Reactions exclusion rules

Criteria and guidelines useful in network elucidation and supplementing the rules derived in this chapter include considerations of steric effects, molecularities of postulated reaction steps, and thermodynamic constraints as well as Tolman s 16- or 18-electron rule for reactions involving transition-metal complexes and the Woodward-Hoffmann exclusion rules based on the principle of conservation of molecular orbital symmetry. Auxiliary techniques that can be brought to bear include, among others, determinations of isomer distribution, isotope techniques, and spectrophotometry. 

This type of restricted, steric selectivity must therefore be introduced into the single-events methodology. Fortunately, a certain number of exclusion rules can be introduced at the reaction network generation step, to take into account this type of restriction. 

Difluoroaminofluorodiazirine (225) extrudes nitrogen at only 75 °C. Intramolecular stabilization gives trifluoromethylenimine (226) added tetrafluoroethylene is cyclopropa-nated. This type of dichotomy is not often found in carbene chemistry alkylcarbenes undergo intramolecular stabilization as a rule, whereas intermolecular stabilization is observed exclusively with alkoxycarbonylcarbenes and with difluorocarbene. In the latter case CF2 attacks its precursor when no other reaction partner is present. 

Miller et al. [9] hypothesized rules on the regioselectivity of addition from the study of the base-catalyzed addition of alcohols to chlorotnfluoroethylene. Attack occurs at the vinylic carbon with most fluorines. Thus, isomers of dichloro-hexafl uorobutene react with methanol and phenol to give the corresponding saturated and vinylic ethers The nucleophiles exclusively attack position 3 of 1,1-dichloro-l,2,3,4,4,4-hexafluoro-2-butene and position I of 4,4-dichloro-l,l,2,3,3,4-hexafluoro-1-butene [10]. In I, l-dichloro-2,3,3,4,4,4-hexafluoro-l-butene, attack on position 2 is favored [J/] (equation 5) Terminal fluoroolefms are almost invariably attacked at tbe difluoromethylene group, as illustrated by the reaction of sodium methoxide with perfluoro-1-heptene in methanol [/2J (equation 6). 

The (V-methyldihydrodithiazine 125 has also been used as an effective formyl anion equivalent for reaction with alkyl halides, aldehydes, and ketones (77JOC393). In this case there is exclusive alkylation between the two sulfur atoms, and hydrolysis to give the aldehyde products is considerably easier than for dithianes. However, attempts to achieve a second alkylation at C2 were unsuccessful, thus ruling out the use of this system as an acyl anion equivalent for synthesis of ketones. Despite this limitation, the compound has found some use in synthesis (82TL4995). 

The combination of carbon-centered radicals usually involves head-to-head (a,a ) coupling. Exceptions to this general rule occur where the free spin can be delocalized into a n-system. The classic example involves the triphenylmethyl radical (13) which combines to give exclusively the a-para coupling product (26), Scheme I.8).27 This chemistry is also seen in cross reactions of 13 with other tertiary radicals.146... 

The hydration of triple bonds is generally carried out with mercuric ion salts (often the sulfate or acetate) as catalysts. Mercuric oxide in the presence of an acid is also a common reagent. Since the addition follows Markovnikov s rule, only acetylene gives an aldehyde. All other triple-bond compounds give ketones (for a method of reversing the orientation for terminal alkynes, see 15-16). With allqmes of the form RC=CH methyl ketones are formed almost exclusively, but with RC=CR both possible products are usually obtained. The reaction can be conveniently carried out with a catalyst prepared by impregnating mercuric oxide onto Nafion-H (a superacidic perfluorinated resinsulfonic acid). ... 

Recently several examples of diolefin crystals in which the reaction behaviour deviates from the topochemical rule have been observed. For example, in the photoreaction of methyl a-cyano-4-[2-(4-pyridyl)-ethenyljcinnamate (2 OMe), the first reaction occurs exclusively at the pyridyl side although the distance between the ethylenic double bonds on the pyridyl side is exactly the same as that between the ethylenic double bonds on the ester side (4.049 A), as shown in Fig. 5 (Maekawa et al., 1991a). A few other unsymmetrical diolefin compounds display the same regioselective behaviour (Hatada, 1989). 

Hydration of unactivated alkynes is an important method for functionalizing this plentiful hydrocarbon source. Therefore, a variety of metal ions have been proposed as catalysts for this reaction, and almost all of the reported additions of water to terminal alkynes follow the Markonikov rule. The hydration of l-aUcynes with Hg(II) salts in sulfuric acid [85], RuCh/aq.HCl [86, 87], K[Ru (edta-H)Cl] 2H20 [88], RhCl,.3H20/aq. HCl [89], RhCl3/NR4 [90], Zeise-type Pt(II) complexes [91-93], and NaAuCl4 [94] produced exclusively methyl ketones (Eq. 6.46). 

Branching at an olefinic carbon atom inhibited the reaction markedly, the most dramatic case being that of 2,3-dimethyl-2-butene. It should be noted that the product in this case is nearly exclusively 3,4-dimethylpen-taldehyde for either cobalt or rhodium catalysis (7). Thus, a general rule that products containing a formyl group attached to a quaternary carbon atom are not formed (49) remains valid. Hydroformylation proceeds only after isomerization has occurred. 

Substitution on the double-bond carbon a to the oxygen increased the difficulty of the reaction, and formyl attachment occurred exclusively on the /3-carbon [Eq. (42)]. Thus, these substituted olefins also followed the rule of Keulemans (49). 

It is apparent that the exclusive application of the donicity-rule may fail to account for the actual reaction which is occurring, since solvation of anions, steric factors, and other specific donor-acceptor interactions as well as the en-tropic effects must also be considered. 

In order to provide more evidence for one of these possible mechanisms, the phenylation reaction was studied with the double-labtlQd N-methyl[l,3- N]pyrimidinium salt as substrate. It was found that the 2-phenylpyrimidine obtained did not show any enrichment, proving that in the ring transformation the benzamidine acts exclusively as an N-C-N donor and has replaced the N(l)-C(2)-N(3) moiety of the pyrimidine ring. This result rules out the occurrence of a cyclization according to route A (Scheme III.54) (76RTC209 95H(40)441]. 

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