Magnesium Boryl Reactivity with 9‐BBN and Ph₃B: Rational B−B′ Bond Formation and Diborane Isomerization

Abstract

Reactions of a magnesium‐based pinacolatoboryl nucleophile with the electrophilic organoboranes, 9‐BBN and Ph3B, provide facile B−B′ single bond formation. Although the Ph3B derivative is thermally stable, when heated, the unsymmetrical diborane(5) anion derived from 9‐BBN is found to isomerize to two regioisomeric species via a proposed mechanism involving dehydroboration of the borabicyclo[3.3.1]nonane and syn‐diboration of the resultant alkenyl carbocycle.

Selective Boryl‐Anion Migration in a Vinyl sp2−sp3 Diborane Induced by Soft Borane Lewis Acids

“…[10] Indeed, transfer of ab oryl nucleophile to an external electrophile is the dominant reactivity pathway reported for B 2 Pin 2 activated by simple carbanions. [10] It is important to extend the chemistry of [(R)B 2 Pin 2 )] À to enable new routes to highly functionalized organoboronates to be discovered. Such routes will be particularly desirable if readily accessible starting materials (e.g.RMgX/B 2 pin 2 )c an be used.…”

“…[23] The addition of B(C 6 F 5 ) 3 (1 equiv) to [2] À (at À78 8 8C) led after 2hto asingle new 11 Bresonance at À3.2 ppm, consistent with an [ROÀB(C 6 F 5 ) 3 ] À species (in contrast, [alkylÀB(C 6 F 5 ) 3 ] À anions have a 11 Br esonance at ca. [10] The addition of BPh 3 (1 equiv) in THF to [2][(THF) n MgBr] generated in situ (at À78 8 8C) resulted in the formation of the desired product [3] À formed by intramolecular {BPin} À transfer (Scheme 2, bottom). The 19 FNMR spectrum confirmed [RO À B(C 6 F 5 ) 3 ] À formation, with ESIMS analysis supporting the formation of an [RO À B(C 6 F 5 ) 3 ] À species derived from ring opening of one BPin moiety in [2] À .With two additional 11 Bresonances observed at 48.0 and 29.2 ppm, we tentatively assign the product as derived from B(C 6 F 5 ) 3 activation of pinacol bound to the four-coordinate boron atom (Scheme 2, top).…”

“…[1,2] This strategy has become ap owerful transitionmetal-free method to borylate organic substrates and generate desirable organoboronate esters.A lkoxides or Nheterocyclic carbenes (NHCs) are the typical LBs employed in the activation of 1, [1][2][3] with the use of carbanions (R À ) having much less precedent, [4][5][6][7][8][9] despite the ability of carbanions to generate amore nucleophilic {BPin} moiety owing to their greater basicity relative to alkoxides and NHCs.Among the limited examples in this area, recent studies have shown that complex A synthesised from 1 and nBuÀMgL (L = bdiketiminato) transfers aboryl anion to boranes to form new unsymmetrical sp 2 Àsp 3 diboranes (Scheme 1a). [10] Indeed, transfer of ab oryl nucleophile to an external electrophile is the dominant reactivity pathway reported for B 2 Pin 2 activated by simple carbanions. [10] It is important to extend the chemistry of [(R)B 2 Pin 2 )] À to enable new routes to highly functionalized organoboronates to be discovered.…”

“…[24] Theo xo-based reactivity of B(C 6 F 5 ) 3 with [2] À was attributed to the high electrophilicity and oxophilicity of this borane.T herefore,s ofter boron electrophiles were explored, in particular BPh 3 ,s ince this borane reacts with complex A to generate [PinB À BPh 3 ] À with no competitive reactivity at the oxo sites reported (Scheme 1a). [10] The addition of BPh 3 (1 equiv) in THF to [2][(THF) n MgBr] generated in situ (at À78 8 8C) resulted in the formation of the desired product [3] À formed by intramolecular {BPin} À transfer (Scheme 2, bottom). Anion [3] À has diagnostic resonances in the 11 BNMR spectrum (34.7 ppm for the CÀ BPin moieties,a nd À9.5 ppm for [C À BPh 3 ] À )a nd in the 1 HNMR spectrum (broad signal at 0.55 ppm for CH(BPin) 2 ), with the formulation further confirmed by accurate mass spectrometry.P erforming the reaction at À78 8 8Cf or 2hand then room temperature for 18 hr esulted in complete consumption of [2] À to yield [3] À (71 %insitu conversion) as the major product.…”

Magnesium Boryl Reactivity with 9‐BBN and Ph3B: Rational B−B′ Bond Formation and Diborane Isomerization

“…ORTEP representations of a) 9;b )10 (ellipsoids set at 25 % probability).I sopropyl methyl groups and hydrogen atoms except H2 and H40 (9)a re removed for clarity. [18] Selected bond lengths []and angles [8 8]: (9)M g1-N1 2.0556(10), Mg1-N2 2.0488(10), Mg1-O1 2.0719(9), B1-B2 1.7203 (18);N 2-Mg1-N1 94.95(4), B1-O1-Mg19 3.15-(7), O1-B1-B2 116.68(10);(10)M g1-O1 2.059(3), Mg-N1 2.043(4), Mg1-N2 2.037(4), Mg1-C36 2.544(5), Mg1-C372 .410(5), B1-B2 1.718(7), C36-B2 1.647(7), C42-B2 1.649(7), C48-B2 1.630(7);N 2-Mg1-N1 96.40 (14), O1-B1-B2 125.2(4), O2-B1-O1 108.1(4), O2-B1-B2 126.6(4), C36-B2-B1 109.3(4), C42-B2-B1 106.6(4), C48-B2-B1 114.4(4). (12)r ing systems;w hereas the initial six-membered rings of 9-BBN are retained in 11,the bicycle has isomerized to acombination of five-and seven-membered rings in 12.In both cases the borohydride anion binds to the magnesium center through acombination of O1ÀMg1 and B2-m 2 -H 2 -Mg1 bridging interactions augmented by af urther close contact with the C À Hb ond of the C36-containing methine unit.…”