The construction of BB bonds has recently become a major goal in synthetic chemistry owing to the intense interest in diboranes(4) as reagents for organic chemistry. Historically, synthetic methods suitable for this transformation are rare and most come with significant disadvantages. This article summarizes recent efforts by our group and others to provide new, mild, functional‐group‐tolerant, and convenient synthetic methods for the formation of electron‐precise BB single bonds.

Alkylidenecyclopropanes containing a chelation assistant at the terminal carbon atom of the olefinic moiety undergo an Os- or Ru-promoted ring expansion reaction to afford metal cyclobutylidene derivatives. The process occurs through a novel mechanism that implies a 1,2-migration of a CH(2) group of the three-membered ring from an olefinic carbon atom to the other one. It takes place, without direct participation of the metal, on a metallaheterocyclopentene intermediate which is generated from an eta(2)-methylenecyclopropane species stabilized by coordination of the chelation assistant.

The reactions of the bis(solvento) complexes [OsCp(MeCN) 2 (P i Pr 3 )]PF 6 (1; Cp = cyclopentadienyl) and [OsTp(κ 1 -OCMe 2 ) 2 (P i Pr 3 )]BF 4 (1a; Tp = hydridotris(pyrazolyl)borate) with allenes have been investigated. Complex 1 reacts with 1-methyl-1-(trimethylsilyl)allene and 1,1-dimethylallene to give the π-allene derivatives [OsCp(η 2 -CH 2 dCdCRMe)(MeCN)(P i Pr 3 )]PF 6 (R = SiMe 3 (2), Me (3)). In fluorobenzene at 80 °C, complexes 2 and 3 are moderately stable and evolve into the isopropenyldiisopropylphosphine derivative [OsCp{κ 3 P,C,C-P i Pr 2 [C(Me)dCH 2 ]}(MeCN)]PF 6 (4) by hydrogen transfer from an isopropyl substituent of the phosphine to the coordinated double bond of the allene. Under an ethylene atmosphere the acetonitrile ligand of 4 is displaced by the olefin. The resulting π-ethylene derivative [OsCp(η 2 -CH 2 dCH 2 ){κ 3 P,C,C-P i Pr 2 [C(Me)dCH 2 ]}]PF 6 ( 5) is obtained through a one-pot synthesis procedure by the stirring of 3 in fluorobenzene at 80 °C under 2 atm of ethylene. Treatment of 2 and 3 with dimethyl acetylenedicarboxylate gives [OsCp{η 2 -CH(CO 2 Me)dCH(CO 2 Me)}{κ 3 P,C,C-P i Pr 2 [C(Me)dCH 2 ]PF 6 (6). The reaction of 1a with 1,1dimethylallene leads to [OsTp(η 2 -CH 2 dCdCMe 2 )(κ 1 -OCMe 2 )(P i Pr 3 )]BF 4 (7). In contrast to its Cp counterpart, complex 7 evolves into the hydride-alkenylcarbyne derivative [OsHTp(tCCHdCMe 2 )-(P i Pr 3 )]BF 4 (8), by means of a double migration of the hydrogen atoms of the terminal CH 2 group of the allene. One of them migrates to the central carbon atom of the allene, and the other one goes to the metal center. The alkenylcarbyne group of 8 is selectively deprotonated in the presence of the hydride ligand to afford the hydride-alkenylvinylidene OsHTp{dCdCHC(Me)dCH 2 }(P i Pr 3 ) (9). The X-ray structures of 2, 5, 8, and 9 are also reported.

A series of iridium dihydroborate complexes [(tBuPOCOP)­IrH­(κ2-H2BHR)] (tBuPOCOP = κ3-C6H3-1,3-[OPtBu2]2; R = Mes = 2,4,6-Me3C6H2; R = Dur = 2,3,5,6-Me4C6H) and [LIrH­(κ2-H2BHDur)] (L = tBuPCP = κ3-C6H3-1,3-[CH2PtBu2]2, L = η5-C5Me5) and an osmium dihydroborate compound [OsH­(κ2-H2BHDur)­(CO)­(PiPr3)2] have been prepared by using two different synthetic strategies. The first approach is based on direct borane coordination to the metal center, whereas the second is based on a salt-elimination protocol using the lithium salts Li­[H3BR] (R = Mes or Dur) and the corresponding metal halides. The compounds have been characterized by multinuclear NMR and IR spectroscopy and X-ray diffraction analysis. The results constitute the first syntheses of κ2-σ:σ-dihydroborate complexes featuring bulky aryl groups.

Complex [OsTp(κ 1 -OCMe 2 ) 2 (P i Pr 3 )]BF 4 (1; Tp = hydridotris(pyrazolyl)borate) reacts with 2vinylpyridine to give the olefin derivative [(3). The rearrangement takes place via the intermediates À OsTp{CH 2 CH(OMe)-C 5 H 4 À N}(P i Pr 3 ) (4) and À OsTp(CHCH-C 5 H 4 À N)(P i Pr 3 ) (5), which have been isolated and characterized. The ruthenium complex [RuTp(κ 1 -OCMe 2 ) 2 (P i Pr 3 )]BF 4 (6) reacts with 2-vinylpyridine, in a similar manner to 1, to give [ À RuTp(η 2 -CH 2 dCH-C 5 H 4 À N)(P i Pr 3 )]BF 4 (7). Like its osmium counterpart, complex 7 is transformed in an alkylidene species [ À RuTp(dCHCH 2 -C 5 H 4 À N)(P i Pr 3 )]BF 4 (10), via the ruthenium analogue to 4 and 5, À RuTp{CH 2 CH(OMe)-C 5 H 4 À N}(P i Pr 3 ) (8), and À RuTp{CHCH-C 5 H 4 À N}(P i Pr 3 ) (9). In contrast to 1 and 6, the cyclopentadienyl derivative [OsCp(CH 3 CN) 2 (P i Pr 3 )]PF 6 (11) reacts with 2vinylpyridine to give the hydride-3-osmaindolizine derivative [ À OsHCp(CHCH-C 5 H 4 À N)(P i Pr 3 )]PF 6 (12), by C-H bond activation of the CH 2 group of the substituent of the heterocycle. Treatment of 12 with NaOMe in THF gives À OsCp(CHCH-C 5 H 4 À N)(P i Pr 3 ) (13), which can also be prepared by reaction of OsCpCl(P i Pr 3 ) 2 (14) with 2-vinylpyridine. Protonation of 13 initially gives the alkylidene cation [ À OsCp(dCHCH 2 -C 5 H 4 À N)(P i Pr 3 )] þ (15), which isomerizes into the olefin species [ À OsCp(η 2 -CH 2 dCH-C 5 H 4 À N)(P i Pr 3 )] þ (16). The latter is transformed into 12 via the hydride-3osmaindolizine intermediate 17 containing the hydride ligand transoid disposed to the nitrogen atom.