Reactions at the Boron-Carbon Double Bond of Methyl(methylidene)boranes

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 [—CA2—BH—CA2—BMe(Hm)2BMe—] (2e; 2:1) or the six‐membered ring [—CA2—BH(Hμ)2BMe—CA2—BH(Hμ)2BMe—] (2f; 1:1) are formed, both containing double hydrogen bridges; the product 2f crystallizes in the space group P1¯. The metallocene trihydrides [Cp2MH3] add to the BC double bond under formation of a double hydrogen bridge to give [Me(A2CH)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—l; 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=CA2 [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=CA2 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′