HETEROCYCLES

Paper | Special issue | Vol. 86, No. 1, 2012, pp. 331-342
Received, 11th May, 2012, Accepted, 15th June, 2012, Published online, 18th July, 2012.
DOI: 10.3987/COM-12-S(N)14

■ Reaction of Grignard Reagents with Diisopropylaminoborane. Synthesis of Alkyl, Aryl, Heteroaryl and Allyl Boronic Acids from Organoc(diisopropyl)aminoborane by a Simple Hydrolysis

Christopher L. Bailey, Chris L. Murphy, Jacob W. Clary, Scott Eagon, Naomi Gould, and Bakthan Singaram*

Department of Chemistry, University of California, 1156 High Street, Santa Cruz, CA 95064, U.S.A.

Abstract

Diisopropylaminoborane (BH2-N(iPr)2) is prepared by reacting lithium diisopropylaminoborohydride (iPr-LAB) with trimethylsilyl chloride (TMSCl). Aliphatic, aromatic, and heteroaromatic (diisopropylamino)boranes are readily synthesized at ambient temperature (0 °C) in 1h by the reaction of Grignard reagents with (BH2-N(iPr)2). Two contending reaction pathways have tentatively been identified. During the mechanistic investigation, bromomagnesium diisopropylaminoborohydride was identified as a byproduct. This borylation reaction can be carried out under Barbier conditions, where (BH2-N(iPr)2) traps the in situ formed Grignard reagent from the corresponding organic halide and magnesium metal. Simple acid hydrolysis of the product organo(diisopropylamino)borane leads to the corresponding boronic acid in good to excellent yield.

1.　INTRODUCTION
In recent years interest in the chemistry of aminoboranes has increased dramatically mostly due to their potential application in hydrogen storage.1-4 Aminoboranes (R1R2N–BH2) are well-known in material science as precursors of BN-based ceramics.5,6 The methods to synthesize aminoboranes include thermally induced dehydrogenation of secondary amine-borane adducts (R1R2HN:BH3) and metal-catalyzed dehydrogenation of the corresponding amine-borane adducts.7-10 Aminoboranes can also be synthesized by the reduction of the corresponding (amino)dihaloboranes.11 Unfortunately, aminoboranes are known to form mixtures of dimers and oligomers, which prevented further purification.12 Consequently, they have been scarcely studied as useful tools for synthetic organic chemistry. It was recently shown that the monomeric dialkylaminoborane, diisopropylaminoborane (BH2-N(iPr)2) 1, can be used as an inexpensive boron source in palladium catalyzed Alcaraz-Vaultier borylation of aryl halides.13 The BH2-N(iPr)2 was prepared through thermal decomposition of the corresponding amine-borane at high temperatures with the concomitant generation of hydrogen gas.
Recently, we developed a mild and practical synthesis of various monomeric and dimeric aminoboranes from the corresponding lithium aminoborohydride (LAB) reagents and trimethylsilyl chloride (Scheme 1).14 LAB reagents are prepared by deprotonating an amine-borane complex, H3B:NHR2, with nBuLi.15 We reported that BH2-N(iPr)2 prepared from the corresponding LAB reagent could also be used in the Alcaraz-Vaultier borylation (Scheme 1). The aryl(diisopropylamino)borane adduct 4 was not isolated but rather converted to the corresponding boronic acid upon acid hydrolysis.14

We also reported that BH2-N(iPr)2 is effective as a boron source for palladium catalyzed cross coupling with aryl iodides, bromides, and triflates.16 Additionally, we found that BH2-N(iPr)2 reduces nitriles at room temperature in the presence of catalytic amount of LiBH4.14,17
owever, very little is known about the reaction of aminoboranes with organometallic reagents, such as organolithium and Grignard reagents. Several years ago, Vaultier reported that Grignard reagents react with chlorobis(diethylamino)boranes to afford a mixtures of monoalkylbis(dialkylamino)boranes and dialkyl(diethylamino)boranes.18 To the best of our knowledge there are no studies describing the reaction of organometallic compounds with BH2-N(iPr)2. Consequently, we investigated the reaction of BH2-N(iPr)2 with various organometallic reagents, such as butyl lithium, phenyllithium, phenylzinc bromide, and phenylmagnesium bromide. Herein, we report the results of our study on the reactions of various organometallic reagents with BH2-N(iPr)2.

2. REULTS AND DISCCUSION
Diisopropylaminoborane BH2-N(iPr)2 1 was easily prepared by the reaction of the corresponding lithium
diisopropylaminoborohydride (LAB) reagent and trimethylsilyl chloride.14 This reaction was quantitative as confirmed by 11B NMR analysis, (δ +35, t, J = 125 Hz). We were interested in determining the compatibility of 1 with various organometallic reagents. When nBuLi was reacted with 1 in a THF solution, multiple additions were observed at both 0 and -78 °C according to 11B NMR analysis. When the milder phenylzinc bromide was reacted with 1, no addition was observed. Subsequently, the compatibility of Grignard reagents with 1 was investigated.
BH2-N(iPr)2 1 was reacted with one equivalent of p-tolylmagnesium bromide 2 at 0 °C in THF and the reaction progress was followed by 11B NMR spectroscopy. After 30 minutes, 11B NMR analysis revealed the absence of 1 (δ +35, t, J = 125 Hz) and the appearance of the single addition product aryl(diisopropylamino)borane (δ +38, d, J = 111 Hz) 4 and bromomagnesium aminoborohydride (BrMgBH3N(iPr)2, δ -22, q, J = 88 Hz) 5.21,19 The 11B NMR spectrum also showed a small amount of the initially formed bromomagnesium aryl(diisopropylamino)borohydride adduct (δ -12, t, J = 78 Hz) 3. Similar results were obtained when p-tolylmagnesium chloride was used (Figure 1).

2.1 INVESTIGATION OF REACTION PATHWAY
A plausible mechanism of borylation involves an initial nucleophilic addition of 2 to the boron atom of 1 forming the bromomagnesium aryl(diisopropylamino)borohydride adduct 3. Once 3 is formed it will either disproportionate forming 4 and hydridomagnesium bromide (HMgBr, Pathway A) or it can transfer a hydride to the starting material 1 forming 4 and 5 (Pathway B). Bromomagnesium diisopropylaminoborohydride 5 can also be formed by a simple addition of HMgBr to the starting material 1.
HMgBr and HMgCl are known compounds and could be made essentially quantitatively from the reaction of isopropylmagnesium halide with pinacolborane in THF.20 Unfortunately, HMgBr will disproportionate readily to MgBr2 and MgH2 in THF.21 Fortunately, HMgCl, does not undergo disproportionation in THF. Consequently, we generated HMgCl in situ from isopropylmagnesium chloride and pinacolborane and reacted it with 1 to determine if HMgCl was capable of transferring a hydride to form chloromagnesium diisopropylaminoborohydride, an analog of 5. Analysis of the reaction mixture by 11B NMR spectroscopy showed the absence of any signals due to aminoborohydride. Due to an expected difference in reactivity between HMgCl and HMgBr this experiment is not conclusive, it does however, point out that the observed bromomagnesium diisopropylaminoborohydride may have resulted from a hydride transfer from intermediate 3 to 1 (Scheme 2).

Interestingly, it was found that only 1.2 equivalents of 1 were required for greater than 95% conversion to the aryl(diisopropylamino)borane. This result strongly indicated that the reaction pathway did not exclusively proceed through Pathway B as two equivalents of BH2-N(iPr)2 would be required to account for the quantitative conversion. Although the reaction mechanism is not fully understood we suggest that the reaction proceeds concomitantly through Pathways A and B.

2.2. SYNTHESIS OF CHLOROMAGNESIUM DIMETHYLAMINOBOROHYDRIDE
In order to verify the chemical shift and coupling constant of 5, we attempted to synthesize authentic halo-magnesium aminoborohydride from diisopropylamine-borane and methylmagnesium chloride in THF. 11B NMR analysis of the reaction mixture showed a small amount of the chloromagnesium aminoborohydride (δ -17, J = 88 Hz) and a number of disproportionation products. When the less sterically demanding dimethylamine-borane was deprotonated with methylmagnesium chloride, chloromagnesium dimethylaminoborohydride (δ -16, J = 88 Hz) was produced quantitatively confirming the identity of 5 (see Supporting Information) (Scheme 3).

2.3. HYDROLYSIS OF ORGANO(DIISOPROPYLAMINO)BORANE
Previously, we have shown that aryl(diisopropylamino)borane compounds can be converted to the corresponding boronic acid via an aqueous quench.20 11B NMR analysis of the water quenched reaction mixture showed the aryl(diisopropylamino)borane adduct and aminoborohydride peaks were replaced by a singlet (δ +30) and a quartet (δ -20, J = 98 Hz) corresponding to the boronic acid and amine-borane complex respectively. Although both the bromomagnesium aminoborohydride and the corresponding amine-borane reagents appear as sharp quartets with virtually identical chemical shifts in their respective 11B NMR spectra, the coupling constants are significantly different.22 The aminoborohydride reagent exhibits 11B NMR J values of between 82-87 Hz. In contrast, the amine-boranes have coupling constants ranging from 95-98 Hz (See Supporting Information).18 The boronic acid was then separated from the amine-borane by an acidic liquid-liquid extraction.

2.4. SYNTHESIS OF BORONIC ACIDS
We were pleased with the finding that organo(diisopropylamino)boranes can be converted to the corresponding boronic acid via an aqueous quench. Boronic acids have wide application in organic synthesis, especially in the formation of C-C bonds through the Suzuki-Miyaura cross coupling reaction.23 The generality of the reaction of Grignard reagents with BH2-N(iPr)2 to generate aryl and alkyl(diisopropylamino)boranes was investigated by using commercially available, representative Grignard reagents.24 The boronic acids, free of magnesium hydride and aminoborohydride, were readily isolated by quenching the reaction mixture with hydrochloric acid (3M) followed by diethyl ether extraction. This methodology was applied to a variety of aryl and alkyl Grignard reagents affording the corresponding boronic acids in essentially quantitative yields (Table 1). With preformed Grignard reagents the reaction was carried out at 0 °C for 1h (entry 1-7). It was interesting to find that phenylmagnesium bromide smoothly converted to the boronic acid in less than 30 min reaction time at -45 °C (entry 11). This result implied that 1h of reaction time was not required at the reaction temperature of 0 °C. However, when the reaction was carried out at -78 °C (entry 12) the reaction mixture would freeze and magnetic stirring would stop. In this case, the reaction flask was removed from the cryogenic conditions, allowed to warm until the magnetic stir bar was free flowing and returned to the -78 °C conditions. Though the reaction was not maintained at a constant temperature the isolated yield of boronic acid was 95%.

Under Barbier-type conditions, a number of aryl halides underwent smooth conversion to the corresponding boronic acids with good yields. In this case the organic halide is added to a mixture of magnesium turnings and BH2-N(iPr)2 1M/THF at 65 °C. The synthesis of boronic acids under the modified Barbier conditions was compatible with a number of arylbromides including 1-bromonapthalene (entry 8), 4-bromoanisol (entry 9) and the heteroaromatic 2-bromothiophene (entry 10). Other substrates, including α-bromostyrene and 9-bromoanthracene were not compatible with the Barbier conditions. When allyl and benzyl halides were subject to the Barbier conditions described above rapid consumption of the magnesium metal was observed. However, subsequent 11B NMR analysis of the reaction mixture showed approximately a 1:1 mixture of the corresponding B-allyl(diisopropylamino)borane and unreacted BH2-N(iPr)2 (entry 13 and 14). What was observed was the rapid consumption of the magnesium metal and the presence of unreacted BH2-N(iPr)2 which can be explained by the homocoupling and reduction of the reactive allyl and benzyl bromides.

3. CONCLUSION
In summary, a simple and mild borylation of aryl and alkyl halides with BH2-N(iPr)2 has been described, under mild Grignard and Barbier conditions. It was found that Grignard reagents add only once to BH2-N(iPr)2. Performing the borylation reaction under Barbier conditions allows the use of a simple one-pot procedure and avoids low temperatures and expensive transition metal catalysts. Diisopropylaminoborane is inexpensively synthesized and is stable in THF for at least one year. The reaction proceeds in excellent yields, affords a single addition product under mild conditions and does not require excess of the boron donor. The organo(diisopropylamino)borane product is hydrolyzed under mild conditions to the corresponding boronic acid. Although the mechanism is not fully understood, we have tentatively identified two possible reaction pathways. In both cases, a hydride acts as a leaving group, either forming HMgBr (Pathway A) or adding to the starting material, BH2-N(iPr)2 (Pathway B). Evidence suggests that the reaction proceeds mainly through Pathway A, as only 1.2 equivalents of aminoborane is required for greater than 95% conversion to the aryl(diisopropylamino)borane. During the mechanistic investigation, bromomagnesium diisopropylaminoborohydride was identified as a byproduct and an analogue was subsequently synthesized from dimethylamine-borane and methylmagnesium chloride.

4. EXPERIMENTAL
General Methods.
All reactions were performed in oven-dried, argon-cooled glassware. The BH2-N(iPr)2 was used as synthesized; it was stored under Ar at room temperature. All Grignard reagents were used as received from Aldrich, they were stored in the bottle received and kept in the refrigerator held at 15 °C. Magnesium metal was used as received from Aldrich. All air- and moisture-sensitive compounds were introduced via syringes or cannula through a rubber septum. Tetrahydrofuran (THF) was freshly obtained from a solvent purification system (Pure Solv MD, Innovative Technology Inc.). NMR spectra were recorded in CDCl3. Chemical shifts are reported relative to TMS (δ = 0) for 1H NMR (500 MHz) and are referred to the CDCl3 resonance (δ = 77) for 13C NMR (125.7 MHz) spectra. Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, p = pentet, br = broad, m = multiplet, app = apparent), coupling constant and integration. Boron NMR samples were recorded at 160.4 MHz and are reported relative to external standard BF3:Et2O (δ = 0).
Synthesis of Bromomagnesium diisopropylaminoborohydride.
Methylmagnesium bromide (0.83 mL, 2.4 M solution in THF, 2.0 mmol) was cooled to 0 °C (ice bath). In a separate 25-mL round-bottom flask diisopropylamine-borane (0.118g, 2.0 mmol) was dissolved in anhydrous THF (2.0 mL). The diisopropylamine-borane/THF solution was added dropwise over 10 min via syringe at 0 °C to the methylmagnesium bromide/THF solution. After 0.5 h of stirring at 0 °C a 0.5 mL aliquot was analyzed via 11B NMR, which showed the solution to be bromomagnesium diisopropylaminoborohydride (δ -22, q, J = 83 Hz).
General Procedure for the Preparation of Arylboronic Acids.
The following procedure for the preparation of p-tolylboronic acid is representative. A 50-mL round-bottom flask equipped with a magnetic stir bar and fitted with rubber septum was charged with BH2-N(iPr)2 (2.4 mL, 2.4 mmol, 1.2 eq.). p-Tolylmagnesium bromide (2 mL, 2 mmol, 1 eq.) was added dropwise over 5 min via syringe while stirring at 0 °C (ice bath). After 1 h, with the reaction still on ice, 3M HCl (5 mL) was added dropwise over 5 min and allowed to stir for 30 min. The reaction mixture was transferred to a separatory funnel and extracted with Et2O (2 x 15 mL). The organic layers were combined and washed with 1M HCl (4 x 15mL), dried over anhydrous MgSO4, and concentrated in vacuo (25 °C, 1 Torr) to afford p-tolylboronic acid as a white powder. For other boronic acids prepared by this method see Table 1.
General Procedure for the Preparation of Arylboronic Acids Under Barbier-type Conditions.
The following procedure for the preparation of 1-napthylboronic acid is representative. A 25-mL round-bottom flask equipped with a condenser and magnetic stir bar was charged with magnesium turnings (0.058 g, 2.4 mmol) and was activated by addition of iodine crystals and warming until iodine sublimed. The flask was cooled to 25 °C and was purged with argon. BH2-N(iPr)2 (2.4 mL, 2.4 mmol) was added to the flask and brought to reflux. 1-Bromonapthalene (1.33 mL, 2.0 mmol) was then added dropwise over five minutes with constant stirring at 65 °C. The reaction was complete after 4 h as evidenced by the disappearance of BH2-N(iPr)2 starting material (δ +35, t, J = 125 Hz), and the appearance of a doublet at (δ 38, d, J = 112 Hz) with the corresponding bromomagnesium aminoborohydride signal (MgBr+ -BH3-NiPr2, δ -28, q, J = 88 Hz). The reaction was then cooled to 25 °C and acidified with 3M HCl (3mL) (CAUTION: hydrogen evolution). After 10 min of stirring the reaction mixture was warmed to 65 °C and stirred for an additional 15 min. The reaction mixture was then transferred to a separatory funnel and extracted with Et2O (3 x 15 mL). The combined organic layers were dried over anhydrous MgSO4, filtered, and concentrated in vacuo (25 °C, 1 Torr) to afford napthylboronic acid as a white solid. The results for the other boronic acids prepared by this method are summarized in Table 1. For copies of the 1H, 13C and 11B NMR spectrum see Supporting Information.
Because of their facile dehydration, boronic acids tend to provide inconsistent melting points. Therefore, the melting points for boronic acids were not taken. (J. Org. Chem., 2011, 76, 3571.)

Phenylboronic acid (Table 1, entries 1, 11, 12);16 White powder; (0.234 g, 95%). 1H NMR (500 MHz, CDCl3): δ 7.52 (t, J = 6 Hz, 2H), 7.61 (t, J = 7 Hz, 1H), 8.26 (d, J = 5.5 Hz, 2H); 13C NMR (125.7 MHz, CDCl3): d 128.0, 132.7, 135.7, carbon attached to boron not observed; 11B NMR (160.4 MHz, CDCl3): d +30.7.
o-Tolylboronic acid (Table 1, entry 2);16 White powder; (0.226 g, 88%). 1H NMR (500 MHz, CDCl3): δ 2.82, 7.27 (m, 2H), 7.459 (dt, J = 1.5, 7 Hz, 1H), 8.22 (dd, J = 7 Hz, 1H); 13C NMR (125.7 MHz, CDCl3): d 23.1, 125.3, 130.7, 132.3, 137.4, 146.4; 11B NMR (160.4 MHz, CDCl3): d +31.9.
p-Tolylboronic acid (Table 1, entry 3);16 White powder; (0.345 g, 95%). 1H NMR (500 MHz, CDCl3): δ 2.44 (s, 3H), 7.32 (d, J = 7.5 Hz, 1H), 8.13 (d, J = 7.5 Hz, 1H); 13C NMR (125.7 MHz, CDCl3): d 21.9, 128.9, 133.7, 135.9, 143.1, carbon attached to boron not observed; 11B NMR (160.4 MHz, CDCl3): d +30.4.
t-Butylboronic acid (Table 1, entry 4);25 White powder; (0.481g, 94%). 13C NMR (125.7 MHz, CDCl3): d 27.8, carbon attached to boron not observed; 11B NMR (160.4 MHz, CDCl3): d +32.7.
Cyclohexylboronic acid (Table1, entry 5);26 White powder; (0.497g, 97%). 13C NMR (125.7 MHz, CDCl3): d 27.3, 27.5, 28.3, carbon attached to boron not observed; 11B NMR (160.4 MHz, CDCl3): d +33.5.
n-Hexylboronic acid (Table 1, entry 6);27 White powder; (0.404g, 95%). 13C NMR (125.7 MHz, CDCl3): d 14.7, 23.4, 24.2, 25.0, 32.5, 32.8; 11B NMR (160.4 MHz, CDCl3): d +34.4.
n-Decylboronic acid (Table 1, entry 7);28 White powder; (0.888g, 95%). 13C NMR (125.7 MHz, CDCl3): d 14.1, 22.7, 23.4, 24.4, 29.4, 29.5, 29.7, 31.9, 32.4, carbon attached to boron not observed; 11B NMR (160.4 MHz, CDCl3): d +33.9.
1-Napthylboronic acid (Table 1, entry 8);16 White powder; (0.253g, 79%). 1H NMR (500 MHz, DMSO-d6): δ 3.44 (brs, 1H), 7.50 (m, 3H), 7.78 (d, J = 5 Hz, 1H), 7.91 (t, J = 9.5 Hz, 2H), 8.36 (brs, OH), 8.42 (dd, J = 8 Hz, 1 Hz, 1H); 13C NMR (125.7 MHz, DMSO-d6): d 128.2, 128.8, 129.1, 132.0, 132.9, 135.7; 11B NMR (160.4 MHz, DMSO-d6): d +30.2.
4-Methoxyphenylboronic acid (Table 1, entry 9);16 White powder; (0.196g, 67%). 1H NMR (500 MHz, CDCl3): δ 3.89 (s, 3H), 7.03 (d, J = 8.5 Hz, 2H), 8.17 (d, J = 8.5 Hz, 2H); 13C NMR (125.7 MHz, CDCl3): d 55.3, 113.7, 137.7, 163.4, carbon attached to boron not observed; 11B NMR (160.4 MHz, CDCl3): d +29.1.
2-Thiopheneboronic acid (Table 1, entry 10);16 White powder; (0.221g, 74%). 1H NMR (500 MHz, CDCl3): δ 7.33 (dd, J = 3.5, 4.5 Hz, 1H), 7.83 (d, J = 4.5 Hz, 1H), 8.06 (d, J = 4 Hz, 1H); 13C NMR (125.7 MHz, CDCl3): d 128.9, 135.1, 139.8, carbon attached to boron not observed; 11B NMR (160.4 MHz, CDCl3): d +27.0.
Allylboronic acid (Table 1, entry 13);29 11B NMR (160.4 MHz, CDCl3): d +42.1 (d, J = 147 Hz), +36.6 (t, J = 129 Hz).
1-Boronic acid ethylbenzene (Table 1, entry 14);30 11B NMR (160.4 MHz, CDCl3): d +42.1 (d, J = 147 Hz, 1H), +36.6 (t, J = 123 Hz, 2H).

ACKNOWLEDGEMENTS
Dedicated to Professor Ei-ichi Negishi on occasion of his 77th birthday.

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