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Oxidation of Aldoses with Bromine! Horace S

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Oxidation of Aldoses with Bromine! Horace S _., JOURNAL OF RESEARCH of the National Bureau of Standa rds- A. Physics a nd Chemistry Vol. 66A, No.3, May- June 1962 Oxidation of Aldoses With Bromine! Horace S. Isbell (February 12, 1962) Rates of oxidation of aldoses with bromine have bee n reappraised and interpreted in th e light of present concepts of conformation and reaction mechanism. It is suggested that differences in t he rates of ox id ation of the a and /3 anomers are largely determined by differences in the free energy req uired by the reactants for passing from the ground state to the complex in the transition state. Structures for t he aldoses in the ground states and in the transition states are postulated, a nd factors affectin g the energy required for reaching the transition states from t he ground states are di scussed. The relative ra tes of ox idation are in accordance with the hypothesis that cach of the aldoses in the ground state has the conformation predicted by Reeves, and, in the transit ion state, has a conformation in which the oxygen atom of the C1-hydroxyl group lies in the plalle formed by t he r in g oxygen atom, C1, C2, and C5. Presumably, this conformation is stabilized by resonance in volving the oxygen atom of the ring. For a ldoses havin g high stability in one cha ir conformation, the rates of oxidat ion of the anomers d iffer widely; in each in stance, t he anomer in which the Cl-hydroxyl group is axial is ox idized more slowly t han the anomer in which this group is equatorial. For aldoses having less stabil ity in a cha ir conformation, the rates of oxidation of t he a nomers difrer lcss widely, but, nevertheless, show a defin ite corrclation with the angular posit ion of the Cl-hydroxyl group relative to the plane of t he ring. For aldoses for which the stabili ty in both chair conformations is so low that they probably exist in a variety of conformations, the rates of ox idat ion of the anomers show little difference and no particular correlation with the angular position of the Cl-hydroxyl group. The presence 0 " absence of an oxygen atom in the rin g is used to account for the large diff erences between the rates of bromin e ox idation of t he aldoses a nd those of deri vati ves of eyclohexanol. Differences in conformation in the t ra nsition statp, associated with t he presence or absence of this oxygen atom, likewi se accoun t for t he fact that t he relative rates of oxidation of t he axial and equatorial isomers in the two classes of compound a re reversed. Because of uncerta in ty as to the anomeri c configurations co mmonl.v assigned to son1(' of t Il e aldoses, the co nfigurations of 22 aldoses were reappra ised. Advantage was taken of t he principle that the anomer p reponderatin g in the equili brium solu tion has l1'ans hydroxyl groups at Cl and C2. Except for crystalli ne D-glyceTo -D-ido-heptose , the assignments of con fi guration based on this p ri nciple agree with t he configurations genera ll y accepted. Classifi ciation of cr ys talline D-glyceTo-D-ido-heptose as an a-D-pyranose necessitates co rrection of ea dier reco rds in which t hi s suga r was co nsid ered to be a /3 -D-py ranose. In accorda nce with the author's earli er formulation, oxid ation of the axial a nomer is beli eved to take place by two co urses: (1 ) di rect ox id ation and (2) conversion to the eq uatori al a nomer by the anomerizat ion reaction and the subsequent oxid ation of this ano mer. The relative importa nce of the two courses is not considered in thi paper . It is poin ted out, however, that the actual difference in t he rates for t he direct ox id ation of the two anomers must be at least as great as that observed For the overall rates of oxidation. 1. Discussion of Prior Work plane or the ring, and Lha.t "a comparison or the reaction rates or alpha fLnd beLa pyranoses should ULilization of bromine-oxidation measurem ents provide inform ation about the con formation of for st u d~T of the ring structure of aldoses began with the pyranose ring" (p . 524 of [6]) . Subsequently, the observation of Isbell and Hudson [1,2] 2 that, through the brillian t work of R eeves [7 ,8], H asscl in slightly acid solution, pyranoses are oxidized to and Ottar [9], Beckett fLnd co-workers [10], fLnd 1,5-la,ctones without cleavage of the ring. Isbell others, factors that affect tho conrormation or thc and Pigman [3,4,5 (pp. 435 to 456)] showed that, pyrfLnoses have become known. Theoretical con­ for many aldoses, the a and (3 anom ers are oxidized siderations fLS well as extensive experimental (hta at widely different rates, and Isbell [6] sought to have established that Isbell fLnd Pigman's slowly classify the anomers of aldoses according to the oxidizable "alpha" anomers have fL chair con form a­ angular position of the Cl-hydroxyl group relaLive Lion wiLh an ftxi al C I-hydroxyl group and their to Lhe phne of a strainless, pyranoid ring. \iVhen rapidly oxidizable "beta" anomers have a chair those studies were made, the concept of conform ation conform ation with ftU equatori.al Cl-hydroxyl group WflS just beginning to emerge. Nevertheless , it [11]. was concluded that the aldopyrfLnoses exist in a Kinetic studies have shown that oxidfttion by defi. nite conformfLtion in which the a and (3 positions bromin e is a complex process. Bunzcl and Mftthews are not s:rmmetricfLlly located with respect to the [12] found that the reaction is rapid in neutral solu­ tion and slow in fLcid. Perlmutter-lIaym fLn and 1 Presen ted, in part, before the Division of Carbohyd rate Chemistry at the P ersky [13] recently eonfu'm ed th e work' of Bunzel l32d M ee ting of the American Chem ica l Society, at New York, N. Y., 0 11 Se p tem­ and Mathews and presented evidence L]mt the ber 8, 1957. ' Figures in brackets indicate the literature references at the end ofthi5 paper. anionic form of D-glu cose is oxidiz ed much faster 233 than D-glucose itself. Bunzel and Mathews also C4 established that the rate of oxidation of pre-equili­ BH ___ _ ~6' C 2 0 /\ C5 brated D-glucose is first order with respect to the I ~ aldose and fr ee bromine. Several investigators have /' I C3 advanced hypotheses to explain the difference in the ~ , J~ rates of oxidation for the a and {3 anomers. Isbell 'Br ' and Pigman [4] showed that oxidation of the sepa­ rate anome1"S is complicated by the simultaneous 2 C4 3 anomerization r eaction. In the oxidation of a-D­ BH -----O-::'\ ~. C5 glucose, form ation of the more reactive {3 anom er and ICI oxidation thereof raises the overall rate of oxidation; :21 C3 in the oxidation of {3-D-glucose, formation of the less H___ f'. - Br----·Br reactive a anomer lowers the overall rate. Thus, th e actual difference in the rates for the direct oxida­ tion of the two anomers must be at least as great as C4 the observed difference in the overall rates. Barker, ,.0 C~ ° /\ C5 Overend, and R ees [14] consider the values for the / ~ H/ : C3 differences in the overall rates to be of little sig­ ,,_____ (H nificance because of the anom eriz ation of the slowly 'Br-l__ Br ) ~ oxidizable an omers, which, the~T believe, takes plaee nearly co mpletely b efore oxidation. Bentley [15] FIGU RE 1. H ypothetical t?"ansilion slales. suggested that the higher activity of the anom e1'S in (a) Intramolecular, base-catalyzed mechanism. whieh the C1-hydroxyl group is in an equatorial (b) Intermoleenlar, base-catalyzed llleehanism. (c) Intra molec ular, concerted mechaniSlll. position arises from the release of strain in the rate­ 1 determining step. H e also noted that the ratios of rates of oxidation for the anom eric aldoses appear to experimen tal data. Structures (a) and (b) arc b e related to th e conform ational stability of the symbolic of either a fast removal of a proton from 1 aldoses. The subj ect was discussed in an excellent the C1-hydroxyl group followed by a rate-determin­ review by Shafiz adeh [16] . ing hydride transfer from C1, or a partly concerted Various luechanisL11s have been proposed for the process in which the proton removal is not a com­ oxidation of aldoses with bromine. Isbell and pletely separate step. Structure (b) is similar to Pigman [4] suggested rapid addition of free bromine one proposed by Swain, Wiles, and Bader [21] for to the ring oxygen atom, followed by slow elim.ination the transition state in the oxidation of 2-propanol 01' hydrogen bromide at a rate depending on the stereo ­ with bromine. Structur e (c) is similar to an inter­ merie position of the C1-hydrogen atom. Lichten mediate proposed by Bark er , Overend, and Rees [22] and Saxe [171 postulated rapid addition of bromine for the oxidation of tert-butyl cycloh exanol, but differs to the anomeric oxygen atom; this is followed by in certain res pects to be discussed in section 3.
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