Pyridyl-amido catalysts have emerged recently with great promise for olefin polymerization. Insights into the activation chemistry are presented in an initial attempt to understand the polymerization mechanisms of these important catalysts. The activation of C1-symmetric arylcyclometallated hafnium pyridyl-amido precatalysts, denoted Me2Hf{N(-),N,C(-)} (1, aryl = naphthyl; 2, aryl = phenyl), with both Lewis (B(C6F5)3 and [CPh3][B(C6F5)4]) and Brønsted ([HNR3][B(C6F5)4]) acids is investigated. Reactions of 1 with B(C6F5)3 lead to abstraction of a methyl group and formation of a single inner-sphere diastereoisomeric ion pair [MeHf{N(-),N,C(-)}][MeB(C6F5)3] (3). A 1:1 mixture of the two possible outer-sphere diastereoisomeric ion pairs [MeHf{N(-),N,C(-)}][B(C6F5)4] (4) is obtained when [CPh3][B(C6F5)4] is used. [HNR3][B(C6F5)4] selectively protonates the aryl arm of the tridentate ligand in both precatalysts 1 and 2. A remarkably stable [Me2Hf{N(-),N,C2}][B(C6F5)4] (5) outer-sphere ion pair is formed when the naphthyl substituent is present. The stability is attributed to a hafnium/eta(2)-naphthyl interaction and the release of an eclipsing H-H interaction between naphthyl and pyridine moieties, as evidenced through extensive NMR studies, X-ray single crystal investigation and DFT calculations. When the aryl substituent is phenyl, [Me2Hf{N(-),N,C2}][B(C6F5)4] (10) is originally obtained from protonation of 2, but this species rapidly undergoes remetalation, methane evolution, and amine coordination, giving a diastereomeric mixture of [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (11). This species transforms over time into the trianionic-ligated [Hf{N(-),C(-),N,C(-)}NR3][B(C6F5)4] (12) through activation of a C-H bond of an amido-isopropyl group. In contrast, ion pair 5 does not spontaneously undergo remetalation of the naphthyl moiety; it reacts with NMe2Ph leading to [MeHf{N(-),N}NMe2C6H4][B(C6F5)4] (7) through ortho-metalation of the aniline. Ion pair 7 successively undergoes a complex transformation ultimately leading to [Hf{N(-),C(-),N,C(-)}NMe2Ph][B(C6F5)4] (8), strictly analogous to 12. The reaction of 5 with aliphatic amines leads to the formation of a single diastereomeric ion pair [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (9). These differences in activation chemistry are manifested in the polymerization characteristics of these different precatalyst/cocatalyst combinations. Relatively long induction times are observed for propene polymerizations with the naphthyl precatalyst 1 activated with [HNMe3Ph][B(C6F5)4]. However, no induction time is present when 1 is activated with Lewis acids. Similarly, precatalyst 2 shows no induction period with either Lewis or Brønsted acids. Correlation of the solution behavior of these ion pairs and the polymerization characteristics of these various species provides a basis for an initial picture of the polymerization mechanism of these important catalyst systems.

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Although tetrahydrofurans and tetrahydropyrans are important structural subunits of many classes of natural products,l comparatively few general synthetic methods are known? Since y-and &lactones are readily available: an efficient and versatile transformation to the ether would significantly extend current methodology. The conversion of a lactone to an ether has been accomplished by hydride reduction to a diol followed by cyclization by way of a monotosylate4 or other activated ester.5

Several macrocyclic lactones of the trichothecene class of compounds exhibit significant anticancer activity.' A common structural subunit in each of these lactones is the sesquiterpene verrucarol(1). Anguidin (2), a more highly oxygenated analogue, also shows inhibitory activity against several cancers., Calonectrin (3), considered to be the biogenetic precursor to verru~arol,~ has recently been isolated.

2579Registry No. 1,73587-50-5; 1 2,4-DNP derivative, 73587-51-6; 2, 73587-44-7; 6 Tl(1) salt, 73597-06-5; diethyl a-ethyl-a-formylsuccinate, 73587-53-8; diethyl a-(ethoxymethylene)succinate, 70145-31-2; diethyl a-formyl-a-isopropylsuccinate, 73587-54-9; diethyl a-(isopropoxymethylene)succinate, 73587-55-0; ethyl a-formyl-a-methylcaproate, 73587-56-1; ethyl a-(methoxymethy1ene)-caproate, 73587-57-2; ethyl a-ethyl-a-formylcaproate, 73587-58-3; ethyl a-(ethoxymethylene)caproate, 73587-59-4; ethyl a-formyl-aisopropylcaproate, 73587-60-7; ethyl a-(isopropoxymethy1ene)-caproate, 73587-61-8; ethyl a-(methoxymethylene)phenylacetate, 15937-30-1; ethyl a-(ethoxymethylene)phenylacetate,The transformation of epoxides into allylic alcohols by use of iodotrimethylsilane and 1,5-diazabicyclo-[5.4.0]undec-5-ene is described. The scope and limitations of this reaction are examined. This method is complementary to the method of Sharpless in the case of trisubstituted epoxides and proceeds under milder reaction conditions than the method employing lithium dialkylamides.