Addition of Enamines or Pyrroles and B(C₆F₅)₃ “Frustrated Lewis Pairs” to Alkynes

Reaction of 1-morpholinocyclohexene with B(C 6 F 5 ) 3 and phenylacetylene gave a mixture of two compounds, [C 6 H 10 N(CH 2 CH 2 ) 2 O][PhCCB(C 6 F 5 ) 3 ] ( 1a ) and C 6 H 9 (2-PhC═C(H)B(C 6 F 5 ) 3 )(N(CH 2 CH 2 ) 2 O ( 1b ). The analogous reaction with ethynylferrocene resulted in only the deprotonation product, the alkynyl-borate salt [C 6 H 10 N(CH 2 CH 2 ) 2 O][CpFe(C 5 H 4 )CCB(C 6 F 5 ) 3 ] ( 2 ). The related reactions of pyrrole, phenylacetylene, and B(C 6 F 5 ) 3 led to the vinyl-borate addition product, HNC 4 H 4 (2-PhC═C(H)B(C 6 F 5 ) 3 ) ( 3 ). In a similar fashion, reaction of N -methylpyrrole, phenylacetylene, and B(C 6 F 5 ) 3 gave a 3:2 ratio of the products MeNC 4 H 4 (2-PhC═C(H)B(C 6 F 5 ) 3 ) ( 4a ) and MeNC 4 H 4 (3-PhC═C(H)B(C 6 F 5 ) 3 ) ( 4b ), while the corresponding reaction of N - tert -butylpyrrole with phenylacetylene and B(C 6 F 5 ) 3 provided a single product, t BuNC 4 H 4 (3-PhC═C(H)B(C 6 F 5 ) 3 ) ( 5a ). Variations in the aryl alkynes produced the corresponding addition complexes, t BuNC 4 H 4 (3-ArC═C(H)B(C 6 F 5 ) 3 ) (Ar = p -C 6 H 4 Br 5b, m -C 6 H 4 Cl 5c, p -C 6 H 4 CF 3 5d, CpFe(C 5 H 4 ) 5e ). Similarly MeNC 4 H 2 (2,5-Me 2 )(3-ArC═C(H)B(C 6 F 5 ) 3 ) (Ar = Ph 6a, p -C 6 H 4 Br 6b, m -C 6 H 4 Cl 6c, p -C 6 H 4 CF 3 6d, CpFe(C 5 H 4 ) 6e ) were derived from 1,2,5-trimethylpyrrole. The species 5a and 5b were shown to rearrange to give t BuNC 4 H 3 (3-ArC═C(H)(C 6 F 5 )B(C 6 F 5 ) 2 ) (Ar = Ph 7a, CpFe(C 5 H 4 ) 7b ). The related complexes RNC 4 H 3 (3-PhC═C(H)(C 6 F 5 )B(C 6 F 5 ) 2 ) (R = SiMe 3 8, Ph 9 ) and MeNC 4 H(2,5-Me 2 )(3-PhC═C(H)(C 6 F 5 )B(C 6 F 5 ) 2 ) ( 10 ) were derived directly from the corresponding reactions of the pyrrole, PhCCH, and B(C 6 F 5 ) 3 . In the case of the reaction of N-tert -butylpyrrole, phenylacetylene, and PhB(C 6 F 5 ) 2, phenyl group migration was observed, affording exclusively the species t BuNC 4 H 3 (3-PhC═C(H)(Ph)B(C 6 F 5 ) 2 ) ( 11 ). Reaction of 5a or 4a/4b and t Bu 3 P resulted in deprotonation and formation of the salts [ t Bu 3 PH][RNC 4 H 3 ( X -PhC═C(H)B(C 6 F 5 ) 3 )] (R = t Bu, X = 3 12; R = Me X = 2 13a, 3 13b ). Reaction of 5a − d or 6a − d with one equivalent of Et 3 PO mediated proton transfer to generate a series of vinyl pyrroles t BuNC 4 H 3 (3-ArC═CH 2 ) (Ar = Ph 14a, p -C 6 H 4 Br 14b, m -C 6 H 4 Cl 14c, p- C 6 H 4 CF 3 14d ) and MeNC 4 H 2 (2,5-Me 2 )(3-ArC═CH 3 ) (Ar = Ph 15a, p -C 6 H 4 Br 15b, m -C 6 H 4 Cl 15c, p -C 6 H 4 CF 3 15d ) and the byproduct Et 3 PO·B(C 6 F 5 ) 3 .