Catalysts for cross-coupling reactions with non-activated alkyl halides.

Despite the problems inherent to metal-catalyzed cross-coupling reactions with alkyl halides, these reactions have become increasingly important during the last few years. Detailed mechanistic investigations have led to a variety of novel procedures for the selective cross-coupling of non-activated alkyl halides bearing beta hydrogen atoms with a variety of organometallic nucleophiles under mild reaction conditions. This Minireview highlights selected examples of metal-catalyzed coupling methods and is intended to encourage chemists to exploit the potential of these approaches in organic synthesis.

"Solventless" continuous flow homogeneous hydroformylation of 1-octene.

The hydroformylation of 1-octene under continuous flow conditions is described. The system involves dissolving the catalyst, made in situ from [Rh(acac)(CO)(2)] (acacH=2,4-pentanedione) and [RMIM][TPPMS] (RMIM=1-propyl (Pr), 1-pentyl (Pn) or 1-octyl (O) -3-methyl imidazolium, TPPMS=Ph(2)P(3-C(6)H(4)SO(3))), in a mixture of nonanal and 1-octene and passing the substrate, 1-octene, together with CO and H(2) through the system dissolved in supercritical CO(2) (scCO(2)). [PrMIM][TPPMS] is poorly soluble in the medium so heavy rhodium leaching (as complexes not containing phosphine) occurs in the early part of the reaction. [PnMIM][TPPMS] affords good rates at relatively low catalyst loadings and relatively low overall pressure (125 bar) with rhodium losses <1 ppm, but the catalyst precipitates at higher catalyst loadings, leading to lower reaction rates. [OMIM][TPPMS] is the most soluble ligand and promotes high reaction rates, although preliminary experiments suggested that rhodium leaching was high at 5-10 ppm. Optimisation aimed at balancing flows so that the level within the reactor remained constant involved a reactor set up based around a reactor fitted with a sight glass and sparging stirrer with the CO(2) being fed by a cooled head HPLC pump, 1-octene by a standard HPLC pump and CO/H(2) through a mass flow controller. The pressure was controlled by a back pressure regulator. Using this set up, [OMIM][TPPMS] as the ligand and a total pressure of 140 bar, it was possible to control the level within the reactor and obtain a turnover frequency of ca. 180 h(-1). Rhodium losses in the optimised system were 100 ppb. Transport studies showed that 1-octene is preferentially transported over the aldehydes at all pressures, although the difference in mol fraction in the mobile phase was less at lower pressures. Nonanal in the mobile phase suppresses the extraction of 1-octene to some extent, so it is better to operate at high conversion and low pressure to optimise the extraction of the products relative to the substrate. CO and H(2) in the mobile phase also suppress the extraction efficiency by as much as 80%.

A synthetic breakthrough into an unanticipated stability regime: a series of isolable complexes in which C6, C8, C10, C12, C16, C20, C24, and C28 polyynediyl chains span two platinum atoms.

The reaction of trans-[PtCl(p-tol){P(p-tol)3}2] (PtCl) and H(C[triple chemical bond]C)2H (cat. CuI, HNEt2) gives PtC4H (82 %), which can be cross-coupled with excess HC[triple chemical bond]CSiEt3 (acetone, O2, CuCl/TMEDA; Hay conditions) to yield PtC6Si (77 %). The addition of nBu4N+F- in wet acetone gives PtC6H (84 %), and further addition of ClSiMe3 (F- scavenger) and excess HC[triple chemical bond]CSiEt3 (Hay conditions) yields PtC(8)Si (23 %). Similar cross-coupling reactions of PtCxH (generated in situ for x>6) and excess H(C[triple chemical bond]C)2SiEt3 give a) x=4, PtC8Si (29 %), PtC12Si (30 %), and PtC16Si (1 %); b) x=6, PtC10Si (59 %) and PtC14Si (7 %); c) x=8, PtC12Si (42 %); and d) x=10, PtC14Si (20 %). Hay homocoupling reactions of PtC4H, PtC6H, PtC8H, and PtC10H give PtC8Pt, PtC12Pt, PtC16Pt, and PtC20Pt (88-70 %), but PtC12H decomposes too rapidly. However, when PtC12Si and PtC14Si are subjected to Hay conditions, protodesilylation occurs in the presence of the oxidizing agent and PtC24Pt (36 %) and PtC28Pt (51 %) are isolated. Reactions of PtC6H and PtC10H with PtCl (CuI, HNEt2) give PtC6Pt (56 %) and PtC10Pt (84 %). The effect of the chain lengths in PtCxPt upon thermal stabilities (>200 degrees C for x< or =20), IR nu(C[triple chemical bond]C) patterns (progressively more bands), colors (yellow to orange to deep red), UV/Vis spectra (progressively red-shifted and more intense bands with epsilon>400,000 M(-1) cm(-1)), redox properties (progressively more difficult oxidations), and NMR spectra (many monotonic trends) are analyzed, including implications for the sp carbon allotrope carbyne. Whereas all other dodecaynes and tetradecaynes rapidly decompose at room temperature, PtC24Pt and PtC28Pt remain stable at >140 degrees C. Crystal structures of PtCxSi (x=6, 8, 10) and PtCxPt (x=6, 8, 10, 12) have been determined.