Group 13 BN dehydrocoupling reagents, similar to transition metal catalysts but with unique reactivity

Abstract

Reactions of the Al III and Ga III bases Al(N i Pr 2 ) 3 and E(NMe 2 ) 3 (E ¼ Al, Ga) with the amine-boranes [ i Pr 2 NHBH 3 ] and [ t BuNH 2 BH 3 ] give the amino-borane monomer [ i Pr 2 N ¼ BH 2 ] (4) and the borazine [ t BuNBH] 3 (5), respectively. This is similar to the results of dehydrocoupling previously seen with single-site Rh I catalysts and appears to occur via intermediate group 13 hydrides, as shown by the isolation of the amido-alane [H 2 Al(m-N i Pr 2 )] 2 (7) in the formation of 4 from Al(N i Pr 2 ) 3 . In general, the outcome of group 13 dehydrocoupling reactions show a marked dependence on the amine-borane used and on the nucleophilic and redox character of the group 13 pre-catalyst. The importance of these factors is seen in the formation of the unusual, delocalised amino-borane [B{(NHBH)N(SiMe 3 )Si(Me 2 ) N(SiMe 3 ) 2 } 3 ] (10) in the non-catalytic reaction of [Ga{N(SiMe 3 ) 2 } 3 ] with [NH 3 BH 3 ], in which coupling of B-N as well as Si-N bonds occurs along with the deposition of Ga metal.Scheme 1 Formation of 1 and 2 by dehydrocoupling with the presumed catalytic intermediate 3.

A tert-butyl-substituted amino-borane bound to an iridium fragment: A latent source of free H2BNtBuH

“…Intrigued that similar chemistry was happening in our system, as evidenced by the formation of H 3 B$N t BuH 2 , we investigated the likely route of formation of H 3 B$N t BuH 2 . At a first approximation it might be expected that H 2 B]N t BuH would trimerise to give the cyclic cyclotriborazane [H 2 BN t BuH] 3 [25]. This could then potentially react with more H 2 B]N t BuH by a hydrogen redistribution reaction (exemplified in Scheme 5) to form [HBN t Bu] 3 and H 3 B$N t BuH 2 .…”

“…To probe this, ideally the cyclotriborazane [H 2 BN t BuH] 3 would be reacted with H 2 B]N t BuH. Although the crystal structure of this material has recently been reported [25], it is only produced in low yields from the Al(NMe 2 ) 3 catalyzed dehydrogenation of H 3 B$N t BuH 2 or by pyrolysis of H 3 B$N t BuH 2 [21]. As an alternative [37] we instead used the sterically less encumbered methylsubstituted cyclotriborazane [H 2 BNMeH] 3 , which is readily prepared [38] (2)) [17].…”

“…In this we find that, when released by addition of excess MeCN, simple trimerisation of H 2 B]N t BuH to form [H 2 BN t BuH] 3 [25] is not occurring, and the ultimate product of the reactions, the borazine [HBN t Bu] 3 , potentially forms via a different route that involves a hydrogen redistribution reaction [16]. [17] and show that the amine-borane is relatively tightly bound to the Ir centre through two 3 centree2 electron bonds.…”

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…Stoichiometric reactions of amine-boranes with aluminium species have revealed possible reaction intermediates although the mechanism of the dehydrocoupling has not been unambiguously determined [139][140][141][142].…”

Experimental and Theoretical Studies of the Potential Interconversion of the Amine–Borane iPr2NH·BH(C6F5)2 and the Aminoborane iPr2N=B(C6F5)2 Involving Hydrogen Loss and Uptake

“…For example, homogeneous systems based on Rh, [11] Ru, [12,13] Re, [14] Ni [15,16] and Ir [17][18][19] are now known, along with those based on Group 2 and 4 metals, [20][21][22][23][24][25] and systems based on p-block metals. [26] In addition, photoactivated catalysts of the first-row transition metals have also recently been reported. [27][28][29] Mechanistic insights into metal-catalysed dehydrocoupling processes have also been provided through various studies demonstrating the coordination of amine-and aminoboranes to metal centres, [11,[30][31][32][33][34][35][36][37][38][39] with the isolation of species implicated in the catalytic cycle.…”