Rearrangement, 1,2-alkyl complexes

A simplified mechanism for the hydroformylation reaction using the rhodium complex starts by the addition of the olefin to the catalyst (A) to form complex (B). The latter rearranges, probably through a four-centered intermediate, to the alkyl complex (C). A carbon monoxide insertion gives the square-planar complex (D). Successive H2 and CO addition produces the original catalyst and the product ... 

Strictly speaking, rj3-allyi complexes are not considered as alkyl complexes however, because certain ligands such as CO will induce the tj3 —> rj1 allyl rearrangement, j3-allyls will be considered as such here for the sake of consistency. 

Neutral and cationic homoleptic (see Homoleptic Compound) tnngsten-isocyanide complexes W(CNR)6 and [W(CNR)y] + (R = alkyl, aryl) are known, and W(CO)e (CNR) (n=l-3) complexes may be prepared from W(CO)6 and excess isocyanide in the presence of catalytic amounts of C0CI2 or PdO. Iso-cyanides are isoelecfronic with CO and also insert into the W-R bonds of alkyl complexes (see Alkyl Complexes). For instance, the alkyl-nitrosyl complexes Cp W(NO)(X)(CH2Bfr) (X = NHBfr, OBfr) react with CNBu to afford -iminoacyl complexes, and the isocyanide complexes, Cp W(CO)2(Me)(CNR) (10, R = alkyl), rearrange to afford either -iminoacyl Cp W(CO)2( -MeCNR)(ll) or ij -l-azaallyl Cp W(CO)2( -CH2CHNR) (12) derivatives, depending upon the reaction conditions (equation 4). ... 

ALLYLIC REARRANGEMENT AlkyL copper-Boron trifluoride complexes. 

Alkyl-allyl complexes of isomeric systems can be interconverted and thus be used in isomerization of vinylcyclopropanes. Ethyl 4-azabicyclo[5.1.0]octa-2,5-diene-4-carboxylate (20) reacts with pentacarbonyliron to give complex 21, which photochemically rearranges to complex 23. Carbonylation of both products 21 and 23 leads to ethyl 9-oxo-2-aza-bicyclo[3.3.1]nona-3,7-diene-2-carboxylate (22). While complex 21 upon heating regenerates the starting material, complex 23 gives the isomeric product 24. In contrast to iron, with rhodium only the endo-complex 25 is formed. ... 

Since 1980, the applications zeolites and molecular sieves in the speciality and fine chemicals increased enormously. Zeolites are being used in the various types of reactions like cyclization, amination, rearrangement, alkylation, acylations, ammoxidation, vapour and liquid phase oxidation reactions. Zeolites and molecular sieves have also been used to encapsulate catalytically active co-ordination complexes like ship-in-bottle and as a support for photocatalytic materials and chiral ligands. Redox molecular sieves have been developed as an important class of liquid and vapour phase oxidation and ammoxidation reactions. We have discussed few typical recent examples of various types of reactions. 

It is nowadays well established that the hydrogenolysis of an alkyl complex involves the initial formation of the corresponding hydride. This may be an unstable transient species, especially when the alkyl precursor includes non-bulky ligands. A monomeric stable [Ln]R complex can lead to an unstable [Ln]H moiety, because the smaller size of the hydride ligand may not ensure the saturation of the co-ordination sphere about the lanthanide. The rearrangement into tris(cyclopentadienyl) derivative may also compete with both dimerisation or reduction (Scheme 4). 

This is the rate-determining step involving heterolytic splitting of the hydrogen molecule and formation of an hydridoruthenium(II) complex. The next step involves rearrangement of the hydrido-7r-olefin complex to a (T-alkyl complex via insertion of the olefin into the metal hydride bond. Finally, electrophilic attack occurs on the metal-bonded carbon... 

The catalyst RhH(CO)2(P03)2 reacts in either an associative or dissociative fashion to form an hydrido-alkene complex which then undergoes a 7T-cr rearrangement to a five-coordinate alkyl complex (21). Alkyl transfer to the coordinated CO gives a square acyl complex (22) which oxidatively adds H2 yielding a dihydroacylrhodium(III) complex (23). This then eliminates the aldehyde and RhH(CO)(P /)3)2. 

NMR studies of the reaction of RhH(CO)(P< >s)3 with an equilibrium mixture of ethylene and carbon monoxide have shown the formation of the alkyl complex and the subsequent rearrangement to the acyl complex (Fig. 33) (51). 

Fio. 33. Formation of the alkyl complex and rearrangement to the acyl complex. 

Fig. 44. Possible mechanisms for formation of ir-complex. (a) cr-ir rearrangement of alkyl complex (b) addition of olefin to an unalkylated complex.

D. Abstraction of H from the OH group of the alkyl complex rearrangement. 

---