Selective carbon–carbon bond formations with alkenylzirconocenes

Alkylzirconocenes in Organic Synthesis: An Overview

Organozirconium chemistry has found extensive applications in organic synthesis since its discovery in the last century. Alkyl­zirconocenes, which are easily generated by the hydrozirconation of alkenes with the Schwartz reagent, are widely utilized for carbon–carbon­ and carbon–heteroatom bond formation. This short review summarizes the progress to date on the applications alkylzirconocenes in organic synthesis.

1 Introduction

2 General Methods for Generating Alkylzirconocenes

3 Transformations of Alkylzirconocenes by Heteroatoms

4 Insertion of Unsaturated Groups into Alkylzirconocenes

5 Transmetalations

6 Cross-Coupling Reactions of Alkylzirconocenes

7 Photochemistry of Alkylzirconocenes

8 Bimetallic Reagents of Zirconium

9 Asymmetric Transformations

10 Applications of Alkylzirconocenes Generated from the Negishi Reagent

11 Conclusions and Outlook

Convergent Evolution of Diastereomeric Mixtures of 5-Methoxy-pentylzirconocenes toward Trans-1,2-substituted Cyclopentanes

The access to 1,2- and 1,1,2-substituted trans cyclopentanes via a sequential hydrozirconation/TMSOTf-mediated cyclization applied to 5-methoxypent-1-enes is presented. Involving a transient carbocation, the reaction was shown to be diastereo-convergent. Possibly performed in a nonracemic version, the reaction proved compatible with a range of functional groups affording a large panel of cyclopentanes.

Ru-Catalyzed Chemo- and Enantioselective Hydrogenation of 2,4-Pentadien-1-ones: Synthesis of Chiral 2,4-Pentadien-1-ols

The asymmetric hydrogenation of 2,4-pentadien-1-ones has been achieved by using trans-RuCl₂[(R)-XylylSunPhos][(R)-Daipen] as a catalyst under basic conditions. This hydrogenation demonstrated exclusive C1-carbonyl selectivity, and thus the conjugated 2,4-diene motifs remained untouched, which provides a synthetically useful method for various chiral 2,4-pentadien-1-ols.

Synthesis of 1,3-disubstituted cyclohexenes from dienylethers via sequential hydrozirconation/deoxygenative cyclisation

The preparation of 1,3-disubstituted cyclohexenes from 3-methoxyhexa-1,5-dienes involving a sequential hydrozirconation/TMSOTf-mediated activation is described.

Straightforward access to spisulosine and 4,5-dehydrospisulosine stereoisomers: probes for profiling ceramide synthase activities in intact cells

A stereoselective synthesis of spisulosine (ES285) and 4,5-dehydrospisulosine stereoisomers is described. Hydrozirconation of 1-pentadecyne with Schwartz reagent, followed by diastereocontrolled addition to L- or D-alaninal afforded the required 2-amino-1,3-diol framework. The resulting sphingoid bases revealed as excellent probes for the profiling of ceramide synthase activity in intact cells. Among the sphingoid bases described in this work, spisulosine (ES285), RBM1-77, and RBM1-73 were the most suitable ones because of their highest acylation rates. These molecules should prove useful to study the role of the different ceramide synthases and the resulting N-acyl (dihydro)ceramides in cell fate.

Synthesis of a library of tricyclic azepinoisoindolinones

Hydrozirconation of 1-hexyne, the addition to in situ prepared N-acyliminium species, and ring-closing metathesis (RCM) were key steps in the preparation of a tricyclic isoindolinone scaffold. An unusual alkene isomerization process during the RCM was identified and studied in some detail. Chemical diversification for library synthesis was achieved by a subsequent alkene epoxidation and zinc-mediated aminolysis reaction. The resulting library products provided selective hits among a large number of high-throughput screens reported in PubChem, thus illustrating the utility of the novel scaffold.

Diastereoselective access to nonracemic 2-cis-substituted and 2,6-cis-disubstituted piperidines

Access to nonracemic amino ketones via a hydrozirconation/transmetalation/acylation sequence applied to Boc-protected 1-aminobut-3-enes is presented. This method was applied to the stereoselective synthesis of cyclic imines (or iminiums) which were diastereoselectively converted into 2-cis-substituted and 2,6-cis-disubstituted piperidines. The potential of this approach in the field of alkaloid synthesis was illustrated by the synthesis of (-)-coniine and (-)-indolizidine 209D. Furthermore, access to indolizidines bearing a quaternary center could also be envisioned through this strategy.

Alkyne-aldehyde reductive C-C coupling through ruthenium-catalyzed transfer hydrogenation: direct regio- and stereoselective carbonyl vinylation to form trisubstituted allylic alcohols in the absence of premetallated reagents

Nonsymmetric 1,2-disubstituted alkynes engage in reductive coupling to a variety of aldehydes under the conditions of ruthenium-catalyzed transfer hydrogenation by employing formic acid as the terminal reductant and delivering the products of carbonyl vinylation with good to excellent levels of regioselectivity and with complete control of olefin stereochemistry. As revealed in an assessment of the ruthenium counterion, iodide plays an essential role in directing the regioselectivity of C-C bond formation. Isotopic labeling studies corroborate reversible catalytic propargyl C-H oxidative addition in advance of the C-C coupling, and demonstrate that the C-C coupling products do not experience reversible dehydrogenation by way of enone intermediates. This transfer hydrogenation protocol enables carbonyl vinylation in the absence of stoichiometric metallic reagents.

Homoallylic amines by reductive inter- and intramolecular coupling of allenes and nitriles

The one-pot hydrozirconation of allenes and nitriles followed by an in situ transmetalation of the allylzirconocene with dimethylzinc or zinc chloride provides functionalized homoallylic amines. An intramolecular version of this process leads to 3-aminotetrahydrofurans and 3-aminotetrahydropyrans.

Formation of C-C Bonds via Iridium-Catalyzed Hydrogenation and Transfer Hydrogenation

The formation of C-C bonds via catalytic hydrogenation and transfer hydrogenation enables carbonyl and imine addition in the absence of stoichiometric organometallic reagents. In this review, iridium-catalyzed C-C bond-forming hydrogenations and transfer hydrogenations are surveyed. These processes encompass selective, atom-economic methods for the vinylation and allylation of carbonyl compounds and imines. Notably, under transfer hydrogenation conditions, alcohol dehydrogenation drives reductive generation of organoiridium nucleophiles, enabling carbonyl addition from the aldehyde or alcohol oxidation level. In the latter case, hydrogen exchange between alcohols and π-unsaturated reactants generates electrophile-nucleophile pairs en route to products of hydro-hydroxyalkylation, representing a direct method for the functionalization of carbinol C-H bonds.

Enantioselective iridium-catalyzed carbonyl allylation from the alcohol oxidation level via transfer hydrogenation: minimizing pre-activation for synthetic efficiency

Existing methods for enantioselective carbonyl allylation, crotylation and tert-prenylation require stoichiometric generation of pre-metallated nucleophiles, and often employ stoichiometric chiral modifiers. Under the conditions of transfer hydrogenation employing an ortho-cyclometallated iridium C,O-benzoate catalyst, enantioselective carbonyl allylations, crotylations and tert-prenylations are achieved in the absence of stoichiometric metallic reagents or stoichiometric chiral modifiers. Moreover, under transfer hydrogenation conditions, primary alcohols function dually as hydrogen donors and aldehyde precursors, enabling enantioselective carbonyl addition directly from the alcohol oxidation level.

Asymmetric Synthesis of 1,2-dioxanes: Approaches to the Peroxyplakoric Acids

The stereospecific intramolecular alkylation of a hydroperoxyacetal provides the basis for the first asymmetric synthesis of the dioxane propionate core of the peroxyplakorates. Chemoselective hydrometallation of an alkyne in the presence of a peroxide is used to introduce a synthon for the polyunsaturated side chains of the peroxyplakorates. The route suggests a general solution for the 1,2-dioxane unit in many peroxide natural products.

Direct vinylation of alcohols or aldehydes employing alkynes as vinyl donors: a ruthenium catalyzed C-C bond-forming transfer hydrogenation

Under the conditions of ruthenium catalyzed transfer hydrogenation, 2-butyne couples to benzylic and aliphatic alcohols 1a-1l to furnish allylic alcohols 2a-2l, constituting a direct C-H vinylation of alcohols employing alkynes as vinyl donors. Under related transfer hydrogenation conditions employing formic acid as terminal reductant, 2-butyne couples to aldehydes 4a, 4b, and 4e to furnish identical products of carbonyl vinylation 2a, 2b, and 2e. Thus, carbonyl vinylation is achieved from the alcohol or the aldehyde oxidation level in the absence of any stoichiometric metallic reagents. Nonsymmetric alkynes 6a-6c couple efficiently to aldehyde 4b to provide allylic alcohols 2m-2o as single regioisomers. Acetylenic aldehyde 7a engages in efficient intramolecular coupling to deliver cyclic allylic alcohol 8a.

Highly enantioselective vinyl additions of vinylaluminum to ketones catalyzed by a titanium(IV) catalyst of (S)-BINOL

We report a novel asymmetric addition of vinyl group to ketones using vinylaluminum reagents catalyzed by in situ prepared Ti(O(i)Pr)(4) complexes of (S)-BINOL to afford diversified tertiary allylic alochols. Varieties of aromatic ketones bearing either an electron-donating or an electron-withdrawing substituent on the aromatic ring were examined to afford products in excellent enantioselectivities of up to 98% ee with high yields. A 10-fold scale-up reaction afforded the product in a similar yield with a comparable enantioselectivity. More importantly, additions of a variety of vinyl reagents including functionalized vinyls were demonstrated, affording tertiary allylic alcohols with good to excellent enantioselectivities of up to 96% ee.

Catalytic carbonyl addition through transfer hydrogenation: a departure from preformed organometallic reagents

Classical protocols for carbonyl allylation, propargylation and vinylation typically rely upon the use of preformed allyl metal, allenyl metal and vinyl metal reagents, respectively, mandating stoichiometric generation of metallic byproducts. Through transfer hydrogenative C-C coupling, however, carbonyl addition may be achieved from the aldehyde or alcohol oxidation level in the absence of stoichiometric organometallic reagents or metallic reductants. Here, we review transfer hydrogenative methods for carbonyl addition, which encompass the first catalytic protocols enabling direct C-H functionalization of alcohols.

Synthesis of Functionalized Isoindolinones: Addition of In Situ Generated Organoalanes to Acyliminium Ions

Addition of in situ generated di- or trisubstituted alkenylalanes to N-acyliminium ions provides rapid access to functionalized isoindolinones. Subsequent ring closing metathesis leads to tricyclic products. These transformations proceed under mild conditions and allow for the convergent synthesis of biologically significant scaffolds from readily available starting materials.

Highly Enantioselective Synthesis of (E)-Allylic Alcohols

The asymmetric addition of alkenylzincs to aromatic and alpha-branched aliphatic aldehydes catalyzed by 1 generated the corresponding (E)-allylic alcohols with >95% ee and good to excellent chemical yields, especially >99.5% ee was observed in the case of 4-CF3-benzaldehyde. Notably, 1 is an effective ligand to catalyze the addition of disubstituted (R2 = R3 = ethyl) and bulky substituted (R2 = H, R3 = tert-butyl) alkenylzincs to benzaldehyde, affording the corresponding allylic alcohols both with 96% ee.

Transition-metal-mediated cascade reactions: the water-accelerated carboalumination-Claisen rearrangement-carbonyl addition reaction

[Chemical reaction: See text] A three-step cascade reaction involving a water-accelerated catalytic carboalumination, a Claisen rearrangement, and a nucleophilic carbonyl addition converts terminal alkynes and allyl vinyl ethers into allylic alcohols containing up to three contiguous asymmetric carbon centers. Stoichiometric quantities of water as an additive increase the rate of the [3,3] sigmatropic rearrangement as well as the diastereoselectivity of the carbonyl addition process. Reaction products contain 1,6-diene functionalities that are readily cyclized to substituted cyclopentenes. An extension of this methodology to a sequence involving a [1,3] sigmatropic shift was feasible with a cyclopropylmethyl vinyl ether substrate.