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Bulky Aluminum Lewis Acids As Novel Efficient and Chemoselective

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Bulky Aluminum Lewis Acids As Novel Efficient and Chemoselective Erschienen in: Synlett ; 1999 (1999), 5. - S. 584-586 https://dx.doi.org/10.1055/s-1999-2667 584 Bulky Aluminum Lewis Acids as Novel Efficient and Chemoselective Catalysts for the Allylation of Aldehydes Andreas Marx, Hisashi Yamamoto* Graduate School of Engineering, Nagoya University, CREST, Chikusa, Nagoya 464-8603, Japan Fax +81(52)789-3222; E-mail: [email protected] position of the bulky phenol ligands resulted in further ac- Abstract: Bulky aluminum Lewis acids such as MAD and MABR celeration of the rate. Thus, methylaluminum bis(4-bro- are introduced as new efficient catalysts in substoichiometric 8,9 amounts for the allylation of aldehydes with allyltributylstannane. mo-2,6-di-tert-butyl-phenoxide) (MABR) performed In addition, chemoselective allylation of a less hindered or aromatic the reaction between 1 and 2 to yield the desired product aldehyde was achieved with catalytic amounts of the carbonyl re- 3 in excellent yield (93%) within 1 h at -20 °C (Table 1, ceptor aluminum tris(2,6-diphenyl-phenoxide) (ATPH). Entry 5). It turned out that MABR is the reagent of choice Keywords: allylation, allyltributylstannane, Lewis acid catalysis, for the catalytic allylation of aldehydes with allyltributyl- MABR, ATPH stannane. It is noteworthy that aluminum tris(2,6-diphe- nylphenoxide) (ATPH)8,9 which is known to complex carbonyl functionalities very tightly due to its bowl-like The Lewis acid promoted allylation of aldehydes using shape11 is capable of promoting catalytic allylation of ben- allystannanes or their less reactive counterparts allylsi- zaldehyde as well (Table 1, Entry 6). Bulkiness of the phe- lanes is a versatile synthetic tool.1 Numerous Lewis acids noxide ligands in the organoaluminum Lewis acids seems ◊ to be a prerequisite for catalytic activity and efficiency. like TiCl4, SnCl4, BF3 EtO2, or AlCl3 promote this reac- tion but they seem to be required in stoichiometric This might be due to decreased unfavorable interactions amounts and there is little precedent for Lewis acid catal- of the bulky aluminum reagent with the reaction products ysis. Recently however, it has been shown that chiral (ROSnBu3) which would result in reduced catalytic effi- (acyloxy)borane,2 silver(I),3 titanium(IV),4 zirconium ciency. (IV),5 and rhodium(III)6 complexes respectively, are ca- pable of catalyzing the reaction of aldehydes with ally- Table 1. Allylation of Benzaldehyde 1 Catalyzed by stannanes or allylsilanes with excellent enantioselectivity, Various Lewis Acids although reaction times are very long (up to 70 hours). OH O 10 mol% Lewis acid Furthermore, these catalysts are often lacking broad sub- SnBu3 CH2Cl2 strate tolerance. Ytterbium trifluoromethanesulfonate has 12 3 been recently introduced as catalyst for the allylation of aldehydes with allyltributylstannane.7 We would like to Entry Lewis Acid Conditions Yield [%] report here the hitherto unknown capacity of bulky orga- (10 mol%) noaluminum reagents as very efficient catalysts for the re- 1 Me3Al -20°C -> 23°C , 12 h 7 action between aldehydes and allyltributylstannanes even 2 (iPrO)3Al -20°C -> 23°C , 12 h traces when they are employed only in catalytic amounts. 3 (p-CH3-C6H4O)3Al -20°C, 3 h 33 4 MAD -20°C, 3 h 87 To evaluate the ability of organoaluminum Lewis acids as 5 MABR -20°C,1 h 93 suitable catalysts for allylation of aldehydes the reaction 6 ATPH -20°C, 3 h 80 between benzaldehyde 1 and allyltributylstannane 2 was t-Bu Ph investigated as a test system. When Me Al or (i-PrO) Al, MAD R = Me 3 3 RMeOAl OAlATPH respectively, were used in substoichiometric amounts (10 2 MABR R = Br 3 mol%) to catalyze this reaction only trace amounts of the t-Bu Ph desired homoallylic alcohol 3 were yielded even after pro- longed reaction times and increased reaction temperatures We further investigated the scope of the MABR catalyzed (Table 1). However, 10 mol% of (p-MeC H O) Al was 6 4 3 reaction of various aldehydes with allyltributylstannane. able to afford the formation of 3 in 33% yield after 3 h re- Table 2 summarizes the results obtained. Characteristic action time at -20 °C. We were delighted by the finding features are: (1) aromatic aldehydes were allylated in ex- that the allylation reaction was accelerated to a great ex- cellent yields within short reaction times (Entries 1-4). tent when the exceptionally bulky Lewis acid methylalu- (2) In the reaction with a linear aliphatic aldehyde pro- minum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD) longed reaction time was necessary and the yield was low- was used.8,9 Ten mol% of MAD catalyzed the formation er (Entry 5). Lower reaction temperature (-78 °C) effected of 3 in 87% yield within 3 h at -20 °C (Table 1, Entry 4).8 the suppression of side reactions and best yields. (3) When Introduction of electron-withdrawing groups in the para- a a-substituted aliphatic aldehyde was employed the cor- Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-0-329625 585 responding homoallylic alcohol was formed in high CHO CHO chemical yield but low stereoselectivity (Entry 6). (4) In 45 the reaction with a,b-unsaturated aldehydes the 1,2-addi- a tion proceeded exclusively (Entry 7). Al Lewis Acid Ratio 6 : 7 (5 mol%) Table 2. Allylation of Various Aldehydes Catalyzed by ATPHb >99 : <1 10 mol% MABR CHO Al CHO Al MABRb 6 : 1 c 10 mol% MABR OH Yb(OTf)3 4.5 : 1 SnBu R O 3 CH2Cl2 R a Determined by 1H-NMR. b The SnBu 3 reaction was performed at -78°C for 36 - 48 h. c The reaction was Entry Aldehyde Conditions Yield [%] performed as described in ref. 5a. OH OH >>> OMe 67 1 CHO -20°C, 45 min 99 Scheme 1 MeO 2 -20°C, 45 min 99 CHO selective allylation of the less hindered aldehyde to form 6 exclusively (Scheme 1). The formation of the homoal- 3 Br CHO -20°C, 45 min 93 lylic aldehyde 7 through allylation of trimethylacetalde- 1 CHO hyde 5 was not detected by means of TLC and H NMR 4 -20°C, 45 min 90 analysis. In marked contrast MABR and Yb(OTf)3 afford- ed the allylation with sufficiently lower chemoselectivity (Scheme 1). CHO 5 Ph -78°C,5 h 75 Selected examples for the catalytic allylation of different aldehydes by ATPH are listed in Table 3. a 6 Ph CHO -95°C -> -78°C, 3h 92 Table 3. Chemoselective Allylation between Two Different Aldehydes with 5 mol% ATPH CHO 7 Ph -20°C, 2 h 90 Entry Aldehydes Homoallylic Alcohol a a As 1:1 mixture of diastereomers. Yield [%] (Ratio) b CHO 1 CHO ATPH has been known to tightly complex carbonyl func- 82 (>99:<1) tionalities in the cavity of its bowl-shape ligand scaffold.11 b It has been reported that ATPH is able to discriminate be- 2 CHO CHO 77 (24:1) tween structurally similar aldehydes and thereby facili- c tates the selective functionalization of a less hindered 3 CHO CHO - (1:3.3) aldehyde as it has been demonstrated for hetero-Diels-Al- der and aldol reactions.12 Recently it has been shown that 4d CHO CHO Diels-Alder reactions between a carbonyl and diene due to 89 (1:10) encapsulation of both substrates by ATPH can be 13 CHO CHO achieved. In all the reported examples stoichiometric c amounts or even an excess of ATPH (up to 2.2 eq.) has to 5 83 (>99:<1) be employed to achieve the desired selectivities. Encour- a Determined by 1H-NMR. b The reaction was performed at -78°C for 36 - aged by our results obtained in the reaction between alde- 48 h. c The reaction was performed at -20°C for 36 - 48 h.d The reaction was performed at -40°C for 48 h. hydes and allyltributylstannane catalyzed by bulky aluminum reagents we investigated whether these re- agents are capable to perform chemoselective catalytic al- Table 3 demonstrates that ATPH used in catalytic lylation of one alcohol out of an equimolar mixture of two amounts is able to promote highly selective allylation of structurally different aldehydes (Scheme 1). two aliphatic aldehyde carbonyl functionalities which dif- In our initial investigation it turned out that treatment of fer in their steric constraints. Thus, hexanal 4 when it is in an equimolar mixture of hexanal 4 and trimethylacetalde- an equimolar mixture with trimethylacetaldehyde or cy- hyde 5 with 5 mol% ATPH (1.00 mmol of each aldehyde clohexanecarboxaldehyde, respectively, is recognized by were treated with 0.10 mmol of ATPH) at -78 °C and sub- the Lewis acid receptor ATPH and selectively allylated in sequent addition of allyltributylstannane resulted in the good yield (Entries 1, 2). Unfortunately, attempted selec- 586 tive functionalization of one aldehyde out of a mixture of (4) (a) Costa, A. L.; Piazza, M. G.; Tagliavini, E.; Trombini, C. hexanal and benzaldehyde led to disappointing results Umani-Ronchi, A. J. Am. Chem. Soc. 1993, 115, 7001. (Entry 3). However, when two aldehydes of similar steric (b) Keck, G. E.; Tarbet, K. H.; Geraci, L. S. J. Am. Chem. Soc. 1993, 115, 8467. (c) Gauthier, D. R.; Carreira, E. M. Angew. demands were employed (benzaldehyde and cyclohexan- Chem. Int. Ed. Engl. 1996, 35, 2363. ecarboxaldehyde) functionalization of the aromatic alde- (5) (a) Bedeschi, P.; Casolari, S.; Costa, A. L.; Tagliavini, E.; hyde proceeded with satisfying selectivity (Entry 4). Umani-Ronchi, A. Tetrahedron Lett. 1995, 36, 7897. Furthermore, chemoselective allylation of two sterically (b) Casolari, S.; Cozzi, P. G.; Orioli, P.; Tagliavini, E.; Umani- different aromatic aldehydes can be achieved.
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