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(H_μ)₂BMe—CA₂—BH(H_μ)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(H_μ)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′ and AN₃. 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.