Regiochem istry

Predict the products of the following reactions, showing both regiochem-istry and stereochemistry where appropriate ... 

An advantage of nickel catalysts over other metal systems is that the properties of the active species can easily be tuned by the addition of suitable ligands. For example, the presence of PPhj was shown to have a direct influence on the regiochem-istry of hydroalumination of 1,1-dimethylindene la [33]. While the reaction of BU2AIH with la gave a 4 1 mixture of regioisomeric products 13a/13b after deuterolytic workup, the same reaction carried out in the presence of PPh, yielded 13a and 13b in a ratio of >99 1 (Scheme 2-14). 

With cyclic alkenes, the reaction proceeds with variable regiose-lectivity and stereoselectivity depending on the substituents, although the overall reaction proceeds in moderate to good yield. The approach is of particular value for those systems where regiochem-istry and stereochemistry are not variable. 

Cycloaddition reactions between alkenes and noncarbohydrate, carbonyl compounds have been described in discussing the reactions of alkenes (see Table I and Scheme 1). The depiction of the excited carbonyl given in Scheme 6 is useful in understanding the regiochem-istry of the cycloaddition process, as it suggests that the electron-deficient oxygen atom in the excited carbonyl will react with the alkene to produce the (more-stable) 1,4-diradical. Table VIII lists cycloaddition reactions in which the excited carbonyl is part of a carbohydrate. 

Resonance effects, on the other hand, can significantly affect the regiochem-istry of the cyclizadon. Resonance delocalization of the unpaired electron of a free radical stabilizes that radical. This is why the allyl radical is much more stable than the //-propyl radical. Thus, if a double bond is substituted with a group capable of providing resonance stabilization to a free radical, it undergoes free-radical addition much more readily than a double bond which cannot provide such resonance stabilization. 

In C70 monoadducts (Fig. 9) the emission maxima appear around 690-710 nm and a shoulder at 750 nm, which is slightly red-shifted compared to C70 [82,83,92], The exact emission band position depends strongly on the regiochem-istry of the monoadduct. The two possible 1,2-isomers (10,10a) with an addend attached near to the pole show similar emission spectra, while in the case of the 5,6-isomer (11) the emission maxima is bathochromic shifted about 8 nm [92], The fluorescence lifetime was determined to be in the range of 1 ns, somewhat higher than that of C70 with 0.65 ns, while the fluorescence quantum yield is 7 X 10 4 and nearly identical to that of C70 [82],... 

An experimental and computational investigation into the photorearrangement of N-cyclopropylimines to yield pyrrolines (Scheme 122) has shown that the, regiochem-istry and stereochemistry can be understood in terms of a mechanism involving barrierless evolution in three different (S2, Si, and So) singlet states and sequential decay through two different (S2/S1 and S1/S0) conical intersection funnels.183... 

If 1-methylcyclohexene is used instead of 4-methyl-3-cyclohexenone, the regiochem-istry inverts so the methyl group appears next to the ester in the product. Why ... 

Modifying the aldehyde function in the pyrroles 1162 into 1,3-dipoles 1163 (nitrone) and 1167 (nitrile oxide) furnished tricyclic heterocycles 1164 and 1168 via intramolecular 1,3-dipolar cycloaddition reactions (Scheme 226) <2000T3013>. None of the isomeric bridged product 1165 is produced despite the preference for that regiochem-istry in the intermolecular reaction. Generated in situ (from the oximes 1166) nitrile oxides 1167 cyclized spontaneously to the dihydroisooxazole 1168 in quantitative yield at room temperature. 

The factors which govern the stereochemistry and regiochem-istry of these reactions... 

How do these cyclase enzymes control the precise regiochem-istry and stereochemistry of these multistep cyclizations The active site of pentalenene synthase consists of a hydrophobic cleft, which is lined with aromatic and nonpolar residues. It is thought that the carbocation intermediates might be stabilized by the formation of tt-cation interactions, with aromatic residues such as phenylalanine, tyrosine, and tryptophan. In pentalenene synthase, replacement of Phe-76 or Phe-77 by Ala gave > 10-fold reduction in activity, which suggests that they may stabilize carbocationic intermediates through Jt-cation interactions. 

The mechanism for polymerization of propylene using Ziegler-Natta catalysts is analogous to that discussed in section 3.7 with ethylene. However, unlike ethylene, propylene can be said to have "head" and "tail" portions and regiochem-istry can vary. More importantly, the orientation (stereochemistry) of the methyl group in the polymer has a dramatic effect on polymer properties. These factors make polymerization of propylene (and other a-olefins) more complex (17). 

Anderson and coworkers emphasized the polarizability of sulfur, which accounts for the regiochem-istry of C—H acidification by sulfur. Epiotis et al. developed a hyperconjugative model involving delocalization of the unshared pair on carbon into the low-lying adjacent S—R antibonding orbital," which accounts for the stereochemical aspects of C—H bond acidification by sulfur. 

Acceptable yields of [3-1-2] cycloadducts were also obtained when l-methyl-6-methylene-tricyclo[3.2.1.0 ]oct-3-en-8-one was reacted with phenyl azide and 2,4,6-trimethylbenzenecar-bonitrile oxide.Interestingly, the two 1,3-dipolar reagents exhibited different regiochem-istry the azide reacted with the endocyclic double bond whereas the A-oxide attacked the exocyclic C-C double bond. ... 

Other structural elements on the scaffold may be relevant in relation to the regiochem-istry of the two key adjacent substituents on... 

Dubois and Fellmann have studied the kinetic regioselectivity in the base-promoted reactions of 2-bu-tanone and 2-pentanone with a series of aliphatic aldehydes (equation 52) results are summarized in Table 2. The data indicate that steric effects play a subtle role in the determination of aldol regiochem-istry in unsymmetrical ketones. Although reaction at C-3 is favored for 2-butanone with all of the aldehydes, pivalaldehyde gives more reaction at C-1 than the other aldehydes studied. Selectivity for reaction at C-3 in 2-pentanone is significantly less with all aldehydes, especially pivalaldehyde. 

The conditions required for each reaction mode are described here, along with the regiochem-istry and simple diastereoselectivity of such processes. It will emerge that although, on occasions. the outcome of the cycloaddition is difficult to predict, the method does provide a route to methylenecyclopentanes complementary to the TMM-Pd method. In particular, the cycloaddition described here is viable both for electron-poor and electron-rich olefins. The survey will not concern itself with the detailed mechanistic implications of the cycloaddition. Suffice it to say that there is ample evidence that the reaction pathway followed is different to that described for the conjunctive reagents above and a TMM-metal intermediate is not proposed,... 

Muller, N. and Matzke, M., U.S. Patent 6 444 828. (Note that under certain conditions the regiochem-istry for the methylhydrazine reaction can be 13 1.)... 

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