A Comparison of the Reactivity of Alkoxide and Alkoxide-Alkanol

Alkoxide-Alkanol Negative Ions with Alkyl- and Alkoxyboranes ... Alkoxide ions R'O- react with alkylboranes (R2)3B to effect both deprotonation of and...
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Organometallics 1986, 5 , 162-167

162

preferential alkoxy loss may not always be a valid criterion for distinguishing between exo and endo isomers of this type.

Conclusions Low-temperature spectroscopic studies show that for where M = Mo attack of alkoxide on [(v~-C,H,)M(CO)~]+, and W, in CH2C1, or pentane, the initial site of attack is the metal. Changes in solvent polarity do not affect the initial attacking site although for solvents with low dielectric constant the rate of reaction increases. No transient adducts are observed in the reaction of the analogous chromium complex. The 7-ex0 ring adducts are the final thermodynamically stable products in all cases, suggesting

a dissociative mechanism for their formation from the transient adducts since some formation of the 7-endo adducts would be expected if the reaction proceeded by an intramolecular rearrangement. NMR studies of the 7-ero-alkoxy ring adducts show that H, lies slightly downfield from and the values of J1,7 = 7.33 and 3.5 Hz confirm exo and endo geometries, respectively. 13C NMR of the exo and endo isomers show differences in chemical shift for the C1,6and C,. Increasing the size of the metal does not significantly affect the shift values of the ring carbons although the carbonyl carbons are shielded. Mass spectra show increasing ease of ionization of the exo ligand across the series from Cr to W.

A Comparison of the Reactivity of Alkoxide and Alkoxide-Alkanol Negative Ions with Alkyl- and Alkoxyboranes in the Gas Phase. An Ion Cyclotron Resonance and ab Initio Study Roger N. Hayes, John C. Sheldon, and John H. Bowie" Departments of Chemistry, University of Adelaide, Adelaide, South Australia 500 1 Received May 15, 1985

Alkoxide ions R'O- react with alkylboranes (R2)3B to effect both deprotonation of and hydride transfer to the borane. Tetrahedral adducts (R10)(R2)3B-are also formed. The decomposing form of this adduct gives (R2)2BO-+ R1R2. Alkanol-&oxide ions (R'O--.HOR') react with alkylboranes to give only (R'O)(R2),B+ R'OH. Alkoxide ions R1O- react with alkoxyboranes [e.g., (R20)8B]to produce tetrahedral species (R'O)(R20)3B-: the decomposing form of which yields R20- + (R'O)(R Ol2B. In addition, the SN2reaction R'O- R2-OB(OR2), R10R2+ (R20)zBO-is observed. Alkanol-alkoxide ions (R'O--.HOR') react with alkoxyboranes in two ways: (i) to form (R10)(R20)3B-ions and (ii)to undergo the alkoxide exchange reaction (R10-HOR') + (R20)3B (R1O-.H.-ORZ)- + (R'O)(R20)2B. When the alkanol-alkoxide reactant ion is unsymmetrical, the smaller alkoxide reacts at boron.

+

-

-

Introduction The differences in reactivities and rates of chemical reactions in the gas phase and in the condensed phase have been often described (see, e.g., ref 1-4). Condensed-phase reactions are dependent in a complex manner upon solvent parameters, and it is not possible to totally mirror these parameters in gas-phase reactions. Yet it is possible to form "solvated" ions [Nu-(HNu),] in the gas phase and to study their rea~tivities.~-'The differences in reactivity of alkoxide ions (RO-) and alkanol-&oxides (RO--.HOR)7 toward carbon2,8and silicon3 substrates have been described. The solvated species are less basic and less nucleophilic than their unsolvated counterparts, and the reactivity of the two species is often quite different. In particular, alkanol-alkoxide ions react with alkoxysilane~~ (and to a lesser extent with carbon ethers*) by reaction 1 (R, IR2). Alkanol-alkoxide addition in the reverse sense (1) DePuy, C. H.; Bierbaum, V. M. Acc. Chem. Res. 1981, 14, 146. (2) Bowie, J. H. Mass Spectrom. Reu. 1984, 3, 1. (3) Hayes, R. N.; Bowie, J. H.; Klass, G. J . Chem. Soc., Perkin Trans. 2 1984, 1167. (4) Tanaka, J.; Mackay, G. I.; Payzant, J. D.; Bohme, D. K. Can. J. Chem. 1976,54, 1643. (5) Bohme, D. K.; Young, L. B. J . Am. Chem. SOC.1970, 92, 7354. (6) Bohme, D. K.; Mackay, G. I.; Payzant, J. D. J . Am. Chem. SOC. 1974, 96,4027. (7) Faigle, J. F. G.; Isolani, P. C.; Riveros, J. M. J. Am. Chem. SOC. 1976,88, 2049 and references cited therein. (8)Hayes, R. N.; Paltridge, R. L.; Bowie, J. H. J . Chem. SOC.,Perkin Trans. 2 1985. 567.

-

is not observed in silicon systems. [R10-.-HOR2]

+ Me3SiOR3

+

[R20---HOR3] Me3SiOR1

1

B,H;

--*

[B,H,-,]-

+ H2

2

B,H;

+

[Bx-1Hy-3]-+ BH3

3

F- + B2H6

BH4-

+ BHzF

4

CD30- +

B,H6D-

+ CDzO

5

CD,O-

+

---*

BH,

+ CD3OBH2

6

In this paper we extend our studies of alkoxide and alkoxide-alkanol reactivity into the area of boron chemistry. Some aspects of the gas-phase negative ion chemistry of boron hydrides and alkylboranes have been described, and these are summarized here. Negative ions from diborane (e.g., B 2 H and ~ B,H,-) have been observed in conventional mass spectra: the decompositions of typical

0276-7333/86/2305-0162$01.50/0 0 1986 American Chemical Society

Organometallics, Vol. 5, No. 1, 1986 163

Alkoxide and Alkoxide-Alkanol Negative Ions R' Me CD3 Et Pr Me Et

Pr t-Bu Me

Table I. Partial ICR Spectra for the Reactions between RIO-(from R'ONO) and RZaB(M) RZ [RIO-1 [M + R'O-] [M - H+] [M + H-]O [RzZBO-] 100 3 23 12b b Me 100 4 25 14ctd d Me 100 1 75 45b b Me 100 1 45 88bpe b Me 100 7 24 9 2 Et 100 2 2 1 3 Et 100 4 3 1 Et 100 2 1 2 Et 100 11 24 33 6 Pr

+

*

The major precursors for these ions are [HNO-e] for [M H-] or [DNO-e] for [M + D-]-see, e.g., Figure 1. [Me,BO-] and [Me3BH-] are both m/z 57. eIn this case [M D-1. [Me2'lBO-] and [MeJOBD-] are both m / z 57. e [Me3BH-]and [MeCH=CHO-] (from PrO-) are both m / z 57.

+

34

I

I Et B =

CHMr

\

,

,

I

'c730-1

Figure 1. ICR spectrum of the system CD30NO/Et3B. For experimental conditions, see Experimental Section. Numbers in parentheses refer to the masses of precursor ions as determined by cyclotron ejection.

boron anions are summarized in sequences 2 and 3.9 The fluoride negative ion reacts with diborane to give a number of product ions including BH4- (reaction 4),1° and it has been demonstrated that BH4- [unlike CH30-12 and C6H; 13]does not transfer H- to formaldehyde in gas-phase experiments.l' Reactions 5 and 6 occur between CD30and diborane.1° Deuterium transfer (from DNO-.)'O and fluoride transfer (from NF,, SF,-., and SF5-)14J5to trimethylborane yield Me3BR- (R = D or F). Alkoxide negative ions deprotonate trimethylborane to form Me2B-=CH2.I0 Results a n d Discussion (A) Trialkylboranes. (1) Reactions with Alkoxide Negative Ions. The ICR spectra obtained for alkoxide ion/trialkylborane systems are recorded in Table I, and a representative spectrum (CD30-/Et3B) is shown in Figure 1. The reactions may be summarized as follows. (i) Alkoxide ions react with trialkylboranes to give both stable (9) Enrione, R. E.; Rosen, R. Inorg. Chim. Acta 1967, 1, 169. (10)Murphy, M. K.; Beauchamp, J. L. J.Am. Chem. SOC. 1976,98, 1433. (11)Kayser, M. M.; McMahon, T. B. TetrahedronLett. 1948,25,3379. (12) Ingemann, S.; Kleingeld, J. C.; Nibbering,N. M. M. J.Chem. SOC., Chem. Commun.1982,1009. Sheldon, J. C.; Bowie, J. H.; Hayes, R. N. Nouu. J. Chim. 1984, 8, 7 9 Sheldon, J. C.; Currie, G. J.; Lahnstein, J.; Hayes, R. N.; Bowie, J. H. Nouu. J. Chim.1985, 9, 205. (13) DePuy, C. H.; Bierbaum, V. M.; Schmitt, R. J.; Shapiro, R. H. J. Am. Chem. SOC. 1978,100, 2920. (14) Rhyme, T. C.; Dillard, J. G. Inorg. Chem. 1971,10, 730. (15) Murphy, M. K.; Beauchamp, J. L. Inorg. Chem. 1977,16, 2437.

I

It31

1

I _ ~~ _

reocllon

coord! n o t e

Figure 2. Ab initio calculations for reactions of MeO- with MeBH2. Calculations on 8,9, and 11 performed with Gaussian 76 (4-31G) and other calculations with Gaussian 82 (4-31G level). Numbers in parentheses refer to structure numbers and numbers in brackets to energies. Solid lines refer to data obtained from fully optimized geometries. (4-31G level). Bond lengths (A) and angles (deg) are as follows: 7 , a = 1.75, b = 1.15, c = 178; 8, d = 1.53, e = 1.25, f = 1.65,g = 109.4; 9, h = 1.22, i = 1.44,j = 1.085, k = 120; 10, 1 = 1.48, m = 2.56, n = 2.88, o = 2.88; 11, p = 1.24, q = 1.295, r = 120.

-

and decomposing (M + RO-) adducta.16 Adducts decompose by the process R'O- + RZ3B R22BO-+ R1R2. (ii) Alkoxide ions deprotonate trialkylboranes to yield (M H+) ions. (iii) (M + H-) ions are observed in the majority of spectra: these ions are shown by cyclotron ejection experiments to be formed mainly from HNO-. and to a lesser extent by transfer of an a hydrogen from the alkoxide ion. We have undertaken a study of the structures of certain reactive intermediates and product ions in the above reaction sequences using ab initio calculations ( a t 4-31G (16) Both 'stable" and 'decomposing" adducts (M + RO-) are considered to have the same structure in this case, viz., tetrahedral geometry. Such adducts are formed in strongly exothermic reactions. A "stable" adduct is one which is detected as a peak in an ICR spectrum-its energy of formation has been dissipated either by collision or by some other (e.g., radiative) process. A 'decomposing" adduct is one which has retained sufficient of ita formation energy in order that energy barrier(s) may be surmounted, and product(s) formed.

164 Organometallics, Vol. 5, No. 1, 1986 R Me Et Pr t-Bu Me

Hayes et al.

Table 11. Partial ICR Spectra for the Reactions between [RO'. *HORY and R'jB (M) R' [RO-I [RO-*-HOR] [M + RO-] [M + H-lb [M - H+1 Me 100 21 15 21 1_ 2_ c Me 100 11 8 93 20c Me 100 5 3 51 7Wd Me 100 32 18 26 17c Et 100 20 11 20 16 ~~

.IR'oBO-1 -

~

r C

d C

4

"Produced by the Riveros reaction7 between RO- (from RONO) and HC02R. *The major precursor for these ions is [HNO;]. and [Me,BH-] are both m / z 57. [Me,BH-] and [MeCH=CHO-] (from PrO-) are both m / z 57.

R' CD3 Et Pr i-Pr BU t-Bu neo-Pen Me Pr

Table 111. Partial ICR Spectra for Reactions R'O- (from R'ONO) and B(ORZ)J(M) R2 [R'O-I [R20-l IM + R'O-1 IM + R20-1 r(R*O).,BO-l Me 87 39 63 100 1 Me 100 25 20 3 100 46 22 Me 25 1 100 29 53 Me 7 0.5 54 100 72 Me 57 1 74 100 19 Me 68 Me 18 67 1 100 36 Et 100 56 29 11 1 Et 100 8 56 10 2

levell' using Gaussian 76lSand 8219). Results are shown in Figure 2 for the model system MeO-/MeBH2. If MeOapproaches the neutral in the vicinity of the methyl group, a transient H-bonded species 7 is formed which is a precursor of both the deprotonated ion 9 and the tetrahedral adduct 8. However we believe that there is also a second channel to 8, in which MeO- skirts the methyl substituent, approaches, and bonds to boron with monotonically decreasing energy.20 Adduct 8 is formed with considerable excess energy (-299 kJ moF, 4-31G),and in order to produce a stable form, collisional deactivation would be necessary. The decomposing form of the adduct 8 eliminates ethane to form H2BO- (11). The reaction is stepwise, proceeding through the intermediacy of solvated methyl anion In Figure 2 we indicate that the solvated ion 7 is formed by nucleophilic addition to a trialkylborane and that this (17) We recognize that accurate representation of negatively charged species requires expanded bases (Radom,L. Mod. Theor. Chem. 1977,4, 333. Chandraseka, J.; Andrade, J. G.; Schleyer, P. v. R. J. Am. Chem. SOC. 1981,103, 5609), preferably with a low-exponent Gausian in each set to represent the diffuse outer region of negative ions. Nevertheless we believe that systematic exploration of the relative energies of intermediates and products at a practical level of approximationis indispensable if reaction sequences (like those shown in Figure 2) are to be evaluated and . .- characterized. . -. .. ..-. -. (18) Binkley, J. S.; Whitehead, R. A.; Hariharan, P. C.; Seeger, R.; Pople, J. A. QCPE 1978, 368. (19) Gordon. M. S.: Binklev. J. S.: Pode, J. A.: Pietro. W. J.: Hehre, W. J. J. Am. Chem. SOC.1982, 104, 2797.

(20) We have not carried out calculations on this channel, but calculations on the model system F/MeBH2clearly show two channels, viz., the formation of an H-bonded intermediate correspondingto 7 (Figure 2) and direct attack of F- on boron. A similar situation occurs for the reaction of MeO- with MeSiHB(Sheldon,J. C; Hayes, R. N.; Bowie, J. H.

J. Am. Chem. SOC.1984,106, 7711).

(21) (a) Analogous gas-phase reactions are observed in other systems, e.g., alkoxides (Tumas, W.; Foster, R. F.;Pellerite, M. J.; Brauman, J. I. J.Am. Chem. Soc. 1983,105,7464. Tumas, W.; Foster, R. F.; Brauman, J. I. J. Am. Chem. Soc. 1984, 106, 4053. Hayes, R. N.; Sheldon, J. C.; Bowie, J. H.; Lewis, D. E. J. Chem. Soc., Chem. Commun. 1984, 1431; Aust. J. Chem. 1985, 38, 1197), tetrahedral adducts formed between alkoxide negative ions and carbonyl systems (e.g. Klass, G.; Underwood, D. J.; Bowie, J. H. A u t . J. Chem. 1981,34,507), and silanes (DePuy, C. H.; Bierbaum, V. M.; Flippin, L. A,; Grabowski, J. J.; Schmitt, G. K.; Sullivan, S. A. J. Am. Chem. SOC.1980, 102, 5012. DePuy, C. H., Bierbaum, V. M.; Damrauer, R. J. Am. Chem. SOC.1984,106,4051. Klass, G.; Trenerry, V. C.; Sheldon, J. C.; Bowie, J. H. A u t . J. Chem. 1981,34, 519). There are subtle differences in the structures of the reactive intermediates (e.g., 10, Figure 2) in the various reaction sequences. (b) A reviewer has suggested that if the alkoxy group is larger than OMe, an elimination reaction may compete with the reaction sequence analogous to 8 10 11 (Figure 2). For example in the case of the system EtO-/MeBH2this would be EtO- + MeBH, (EtO)(Me)BH2- H,BO+ CH, + CSH4.

- -

[Me2BO-]

-

-

\

'

I '

I

I

i

\ '

'

r

;

~

-oaci,op w o r d r o t e

Figure 3. Ab initio calculations for reactions of MeO- with MeOBH2. Calculations on 15 and 17 performed with Gaussian 76 (4-31G) and other calculations with Gaussian 82. Numbers in parentheses refer to structure numbers and numbers in brackets to energies. Solid linea refer to data obtained from fully optimized geometries. Bond lengths (A) and angles (deg) are as follows: 14, a = 1.89, b = 1.09, c = 1.47, d = 207; 15, e = 1.25, f = 1.49, g = 109.4; 16, h = 2.11, i = 1.80.

species may then rearrange to tetrahedral ion 8 in a strongly exothermic process. We have been unable to identify such solvated species by solvent-exchange experiments. For example if the (M + MeO-) species formed from MeO- and Me3B is allowed to react with CDBODor EtOH, neither (M + CD,O-) nor (M + EtO-) ions are produced. Stable (M + RO-) species are thus likely to correspond to tetrahedral structures. (2) Reactions with AlkoxideAlkanol Negative Ions. The ICR spectra obtained from reactions between alkoxide-alkanol negative ions and trialkylborane are listed in Table 11. Alkoxide-alkanol ions (RO-.-HOR)' yield stable adducts (M + RO-) with trialkylboranes. The relative yield of stable (M + RO-) ions is greater when the reactant ion is (RO---HOR) rather than RO-, since the

Organometallics, Vol. 5, No. 1, 1986 165

Alkoxide and Alkoxide-Alkanol Negative Ions

R'

R2

Et Et Et Me Me Me Et Pr Et

Et Bu Et Pr Me Et Et Pr Bu

Table IV. Partial ICR Spectra for Reactions between [R'O *H* *OR*]- and RSB(OR'), (M) [R'O-. [R20*[R'O-* [M+ [M+ R3 R4 [R'O-] [R'O-] [R30-] H-*OR2]-' H*..0R4]-b H.-OR']-' R'O-] R'0-1 d 4d 100 (96) 20 42 Me Me 96 23 18 8 100e 1 4 37 Me Me 68 d 71d 21 34 65 (100) Me Bu 100 2 35 4 2 3 20 100 6 Et Et 11 d 14d (100) 11 39 Et Bu 100 7 100 99 17 Et Bu 7 15 2 3 . 2 d 8d Pr Me (100) 26 76 100 d, f 7d 13 (63) Pr Me 63 21 100 100 53 f 2 1 4 Pr Me 13 60

[M+ R40-] 5 e 5 5 1

0.5

.-

[R'O H...0R2]- is produced by the Riveros reaction between R20- and HC02R'. [R20.-H-.0R4]- is produced by a specific alkoxideexchange reaction between [R10-.H-.0R2]- and R3B(OR4)2. [R'O--H-.OR*]- is produced by the Riveros reaction between R'O- and HC02R1 (R40- is produced by the reaction between R20- and R3B(OR')2). dSince R' = R2 in this case: [R10.-H.-OR4]- is the same as [R20...H...0R4]- e [EtO-H-.OBu]- and [M MeO-] are both m/z 119. 'No alkoxide-exchange reaction is observed between [R'0.-H--OR2]and R3B(OR4)2. a

+

.

R' Et Et Et Et CD3 t-Bu Me Me Me Me Me Me Me Me

R2 CD3 Et Pr i-Pr Bu t-Bu Me Pr Me Et Bu Me Et Pr

Table V. Partial ICR Spectra for Reactions between [R'O H OR2]- and B(OR3)8(M) [R'O-* [R20*[R'O-* R3 [R'O-] [R'O-] [R30-] H*-OR2]-' H***OR3]-b H-OR3]-' [M + R'O-] [M + R'O-1 16 5 17 19 100 3 Me 25 95 13 d 1Od 49 (100) Me 100 11 58 36 8 33 5 13 Me 100 0.5 6 9 17 (100) 34 53 Me 100 78 21 1 17 4 27 (100) Me 100 36 (100) 38 36 Me 100 (100) 46 12 d 5d 88 Et 100 24 Et 100 51 49 3 33 31 72 d 5d 1 (100) 6 31 Pr 100 29 Pr 100 14 43 12 22 8 7 39 100 63 9 82 15 3 Pr 8 d 13d (80) 2 100 Bu 80 12 20 21 11 88 Bu 100 23 22 Bu 100 24 8 88

[M + R30-] 17 11 15 9 8 26 51 37 0.4 8

[R10.-H-.0R2]- is produced by the Riveros reaction between R20- and HC02R'. [R20.-H-.OR3]- is produced by a specific alkoxideexchange reaction between [R10-.H-.0R2]- and B(OR3),. [R'O-.H.-OR3]- is produced by a Riveros reaction between R30- and HC02R' (R30- is produced by the reaction of R20- with B(OR3),). dSince R' = R2 in this case: [R'O-.H--OR3]- is the same as [R20-.H...0R3]- .

eliminated molecule of ROH takes off some of the excess energy of the reaction effectively stabilizing the (M + RO-) species. Alkoxide-alkanol ions do not deprotonate trialkylboranes, they do not transfer hydride ion to the neutral, and they do not act as precursors to R2BO-ions. We were able to demonstrate attachment of (RO---DOR) to hydrogen in >CH-CO- systems because of the occurence of a specific H / D equilibration reaction.22 The analogous intermediates for (RO--.DOR)/Me3B are 12 and 13. In an ICR experiment, the reaction between (EtO-

are recorded in Figure 3 for the model system MeO-1 (MeO)BH,. An alkoxide ion approaching a methoxyborane (rotating through its center of mass) encounters the hydrogens of a methyl group and may form an initial hydrogen-bonded intermediate (e.g., 14, Figure 3).23 Such an intermediate is a precursor for products formed in these systems. Products are summarized as follows. (i) Pronounced peaks corresponding to stable four-coordinate boron anions are observed for all systems studied. The reactions are strongly exothermic, and the product ion must be stabilized by collisional deactivation. The high yields of stable (M RO-) adducts (e.g., 15, Figure 3) in these systems should be contrasted with the small abundances of analogous peaks in trialkylborane systems. Perhaps the difference is due to the stabilizing effect of oxygen atoms of the alkoxyborane system from dative ?r bonding into the vacant 2p orbital.24 (ii) Alkoxide displacement (through, e.g., 15, Figure 3) is the major reaction of these systems. The liberated alkoxide ion also undergoes reaction with the alkoxyborane to yield a four-coordinate boron ion. (iii) All systems show the reaction R1O- + (R20)3B (R20)2BO-+ R10R2. This reaction could, in principle, occur by two mechanisms, viz., a 1,Zelimination from an excited four-coordinate intermediate [ (R20)3B(OR1)]-(cf. 8 -+ 11, Figure 2) or an SN2reaction at carbon. In order

+

IG

13

--D.-OPr)- and Me3B yielded (Me3BOEt)- and (Me3BOPr)-. No H/D equilibration occurred; therefore, we can provide no experimental evidence for the intermediacy of 12 in the reaction pathway. Nevertheless we believe that ions analogous to 12 are initial intermediates in reaction sequences (of this type) which lead to fourcoordinate boron negative ions (cf. also Figure 2). (B) Alkoxyboranes. (1) Reactions with Alkoxide Negative Ions. The ICR spectra obtained from reactions between alkoxide negative ions and trialkylborates are listed in Table 111. Ab initio calculations (at 4-31G level) (22) Klass, G.; Sheldon, J. C.; Bowie, J. H. Aust. J. Chem. 1982, 35, 2471.

+

(23)There is also a strong possibility that MeO- may directly attack boron in this system, but we have not carried out calculations on this possible reaction channel. (24) Cotton, F. A.; Wilkinson, G. "Advanced Inorganic Chemistry. A ComprehensiveText", 3rd ed.; Wiley Interscience: New York, 1972; p 223.

166 Organometallics, Vol. 5, No. 1, 1986

Hayes et al.

Table VI. Partial ICR Spectra for the Reactions [R'O

...

R' Me Me Me

R2 Me Et Pr

[R'O[R20-] [R'O***H*-OR2]-" HOR']*

[R'O-] 100

OR2]- and EtBOCH2CH,0 (M) IM + [M + [M + ( R ~ ... OH... R'O-I R20-] OR2)-]

H

60 59 68

44

13 19

66 84

72 100

5 2

2 3 4

17 36

IM + (RQHOR')]

...

1 1

[R10--H-OR2]- is produced by the Riveros reaction between R20- and HC02R'. [R'O--.HOR'] is produced by the Riveros reaction between R'O- and HC02R' (R'O- is produced by the reaction between R20- and HCO2R').

to differentiate experimentally between these two possibilities, Me180- was allowed to react with trimethoxyborane. No l80was incorporated into the peak corresponding to (Me0)2BO-: the reaction therefore proceeds 16 17, Figure 3). by an SN2 mechanism (e.g., 14 (2) Reactions with Alkoxide-Alkanol Negative Ions. Results obtained for reactions of (R10-.H--OR2)-with both dialkoxyboranes and alkylborates are listed in Tables IV and V. In summary: (i) alkanol-alkoxide negative ions (R10-.H-.0R2)- react with alkoxyboranes to yield stable species (M + R1O-) and (M + R20-); (ii) the only other reaction observed is the alkoxide exchange reaction 18 (R1 IR2, at least either R1 or R2 must be ethyl or larger and R3 is either alkyl or OR4). (R10*-H-*OR2)-+ R3B(OR4)2 (R20--H***OR4)+ R3B(OR4)(OR1)

- -

I "e"

~"

* 2.

I c 3

-+

18

+

-

(R'O-*H*-OR2)- R3B(OR4)2 (R10--H.-OR4)-

+ R3B(OR4)(OR2)

19

In this reaction the smaller alkoxide ion reacts at boron and the larger alkanol forms part of the new alkanol-alkoxide ion. We were not able to detect (by cyclotron ejection experiments) addition in the reverse direction (i.e., reaction 19).% This specificity is similar to that observed for analogous reactions in Si-OR system^:^ in contrast, addition in both directions occurs for C-OR reactions with that analogous to 18 being the major process.8 The yield of product alkanol-alkoxide ion increases with elaboration of R' and R2and decreases with elaboration of R4in accord with previous results in carbon8 and silicon3 systems. A number of possible mechanisms can be proposed for the alkoxide-exchange reaction of alkoxyboranes, but the most plausible is that shown in sequence 20. We have R

503OR^ /I

,4

'E-OR

R4@'

I,-

-

2

4 -

( R o...H*+.oR

)

+

O ,R'

-E

OR^

-

1

already shown that analogous processes occur for carbon8 and silicon3 ether systems, e.g., the unsymmetrical precursor (R'O-H-OR2)- reacts with cyclic silyl ethers in the specific manner shown in reaction 21. If such a mechanism (e.g., 20) is operating in boron systems, it should be possible to observe reaction 22 for cyclic boronates. The ICR spectra of (R10.-H-OR2)and I EtBOCH2CH20systems are recorded in Table VI and Figure 4. The situation is best described by reference to Figure 4. The reactant ion (MeO-H-.OEt)- ( m / z 77, from EtO- and HC02Me)reacts with the cyclic boronate to yield both (M + MeO-) and (M EtO-) ions. In addition, it reacts to form an adduct [M + (MeO-H.-OEt)-] ( m / z 177). In order to determine the structure of m / z 177, a solvent-exchange reaction was carried out. Addition of propanol to the system resulted in solvent exchange with [M + (MeO.-H--OEt)-] ion to form exclusively the species [M + (MeO-H--OPr)-]. Thus the methoxide ion is bound to boron, and the ethanol residue is hydrogen bonded to the oxy moiety as shown in Figure 4 (cf. reactions 20 and i

+

22).26

21 ~ 0 0 - r ; ) .

Figure 4. ICR spectrum of the system EtONO/HCO,Me/ EtBOCH2CH20. For experimental conditions, see Experimental Section. Numbers in parentheses refer to the masses of precursor ions as determined by cyclotron ejection: (A) (MeO--HOMe), are formed by the Riveros reaction between MeO- and HC0,Me; formed by the alkoxide (B) [Et(MeO)B-O(CH,),O---HOMe], exchange reaction from (MeO--HOMe) ( m / z 63).

.HOR'

Conclusions Alkoxide and alkoxide-alkanol ions show quite different reactivity toward trisubstituted boranes-in summary: (i) both alkoxide and alkoxide-alkanol ions react with trisubstituted boranes to yield tetrahedral ions ROBR3,with the latter reactant ion providing the more abundant product peaks; (ii) alkoxide-alkanol ions undergo a specific alkoxide exchange reaction with alkoxyboranes, e.g.,

22 (25) We have not carried out ab initio calculations for this reaction. It is certcunly close to thermoneutral, but we do not know if the specificity is due to internal barriers for the stepwise reactions.

(26) A reviewer has suggested that the solvated product ion shown in sequence (22) could be in equilibrium with (R10)(RzO)EtB-O(CH2)?OH. This is possible: our 'solvent-switching" experiment merely establishes the presence and identity of the solvated ion.

Organometallics, VoE. 5, No. 1, 1986 167

Alkoxide and Alkoxide-Alkanol Negative Ions

+

-

+

(R'O***H-*OR2)- >B-OR3 (R20.-*H.-OR3)- >B-OR' when R' IR2; (iii) alkoxide ions react with both alkyl- and alkoxyboranes by reaction channels not available for the solvated ions: e.g., deprotonation of alkylboranes, hydride transfer to alkylboranes, 1,2-elimination reactions (e.g., MeO- + R3B R2BO- + MeR), alkoxide displacement with alkoxyboranes and SN2reactions with alkoxyboranes.

-

Experimental Section ICR spectra were measured with a Dynaspec ICR 9 spectrometer equipped with a three-section cell. Spectra were obtained a t 70 eV (primary negative ions formed by dissociative secondary electron capture). Other reaction conditions: 4 2 = 125.0 ~ kHz, ion current in A range, emission current 0.2 MA,and ion transit time 1 x a. Precursor ions in reaction sequences were determined by the cyclotron ejection technique. Pressure conditions: (i) in RO-/borane or borate experiments, both RONO (the precursor of RO-) and the borane (borate) = 5 X lo4 torr (total pressure = 1 X torr); (ii) for (RO--HOR)/borane (borate) experiments, RONO = 5 X lo4 torr, HCOzR = 5 X 10" torr, and the borane/borate = 1 X torr (total pressure = 2 x torr). Alkyl nitrites were prepared from the appropriate alcohol and sodium nitrite by a reported method.27 Formates, including methyl [2H]formate,were prepared by the method of Stevens and Van Ea.% All boranes, boronates, and borates were prepared by (27)Noyes, W. A. Org. Synth. 1936,16, 108.

reported procedures: trimethylborane,m t r i e t h y l b ~ r a n e ,tri~~ propylborane,30methyl methy1boronate:l ethyl ethylboronate,3l butyl ethylboronate,32 methyl propylboronate,32 2-ethyl-1,3,2dioxaborolane,33trimethyl borate,%triethyl and tripropyl borate.36

Acknowledgment. This work was carried out with the aid of a grant from the Australian Research Grants Scheme. We thank the University of Adelaide Computing Centre for facilities. Registry No. MeO-, 3315-60-4; CD30-, 51679-31-3; EtO-, 16331-64-9; PrO-, 26232-83-7; t-BuO-, 16331-65-0; i-PrO-, 15520-32-8; BuO-, 26232-84-8; neo-Pen0-, 55091-58-2; Me3B, 593-90-8; Et3B, 97-94-9; Pr38, 1116-61-6; B(OMe)3, 121-43-7; B(OEt)3,150-46-9;MeB(OMe)2,7318-81-2; EtB(Oet)2,53907-92-9; EtB(OB&, 3027-60-9;PrB(OMe)2,2938-87-6; B(OPr),, 688-71-1;

.

B(OBU)~, 688-74-4; EtBOCH2CH20, 10173-38-3.

(28)Stevens, W.; Van Es,A. R e d . Trau. Chim. Pays-Bas 1964,83, 1287. (29)Brown, H.C.J. Am. Chem. SOC.1945,67, 374. (30)Rosenblum, L. J. Org. Chem. 1960,25, 1652. (31)Wiberg, E.;Kruerke, U. Z . Naturforsch., E: Anorg. Chem., Org. Chem., Biochem., Biophys. Bid. 1953,8B, 608. (32)Mikhailov, B. M. and Blokhima, A. N. Izu. Adad. Nauk SSSR, Ser. Khim. 1962,827;Chem. Abstr. 1963,58,5706g. (33)Laurent, J. P.;Bonnet, J. P. C. R. Seances Acad. Sci., Ser. C 1900, 262. 1109. (34)Schlessinger, H.I.; Brown, H. C.; Mayfield, D. L.; Gilbreath, J. R.J.Am. Chem. SOC.1953,75, 213. (35)Lappert, M. F. Chem. Reu. 1956,56, 959.