Exchange metal

Reaction (A) is a reversible metal-metal exchange. Reaction (B) is a reversible halogen-metal exchange. Reaction (C), which is of synthetic promise, takes place with catalytic quantities of RLi compound. The mechanism of this catalyzed reaction follows from the addition of the reversible M-M reaction (A) and the reversible X-M reaction (B). 

The halogen-metal exchange reaction can also be undesirable at times. One illustration, selected from a number, is the attempted coupling between pentafluorophenyllithium and periluorinated heptyl iodide, from which there is obtained an excellent yield of pentafluoroiodobenzene, with no significant coupling. 

The metal-metal interconversion reaction, as we first designated this group of reactions, was initially concerned largely with interaction of two organometallic compounds. 

The reaction is of broad utility for the preparation of a wide variety of organometallic compounds. Attention might be directed to a review article with Jones 35) on Methods of Preparation of Organometallic Compounds. This was published in 1954 and has the large total of 471 references. Since that time, of course, the methods of preparation have expanded greatly and part of this is due to the relatively recent high activity in the area of transition metal types. 

The simple descriptive reaction given as an illustration at the beginning of this section is incomplete on two counts. First, we have shown that some 

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Alkynyl anions are more stable = 22) than the more saturated alkyl or alkenyl anions (p/Tj = 40-45). They may be obtained directly from terminal acetylenes by treatment with strong base, e.g. sodium amide (pA, of NH 35). Frequently magnesium acetylides are made in proton-metal exchange reactions with more reactive Grignard reagents. Copper and mercury acetylides are formed directly from the corresponding metal acetates and acetylenes under neutral conditions (G.E. Coates, 1977 R.P. Houghton, 1979). 

There are a wide variety of methods for introduction of substituents at C3. Since this is the preferred site for electrophilic substitution, direct alkylation and acylation procedures are often effective. Even mild electrophiles such as alkenes with EW substituents can react at the 3-position of the indole ring. Techniques for preparation of 3-lithioindoles, usually by halogen-metal exchange, have been developed and this provides access not only to the lithium reagents but also to other organometallic reagents derived from them. The 3-position is also reactive toward electrophilic mercuration. 

Carbocyclic substitution can also be achieved by first introdueing a reactive organomelallic substituent. Preparation of organolithium reagents can be done by one of the conventional melhods. especially halogen-metal exchange or directed lithiation. Table 14.2 gives examples. 

Two types of hydrogen replacement are discussed here (1) the base-induced hydrogen-deuterium exchange reactions and (2) the hydrogen-metal exchange reactions. 

In conclusion, it appears that in neutral or weakly acidic conditions only the methyl in the 2-position shows pseudoacidic behavior. The same conclusion can be drawn from the base-induced hydrogen-metal exchange reactions discussed in Section III.5.B. 

Each of these intermediates can be hthiated in the 2-position in good yield. The reactivity toward hthiation is due to the inductive effect of the nitrogen atom and coordination by oxygen from the N-substituent. A wide variety of electrophiles can then carry out substitution at the 2-position. Lithiation at other positions on the ring can be achieved by halogen—metal exchange 3-hthio and 5-hthioindoles have also been used as reactive intermediates. 

Magnesium haUde and alkyl magnesium haUde precipitate and the alkyl magnesium compound remains in solution. Filtration (qv) followed by drying the filtrate yields soHd magnesium alkyl (11). Another preparation method is that of metal exchange using mercury alkyl in ether. 

The trends in total world mine production rates from 1987 to 1992 are evident in Table 3. An 8-yr averaging shows ca 2% growth in annual consumption. The average price of nickel has varied from year to year the actual price more than doubled from 1985 to 1988. However, third quarter 1993 prices dropped below mid-1980 prices to < 4.50/kg. Based on the 1992 world nickel consumption level of 813,900 t and the average annual London Metal Exchange (LME) nickel price, the 1992 monetary value for the nickel mining and refining industry would be approximately 6 x 10 . ... 

The 6a-halo- and 6,6-dihalo-penicillanates have been shown to undergo halogen-metal exchange to form enolates which can then react with acetaldehyde to form 6-(l-hydroxyethyOpenicillanates (Scheme 34) (77JOC2960, 79TL3805). From (41) the isomeric products were obtained in the ratio (48) (49) (50) = 24 49 27. From (42) the isomeric products were obtained in the ratio (51) (52) (53) = 39 1.5 60. Product ratios were very... 

Pyran, 4-arylimino- C NMR, 3, 585 Pyran, 4-arylimino-2,6-dimethyl-synthesis, 3, 762 Pyran, 2-aryloxytetrahydro-X-ray studies, 3, 621 Pyran, 4-benzyl-isomerization, 3, 666 Pyran, 3-bromodihydro-synthesis, 3, 769 Pyran, -bromodihydro-halogen-metal exchange with t-butyllithium, 1, 474 Pyran, 2-bromotetrahydro- H NMR, 3, 579... 

The halogen-metal exchange reaction was pioneered by Gilman and co-... 

The halogen migration is completely suppressed by halogen-metal exchange when the chloroethynyl group is in position 5 of the pyrazole ring. The concentrations of 3-pyrazolyl and 4-pyrazolyl anions are probably small, and they cannot compete with NH2 anions for chlorine bonded to the acetylenic carbon. 

Dialkylquinolinyl boranes 83 and 86 were prepared from halogen/metal exchange of 3-bromoquinoline (70) with n-BuLi followed by quenching with Et2BOMe and Br-9-BBN, respectively. They are then coupled with bromides 84 and 87 to give 3-substituted quinoline derivatives 85 and 88, respectively (85H2375). 

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