Snapshots of magnesium-centred diborane heterolysis by an outer sphere SN2 process

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Abstract

Reactions of the β-diketiminato magnesium diboranate derivative, [(BDI)Mg{pinB(n-Bu)Bpin}] (BDI = HC{(Me)CNDipp} 2; Dipp = 2,6-i-Pr 2C 6H 3), with N,N′-dialkyl and N,N′-diaryl carbodiimides provided the corresponding C-borylated magnesium borylamidinates. This reactivity occurs with the displacement of n-BuBpin and with the apparent addition of a nucleophilic {Bpin} anion to the electrophilic unsaturated carbodiimide carbon centres. In contrast, while analogous reactions of [(BDI)Mg{pinB(n-Bu)Bpin}] with N-alkyl or N-aryl aldimines and ketimines also resulted in facile displacement of n-BuBpin, they provided the organomagnesium products of {Bpin} addition to the imine nitrogen atom rather than the more electrophilic trigonal imine carbon. Computational assessment by density functional theory (DFT) indicated that, although the energetic differences are marginal, the organomagnesium products may be considered as the kinetic outcome of these reactions with respect to the generation of alternative amidomagnesium regioisomers. This latter deduction was borne out by the thermally-induced conversion of two such organomagnesium species to their C-borylated amidomagnesium isomers, both of which occur with negligible entropies of activation indicative of purely intramolecular processes. Detailed analysis by DFT of the reaction of [(BDI)Mg{pinB(n-Bu)Bpin}] with PhNCHPh indicated that B-N bond formation is initiated by attack of the imine nitrogen at the three-coordinate boron atom of the diboranate anion rather than the more crowded magnesium centre. Consistent with an effectively spontaneous reaction, the resultant cleavage of the B-B bond of the diboranate unit is accomplished via the traversal of two very modest barriers of only 8.3 and 6.7 kcal mol -1. This analysis was also supportive of a subsequent intramolecular B-N to B-C isomerisation process. Of greater general significance, however, the addition of the {Bpin} - anion to the reducible aldimine is best rationalised as a consequence of the electrophilic character of this three-coordinate boron centre rather than any intrinsic nucleophilicity associated with the B-B bond of the [pinBB(n-Bu)pin] - anion.

Original languageEnglish
Pages (from-to)6672-6682
Number of pages11
JournalChemical Science
Volume10
Issue number27
Early online date24 May 2019
DOIs
Publication statusPublished - 21 Jul 2019

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Snapshots of magnesium-centred diborane heterolysis by an outer sphere SN2 process. / Pecharman, Anne-Frederique; Hill, Michael S.; McMullon, Grace; McMullin, Claire; Mahon, Mary.

In: Chemical Science, Vol. 10, No. 27, 21.07.2019, p. 6672-6682.

Research output: Contribution to journalArticle

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abstract = "Reactions of the β-diketiminato magnesium diboranate derivative, [(BDI)Mg{pinB(n-Bu)Bpin}] (BDI = HC{(Me)CNDipp} 2; Dipp = 2,6-i-Pr 2C 6H 3), with N,N′-dialkyl and N,N′-diaryl carbodiimides provided the corresponding C-borylated magnesium borylamidinates. This reactivity occurs with the displacement of n-BuBpin and with the apparent addition of a nucleophilic {Bpin} anion to the electrophilic unsaturated carbodiimide carbon centres. In contrast, while analogous reactions of [(BDI)Mg{pinB(n-Bu)Bpin}] with N-alkyl or N-aryl aldimines and ketimines also resulted in facile displacement of n-BuBpin, they provided the organomagnesium products of {Bpin} addition to the imine nitrogen atom rather than the more electrophilic trigonal imine carbon. Computational assessment by density functional theory (DFT) indicated that, although the energetic differences are marginal, the organomagnesium products may be considered as the kinetic outcome of these reactions with respect to the generation of alternative amidomagnesium regioisomers. This latter deduction was borne out by the thermally-induced conversion of two such organomagnesium species to their C-borylated amidomagnesium isomers, both of which occur with negligible entropies of activation indicative of purely intramolecular processes. Detailed analysis by DFT of the reaction of [(BDI)Mg{pinB(n-Bu)Bpin}] with PhNCHPh indicated that B-N bond formation is initiated by attack of the imine nitrogen at the three-coordinate boron atom of the diboranate anion rather than the more crowded magnesium centre. Consistent with an effectively spontaneous reaction, the resultant cleavage of the B-B bond of the diboranate unit is accomplished via the traversal of two very modest barriers of only 8.3 and 6.7 kcal mol -1. This analysis was also supportive of a subsequent intramolecular B-N to B-C isomerisation process. Of greater general significance, however, the addition of the {Bpin} - anion to the reducible aldimine is best rationalised as a consequence of the electrophilic character of this three-coordinate boron centre rather than any intrinsic nucleophilicity associated with the B-B bond of the [pinBB(n-Bu)pin] - anion.",
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T1 - Snapshots of magnesium-centred diborane heterolysis by an outer sphere SN2 process

AU - Pecharman, Anne-Frederique

AU - Hill, Michael S.

AU - McMullon, Grace

AU - McMullin, Claire

AU - Mahon, Mary

PY - 2019/7/21

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N2 - Reactions of the β-diketiminato magnesium diboranate derivative, [(BDI)Mg{pinB(n-Bu)Bpin}] (BDI = HC{(Me)CNDipp} 2; Dipp = 2,6-i-Pr 2C 6H 3), with N,N′-dialkyl and N,N′-diaryl carbodiimides provided the corresponding C-borylated magnesium borylamidinates. This reactivity occurs with the displacement of n-BuBpin and with the apparent addition of a nucleophilic {Bpin} anion to the electrophilic unsaturated carbodiimide carbon centres. In contrast, while analogous reactions of [(BDI)Mg{pinB(n-Bu)Bpin}] with N-alkyl or N-aryl aldimines and ketimines also resulted in facile displacement of n-BuBpin, they provided the organomagnesium products of {Bpin} addition to the imine nitrogen atom rather than the more electrophilic trigonal imine carbon. Computational assessment by density functional theory (DFT) indicated that, although the energetic differences are marginal, the organomagnesium products may be considered as the kinetic outcome of these reactions with respect to the generation of alternative amidomagnesium regioisomers. This latter deduction was borne out by the thermally-induced conversion of two such organomagnesium species to their C-borylated amidomagnesium isomers, both of which occur with negligible entropies of activation indicative of purely intramolecular processes. Detailed analysis by DFT of the reaction of [(BDI)Mg{pinB(n-Bu)Bpin}] with PhNCHPh indicated that B-N bond formation is initiated by attack of the imine nitrogen at the three-coordinate boron atom of the diboranate anion rather than the more crowded magnesium centre. Consistent with an effectively spontaneous reaction, the resultant cleavage of the B-B bond of the diboranate unit is accomplished via the traversal of two very modest barriers of only 8.3 and 6.7 kcal mol -1. This analysis was also supportive of a subsequent intramolecular B-N to B-C isomerisation process. Of greater general significance, however, the addition of the {Bpin} - anion to the reducible aldimine is best rationalised as a consequence of the electrophilic character of this three-coordinate boron centre rather than any intrinsic nucleophilicity associated with the B-B bond of the [pinBB(n-Bu)pin] - anion.

AB - Reactions of the β-diketiminato magnesium diboranate derivative, [(BDI)Mg{pinB(n-Bu)Bpin}] (BDI = HC{(Me)CNDipp} 2; Dipp = 2,6-i-Pr 2C 6H 3), with N,N′-dialkyl and N,N′-diaryl carbodiimides provided the corresponding C-borylated magnesium borylamidinates. This reactivity occurs with the displacement of n-BuBpin and with the apparent addition of a nucleophilic {Bpin} anion to the electrophilic unsaturated carbodiimide carbon centres. In contrast, while analogous reactions of [(BDI)Mg{pinB(n-Bu)Bpin}] with N-alkyl or N-aryl aldimines and ketimines also resulted in facile displacement of n-BuBpin, they provided the organomagnesium products of {Bpin} addition to the imine nitrogen atom rather than the more electrophilic trigonal imine carbon. Computational assessment by density functional theory (DFT) indicated that, although the energetic differences are marginal, the organomagnesium products may be considered as the kinetic outcome of these reactions with respect to the generation of alternative amidomagnesium regioisomers. This latter deduction was borne out by the thermally-induced conversion of two such organomagnesium species to their C-borylated amidomagnesium isomers, both of which occur with negligible entropies of activation indicative of purely intramolecular processes. Detailed analysis by DFT of the reaction of [(BDI)Mg{pinB(n-Bu)Bpin}] with PhNCHPh indicated that B-N bond formation is initiated by attack of the imine nitrogen at the three-coordinate boron atom of the diboranate anion rather than the more crowded magnesium centre. Consistent with an effectively spontaneous reaction, the resultant cleavage of the B-B bond of the diboranate unit is accomplished via the traversal of two very modest barriers of only 8.3 and 6.7 kcal mol -1. This analysis was also supportive of a subsequent intramolecular B-N to B-C isomerisation process. Of greater general significance, however, the addition of the {Bpin} - anion to the reducible aldimine is best rationalised as a consequence of the electrophilic character of this three-coordinate boron centre rather than any intrinsic nucleophilicity associated with the B-B bond of the [pinBB(n-Bu)pin] - anion.

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