Hydrogen halide acceptor

Compounds containing a basic nitrogen, such as 2-chloroaniline. may function as their own hydrogen-halide acceptor (4). The rate of reduction is greatly retarded by added hydrogen chloride. 

Dihydro-2//-pyrido[ 1,2-a]pyrimidin-2-one 675 has also been used in the preparation of binary photographic sensitizers as a nonbasic hydrogen halide acceptor (93EUP565074). 

It is essential in this preparation to start from the preformed tri-ethylamine-trihaloborane complex. Addition of triethylamine as a hydrogen halide acceptor at a later stage of the reaction (i.e. to the pentafluotoanilino-dihaloborane) does not yield the N-tris(perfluoroaiyl) compound. Rather, a mixture of products is obtained from which the desired compound cannot be isolated in a pure state. Fluoroorgano-borazine derivatives described so far are listed in Table 1. The table also includes pentafluorophenoxy- and pentafluoroanilinoborazines. 

A third basic procedure of synthesizing 1,3,2-diazaboracyloalkanes involves the interaction of aliphatic a, co-diamines with boron trihalides in the presence of a tertiary amine as hydrogen halide acceptor as is illustrated by Eq. 5 15>17). 

Two equivalents of amine or ammonia are needed to replace one halogen atom the amine also functions as the hydrogen halide acceptor. In some cases the aminophosphazene formed in the reaction has a base strength comparable to that of the parent amine, and it is subsequently isolated as a hydrohalide adduct (see Section IV, A,2). A tertiary base, such as triethylamine or pyridine, can also be employed as a hydrogen halide acceptor. 

Aliphatic or alicyclic fluoroformates may be prepared continuously from the reaction of the corresponding alcohols with a mixture of carbonyl halides e.g. COFj, COCIF and COClj at -20 to +80 "C in the presence of isobutene, which acts as a hydrogen halide acceptor). The process was illustrated using methanol, t-butanol and cyclohexanol as examples [1118]. 

Polyenephosphonium halides can also be reacted with carbonyl compounds in a variant of the Wittig reaction [89] in which oxiranes function as hydrogen halide acceptors. Carbonyl olefination of 79 with 58 in butylene oxide readily leads to 406, which is isolated as described above, in a yield of 83%. 

Thiol derivatives of silicon, germanium, tin and lead can readily be prepared from a halide, usually chloride, and a thiol in the presence of a hydrogen halide acceptor or a metal thiolate. Various illustrative examples... 

The thiol derivatives of the other main groups elements are often prepared from their halides using the thiol in the presence of a hydrogen halide acceptor or by using a metal thiolate, such as lead, where R is a main group element. 

Hydrogen cyanide —, elimination of — s. De-hydrocyanation Hydrogen fluoride/boron fluoride 16, 833 Hydrogen halide acceptor, formamide as — 16,174 Hydrogen iodide 16, 570, 627, 983 17,18,32,119 Hydrogenolysis (s. a. Hydrocarbons, Replacement by hydrogen)... 

Towards a simple Lewis base, for example the proton, phosphine is a poorer electron donor than ammonia, the larger phosphorus atom being less able to form a stable covalent bond with the acceptor atom or molecule. Phosphine is, therefore, a much weaker base than ammonia and there is no series of phosphonium salts corresponding to the ammonium salts but phosphonium halides. PH4X (X = Cl, Br, I) can be prepared by the direct combination of phosphine with the appropriate hydrogen halide. These compounds are much more easily dissociated than ammonium halides, the most stable being the iodide, but even this dissociates at 333 K PH4I = PH3 -t- HI... 

Alcohols react with metal hydrides, MH, and with hydrogen halides, HX, but in very different ways. Proton transfer is involved in both reactions, but different molecules act as the proton donor and acceptor. 

Mechanistic studies on this system have demonstrated that the reaction does not follow a simple stepwise condensation pathway as first assumed. Instead, the authors have proposed a divergent mechanism by which the production of 2 and 3 are linked by a common intermediate (see Scheme 3). In such a mechanism, hydrogen-bonding interactions between the donor molecular moieties and the templating halide (acceptor) play an important role. 

The nucleophilic addition of alcohols [130, 204-207], phenols [130], carboxylates [208], ammonia [130, 209], primary and secondary amines [41, 130, 205, 210, 211] and thiols [211-213] was used very early to convert several acceptor-substituted allenes 155 to products of type 158 and 159 (Scheme 7.25, Nu = OR, OAr, 02CR, NH2, NHR, NRR and SR). While the addition of alcohols, phenols and thiols is generally carried out in the presence of an auxiliary base, the reaction of allenyl ketones to give vinyl ethers of type 159 (Nu = OMe) is successful also by irradiation in pure methanol [214], Using widely varying reaction conditions, the addition of hydrogen halides (Nu= Cl, Br, I) to the allenes 155 leads to reaction products of type 158 [130, 215-220], Therefore, this transformation was also classified as a nucleophilic addition. Finally, the nucleophiles hydride (such as lithium aluminum hydride-aluminum trichloride) [211] and azide [221] could also be added to allenic esters to yield products of type 159. 

The halide catalysts are electron acceptors, and, in the absence of hydrogen halide promoter, the active complex is presumably formed by the addition of the catalyst to the olefin (Hunter and Yohe, 17 cf. Whitmore, 18) ... 

The need for a hydrogen chloride acceptor was shown by Bissinger and Kung [7], who found that primary and secondary sulfites decompose to the halide if hydrogen chloride is not removed (Eq. 8). The effect of hydrogen... 

Carbocations formed through protonation of alkenes by proton acids are usually assumed as intermediates in alkylation with alkenes. Metal halides, when free of protic impurities, do not catalyze alkylation with alkenes except when a cocatalyst is present. It was shown that no neat conjugate Friedel-Crafts acids such as HA1C14 or HBF4 are formed from 1 1 molar compositions in the absence of excess HC1 or HF, or another proton acceptor.163-166 In the presence of a proton acceptor (alkene), however, the Lewis acid halides—hydrogen halide systems are readily able to generate carbocations ... 

Although solvents may be classified as donor solvents (Lewis bases) and acceptor solvents (Lewis acids), most of the more widely used nonaqueous solvents are donor solvents. Some acceptor solvents, such as S02, BrF3, AsC13, or the liquid hydrogen halides, have proved to be useful in coordination chemistry.13"16 Ionization is promoted in a donor solvent by solvation of cations and in an acceptor solvent by solvation of anions. For example, arsenic(m) iodide is ionized in a donor solvent D according to the reaction... 

Despite its wide application in glycoside synthesis, the Koenigs-Knorr reaction suffers from several disadvantages (i) the glycosyl halides are unstable, (ii) excess toxic heavy metals are needed to activate the donor, and (iii) a desiccant (to absorb any liberated water) and an acid acceptor (to absorb the liberated hydrogen halide and can be a promoter itself) are often needed to increase the yield and suppress side reactions. 

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