The combination of gold and silver salts catalyzed the addition of active methylenes to dienes, triene, and cyclic enol ethers with high regioselectivity. The catalysis provides a synthetically useful method for functionalized carbocycles and heterocycles through a highly atom-economical carbon-carbon bond formation. [reaction: see text]
[reaction: see text] A highly efficient annulation of phenols and naphthols with dienes was developed by using a combination of AuCl(3)/AgOTf as catalyst. The annulation generated various benzofuran derivatives under mild conditions rapidly.
Abstraction of Cl from the complexes (indenyl)Ni(PPh3)Cl generates cationic species that are effective precatalysts for the hydrosilylation of some olefins and ketones. For instance, the mixture of (1-Me-indenyl)Ni(PPh3)Cl and NaBPh4 (or methylaluminoxane) reacts at room temperature with ca. 100 equiv. each of PhSiH3 and styrene to produce [1-phenyl-1-ethyl](phenyl)silane, PhCH(CH3)(SiPhH2), in 50%80% yield. The same system can also catalyze the hydrosilylation of 1-hexene and norbornene, but the products arising from these substrates consist of mixtures of regio- and stereoisomers. On the other hand, ketone hydrosilylation is regiospecific, giving the corresponding silyl ethers in high yields. A number of experimental observations have indicated that the initially generated Ni-based cation is not the catalytically active species. Indeed, the cationic initiators may be replaced by LiAlH4 or AlMe3, which generate the corresponding Ni-H or Ni-Me derivatives, respectively. Moreover, the observed regioselectivity for the addition of PhSiH3 to styrene (i.e., predominant addition of the silyl fragment to the α-C) is opposite of what would be expected if the reaction mechanism involved carbocationic intermediates. A new mechanism is proposed in which the active species is a Ni-H species originating from the transfer of H from PhSiH3 to the initially generated Ni cation. Key words: hydrosilylation, nickel indenyl complexes, cationic complexes, hydride intermediates.
We have now determined that the mutant designated P86A in this paper is, in actuality, P56A. We attribute this error to a visual mistracking along the DNA sequence, such that primers were constructed to alter the proline at position 56. P56 lies on a surface-exposed loop in the substrate-binding domain, ca. 18 Å away from the bound trifluoroethanol; the corresponding position is either deleted or a proline in the mesophilic and psychrophilic alcohol dehydrogenases, respectively. A comparison of the properties of P86A and P56A will be published at a later date.
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