Organometallics 1986, 5, 158-162
158
Formation of Low-Temperature Adducts on Nucleophilic Addition to Tricarbonyltropyliummetal Complexes David A. Brown,' Noel J. Fitzpatrick, William K. Glass, and Thomas H. Taylor Department of Chemistry, University College, Belfield, Dublin 4, Ireland Received April 22, 1985
Low-temperature spectroscopic techniques were employed to explore the reaction between the title cations and alkoxide ions. The low-temperature adducts observed were characterized by IR and NMR spectroscopy. A metal attack adduct is the initial species formed by the Mo and W cations. The initial site of attack is unaffected by changes in solvent polarity. No adducts are observed in the reaction with the chromium cation. In all cases, the thermodynamically stable product is the (7-exo-q-1,6-C7H70R)M(CO), complex, where R = Me and Et and M = Cr, Mo, and W, for which mass spectra, NMR, and IR data are presented. T h e simulated NMR spectrum of (7-exo-v-1,6-C7H70CH3)Cr(CO), is given.
Introduction
lytical laboratory of this department. Preparation of (7-exo-1,6-C7H70CH3)Mo(CO), (VI). AdT h e factors influencing t h e regioselectivity of nucleoditon of a solution of sodium (0.1 g, 4.35 mmol) in methanol (15 philic attack on cations of the type [(v~-C~H~)M(CO)~]BF~, mL) to a well-stirred suspension of [ (q7-C7H7)Mo(C0),]BF,(0.8 where M = Cr, Mo, and W, are still unclear, but in a g, 2.24 "01) in methanol (10 mL) gave an initial red colour before number of related cases, where t h e final product is either formation of an orange solution. After the solution was stirred for 10 min, the solvent was removed. Extraction of the residue the ring a d d u c t or a carbonyl substitution product, eviwith petroleum spirit (40-60 "C)and subsequent solvent removal, dence has been presented for t h e existence of a d d u c t s followed by recrystallization from hexane gave orange needles of involving initial attack at other sites, for example, t h e ( ~ - ~ x o - ~ - ~ , ~ - C ~(0.41 H ~ g,~62%). C H J Prolonged M O ( C Ostirring )~ metal or carbonyl In a recent note, we reported during reaction resulted in significant loss of yield. The product briefly o n t h e existence of low-temperature, kinetically (VI) is stable for several months if stored at 0 OC under nitrogen. controlled, a d d u c t s observed during the reaction of alkPreparation of ( ~ - ~ x o - ~ - ~ , ~ - C ~ H(VII). ~ O C HThe ~)W(CO)~ oxide ions with [(q7-C7H,)M(C0)3]BF4, where M = Mo and (0.8 g, 1.79 above procedure)was applied to [(q7-C7H7)W(C0),]BF4 W.4 A theoretical approach t o reactivity s u p p o r t s t h e mmol). Recrystallization from hexane yielded orange/red needles existence of such a d d u c t s since initial metal attack is (0.36 g, 52%). The product was stable for short periods at room temperature and decomposed after approximately 2 weeks when predicted for these cation^.^^^ stored under nitrogen at 0 "C. In this paper we present detailed spectroscopic evidence Preparation of ( ~ - ~ X O - ~ - ~ , ~ - C ~ H ~(VIII). OC~H~)C~(C for these a d d u c t s together with t h e preparation a n d A solution from sodium (0.1 g, 4.35 "01) in ethanol (10 mL) was spectroscopic characterization of a range of 7-exo-alkoxy added to a well-stirred suspension of [(q7-C7H7)Cr(C0)3]BF4 (0.8 products not reported previously. g, 2.55 mmol) in ethanol (15 mL). No intermediate colors were observed. After the solution was stirred for 30 min, the solvent Experimental Section was removed. Extraction of the residue with petroleum spirit (40-60 "C) and recrystallization from the same solvent yielded General Information. Published methods were used to the product as yellow/orange needles (0.35 g, 50%). prepare [(q7-C7H7)M(CO),]BF,,where M = Cr,' Mo, and W,B C~H The S)MO(CO)~ 7-exo- and ~ - ~ ~ ~ O - ( ~ ~ , ~ - C ~ H ~ (I -OV). C HSodium , ) C ~ ( C O Preparation ) ~ ~ ~ of ( ~ - ~ x ~ - ~ ~ , ~ - C ~ H ~ O(IX). above method was applied to [(q7-C7H7)Mo(C0),]BF,,(1.0 g, 2.80 alkoxide solutions were freshly prepared prior to use by dissolving mmol). An initial red color was observed before formation of an the required amount of sodium in the respective alcohol. All orange solution. Recrystallization, at -70 "C, from petroleum spirit solvents were dried and deoxygenated before use. Reactions and yielded ( ~ - ~ x o - ~ - ~ , ~ - C ~ H ~asOaCyellow/orange ~H~)MO(CO)~ workup, including chromatography, were carried out under oxpowder (0.2 g, 33%). The product was stable for approximately ygen-free nitrogen. Infrared spectra were recorded on a Per1 week if stored a t 0 "C under nitrogen. kin-Elmer 283B spectrophotometer linked to a Perkm-Elmer 3500 Attempted Preparation of (7-end0-q-1,6-C7H70CH3)Modata station. 'H NMR spectra were recorded on JEOL PSlOOFT (CO)%7-endo-C7H70CH," (3.1 g, 25.4 mmol) was added to a and JEOL GX270FT spectrometers. 13C NMR spectra were solution of (CH3CN),Mo(CO), (4.0 g, 13.2 mmol) in THF (80 mL). recorded on a JEOL GX270FT spectrometer. Mass spectra were The mixture was heated under reflux for 30 min and the solvent recorded on a VG micromass 7070H linked to an INCOS 2400 removed under reduced pressure. Extraction of the residue with data system. Microanalyses were performed by the microanapetroleum spirit (60-80 OC)-ether (1:3) and chromatography on neutral alumina gave (CH&N)MO(CO)~, a yellow solid, as the as major product with the dimeric complex [ (q7-C7H7)Mo(C0),l2 (1) Cowles, R. J.; Johnson, B. F. G.; Lewis, J. J. Chem. Soc., Chem. a minor product (as confirmed by microanalysis). None of the Commun. 1969,392. endo complex could be detected although the presence of the (2) Powell, P.; Russell, L.; Styles, E.; Brown, A.; Howarth, D.; Moore, P. J . Organomet. Chem. 1978,149, C1. dimer suggests that the endo isomer is initially formed. The use (3) Brown, D. A.; Chawla, S. K.; Glass, W. K. Inorg. Chim. Acta 1976, of milder reaction conditions failed to give the product. 19, L31. Low-Temperature Reaction of [(q7-C7H7)M(C0)3]BF4 with (4) Brown, D. A.; Fitzpatrick, N. J.; Glass, W. K.; Taylor, T. H. J . NaOCH3. To the cation (M = Cr, Mo, W (0.7 g)), slurried in Organomet. Chem. 1984,275, C9. CHzC12(15 mL) at -15 O C was added a slurry of sodium methoxide (5) Brown, D. A.; Fitzpatrick, N. J.; McGinn, M. A. J . Organomet. Chem. 1984,275, C5. in CH2C12at -15 O C . The sodium methoxide was prepared freshly ( 6 ) Brown, D. A.; Fitzpatrick, N. J.; McGinn, M. A.; Taylor, T. H., as a solid from sodium (1.5 g, 65.2 mmol) in CH30H (20 mL) and preceding paper in this issue. subsequent solvent removal. After the solution was stirred briefly (7) Munro, J. D.; Pauson, P. L. J . Cher (1min), a sample was removed for immediate IR analysis in the (8)Tab, C. P.; Augi, J. M.; Kn .. (9) Pauson, P. L.; Smith, G. H.; Val 1057. (10) Pauson, P. L.; Todd, K. H. J. Chem. SOC.C 1970, 2315
0276-7333/86/2305-Ol58$01.50/0
(11)Doering, W. von E.; Knox, L. H. J. Am. Chem. SOC.1954, 76,3203.
0 1986 American Chemical Society
Organometallics, Vol. 5, No. 1, 1986 159
Tricarbonyltropyliummetal Complexes
Table I. Analyses and IR Spectra of Starter Cations, Low-Temperature Adducts, and Thermodynamically Stable Products (M = Cr, Mo, W)Formed by Reaction of Alkoxide with the Cations anal. found (calcd) IR spectra YCO, cm-' complex no. C H CHzClz pentane [(C7H7)Cr(C0)31BF4 I 38.2 (38.2) 2.7 (2.2) 2071, 2028, 1990 [(CJ%)Mo(C0)31BF, I1 33.5 (33.5) 2.1 (2.0) 2081, 2025, 1976 1.5 (1.6) 2075, 2010, 1952 [(C7H7)W(C0)31BF4 I11 26.8 (26.9) ~ - ~ ~ o - ( O C H ~ C , H , ) C ~ ( C O ) ~ IV 50.6 (51.1) 1996, 1937, 1917 3.6 (3.9) 1987, 1922, 1896 V 51.3 (51.1) 3.9 (3.9) 1983, 1918, 1885 1998, 1939, 1909 3.4 (3.3) VI 43.9 (43.7) 2000, 1941, 1919 1996, 1926, 1896 2000, 1936, 1912 2.9 (2.6) 1991,1919,1889 VI1 34.2 (33.9) VI11 52.5 (52.8) 1995, 1935, 1915 4.3 (4.4) 1983, 1919, 1891 IX 45.7 (45.6) 2000, 1940, 1918 3.9 (3.8) 1993, 1921, 1893 X 2015, 1974 2005, 1957 XI 1986, 1631 XI1 2002, 1955 2010, 1971 1983, 1630 2004, 1957 1984, 1630
Figure 2. Possible structure of fluxional adduct resulting from metal attack.
Results and Discussion A typical series of IR spectra are shown in Figure 1for
F i g u r e 1. Low-temperature spectra of reaction between [ (1'C7H7)Mo(CO)3]+and NaOCH3 recorded on a Perkin-Elmer 283B IR spectrometer: (a) initial spectrum after 1 min, (b) second sample after 10 min, (c) second sample after 30 min, (d) second sample after 50 min, and (e) third sample after 3 h. region 1550-2200 cm-', using low-temperature cells. After 10 min, a second sample was removed from the reaction mixture for IR analysis. This sample was allowed to warm up in the IR cells and was rerun after 30 and 50 min. A third sample was removed after 3 h and its IR spectrum obtained. Similar reactions were performed by (a) using pentane as solvent in place of CHzClz and (b) using NaOC2H, in place of NaOCH, as nucleophile.
the low-temperature reaction between [ (q7-C7H7)Mo(CO)3]BF4and CH,O- in CH2C12. Similar spectra were obtained for both cations and alkoxide ions in CHzClzand n-pentane. The results are summarized in Table I. The peaks at 2005 and 1957 cm-l, which disappear within 45 min, are assigned to an alkoxymetal adduct (X) formed by direct attack (or association) of the CH30- at the metal atom. This then dissociates to give both the stable ?-ex0 ring product VI, with peaks at 1996,1926, and 1896 cm-', and the carbomethoxy species (q7-C7H7)Mo(C0)2(C02Me)(XI), with bands at 1986 and 1631 cm-'. The carbomethoxy compound XI is stable for about 24 h at low temperatures but rearranges rapidly to the ring product VI at room temperature and could not be isolated pure. Complementary 'H NMR studies in CD2Clzshowed, at the very early stages of reaction, a sharp singlet at 5.34 ppm, assigned to X, which disappears with time to give the 7-ex0 ring adduct VI. No ring proton peak of XI was observed although it may be obscured by a solvent peak at 5.32 ppm, but a small singlet at 2.91 ppm may be assigned to the methoxy group of XI. The initial appearance of the sharp singlet at 5.34 ppm suggests equivalent ring protons in X, implying either rapid fluxionality of the ring (Figure 2) or possibly a 20-electron system as shown in the reaction scheme in Figure 3. The exclusive formation of the 7-exo ring adduct VI suggests a dissociative rearrangement of X and XI since intramolecular rearrangement would yield a measurable quantity of the 'I-endo ring adduct. In the case of related complexes addition of alkoxide to [ (C7H9)Fe(C0)3]BF412 and [(C6H7)0s(C0)3]BF,13 results (12) Brown, D. A.; Glass, W. K.; Hussein, F. M. J . Organomet. Chem. 1980,186, C58.
(13) Bryan, E. G.;Burrows, A. L.; Johnson, B.F.; Lewis, J.; Schiavon, G. M. J. Organomet. Chem. 1977,129, C9.
160 Organometallics, Vol. 5, No. I , 1986
Brown et al.
Table 11. Proton S h i f t s and J Values for Cycloheptatriene, Methoxycycloheptatriene, and (q-1,6-C,H70R)M(C0)3 chemical shift values, ppm J values, Hz comDlex H3,4 H z , ~ HI,B Hi OCHB OCH2 CH3 J3,4 J2,3=4,5 J1,2=5,6 J1.?=6,? 6.6 6.2 5.4 2.2 6.7 6.2 5.5 3.4 3.4 8.6 7.0 9.3 4.4 5.9 5.1 4.1 4.3 3.0 8.1 7.5 8.5 7.3 6.0 4.7 3.3 3.2 3.4 8.0 b 8.8 3.5 5.9 5.1 4.0 4.4 3.2 1.0 6.1 4.8 3.4 3.3 3.5 1.2 5.9 5.2 4.2 4.3 3.1 8.1 7.8 8.5 7.3 5.9 5.2 4.2 4.4 3.3 1.0 7.9 b 8.4 7.1 5.9 5.1 4.2 4.3 3.1 8.1 7.6 8.5 7.4 Pauson, P. L.; Todd, K. H. J. Chen. SOC.C 1970, 2317. *Coupling constants could not be measured due to difficulties in decoupling at H1.6
-+
NaOMe
L/M\
-lSCC
4
J/
CH2C'2
Y
Figure 3. Reaction scheme for the low-temperature reaction between [ (q7-C7H7)Mo(C0),]' and NaOCH3in CHzClzsolvent. in the formation of carboalkoxy adducts which rearrange dissociatively on heating to give the 5-ex0 ring adduct. Analogous results were obtained for the [ (q7-C7H7)W(CO),]+ cation in CH2C12. The infrared uco peaks for the alkoxymetal XII, the carboalkoxy XIII, and the "-ex0 ring product are given in Table I. The rate of reaction was faster that that of the Mo complex with peaks due to all three adducts present in the initial spectrum. The final spectrum, recorded after 35 min, showed only those peaks due to the 7-ex0 ring adduct VI1 along with some peaks due to decomposition. In pentane the rate of reaction increased and no carbomethoxy species were observed. However, the change in solvent polarity had no effect on the initial site of attack (metal attack). The results of the analogous reaction with the tungsten cation I11were similar and are given in Table I. The reaction of ethoxide in CH2C12,at -15 OC, with the Mo cation I1 also gave metal (XIV), carbonyl (XV), and ring attack (VI) although the rate of reaction was faster than that of the analogous methoxide reaction. Further qualitative evidence for adducts in the reaction between alkoxide and the Mo and W cations was observed when preparing the 7-exo-alkoxy ring products in the respective alcohols. On additon of alkoxide to the orange (14) King, R. B.; Fronzaglia, A. Inorg. Chem. 1966, 5, 1837.
cation slurry in alcohol, a clear deep red color formed which persisted for approximately 20 s at room temperature before the formation of the final orange solution which is characteristic of the 7-ex0 ring adduct. In contrast, no adducts were detected with [(q7-C7H7)Cr(CO),]+ in either CH2C12or pentane as solvent and only the 7-exo-alkoxyring product was obtained. This is consistent with the fact that metal attack products are rare for the chromium cation; Le., no reaction occurs with halide ions whereas the Mo and W cations readily form (q7C7H7)M(CO)21.The anomalous behavior of the Cr cation cannot be explained by steric factors or the suggestion that the tropylium ring is more electrophilic than in the Mo and W cations which is contradicted by the charges calculated in the preceding paper (Table 11). It was shown also that for the three cations [(q7-C7H7)M(C0),]',where M = Cr, Mo, and W, the metal is the most favorable initial site of nucleophilic attack although the difference in interaction energies between ring and metal attack is much smaller for the Cr cation than the Mo and W analogues.
Spectroscopic Results for 7-Exo and 7-Endo Ring Adducts Both NMR15 and mass spectroscopy16have been used to distinguish between the 7-ex0 and (7-endo-q-1,6C7H7L)M(CO),,where M = Cr, Mo, and W. lH NMR Spectra. The substitution of a proton by a methoxy group at the 7-position of cycloheptatrieneresults in little change in the lH NMR spectrum. Two-dimensional J-resolved studies projected along the F1 domain, for the 7-methoxycycloheptatriene, showed three singlets, for the olefinic protons, indicating that H, = H4, H2= H5 and H1 = H,. Addition of the Cr(CO), moiety to form the (7-endo-q-l,6-C,H70CH3)Cr(CO), complex does not affect the equivalence of these protons but each set of protons is shielded to a different extent. The X-ray structure of (q-1,6-C4H8)Mo(CO)3,17 assumed to be similar to (7-exoq-1,6-C7H70R)Mo(C0),,shows that the metal atom is not equidistant from the olefinic carbons with the M-C3,4 distance 2.31 A, M-C2,6 distance 2.35 A, and the M-C,,6 distance 2.45 A. This "ring slippage" may be a contributing factor toward the unequal shielding of the olefinic protons. Increasing the size of the metal from Cr to W in the 7exo-alkoxy complexes has little or no effect on the chemical shift of any of the protons. However, comparison between the 7-ex0 and 7-endo chromium complexes shows a marked difference in chemical shift. This deshielding of the endo ligand has been observed previously with related complexesl8 and suggests some direct interaction between the (15) Pauson, P. L.; Smith, G. H.; Valentine, J. H. J. Chem. SOC.C 1967, 1061. (16) Muller, J.; Fenderl, K. Chem. Ber. 1970, 103, 3128. (17) Dunitz, J. D.; Pauling, P. Helu. Chim. Acta 1960,43, 2188.
Tricarbonyltropyliummetal Complexes
Organometallics, Vol. 5, No. I , 1986 161
NMR Shift Values for Cycloheptatriene, Methoxycycloheptatriene, and (q-1,6-C7H70R)M(CO)3 chemical shifts, ppm comp1ex c3,4 c2,5 c1,6 c7 OCH3 OCHp CH3 (co)3 131.3 127.3 121.3 28.7 C7Hs" 130.9 125.3 123.0 77.8 56.3 C7H70CH3 94.7 61.7 73.8 56.8 b 7-end0-0CH~C~H,Cr(C0)~ 97.3 98.3 67.2 70.6 52.5 230.3 ~ - ~ X O - O C H , C ~ H ~ C ~ ( C O 99.5 )~ 98.3 67.7 68.7 60.0 15.1 230.9 ~ - ~ X O - O C ~ H , C ~ H ~ C ~ ( C 99.4 O)~ 68.7 72.6 52.8 217.9 ~-~xo-OCH~C~H~MO(CO 101.0 )~ 97.0 Table 111.
"Mann, B. E. J. Chem. Soc., Chem. Commun. 1978, 976. *No resonance was observed for the carbonyl carbons.
Table IV. Mass Spectra Values for (q-1,6-C7H70R)M(CO),, Where M = Cr, Mo, and W and R = CHI and C2H5 7-exo7-exo7-exo7-exo7-endo7-exo(C7H70CH3)(C7HjOCH3)(C7H70CH3)(C7H70CH3) (C7HjOCZH5)(C7HjOCZH5)Mo(C0)3 W(co)3 Cr(C0I3 Cr(C0h Cr(C0)S Mo(C0)3 amu intens amu intens amu intens amu intens amu intens amu intens fragment 318 20.70 C,H,ORM(CO)nt 258 35.48 258 13.10 304 47.80 390 53.40 272 7.20 230 362 8.17 244 3.30 290 2.65 230 9.74 7.61 276 C;H;ORM(CO);+ 8.30 18.41 0.49 227 1.26 273 359 28.80 227 2.41 273 9.79 W7M(C0)3+ 227 CjH70RM(CO)+ 202 23.70 202 4.97 248 20.49 334 16.50 216 6.17 262 6.34 25.98 0.15 199 8.51 245 331 35.52 199 6.76 245 9.68 C7H7M(C0)zt 199 48.77 174 15.41 220 C7H70RMt 174 100.00 306 81.72 188 15.96 234 18.58 0.85 171 303 53.24 171 7.13 217 11.38 C7H7M(CO)+ 171 11.41 217 86.95 159 25.24 205 C7H70M' 159 100.00 26.34 291 54.85 159 12.14 205 16.66 21.19 81.23 C7HsM+ 144 144 24.57 190 276 43.04 144 74.38 190 4.86 4.69 143 CjH7Mt 143 20.94 189 67.80 275 83.98 143 16.46 189 29.67 C7H6Mt 142 4.84 142 1.71 188 47.26 274 60.03 142 4.20 188 20.06 50.91 84.25 91 100.00 91 100.00 91 100.00 91 100.00 91 C7H7; 91 15.88 2.55 78 2.90 78 78 3.30 78 14.04 78 26.90 C6H6+ 78 65 36.57 65 32.64 65 13.13 14.34 13.36 65 18.89 65 C5H5 65 11.65 H+ 52 82.19 52 43.31 98 184 52 28.65 98 9.36
metal and 7-endo-alkoxy ligand. The main points of interest with the coupling constants involves the J1,7=;6,7 and, to a lesser extent, the J1,2=5,6 values (Table 11). These values remain unchanged at 7.33 and 8.50 Hz, respectively, for the 7-exo-methoxy series, irrespective of the metal. The accuracy of the coupling constants and chemical shift values were confirmed by computer-simulated spectra using the NMRIT progam.lg The expanded FTlOO 'H NMR spectrum of the seven ring protons of the (7-exo.rl-1,6-C7H70CH3)Cr(C0)3 complex and its computer-simulated spectrum are shown in Figure 4. 13C N M R Spectra. Comparison of the 13C NMR spectra of cycloheptatriene and 7-methoxycycloheptatriene shows that C7 is deshielded by the methoxy group by ca. 50 ppm. In (7-endo-~pl,6-C~H~OCH~)Cr(CO)~ the ring carbons are shielded but, as with the proton spectra, not equally (Table 111). Comparison with the exo isomer shows, once again, differences at the c1,6and C7positions with c1,6of the exo complex deshielded by 5.5 ppm more than the endo isomer while the C7is shielded by 3.2 ppm in accord with the general assumption that the metal deshields the endo ligand in metal complexes.20 Replacement of Cr with Mo in the 7-ex0 series results in little change in the ring I3C chemical shifts although the carbonyl carbons are shielded by 12 ppm. Mass Spectra. The mass spectra of the exo and endo isomers of ( V - ~ , ~ - C ~ H ~ O C H ~ )showed C~(CO that ) ~ preferential elimination of the three carbonyls occurs in the endo case whereas loss of the methoxy group occurs only in the exo case. Such differences in mass spectra were proposed as a means of distinguishing the isomers.16 In (18) Brown, D. A.; Fitzpatrick, N. J.; Glass, W. K.; Sayal, P. K. Organometallics 1984, 3, 1137. (19) Detar, D. F. 'Computer Programs for Chemistry"; W. A. Benjamin: New York, 1968; Vol. 1. (20) Bandara, B. M. R, Birch, A. J.; Raverty, W. D. J. Chem. SOC., Perkin Trans. 1982, 1745.
7 - E X O - OME
J
;Ipm
5.5
CR
C7H7
,
,
,
,
(CO1
3
,
,
l
5.3
,
,
,
1.5
,
,
,
,
,
,
I .3
Figure 4. (a) Expanded 'H NMR spectrum and (b) computershowing OCH~)C~(CO) simulated spectrum of ( ~ - ~ x o - ~ - ~ , ~ - C ~ H ~ the ring protons.
the present ( ~ - ~ X O - . ~ ~ - ~ , ~ - C series, ~ H ~where O R )M M(CO)~ = Cr, Mo, and W and R = CH3and C2H5,the preferential loss of the alkoxy ligand occurs in accordance with the above suggestion (Table IV). However, the increasing intensity of the (M - OCH,)' fragment across the series Cr to W indicates increasing ease of removal of the CH30group, suggesting that for the endo isomers alkoxy loss may also be more significant in the Mo and W cases. Thus
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 attack of alkoxide on [(v~-C,H,)M(CO)~]+, where M = Mo 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. A m . 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
