Ethyl‐substituierte 2,5‐Dihydro‐1,2,5‐oxadiborole

Abstract

Ethyl-substituted 2,5-Dihydro-1 ,2,5-oxadiboro1esl1~Four N-Lewis base-(1,2,5-oxadiboroles) rac-5(Do) [Do = ammonia (NH,), quinuclidine (a), pyridine (Py), y-picoline (yPic)] of k(Et)OB(Et)C(Et)=e(Et) (5, identical with 11) are prepared by reaction of cis-EtzBC(Et)=C(Et)BEt2 (A) with KNHz via the tetraethyl-substituted (m) heterocycles 1-4. k(Et)N(SiMe,)B(Et)C(Et)=C(Et) (4) reacts with water with elimination of Me,SiOH to give rac-5(NH3). The latter undergoes exchange reactions with various N-bases (Py, yPic, Q) quantitatively forming the 1: 1 addition compounds racS(Do) [Do = yPic (X-ray structure analysis)]. rac-5(NH3) reacts with Ph3B to yield H3N-BPh3 and the unstable compound 5, which dimerises t' o (5)* and/or reacts with (EtBO) elimination to afford the boroxin (EtBO), and, presumably,to EtPbMe, and presumably 5 or to the rearranged heterocycle k(Et)OB(Et)C(Et,)k(Et, PbMe,) (?).Tetraorgano-substituierte 2,5-Dihydro-1,2,5-0xadiborole sind cyclische Vinyldiboroxane, deren Verwendung als Reagenzien zur 1,2-Vinyloborierung von Carbonyl-Verbindungen und als q4-(2e-Fiinfring)-Liganden von Ubergangsmetallfragmenten interessant ist. Wir wollten feststellen, ob und wie (-)-Heterocyclen hergestellt und die aufgrund von Literat~rangaben".~] vermutlich instabilen Fiinfringe in situ aus geeigneten Depotverbindungen freigesetzt werden konnen. 2,s-Dihydro-1,2,5-thiadiborole sind demgegeniiber stabile Verbindungen[2bl. 2,5-Dihydro-l,2,5-oxadiborole mit weiteren elektronenspendenden Substituenten wie z.B. rnit exocyclischen OR-oder NR2-Resten an den Roratomen haben wir nicht untersucht, da derartige Verbindungen eine vergleichsweise geringe Reaktivitat haben durften.Uber Lewisbase-freie 2,5-Dihydro-l,2,5-oxadiborole ist nur vorlaufig berichtet worden, so uber das 2,5-Dichlor-3,4dimethyl-Derivat I[2] sowie die all-Ethyl-Verbindung IIL3]. Da wegen fehlender zusatzlicher Verlautbarungen die ( m ) -C y c l e n ohne Komplexpartner offensichtlich instabil sind, haben wir zunachst versucht, Lewisbase-Addukte der Organo-substituierten 2,5-Dihydro-1,2,5-oxadiborole praparativ zu gewinnen. Wir berichten hier uber Synthesewege zu der perethylierten Verbindung I1 (im folgenden rnit 5 bezeichnet) aus 3,4-Bis(diethylboryl)-cis-3-hexen (A)[4,51 und aus dem Kaliumsalz B, das aus A rnit Kaliumhydroxid nach G1. (a) leicht zuganglich id6].

Chemoselective Boron–Carbon Bond Cleavage by Hydroboration of Borirenes

“…As a result of the four‐coordinate boron atom the C1B2 distance [1.678(2) Å] in 5 is elongated with respect to the C2B1 distance [1.575(2) Å]. This corresponds to the relative bond length disparity observed in the tri‐/tetracoordinate boron atoms in 2,5‐dihydro‐1,2,5‐oxadiboroles [CB borane 1.571(3) Å; CB borate 1.615(3) Å] 15. The B1–N1 distance [1.422(2) Å] lies in the range expected for BN double bonds and is comparable to the BN distance in 1‐[bis(trimethylsilyl)amino]‐2,3‐bis(trimethylsilyl)borirene [1.421(4) Å] 10a.…”

“…This corresponds to the relative bond length disparity observed in the tri-/tetracoordinate boron atoms in 2,5-dihydro-1,2,5-oxadiboroles [C À B borane 1.571(3) ; C À B borate 1.615 (3) ]. [15] The B1-N1 distance [1.422 (2) (2) , 1.87(2) and 2.04(2) ] in lithium-9-borata-bicyclo- (6) were reported by Nçth et al As in 5, the lithium atom in 6 is coordinated by one Et 2 O molecule and its short LiÀH distances were interpreted as a result of agostic interactions. [16] The interaction of H1 with Li1 is slightly weaker [1.985(2) ] than those to H2 or H3, which is also reflected in the longer Li1 À B1 distance [2.505(1) ] relative to the Li1 À B2 distance [2.186 (2) ].…”