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Journal of Molecular Structure: THEOCHEM 802 (2007) 11–16

Theoretical investigation of the reducing capacity of sodiumborohydride and sodium acetoxyborohydride derivatives

Melina Alexandre Machado, Antonio Claudio Herrera Braga, Rogerio Custodio *

Instituto de Quımica, Universidade Estadual de Campinas, Barao Geraldo, CP 6154, 13084-971 Campinas, Sao Paulo, Brazil

Received 17 June 2006; received in revised form 16 August 2006; accepted 4 September 2006Available online 8 September 2006

Abstract

The reducing capacity of sodium borohydride and its acetoxy derivatives was studied. Density functional theory using four differentfunctionals were used to investigate the enthalpies, charges and molecular structures of four distinct reactions associated with hydriderelease. The theoretical results in the gas phase reinforce the experimental observations that the acetoxyborohydride derivative reducingcapacities are a consequence of both the inductive electron-withdrawing ability of the acetoxy group and the steric bulk surrounding theBAH bond. The electron acceptor effect of the acetoxy group provided a linear relation between the boron GAPT charges and the enthal-py necessary to remove the hydride from sodium borohydride, justifying the smaller or even nonexistent reductor capacity of moresubstituted borohydrides.� 2006 Elsevier B.V. All rights reserved.

Keywords: Borohydride; Acetoxyborohydride; Density functional theory; Enthalpy; Substituent effect; Reduction efficiency

1. Introduction

The use of alkali metal borohydrides as reducing agentsin organic chemistry is well documented. Sodium borohy-dride (NaBH4), since the initial preparation in 1946, hasbeen used in the reduction of many different organic com-pounds [1]. Due to its hydride donor properties, sodiumborohydride was initially believed to be incompatible witha highly acidic medium. However, in 1963, the use of acombination of sodium borohydride and acetic acid wasreported [2]. After the initial report, the combination ofsodium borohydride and carboxylic acids has become aversatile reagent for organic synthesis [3]. Thus, the pio-neering work of Gribble [4] on sodium borohydride reduc-tions in anhydrous carboxylic acids was very importantand showed that many reductions, which either do notoccur or result in poor yields with sodium borohydride

0166-1280/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2006.09.001

* Corresponding author. Tel.: +55 19 35213104; fax: +55 19 35213023.E-mail address: roger@iqm.unicamp.br (R. Custodio).

alone, take place readily when a carboxylic acid is usedas solvent or is present in stoichiometric amounts.

The combination of sodium borohydride and carboxylicacids has yielded interesting series of reducing agents.These acyloxyborohydride species efficiently reduce andN-alkylate an enormous series of functional groups [4].

We have long had interest in the combination sodiumborohydride/glacial acetic acid [5]. This combination pro-duces acetoxyborohydride species of different hydride-do-nating abilities that can be analyzed as a consequence ofboth the inductive electron-withdrawing ability of the acet-oxy group, which strengthens the BAH bond, and the stericbulk surrounding the BAH bond. These data indicate theorder of decreasing hydride-donating ability shown inScheme 1.

Despite the wide application of these reagents, very littleeffort has been made to understand quantitatively the effi-ciency of the reducing capacity of the species generated bythe combination sodium borohydride and acetic acid. Inorder to understand more about this reducing system, weestablished as our objective an analysis of the hydride-do-nating ability of the several acetoxyborohydride species

H3BOAc- > H2B(OAc)2- > HB(OAc)3

-

Scheme 1. Decreasing order of hydride-donating ability.

12 M.A. Machado et al. / Journal of Molecular Structure: THEOCHEM 802 (2007) 11–16

obtained from this combination, using density functionaltheory.

2. Computational methods

In this work density functional theory was used to opti-mize the molecular structures and changes in enthalpies ofreactions involving substituted borohydrides.

Four different reactions were investigated to analyze theenthalpy change in possible combinations of acetoxybora-ne derivatives and sodium hydride. The reactions are iden-tified as (where A represents species having boron):

reaction I : AH�ðgÞ ! AðgÞ þH�ðgÞ DH 0298KðIÞ

reaction II : AHNaðgÞ ! AH�ðgÞ þNaþðgÞ DH 0298KðIIÞ

reaction III : AðgÞ þNaHðgÞ ! AHNaðgÞ DH 0298KðIIIÞ

reaction IV : AðgÞ þNaHðgÞ ! AH�ðgÞ þNaþðgÞ DH 0298KðIVÞ

Reaction I represents the dissociation enthalpy of the boro-hydride anion (AH�) forming hydride (H�) and the neutralboron compound (A) and does not consider the presence of acation. Reaction II considers dissociation of sodium borohy-dride and sodium acetoxyborohydride derivatives, forming aborohydride anion or an acetoxyborohydride derivatives(AH�) and a sodium cation. Thus, reaction II reflects theaffinity of the borohydride anion or the acetoxyborohydridederivatives for the counter-ion. Reaction III is related to theinverse of reaction II. It represents the affinity of boron com-pounds for sodium hydride or, from the opposite direction, itrepresents the dissociation of sodium borohydride into aboron compound and sodium hydride. Reaction IV repre-sents hydride transfer from sodium hydride to the boroncompound, forming the borohydride anion or acetoxyboro-hydride derivative anions and a sodium cation.

The enthalpies of reactions I, II and III were calculatedfrom the differences in the absolute enthalpies (H) of prod-ucts and reagents:

DH 0298 K ¼

XH 0

products �X

H 0reagents ð1Þ

The enthalpy of reaction IV was calculated by applyingHess’ law using enthalpies from reactions II and III. Theabsolute enthalpy of each compound was calculated usingthe well-known equation:

H 0298 KðXÞ ¼ EelecðXÞ þ Evib

298 KðXÞ þ Erot298 KðXÞ

þ Etrans298 KðXÞ þ pV ð2Þ

where Eelec(X) represents the electronic energy plus thenuclear repulsion of compound X. The structures of theneutral and ionic species are fully optimized at differentlevels of theory. Evib is the vibrational energy computedusing the harmonic approximation, standard statisticalmechanical formulas and information from density

functional calculations. Erot and Etrans are rotational andtranslational contributions, respectively, and were comput-ed classically. The pV contribution is replaced by nRT,using the ideal gas law, where n is the stoichiometric coef-ficient of the chemical reaction, R is the ideal gas constantand T the temperature in Kelvin. The temperature is main-tained in 298 K for all the calculations.

The compact effective core potentials (CEP) developedby Stevens, Basch and Krauss [6] were used along with fourdifferent functionals for all systems: B3LYP, B3PW91,PBEPBE and LSDA (Vosko, Wilk and Nusair correlationfunctionals and the Slater exchange functional with a the-oretical coefficient of 2/3). The anionic character of somespecies and the excellent results obtained for proton affini-ties of similar species [7] suggest the use of the CEP-31++G**basis set. Sets of 5d functions are used whenpolarization is considered using either CEP. The effect ofall electrons in the optimized structures and enthalpies ofthe four reactions was tested only with the B3LYP func-tional. All the calculations were carried out with the Gauss-ian/2003W program [8].

Two different methods were used to determine the atom-ic charges of the compounds: (a) Mulliken population anal-ysis and (b) the generalized atomic polar tensor (GAPT)[9,10]. GAPT considers that the effective atomic chargesare obtained from the isotropically averaged atomic polartensor:

QA ¼1

3

olx

oxA

þoly

oyA

þ olz

ozA

� �ð3Þ

where QA is the effective charge of atom A, lx, ly and lz

are components of the dipole moment and xA, yA and zA

are the Cartesian coordinates of nucleus A. Applicationsof GAPT have presented interesting correlations withexperimental properties allowing the interpretation of ten-dencies through simple physical pictures [11]. Anotherimportant aspect is that GAPT has proved to be not verysensitive to the basis set or correlation effects [10]. There-fore the correlation between atomic charges and changesin enthalpies may be analyzed.

3. DFT calculations and enthalpy accuracies

Table 1 shows the results of enthalpies for the four reac-tions studied, differentiating the effect of borane and substi-tuted boranes with one, two and three acetoxy groups.Before analyzing the aspects related to the process ofhydride transfer in substituted boron composites, it is con-venient to evaluate the dependence of the results obtainedwith different basis sets and functionals. Experimentalenthalpies for the four reactions were not available, whichchallenges a quantitative evaluation of the theoreticalenthalpies and the recommendation of specific functionalsand basis sets to be used in these cases. However, the quan-titative behavior of the calculated data allows estimation ofthe degree of confidence of the results.

M.A. Machado et al. / Journal of Molecular Structure: THEOCHEM 802 (2007) 11–16 13

Table 1 shows the great dependence of enthalpies for thefour reactions with the inclusion of diffuse functions andeffects originating from the use of specific functionals.The most significant discrepancies among the calculatedenthalpies, originating from regarding the average of allresults, occur from application of the B3LYP/CEP-31G(d,p) and LSDA/CEP-31++G(d, p) levels. The formerincludes effects of correlation and exchange from thehybrid B3LYP functional, but does not include diffusefunctions. The last also includes these diffuse functionsand exchange effects from approximate Slater functionalswith a theoretical coefficient of 2/3. More significant devi-ations regarding the average are obtained from the LSDAfunctional than with calculation using B3LYP and no dif-fuse functions. A similar behavior is obtained in calcula-tions carried out with the other three functionals,suggesting that LSDA calculations have to be used withcaution for these systems.

Arbitrarily considering the data obtained from theB3LYP/CEP-31++G(d,p) calculations as reference for thefour reactions, it was verified that the differences betweenthe calculations using the same basis set and PBEPBE andB3PW91 functionals or B3LYP calculations with aug-cc-pVDZ augmented all electron basis sets present acceptableagreement. For reaction I the average deviation regardingB3LYP/CEP-31++G(d,p) calculations corresponds to�2.1 kcal/mol (B3LYP/aug-cc-pVDZ), �5.00 kcal/mol(PBEPBE/CEP-31++G(d,p)) and �3.3 kcal/mol (B3PW91/CEP-31++G(d,p)), while B3LYP/CEP-31(d,p) and

Table 1Theoretical enthalpies (in kcal/mol) for reactions involving borane and acetoxyof 298.15 K

Method Reaction BH3

B3LYP cep-31G(d,p) I 59.79II 133.02III �53.08IV 79.94

B3LYP cep-31++G(d,p) I 67.54II 126.55III �53.84IV 72.71

B3LYP aug-cc-pVDZ I 71.82II 129.80III �53.51IV 76.29

B3PW91 cep-31++G(d,p) I 71.67II 127.50III �58.10IV 69.40

PBEPBE cep-31++G(d,p) I 73.71II 127.14III �60.60IV 66.54

LSDA cep-31++G(d,p) I 82.34II 126.26III �69.46IV 56.80

LSDA/CEP-31++G(d, p) present and average deviation of6.5 kcal/mol and �12.6 kcal/mol, respectively. A similartrend is observed for reaction III where the deviation withrespect to B3LYP/aug-cc-pVDZ, PBEPBE/CEP-31++G(d,p) and B3PW91/CEP-31++G(d,p) calculations are1.9 kcal/mol, �4.0 kcal/mol and �3,3 kcal/mol. ForB3LYP/CEP-31 G(d,p) and LSDA/CEP-31++G(d,p) cal-culations the differences are significantly larger with devia-tions of 5.1 kcal/mol and �12.9 kcal/mol, respectively.Reaction IV corresponds to a combination of enthalpies ofreaction II and III, which justifies deviations with the sametrends as these reactions. The average deviations usingB3LYP/CEP-31G(d,p) (�6.0 kcal/mol), B3LYP/aug-cc-pVDZ (�5.7 kcal/mol), PBEPBE/CEP-31++G(d,p)(�5.0 kcal/mol) and B3PW91/CEP-31++G(d,p) (�2.5 kcal/mol) are similar. It was verified that the combination ofreactions II and III increases the deviations for B3LYP cal-culations with respect to the all electron calculations andthe results with the PBEPBE functionals. On the other hand,the calculations without diffuse functions preserve theiraverage deviations, in relation to B3LYP/CEP-31++G(d,p) calculations, of around 5–6 kcal/mol.

In general, the consistency of the results suggest the useof diffuse functions and considers that the functionals used,with exception the LSDA functional, present lesser devia-tions among themselves of between 5 and 6 kcal/mol. Thissignificant difference also suggests the use of more rigorouscalculations for the attainment of more accurate results forthe species involved. Since the calculated enthalpies are far

borane derivatives obtained from different levels of theory at a temperature

BH2OAc BH(OAc)2 BH(OAc)3

61.14 68.57 75.59119.85 140.16 139.20�41.26 �69.01 �75.07

78.59 71.15 64.13

67.98 73.77 81.84134.11 134.50 133.51�61.84 �68.02 �75.10

72.27 66.48 58.42

70.32 75.19 82.32137.99 138.16 137.96�60.20 �65.24 �72.18

77.79 72.92 65.78

71.37 76.91 84.34135.10 134.82 134.40�65.41 �70.66 �77.67

69.70 64.16 56.73

73.24 78.83 85.32133.17 132.19 132.01�66.16 �70.78 �77.08

67.01 61.42 54.93

79.95 87.17 91.97133.49 131.61 134.29�74.30 �79.63 �87.11

59.20 51.98 47.18

14 M.A. Machado et al. / Journal of Molecular Structure: THEOCHEM 802 (2007) 11–16

larger than the observed deviations and the relative valuesbetween borane and tri-substituted borane are larger than5 kcal/mol for the same reaction (see Table 1), the analysisof the enthalpy trend can proceed without compromisingthe prediction of compound reactivity.

4. Borohydride dissociations

Considering the accuracy presented by the above results,in the present section the trend of enthalpies will be evalu-ated for the four reactions analyzing only the results pre-sented in Table 1 obtained with B3LYP, PBEPBE andB3PW91 functionals and the CEP-31++G(d,p) basis set.

Since the reducing trend proposed by Gribble [4] focus-es on release of the hydride ion by borane and substitutedboranes from borohydride and substituted borohydride,our initial analysis will concentrate on reaction I. Table1 shows that the process of releasing hydride ion fromthe borohydride anion (reaction I) is an endothermic pro-cess with enthalpies oscillating between 67 kcal/mol(B3LYP) and 74 kcal/mol (PBEPBE). The substitutionof a hydrogen in the borohydride anion by a single acet-oxy group, which is an acceptor of electrons, numericallydoes not introduce a significant effect in terms of enthalpystability. The enthalpy variation remains nearly the same,suggesting that the influence of acetoxy as an electronacceptor and the steric effect presented by the carbonylgroup in the neighborhood of the hydride acceptor centerare small.

The substitution of another hydrogen bonded to boronby a second acetoxy group provides an enthalpy increase of5.0 kcal/mol, reducing the ease of release of the hydrideion, independent of the functional used. For example, theenthalpy calculated using the B3LYP method is modifiedfrom 68.0 kcal/mol for the reaction involving BH2OAc to73.8 kcal/mol for the reaction with BH(OAc)2. The samedifference can be observed using either PBEPBE orB3PW91. The substitution of a third hydrogen bonded toboron produces a more significant variation of enthalpy,being around 8.0 kcal/mol for B3LYP, 7.4 kcal/mol forB3PW91 and 6.5 kcal/mol for PBEPBE. This large enthal-py variation for reaction I with the increases in the numberof acetoxy substituents is a direct consequence of the

Table 2Mulliken and GAPT atomic charges on boron and hydrogen bonded to borontheory with CEP-31++G(d,p)

Method QMulliken

ðBH�4 ÞQMulliken

(BH2OAc�)QMulliken

ðBHðOAcÞ�2 ÞQMullik

ðBHðOB3LYP B 0.176 0.354 1.218 0.985

H �0.294 �0.158 �0.117 0.417

B3PW91 B 0.045 0.267 1.141 0.802H �0.261 �0.138 �0.112 0.405

PBEPBE B �0.117 0.193 1.088 0.657H �0.221 �0.109 �0.093 0.433

interaction effect of hydrogen bonded to boron with thesubstituent.

Table 2 shows the Mulliken and GAPT charges calculat-ed using the three functionals. GAPT charges of the boronatom calculated by any of the three functionals indicate asystematic reduction of electronic density on boron as thenumber of acetoxy substitutes increases. The oppositeoccurs for the GAPT charges on hydrogen atoms bondedto boron indicating a deficiency of hydrogen electronicdensity with the increase in the number of substituents.Mulliken charges present the same trend for hydrogenbonded to boron, even though the charges on the boronatom present larger electronic densities for the three acet-oxy substituents.

These results indicate that the larger the number of acet-oxy substituents the more difficult the elimination of thehydride, because significant transfers of the electron densi-ty occur towards the acetoxy substituents. These results arein agreement with the proposal of Gribble regarding a less-er hydride-donating character with the increase of the num-ber of acetoxy groups [4]. Experimentally, a better reducingenvironment is observed for the hydrides with smallernumbers of acetoxy substituents, whereas for the largestnumber of acetoxy substituents the reactivity is significant-ly lower. This was observed clearly when analyzing theresults obtained in b-enamino ketone reductions. Whenthe reaction is carried out using NaBH4/HOAc, we obtainproducts from total reduction (c-amino alcohols), however,when the reducing agent used is NaBH(OAc)3/HOAc, apartial reduction is observed and the products are the cor-responding Mannich bases [5].

Curiously, the most stable structure for the triacetoxy-borohydride anion, BHðOAcÞ�3 , presents the hydrogenatom bonded to boron close to the carbonyl oxygen atoms.Intuitively, the expected structure for this conformer sug-gests a larger distance of the hydrogen with respect to thecarbonyl oxygens. However, the considerable charge reduc-tion of the hydrogen in the anion favors an approachbetween this atom and the carbonyl oxygens. This particu-lar conformation justifies the highest stability of the tri-substituted borohydrides in the presence of a counter-ionsuch as sodium, suggesting that, in this case, the steric bulkis a consequence of better electronic distribution (Fig. 1).

in borane and substituted boranes, obtained from three different levels of

en

AcÞ�3 ÞQGAPT

ðBH�4 ÞQGAPT

(BH2OAc�)QGAPT

ðBHðOAcÞ�2 ÞQGAPT

ðBHðOAcÞ�3 Þ0.617 0.950 1.284 1.625�0.404 �0.360 �0.344 �0.269

0.555 0.901 1.242 1.589�0.390 �0.350 �0.338 �0.262

0.481 0.842 1.176 1.537�0.370 �0.339 �0.327 �0.255

Fig. 1. Two different views of the three-acetoxy borohydride anion structure calculated at the B3LYP/CEP-31++G(d,p).

M.A. Machado et al. / Journal of Molecular Structure: THEOCHEM 802 (2007) 11–16 15

Thermochemically the formation of hydrides in vacuumcan be considered from different alternatives. The diagrampresented in Fig. 2 corresponds to the enthalpies of the fourreactions of the processes mentioned above. The samequalitative behavior is observed for Gibbs free energiesshowing that the stability of the path can be discussed interms of enthalpies. In general, the reaction between bor-ane or substituted borane and sodium hydride producessodium borohydride or its acetoxyborohydride derivativesas extremely stable species with respect to the reagents(reaction III) and the same behavior is observed regardingthe compounds presented by the other reactions. Theenthalpy for reaction III is considerably exothermic withrespect to the reagents, whereas any reaction that providesion formation in a medium without solvent is considerablyendothermic.

The enthalpy variation involved in hydride ion dissocia-tion from the reagents only corresponds to the dissociationof the sodium hydride and must remain constant. Howev-er, Fig. 2 suggests that in vacuum hydride ion releaseshould occur from the sodium borohydride or the substi-tuted sodium borohydride. The variation of the enthalpyinvolved in hydride ion release from sodium borohydride

-60

-40

-20

0

20

40

60

80

100

120

140

ΔH0

I

ΔH0

II

ΔH0

III

Na+(g)A(g) + H-(g)

Na+(g) + AH-(g)

AHNa(g)

A(g) + NaH(g)

path

ΔH0

IV

ΔH

0 reac

tions

/ kc

al.m

ol-1

Fig. 2. Relative enthalpies of the reactions involving boron compoundscalculated at the B3LYP/CEP-31++G(d,p).

can be considered as the enthalpy sum taking reaction Iplus II or reaction I plus IV, then subtracting the reactionIII enthalpy. Considering the latter alternative, since theenthalpy of hydride ion release from the reagents is con-stant ðDH 0

IV þ DH 0I ¼ constantÞ, the necessary heat to liber-

ate the hydride ion from sodium borohydride must dependonly on the stabilization energy ð�DH0

IIIÞ. Therefore, thevariation of enthalpy for this process can be written as:

AHNaðgÞ ! AðgÞ þNaþðgÞ þH�ðgÞ;

DH 0 ¼ DH 0IV þ DH 0

I � DH 0III ¼ DH 0

I þ DH 0II

Table 3 shows the greater endothermic character for therelease of hydride ion as the number of acetoxy substitu-ents increases, in agreement with the proposal of Gribble[4]. Two extremely interesting aspects are the quantitativebehavior of DH0 and its relation to charge distribution.Regarding to DH0, a linear increment of approximately6 kcal/mol is observed with the increment in the numberof acetoxy substituents (see Table 3). This constant incre-ment in the enthalpy of hydride ion release from sodiumborohydride is also compatible with the positive chargeincrement presented by the boron atom in the borohydride,calculated with GAPT (Fig. 3). This linear behavior of theGAPT charges with respect to variation of enthalpy in pro-cesses of ion release has been observed in different systems[7,9] and allows establishing quantitative relationshipsbetween the properties. Table 4 shows the coefficients ofthe linear regression for the variation of enthalpy andGAPT charges on the boron atom (dGAPT) for borohydrideanion and substituted borohydride, obtained using

Table 3Theoretical enthalpies (in kcal/mol) of reactions releasing hydride fromsodium borohydride (DH 0 ¼ DH 0

I þ DH 0IIÞ and substituted sodium boro-

hydrides, obtained from different levels of theory at a temperature of298.15 K

Method BH3 BH2OAc BH(OAc)2 B(OAc)3

B3LYPcep-31++G(d,p) 194.09 202.09 208.27 215.35B3PW91cep-31++G(d,p) 199.17 206.47 211.73 218.74PBEPBEcep-31++G(d,p) 200.85 206.41 211.02 217.33

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8190

195

200

205

210

215

220

B3LYP B3PW91 BPEBPE

H /

kcal

.mol

0-1

Boron GAPT charge

Fig. 3. Linear correlation between GAPT charges and enthalpy variationinvolving hydride release from sodium borohydride.

Table 4Linear regression for enthalpies versus the net charge on the boron atom(dGAPT) for the anionic form of borane and substituted boranes, obtainedusing B3LYP/CEP-31++G(d,p), PBEPBE/CEP-31++G(d,p) andB3PW91/CEP-31++G(d,p)

Method aa ba rb SDc

PBEPBE/CEP-31++G(d,p) 193.32 15.44 0.9990 0.3909B3PW91/CEP-31++G(d,p) 189.11 18.58 0.9984 0.5773B3LYP/CEP-31++G(d,p) 181.64 20.83 0.9987 0.5621

a DH 0298 K ¼ aþ b:dGAPT.

b Correlation coefficient.c Standard deviation.

16 M.A. Machado et al. / Journal of Molecular Structure: THEOCHEM 802 (2007) 11–16

B3LYP/CEP-31++G(d,p), PBEPBE/CEP-31++G(d,p)and B3PW91/CEP-31++G(d, p). Both the linear regressionparameters and the behavior presented in Fig. 3 show greatquantitative similarities between the results calculatedusing the three functionals. Mulliken’s charges presentthe same trend although not providing so good linearity.

Obviously the above analysis must undergo significantalterations with the inclusion of the solvent effect. Theion solvation energy must stabilize the products from thereactions I, II and IV, eventually resulting in release ofhydride ion as an exothermic process. However, we believethat the agreement between the experimental reducingbehavior of borohydrides and the enthalpies is preserved,confirming Gribble’s original proposal. Calculationsincluding solvent effects in the determination of the reac-tion enthalpies are in progress.

5. Conclusion

Density functional theory was used to estimate and tointerpret the reducing capacity of sodium borohydrideand sodium acetoxyborohydride derivatives. Althoughthe calculations were carried out in the absence of the sol-vent effect, it was observed that the release process of thehydride ion can occur from the borohydride anion or from

sodium borohydride, which presents a considerable stabil-ity in relation to the other possibilities analyzed.

The behavior observed for the enthalpy variations of thereactions involving hydride ion release from borohydrideand/or acetoxyborohydride anions or from sodium boro-hydride and/or sodium acetoxyborohydride derivativespresents a consistent trend with the experimental results.In short, the larger the number of acetoxy substituents inthe compound, the more difficult is hydride ion release.This difficulty, especially for the triacetoxy-substitutedcompound, is a consequence of the positive charge increaseon the boron atom. A linear relationship was observedbetween the enthalpy variation of the releasing reactionof the hydride ion from sodium borohydride and theGAPT charges on the boron atom. Another factor thataffects the release of the hydride ion is the associated geom-etry of the substituted borohydride ions.

Acknowledgements

Financial support from CNPq (Conselho Nacional deDesenvolvimento Cientıfico e Tecnologico) and FAPESP(Fundacao de Amparo a Pesquisa do Estado de Sao Paulo)is acknowledged. We thank Prof. Carol H. Collins for herkind attention in revising this manuscript.

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