We recently reported that molybdenum pentacarbonyl will cyclize 1-alkyn-4-01s to the isomeric 2,3-dihydrofurans.2.3 Although the generation of Mo(C0)s from trimethylamine N-oxide and molybdenum hexacarbonyl produces a modestly catalytic reagent for this cycloisomerization, we have found that EtSN: Mo(C0)s is best prepared by photolysis Of Mo(CO)6 in a mixture of diethyl ether and triethylamine (Rayonet lamp, 350 nm, pyrex filter, eq l).* Subsequent addition of alkynol l5 (dark reaction) hv (Pyrex filter),and stirring for ca. 18 h gives dihydrofuran 26 (89% optimized yield from 26 mol % Mo(CO)~, eq 2). This reaction system is critically dependent on the presence of a tertiary amine, in that photogenerated molybdenum pentacarbonyl-diethyl ether leaves 1 unchanged. A practical advantage of supplanting trimethylamine N-oxide decarbonylation with the photochemical method is that cyclic products are more effectively removed from inorganic byproducts, thus resulting in higher isolated yields. Attempts to follow the mechanistic course of this reaction by deuterium-labeling of either the alcohol or alkynyl hydrogens of 1 were complicated by equilibration of the deuterium atom between these two moderately acidic functional groups upon standing in the absence of triethylamine or molybdenum carbonyls.' However, our current evidence for the intermediacy of molybdenum carbene anion 4 as a catalytic intermediate includes the isolation of the apparent aldol condensation-dehydration* products 5 and 6 (Scheme 1). Reaction of 1 with 1 equiv of
The amplitude and phase for coherent regeneration in hydrogen and deuterium have been measured for six momentum bins in the range 3.5-10.5 GeV/c. Over this region the phase,
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