Current Fluoroorganic Chemistry - American Chemical Society

Chapter 3

An Issue of Synthesis Strategy: To Make or Rather to Transform Organofluorine Compounds

Downloaded by PURDUE UNIV on September 2, 2014 | http://pubs.acs.org Publication Date: January 11, 2007 | doi: 10.1021/bk-2007-0949.ch003

Manfred Schlosser, Qian Wang, and Fabrice Cottet Institute of Chemical and Engineering Sciences, Ecole Polytechnique Fédérale, CH-1015 Lausanne, Switzerland

Virtually all organofluorine compounds are man-made. This fact immediately brings up the critical question of how and at what moment to integrate fluorine into the carbon backbone. One may import the featured halogenfroman inorganic source which can be either a nucleophilic (e.g., hydrogen fluoride) or an electrophilic reagent (e.g., perchloryl fluoride). Alternatively, one may deliver the required number of fluorine atoms attached to a C species (e.g., as a trifluoromethyl group). Finally, one may startfroma commercial compound which carries the right number of fluorine atoms at the right places and which just needs to be fitted with the lacking functionality. The latter approach is particularly appealing if diversity-oriented, as opposed to target-oriented, synthesis is the objective. 1

© 2007 American Chemical Society

In Current Fluoroorganic Chemistry; Soloshonok, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

39

40


Fluorine is undeniably overrepresented in the life science arena. According to recent estimates (7), up to, respectively, 20 and 40 % of new low molecularweight active principles in the pharmaceutical and agricultural (phytosanitary) sectors harbor one or several atoms of fluorine. Subtle changes in shape and size, acidity, dipole moments, polarizability, lipophilicity, transport behavior and chemical and metabolic stability (or fragility) have been evoked to explain why the "dope" of the lightest halogen is so effective in fine-tuning biological properties and, as a corollary, in improving the chances of scoring a hit by almost tenfold (2). Paradoxically, only a handful of organofluorine compounds occurs in nature and even those only in insignificant amounts. Thus, all commercial fluorinecontaining substances are man-made. This raises a principal question in relation with synthesis and manufacture planning. Should the fluorine label be introduced at the very beginning or at the very end or somewhere in the middle of the product assembly chain ? Often people opt for one of the two extremes. They either attach the halogen to a prefabricated carbon backbone as late as possible or they elaborate a fluorine-adorned starting material in a multistep sequence to the desired final product. Whatever the practical approach, its merits and shortcomings have to be evaluated on a from case to case basis.

The Direct Fluorination The ultimate precursor to any fluorinating reagent is the fluorspar (apatite, calcium fluoride) from which, among other things, hydrofluoric acid, sulfur tetrafluoride and elemental fluorine are derived. Any laboratory or pilot plant that disposes of the appropriate equipment and know-how to handle such corrosive liquids or gases feels tempted to make their own organofluorine chemicals. Whereas hydrogen fluoride (J) and potassium fluoride, for "halex" reactions (4), can be most advantageously applied on an industrial scale, academic researchers are more inclined to work with small quantities and to use more sophisticated reagents.

Electrophilic Fluorinating Reagents The fluorination of electron-rich carbon entities can be readily achieved with perchloryl fluoride (5). This versatile reagent was first employed to convert diethyl malonate into diethyl fluoromalonate (1) and diethyl difluoromalonate (2). As the intermediate enolates were set free under acid-base equilibrating conditions, inevitably mixtures of zero-, mono- and difluorinated products were obtained (6).


41 Na-OR 9 FCIO3 ZT-** Na CH(COOR)

Na-OR

Q

CH (COOR) 2

2

2

[OR = OC H ] 2

5

* CHF(COOR)

2

9

FCI0

Q N

a CF(COOR)

3

*- CF (COOR)

2

2

1

2

2

Such "fluorine scrambling" can be prevented if one produces an organometallic compound irreversibly in an ethereal solvent. In this way, fluorocyclooctatetraene (3), a model compound for dynamic nmr studies, was accessed from bromocyclooctatetraene via the lithium species, if in just 10 % yield (7).


3

Prior to this landmark work, 2-thienyllithium, 6-methyl-2-thienyllithium and 2-benzothienyllithium, all generated from the halogen-free heterocycles by permutational hydrogen-metal rather than halogen-metal interconversion, provided the 2-fluoro derivatives 4a - 4c in 44 - 70 % yield when treated with perchloryl fluoride (8). It remains obscure why the authors failed to convert phenyllithium into fluorobenzene under the same conditions (8).

[ a : R = R* = H; b: R = H C, R' = H; c : R + R' = H C ] 3

4

e

Actually, the method proved generally applicable (9). This was demonstrated by the preparation of open-chain or cyclic fluoroalkanes (5 - 7; 39 - 83 %), a fluoroalkene (8; 45 %), several fluoroarenes (9 - 11; 42 - 94 %) and, always in aprotic medium, oc-fluoroacetophenone (12; 44 %). In the case of o> fluorophenylacetylene, too reactive to survive in the presence of strong nucleophiles, only its selfcondensation product l,4-diphenyl-l,3-butadiyne (13; 32 %) was isolated in moderate yield (9).


42


Perchloryl fluoride has the bad reputation of being explosive. This is not quite exact. When left in contact with ethereal solvents or amines, it triggers a radical chain process producing chloric acid. The latter behaves like a time bomb. A t any moment it may disproportionate exothermally, under detonation, to hydrochloric acid and perchloric acid. To preclude any hazards one has to monitor continuously the consumption of the reagent (most simply by using a wet, potassium-iodide impregnated paper strip) and to avoid rigorously its accumulation in the presence of aprotic organic solvents. Safety considerations fostered the development of Af-fluorinated surrogates, in particular W-fluorosulfonamides (10, 11), N-fluoroquinuclidinium salts (12) and A^chloromethyl-N-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (75). However, all these reagents suffer from drawbacks. Except those which are commercial, they have to be prepared using elemental fluorine. Most of them are insufficiently soluble in ethereal media and many other solvents and, all in all, they are only moderately reactive. The fluorinated camphenes Z-14, £ 14 and 15 were required for an olfactive comparison with the halogen-free parent substance (14, 75). Whereas 10,10-difluorocamphene (15) was easily obtained by Horner-Wittig reaction from camphenilone, the fluorination of (Z)and (£)-10-camphenyllithium, generated by halogen/metal permutation with butyllithium from (Z)-10-bromocamphene or (7£)-10-iodocamphene, succeeded solely when N-te^butyl-W-fluorobenzenesulfonamide (lib) was employed (75/

Nucleophilic Fluorination Reagents A most convenient laboratory-scale reaction is the vicinal bromofluorination using N-bromosuccinimide in the presence of an excess of hydrofluoric acid, either neat or complexed with pyridine or an aliphatic amine. In this way,


43


(2-bromo-l-fluoroethyl)arenes and, after base-mediated dehydrobromination, (2-bromo-l,l-difluoroethyl)arenes (16) became readily available from styrenes (16). Alternatively, they could be prepared by heating oc-bromomethyl aryl ketones with diethylaminosulfur trifluoride [DAST] (17).

Although they reacted quite reluctantly, the (2-bromo-l,l-difluoroethyl)arenes (16) were amenable to nucleophilic substitutions. The thioethers or primary alcohols thus obtained were oxidized by means of the Pummerer method or the Swern reagent to the corresponding a-aryl-a,a-difluoroacetaldehydes (17) (18, 19).

F

OCOCH

3

F

OH

One of these p,P-difluoroacetaldeydes was elaborated to a-amino-P-(4-chlorophenyl)-P,P-difluoropropionic acid (18; also obtained in enantiomerically enriched form). p-(4-Chlorophenyl)-P,p-difluoro-a-hydroxypropionic acid was formed as a by-product in the critical reductive amination of the key intermediate P-(4-chlorophenyl)-P,P-difluoro-a-oxo-propionic acid. The oc-oxo acid proved to be a potent and selective inhibitor of arogenate dehydrogenase, the enzyme that catalyzes the last step of the phenylalanine biosynthesis (19).



44

Package Deal

A trifluoromethyl substituent incorporated in an aliphatic, aromatic or heterocyclic structural motif can be ordinarily created by treating a carboxy function with sulfur tetrafluoride in the presence of anhydrous hydrogen fluoride (20). However, the entire C F group may instead be delivered as a package from a suitable precursor, in particular the so-called Prakash-Ruppert reagent (21) trimethyl(trifluoromethyl)silane, to the target compound where it displaces a bromine or iodine atom. This attractive alternative option is based on the pioneering work of mainly Japanese researchers (22 - 26) and has been optimized recently (27). 3

COOH

f\

J\S\f

CF

Br(I)

3

—^— • HF

l*S d\J\f

Cul + KF

v / W

Our main objective was to extend the scope of the method toward sterically hindered arenes, a topic being featured elsewhere (2), and, on the other hand, to heterocyclic substrates. Thus, for example, 3-chloro-2-(trifluoromethyl)pyridine (46 %), 5-chloro-2-(trifluoromethyl)pyridine (72 %), 2-chloro-6-(trifluoromethyl)pyridine (89 %), 2-chloro-3-(trifluoromethyl)pyridine (91 %), 2-chloro-4(trifluoromethyl)pyridine (90 %), 3-chloro-4-(trifluoromethyl)pyridine (69 %), 3-bromo-5-(trifluoromethyl)pyridine (72 %), 2-bromo-5-(trifluoromethyl) pyri dine (42 %), 2-bromo-6-(trifluoromethyl)pyridine (93 %), 3-bromo-5-chloro-2(trifluoromethyl)pyridine (69 %), 3,5-dibromo-2-(trifluoromethyl)pyridine (87 %) and 5-bromo-2-chloro-4-(trifluoromethyl)pyridine (64 %) were prepared (27, 28). As a systematic comparison had revealed (27), the I/CF displacement occurs more readily than the B r / C F displacement and thus provides higher 3

3


45 yields. Moreover, bromoiodoarenes can be selectively converted into bromo(trifluoromethyl)arenes which subsequently may be subjected to a heavy halogen/metal permutation followed by fimctionalization (28).

!,

tt ' rr ' n cf

tx" *xx°

Cl

B r ^ V Br^N^CF, Sj^CF S|"SF, %"Sl The C i F "package delivery" option is not restricted to trifluorometh ylation (n = 3). A famous industrial process is based on chlorodifluoromethane (n = 2) as a fluorine carrier. Upon deprotonation with sodium hydroxide and ensuing elimination of sodium chloride, it sets free difluorocarbene which readily combines with phenolates to afford aryl difluoromethyl ethers (29) which often exhibit intriguing biological activities (lb, 30). Dibromofluoromethyllithium can be readily produced from tribromofluoromethane (n = 1) by butyllithium-promoted halogen/metal permutation (31). The addition or condensation products obtained after electrophilic trapping should be amenable to selective debromination using, for example, tributyltin hydride as the reducing agent. In situ generated fluoromethoxide enables the straightforward preparation of fluoromethyl carboxylates (32). The structure of tris(dimethylamino)sulfonium trifluoromethoxide (33) and reactions of trifluorothiolate (34) have to be also mentioned in this context. \


CF

'

3

n

Fimctionalization of Fluorinated Core Compounds

It is amazing how many fluoroarenes and fluoroheterocycles can be ordered from specialized suppliers at affordable prices. However, most of such core compounds are devoid of functional groups and need to be modified first before they qualify as building blocks in pharmaceutical or agricultural research. Organometallic intermediates are ideal for this purpose as they offer an almost illimited choice of possible transformations. The standard electrophile used to trap such species is of course carbon dioxide. A carboxylic acid, usually well crystalline, is isolated after neutralization. But in the same way one can employ N, Af-dimethylformamide, aldehydes or ketones, oxiranes, sulfur dioxide, thionyl or sulfuryl chloride and a variety of phosphorus halides to access aldehydes,


46


alcohols, sulfinic or sulfonic acids, phosphinic or phosphonic acids and numerous other classes of useful derivatives. Product versatility is one big advantage of the organometallic approach to functionalized fluoro compounds, its positional flexibility another. Let us assume we have identified a lead structure and wish to couple it with a fluoroindolecarboxylic acid as a complementary subunit. Even i f we confine the halogen and carboxy group to the benzo ring, there are still 12 possible regioisomers (for examples, see the end of this Chapter). Unless we know (how ?) in advance which substitution pattern satisfies the requirements best, we should make all possible combinations and test the resulting samples empirically. Such a regiochemically exhaustive fimctionalization is a priori a challenge to organic synthesis. To facilitate the task we have developed the "toolbox methods" (35). They will be illustrated below by representative examples. The most acidic site of 2-fluoropyridine is the 3-position. When treated consecutively with lithium diisopropylamide in tetrahydrofiiran and dry ice, the acid 19a was isolated in 84 % yield after neutralization (36). The 3-position was blocked by the introduction there of a chlorine atom using 1,1,2-trichloro-1,2,2trifluoroethane as the reagent (85 % yield) which at the same time activated the 4-position. Another metalation, this time performed with butyllithium at -75 °C, and trapping with dry ice afforded 3-chloro-2-fluoropyridine-4-carboxylic acid and, after reductive dechlorination 2-fluoropyridine-4-carboxylic acid (19b; 62 % yield over all). Whereas chlorine activates adjacent positions, trialkylsilyl groups screen them by steric hindrance. Thus, metalation of 3,4-dichloro-2fluoropyridine and 3-chloro-2-fluoro-4-(trimethylsilyl)pyridine (formed by reaction of the 4-lithiated intermediate with trichlorotrifluoroethane and chlorotrimethylsilane in 80 and 90 % yield, respectively) followed by carboxylation and deprotection gave 6-fluoropyridine-3-carboxylic acid (19c; 57 % over all) and 6-fluoropyridine-2-carboxylic acid (19d; 22 % over all) (36).



47 The skilful deployment of protective groups was also crucial for the regioexhaustive fimctionalization of 3-fluorophenol, the methoxymethyl acetal of which was rapidly attacked by sec-butyllithium at the 2-position, as expected, leading to 2-fluoro-6-hydroxybenzoic acid (20a; 93 %). The two acids 20b and 20c became available via the same intermediate 2-chloro-l-fluoro-3-(methoxymethyl)benzene, a substrate blessed with "optional site selectivity" (57). Whereas butyllithium deprotonated the oxygen-adjacent 4-position (after deprotection 66 % of acid 20b over all), lithium 2,2,6,6-tetramethylpiperidide (LITMP) attacked exclusively the fluorine-adjacent 6-position (61 % of acid 20c over all). The access to the last acid 20d (26 % over all) featured [(2-fluoro-6(methoxymethoxy)phenyl]trimethylsilane as the key intermediate. This was sub jected to metalation with the L I C - K O R superbase, chlorination, new metalation (with LITMP), carboxylation and deprotection (38).

OR

^

OR

OR

OR

fY - (fY V^COOH OH 20a / /

20d

0 H

lY - l Y - lY

Li^V^CI OR

OR \

HOOC"V^ OR

/ \

/

ci OR

OR

OR

Li F

COOH . ^ F

y x i

y x i

OR

OR

HOOC'Y OH 20b COOH ^ , F

OH 20c

[ OR = OCH OCH ] 2

3

a-Alkoxyalkyl entities such as methoxymethyl or 2-tetrahydropyranyl are excellent protective groups for phenolic hydroxyls as they can be easily attached and removed. Moreover, compared to simple alkyl groups, they accel erate orfAo-metalation. It is nevertheless fortunate to dispose of a complementary option. The triisopropyl group sterically and electronically shields the vicinity of the phenolic oxygen atom and thus directs the attack of the base (in the given context always sec-butyllithium complexed with N,N,N\N",N"-pentemethy\diethylenetriamine) to a remote position. Thus, depending on the protective group employed, 2-, 3- and 4-(trifluoromethoxy)phenols (21a - 21c; scheme overleaf) were functionalized either at an O H - or an OCF -adjacent site (40). 3

This peripheral control of site selectivity became the cornerstone of the regioexhaustive fimctionalization of 3-chlorophenol. Chlorine, which had played


48 OH HOOC JÔCF3

OR Li^JÔCF,

COOH HO JvÔCF

Li RO.JsÔCF,

>

OH

HOOC^T OH



49 As the last examples show convincingly, the "toolbox methods" (23) are not only applicable to fluoro and trifluoromethyl bearing aromatic and heterocyclic substrates but also to chloro and oxygen substituted analogs. The site selective transformation of 4,7-dichloroquinoline, the precursor to the antimalarial chloroquine, sustains this claim. The substrate offering threefold "optional site selectivity", the three acids 23a (74 %), 23b (17 %) and 23c (54 %) were selectively produced by merely varying the metalating reagent (LITMP in diethyl ether, butyllithium in the presence of lithium 2-dimethylaminoethoxide in diethyl ether and lithiumdiisopropylamide in tetrahydrofuran, respectively). In order to synthesize acid 23d (12 % over all), one had to "silence" consecutively the 3- and 8-position by attaching trimethylsilyl groups. Thereafter deprotonation occurred exclusively at the 6-position (41). In order to access the remaining acid 23e, one would have to introduce a 5-bromine atom and to promote its dislocation to the 4-position by deprotonation-triggered heavy halogen migration (23) before terminating in halogen/metal permutation, carboxylation and protodesilylation.

COOH

Until recently not a single fiinctionalized derivative of 4-, 5-, 6- or 7fluoroindole had been reported. We have elaborated a convenient entry to all twelve regioisomers wherein both the functional group and the halogen are accommodated at the benzo ring (42). Only 6-fluoroindole underwent metalation directly, whereas the three other isomers, unless iV-silyl protected, proved inert toward even strongest bases. Thus, the 6-fluoroindole-7-carboxylic acid (24a, 60 %) was obtained in a most straightforward manner (42). After attachment of a triisopropylsilyl group


50


metalation took place instead at the sterically unhindered 5-position thus paving the way to the acid 24b (71 % over all). Treatment of the 5-lithiated intermediate with iodine gave a 5-iodo derivative (59 %) which was converted into its 4-iodo isomer (88 %) by deprotonation-triggered heavy halogen migration. Halo gen/metal permutation followed by carboxylation and deprotection eventually provided 6-fluoroindole-4-carboxylic acid (24c; 90 %) (42).

A Synopsis "Methodology", which means the knowledge and critical evaluation of methods, is the basis for synthesis planning in any field, organofluorine chemistry included. What makes the latter area special are coincidences such as the notorious lability of fluorine labels in most aliphatic and many heterocyclic structures and inconveniences in the handling of high-performance reagents. Therefore, practical progress is especially welcome. One frequently recommended strategy is to minimize fluorine-related trouble by transferring the halogen to an organic backbone as late as possible, in other words toward the end of a reaction sequence. This advice is particularly apt when a single fluorine atom has to be introduced into a prefabricated skeleton or when a C i F entity is to be attached packagewise to a substrate. n

Other rules apply when the fluorine label is already incorporated into a commercially available starting material. The elaboration of such a core compound to the targeted final product by a sequence of reaction steps can be


51 accomplished in various ways. Being simple, safe and cheap, organometallic methods, as exemplified in the preceding Chapter, offer unique advantages in this respect, in particular functional group versatility (43, 44) and regiochemical flexibility (44).


Acknowledgment This work was supported by the Schweizerische Nationalfonds zur FcJrderung der wissenschaftlichen Forschung, Bern (grants 20-55303-98, 2063'584-00 and 20-100'336-02).

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53 Chem. 2004, 3793 - 3798; (c) Cottet, F.; Schlosser, M.; Tetrahedron 2004, 60, 3793 - 3 7 9 8 . (a) Clark, R. F.; Simons, J. H . ; J. Am. Chem. Soc. 1995, 77, 6618 6618; (b) Miller, T. G.; Thanassi, J. W.; J. Org. Chem. 1960, 25, 2009 2012; (c) Langlois, B.; J. Fluorine Chem. 1988, 41, 247 - 261. Leroux, F.; Jeschke, P.; Schlosser, M.; Chem. Rev. 2005, 105, 827 - 856. (a) Kuroboshi, M.; Yamada, N.; Tokebe, Y.; Hiyama, T.; Synlett 1995, 987 - 988; (b) Shimizu, M . ; Yamada, N.; Takebe, Y . ; Hata, T.; Kuroboshi, M . ; Hiyama, T.; Bull. Soc. Chim. Jpn. 1998, 71, 2903 - 2921. Schlosser, M.; Limat, D.; Tetrahedron 1995, 51, 5807 - 5812. Farnham, W . B.; Smart, B . E.; Middleton, W . J.; Calabrese, J. C.; Dixon, D. A.; J. Am. Chem. Soc. 1985, 107, 4565 - 4567. Adams, D . J.; Clark, J. H . ; Heath, P. A.. Hausen, L . B . ; Sanders, V . C.; Tavener, S. J.; J. Fluorine Chem. 2000, 101, 1 8 7 - 191. Schlosser, M.; Angew. Chem. 2005, 116, 380 - 398; Angew. Chem. Int. Ed. Engl. 2005, 44, 376 - 3 9 3 . Bobbio, C.; Schlosser, M.; J. Org. Chem. 2005, 70, 3039 - 3045. (a) Katsoulos, G.; Takagishi, S.; Schlosser, M.; Synlett 1991, 731 - 732; (b) Mongin, F.; Maggi, R.; Schlosser, M.; Chimia 1996, 50, 650 - 652. Marzi, E . ; Bobbio, C.; Cottet, F.; Schlosser, M.; Eur. J. Org. Chem. 2005, 2116 - 2123. (a) Schlosser, M.; Strunk, S.; Tetrahedron Lett. 1984, 25, 741 - 744; (b) Schlosser, M.; Pure Appl. Chem. 1988, 60, 1 6 2 7 - 1634; (c) Schlosser, M . ; Modern Synth. Methods 1992, 6, 227 - 271. Marzi, E.; Schlosser, M.; Tetrahedron 2005, 61, 3393 - 3401. Schlosser, M.; Marull, M.; unpublished results (2003). (a) Schlosser, M . ; Ginanneschi, A . ; Leroux, F.; Eur. J. Org. Chem., in press (2006); (b) Ginanneschi, A.; Doctoral Dissertation, Ecole Polytechnique Fédérale, Lausanne (2005). Schlosser, M.; Gorecka, J.; Castagnetti, E.; Eur. J. Org. Chem. 2003, 452 - 462. Schlosser, M. (ed.); Organometallics in Synthesis : A Manual, 2nd edition, Wiley, Chichester, 2004.


Current Fluoroorganic Chemistry - American Chemical Society

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