Rhenium-Catalyzed Addition of β-Enamino Esters to Allenes

Abstract

Treatment of β-enamino esters with terminal allenes in the presence of a catalytic amount of a rhenium complex, ­[ReBr(CO)₃(thf)]₂, gave α-alkenylated β-imino or β-enamino esters. In this reaction, a new carbon-carbon bond is formed between the active methylene moiety of the β-enamino esters and the β-carbon of the terminal allenes.

Key words

rhenium - β-enamino ester - allene - nucleophilic addition

References and Notes

• For reviews, see:
• 1a Houpis IN. Lee J. Tetrahedron  2000,  56:  817 
• 1b Beller M. Seayad J. Tillack A. Jiao H. Angew. Chem. Int. Ed.  2004,  43:  3368 
• 1c For example, there is a report on gold- and silver-catalyzed addition of active methylene compounds to styrenes, see: Yao X. Li C.-J. J. Am. Chem. Soc.  2004,  126:  6884 
• 1d For an example of an intramolecular reaction of 7-octene-2,4-dione, see: Qian H. Widenhoefer RA. J. Am. Chem. Soc.  2003,  125:  2056 
• 2a Kuninobu Y. Kawata A. Takai K. Org. Lett.  2005,  7:  4823 
• 2b For an indium-catalyzed example, see: Nakamura M. Endo K. Nakamura E. J. Am. Chem. Soc.  2003,  125:  13002 
• For the nucleophilic addition of β-enamino esters to terminal alkynes, see:
• 3a Fujimoto T. Endo K. Tsuji H. Nakamura M. Nakamura E. J. Am. Chem. Soc.  2008,  130:  4492 
• 3b Chun YS. Ko YO. Shin H. Lee S.-g. Org. Lett.  2009,  11:  3414 
• There have been several reports on the nucleophilic addition of active methylene compounds to activated allenes. See:
• 4a Paik YH. Dowd P. J. Org. Chem.  1986,  51:  2910 
• 4b Lu C. Lu X. Org. Lett.  2002,  4:  4677 
• 4c Patil NT. Pahadi NK. Yamamoto Y. Synthesis  2004,  2186 
• 4d Huang X. Shen R. Synthesis  2006,  2731 
• 5 There has been a report on the nucleophilic addition of active methylene compounds to unactivated allenes. See: Yamamoto Y. Al-Masum M. Fujiwara N. Asao N. Tetrahedron Lett.  1995,  36:  2811 
• There have been several reports on the intramolecular nucleophilic addition of active methylene compounds to allenes. See:
• 6a Meguro M. Kamijo S. Yamamoto Y. Tetrahedron Lett.  1996,  37:  7453 
• 6b Trost BM. Michellys P.-Y. Gerusz VJ. Angew. Chem. Int. Ed.  1997,  36:  1750 
• For reviews on the transformations of allenes, see:
• 7a Ma S. Chem. Rev.  2005,  105:  2829 
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• 7c Ma S. Aldrichimica Acta  2007,  40:  91 
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• 11 Yudha SS. Kuninobu Y. Takai K. Angew. Chem. Int. Ed.  2008,  47:  9318 
• 15 There is a report that a mono-alkylated allene coordinates to a rhenium center at the internal olefinic moiety of the allene. Therefore, we postulated a similar intermediate - an allene coordinated to a rhenium center at the internal olefinic position. See: Casey CP. Brady JT. Organometallics  1998,  17:  4620 
• We have already reported on the formation of rhenacycle intermediates. See:
• 16a Kuninobu Y. Kawata A. Takai K. J. Am. Chem. Soc.  2006,  128:  11368 
• 16b Kuninobu Y. Yu P. Takai K. Chem. Lett.  2007,  36:  1162 
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• 16d Kuninobu Y. Kawata A. Nishi M. Takata H. Takai K. Chem. Commun.  2008,  6360 ; See also: refs. 2 and 9

8

This reaction did not proceed using Re₂(CO)₁₀, ReCl₃(PMe₂Ph)₃, ReCl₃(NCMe)(PPh₃)₂, ReCl₃O(PPh₃)₂, Mn₂(CO)₁₀, MnBr(CO)₅, PdCl₂, PtCl₂, AuCl, AuCl₃, or GaCl₃.

9

α-Alkenylated β-imino ester 4a was obtained selectively as only the E-form. One of the possible reasons for this selectivity is that E-4a is thermodynamically more stable than the Z-form of 4a.

10

Two equivalents of allene 2a are necessary to produce a mixture of α-alkenylated β-imino esters 3a and 4a in high yield because of the polymerization of 2a.

12

The reaction did not proceed when an N-alkyl β-imino ester [ethyl (Z)-2-methyl-3-(propylamino)-2-butenoate] was employed as a substrate.

13

6-Vinylideneundecane (1,1-disubstituted allene), trideca-6,7-diene (internal allene), 1-(propa-1,2-dienyl)benzene (phenyl-substituted allene), and benzyl buta-2,3-dienoate (ester-substituted allene) did not promote the reaction.

14

The reaction did not proceed with reactive alkenes, such as styrene or norbornene, in place of allenes.

17

The products 3 and 4 are quite different from cyclopentenes, which are derived from β-keto esters and terminal allenes (see, ref. 9). The reason is not clear. One possible reason for the difference is that the formation of an allylic intermediate (step 3 or 5) by the flow of electrons from a nitrogen atom is easier than with an oxygen atom. Therefore, in the case of β-enamino esters, α-alkenylated products were formed instead of the formation of cyclopentenes by intramolecular nucleophilic cyclization.

18

Typical Procedure for Rhenium-Catalyzed Addition of β-Enamino Esters to Allenes. A mixture of β-enamino ester (0.250 mmol), allene (0.500 mmol), [ReBr(CO)₃(thf)]₂ (5.3 mg, 0.0063 mmol), and toluene (0.25 mL) was stirred at 135 ˚C. After stirring for 12 h, [RhCl(cod)]₂ (3.1 mg, 0.0063 mmol) was added to the crude mixture. After the mixture was stirred at 135 ˚C for 12 h, the solvent was removed in vacuo. The product was isolated by column chromatography on silica gel, which was pre-treated with Et₃N, using
n-hexane-ethyl acetate (30:1) as an eluent.

19

Ethyl 2-methyl-2-(1-phenyliminoethyl)-3-(3-phenyl-propyl)-3-butenoate (3a) and ethyl 2,3-dimethyl-6-phenyl-2-(1-phenyliminoethyl)-3-hexenoate (4a). ^¹H NMR (400 MHz, CDCl₃): δ = 1.26 (t, J = 7.2 Hz, 3H), 1.56 (s, 3H), 1.66 (s, 3H), 1.67 (s, 3H), 2.42 (q, J = 7.3 Hz, 2H), 2.71 (t, J = 7.3 Hz, 2H), 4.19 (q, J = 7.2 Hz, 2H), 5.08 (s, 1H, 3a), 5.15 (s, 1H, 3a), 5.39 (t, J = 7.3 Hz, 1H, 4a), 6.63 (d, J = 8.7 Hz, 2H), 7.03 (t, J = 7.5 Hz, 1H), 7.17-7.31 (m, 7H); ^¹³C NMR (100 MHz, CDCl₃): δ (4a) = 14.1, 14.6, 17.5, 20.9, 30.0, 35.4, 60.8, 62.9, 118.7, 123.0, 125.8, 127.1, 128.3, 128.5, 128.9, 135.4, 141.8, 151.4, 171.6, 173.6; IR (nujol): 3061, 2930, 2251, 1731, 1595, 1453, 1365, 1249, 1096, 910, 802, 733, 699, 648 cm^-¹; HRMS: m/z [M + Na]⁺ calcd for C₂₄H₂₉NO₂Na: 386.2096; found: 386.2104.