Organosubstituierte 1,2‐Diborylalkene — Herstellung und Photoisomerisierung

Substituted 1,2‐Diborylalkenes — Preparation and Photoisomerization The organo(halo)substituted cis ‐1,2‐diborylalkenes cis 1‐R 2 BC(R)C(Et)BEt 2 (Z)‐ 1 to (Z)‐ 14 are prepared [route I from Na[R 3 BCCR] and ClBR 2 ; route II from cis 1‐R 3 SnC(R)C(Et)BEt 2 and R n BHal 3+ n ′ n 0–2; (Z)‐ 11 (route III): cis 1‐ClB(Ph)CHC(Et)BEt 2 and Me 3 SiOMe]. Their 11 B‐and 13 C‐NMR data show the degree of pz(B)/(CC) π overlap and are compared with the corresponding data of the monoboryl alkenes [(Z)‐ 15, 16 , (Z)‐ 17 , (Z)‐ 23 , (Z)‐ 24 , (Z)‐ 26 ], the cis ‐2‐boryl‐1‐silylalkenes [( E )‐ 18 , ( E )‐ 19 , ( E )‐ 27 , ( E )‐ 28 ], and the cis ‐2‐boryl‐1‐stannylalkenes [( E )‐ 20 to ( E )‐ 22 ]. — Compounds (Z)‐ 3 , (Z)‐ 4 , (Z)‐ 9 , ( E )‐ 18 , and ( E )‐ 19 with one or two nearly coplanar pz(B) and pz(CC) groups ( pl 1‐R 2 B) are isomerized by UV irradiation to their equilibria with the corresponding trans isomers ( E )‐ 3 , ( E )‐ 4 , ( E )‐ 9 , (Z)‐ 18 , and (Z)‐ 19 , resp., with one or two nearly orthogonal pz(B) and pz(CC) groups ( or 1‐R 2 B). Irradiation of (Z)‐ 9 gives the isomer pair ( Z / E )1‐iso‐ 9 by a Ph/Et exchange between the two boron atoms.