactions were readily available. However, current progress in the field is still hampered by a lack of universal design concepts and mechanistic understanding. Herein we report that the doubly arylene-bridged diborane(6) 1H2 and its B[double bond, length as m-dash]B-bonded formal deprotonation product Li2[1] can activate the particularly inert C(sp3)-H bonds of added H3CLi and H3CCl, respectively. The first case involves the attack of [H3C]- on a Lewis-acidic boron center, whereas the second case follows a polarity-inverted pathway with nucleophilic attack of the B[double bond, length as m-dash]B double bond on H3CCl. Mechanistic details were elucidated by means of deuterium-labeled reagents, a radical clock, α,ω-dihaloalkane substrates, the experimental identification of key intermediates, and quantum-chemical calculations. It turned out that both systems, H3CLi/1H2 and H3CCl/Li2[1], ultimately funnel into the same reaction pathway, which likely proceeds past a borylene-type intermediate and requires the cooperative interaction of both boron atoms.

Deprotonation of a Seemingly Hydridic Diborane(6) To Build a B-B Bond.

Deprotonation of the doubly arylene-bridged diborane(6) derivative 1H2 with (Me3 Si)3 CLi or (Me3 Si)2 NK 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 (Me3 Si)3 CLi 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.