Metalation-boronation

The reactions described so far can be considered as alkylation, alkenylation, or alkynylation reactions. In principle all polar reactions in syntheses, which produce monofunctional carbon compounds, proceed in the same way a carbanion reacts with an electropositive carbon atom, and the activating groups (e.g. metals, boron, phosphorus) of the carbanion are lost in the work-up procedures. We now turn to reactions, in which the hetero atoms of both the acceptor and donor synthons are kept in a difunctional reaction produa. 

Metal-boron compounds — problems and perspectives. G. Schmid, Angew. Chem., Int. Ed. Engl., 1970, 9, 819-830 (89). 

Some group-IB and -IIB boranes or carboranes, especially of Cu, possess M — H—B structure units and are not discussed here because no direct metal-boron bond exists. 

The formation of boron-group IB bonds succeeds in two ways by transfer of a boryl group from metal-boron compounds to other metals, and by reaction of anionic boranes or carboranes with transition-metal halides. 

A series of Rh and Ir complexes is also basic enough to add boron halides, although the metal-boron bonds are weak, e.g., dppe and PPh3-Rh complexes serve as electron-pair bases when they react with BF3 and BCI3 . Table 1 summarizes the results. 

The series of neutral B-containing carbocycles as complex ligands within the theme of metal-boron donor-acceptor relationships concludes with the 4,5-dihydro-borepines. l-Phenyl-4,5-dihydroborepine is able to substitute pyridine, acetonitrile or CO... 

Dihydro-l,2,5-thiadiboroles (Et2C2B2R2S) have remarkable acceptor properties toward metal-ligand moieties. Dihydrothiadiborole-derived complexes can be achieved thermally or photochemically, e.g., Co and Fe sandwich, triple-decked and even tetradecked complexes are known, all having direct metal-boron bonds. 

Metal-boron ratio Metal borides and structure types Boron aggregation... 

Critical assessment of phase-diagram data employing the thermodynamics of the phase equilibria in the form of phase-diagram modeling provides consistent thermochemical data (including also the liquid solution) for binary metal-boron systems such as V—B Cr—B, Mn—B, Fe —B, Co—B, Ni—Mo—B and Nb—B, W—B, A1 — B Despite these efforts, reinvestigation of binary systems is... 

Boron resonances (Table III) appear in the range expected for sandwich-type complexes with metal-boron bonding (for reference data see Ref. 50). In cases where pairs of B-methyl and B-phenyl derivatives (as, e.g., 9 and 17) are known, the nB resonance of the methyl compound is at lower field by 2.0-3.7 ppm (average, 2.6 ppm). 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

Just as for group 5, 6, and 7 ( -CsF MCU species, Fehlner has shown that BH3-THF or Li[BH4] react with group 8 and 9 cyclopentadienyl metal halides to result in metallaborane clusters, many of them having a metal boron ratio of 1 3 and 1 4, and much of the synthetic chemistry and reactivity shows close connections with the earlier transition metals. The main difference between the early and later transition metallaboranes that result is that the latter are generally electron precise cluster species, while as has been shown, the former often adopt condensed structures. Indeed, as has been pointed out by King, many of the later transition metallaborane clusters that result from these syntheses have structures closely related to binary boranes and, in some cases, metal carbonyl clusters such as H2Os6(CO)18.159... 

Another very useful rule for classifying the structures of polyboranes and hetero-boranes as well as many metal boron cluster compounds and their derivatives has been developed by Rudolph, Williams, Mingos and Wade (see Chapter 1.1.2) [4]. Today these are generally termed the Wade rules. They can be derived from the structures and electronic requirements of closed polyhedral boranes, such as an octahedron or an icosahedron, which are present in the anions B6H62 and B,2 H, 22. Since there are only exopolyhedral B-H bonds the number of electron... 

Direct synthesis from the metal, boron and hydrogen at 825-975 K and 30-150 bar Hj has yielded the lithium salt, and such a method is generally applicable to groups lA and llA metals (Goerrig, 1958). The reaction involving either the metal or the metal hydride, or the metal together with triethylborane in an inert hydrocarbon has formed the basis of a patent. 

The recent work of Grimes and his co-workers 14, 26, 84, 96-96b, 98, 119, 191, 192) has shown the smaller carboranes to be particularly fertile sources of metal-boron-carbon clusters, of which a few further examples are shown in Fig. 19. 

Stokinger HE The halogens and the non-metals boron and silicon. In Clayton GD, Clayton FE (eds) Patty s Industrial Hygiene and Toxicology, 3rd ed, rev, Vol 2B, Toxicology, pp 2965-2968. New York, Wiley-Interscience, 1981... 

Type of boron ligand Metal complex or complex unit partner Substituents at the boron Metal- boron ratio Ref. 

Rogl, P. and G. Eftenberg Phase Diagrams of Ternary Metal-Boron-Carbon Systems, ASM International, Materials Park, OH. 1998. 

Detailed studies have been carried out with metal-boron (Ni-B, Pd-B, Pt-B) and metal-phosphorous (Ni-P, Pd-P) films prepared by radiofrequency sputtering269-272. Pt hardly interacts with boron and shows the low selectivity of the pure metal269. Interaction between Ni or Pd and the metalloids results in a change in the electron density of... 

---