Halometallation

The kinetic product distribution can be further shifted in excess reagent to get organozirconium derivatives with high regioselectivity. Even subtle steric differences such as in 2-pentyne ensure high selectivity [Eq. (6.77), a]. As Eq. (6.77) demonstrates, hydrozirconation of alkynes is an exclusive syn process. 

Reactive metal halides are known to add to unsaturated compounds to form 1,2 halometal compounds. Addition of boron and aluminum halides is of particular interest. 

Boron trihalides, particularly BBr3, add to alkenes in a reaction (haloboration) that is usually more complicated than hydroboration.465 Several compounds can be isolated as a result of secondary transformation of the primary addition product 466 

Z compounds are formed469 (the halogen and boron substituents are in cis position) 470 

With increasing reactivity of the boron halide, however, trans addition can also be observed BI3 yields a 1 1 mixture of Z and E compounds in the addition to phenylacetylene.469 However, the tram isomer is formed when BBr3 adds to acetylene 468 

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Many of the compounds in higher oxidation states are reactive, and for moisture-sensitive solids that cannot be crystallized, some of the bond lengths quoted in Table 2.1 are from EXAFS measurements [24], Raman spectroscopy is likewise well suited to studying such reactive compounds, and vibrational data for halometallates are given in Table 2.2 trends illustrated include the decrease in frequency as the oxidation state of the metal decreases, and similarly a decrease in vibrational frequency, for a given oxidation state, with increasing mass of the halogen. 

Mixed halometallates [28] can be synthesized RhCl6Br l from the reaction of RhCl - with HBr, or RhBr - with HC1. Individual isomers have been identified in solution by 103 Rh NMR, which can even distinguish between stereoisomers (Figure 2.2) and shows isotopic splitting (Figure 2.3). 

So far no [Ni (Bu2dfc)3) complexes are found with any halometallate anion apart from [FeXj] (X = Cl, Br). 

Only a few recent examples of halogen bonding of halogenated TTFs with halometallate anions have been described and this most probably represents a broad development field in this area. Indeed, these halometallate salts offer a wide variety of complexes and specifically here a large variation of the ratio of anion charge vs number of halide atoms, as already discussed above in the comparison between Br and 13 salts. This point is illustrated by the superb structures of two salts of EDT-TTF-I2 with the polymeric one-dimensional PbV (Fig. 9) and two-dimensional (Pbs/eDi/eh) anions where shorter C -1- -Ianion contacts are observed with the linear Pbl3 chains... 

In the preparation of (2R,33, 7a3 )-2-phenyl-3-methyl-5-oxo-2,3,5,6,7,7a-hexahydropyrrolo-[2,T ]-oxazole 248, the reaction time is the most important parameter in the /ra r-halometallation. For instance, by stirring 246 with 1 or 2equiv of fer7-butyllithium in EtzO at — 78 °C for 30 min, 247 and the bicyclic lactam 248 were obtained as an equimolar mixture. The latter substance was isolated as a single product when the reaction time was increased to 120min for the reactions of 2-(2 -bromoethyl)oxazolidine 246 with fert-butyllithium (Scheme 36) <1996T10761>. 

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