Improved Catalytic Deoxygenation of Vicinal Diols and Application to

Chapter 11

Improved Catalytic Deoxygenation of Vicinal Diols and Application to Alditols

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Kevin P. Gable and Brian Ross Department of Chemistry, Oregon State University, Corvallis, OR 97331

Judicious modification of the ligand environment in L R e O complexes allows successful catalytic deoxygenation of vicinal diols using PPh as terminal reductant. There is a steric bias toward diol units at the end of a linear chain (versus internal diols) and for erythro internal diols over threo. The potential for synthetic applications is discussed. 3

3

Introduction Generation of organic compounds from feedstocks available in Nature usually requires manipulation of oxygen content. Most commodity chemicals (and consequently the vast majority of fine chemicals derived from them) are currently prepared by oxidation of petrochemical feedstocks (1). The reactions needed to generate the desired functionality require chemo- and regioselective introduction of oxygen atoms; while stereoselective oxygenation has not been important in commodity chemical production, it often becomes an issue in fine chemical production as in the pharmaceutical industry. Control over these selectivity issues coupled with the need to avoid overoxidation has created interest in understanding oxygen atom transfer reactions over the past decade (2). Consideration of renewable feedstocks as petrochemical substitutes also leads one immediately to the problem of manipulating oxidation levels, but in a different sense. Carbohydrates are the most common primary class of compounds available from renewable biomass (5), yet these compounds are © 2006 American Chemical Society

In Feedstocks for the Future; Bozell, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

143


144 much more highly oxygenated than desired for many uses. The challenge thus becomes removal of oxygenated functionality; again, regio-, chemo- and stereoselectivity are desired control elements. Although there are reactivities that can be used to accomplish deoxygenation (e.g., dehydration/hydrogenation (4)% these often require extreme conditions and lack the degree o f control that would allow retention of any of the molecular complexity provided in the original feedstock. Specific atom-transfer reactions can thus open new avenues for manipulation of these feedstocks. Scheme 1 illustrates a comparison of possible routes from feedstock chemicals to useful commodities based on both petroleum and biomass feedstocks. In these examples, a simple 3-carbon feedstock is chosen as the ultimate precursor; manipulation of the oxidation level illustrates either selective introduction of oxygen into propane or selective removal of oxygen from glycerol.

OH OH

OH

\

ο

Scheme 1. Comparison of oxidative and reductive routes to common commodity chemicals containing three carbons.

One may anticipate similar schemes based on commonly available compounds with specific carbon numbers: tartrates leading to C compounds; ribose or xylose leading to C 5 compounds; glucose, fructose, mannose or 4


145 galactose leading to C compounds. (While alditols are not, strictly speaking, carbohydrates, their production from true carbohydrates justifies their inclusion in this discussion. Glycerol itself is a true biomass-derived feedstock, being a hydrolysis byproduct in the production of biodiesel from renewable triglycerides (5).) 6


Rhenium-mediated deoxygenations Work in our laboratory has for some time focused on rhenium-mediated Oatom transfer reactions (6). The initial discoveries by Davison and by Herrmann that some Re(V) diolate complexes undergo thermal cycloreversion (7) (Eq. 1) led us to perform a series of mechanistic investigations. Our observations revealed substantial asynchronous character to the bond-cleavage process, but in the end the mechanism appeared to be kinetically concerted. Several other valuable observations accompanied these studies. The impact of the ancillary ligand appeared to be negligible (8), in that the activation enthalpy for Tp'ReO(OCH CH 0) (Τρ' = hydrido-/m-(3,5-dimethylpyrazolyl)borate) was only about 4 kcal/mol higher than that of the corresponding Cp* compound; Davison also noted the similarity between our results and those where L = {CpCo{(RO) P=0) } (9). The cycloreversion was studied predominantly in nonpolar solvents (e.g., benzene), but investigation of solvent effects noted only a doubling of rate on going to acetone, clearly discounting formation of a polar intermediate. 2

2

2

3

During these studies, Cook and Andrews published the first observation of catalytic rhenium-mediated diol deoxygenation (10) (Eq. 2). Their studies were specifically directed toward alditol manipulation. These results showed a turnover frequency for 1,2-phenylethanediol that was consistent with the stoichiometric rate of diolate cycloreversion, indicating that reduction of Cp*Re0 and subsequent cyclocondensation of the diol on the rhenium were 3


146 both rapid reactions. While successful catalytic turnover of a series of alditols was observed, in each case the total turnover number was limited by precipitation of a purple solid the investigators suggested might be an overreduced Re(III) species.

\ Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: January 12, 2006 | doi: 10.1021/bk-2006-0921.ch011

OH

FA-

0H

(2)

b

or (cat.) PPh

3

For a variety of reasons, we began to investigate epoxide deoxygenation. Use of the reduced Re(V) dimer {(Cp*ReO) ^-0) } led to high yields of alkene (11). However, when the reaction was performed using less than stoichiometric amounts of rhenium, poor turnover was observed; as in the case of Cook & Andrews, we observed a purple solid forming. Closer investigation revealed that this solid was not a Re(III) compound, but that it formed on conproportionation of {(Cp*ReO) ^-0) } with C p * R e 0 (Scheme 2). Crystal lographic characterization (12) revealed the compound was an ionic tetranuclear cluster (with perrhenate as the counterion). The structure is shown in Fig. 1. Accompanying this tetranuclear species was also a green, dicationic cluster that had first been observed by Herrmann during aerobic reactions between C p * R e 0 and PPh (75); we demonstrated that a variety of oxidants could convert the purple tetranuclear cluster into the green trinuclear species. A critical key to generating a long-lived catalyst was clearly the development of a ligand system that could inhibit clustering while maintaining the substrate's access to the metal center. The prior observation that the pyrazolylborate ligand was so similar to Cp* in diolate cycloreversions made it a prime candidate. The methylated Τρ' ligand is unusual in that its cone angle is 255° (14), but the interstices of the three pyrazole rings still allow access to the metal. Tp'Re0 successfully catalyzes transfer of oxygen from epoxides to triphenylphosphine (Eq. 3) (75). The system is largely insensitive to other functional groups (16), although unprotected alcohols condense to form an inactive Re(V) bis-alkoxide, and good O-atom donors such as nitro groups also interfere. The system imposes several tradeoffs compared to the Cp* system: the trioxo precatalyst is more air- and thermallystable than C p * R e 0 ; the stoichiometric reaction rates are, in general, slower; and the system is much more sensitive to steric properties of the epoxide for obvious reasons. 2

2

3

2

2

3

3

3

3


147 Ο

2 Cp*Re0

{Cp'ReOfefo-Ofc} \

Cp*Re0

\

R ^

Cp*Re0

2

/

3

r'

Cp* Ο—Re-Ο

—ΙΘ 1

PPho Re0

4

0=PPh


Cp'Re^

purple

3

^pReCp»

3

\?/ Re0

3

Cp*Re0

Cp*Re^

" Ί

3

+

2

(Re0 ") 4

g r e e n

2

i£o%\ Cp*Re^

^ReCp*

Scheme 2. Catalytic deoxygenation of epoxides with Cp*Re0 /PPh 3

3

Figure J. Cationic rhenium cluster formed by conproportionation of Cp*Re0 with f(Cp*ReO) fr-0) }.

3

2

2


148

R = Me, C H Ph, CH OtBu, CH OTMS COR, CH OCOR CF 1 0

2 1

2

(3)

2

2

3

ο

Λ


Ri

R

2

Ri

PPr.3

R

2

We next returned to catalytic deoxygenation of vicinal diols. Phenylethanediol was the first substrate chosen (in all stoichiometric reactions, the phenyl group has a significant accelerating effect). At 5 mol-percent rhenium, successful reduction was complete after 4 days at 100° C. The solvent used was toluene-d . One concern (Scheme 3) was that the equivalent of water generated by cyclocondensation of the diol with rhenium could have an inhibitory effect as it accumulated; this appeared not to be the case over the first 90% reaction. Independent experiments examining hydrolysis of related diolates had suggested the equilibrium constant for cyclocondensation is at least 100. 8

Scheme 3. Catalytic deoxygenation of vicinal diols with Tp'Re0 . 3

One may predict the turnover frequency based on our earlier measurement of diolate cycloreversion. The observed rate constant for stoichiometric extrusion of styrene from diolate is 8.61 χ IO" s" , predicting a maximal turnover frequency at the observed [Re] of 0.009 M of 7.77 χ 10 " M-s" . A n intermediate measurement at 1002 minutes showed 33% conversion of 0.18 M diol, for a turnover frequency of approximately 9.9 χ IO* M-s" , within experimental error of the predicted value. It is of note that Arterburn and coworkers have developed a heterogenized rhenium catalyst that is also effective for diol deoxygenation (17). 5

1

7

7

1

1


149


Alditol substrates Glycerol was the next target substrate. Here we had the added potential challenge that the substrate has a low solubility in the toluene solvent. However, this did not inhibit formation or reaction of the diolate, indicating rapid mass transport relative to the (slow) cycloreversion of diolate. This substrate reacts more slowly than phenylethanediol; at 121° C, a reaction mixture containing comparable concentrations requires 5 days to reach completion. This is consistent with the electronic effect of switching substituents from an aromatic to an aliphatic group; there may be some impact from product inhibition (though condensation of the vicinal diol is expected to be preferred over formation of a bis-alkoxide until late in the reaction). Again, the presence and generation of water does not appear to have any significant impact at low concentration. Use of erythritol begins to explore the factors that control regiochemistry of the catalytic reaction. Here, cyclocondensation can lead to either a 1,2-diolate (Scheme 4; "terminal") or a 2,3-diolate ("internal"). The former will produce 3,4-dihydroxy-l-butene, while the latter will produce 1,4-dihydroxy-2-butene (presumably as the cis isomer). Secondary reduction of the terminal product will also produce 1,3-butadiene.

HOCH2 / HOCH2 syn

anti

Scheme 4: Regio- and stereoisomerism in erythritolate complexes A l l three products are observed (Eq. 4); the initial selectivity for terminal deoxygenation is reasonably high: the ratio of (1-ene + butadiene)/(2-ene) is 8.8:1 at 40% conversion (24 h at 121° C; 5 mol-per cent rhenium). The 1ene:butadiene ratio is still high at this stage at 7.2:1.6. As further reaction occurs, the butadiene proportion rises, and a slight drop in the total term inai: internal reaction product ratio is seen.


150 OH i—OH

5 mol-% Tp'ReO;

-OH

1.1 eq. P P h

OH

3

1.0

-OH L-OH

1 2 1 ° C 24 h toluene-d , 40% 8

7.2

OH

(4)

OH


1.6

Use of threitol leads to a very different outcome (Eq. 5). The reaction is slower (21% conversion after 24 h at 121° C), and there is no detectable internal alkene produced. The proportion of butadiene is also higher; 1-ene/butadiene = 1.4:1 at this point. OH

HO

(5)

Consideration of structure for the diolates highlights the likely origin of the differences in selectivity. Stoichiometric reductive cyclocondensation of glycerol with Tp'Re0 /PPh gives two diolate isomers in roughly equivalent amounts. 1-D and 2-D N M R experiments allow almost total assignment of the spectroscopic signals. The C H O H signals for the two isomers are at 4.03 and 4.42 ppm. Sequential nOe experiments reveal that irradiating a methyl signal at 2.33 ppm leads to enhancement of the signal at 4.42 ppm (as well as the other syn diolate proton further downfield). Irradiation of a nearby methyl signal at 2.30 ppm results only in enhancement of diolate signals at 5.53 ppm, and not at the hydroxymethylene position. The consequences of this full assignment are that the two stereoisomers are formed in approximately equal amounts. They react at different rates, though; after heating a sample to 100° C for 15 h, the remaining diolate was enriched in the exo isomer. Further experiments are needed to establish relative rates and to what degree interconversion can occur under stoichiometric and catalytic conditions. A similar analysis of structure for the erythritolates is more complex in that four different stereoisomers are anticipated as noted above. One aid to structural analysis lies in the symmetry properties of different isomers. The 2,3-diolates are C -symmetry, in which two of the ligand pyrazole rings are equivalent. The 3

3

2

s


151 1,2-diolates are nonsymmetric, and the pyrazoles are all unique. A count of the methyl signals for the ligand thus provides some sense of which isomers are present; a total of 20 signals are possible for the four isomers and 19 are readily evident. More significantly, one may discern 10 major signals, indicating that 1,2- and 2,3-isomers are present in roughly equivalent amounts: the C\ isomers each have six unique signals, and the C isomers have 4 in a 2:2:1:1 integral ratio. Given the difference between the selectivity outcome for cyclocondensation and that for production of terminal vs. internal alkenes, we can conclude that different diolates will have different reactivities. This is self-evident from our prior kinetic studies in that each alkyl substituent has a slight decelerating effect; one expects a preference for terminal deoxygenation over internal unless the internal diol has an electron-withdrawing substituent. Less important will be the difference in reactivity between syn and anti diolates, although if one or the other is particularly less reactive, it risks developing a thermodynamic sink that effectively removes active catalyst from the system. Further work is needed to quantitatively establish rates of reaction under stoichiometric conditions in order to derive a predictive model for selectivity. Preliminary work on higher alditols confirms that catalytic deoxygenation occurs, although full characterization is not complete. Xylitol shows a mixture of mono- and bis-alkene reduction products (with terminal selectivity). A protected mannitol, the 3,4-O-acetonide, also shows a degree of double reduction.


s

Prospects and applications We have successfully demonstrated the principle of selective catalytic reductive deoxygenation of vicinal diols. The challenges inherent to this transformation have hindered development of carbohydrate-based organic feedstock chemistry. While there remain aspects of our chemistry that keep it a laboratory methodology, there is now a clear route to development. The two obvious drawbacks to these rhenium-mediated reactions are the expense of the catalyst (rhenium being one of the rarer transition metals) and the disadvantage of using PPh as the terminal reductant. Preliminary experiments with Ph SiH suggest that silanes are promising reductants that could more easily be recycled; one would ideally like to develop a scheme wherein H or CO was the terminal reductant. Achieving this will strongly depend on the thermodynamic properties of the M=0 bonds produced in diolate cycloreversion; published data suggest the current ligand/metal system may not allow direct reduction by either. A less significant drawback is that the temperatures required for diolate cycloreversion are relatively high. Given that switching from Cp* to Τρ' induced an increase in AG* of 2-4 kcal/mol, it appears probable that judicious ligand design would allow both a decrease in the activation barrier while maintaining 3

3

2



152 the necessary steric bulk to inhibit the clustering reactions that occurred with the Cp* complexes. More work is clearly needed to establish the basis for internal/terminal reactivity and to document the capacity for diolate isomerization under the reaction conditions. Again, ligand design would appear to allow enhancement of the selectivity for formation of a terminal diolate, and a preliminary conclusion we can draw from the reactivity of erythritol is that the terminal diolate appears to be the more reactive intermediate. Several important applications can be immediately anticipated i f these issues can be resolved. Scheme 5 illustrates potential conversions of mannitol (or, indeed, sorbitol) derivatives into linear and cyclic Ce compounds.

CH OH 2

QUO

•

Cyclopentanoid compounds

Scheme 5. Compounds available from bisdeoxygenation of mannitol or sorbitol. Another immediate application would be direct conversion of glucose and other hexoses to glycals (Scheme 6) without protection/deprotection (18), Again, establishing the selectivity for cyclocondensation across cis vs. trans vicinal diol units must be established; the trans-dio late s presumably will not cyclorevert, and dynamic interconversion of isomeric diolates may become an advantage. Solubility and choice of solvent becomes a significant issue here; development of a water-soluble catalyst would open an important means of optimizing these as practical syntheses.



153

OH

Galactose

Scheme 6. Possible generation of glycols from hexoses. Finally, it must be recognized that recent developments in organic synthetic strategy open other applications. Alkene metathesis has become a standard strategy for synthesis of cyclic compounds (19). Given that our methodology gives rise to new alkene-containing synthons derived from the natural chiral pool, direct entry into chiral oxygen-containing rings becomes feasible (20), and use in constructing chiral carbocycles is possible with minor elaboration (Scheme 7).



154

Scheme 7. Strategy for using deoxygenated products in metathesis-driven ring construction.

Conclusion In contrast to petroleum-based chemistry, a common challenge in utilizing biomass feedstocks for fine chemical production is to reduce molecular complexity by specifically removing oxygenated functionality. The work described here advances this goal, demonstrating that a robust catalyst can achieve net reductive deoxygenation of the vicinal diol group. While important challenges remain, we have begun to understand the principles that govern selectivity.

Acknowledgment We thank the National Science Foundation (CHE-0078505) for partial support of this work.

References 1. 2.

Organic syntheses by oxidation with metal compounds Mijs, W. J.; de Jonghe, C. R. H . I., Eds. Plenum: New York, N Y , 1986. a. Espenson, J. H.; Adv. Inorg. Chem. 2003, 54, 157-202. b. Romão, C. C.; Kühn, F. E.; Herrmann, W. A . Chem. Rev. 1997, 97, 3197-3246.


155 3. 4.


5. 6. 7.

8. 9.

Carbohydrates as Organic Raw Materials II, Descotes, G., ed. V C H : New York, 1993. Mehdi, H . ; Horvath, I. T.; Bodor, A . Abstracts of Papers, 227 ACS National Meeting, CELL095. American Chemical Society: Washington, 2004. Ma, F.; Hanna, M . A . Bioresource Tech. 1999, 70, 1-15. Gable, K . P. Adv. Organomet. Chem. 1997, 41, 127-161. a. Pearlstein, R. H . ; Davison, A . Polyhedron 1988, 7, 1981-1989. b. Thomas, J. Α.; Davison, Α.; Inorg. Chim. Acta 1991, 190, 231-235. c. Herrmann, W. Α.; Marz, D.; Herdtweck, E.; Schäfer, Α.; Wagner, W.; Kneuper, H . J. Angew. Chem., Int. Ed. Engl. 1987, 26, 462-464. Gable, K . P.; AbuBaker, Α.; Zientara, K . ; Wainwright, A . M . Organometallics 1999, 18, 173-179. Cook, J. Α.; Davis, W. M . ; Davison, Α.; Jones, A . G.; Nicholson, T. L . ; Simpson, R. D . Abstracts of the 207 National Meeting of the American Chemical Society, INOR066. American Chemical Society: Washington, 1994. Cook, G . K . ; Andrews, M . A . J. Am. Chem. Soc. 1996, 118, 94489449. Gable, K . P.; Juliette, J. J. J., Gartman, M . A . Organometallics 1995, 14, 3138-3140. Gable, K . P.; Zhuravlev, F. Α.; Yokochi, A . F. T. J. Chem. Soc., Chem. Commun. 1998, 799-800. Herrmann, W. Α.; Serrano, R.; Ziegler, M . L . ; Pfisterer, H . ; Nuber, B . Angew. Chem., Int. Ed. Engl. 1985, 24, 50-51. Bergman, R. G . ; Cundari, T. R.; Gillespie, A . M . ; Gunnoe, T. B . ; Harman, W. D.; Klinckman, T. R.; Temple, M . D . ; White, D . P. Organometallics 2003, 22, 2331-2337. Gable, K . P.; Brown, E. C. Organometallics 2000, 19, 944-946. Gable, K . P.; Brown, E. C. Synlett 2003, 2243-2245. Arterburn, J. B.; L i u , M . ; Perry, M . C. Helv. Chim. Acta 2002, 85, 3225-3236. Roth, W.; Pigman, W., in Methods in Carbohydrate Chemistry, Whistler, R. L . ; Wolfrom, M . L.; BeMiller, J. N . , Eds. V o l . 2, pp. 405 408. Academic Press: New York, 1963. Furstner, Α.; Top. Catal. 1998, 4, 285-299. Voight, Ε. Α.; Rein, C.; Burke, S. D. J. Org. Chem. 2002, 67, 84899499. th

th

10. 11. 12. 13. 14.

15. 16. 17. 18.

19. 20.


Improved Catalytic Deoxygenation of Vicinal Diols and Application to

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