Redox-triggered C-C coupling of alcohols and vinyl epoxides: diastereo- and enantioselective formation of all-carbon quaternary centers via tert-(hydroxy)-prenylation.

Iridium catalyzed primary alcohol oxidation triggers reductive C-O bond cleavage of isoprene oxide to form aldehyde-allyliridium pairs that combine to form products of tert-(hydroxy)-prenylation, a motif found in >2000 terpenoid natural products. Curtin-Hammett effects are exploited to enforce high levels of anti-diastereo- and enantioselectivity in the formation of an all-carbon quaternary center. The present redox-triggered carbonyl additions occur in the absence of stoichiometric byproducts, premetalated reagents, and discrete alcohol-to-aldehyde redox manipulations.

Based on a pattern of reactivity wherein alcohol oxidation triggers reductive C–C coupling,[1] a suite of catalytic enantioselective transformations for polyketide construction were developed in our laboratory (alcohol C–H allylation,[2] crotylation,[3,4] and propargylation[5]). As illustrated in several total syntheses, these merged “redox-construction” protocols streamline polyketide construction, enabling the most concise routes reported, to date, to members of several natural product families.[1] To access substructures evident in terpenoid natural products, related protocols for alcohol C–H n-prenylation[6a] and tert-prenylation[6b] were developed. Despite the ubiquity of the tert-(hydroxy)-prenyl motif, which is found in over 2000 terpenoid natural products, enantioselective carbonyl tert-(hydroxy)-prenylation remains an unmet challenge (Figure 1).[7,8] Here, we disclose a catalytic protocol for direct regio-, diastereo-, and enantioselective C–C coupling of primary alcohols and isoprene oxide to form products of tert-(hydroxy)-prenylation, and related aldehyde-isoprene oxide reductive couplings mediated by isopropanol. These alcohol C–C couplings occur in the absence of stoichiometric byproducts and represent the first enantioselective umpoled carbonyl allylations through the use of vinyl epoxides.[7−9]

Figure 1

The tert-(hydroxy)-prenyl motif is found in over 2000 terpenoid natural products, yet direct methods for its regio- and stereoselective construction were hitherto unknown.

The prospect of utilizing vinyl epoxides in metal catalyzed C–C couplings of primary alcohols is rendered uncertain by competing epoxide ring opening to furnish products of O-allylation,[9−12] as well as the propensity of vinyl epoxides to engage in metal catalyzed ring expansion.[13] Remarkably, exposure of p-bromobenzyl alcohol 1a to isoprene oxide 3a in the presence of the chromatographically purified iridium–SEGPHOS complex (R)-Ir-Ib in THF (0.5 M) at 60 °C led to a promising 37% yield of the desired product of tert-(hydroxy)-prenylation 4a as a single regioisomer with 2:1 anti-diastereoselectivity and 93% enantiomeric excess (Table 1, entry 1). Use of K3PO4 (5 mol %) under these conditions improved conversion, providing 4a in 90% yield with 4:1 anti-diastereoselectivity and 93% enantiomeric excess (Table 1, entry 2). Although the reaction does not proceed at ambient temperature, excellent conversion is observed at 45 °C (Table 1, entries 3 and 4, respectively). Higher temperatures erode conversion and enantiomeric enrichment (Table 1, entry 5). At this stage, to identify optimal structural features of the catalyst, a series of eight chromatographically purified iridium complexes were evaluated (Table 1, entries 7–14). It became evident that, to achieve optimal energetic partitioning of the transient (E)- and (Z)-σ-allyliridium intermediates in the transition state for aldehyde addition, use of a more electron-deficient C,O-benzoate complex that embodies greater steric demand is required. This ultimately led to the identification of Ir-IVb, the (R)-Tol-BINAP modified C,O-benzoate complex derived from 4-CN-3-NO2-benzoic acid, as the catalyst of choice (Table 1, entry 7). Using Ir-IVb, adduct 4a was obtained in 91% yield, 40:1 anti-diastereoselectivity, and 94% enantiomeric excess.

Table 1

Selected Optimization Experiments in the Iridium Catalyzed tert-(Hydroxy)-Prenylation of p-Bromobenzyl Alcohol 1a with Isoprene Oxide 3aa

Yields are of material isolated by silica gel chromatography. Ir-Ia (ref (6b)) and Ir-IIIa (ref (2a)) are characterized by X-ray diffraction. See Supporting Information for further details.

K3PO4 was omitted.

48 h.

Under optimized conditions that utilize Ir-IVb as the catalyst, primary benzylic alcohols 1a–1c, allylic alcohols 1d–1f, and aliphatic alcohols 1g–1i are converted to the corresponding products of tert-(hydroxy)-prenylation 4a–4i (Table 2). Good to excellent isolated yields were accompanied by uniformly high levels of anti-diastereo- and enantioselectivity. Corresponding reactions conducted from the aldehyde oxidation level were achieved with isopropanol as a terminal reductant (Table 3). Here, as shown in the tert-(hydroxy)-prenylation of aldehyde 2a (Scheme 1), anti-diastereoselectivity was found to increase with decreasing concentration. This effect suggests a Curtin–Hammett scenario is operative wherein the relative thermodynamic stabilities of the (Z)- and (E)-σ-allyliridium intermediates do not solely determine diastereoselectivity. Rather, rate-determining carbonyl addition occurs more rapidly from the (E)-σ-allyliridium intermediate, requiring lower concentrations to retard the rate of carbonyl addition with respect to (E/Z)-isomerization of the allyliridium intermediate, which is required to replenish the (E)-σ-allyliridium intermediate from the unreacted (Z)-σ-allyliridium isomer.[14] This effect is especially pronounced in reactions conducted from the aldehyde oxidation level, as the aldehyde concentration is higher throughout the course of the reaction, accelerating carbonyl addition. Concentration dependent kinetic resolution in the ionization of isoprene oxide 3a, a chiral racemic material, can be excluded as recovered 3a displays low levels of enantiomeric enrichment. Carbonyl addition by way of the (E)-σ-allyliridium intermediate may be faster due to minimization of dipole–dipole interactions. Reaction by way the (Z)-σ-allyliridium intermediate may be slow due to internal coordination of the hydroxyl group to iridium.

Table 2

Regio-, Diastereo-, and Enantioselective Iridium Catalyzed tert-(Hydroxy)-Prenylation of Alcohols 1a–1i with Isoprene Oxide 3aa

Yields are of material isolated by silica gel chromatography.

THF (1.0 M).

3a (400 mol %).

THF (0.33 M).

60 °C. See Supporting Information for further details.

Table 3

Regio-, Diastereo-, and Enantioselective Iridium Catalyzed tert-(Hydroxy)-Prenylation of Aldehydes 2a–2i with Isoprene Oxide 3aa

Yields are of material isolated by silica gel chromatography.

THF (1.0 M).

THF (0.33 M).

35 °C.

THF (0.1 M).

60 °C.

70 °C. See Supporting Information for further details.

Scheme 1

Concentration Dependent anti-Diastereoselectivity As Illustrated in the Iridium Catalyzed tert-(Hydroxy)-Prenylation of Aldehyde 2a with Isoprene Oxide 3a and General Catalytic Mechanism

Yields are of material isolated by silica gel chromatography.

To further probe the scope of this process, the unprotected malic acid derived diol 1j was reacted with isoprene oxide 3a. As observed in related C–H allylations of glycols and higher polyols,[2f] primary alcohol dehydrogenation is kinetically preferred. Thus, using (R)-Ir-IVb and (S)-Ir-IVb as catalysts, site-selective dehydrogenation enables tert-(hydroxy)-prenylation of 1j to form 4j and iso-4j, respectively, with excellent levels of diastereoselectivity, as determined by 1H NMR analysis (eqs 1 and 2).

Vinyl epoxides beyond isoprene oxide also participate in redox-triggered C–C coupling. For example, the reaction of butadiene oxide 3b with p-bromobenzyl alcohol 1a delivers the product of (hydroxymethyl)-allylation 5a in 63% yield, 5:1 anti-diastereoselectivity, and 94% enantiomeric excess (eq 3). The reaction of myrcene oxide 3c(15) with p-bromobenzyl alcohol 1a delivers the product of tert-(hydroxy)-geranylation 5b in 94% yield, 40:1 anti-diastereoselectivity, and 87% enantiomeric excess (eq 4).

In summary, we report the anti-diastereo- and enantioselective redox-triggered C–C coupling of vinyl epoxides and alcohols to form products of carbonyl tert-(hydroxy)-prenylation. This process enables formation of an all-carbon quaternary center in the absence of stoichiometric byproducts with uniformly high levels of anti-diastereo- and enantioselectivity, even in the case of unprotected diols. This methodology, used in combination with recently reported iron catalyzed reductive diene cyclizations,[16] should broaden access to the >2000 terpenoid natural products incorporating the tert-(hydroxy)-prenylation motif. Studies of this type are underway and will reported in due course.