Reactions at the boron-carbon double bond of methyl(methylidene)boranes

The addition of Lewis bases L to the methyl(methylidene)boranes MeB=CA(2) (A = SiMe3) and MeB=CAA' (A' = SiMe2Cl) gives the adducts MeB(L)=CA(2) (1a, b; L = trimethylpyridine, PMe3) and MeB(L)=CAA' (1c, d; L = di- and trimethylpyridine), respectively. Alcohols and amines FIX are added to the BC double bond to give boranes MeB(X)-CHA(2) (8a-c; X = OiPr, OtBu, NiPr2); MeB=CAA' and HNMe2 react in the ratio 1:2, yielding MeB(X)-CHA(SiMe2X) (2d; X = NMe2). From MeB=CA(2) and BH3, the five-membered ring [-CA(2)-BH-CA(2)-BMe(Hm)(2)BMe-] (2e; 2:1) or the six-membered ring [-CA(2)-BH(H-mu)(2)BMe-CA(2)-BH(H-mu)(2)BMe-] (2f; 1:1) are formed, both containing double hydrogen bridges; the product 2f crystallizes in the space group P (1) over bar. The metallocene trihydrides [Cp2MH3] add to the BC double bond under formation of a double hydrogen bridge to give [Me(A(2)CH)B(H-mu)(2)MCp2] (2g, h; M = Nb, Ta). MeB=CA(2) can be chloroborated, -stannated, and -phosphated with E-Cl to yield the boryldisilylmethanes MeB(Cl)-CA(2)-E (2i-l; E = EtClB, tBuClB, Me2ClSn, Cl2P). The alkyloboration and -alumination with E-R leads to the boryldisilylmethanes MeBR-SiA(2)-E (2m-o; E-R = Me2B-Me, Et2B-Et, Cl2Al-Et) and the bromination to MeB(Br)-CA(2)Br. (2+2) Cycloadditions are achieved, when MeB=CA(2) is reacted with unsaturated molecules a=b, yielding four-membered rings [-BMe-CA(2)-b-a-] [4a-d; a=b = fluorenone, bis(methoxycarbonyl)ethyne (reacting at both of the C=O bonds), phenylisocyanate (reacting at the C=O bond), N-isopropylacetoneimine], or with triple bond systems RC=Z, yielding four-membered rings [-BMe-CA(2)-Z=CR-] (4e-g; RCdropZ = PhCdropCPh, ACdropCCl, tBuCdropP). With a series of six molecules with an element-oxygen double bond, a primary (2+2) cycloaddition is followed by a metathetical splitting of the transient four-membered rings 4h-m. One of the metathesis products is MeBdropO. which is identified as boroxene (MeBO)(3), the other component is an alkene RR'C=CA(2) [starting from MeB=CA(2) and PhCHO, PhC(Me)O] or an alkene RR'C=CAA' (starting from MeB=CAA' and PhCHO, tBuCHO) or the methylidene phosphorane Ph3P=CA(2) (starting from MeB=CA(2) and Ph3PO) or the dicarbadicobaltatetrahedrane [(CA)(2){Co(CO)(3)}(2)] {starting from MeB=CA(2) and [Co-2(CO)(8)]}. The (2+3) cyclodaddition of MeB=CA(2) to the azide X2PN3 (X = NiPr2) as 1,3-dipole gives the five-membered ring [=BMe-CA(2)-N=N-NX=] (5a) and to RN3 the rings [=BMe-CA=N-NA-NR=] (5'b, c; R = iBu, A; formed from the cycloadducts 5b, c by migration of A); analogously, [=BMe-CA'=N-NA-NA=] (5'd) is formed from MeB=CAA' and AN(3). Finally, the nitrone O-NMe=CHPh and MeB=CA(2) or MeB=CAA' give the corresponding (2+3) cycloadducts 5e, f, respectively. All of the products were characterized by their H-1, B-11, and C-13 NMR spectra.