C-Propargylation Overrides O-Propargylation in Reactions of Propargyl Chloride with Primary Alcohols via Rhodium Catalyzed Transfer Hydrogenation

Graphical abstract

A Path Less Travelled. The canonical S_N2 behavior displayed by alcohols and activated alkyl halides in basic media (O-alkylation) is superseded by a pathway leading to carbinol C-alkylation under the conditions of rhodium catalyzed transfer hydrogenation. Racemic and asymmetric propargylations are described.

The merger of carbonyl addition and transfer hydrogenation has enabled a new class of metal catalyzed C-C couplings wherein lower alcohols are convert directly to higher alcohols.¹ Three mechanistic pathways are corroborated, in which alcohol dehydrogenation mediates (a) C-C π-bond hydrometalation (Ir^I, Ru^II), (b) metalacycle transfer hydrogenolysis (Ru⁰, Os⁰) or (c) C-X bond reductive cleavage (Ir^I).^[1] In the latter context, iridium-based catalysts operate exclusively, promoting the coupling of primary alcohols with a diverse array of allylic carboxylates^[2a-d] and related pronucleophiles, such as vinyl epoxides^[2e] and vinyl aziridines.^[2f] The identification of metal catalysts beyond iridium that promote alcohol C-H functionalization via C-X bond reductive cleavage pathways should enable further expansion of scope. Rhodium-based catalysts, being isostructural with respective to iridium, were viewed as promising candidates. However, rhodium is used less frequently than iridium in transfer hydrogenation,^[3] with the vast majority of examples involving analogues of the classic ruthenium-based system RuCl(Tsdpen)(η⁶-arene).^[4,5b-e] Indeed, rhodium analogues of the broadly utilized cyclometalated π-allyliridium ortho-C,O-benzoate complexes developed in our laboratory have not yet proven effective (Figure 1).

Figure 1.

Rhodium vs iridium catalysts for redox-triggered carbonyl addition.

As the pronucleophile serves as oxidant in all transfer hydrogenative couplings, we reasoned that more easily reduced pronucleophiles might be accommodated by rhodium, which is a weaker reductant than iridium.^[6] Accordingly, we turned our attention to the redox-triggered coupling of primary alcohols with propargyl chloride with the goal of developing methods for enantioselective carbonyl propargylation (Figure 2).^[7-15] In earlier work from our laboratory,^[16] an iridium-catalyzed transfer hydrogenative carbonyl propargylation was developed (eq. 1),^[16c] but suffered from two severe limitations in scope: (a) trialkylsilyl substitution was required at the acetylenic terminus of the propargyl chloride, and (b) only benzylic alcohols would participate in C-C coupling. To determine whether rhodium catalysts could overcome these restrictions, a series of experiments were performed (Scheme 1). The rhodium analogue of the optimal iridium catalyst identified for the coupling of the silyl-terminated propargyl chloride 1a with benzyl alcohols was prepared and evaluated in the coupling of benzylic alcohol 2b with the unsubstituted propargyl chloride 1b. The desired product, homopropargyl alcohol 3b, was obtained in 23% yield (eq. 2). Further improvements in the yield of 3b were obtained by conducting the reaction at 40 °C in toluene – a remarkably low temperature for alcohol dehydrogenation – using a neutral rhodium-BINAP catalyst and increasing the loading of propargyl chloride 1b (1000 mol%). Applied in concert, these changes enabled formation of homopropargyl alcohol 3b in 80% yield (eq. 3). Reactions conducted at lower loading of 1b (500 mol%) under otherwise identical conditions led to a modest but significant decrease in the yield of 3b (10-15% lower). A comparable decrease in the yield of 3b is observed upon omission of 2-propanol. Remarkably, products of O-propargylation via S_N2 substitution were not observed.^[17]

Figure 2.

Selected methods for enantioselective carbonyl propargylation.^[17-15]

Scheme 1.

Selected experiments in the development of rhodium catalyzed transfer hydrogenative propargylation.^a

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

To establish the generality of these conditions, in particular, the ability to engage aliphatic alcohols in C-propargylation, these conditions were applied to primary alcohols 2a-2o (Table 1). Benzylic alcohols 2a-2h, including ortho-substituted benzylic alcohol 2f and heteroaromatic benzylic alcohol 2h were converted to the corresponding homopropargylic alcohols 3a-3h in good to excellent yield. Remarkably, allylic alcohols 2i and 2j were converted to homopropargyl alcohols 3i and 3j without competing internal redox isomerization.¹⁸ Most importantly, aliphatic alcohols 2k-2o were converted to the homopropargylic alcohols 3k-3o, respectively, in good yield. Finally, as illustrated in the conversion of dehydro-2b to homopropargyl alcohol 3b, these conditions are applicable to the 2-propanol mediated reductive coupling of propargyl chloride 1b with aldehydes (eq. 4).

Table 1.

Rhodium catalyzed C-C coupling of propargyl chloride 1b with alcohols 2a-2o to form homopropargyl alcohols 3a-3o.^a

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

^b2-PrOH was omitted.

(Eq. 4)

Products 3a-3o were generated using a racemic rhodium-BINAP catalyst (Table 1). Using enantiomerically pure BINAP, moderate levels of enantioselectivity are observed (40-55%ee). Efforts to improve enantioselectivity while maintaining high levels of conversion have, thus far, been unrequited. Hence, match-mismatch effects in the C-propargylation of the enantiomerically enriched α-stereogenic amino alcohol 2p were explored (eq. 5). First, to establish the intrinsic diastereofacial bias, the rhodium catalyst modified by racemic BINAP was used. The anti- and syn-diastereomers 3p and epi-3p are formed in a 2.3:1 ratio, respectively. This diastereofacial bias suggests intervention of an internal NH-O hydrogen bond in the transient aldehyde dehydro-2p, which directs carbonyl addition to the sterically less encumbered face of the chelate. When the propargylation is conducted using (S)-BINAP, the mismatched case, 3p and epi-3p are in formed in a 1:2.6 ratio, respectively. When the reaction is conducted using (R)-BINAP (the matched case), 3p and epi-3p are formed in a 5:1 ratio, respectively. It was reasoned that reactions conducted in a lower dielectric medium more conducive to hydrogen bonding would display higher diastereoselectivities. Indeed, in toluene, 3p and epi-3p are in formed in an 11:1 ratio, respectively, albeit in slightly lower yield.

Table 2.

Rhodium catalyzed C-C coupling of propargyl chloride 1b with α-amino alcohols 2p-2u to form homopropargyl alcohols 3p-3u.^a

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

^bToluene (1.0 M)

(Eq. 5)

These conditions were applied to C-propargylation of enantiomerically enriched α-amino alcohols 2p-2u (Table 2). Due to solubility issues, it was necessary to use THF as solvent. Nevertheless, the products of asymmetric C-propargylation 3p-3u were formed in a stereoselective manner, with diastereoselectivities increasing with increasing size of the α-substituent. HPLC Analysis of adduct 3p prepared through asymmetric propargylation was compared to a mixture of all 4 stereoisomers, revealing that racemization of the transient aldehyde does not occur (see Supporting Information).

Although at this early stage precise details of the catalytic mechanism are unknown, a very simple working model has been proposed as a basis for further refinement (Scheme 2). It is postulated that catalysis is initiated by oxidative addition of propargyl chloride 1b to rhodium complex I to form the η¹-allenylrhodium(III) complex III. Precedent for this step in the catalytic mechanism is found in the oxidative addition of propargyl chloride 1b to Vaska’s complex, which provides well defined η¹-allenyliridium(III) complexes.¹⁹ Equilibration may occur between η¹-allenylrhodium(III) complex III and propargylrhodium(III) complex II, however, the η¹-allenylmetal isomers are thermodynamically preferred.²⁰ Complex III undergoes substitution with benzyl alcohol 2a to form the rhodium(III) alkoxide complex IV, which upon β-hydride elimination generates the rhodium(III) hydrochloride complex V and the aldehyde dehydro-2a. At this stage, complex V may undergo C-H reductive elimination to form propyne, which may account for the requirement of relatively high loadings of propargyl chloride 1b. An ion consistent with the molecular weight of propyne (or allene) is observed by GC-MS analysis of aliquots taken from reaction mixtures in the coupling of propargyl chloride 1b and benzyl alcohol 2a. This pathway, which effects net catalytic transfer hydrogenolysis of propargyl chloride 1b, delivers unreacted aldehyde, which is converted back to the alcohol by 2-propanol-mediated reduction, reinitiating the catalytic cycle. Nonconjugated aldehydes derived from alcohols 2k-2u appear more reactive toward addition. Hence, 2-propanol is not required. Alternatively, complex V may eliminate HCl with the assistance of base, as documented in stoichiometric transformations.²¹ The latter pathway delivers the η¹-allenylrhodium(I) complex VI, which coordinates aldehyde to form complex VII, which, in turn, triggers carbonyl addition to generate the homoallylic rhodium(I) alkoxide VIII. Protonolytic cleavage then releases the homopropargylic alcohol 3a and regenerates complex I to close the catalytic cycle. The stereochemistry of the octahedral complexes II-V should be considered tentative.

Scheme 2.

General catalytic mechanism for rhodium catalyzed alcohol C-propargylation..^a

In summary, new reactivity is the most fundamental basis for innovation in the field of chemical synthesis. Here, using the concepts of C-C bond formation transfer hydrogenation pioneered in our laboratory, the canonical S_N2 behavior displayed by alcohols and activated alkyl halides in basic media (O-alkylation) is superseded by an alternate pathway leading to products of carbinol C-alkylation. This method enables direct conversion of primary alcohols, including simple aliphatic alcohols, to secondary homopropargyl alcohols using inexpensive, commercial reagents. More broadly, these studies further demonstrate how the native reducing features of alcohol reactants can mediate reductive carbonyl addition, thus bypassing preformed carbanions or stoichiometric metallic reductants.