An improved synthesis of B3(NMe& (2), B4(NMe& (l), and B6(NMe& (4) is reported. From each compound two of the four terminal dimethylamino groups are readily replaced by halogen atoms yielding a,o-dihalides B,(NMe2)nX2 (n = 3 , 4 ; X = C1, Br, I). These are used in turn to replace the substituent X by groups like RO, RS, RHN, R2P, and R. Crystal struc-ture determinations on several of these compounds reveal as a common feature planar (C2N)BB2 and (C,N)B(B)X groups which are arranged in an approximately orthogonal manner to one another with NBBN dihedral angles ranging from 69 to 104.6'. Consequently, there is no significant 7c bonding between the boron atoms. NMez NMez m e , Me,N I l l \ B / B \ B / B , B / B \ I l l me, We, m e , me,While compounds 1 to 3 can be separated by fractional distillation in vacuo the hexaborane 4 can only be isolated by short-path distillation as a very viscous brown oil. The components of the dark brown, glassy residue which is left after fractional distillation have not yet been identified.It is imperious that distillation of the (dimethy1amino)polyboranes is carried out at the lowest possible temperature because the compounds 1, 2, and 4 decompose on heating by releasing B(NMe2)3 as the most volatile compound.
Several transamination reactions of B2(NMe2)4 (1a) with secondary amines have led to mixed tetraaminodiborane(4) compounds B2(NMe2)4 ‐ n(NR2)n (2–4), and B2(NC5H10)4 (1d) has been characterized by an X‐ray structure analysis which reveals the presence of a rather long B–B bond (1.75 Å). However, tetraaminodiboranes(4) of type R2N(Me2N)B‐B(NMe2)NR2 are more readily accessible from LiNR2 and B2(NMe2)2Cl2. Similarly, amination of B2(NMe2)2Cl2 with N,N'‐dimethylethylenediamine (7) yields B‐[bis(dimethyl‐amino)boryl]‐N,N'‐dimethyl‐1,3,2‐diazaborolidine (8), while reactions with Li(Me)N–CH2–CH2–N(Me)Li (9) lead also to 2,3‐bis(dimethylamino)‐1,4‐dimethyl‐1,4,2,3‐diazadiborinane (10) as the kinetically controlled product. This is further substantiated by the reaction of the B2(NMe2)2Br2 with 9 which gives exclusively the corresponding 1,4,2,3‐diazadiborinane 11. Diborane(4) dihalides B2(NMe2)2X2 (X = Cl, Br) react only in a 1:1 ratio with tmp‐B=N‐CMe3 (13) leading to 14a, b. However, both a 1:1 and a 1:2 methoxyboration of 13 has been observed with B2(OMe)4 with formation of 15 and 16.
Two representatives of the three membered B2N-ring system have been obtained by dehalogenation of diborylamines Me3CN[BHalNR2]2. In solution there is no restricted rotation at the exocyclic BN bond, even at low temperature. In contrast, the X-ray structure analysis of the bis-tetramethylpiperidino derivative 10a reveals alternation of short and long BN bonds; one of the tetramethylpiperidino groups stands nearly coplanar with the B2N-ring system, the other, however, is almost perpendicular to the ring plane. The ring contains a short BB bond (1.612 Å).
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