2,6-bis(diisopropylphosphinylmethyl)-pyridine borane complex

In a 0.5 L round 3-necked flask, a solution of diisopropylphosphine-borane complex 11.2 g (84.5 mmol; 2.4 eq.) in anhydrous THF (0.2 L) was cooled to 0 ^oC on ice bath. 39.0 mL of 2 M nBuLi in cyclohexane [Aldrich] (78 mmol; 2.2 eq.) was added in two portions dropwise over a 30 min period with vigorous stirring. The cooling bath was then removed and the solution was stirred at ambient temperature for 1 hour, then cooled back to 0 ^oC on ice bath. Under flow of Ar, the side-neck of the flask was opened and solid 2,6-bis(chloromethyl)-pyridine 6.20 g [TCI] (35.2 mmol) was rapidly added in one portion. The mixture was stirred overnight (14 hours) under Ar without replenishing ice in the bath, to room temperature. (By HPLC analysis, the reaction was complete within 7 hours at room temp). The reaction was quenched by addition of D.I. water 0.2 L and toluene 100 mL, the organic phase was separated and the aqueous phase was extracted with toluene 2 x 150 mL. The combined organic phases were dried by gradual addition of magnesium sulfate, filtered and concentrated on rotovap. The oily residue (19 g) was dissolved in toluene 80 mL and slowly re-evaporated from ambient bath. The resulting solid crystalline residue was suspended in hexane (30 mL), the product was collected by filtration, rinsed with additional small portions of hexane (3 x 20 mL) and dried by suction. A second small fraction of pure product crystallized from the concentrated hexane supernatants. The combined white solids were re-dried on highvac overnight, to remove remaining trace of iPr2PH.BH3 (which is volatile at 0.05 Torr). The yield was 12.26 g of a white crystalline solid (95% of theory).

¹H(C₆D₆, 600MHz): 6.974(m, 2H), 6.953(m, 3H), 2.904(d, 11.4 Hz, 4H), 1.797(m, 4H), 1.5-0.7(very br q, 3H), 0.988(m, 12H); ³¹P(C₆D₆, 243 MHz): 37.13(d, 68 Hz, 2P), ¹¹B(C₆D₆, 193 MHz): -43.64(m, 2B); ¹³C(C₆D₆, 150MHz): 154.71(d, 4Hz, 2C), 136.57(s, 1C), 123.18(d, 4.6 Hz, 2C), 30.75(d, 25.5 Hz, 2C), 22.03(d, 30.8 Hz, 2C), 17.05(d, 9.3Hz, 4C)

2-(di-tert-butylphosphinomethyl)-6-phenyl pyridine borane complex

A solution of 2-methyl-6-phenyl pyridine 6.48 g (38.3 mmol) in anhydrous THF 250 mL under Ar was cooled to -78 ^oC and a solution of tert-BuLi 1.9 M in pentane 20 mL (38.0 mmol) [TCI] was added dropwise from a glass gas-tight Hamilton syringe with Teflon tipped plunger, with vigorous stirring over a 15 min period. The reaction mixture turned dark purple. The mixture was stirred at -78 ^oC for 2 hours. Neat t-Bu₂PCl 96% 7.55g (40.1 mmol) [Aldrich] was added in one portion and the mixture was stirred at -78 ^oC for additional 2 hours. The dry ice/acetone cooling bath was then allowed to expire and attain room temperature (over a half-day period). The reaction mixture was finally stirred at 20 ^oC for extra 14 hours. Neat borane-dimethylsulfide complex 90% 4.00 g (47 mmol) [TCI] was then added in one portion and the reaction mixture was stirred at room temperature for 30 min. ³¹P-NMR analysis of a sample from the reaction mixture (diluted with C₆D₆) indicated complete borane-phosphine complex formation. The reaction mixture was then carefully quenched by gradual addition of water 100 mL (gas evolution!), the mixture was stirred for 30 min to complete the excess borane-DMS decomposition, and saturated NaCl aq. solution 50 mL was added in the end. The organic phase was separated and the aqueous phase was repeatedly extracted with toluene, 3 x 150mL. The combined organic phases were dried (MgSO₄, 1 hour stirring), filtered and concentrated from ambient water bath down to about 50 mL volume. The concentrated solution was filtered through a sintered glass Buchner funnel to remove a small amount of insoluble materials. The filtrates were concentrated down, to obtain 14.5g of yellow oil. The crude product was dissolved in hexane 80 mL, seeded by scratching, and allowed to crystallize for 1 hour, the precipitate (8.1 g) was collected by filtration, rinsed with hexane and dried by suction and on highvac. A second crop (2 g) crystallized by concentrating the supernatants. The supernatants obtained after the second crop were evaporated, the residue was re-evaporated from benzene 10 mL. The resulting oil (3 g) was dissolved in pentane 10 mL, seeded and allowed to crystallize in a refrigerator at +4 ^oC overnight, to produce additional 1.0 g of pure crystalline product (washed with pentane). After vacuum drying, the combined yield was 11.125 g of a pale yellow crystalline solid (89% of theory).

¹H (CDCl₃, 600 MHz): 7.986(m, 2H), 7.669(t, 7.8 Hz, 2H), 7.582(d, 7.8Hz, 1H), 7.500(app d, 7.8Hz), 7.455(m, 2H), 7.393(m, 1H), 3.429(d, 12.0 Hz, 2H), 1.302(d, 12.0 Hz, 18H), 0.511(very bq, 106.8 Hz, 3H); ¹³C (CDCl₃, 150 MHz): 156.19, 156.35, 139.36, 136.80, 128.96, 128.81(2C), 126.83(2C), 124.46, 118.50, 32.93(d, 25.2 Hz, 2C), 29.57(d, 23.0Hz), 28.31(6C); DEPT (CDCl₃, 150 MHz): 136.80, 128.96, 128.81(2C), 126.83(2C), 124.46, 118.50, 29.57(d, 23.0Hz, CH₂), 28.31(6C); ³¹P (CDCl₃, 243 MHz): 47.88(app d, 71.7Hz); ¹¹B (CDCl₃, 192.6 MHz): -43.37(m)

¹H (d₆-acetone, 600 MHz): 8.134(m, 2H), 7.770(m, 2H), 7.482(m, 3H), 7.418(m, 1H), 3.502(d, 12.0 Hz, 2H), 1.326(d, 12.0 Hz, 18H), 0.500(very bq, 100.8 Hz, 3H); ¹³C (d₆-acetone, 150 MHz): 156.62, 156.37, 140.00, 137.49, 129.69, 129.47(2C), 127.41(2C), 125.26, 118.70, 33.32(d, 25.0 Hz, 2C), 28.44(6C); a signal of benzylic CH₂ overlaps with the d₆-acetone multiplet; ³¹P (d₆-acetone, 243 MHz): 48.500(q, 60.3 Hz); ¹¹B (d₆-acetone, 192.6 MHz): -43.04(m)