Reactions of substituted pyridines with electrophilic boranes

The lutidine derivative (2,6-Me(2))(4-Bpin)C(5)H(2)N when combined with B(C(6)F(5))(3) yields a frustrated Lewis pair (FLP) which reacts with H(2) to give the salt [(2,6-Me(2))(4-Bpin)C(5)H(2)NH][HB(C(6)F(5))(3)] (1). Similarly 2,2'-(C(5)H(2)(4,6-Me(2))N)(2) and (4,4'-(C(5)H(2)(4,6-Me(2))N)(2) were also combined with B(C(6)F(5))(3) and exposed to H(2) to give [(2,2'-HN(2,6-Me(2))C(5)H(2)C(5)H(2)(4,6-Me(2))N][HB(C(6)F(5))(3)] (2) and [(4,4'-HN(2,6-Me(2))C(5)H(2)C(5)H(2)(2,6-Me(2))N] [HB(C(6)F(5))(3)] (3), respectively. The mono-pyridine-N-oxide 4,4'-N(2,6-Me(2))C(5)H(2)C(5)H(2)(2,6-Me(2))NO formed the adduct (4,4'-N(2,6-Me(2))C(5)H(2)C(5)H(2)(2,6-Me(2))NO)(B(C(6)F(5))(3)) (4) which reacts further with B(C(6)F(5))(3) and H(2) to give [(4,4'-HN(2,6-Me(2))C(5)H(2)C(5)H(2)(2,6-Me(2))NO)B(C(6)F(5))(3)] [HB(C(6)F(5))(3)] (5). In a related sense, 2-amino-6-CF(3)-C(5)H(3)N reacts with B(C(6)F(5))(3) to give (C(5)H(3)(6-CF(3))NH)(2-NH(B(C(6)F(5))(3))) (6). Similarly, the species, 2-amino-quinoline, 8-amino-quinoline and 2-hydroxy-6-methyl-pyridine were reacted with B(C(6)F(5))(3) to give the products as (C(9)H(6)NH)(2-NHB(C(6)F(5))(3)) (7), (C(9)H(6)N)(8-NH(2)B(C(6)F(5))(3)) (8) and (C(5)H(3)(6-Me)NH)(2-OB(C(6)F(5))(3)) (9), respectively; while 2-amino-6-picoline, 2-amino-6-CF(3)-pyridine, 2-amino-quinoline, 8-amino-quinoline and 2-hydroxy-6-methyl-pyridine react with ClB(C(6)F(5))(2) to give the species (C(5)H(3)(6-R)NH)(2-NH(ClB(C(6)F(5))(2))) (R = Me (10), R = CF(3) (11)) (C(9)H(6)NH)(2-NH(ClB(C(6)F(5))(2))) (12), (C(9)H(6)N)(8-NH(2)ClB(C(6)F(5))(2)) (13) and (C(5)H(3)(6-Me)NH)(2-OClB(C(6)F(5))(2)) (14), respectively. In a similar manner, 2-amino-6-picoline and 2-amino-quinoline react with B(C(6)F(5))(2)H to give (C(5)H(3)(6-Me)NH)(2-NH(HB(C(6)F(5))(2))) (15) and (C(9)H(6)NH)(2-NH(HB(C(6)F(5))(2))) (16). The corresponding reaction of 8-amino-quinoline yields (C(9)H(6)N)(8-NHB(C(6)F(5))(2)) (17). In a similar fashion, reaction of 2-amino-6-CF(3)-pyridine resulted in the formation of (18) formulated as (C(5)H(3)(6-CF(3))N)(2-NH(B(C(6)F(5))(2)). Finally, treatment of 15 with iPrMgCl gave (C(9)H(6)N)(2-NH(B(C(6)F(5))(2))) (19). Crystallographic studies of 1, 2, 4, 6, 7, 10, 11, 12 and 15 are reported.

Synthesis and reactivity of alkynyl-linked phosphonium borates

The phosphine tBu(2 PC[triple bond]CH (1) was reacted with B(C(6)F(5)) to give the zwitterionic species tBu(2)P(H)C[triple bond]CB(C(6)F(5))(3) (2). The analogous species tBu(2)P(Me)C[triple bond]CB(C(6)F(5))(3) (3), tBu(2)P(H)C[triple bond]CB(Cl)(C(6)F(5))(2) (4), tBu(2)P(H)C[triple bond]CB(H)(C(6)F(5))(2) (5), and tBu(2)P(Me)C[triple bond]CB(H)(C(6)F(5))(2) (6) were also prepared. The salt [tBu(2)P(H)C[triple bond]CB(C(6)F(5))(2)(THF)][B(C(6)F(5))(4)] (7) was prepared through abstraction of hydride by [Ph(3)C][B(C(6)F(5))(4)]. Species 5 reacted with the imine tBuN=CHPh to give the borane-amine adduct tBu(2)PC[triple bond]CB[tBuN(H)CH(2)Ph](C(6)F(5))(2) (8). The related phosphine Mes(2)PC[triple bond]CH (9; Mes=C(6)H(2)Me(3)) was used to prepare [tBu(3)PH][Mes(2)PC[triple bond]CB(C(6)F(5))(3)] (10) and generate Mes(2)PC[triple bond]CB(C(6)F(5))(2). The adduct Mes(2)PC[triple bond]CB(NCMe)(C(6)F(5))(2) (11) was isolated. Reaction of Mes(2)PC[triple bond]CB(C(6)F(5))(2) with H(2) gave the zwitterionic product (C(6)F(5))(2)(H)BC(H)=C[P(H)Mes(2)][(C(6)F(5))(2)BC[triple bond]CP(H)Mes(2)] (12). Reaction of tBu(2)PC[triple bond]CB(C(6)F(5))(2), a phosphine-borane generated in situ from 5, with 1-hexene gave the species [tBu(2)PC[triple bond]CB(C(6)F(5))(2)](CH(2)CHnBu)[tBu(2)PC[triple bond]CB(C(6)F(5))(2)] (13) and subsequent reaction with methanol or hexene resulted in the formation of [tBu(2)P(H)C[triple bond]CB(C(6)F(5))(2)](CH(2)CHnBu)[tBu(2)PC[triple bond]CB(C(6)F(5))(2)](OMe) (14) or the macrocycle {[tBu(2)PC[triple bond]CB(C(6)F(5))(2)](CH(2)CH(2)nBu)}(2) (15), respectively. In a related fashion, the reaction of 13 with THF afforded the macrocycle [tBu(2)PC[triple bond]CB(C(6)F(5))(2)](CH(2)CHnBu)[tBu(2)PC[triple bond]CB(C(6)F(5))(2)][O(CH(2))(4)] (16), although treatment of tBu(2)PC[triple bond]CB(C(6)F(5))(2) with THF lead to the formation of {[tBu(2)[triple bond]CB(C(6)F(5))(2)][O(CH(2))(4)]}(2) (17). In a related example, the reaction of Mes(2)PC[triple bond]CB(C(6)F(5))(2) with PhC[triple bond]CH gave {[Mes(2)PC[triple bond]CB(C(6)F(5))(2)](CH[triple bond]CPh)}(2) (18). Compound 5 reacted with AlX(3) (X=Cl, Br) to give addition to the alkynyl unit, affording (C(6)F(5))(2)BC(H)=C[P(H)tBu(2)](AlX(3)) (X=Cl 19, Br 20). In a similar fashion, 5 reacted with [Zn(C(6)F(5))(2)]⋅C(7)H(8), [Al(C(6)F(5))(3)]⋅C(7)H(8), or HB(C(6)F(5))(2) to give (C(6)F(5))(3)BC(H)=C[P(H)tBu(2)][Zn(C(6)F(5))] (21), (C(6)F(5))(3)BC(H)=C[P(H)tBu(2)][Al(C(6)F(5))(2)] (22), or [(C(6)F(5))(2)B](2)HC=CH[P(H)tBu(2)] (23), respectively. The implications of this reactivity are discussed.

Organometallic frustrated Lewis pair chemistry

Frustrated Lewis pairs are playing an increasingly important role in organometallic chemistry. Examples are presented and discussed where organometallic systems themselves serve as the Lewis base or Lewis acid components in frustrated Lewis pair chemistry, mostly through their attached functional groups. Activation of dihydrogen takes place easily in many of these systems. This may lead to the generation of novel catalyst systems but also in many cases to the occurrence of specific reactions at the periphery of the organometallic frameworks. Increasingly, FLP reactions are used to carry out functional group conversions in organometallic systems under mild reaction conditions. The limits of typical FLP reactivity are explored with selected organometallic examples, a discussion that points toward new developments, such as the discovery of facile new 1,1-carboboration reactions. Learning more and more about the broad spectrum of frustrated Lewis pair chemistry helps us to find novel reactions and applications.