Boryl complexes

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

The enantioselective P-borylation of a,P-unsaturated esters with (Bpin) was studied in the presence of various [CuCl(NHC)] or [Cu(MeCN)(NHC)] (NHC = chiral imidazol-2-ylidene or imidazolidin-2-ylidene) complexes. The reaction proceeds by heterolytic cleavage of the B-B bond of the (Bpin), followed by formation of Cu-boryl complexes which insert across the C=C bond of the unsaturated ester. Best yields and ee were observed with complex 144, featuring a non-C2 symmetric NHC ligand (Scheme 2.31) [114]. 

One most important observation for the mechanistic discussion is the oxidative addition/insertion/reductive elimination processes of the iridium complex (31) (Scheme 1-10) [62]. The oxidative addition of catecholborane yields an octahedral iridium-boryl complex (32) which allows the anti-Markovnikov insertion of alkyne into the H-Ir bond giving a l-alkenyliridium(III) intermediate (34). The electron-... 

A catalytic cycle proposed for the metal-phosphine complexes involves the oxidative addition of borane to a low-valent metal yielding a boryl complex (35), the coordination of alkene to the vacant orbital of the metal or by displacing a phosphine ligand (35 —> 36) leads to the insertion of the double bond into the M-H bond (36 —> 37) and finally the reductive elimination to afford a hydroboration product (Scheme 1-11) [1]. A variety of transition metal-boryl complexes have been synthesized via oxidative addition of the B-H bond to low-valent metals to investigate their role in cat-... 

Cross-coupling to form carbon heteroatom bonds occurs by oxidative addition of an organic halide, generation of an aryl- or vinylpalladium amido, alkoxo, tholato, phosphido, silyl, stannyl, germyl, or boryl complex, and reductive elimination (Scheme 2). The relative rates and thermodynamics of the individual steps and the precise structure of the intermediates depend on the substrate and catalyst. A full discussion of the mechanism for each type of substrate and each catalyst is beyond the scope of this review. However, a series of reviews and primary literature has begun to provide information on the overall catalytic process.18,19,22,23,77,186... 

The remarkable stability of iridium-boryl complexes, as a function of the substituents on boron, is most likely responsible for the unique behavior of iridium in metal-promoted B-addition to unsaturated molecules. 

It is no coincidence that the first X-ray structure analysis of a transition metal-boryl complex [8] was made on the iridium complex jac-[IrH2(BC8Hi4)(PMe3)3] (2) [9] derived from the B—H activation of the 9-borabicyclo[3.3.1]-nonane (9-BBN)... 

Figure 4.13 Mechanism of reduction of C02 to CO catalyzed by Cu(l) boryl complex [99],...

Recently, density functional calculations were performed to determine the nature and stereochemistry of the olefin insertion into the Cu-B bond of (NHC)Cu boryl complexes (NHC = iV-heterocyclic carbene). The theoretical calculations confirm that the mechanism of insertion involves a nucleophilic attack of the boryl ligand on the coordinated olefin. Furthermore, the hyperconjugation of Cu-C (bond angles, which was also experimentally confirmed by the X-ray diffraction studies of these boryl-copper complexes <2007OM2824>. 

The significant interest that the prospects of transition metal complexes of boron received over the past few years is not only due to a hitherto unknown type of metal-boron linkage. Boryl complexes in particular became a highly rewarding target due to their potential application for the functionalisation of hydrocarbons. They are well known to be key intermediates in the metal catalysed hydroboration and related reactions,58 65... 

Protic reagents such as primary amines, alcohols, and water led to the corresponding substituted borylene complexes [n-BX (C5H4Me) Mn(CO)2 2] (13, X = NHtBu 14, X = NHPh 15, X = OMe 16, X = OEt 17, X = Oz Pr 18, X = OH) in high yields of up to 94% (Fig. 3).100 102 It should be noted that due to the kinetic lability of the metal-boron bond such reactions at the boron center with retention of the M-B linkage are very rare for both boryl- and borylene complexes and were observed subsequently by Roper only in the case of one particular boryl complex.103... 

Abbreviations a, from monoboranes b, from diboranes(4) c, by borylene transfer d, from boryl complexes e, from borylene complexes x, no X-ray data available y, no "B-NMR data available. 

Roper, however, succeeded in converting the osmium boryl complex [Cl2BOsCl(CO)(PPh3)2] (E) into the terminal borylene complex [Os (=BNHC9H6N)Cl2(CO)(PPh3)2]147 (26) upon reaction with 8-aminoquino-line according to Fig. 7. Compound 26 is noteworthy as it represents the first example of a base stabilized terminal borylene complex of the type [LXM = B(L)-R] (IVa, Fig. 1). 

As already indicated, the chemistry of terminal borylene complexes is as yet almost unexplored. In addition to the photochemically induced borylene transfer, which was already discussed in Chapter 3.2, studies of the reactivity of terminal borylene complexes are restricted to two recent reports by Roper.147,148 The base-stabilized borylene complex [Os (=BNHC9H6N)Cl2(CO)(PPh3)2] (26) undergoes a reaction with ethanol to yield the ethoxy(amino)boryl complex [Os B(OEt)NHCgH6N Cl(CO) (PPh3)2] (35) according to Eq. (13) with a 1,2-shift of the quinoline nitrogen atom from the boron to the osmium center. The alcoholysis of 26 indicates that even the boron atom in base-stabilized borylene complexes displays some electrophilic character—a fact already predicted by a theoretical study.117... 

A detailed comparison of the spectroscopic and structural parameters of the base stabilized borylene complexes 30, 36, 37, and 38 with those of tethered boryl complexes such as F established their close structural relationship and led the authors to the statement that a formal distinction between those species is somewhat artificial. Their reactivity, however,... 

Boron compounds, subvalent, 193-194 Boron-containing compounds, See also Diboron compounds Boryl complexes, 165 Borylene complexes bridged, 166-176 molecular structure data, 172 reactivity, 170-171 spectroscopic and structural aspects, 172-176... 

Organoboron compound, geminal, 219 Organodiboron derivatives, 193 Organodielement halides, 79-83 Organoelement halides, 99 Os-Cl exchange, 186 Osmium boryl complexes, 179 Oxidation, ionic liquids, 278-279... 

Comparatively recent is the isolation of stable metal-boryl complexes by the oxidative addition of B—B bonds, e.g., to Pt°or Rh1 ... 

The insertion of ethylene into Pdn allyl complexes with hemilabile P—O ligands gives (l -T -pentenyl complexes.187 The insertion of alkenes into M—C bonds of T -iminoacyl and rf-pyridyl,188 into Rh—B bonds of boryl complexes,189 into M—Si,190 M—P,191 and M—O bonds is also known, an example for the latter being... 

Initial reports on the borylation of alkanes using isolated transition-metal-boryl complexes date back to 1995, when Hartwig showed that Cp Re(CO)2(Bpin)2 converts pentane to 1-borylpentane with high regioselectivity. " The catalytic C-H borylation of alkanes with Cp Re(CO)3 using photochemical activation was demonstrated soon thereafter (equation 25). Also, an efficient thermal process that involves the use of rhodium catalysts has since been developed (equation 26). It is interesting to note that this methodology is not restricted to small molecules, but has recently been exploited for the direct side-chain functionalization of polyolefins. ... 

---