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Hydroacylation is a type of organic reaction in which an aldehyde is added over an alkene or alkyne bond. The reaction product is a ketone. The reaction requires a metal catalyst and intramolecular reaction is favored over an intermolecular one. [1]

Hydroacylation General

Mechanism

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In a general reaction mechanism step one in hydroacylation is oxidative addition of the metal into the aldehyde carbon-hydrogen bond followed by side-on addition of the alkene, then followed by reductive elimination. A lurking side-reaction is decarbonylation from the acyl metal hydride RCH2(CO)MH to the alkane RCH3 and M(CO) via the RCH2M(CO)H intermediate.

Hydroacylation reaction Mechanism


History

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Under the background of atom economy brought up decades ago, C-H bond activation and C-C bond forming became the main focus of many synthesists, especially those catalyzed by transition metal complexes.[2] Transition metal catalyzed hydroacylation is a good example, which involves the cleavage of C-H bond of aldehyde and formation of C-C bond between the carbon of the original C-H bond and the incoming alkyne of alkene, producing ketone with no atom waste[3]. [4] The intramolecular alkene hydroacylation is the first and most developed exploration in this field, reported in 1972 by Sakai.[5] Since then the intermolecular alkene hydroacylation was fast developed with various systems employing low catalyst loading and mild conditions. Later on the hydroacylation for alkyne is less studied compared with that of alkene. At the beginning, the Wilkinson’s catalyst was mostly used, and a lot of mechanic studies concern about this system were reported. After that more effective and selective catalysts were developed to suppress the decarbonylation, the main pathway in hydroacylation, mainly with rhodium or limited ruthenium and cobalt used in intermolecular alkene acylation. Now, chemists in this field are trying to conquer the to get rings other than 5 carbons through intramolecular acylation and increase the region- and enantio- selectivity of catalysts.


Intramolecular Hydroacylation

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Alkene Intramolecular Hydroacylation

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The intramolecular alkene hydroacylation is the first developed hydroacylation, and its development witnessed the growth of both catalysts and methodology in this field. At the beginning, to obtain cyclic ketone from alkene demanded stoichiometric amount of transition metal complexes. And it’s the only one in the four hydroacylations that once passed through that phase of development. The other three hydroacylations’ development, in some sense, is built on that on intramolecular alkene hydroacylation. Also, the main focus of intramolecular hydroacylation was synthesis of cyclopentanone at first, which then switched to larger ring synthesis.

Stoichiometric Systems

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The first hydroacylation is made by Sakai using stoichiometric amount of Wilkinson’s catalyst treating 4-enals at room temperature.[6]

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Then Milstein successfully isolated and characterized the intermediate in the reaction between 4-pentenal and Wilkinson’s catalyst at room temperature. Heating this intermediate gave a cyclopentanone in 72% yield. Though a few turnovers achieved, this is the first example of catalysis that was independent of the existence of ethylene.[7]

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Catalytic Systems

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Cyclopentanone Synthesis

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Both Miller and Larock [8][9]observed the need of ethylene saturated solvent for the yield of cyclopentanone. Miller achieved 72% yield with substoichiometric amount of rhodium complexes in the ethylene saturated solvent. Based on the work of Miller, Larock found the efficient catalyst were generated in situ from [Rh(COD)Cl2] and P(4-MeC6H4)3,P(4-MeOC6H4)3, or P(4-Me2NC6H4)3. Also, Larock discovered the yield was not affected by the existence of water, even though the catalyst was believed to be deactivated by the oxygen in it. Remarkably, in Larock’s research he managed to decrease to catalyst loading to 10 mol %. Moreover, if the catalyst loading increased to as high as 50 mol % in methylene chloride at room temperature, Larock’s system could synthesize a variety of cyclopentanone products.

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One of the breakthrough in catalyst design for alkene hydroacylation is the development of cationic rhodium complexes, which allowed routinely low catalyst loading to be possible. For this, Bosnich was the main contributor. After examining a lot of catalyst system, he found the optimal one was [Rh(dppe)]ClO4. This catalyst was responsible for effective hydroacylation of a variety of substituted 4-pentenals.[10]

Besides Bosnich’s catalyst, Peters reported that zwitterionic Rh complexes was also an effective catalyst for intramolecular alkene hydroacylation. At the same time, this catalyst possessed better tolerance to coordinating solvents as THF, acetonitrile or benzene.[11]

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The third kind of catalyst reported by Morgan and Kundu to be effective in intramolecular hydroacylation is neutral bimetallic rhodium/titanium complexes. This is made in situ from the corresponding hydroxyphosphine, Ti(OiPr4), and [Rh(COD)Cl]2. This catalyst, compared with Bosnich’s catalyst, is slower in rates and lower in yields, but produces less side products. [12]


Larger Ring Synthesis

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The two main challenges for synthesizing cyclic ketone larger than 5-membered ring are slower rates and increasingly dominant pathway, decarbonylation. However there are still many successful examples that managed to circumvent these two problems.

6-Membered Ring
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The first cyclohexanone was made accidently by using rigid carbohydrate-derived scaffold with [RhCl(PPh3)2]2, anticipating cyclopentanone as the product. According to the author, the ring strain resulted from the formation of cyclopentanone is the main reason for this phenomenon. This explanation was reinforced by latter experiment.[13]

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7-Membered Ring
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Mori has successfully synthesized the cycloheptanone with dienal and rhodium(I) catalyst. The proposed mechanism is showed below. Mori also found the geometry of diene had important influence on the distribution of products.[14]

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8-Membered Ring
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The eight-membered ring ketone was made by the incorporation of cyclopropane-containing substrate with cationic Rh catalyst under ethylene atmosphere. Below is the proposed mechanism by Shair.[15]

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Stereoselective Reactions

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With the development of the analytical equipment and more and more findings in naturally bioactive but stereochemically unique molecules, the demand for catalysts to be more stereoselective is also rising, especially the diastereoselectivity and enantioselectivity of catalysts are most investigated.

Diastereoselective Systems

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The first people investigated the diastereoselectivity were Sakai and his co-workers.[16] In their studies they found that a correspondingly high loading of Wilkinson’s complex made the toleration for a number of substituents possible, such as keto-, chloro- and hydroxyl groups. And the products, in all cases, were sorely cis diastereomers.

Later, they found that chiral Rh(Ⅰ) were able to explore double stereoselection, that was using neutral then cationic rhodium complexes.[17] For example, the cationic complex Rh(BINAP)ClO4 permitted the selective of all 4 possible stereoisomers of3,4-disubstituted cyclopentanone. An almost same selectivity was observed with neutral rhodium catalyst, though in much high catalyst loading.

Enantioselective Systems

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The first achievement in enantioselective intramolecular hydroacylation was made by James and Young.[18] However, the selectivity (66% enantiomeric excess) was achieved at the expense of low conversion, 17%.

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As for the first example of enantiomerically pure catalysts in hydroacylation, Sakai [19]used Rh(DIMPC)Cl, DIMPC standing for trans-1,2-bis-[(diphenylphosphino)methyl]cyclohexane, got relatively high enantioselectivity with correspondingly high catalyst loading at room temperature.

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Later, both Sakai and Bosnich confirmed that cationic rhodium complexes with chiral ligands attached were able to make high enantioselectivity at low catalyst loading feasible. Especially Bosnich’s[20] group investigated a variety of chiral ligands like BINAP, chiraphos and Duphos. Also in these experiments, a large range of substituents can be tolerated with conversion over 95% and lowest enantioselectivity over 70%. More importantly, they found the optimal ligand match for individual group of substrates for the best yield and enantioselectivity. For example, according to Bosnich’s study, the BINAP was the best ligand for substrates with tertiary or water and ketone substituents, and Duphos was superior for substrates with primary or secondary alkyl groups.

Interestingly, the Sakai and Suemune[21] group developed an enantioselective desymmetrization approach by using substrates which could have 2 stereocenters. They found cationic rhodium perchlorate BINAP complexes gave trans-configured products in both good yield and selectivity, while at the same time the corresponding neutral complex was able to provide the cis-configured products, again in excellent yield and selectivity. From the research findings above, scientists can synthesize any of the four possible product stereoisomers by academic collection of catalysts.

Applications of Intramolecular Alkene Hydroacylation

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The two main contributions of studies in intramolecular alkene hydroacylation are synthesis and catalyst development. In fact, a lot of key intermediates of natural products syntheses are made through intramolecular hydroacylation. And the finding of more effective and selective catalysts shed the light for other synthesis too. The very first application of intramolecular alkene hydroacylation was the construction of cyclopentanone with noncyclic precursors. Sakai, the first explorer of hydroacylation, used limonene-10-ol from terpene to get enantiomerically pure enal.[22] Then he treated the enal with Wilkinson’s catalyst to achieve a single diastereomer ketone, which was later advanced to a key intermediate in the synthesis of 11-deoxyprostaglandin.

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What worth mentioning about recent application of intramolecular hydroacylation is that Dong’s group[23] successfully synthesized benzo-fused medium-ring heterocycles. They used chelation control to get seven- and eight-membered rings. Also, Dong found that for certain substrate different type of ligand can determine different size of the product. For example, When the sulfide was treated with a Me-DuPhos catalyst, the product was seven-membered ketone. At the same time, if the same substrate was treated with a catalyst featuring the ligand (2S,4S)-2,4-bis-(diphenylphosphino) pentane (BDPP), then the eight-membered ring ketone was the main product.[24][25]

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Sato group also utilized the intramolecular diene hydroacylation as important step forming the key seven membered ring in their cascade process to synthesize the epiglobulol.[26]

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Alkyne Intramolecular Hydroacylation

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Intramolecular alkyne hydroacylation is the least studied among all the hydroacylations. It is partially due to the restricted variety in ring formation compared with intermolecular hydroacylation (only 5 or 6 atoms-ring is formed). Also the products’ lacking of stereogenic centers (C-C double bond is formed) makes chemists less interested in alkyne hydroacylation than its counterparts, the alkene-based one.

Catalytic Achiral Systems

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At the beginning, many attempts using Wilkinson’s complexes failed with low yield. Then scientists (mainly Fu group) successfully cyclized 4-alkynals with cationic rhodium complexes.[27] The [Rh(dppe)2(BF4)2] in acetone at room temperature can tolerate a wide range of substituents with high yields. It was found that the selection of catalyst and its corresponding solvent is important like the combination of methylene chloride and BINAP-derived catalyst.[28]

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For the cyclization of 4-alkynal, the catalyst has to undergo an unusual trans addition. However, Tanaka group found the hydroacylation of 5-or 6-alkynal involves cis addition to alkynes. At the same time, the correspondingly modified catalyst, [Rh(BINAP)]BF4, is less effective, resulting reduced yield when at room temperature.[29]

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The interesting finding is that the 6-alkynals can also be catalyzed under nearly the same condition as 3- or 4-alkynals, as described below. Before this, the hydroacylation for 6-membered ring was only achieved by conformational restrictions or chelation assistance. But, the trial to form 7-membered ring was unsuccessful.

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Enantioselective Systems

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To achieve good enantioselectivity, the Fu group used the enantiomerically pure Rh catalyst and investigated the kinetic resolution and desymmetrization process. They found racemic 4-alkynals with substituents in its 3-position underwent smooth kinetic resolution, thus giving good enantioselectivity to enantiomerical cyclopentenons.[30]

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Also, in Fu’s research the Tol-BINAP-derived catalyst allowed a better selectivity for the same enantiomerically enriched cyclopentenons.

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What’s worth mentioning in Fu’s research is the rare catalytic parallel kinetic resolution.[31] Using the Tol-BINAP-derived Rh catalyst to treat racemic 4-alkynal produced two cyclic ketones (pent- and but-) and both of them were in high enantiomeric excess.

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Intermolecular Hydroacylation

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Success in intramolecular hydroacylation led to the development of the intermolecular analogue. This move to intermolecular systems has presented the new challenges, as well as being disfavoured by entropy. Systems that worked for intramolecular reactions would not neccesarily work for the intermolecular processes. In combining two different molecules at the catalyst there needs to be two different coordination events which happend independently of each other, opening up the potential window of time that exists after the aldehyde coordinates and before the alkene or alkyne latches on. In this time period the intermediate species that is formed is unstable can can undergo decomposition. The most likely path of decomposition at this step of the reaction is the reductive decarbonylation. Another possible side reaction is the aldol reaction, this has been found to occurs when alkyl aldehydes are employed.

To minimize decarbonylation there is the use of high carbon monoxide pressures to disfavour the forward reaction of decarbonylation. These reactions are generally run in coordinating solvents to stabilizing the acyl-hydride intermediate that can undergo decarbonylation or excess alkene or alkyne is employed for the same purpose. Chelation of the aldehyde to metal centre, or chelation of the alkene to the metal centre has also shown great worth as a strategy to deal with decarbonylation. The aldol process can be suppressed by changing certain reaction conditions.[32]

Rhodium

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Acyl-Hydride Intermediate

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It had been proposed that by coordinating the aldehyde to the metal centre an acyl-rhodium (III) intermediate should be formed, and an acyl hydride intermediate of this sort was isolated by Suggs in 1978.[33] He also showed that the intermediate was active in the hydroacylation reaction, reporting a 55% product yield. He found that decarbonylation does not happen at room temperature, proposing the reason for this is the instability of the four membered ring intermediate chelating alkyl that would need to form .

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Major Approaches In Intermolecular Hydroacylation

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Aldehyde Chelation

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The first intermolecular hydroacylation was described by Miller[34] in 1980, enals were employed as the aldehyde and the reaction was a coupling of an enal and ethylene. The intramolecular product cyclopentanone was detected as a product at less than 1%. They found that saturated aldehydes resulted in no product formation.

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After the initial findings by Miller, Vora showed that Z-4 heptanals under identical conditions could be combined with ethylene to form 6-nonen-3-one. [35]

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Leading to the conclusion that internal alkene coordination to the metal centre, esentially chelation of the aldehyde, is necessary for the reaction to occur.

O-chelation
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Chelating Intermediate

Salicylaldehyde as the aldehyde in conjunction with various alkenes and alkynes, using a catalyst produced from [RhCl(COD)]2 and 1,1’-bis(diphenylphosphino)ferrocene (dppf) has been studied and it was found that reactivity was high for vinyltriethylsilane and allenes, also, minimal decarbonylation was observed, this was attributed to the proposed chelating intermediate.[36][37]

A similar system using salicylaldehyde as the aldehyde and 1,4 and 1,5-dienes has been established and is able to attain good yields under mild conditions. 1,6-dienes were found to produce considerably lower yields. This system uses Wilkinson's catalyst in 20% catalyst loading at room temperature. A drawback is that the branched isomer is formed over the linear isomer, a 4:1 branched to linear distribution is observed. They also showed that the reaction was possible in good yields with other 2-hydroxy aldehydes, but that the absence of the hydroxyl group would not produce the hydroacylation adduct.[38]

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S-chelation
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Similar to O-chelation is the method of S-chelation, where a sulfur containing group on the aldehyde is used with the purpose of creating a chelating aldehyde. β–methylsulfide substituted propanal and β-thioacetal substituted aldehydes have been shown to produce good yields when in conjunction with functionalized electron poor alkenes using [Rh(dppe)]ClO at 50°C . This system also showed good results with terminal alkynes; being selective for linear isomers with relatively good E-selectivity and internal alkynes could be employed but do require longer reaction times.[39] Neither of these system was functional for neutral alkenes such as 1-octene. A catalyst produced from (Rh(COD)Cl)2 with bis[(2-diphenylphosphino)phenyl] ether (DPEphos) and Ag(ClO4) in acetone was found to be able to catalyze electron neutral alkenes such as 1-hexene with β-MeS-propanal, in 70% yield. This is attributed to the flexibility and hemilability of the DPEphos ligand. Allowing it to adopt a variety of coordination modes and geometries, stabilizing the catalyst and resulting in a longer lived catalyst and increased activity. This system was also active for alkynes such as 1-hexyne. [40]

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This rhodium DPEphos catalyst could also react enones with simple β-S-aldehydes, whereas the dppe derived rhodium catalyst under similar conditions in the past gave reductive aldol products instead. The success was again attributed to the DPEphos ligand having many different binding modes.

Picolyl Imines

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The use of picolyl imines is quite noteworthy because of the good yields that is attainable, the small amounts of rhodium necessary to catalyze the reaction, and the production of a hydroacylation adduct that is substituent free. The picolyl imine could be formed in situ from an aldehyde and an amine. The catalytic system was optimized by Jun et al, they treated a mixture of aldehyde and alkene with Wilkinson’s complex at 2 mol%, picolyl amine at 20 mol%,benzoic acid at 6mol% and aniline at 60 mol% in toluene at 130°. Good yields were obtained for various aldehyde and alkenes, as well as heteroatomic aldehydes.[41] (scheme 7) Aldol byproducts were also observed when alkyl aldehydes were used as the substrate but by changing the conditions of reaction in using cyclohexylamine instead of aniline and para-trifluormethylbeenzoic acid instead of benzoic acid, aldol side-product formation could be stopped.[42]

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Alkene-chelation

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Bidentate alkene intermediate

Stabilization of the acyl-rhodium intermediate can be achieved by using a bidentate alkene. N,N-dialkyl acrylamides as the alkene can be used with a range of alkyls and aryl aldehydes. The proposed intermediate has the bidentate alkene chelating to the metal.[43]

Ruthenium

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Ru catalyzed hydroacylation of a diene

RuH(CO)(PPh3)3 is used as a catalyst for the addition of 1,3-dienes to aldehydes at relatively low catalyst loadings, 5 mol % at 90° C in toluene. Variety of dienes and aryl and alkyl aldehydes were found to produce branched β,γ-unsaturated ketones as products at good yields.[44] The proposed mechanism of this is different from that of rhodium catalyzed hydroacylation in that the initial step is the diene addition to the ruthenium hydride to form a π-allyl-ruthenium intermediate. To this intermediate the aldehyde can add and through a six membered transition state a ruthenium alkoxide is formed, which can undergo beta-hydride elimination forming the hydroacylation adduct and regenerating the ruthenium hydride.


Cobalt

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Cobalt hydroacylation catalyst

Cobalt (I) bis –olefin complexes have been shown to effectively catalyze addition of aromatic and alkyl aldehydes to vinyl trialkylsilanes, activity is shown for a broad range of aldehydes with vinyltrimethylsilane. The trimethylsilane substitution on the alkene is necessary as this allows for facile dissociate of the alkenes form the metal centre. While the conditions of the reaction is favourable, room temperature, low catalyst loadings, and no requirement for a CO atmosphere; a drawback to this system is the requirement of vinyl silanes.[45]


Nickel

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Nickel (0) catalysts have been shown effective for alkyne but not alkene hydroacylation. [Ni(COD)2] at 5 mol% could combine simple alkyl and aryl aldehydes with internal alkynes in good yields, to preferentially form the E-isomer of the enone product. Similarly to rhodium catalyzed alkene hydroacylation the mechanism of this reaction is also proposed to go through an acyl nickel hydride intermediate.[46]

Enantioselective

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The use of different phosphine ligand systems can lead to formation of preferentially endo or exo-isomers.

This can be explained by the bidentate ligand causing the norbornadiene to bind in only one way, a monodentate manner, leading to the exo selectivity. While the endo selectivtity could be achieved with the use of a monodentate phosphoramidate ligand.[47]


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