Metalated nitriles: cation-controlled cyclizations.

Stereodivergent, cation-controlled alkylations are exceptionally appealing. The appeal stems from the inherent efficiency of controlling nucleophile geometry simply by judicious cation selection. Directing a single precursor to two structurally different organometallics for stereodivergent alkylations is particularly challenging, potentially explaining why cation-controlled alkylations are relatively rare.1 Metalated nitriles are ideal organometallics for stereodivergent alkylations because of their chameleon-like structural preferences.2 At least three structurally distinct metalated nitriles are selectively accessed simply through choice of solvent, temperature, and ligand. Solution and X-ray3 analyses identify the three main metalated nitrile structures as spanning a range of geometries at the nucleophilic carbon: planar N-metalated nitriles 1,4 partially pyramidal carbanions 2,5 and tetrahedral C-metalated nitriles 36 (Figure 1). Figure 1 Structural Classes of Metalated Nitriles Conceptually, two different geometries of metalated nitriles are accessible through stereodivergent deprotonations leading to a C-metalated nitrile and the corresponding inverted nitrile stabilized carbanion. The relatively low inversion barrier of metalated nitriles5a and the dramatic influence of chelation on metalated nitrile geometry7 suggested using internal chelation with mono- or divalent cations to favor two distinct metalated nitrile geometries. Specifically, metalated nitriles derived from γ-hydroxy nitriles such as 5 (Scheme 1) permit two coordination modes by virtue of a metal's capacity to form one or two formal bonds. Internal coordination in the dilithiated nitrile 10 favors a pyramidal anion with an equatorial nucleophilic orbital, whereas the corresponding C-magnesiated nitrile 8 possess an axial nucleophilic C-Mg bond (Scheme 1). Scheme 1 Cation-Dependent Cyclizations of Pyramidal C-Magnesiated and N-Lithiated Nitriles. The facile synthesis of 5 makes this nitrile an ideal prototype for pursuing a cation-selective cyclization. Sequential addition of MeMgCl and 4-pentenylmagnesium bromide8 to oxonitrile 49 provides an alkenenitrile intermediate which, after ozonolysis, reduction and chlorination, provides an expedient synthesis of nitrile 5 (Scheme 1). Mounting experimental evidence10 indicates that addition of excess i-PrMgCl to this type of γ-hydroxynitrile (5) triggers deprotonation to a halomagnesium alkoxide (6) that subsequently engages in a halogen-alkyl exchange to afford an alkylmagnesium alkoxide (7).11 Forming the axial alkylmagnesium alkoxide 7 conveniently anchors the basic iso-propyl group for a directed, internal deprotonation while preventing alkyl addition to the nitrile group. Cyclization of the resulting C-magnesiated nitrile 8 affords the cis-decalin 912 as the sole stereoisomer. The stereochemistry is consistent with a retentive SNi displacement resulting from attack of the carbon-magnesium bond on the C-Cl σ* orbital via a 3-centered transition structure (8).10 The stereodivergent cyclization of 5 to the trans-decalin 11 was pursued via the putative lithiated nitrile 10 with the opposite configuration at the nitrile-bearing carbon. Extensive experimentation identified BuLi as the optimum base, with a slight excess triggering the cyclization of 5 to the diastereomeric trans-decalin 11.12 Exclusive formation of the trans-decalin 11 requires overlap of an equatorally oriented orbital with the σ* orbital of the C-Cl bond. The cyclization stereochemistry is consistent with forming a pyramidal nitrile anion in which internal chelation between the alkoxy lithium cation and the π-electrons of the nitrile7b,13 defines the stereochemistry at the nitrile-bearing carbon. Extending the cation-controlled cyclization strategy to the synthesis of cis and trans bicyclo[5.4.0]undecanes is surprisingly challenging (Scheme 2). Related cyclizations of ω-haloalkylnitriles are remarkably difficult for 7-membered carbocycles relative to corresponding ring closures of the 3–6 membered analogs.14 Consistent with this difficulty, sequential addition of MeMgCl and 5-chloropentylmagnesium iodide15 to oxonitrile 4 generates the chloroalkylnitrile 13 without premature cyclization of the intermediate C-metalated nitrile 12. Regenerating 12, through addition of i-PrMgCl to 13, fails to induce any cyclization even in refluxing THF! Coaxing the cyclization requires prior conversion to the corresponding iodide 14, with cyclization of the C-magnesiated nitrile 15 requiring heating in refluxing THF to afford the cis fused16 nitrile 16.17 Scheme 2 Stereodivergent Cyclizations of Pyramidal of C-Magnesiated and N-Lithiated Nitriles to Bicyclo[5.4.0]undecanes. Diverting the cyclization manifold through the use of BuLi results in a slow, but relatively efficient, cyclization of 13 to the trans-lactone 19 (Scheme 2). Installation of the trans-ring junction stereochemistry is consistent with cyclization via the pyramidal, internally coordinated nitrile 17. Presumably this "dianion" is more reactive than the corresponding C-magnesiated nitrile 12 since cyclization does not require heating or the more electrophilic iodide leaving group. Complete cyclization at room temperature requires 4 days, during which the alkoxide 18 resulting from cyclization attacks the proximal nitrile to afford the lactone 19 after hydrolysis. The stereodivergent decalin and bicyclo[5.4.0]undecane cyclizations stimulated an analogous cyclization with an allylic electrophile to introduce an exo-methylene substituent as a potential handle for subsequent synthetic ventures. Access to the cyclization precursor 20 was readily achieved through sequential addition of MeMgCl and 3-methyl-3-butenylmagnesium bromide to oxonitrile 4 followed by selective chlorination18 in the presence of the unprotected tertiary alcohol. Addition of excess i-PrMgCl to nitrile 20 initiates a deprotonation cascade leading to the cis-decalin 22 whose stereochemistry was secured by X-ray crystallography (Figure 2). Figure 2 ORTEP Diagram of cis-Decalin 22 Employing BuLi for the deprotonation of 20 triggers a particularly facile cyclization to the trans-decalin 24. In contrast to the room temperature SNi displacement of the alkylchloride 10 (Scheme 1), cyclization of the putative dilithiated nitrile 23 (Scheme 3) proceeds at −78 °C and was complete within 2 h. The facility of metalated nitrile cyclizations with allylic electrophiles suggests a possible extension of this strategy in sterically demanding cyclizations. Scheme 3 Stereodivergent Allyl chloride Cyclizations Stereodivergent cyclizations of γ-hydroxynitriles are readily achieved by judicious choice of mono- or divalent cations in the intermediate metalated nitriles. Hydroxyl directed deprotonations with Grignard or organolithium reagents provides an expedient entry to two putative metalated nitriles that differ primarily in the geometry at the nitrile-bearing carbon. Cyclizations initiated with i-PrMgCl consistently afford cis-fused decalins or bicyclo[5.4.0]undecanes whereas BuLi affords the trans-fused diastereomers. In each instance the stereoselectivies are consistent with generating C-magnesiated nitriles through i-PrMgCl deprotonation and an internally coordinated nitrile anion by deprotonating with BuLi. Synthetically, the stereodivergent cyclizations provide an effective method for diverting a single precursor to cis-and trans-bicyclic nitriles through stereoselective alkylation at the quaternary, ring junction stereochemistry.