Brominolysis

The rates of brominolysis of a series of 1,2-diarylcyclopropanes under conditions where the rate is determined by Br- attack and leads to l,3-dibromo-l,3-diarylpro-panes are given below. 

Another synthesis, described in detail in Scheme 12, was devised specifically for the introduction of deuterium at both ends of l-deoxy-D-t/ireo-pentulose.21,22 Stannylene methodology was used twice, first for glycol splitting with phenyliodonium diacetate, under strictly neutral conditions (necessary to preserve the benzylidene acetal), and secondly to convert the sequence -CHOH-CD3 to CO-CD3 by brominolysis. The final, labeled pentulose was l-deo y-D-threo-(1-2H3, 5-2H)pentulose. 

Eq. 34). The resultant p-mercurioalkyl peroxides can often be demercurated with sodium borohydride (Eq. 35), or by brominolysis (Eq. 36) without substantial cleavage of the 0-0 bond. Both peroxymercuration and demercurations occur rapidly under mild conditions 48). 

In the first systematic study on nucleophilic substitutions of chiral halides by Group IV metal anions, Jensen and Davis showed that (S )-2-bromobutane is converted to the (R)-2-triphenylmetal product with predominant inversion at the carbon center (Table 5)37. Replacement of the phenyl substituents by alkyl groups was possible through sequential brominolysis and reaction of the derived stannyl bromides with a Grignard reagent (equation 16). Subsequently, Pereyre and coworkers employed the foregoing Grignard sequence to prepare several trialkyl(s-butyl)stannanes (equation 17)38. They also developed an alternative synthesis of more hindered trialkyl derivatives (equation 18). 

Like chlorinolysis, the brominolysis and iodinolysis of diorganyl ditellurides offer a facile route to tellurium tribromides and triiodides. ... 

Regioselective oxidation by brominolysis of dioxastannolane obtained from methyl / -L-ara-binopyranoside made it possible to accomplish (after subsequent oximation and reduction) a facile synthesis of 4-amino-4-deoxy-L-arabinose, an amino-sugar found in lipopolysaccharides of some Salmonella R. mutant strains <83CPB3778>. 

The intervention of the 1,2-methoxonium ion (141) was postulated, leading to compound 143. However, the main course of reaction in the brominolysis of 140 was rationalized by invoking an elimination to give an intermediate enol acetate (142), which subsequently underwent acetoxybromination to form 144. 

The generation of hydroxy ketones by the brominolysis of stannylenes has been used several times in total synthesis. Experiment G [see Eq. (15)] describes a key step in the total synthesis of the antibiotic (+)-spectinomycin [12,13]. It is remarkable that the two oxygen atoms bound to the tin atom originate from hydroxyl groups, which are port of different functions, a hemiketal and a secondary alcohol. The oxidation is selective for one alcoholic function ont of three. The same product was obtained by W-bromosuccinimide oxidation of the tributylstarmyl ether. 

Bromoboration (Z)-/>2-dihalo-l -alkenes1 Reaction of a terminal alkyne (1 -octyne) with BBr3 in CH2C12 results in a p-bromoalkenylborane, which undergoes brominolysis or iodinolysis with retention to provide (Z)-l,2-dihalo-l-alkenes (equation I). 

The stereochemical course of the brominolysis of cis- and traMJ-4-methyl-cyclohexylmercuric bromide (I) and of optically active sec.-butylmercuric bromide (II) has been comprehensively studied by Jensen et al.1,2. Some of their results... 

More recent results3,4 on the brominolysis of optically active sec.-butylmercuric bromide are in accord with the results obtained by Jensen et al. details are given in Table 1. 

SECOND-ORDER RATE COEFFICIENTS (I.mole-1.SeC ) AND ACTIVATION PARAMETERS FOR THE BROMINOLYSIS OF BENZYLMERCURIC CHLORIDE BY BROMINE IN THE PRESENCE OF BROMIDE ION29... 

BROMINOLYSIS OF JPC.-BUTYLMERCURIC BROMIDE BY BROMINE IN SOLVENT CARBON TETRACHLORIDE... 

The above brominolysis was shown to follow overall first-order kinetics (first-order in bromine and zero-order in sec.-butylmercuric bromide) under constant illumination, and presumably proceeds by some free-radical mechanism3. 

Addition of a small quantity of methanol to the carbon tetrachloride results in the rate of brominolysis becoming independent of the degree of illumination,... 

Nearly all of the kinetic work in this field is due to Gielen and Nasielski and to Tagliavini, who have investigated the brominolysis and iodinolysis of tetraalkyltins using a variety of solvents. Since the kinetic methods and the results depend largely on the particular solvent, the following discussion is drawn up on the basis of the solvents employed. 

BROMINOLYSIS OF TETRAALKYLTINS AND TRIALKYLTIN BROMIDES BY BROMINE, AND BROMINOLYSIS OF TETRA ALK YLTINS BY Br2/Br-, IN... 

The values of k2 and k°2s given in Tables 16 and 17 for brominolysis of tetra-n-propyltin and tetra-n-butyltin enable a value for K, the equilibrium constant for reaction (32), to be estimated using equation (31). Under the conditions given in Table 17, K is found to be 82 l.mole-1, in fair agreement with the reported22 value of 52 l.mole-1. at 25 °C (see Table 2, p. 139). 

The brominolysis of a number of unsymmetrical tetraalkyltins was later investigated3 5, and rate coefficients for cleavage of given alkyl-tin bonds determined by the methods described previously. In this later work35, a value of k°2s = 1.42... 

OBSERVED SECOND-ORDER RATE COEFFICIENTS, /c bs, FOR BROMINOLYSIS OF TETRAALKYLTINS BY BROMINE IN SOLVENT ACETIC ACID AT 20 °C IN THE PRESENCE OF 0.1 M... 

Gielen and Nasielski43 suggested that reaction (37) proceeded through a cyclic transition state of the SE2(cyclic) type, for which we may write (X) as a possible structure. In view of later work35 on the related brominolysis of tetraalkyltins in solvent chlorobenzene, it would seem possible that reaction (37) might proceed by mechanism SE2(co-ord) (cf reactions (41) and (42), p. 158). 

The brominolysis of a number of unsymmetrical tetraalkyltins was also studied35 and second-order rate coefficients for the cleavage of given alkyl-tin bonds determined by combination of rate studies with product analyses. Details are given in Table 21 and it should be noted that for all of the unsymmetrical tetraal-kyls listed in this table only the second-order term in equation (40) is of importance35. It is convenient now to discuss the two terms in equation (40) separately. 

We might, therefore, use these values of AT(46) to provide an estimate of substituent effects on the values of K(Al), whilst using the data in Tables 21 and 22 to deduce substituent effects on the values of k1 and on the overall rate coefficient, k°2bs. At the same time, it is convenient also to refer to Table 24, p. 164, which contains data on the related brominolysis of tetraalkyltins in the non-polar solvent carbon tetrachloride. The total substituent effects may be broken down into three sections. 

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