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Easy access to nucleophilic boron through diborane to magnesium boryl metathesis

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ABSTRACT

Organoboranes are some of the most synthetically valuable and widely used intermediates in organic and pharmaceutical chemistry. Their synthesis, however, is limited by the behaviour of common boron starting materials as archetypal Lewis acids such that common routes to organoboranes rely on the reactivity of boron as an electrophile. While the realization of convenient sources of nucleophilic boryl anions would open up a wealth of opportunity for the development of new routes to organoboranes, the synthesis of current candidates is generally limited by a need for highly reducing reaction conditions. Here, we report a simple synthesis of a magnesium boryl through the heterolytic activation of the B–B bond of bis(pinacolato)diboron, which is achieved by treatment of an easily generated magnesium diboranate complex with 4-dimethylaminopyridine. The magnesium boryl is shown to act as an unambiguous nucleophile through its reactions with iodomethane, benzophenone and N,N′-di-isopropyl carbodiimide and by density functional theory.

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Single crystal X-ray structures of compounds 9 and 10 and the calculated frontier molecular orbitals of compound 9.Thermal ellipsoid plots of (a) 9 and (b) 10 at 25% probability level; hydrogen atoms and iso-propyl methyl groups are omitted for clarity. Selected bond distances (Å) and angles (°): 9, Mg(1)–O(1) 2.0768(8), Mg(1)–O(3) 1.9461(8), Mg(1)–N(1) 2.0688(9), Mg(1)–N(2) 2.0592(9), B(1)–B(2) 1.7503(18), C(42)–B(2) 1.6114(17), N(2)–Mg(1)–N(1) 97.28(4), O(3)–Mg(1)–O(1) 90.01(3). 10, Mg(1)–O(1) 2.0563(17), Mg(1)–O(3) 1.9376(17), Mg(1)–N(1) 2.0515(19), Mg(1)–N(2) 2.0665(19), B(1)–B(2) 1.722(4), B(2)–B(3) 1.746(4), N(1)–Mg(1)–N(2) 95.21(8), O(3)–Mg(1)–O(1) 90.28(7), B(1)–B(2)–B(3) 100.3(2). (c) Natural bond orbitals and energies of the frontier molecular orbitals of 9.
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f3: Single crystal X-ray structures of compounds 9 and 10 and the calculated frontier molecular orbitals of compound 9.Thermal ellipsoid plots of (a) 9 and (b) 10 at 25% probability level; hydrogen atoms and iso-propyl methyl groups are omitted for clarity. Selected bond distances (Å) and angles (°): 9, Mg(1)–O(1) 2.0768(8), Mg(1)–O(3) 1.9461(8), Mg(1)–N(1) 2.0688(9), Mg(1)–N(2) 2.0592(9), B(1)–B(2) 1.7503(18), C(42)–B(2) 1.6114(17), N(2)–Mg(1)–N(1) 97.28(4), O(3)–Mg(1)–O(1) 90.01(3). 10, Mg(1)–O(1) 2.0563(17), Mg(1)–O(3) 1.9376(17), Mg(1)–N(1) 2.0515(19), Mg(1)–N(2) 2.0665(19), B(1)–B(2) 1.722(4), B(2)–B(3) 1.746(4), N(1)–Mg(1)–N(2) 95.21(8), O(3)–Mg(1)–O(1) 90.28(7), B(1)–B(2)–B(3) 100.3(2). (c) Natural bond orbitals and energies of the frontier molecular orbitals of 9.

Mentions: Treatment of compound 7 with one equivalent of B2pin2 provided a single new species (9) (Fig. 2). Compound 9 displayed two resonances in its 11B NMR spectrum at δ 37.3 and 10.4 p.p.m. indicative of both three-coordinate (sp2) and four-coordinate (sp3) boron, respectively. These data are strongly reminiscent of the chemical shifts reported for the species formed, though not isolated, by the addition of tert-BuLi to B2pin2 in THF solution (δ 39.1, 6.4 p.p.m.)47. Crystallographic analysis confirmed compound 9 to be a magnesium complex of a [pinB-Bpin(n-Bu)]− anion coordinated to Mg(1) through the O(1) and O(3) centres of the two pinacolato boron moieties (Fig. 3a). In common with the comparable distances within compounds such as 5 and 6, the B–B bond of compound 9 [1.7503(18) Å] is only marginally elongated in comparison to that reported for B2pin2 itself determined at both high [295 K; 1.716 Å] and low [120 K, 1.7041(15) Å] temperatures48.


Easy access to nucleophilic boron through diborane to magnesium boryl metathesis
Single crystal X-ray structures of compounds 9 and 10 and the calculated frontier molecular orbitals of compound 9.Thermal ellipsoid plots of (a) 9 and (b) 10 at 25% probability level; hydrogen atoms and iso-propyl methyl groups are omitted for clarity. Selected bond distances (Å) and angles (°): 9, Mg(1)–O(1) 2.0768(8), Mg(1)–O(3) 1.9461(8), Mg(1)–N(1) 2.0688(9), Mg(1)–N(2) 2.0592(9), B(1)–B(2) 1.7503(18), C(42)–B(2) 1.6114(17), N(2)–Mg(1)–N(1) 97.28(4), O(3)–Mg(1)–O(1) 90.01(3). 10, Mg(1)–O(1) 2.0563(17), Mg(1)–O(3) 1.9376(17), Mg(1)–N(1) 2.0515(19), Mg(1)–N(2) 2.0665(19), B(1)–B(2) 1.722(4), B(2)–B(3) 1.746(4), N(1)–Mg(1)–N(2) 95.21(8), O(3)–Mg(1)–O(1) 90.28(7), B(1)–B(2)–B(3) 100.3(2). (c) Natural bond orbitals and energies of the frontier molecular orbitals of 9.
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f3: Single crystal X-ray structures of compounds 9 and 10 and the calculated frontier molecular orbitals of compound 9.Thermal ellipsoid plots of (a) 9 and (b) 10 at 25% probability level; hydrogen atoms and iso-propyl methyl groups are omitted for clarity. Selected bond distances (Å) and angles (°): 9, Mg(1)–O(1) 2.0768(8), Mg(1)–O(3) 1.9461(8), Mg(1)–N(1) 2.0688(9), Mg(1)–N(2) 2.0592(9), B(1)–B(2) 1.7503(18), C(42)–B(2) 1.6114(17), N(2)–Mg(1)–N(1) 97.28(4), O(3)–Mg(1)–O(1) 90.01(3). 10, Mg(1)–O(1) 2.0563(17), Mg(1)–O(3) 1.9376(17), Mg(1)–N(1) 2.0515(19), Mg(1)–N(2) 2.0665(19), B(1)–B(2) 1.722(4), B(2)–B(3) 1.746(4), N(1)–Mg(1)–N(2) 95.21(8), O(3)–Mg(1)–O(1) 90.28(7), B(1)–B(2)–B(3) 100.3(2). (c) Natural bond orbitals and energies of the frontier molecular orbitals of 9.
Mentions: Treatment of compound 7 with one equivalent of B2pin2 provided a single new species (9) (Fig. 2). Compound 9 displayed two resonances in its 11B NMR spectrum at δ 37.3 and 10.4 p.p.m. indicative of both three-coordinate (sp2) and four-coordinate (sp3) boron, respectively. These data are strongly reminiscent of the chemical shifts reported for the species formed, though not isolated, by the addition of tert-BuLi to B2pin2 in THF solution (δ 39.1, 6.4 p.p.m.)47. Crystallographic analysis confirmed compound 9 to be a magnesium complex of a [pinB-Bpin(n-Bu)]− anion coordinated to Mg(1) through the O(1) and O(3) centres of the two pinacolato boron moieties (Fig. 3a). In common with the comparable distances within compounds such as 5 and 6, the B–B bond of compound 9 [1.7503(18) Å] is only marginally elongated in comparison to that reported for B2pin2 itself determined at both high [295 K; 1.716 Å] and low [120 K, 1.7041(15) Å] temperatures48.

View Article: PubMed Central - PubMed

ABSTRACT

Organoboranes are some of the most synthetically valuable and widely used intermediates in organic and pharmaceutical chemistry. Their synthesis, however, is limited by the behaviour of common boron starting materials as archetypal Lewis acids such that common routes to organoboranes rely on the reactivity of boron as an electrophile. While the realization of convenient sources of nucleophilic boryl anions would open up a wealth of opportunity for the development of new routes to organoboranes, the synthesis of current candidates is generally limited by a need for highly reducing reaction conditions. Here, we report a simple synthesis of a magnesium boryl through the heterolytic activation of the B–B bond of bis(pinacolato)diboron, which is achieved by treatment of an easily generated magnesium diboranate complex with 4-dimethylaminopyridine. The magnesium boryl is shown to act as an unambiguous nucleophile through its reactions with iodomethane, benzophenone and N,N′-di-isopropyl carbodiimide and by density functional theory.

No MeSH data available.