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

Abstract

The addition of Lewis bases L to the methyl(methylidene)-boranes MeB=CA2(A = SiMe3) and MeB=CAA′ (A′ = SiMe2Cl) gives the adducts MeB(L)=CA2 (1a, b; L = trimethylpyridine, PMe3) and MeB(L)=CAA′ (1c, d; L = di- and trimethylpyridine), respectively. Alcohols and amines HX are added to the BC double bond to give boranes MeB(X)-CHA2 (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=CA2 and BH3, the five-membered ring [-CA 2-BH-CA2-BMe(Hm)2BMe-] (2e; 2:1) or the six-membered ring [-CA2-BH(Hμ)2BMe-CA 2-BH(Hμ)2BMe-] (2f; 1:1) are formed, both containing double hydrogen bridges; the product 2f crystallizes in the space group P1̄. The metallocene trihydrides [Cp2MH3] add to the BC double bond under formation of a double hydrogen bridge to give [Me(A 2CH)B(Hμ)2MCp2] (2g, h; M = Nb, Ta). MeB=CA2 can be chloroborated, -stannated, and -phosphated with E-Cl to yield the boryldisilylmethanes MeB(Cl)-CA2-E (2i-1; E = EtClB, tBuClB, Me2ClSn, Cl2P). The alkyloboration and -alumination with E-R leads to the boryldisilylmethanes MeBR-SiA2-E (2m-o; E-R = Me2B-Me, Et2B-Et, Cl2Al-Et) and the bromination to MeB(Br)-CA2Br. (2+2) Cycloadditions are achieved, when MeB=CA2 is reacted with unsaturated molecules a=b, yielding four-membered rings [-BMe-CA2-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-CA2-Z-CR-] (4e-g; RC≡Z = PhC≡CPh, AC≡CCl, tBuC≡P). 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 MeB≡O, which is identified as boroxene (MeBO)3, the other component is an alkene RR′C=CA 2 [starting from MeB=CA2 and PhCHO, PhC(Me)O] or an alkene RR′C=CAA′ (starting from MeB=CAA′ and PhCHO, tBuCHO) or the methylidene phosphorane Ph3P=CA2 (starting from MeB= CA2 and Ph3PO) or the dicarbadicobaltatetrahedrane [(CA)2{Co(CO)3}2] {starting from MeB=CA2 and [Co2(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-CA2-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=CA2 or MeB=CAA′ give the corresponding (2+3) cycloadducts 5e, f, respectively. All of the products were characterized by their 1H, 11B, and 13C NMR spectra.