Alpha-halogenation

Another group of organic halides which are especially reactive toward displacement by Grignard reagents are those with the halogen alpha to an oxygen or a sulfur. Several examples arc summarized in Table 2.3. 

Sulfur tetrafluoride [7783-60-0] SF, replaces halogen in haloalkanes, haloalkenes, and aryl chlorides, but is only effective (even at elevated temperatures) in the presence of a Lewis acid catalyst. The reagent is most often used in the replacement of carbonyl oxygen with fluorine (15,16). Aldehydes and ketones react readily, particularly if no alpha-hydrogen atoms are present (eg, benzal fluoride [455-31-2] from benzaldehyde), but acids, esters, acid chlorides, and anhydrides are very sluggish. However, these reactions can be catalyzed by Lewis acids (HP, BF, etc). 

Support-phase changes or loss of surface area are, of course, irreversible, and replacement of the catalyst may be appropriate. Catalyst damage may take the form of phase changes to the alumina support from gamma to theta or alpha phase. The last is catalyticaky inert because of insignificant surface area. Theta alumina has a low surface area (< 100 /g) relative to gamma alumina (180 m /g) and has poor halogen retention. 

Replacement of hydrogen with halogen can be carried out in the alpha position of fluorinated ethers, amines, aldehydes, or nitriles In 2,2,3,4,4,4 hexafluoro-bulyl methyl ether, chlorination occurs predominantly at the methyl, however, bromination occurs mostly at the internal position of the fluorobutyl group 133] (equation 20)... 

The action of methyl iodide on chloro-substituted heterocychcs usually results, in addition to quatemization, in the replacement of the chlorine by iodine if the halogen is in a position alpha or gamma to the quaternary center, particularly so in the former case. This type of replacement is less likely if dimethyl sulfate is used but may still occur to give a sulfate betaine (see Section IV, C). An easy method to obtain a-halogeno-quatemary salts has recently been discovered by Balli and Kersting who reacted the readily accessible triethyloxonium borofluoride 12 with a variety of bases. The very... 

Emilio Gino Segre (1905-1989 Nobel Prize for physics 1959), together with Dale Raymond Corson ( 1914) and Kenneth Ross Mackenzie ( 1912) obtained the element in tiny amounts by bombardment of bismuth with alpha particles. Halogen with no stable isotope. 

Products Derived from alpha-Halogenated Glutaric Acids," JCS, 119 (1921) 305329, "Pt. II. Products Derived from alpha-Halogenated Adipic Acids," 951970. 

Ingold, Christopher K. "The Conditions Underlying the Formation of Unsaturated and of Cyclic Compounds from Halogenated Open-Chain Derivatives. Pt. I. Products Derived from alpha-Halogenated Glutaric Acids, Pt. II. 

American chemists Dale R. Corsun, K. R. Mckenzie, and Emilio Segre Member of the halogen group and similar to iodine first produced by bombarding bismuth with alpha particles, but results naturally as uranium and thorium isotopes decay. 

P. Bey, J.P. Vevert, V. Van Dorsselaer, M. Kolb, Direct synthesis of alpha-halogen-omethyl-alpha-amino acids from the parent alpha-amino acids, J. Org. Chem. 44 (1979) 2732-2742. 

Aromatics with Halogenated Side Chain Aromatics with Halogenated Side Chain Benzyl Chloride Chlorotoluene, Alpha DDT Trichloro(chlorophenyl,4-bis) Ethane Arsenic... 

Benzyl chloride Chlorotoluene, alpha under Aromatics with Halogenated Side Chain Beryllium... 

In 1940 D. R. Corson, K. R. Mackenzie, and E. Segre at the University of California bombarded bismuth with alpha particles (26, 27). Preliminary tracer studies indicated that they had obtained element 85, which appeared to possess metallic properties. The pressure of war work prevented a continuation of these studies at the time. After the war, the investigators resumed their work, and in 1947 proposed the name astatine, symbol At, for their element. The name comes from the Greek word for unstable, since this element is the only halogen without stable isotopes (28). The longest lived isotope is At210 with a half-life of 8.3 hours and a very high activity. 

Another previously unknown element, astatine (element 85, the heaviest of the halogens) was made at Berkeley in 1940 by bombarding bismuth with alpha particles. Again, Segre was among the team of chemists who showed it was a new element. 

Hypohalites IRO-Hal) are similar to nitrates (see p. 155 in their photochemical behaviour. Ultraviolet irradiation gives an (n,Ji > excited state that cleaves to form an alkoxy radical and a halogen atom. The radical may undergo alpha-cleavage before recombination with the halogen atom occurs, and this accounts for the formation of 5-iodopentanal (5.69) from the hypoiodite of cydopentanol such hypoiodites are generated in situ from the alcohol. Iodine and merturyfll oxide. In open-chain systems the alkoxy radical can... 

As a metal, Np has a relatively low melting point ( -640°C), is very dense (20.45 g/cnr), and is ductile. The alpha form reacts with hydrogen, carbon, oxygen, sulfur, the halogens, and phosphorus to yield a number of binary compounds. 

Halogenation of saturated aldehydes and ketones usually occurs exclusively by replacement of hydrogens alpha to the carbonyl group ... 

This bromination reaction results exclusively in alpha substitution and therefore is limited to carboxylic acids with a hydrogens. Chlorine with a trace of phosphorus reacts similarily but with less overall specificity, because concurrent free-radical chlorination can occur at all positions along the chain (as in hydrocarbon halogenation see Section 4-6A). 

Several of the alpha-ethyl analogues of ARIADNE were N,N-dialky lated, and were target compounds for halogenation with radio-iodine or radio-fluorine, for evaluation as potential brain blood-flow indicators. In these studies, all examples followed a common flow diagram. The reaction of the appropriate benzaldehyde... 

Halogens add to ketones at the alpha carbon in the presence of a base or an arid-When a base is used, it is dffieult to prevent haJogdnation at more than one of the alpha positions. The base is also consumed by the reaction with water as a by-product, whereas the acid acts as a true catalyst and is not consumed. 

When a base is used with a methy] ketone, the alpha carbon will become completely halogenated. This trihalo product reacts further with the base to produce a carboxylic acid and a haloform (chloroform, CHCI j bromoform, CHBr, or iodoform, CHIj). This is called the Haloform Reaction. 

The structural feature was later applied to alpha-halogenated calixarenes, allowing extraction from more acidic media.212... 

C-6-Nonsubstituted penicillin derivatives have received considerable attention as a new 6< ta-lactamase inhibitor. In this connection, reductive removal of the halogen atoms attached to the alpha-carbon atom of the 6efa-lactam ring has been performed. Electroreduction of bromo- and chloro-penicillins 59 (n = 0-2) in DMF/AcOH/... 

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