Homogeneous alkane dehydrogenation and arene hydrogenation at niobium metal centers

The facile intramolecular dehydrogenation of a substituent alkyl group of a 2,6-diisopropylphenoxide ligand at a niobium metal center has been demonstrated. Room temperature reduction of Nb(OAr-2,6-Pr$\sp{\rm i}\sb2)\sb3$Cl$\sb2$ with 2 equiv. of Na/Hg results in the formation of the metallacyclopropane complex, Nb(OAr-2,6-Pr$\rm\sp{i}\sb2)\sb2 (OC\sb6H\sb3Pr\sp{i}$-$\eta\sp2$-CMe=CH$\sb2$)(thf). This complex will undergo coupling reactions with a number of unsaturated molecules. The metallacyclopropane ring in Nb(OAr-2,6-Pr$\rm\sp{i}\sb2)\sb2 (OC\sb6H\sb3Pr\sp{i}$-$\eta\sp2$-CMe=CH$\sb2$)(thf) will also react with protic reagents, which attack the vinyl methylene carbon affording simple five-membered metallacycles. The alkylation chemistry of arylphenoxyniobium(V)chlorides has produced a variety of alkyl derivatives. Under a high pressure of H$\sb2$ gas, several niobium(V) alkyls undergo hydrogenolysis to give the corresponding hydrido intermediates, which then carry out intramolecular arene hydrogenation. The di- and trialkyl derivatives of 2,6-diisopropylphenoxyniobium(V) have been found to be resistant to this type of reactivity. Homogeneous arylphosphine hydrogenation has been achieved by using $\rm Nb(OC\sb6H\sb3$-2-Ph-6-$\eta\sp4$-$\rm C\sb6H\sb7)(OAr$-2,6-Ph$\sb2)\sb2,$ Nb(OAr-2,6-Ph$\rm \sb2)\sb3(Me)\sb2$ and Nb$\sb2(\mu$-CSiMe$\rm\sb3)\sb2(CH\sb2SiMe\sb3)\sb4$ as catalyst precursors. The reactivity of these catalyst precursors with monodentate arylphosphines has been shown to be in the following order: $\rm PMePh\sb2 \u3e PEtPh\sb2\approx PBzPh\sb2$ and $\rm PMePh\sb2 \u3e PMe\sb2Ph\approx PPh\sb3.$ A study of the sequential first order kinetic hydrogenation reactions of $\rm PRPh\sb2\to PRPhCy\to PRCy\sb2$ by using a constant catalyst concentration has proven the first step to be 1.8 times faster than the second step (statistic is 2.0). The introduction of a para-methyl substituent to the arene ring slows down the reaction rate by five times and the product distribution favors the formation of the cis isomers over the trans by a ratio of $\approx$9:1. Similar reactivity of Nb(OAr-2,6-Ph$\rm\sb2)\sb2Bz\sp\prime\sb3(Bz\sp\prime = CH\sb2C\sb6H\sb4$-p-Me) with arylphosphines has also been seen. The trialkyl compound will undergo catalytic hydrogenation of a number of mono-, bi- and polydentate arylphosphines. The greater reactivity of the niobium(V) trialkyls has led to the formation and isolation in high yields of a variety of useful cyclohexylphosphines. A particularly important ligand, dcpm, has been synthesized on a large scale (25.5 g, $\u3e$96% yield) and the process has shown a great amount of commercial potential. Besides the reduction of arylphosphines, the trialkyl compound can also catalytically convert polyaromatic hydrocarbons to their partially hydrogenated counterparts. Substrates such as naphthalene, 1- and 2-methyl substituted naphthalene, anthracene, phenanthrene, acenaphthylene and acenaphthene are all successfully hydrogenated. Stereoselectivity is also an advantage of this process. The trialkyl compound carries out the hydrogenation of a series of perdeutero polyaromatics substrates. $d\sb{10}$-Naphthalene is converted to give all cis-$\rm H\sb4D\sb{10}$-tetralin; $d\sb{14}$-acenaphthene is found to produce a 100% of the all cis-$\rm H\sb4D\sb{14}$-derivatives. $d\sb{14}$-Anthracene gives a symmetrical $\rm H\sb8D\sb{14}$-hydrogenated counterpart, in which each hydrogenated ring contains all four hydrogen atoms on the same face. (Abstract shortened by UMI.