Diastereo- and Enantioselective Iridium Catalyzed Coupling of Vinyl Aziridines and Alcohols: Site-Selective Modification of Unprotected Diols and Synthesis of Substituted Piperidines

Abstract

The chiral cyclometalated π-allyliridium ortho-C,O-benzoate complex (R)-Ir-VIb derived from [Ir(cod)Cl]₂, allyl acetate, 4-cyano-3-nitrobenzoic acid, and (R)-MeO-BIPHEP catalyzes the coupling of N-(p-nitrophenylsulfonyl) protected vinyl aziridine 3a with primary alcohols 1a–1l to furnish branched products of C-C bond formation 4a–4l with good levels of anti-diastereo- and enantioselectivity. In the presence of 2-propanol, but under otherwise identical conditions, vinyl aziridine 3a and aldehydes 2a–2l engage in reductive coupling to furnish an equivalent set of adducts 4a–4l with roughly equivalent levels of anti-diastereo- and enantioselectivity. Using the enantiomeric iridium catalysts, vinyl aziridine 3a reacts with unprotected chiral 1,3-diols 1m–1o in a site-selective manner to deliver the diastereomeric products of C-allylation syn-4m, 4n, 4o and anti-4m, 4n, 4o, respectively, with good isolated yields and excellent levels of catalyst-directed diastereoselectivity. These adducts were directly converted to the diastereomeric 2,4,5-trisubstituted piperidines syn-5m, 5n, 5o and anti-5m, 5n, 5o.

Introduction

Redox-triggered carbonyl addition via transfer hydrogenation enables direct primary alcohol C-H functionalization in the absence of premetalated reagents or discrete alcohol to aldehyde redox reactions.¹ Enantioselective variants of these processes are remarkably broad in scope, encompassing alcohol C-H allylation,² crotylation,³ reverse prenylation,⁴ and numerous other allylative^5–14 and propargylative¹⁵ transformations (Figure 1).^16,17 An especially important and unique feature of these processes involves the ability to engage unprotected diols and higher polyols in site-selective primary alcohol C-H allylation.^2f,g,14,18 This capability stems from a high kinetic preference for primary vs secondary alcohol dehydrogenation, despite the greater endothermicity of the former.^19,20

Figure 1.

Direct, byproduct-free catalytic enantioselective coupling of vinyl cyclopropanes, vinyl epoxides and vinyl aziridines with primary alcohols.

The prospect of expanding technologies that streamline or eliminate the use of protecting groups^21a and redox reactions^21b in chemical synthesis compelled us to develop related site-selective redox-triggered carbonyl additions. Toward this end, we recently found that vinyl epoxides, such as isoprene oxide, will engage unprotected diols in site-selective C-C coupling to form products of tert-(hydroxy)prenylation.¹⁴ Inspired by these outcomes and the importance of nitrogen containing functional groups in pharmaceutical ingredients,²² in particular, the ubiquity of substituted piperidines,²³ we commenced exploration of vinyl aziridines as nucleophilic partners in redox-triggered carbonyl addition with primary alcohol proelectrophiles, including unprotected diols. We were further motivated by the fact that enantioselective umpoled allylations of aldehydes with vinyl aziridines to form products of carbonyl (α-aminomethyl)allylation have not been described.²⁴

Here, we report that the cyclometalated π-allyliridium ortho-C,O-benzoate complex (R)-Ir-VIb catalyzes the C-C coupling of N-(p-nitrophenylsulfonyl) protected²⁵ vinyl aziridine 3a with primary alcohols 1a–1l to furnish branched products of (α-aminomethyl)allylation 4a–4l with excellent levels of anti-diastereo- and enantioselectivity. Further, using enantiomeric iridium catalysts, vinyl aziridine 3a reacts with unprotected chiral 1,3-diols 1m–1o in a site-selective manner to form the diastereomeric products of C-allylation syn-4m–4o and anti-4m–4o, respectively, in good isolated yields and excellent levels of catalyst-directed diastereoselectivity. Under Mitsunobu reaction conditions, these adducts are directly converted to the diastereomeric 2,4,5-trisubstituted piperidines syn-5m–5o and anti-5m–5o (Figure 1).

Research Design and Methods

The feasibility of engaging vinyl aziridines in metal catalyzed C-C couplings with primary alcohols was rendered uncertain by competing electrophilic of O-allylation of the resulting π-allyliridium complexes,^26,27 as well as the propensity of vinyl aziridines to participate in metal catalyzed ring expansion.²⁸ Despite these potential liabilities, our initial experiments revealed promising results. Specifically, a series of chromatographically purified cyclometalated π-allyliridium ortho-C,O-benzoate complexes derived from [Ir(cod)Cl]₂, allyl acetate, 4-substituted-3-nitro-benzoic acids, and axially chiral chelating phosphine ligands were assayed for their ability to catalyze the C-C coupling of vinyl aziridine 3a with benzyl alcohol 1a (Table 1). Complexes (R)-Ir-Ia - (R)-Ir-Id, which incorporate BINAP-type ligands provided uniformly low isolated yields of coupling product 4a, although high levels of enantiomeric excess were observed (Table 1, entries 1–4). Complexes (R)-Ir-IIa and (R)-Ir-IIb, which incorporate (R)-SEGPHOS, enforced higher levels of conversion (Table 1, entries 5 and 6), suggesting ligands that incorporate alkoxy-substituted biaryl backbones might be efficacious. Complex (R)-Ir-IIc, which incorporates (R)-DM-SEGPHOS, gave only trace quantities of 4a (Table 1, entry 7), hence, subsequent experiments focused solely on diphenylphosphino-substituted ligands. As amply demonstrated in prior studies,^2–15 the 4-substituent of the C,O-benzoate moiety can influence both conversion and selectivity. In the case of complexes (R)-Ir-IIIa,b (R)-Ir-IVa,b, (R)-Ir-Va,b, and (R)-Ir-VIa,b which incorporate (R)-SYNPHOS, (R)-C3-TUNEPHOS, (R)-Cl,MeO-BIPHEP and (R)-MeO-BIPHEP ligands, respectively, those modified by 4-cyano-3-nitrobenzoate moieties promote uniformly higher conversion than the corresponding 4-chloro-3-nitrobenzoate complexes (Table 1, entries 8–15). From these studies, it was found that (R)-Ir-VIb, which incorporates (R)-MeO-BIPHEP (the “Roche ligand”),²⁹ provided the most favorable results, delivering adduct 4a in 85% yield with a 5:1 anti-diastereoselectivity and 96% enantiomeric excess (Table 1, entry 15). At lower temperature, a slight improvement in enantioselectivity was observed, but the isolated yield decreased significantly (Table 1, entry 16). Conversely, at higher temperature, conversion improved but enantioselectivity declined (Table 1, entry 17). Whereas decreased loadings of vinyl aziridine 3a (100 mol%) diminished the isolated yield of adduct 4a (Table 1, entry 18), the use of excess vinyl aziridine 3a (300 mol%) provided adduct 4a in 96% isolated yield with 5:1 anti-diastereoselectivity and 97% enantiomeric excess (Table 1, entry 19). The use of alternate reaction solvents, such as dioxane or toluene, or omission of potassium phosphate led to substantially diminished yields.

Table 1.

Selected optimization experiments in the redox-triggered C-C coupling of aziridine 3a with benzyl alcohol 1a to form p-nitrophenylsulfonyl protected aminoalcohol 4a.^a

Entry	3a (mol%)	[Ir]	T(°C)	4a yield (dr, ee%)
1	200	(R)-Ir-Ia	60	15% (2:1, 90)
2	200	(R)-Ir-Ib	60	20% (2:1, 91)
3	200	(R)-Ir-Ic	60	14% (2:1, 94)
4	200	(R)-Ir-Id	60	14% (2:1, 91)
5	200	(R)-Ir-IIa	60	26% (8:1, 91)
6	200	(R)-Ir-IIb	60	69% (6:1, 90)
7	200	(R)-Ir-IIc	60	trace conversion
8	200	(R)-Ir-IIIa	60	28% (4:1, 93)
9	200	(R)-Ir-IIIb	60	86% (4:1, 90)
10	200	(R)-Ir-IVa	60	50% (7:1, 84)
11	200	(R)-Ir-IVb	60	61% (3:1, 95)
12	200	(R)-Ir-Va	60	28% (3:1, 96)
13	200	(R)-Ir-Vb	60	65% (3:1, 94)
14	200	(R)-Ir-VIa	60	51% (5:1, 96)
15	200	(R)-Ir-VIb	60	85% (5:1 96)
16	200	(R)-Ir-VIb	45	70% (5:1 97)
17	200	(R)-Ir-VIb	80	88% (2:1 85)
18	100	(R)-Ir-VIb	60	36% (5:1 95)
⇨19	300	(R)-Ir-VIb	60	96% (5:1 97)

^aYields are of material isolated by silica gel chromatography. See Supporting Information for further details.

To evaluate the scope of the iridium catalyzed (α-aminomethyl)allylation, diverse primary alcohols 1a–1l (100 mol%) were exposed to vinyl aziridine 3a (300 mol%) in the presence of complex (R)-Ir-VIb (5 mol%) and substoichiometric potassium phosphate (5 mol%) in THF (0.2 M) at 60 °C (Table 2). As illustrated in the conversion of 1a–1f to 4a–4f, a range of benzylic alcohols participate in C-C coupling, including alcohol 1f, which incorporates a Lewis basic pyridyl-substituent. Cinnamyl alcohol 1g, prenyl alcohol 1h and geraniol 1i are transformed to adducts 4g–4i, illustrating use of allylic alcohols as coupling partners. Internal redox isomerization of allylic alcohols 1g–1i to form aldehydes was not observed.³⁰ Finally, as demonstrated by the conversion of heptanol 1j, O-benzyl 1,3-propane diol 1k and cyclopropyl methanol 1l to adducts 4j–4l, respectively, aliphatic alcohols participate in coupling. In each case examined, good to excellent yields, good anti-diastereoselectivities and uniformly high enantioselectivities were observed.

Table 2.

Regio-, diastereo- and enantioselective iridium catalyzed (α-aminomethyl)allylation of alcohols 1a–1l with vinyl aziridine 3a.^a

^aYields are of material isolated by silica gel chromatography.

^b48 h. See Supporting Information for further details.

Related reductive couplings of vinyl aziridine 3a with aldehydes 2a–2l also were evaluated (Table 3). Using 2-propanol (300 mol%) as terminal reductant under conditions otherwise identical to those employed in the coupling of alcohols 1a–1l, aryl aldehydes 1a–1f were converted to adducts 4a–4f, respectively. Similarly, the α,β-unsaturated aldehydes cinnamaldehyde 2g, prenyl aldehyde 2h and geranial 2i were transformed to adducts 4g–4i, respectively. Finally, the aliphatic aldehydes 2j–2l provided adducts 2j–2l, respectively. Good to excellent yields, good anti-diastereoselectivities were accompanied by uniformly high enantioselectivities in each case. These data demonstrate that (α-aminomethyl)allylation via transfer hydrogenation proceeds with equal facility from the alcohol or aldehyde oxidation levels. The relative and absolute stereochemical assignment of adducts 4a–4l was made in analogy to the structural assignment of adducts 4b, which was determined by single crystal X-ray diffraction analysis. Based on the collective data,^1a a general catalytic mechanism accounting for the reaction of primary alcohols 1a–1l with vinyl aziridine 3a to form adducts 4a–4l is given (Scheme 1). It should be noted that attempted preparation of N-nosyl vinyl aziridines substituted at the allylic position failed due to spontaneous ring opening, and N-carbamoyl substituted vinyl aziridines underwent ring expansion upon exposure to coupling conditions.

Table 3.

Regio-, diastereo- and enantioselective iridium catalyzed reductive (α-aminomethyl)allylation of aldehydes 2a–2l with vinyl aziridine 3a.^a

^aYields are of material isolated by silica gel chromatography. See Supporting Information for further details.

Scheme 1.

General catalytic mechanism for iridium catalyzed (α-aminomethyl)allylation of alcohols 1a–1l with vinyl aziridine 3a.

Having demonstrated the feasibility and scope of the coupling of primary alcohols 1a–1l, the catalyst-directed diastereo- and site-selective C-C coupling of unprotected diols 1m–1o with vinyl aziridine 3a was investigated (Table 4).^2f,g,14,18 Conditions used in the coupling of primary alcohols 1a–1l were not directly transferable. Longer reaction times were required. Additionally, the optimal pairing of diol 1m–1o and catalyst was case dependent. For example, in the coupling of diol 1m with vinyl aziridine 3a, the iridium catalyst (R)-Ir-Vb, which is modified by (R)-Cl,MeO-BIPHEP, gave slightly better results than catalyst (R)-Ir-VIb, which is modified by (R)-MeO-BIPHEP. In each case, the products of (α-aminomethyl)allylation syn-4m, syn-4n and syn-4o were formed in good yield and with good levels of catalyst-directed diastereoselectivity. In a parallel set of experiments, the unprotected diols 1m–1o were reacted with the respective enantiomeric iridium catalysts. The diastereomeric adducts anti-4m, anti-4n and anti-4o were formed in good yields with excellent levels catalyst-directed diastereoselectivity. The preparation of syn-4o and anti-4o, which incorporate pyridine rings, underscores the high level of functional group compatibility displayed by the iridium catalysts.

Table 4.

Catalyst-directed diastereo- and site-selectivity in the iridium catalyzed (α-aminomethyl)allylation of unprotected 1,3-diols 1m–1o with vinyl aziridine 3a.^a

^aYields are of material isolated by silica gel chromatography. See Supporting Information for further details.

To illustrate the utility of this methodology, the diol coupling products syn-4m, syn-4n and syn-4o were exposed Mitsunobu reaction conditions (Table 5).³¹ Cyclization proceeded most efficiently using DIAD (disisopropyl azodicarboxylate), delivering the 2,4,5-trisubstituted piperidines syn-5m, syn-5n and syn-5o as single stereoisomers. Similarly, exposure of the diol coupling products anti-4m, anti-4n and anti-4o to Mitsunobu conditions led to formation of the diastereomeric 2,4,5-trisubstituted piperidines anti-5m, anti-5n and anti-5o (Table 5). In all cases, cyclization proceeded with complete inversion of the carbinol stereocenter in good to excellent yield. Finally, as illustrated in the conversion of piperidine syn-5n to the free secondary amine syn-6n, the N-(p-nitrophenylsulfonyl) protecting group²⁵ may be removed in an efficient, selective manner (eq. 1).

Table 5.

Conversion of syn-4m, 4n, 4o and anti-4m, 4n, 4o to diastereomeric 2,4,5-trisubstituted piperidines syn-5m, 5n, 5o and anti-5m, 5n, 5o.^a

^aYields are of material isolated by silica gel chromatography. See Supporting Information for further details.

(eq. 1)

Conclusions

In summary, we report the first enantioselective umpoled allylations of aldehydes with vinyl aziridines to form products of carbonyl (α-aminomethyl)allylation. These processes may be conducted efficiently from the alcohol or aldehyde oxidation level, delivering products of C-C bond formation in good to excellent yield, good levels of anti-diastereoselectivity and uniformly high levels of enantioselectivity. Further, using enantiomeric iridium catalysts, vinyl aziridine 3a reacts with unprotected chiral 1,3-diols 1m–1o in a site-selective manner to form the diastereomeric products of C-allylation syn-4m, syn-4n and syn-4o and anti-4m, anti-4n, anti-4o, respectively, in good isolated yields and excellent levels of catalyst-directed diastereoselectivity; a capability that enables concise access to 2,4,5-trisubstituted piperidines. Future work will focus on related methods for the direct site-selective modification of polyols including natural products.