Replacement preferential

Selective monohalo alkylations were achieved when chlorobromoalkanes such as l-chloro-S-bromo-S-methjIbutane were used as halo alkylating agents (47). The bromine is replaced preferentially ... 

Fluorination of aliphatic hydrocarbons with cobalt trifluoride gives complex mixtures Isobutane (2-methylpropane) fluorinated at 140-200 °C affords a mixture of 30 products of different degrees of fluorination and of isobutane as well as butane skeletons. The tertiary hydrogen is replaced preferentially Products containing 5-10 atoms of fluorine including a small amount of perfluoroisobutane were isolated [10]. 

Similarly, copper salts (cupric and cuprous) facilitate the reaction of aryl halides with trialkyl phosphites in the formation of dialkyl arylphosphonates under conditions like those found in nickel systems.37-39 Again, the copper salts appear to undergo an initial reaction with the phosphites to form a complex that subsequently undergoes reaction with the aryl halide. The requirement for copper is also similar to that for nickel saltstonly a catalytic amount is needed. Further, a preference among halides on the aromatic ring is noted iodide is replaced preferentially to other halides (Figure 6.10).40... 

When both fluorine and another halogen atom are attached to a C = C group, fluorine is usually replaced preferentially ... 

In l,2,3,4-tetrafluoro-5-halobenzenes 1 and l,2.3,5-tetrafluoro-4-halobenzenes 4 the fluorine atoms in the 4-position relative to hydrogen are replaced preferentially to those in the 4-position relative to the halogen atom.83... 

Debrominotion. Horner and co-workers used the reagent for reductive displacement of bromine or iodine atoms in ortho or para positions of phenols. An ortho halogen can be replaced preferentially. A mixture of the phenol, one equivalent OH OH... 

Nitro-6-chloropyridazine 1-oxide reacts with nucleophiles with displacement of the 6-chlorine atom (even with acetyl bromide), except with sodium methoxide. Merer, the 3-nitro group is diplaced. Nucleophilic substitution with amines was investigated for various monoalkoxydihalo-or dialkoxyhalopyridazines. Normally, the chlorine atom is replaced, but in 3-chloro-4,6-dialkoxy- or 4-alkoxy-3,6-dichloropyridazines the alkoxy group may be replaced preferentially or concurrently. The 6-chlorine atom of methyl (3,6-dichloropyridazin-4-yl)-acetate was displaced by hydrazine or on hydrolysis. ... 

There are numerous investigations of the reactivity of di- and polyhalo-pyridazines, particularly polyfluoropyridazines. Aminolysis of l-phenyl-4,5-dichloropyridazin-6-one has been studied in detail. In this and other reactions with nucleophiles, the halogen atom at position 4 is substituted preferentially, although a mixture of 4-amino and 5-amino derivatives is formed in the reaction between 4,5-dihalopyridazin-6-ones and ammonia or amines. It has been now firmly established that displacement reactions on 3,6-dichloropyridazine 1-oxide with sulfur nucleophiles take place at position 6 in contrast to nitrogen or oxygen nucleophiles, where the 3-chlorine atom is replaced preferentially. In connection with the previously observed self-condensation of 3-chloro-6-methylpyridazine to a tricyclic product, the reaction between 3,6-dichloropyridazine and pyridine N-oxides was investigated. 3,6-Dichloropyridazine with 2-methylpyridine N-oxide at 100°-110°C affords three compounds (171, 172, and 173). With 2,6-dimethylpyridine N-oxide, an ether (174) is also formed. The isolation of... 

When the alkyl and aryl groups are both bound to the bismuth atom as mixed bismuthines, the alkyl ligand is replaced preferentially by halogen [85CB1031, 92BCJ3504]. 

Taurine and some ditaurine can be obtained by reacting 256 parts by weight of sodium hydroxyethanesulfonate with 800 parts of 25 per cent ammonia liquor at 210°C for 3 hr. The addition of 10 parts of sodium sulfite aids materially in increasing the conversion, which falls off as the quantity of sodium sulfite is reduced. The synthesis is of particular interest, because it shows that, in aliphatic compounds containing both sulfonic and hydroi l groups, the —OH group can be replaced preferentially by... 

Dichloro-l,4-benzoquinone reacts similarly (75CB693 83JMC876). Facile nucleophilic substitution of a chlorine atom is possible and chlorine at position 5 is replaced preferentially (83JMC876). Some substituted indazole-... 

For example, suppose we wish to prepare cis- and /rawr-amminedichloronitro-platinate(II). How would we proceed The procedure is outlined in Figure 5.12. If we start with the tetrachloroplatinate(II) and treat it first with ammonia and then with nitrite, the cis isomer is prepared in high purity. Note that when ammine-trichloroplatinate(II) is treated with nitrite, a chloride trans to another chloride is replaced preferentially to the chloride trans to ammine. This takes advantage of the fact that chloride is higher in the trans-directing series than is ammonia. On the other hand, when trichloronitroplatinate(II) is treated with ammonia, the chloride... 

The chemoselectivity of these metal-free Arbuzov-type reactions was investigated by Janeba (Scheme 4.215) [354]. Using a heterocycle that had both chloro and fluoro substituents, the authors found that simply heating the neat reagents using a focused microwave reactor without the addition of a metal catalyst afforded the phosphonylated material in moderate yields. Of interest was the observation that the fluoro group was replaced preferentially to the chloro. In addition to this example, a number of mono- and bisphosphonated compounds were prepared and characterized. 

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