Catalytic asymmetric α-Iminol rearrangement: new chiral platforms.

A series of 19 different asymmetric catalysts were screened in an effort to identify the first chiral catalyst for the rearrangement of α-hydroxy imines to α-amino ketones involving a 1,2-carbon shift. Although aluminate complexes of VAPOL, VANOL, and 7,7'-(t)Bu2VANOL were quite effective catalysts giving up to 88% ee, the ne plus ultra catalyst for this reaction was found to be a zirconium complex of VANOL which gives 97 to >99% ee for the majority of the substrates examined. An X-ray diffraction study of the catalyst reveals that the zirconium exists as a homoleptic complex with three VANOL ligands and two protonated N-methyl imidazoles.

The first examples of the α-iminol rearrangement involving a 1,2-carbon shift were reported by Prins and Shoppee in 1943 (Scheme 1).[1] A review of the subsequent history appeared in 2003,[2] and there have been a number of examples since.[3] Most often these reactions have been effected thermally (∼150–200 °C) and several others with Brønsted acids (TsOH, HCO2H, CH3CO2H, HCl, H2SO4),[2,3] and in some cases alkoxide bases have been used.[2] The reaction can also be promoted by transition-metal Lewis acids including scandium, copper, and zinc triflates.[3f,3j,3r] There has been only one report with a chiral catalyst. A nickel pybox complex and a number of chiral lanthanide complexes were found to facilitate the reaction, however, all gave racemic products.[3a] There is one example of a catalytic asymmetric Amadori–Heyns rearrangement which involves a 1,2-hydrogen shift via an intermediary enol.[3t]

Scheme 1

Our interest in the α-iminol rearrangement arose from our work with chiral catalysts for reactions of imines including aziridinations,[4] aza-Cope rearrangements,[5] heteroatom Diels–Alder reactions,[6] and the Ugi reaction.[7] These reactions involve a BOROX catalyst consisting of an ion-pair containing a boroxinate chiral anion with the gegen cation derived from a protonated substrate.[8] We were thus disappointed to find that the VANOL and VAPOL BOROX catalysts 1 and 3 are essentially ineffectual for the α-iminol rearrangement of 22a with the latter giving 23a as a racemic product (Table 1, entries 4 and 7). The VANOL BOROX catalyst gives only 17% ee. There is not a significant background reaction since simply heating 22a for 42 h at 80 °C gives <5% yield of the product (entry 1). However, the α-iminol rearrangement of 22a does occur slowly at 150 °C providing racemic 23a in 28% yield in 2 h with 79% conversion (entry 2). The induction can be increased to 62% ee if the VANOL BOROX catalyst is assembled from 2,4,6-tri-t-butyl phenol rather than phenol, however, the reaction is exceedingly slow even with 25 mol % catalyst (entry 6). The VANOL and VAPOL hydrogen phosphate catalysts and their derivatives have proven to be very effective chiral Brønsted acid catalysts for a number of reactions.[9−19] A number of catalysts in this class were screened (4–9, Scheme 2), and it was found that they were not very effective (entries 8–13).

Table 1

Catalyst Screen fot the α-Lminol Rearrangement of α-Hydroxy Imine 22aa

All reactions were carried out under nitrogen except where indicated.

For protocols for the preparation of the various catalysts see the SI.

Isolated yield. Yields in parentheses are NMR yields with Ph3CH as internal standard.

Determined by HPLC.

71% of the starting material remains.

Performed under air in a screw cap vial.

30% of the starting material remains and a side-product was formed in 50% yield that is tentatively identified as the imine resulting from oxidation of 23a.

25 mol % PhCO2H was added after the precatalyst was prepared. See ref (5).

The substrate was added to a solution of 15 in mesitylene that had been preheated to 160 °C.

5 mol % of a 1:1 mixture of Zr(OPr) + (HOPr) and N-methylimidazole.

79% of 22a was unreacted.

Scheme 2

Given the fact that α-iminol rearrangement is known to be accelerated with either an acid or base,[1,2] we decided to examine Shibasaki’s amphoteric catalyst BINOL Al 10 (ALB) which is very effective in delivering high asymmetric induction in asymmetric Michael addition reactions.[20] The lithium/aluminum catalyst 10 prepared from BINOL only provided a very sluggish reaction. The corresponding aluminum catalysts prepared from VANOL and VAPOL have not been previously reported. These catalysts were generated according to Shibasaki’s procedure from 2 equiv of the ligand and one of LiAlH4 and were found to be very effective catalysts. Both the VANOL and VAPOL catalysts 11 and 13 gave quantitative yields of 23a, and coincidently, both gave 68% ee which was the highest induction that had observed up to this point. The induction could be increased to 88% ee with no loss in reactivity with the aluminate catalyst 12 derived from 7,7′-di-t-butyl VANOL (entry 17).[21]

We had previously found that a VAPOL zirconium catalyst was very effective in promoting the Mannich reaction of imines with ketene acetals.[22] In the present investigation, these were found to be the optimal catalysts for the α-iminol rearrangement. The (R)-VANOL Zr catalyst 15 gives (S)-23a in 96% yield and 97% ee after 1 h at 80 °C (entry 20). The corresponding titanium catalyst 14 gives excellent asymmetric induction as well, but the turnover is much slower (entry 19). The corresponding VAPOL Zr catalyst 17 gives very poor asymmetric induction (entry 24). Unlike their corresponding aluminum catalysts, the VANOL and Bu2VANOL zirconium catalysts 15 and 16 give the same degree of asymmetric induction, but the latter is a bit slower (entries 20 and 23 vs 16 and 17). The BINOL zirconium catalyst 19 is highly effective in the catalytic asymmetric Mannich reaction[23] but did not prove to be an effective catalyst for the α-iminol rearrangement (entry 26). The catalyst loading for the VANOL Zr catalyst 15 can be lowered to 0.5 mol % and gives 23a in 96% yield with 95% ee in 60 min at 80 °C (entry 21). The most remarkable aspect of catalyst 15 is that its reactions can be carried out in the presence of air in a screw-cap vial (Table 1, entries 19–24). The reaction can even be carried out in the presence of air at 160 °C to give 23a in 95% yield with 89% ee in 30 s. The catalyst is remarkably stable as a solution of 15 in toluene can be allowed to stand for more than 5 months in the presence of air at room temperature and will still give a 98% yield of 23a in exactly the same asymmetric induction (97% ee) as freshly prepared catalyst (under the conditions in Table 2, entry 1).

Table 2

Scope of the α-Iminol Rearrangement of α-Hydroxy Imines 22a–qa

entry	series	R	% yield 21b	% yield 22b	% yield 23b	% ee 23c
1	a	C6H5	80	89	94	97
2	b	2-MeC6H4	83	88	88	84
3	c	2-iPrC6H4	56	–	95	–54d
4	d	3-MeC6H4	77	84	96	98
5	e	3-ClC6H4	67	100	92	–93e
6	f	4-MeC6H4	78	93	92	98
7	g	4-nBuC6H4	73	93	95	99
8	h	4-iPrC6H4	49	95	98	99
9	i	4-CyC6H4	69	93	97	94
10	j	4-tBuC6H4	61	98	100	>99
11	k	4-PhC6H4	77	95	100	>99
12	l	4-MeOC6H4	63	97	90	98
13	m	4-FC6H4	99	95	97	> –99d
14	n	4-CF3C6H4	53	90	≤5f	–
15	n	4-CF3C6H4	53	90	74g,h	73
16	o	4-MeC(O)C6H4	55	88	100i	97
17	p	n-hexyl	71	88	95	89
18	q	benzyl	46	100	98	>99
19	r	cyclohexyl	57	88	97	98

Unless otherwise specified, all reactions were run with 2.5 mol % (R)-15 in toluene at 0.3 M in 22 (0.1 mmol) at 60 °C for 1 h under air and went to 100% completion. The catalyst was prepared by stirring a toluene solution of Zr(OPr)4{HOPr) with 2 equiv of (R)-VANOL and 1 equiv of N-methylimidazole at 25 °C for 30 min.

Isolated yield.

Determined by HPLC. Negative sign means that (R)-23 is formed.

(S)-VANOL was used.

Reaction at 70 °C for 6 h with 5 mol % of (S)-15 under N2.

This reaction gives an 18% yield of imine 24n after 19 h at 60 °C along with a 70% recovery of 22n.

Reaction at 70 °C for 2.5 h with 10 mol % catalyst.

The reaction mixture was degassed by the freeze–thaw method.

Reaction at 0.1 M in 22 for 3 h.

The components of the catalyst are the ligand, Zr(OPr)4(HOPr), and N-methylimidazole in a 2:1:1 ratio. Variations in the ratio of ligand to zirconium find that there is no change in induction when it is raised from 1:1 to 2:1 and to 3:1, but the yield does increase from 67 to 98% (see Supporting Information). However, the amount of N-methylimidazole has a dramatic effect on the rate of the reaction; without imidazole there is no reaction but with 1 equiv the reaction goes to completion in 1 h. Increasing the amount of N-methyl imidazole past 1 equiv slows down the reaction; the yield drops to 70% with 2 equiv and to 8% with 20 equiv.

A survey of the scope of the α-iminol rearrangement is presented in Table 2. The imines 22 were prepared from the aldehydes 21 which in turn were prepared by Grignard or organolithium addition to the commercially available acetal 20. Exceptionally high asymmetric inductions were observed over a broad range of substrates including aryl groups with both electron-rich and electron-poor substituents (i.e., entries 12 and 16). The only really problematic aryl substituent was the para-trifluoro-methyl group. This substrate reacted very slowly, and after 30% conversion the α-amino ketone 23n was not detected. Instead the imine 24n, resulting from oxidation of α-amino ketone 23n, was isolated in 18% yield. All of the reactions of all of the substrates in Table 2 were carried out in the presence of air, but it was found that that the trifluoromethyl substituent 22n required an inert atmosphere. When the reaction of 22n was deoxygenated, the α-amino ketone 23n could be isolated in 74% yield and 73% ee (70 °C, 10 mol % catalyst, 2.5 h), and the imine 24n could not be detected. 1,2-Migrations of aliphatic groups were also highly efficient and stereoselective. Benzyl and cyclohexyl substituents gave >99 and 98% ee, respectively, while the rearrangement of 22p with R = n-hexyl gave 23p in 89% ee and 95% yield. The absolute configuration of 23a was determined by chemical correlation with the methyl ester of (R)-phenyl glycine, and the other products were assumed to be of the same antipode (see Supporting Information (SI)). It was also found that it was not necessary to carry out the reaction on the preformed imine. The reaction of aldehyde 21j with aniline gives the rearranged product (R)-23j in the same yield and % ee as when starting with the preformed imine 22j (eq 1).

A number of imines made from substituted anilines were also investigated with the conditions in Table 2 (Scheme 3).[24] The scale could be increased 40-fold over that used in Table 2 to give 1.22 g of α-amino ketone 23s in 85% yield and 98% ee with 1.5 mol % catalyst at 80 °C in 2 h. The optical purity of a sample of 23s that was 95% ee could be enhanced to ≥99% ee by crystallization with 82% recovery. The synthetic utility of α-amino ketone 23s was demonstrated in the synthesis of the amino alcohols 25–27 all of which are important as chiral ligands in asymmetric synthesis and catalysis.[25] Reduction of 23s with super hydride gives the amino alcohol 25 with >50:1 selectivity for the anti-diastereomer. This compound was obtained in 92% ee, but this could be enhanced to >99% ee by crystallization with 83% recovery. The reduction of 23s with super hydride could also be used to deliver the syn-diastereomer 27 with >50:1 stereoselectivity if the amino group was first converted to an acetamide.[26,27] Finally, Grignard addition of a phenyl group to 23s can be used to gain facile access to the triphenyl amino alcohol 26.

Scheme 3

A sample of the catalyst prepared from a ratio of VANOL:Zr:Im = 2:1:1 was grown from bromobenzene, and single crystals were obtained and characterized by X-ray diffraction as the complex 28 in Figure 1. The solid-state structure revealed that the zirconium exists as a six-coordinate homoleptic complex with three VANOL ligands and is charge balanced with two protonated N-methyl imidazoles. The protonated imidazoles are not H-bonded to the dianionic core but rather are H-bonded to water molecules which in turn are H-bonded to the oxy-zirconium core. The imidazolium hydrogen was located, but the protons on water were not. The N–O distance for the H-bond of the protonated imidazole in the foreground is 2.701 Å, and the O–O distance for the H-bond for the associated water molecule to the anionic core is 2.689 Å. The same solid-state structure was observed for crystals grown from a 3:1:2 mixture of VANOL:Zr:Im. Each unit cell contains two zirconium centers. A number of structures of rare earth complexes have been reported with three BINOL ligands but not for zirconium.[20,28] This is the first homoleptic complex of zirconium derived from three bis-phenol ligands.

Figure 1

Structure of Zr((S)-VANOL)3(NMI)228 (hydrogens and bromobenzene not shown).

Crystals of zirconium complex 28 (2.5 mol %) were found to catalyze the α-iminol rearrangement of 22a to give the amino ketone 23a in 96% yield and 98% ee under the conditions in Table 2, entry 1. Whether complex 28 (Scheme 4) is the actual catalyst in solution remains to be determined as does the mechanism for this reaction. Possibilities to be considered are (1) the activation of the imine and the alcohol by hydrogen-bond interactions after proton exchange between the α-iminol and an imidazole and (2) the Lewis acid/Brønsted acid activation of the imine by the zirconium and the alcohol by an alkoxide ligand of the zirconium.

Scheme 4