Intramolecular [3 + 2]-cycloadditions of azomethine ylides derived from secondary amines via redox-neutral C-H functionalization.

Azomethine ylides are accessed under mild conditions via benzoic acid catalyzed condensations of 1,2,3,4-tetrahydroisoquinolines or tryptolines with aldehydes bearing a pendent dipolarophile. These intermediates undergo intramolecular [3 + 2]-cycloadditions in a highly diastereoselective fashion to form polycyclic amines with four new stereogenic centers. Challenging substrates such as piperidine, morpholine, and thiomorpholine undergo the corresponding reactions at elevated temperatures.

[3 + 2]-Cycloadditions of azomethine ylides, in particular when performed in an intramolecular setting, are ideally suited for the purpose of rapidly constructing highly substituted polycyclic amines.[1] Among the methods that are available to generate nonstabilized azomethine ylides in situ, these dipolar species are most frequently prepared via decarboxylative condensation of aldehydes with amino acids such as proline and sarcosine.[1] Examples of azomethine ylide formation from simple, unfunctionalized cyclic amines and their subsequent dipolar cycloadditions remain rare and require relatively high reaction temperatures for even the most activated amines such as 1,2,3,4-tetrahydroisoquinoline (THIQ). Here we report a method to access azomethine ylides from a range of simple secondary amines and their application to intramolecular [3 + 2]-cycloadditions.

Given our continuing interest in developing methods for the redox-neutral α-C–H bond functionalization of amines,[2−6] we were inspired by seminal studies of Grigg et al., who showed that THIQ reacts with aldehyde 1a to form polycyclic amine 2a via nonstabilized azomethine ylide 3 (Scheme 1).[7,8] Although this transformation requires relatively high reaction temperatures (reflux in xylenes), product 2a was obtained as a single diastereomer. The rate limiting step of this reaction is thought to be the generation of the azomethine ylide 3.[7] If conditions could be identified that would allow for the generation of the required azomethine ylide under milder conditions, the scope and applicability of such transformations could be expanded dramatically, possibly paving the way for the development of asymmetric variants.

Scheme 1

Pioneering Work by Grigg and Co-workers

We set out to identify the mildest conditions possible under which the reaction of aldehyde 1b with THIQ would proceed with a synthetically useful rate (Table 1). A reaction conducted under reflux in toluene required 24 h to reach completion and provided [3 + 2]-cycloaddition product 2b in 63% yield (Table 1, entry 1). This finding is in excellent agreement with the studies of Grigg and co-workers,[7] with the increase in reaction time being fully expected based on the reduction in reaction temperature (138 vs 111 °C). As it has been shown that benzoic acid facilitates amine α-functionalization via intermediate azomethine ylides,[2d] we tested this compound as an additive. Indeed, benzoic acid used at a 20 mol % loading led to marked rate acceleration with the reaction being completed after 2 h (Table 1, entry 2). In addition, 2b was obtained with an increased yield of 84%. It was found that the reaction proceeded efficiently at temperatures down to 60 °C (Table 1, entries 3–5). However, little product formation was observed after 24 h in an experiment that was conducted at 50 °C (Table 1, entry 6).

Table 1

Evaluation of Reaction Conditionsa

entry	PhCOOH (mol %)	molecular sieves	temp [°C]	time [h]	2b, yield (%)
1	–	–	reflux	24	63
2	20	–	reflux	2	84
3	20	–	100	2	72
4	20	–	80	3	75
5	20	–	60	24	82
6	20	–	50	24	NR
7	–	3 Å	reflux	16	67
8	–	3 Å	70	24	65
9	–	3 Å	60	24	NR
10	20	3 Å	60	2	91
11	20	4 Å	60	2	89
12	20	3 Å	50	3	92
13	20	3 Å	rt	12	96
14	10	3 Å	rt	18	68
15	50	3 Å	rt	8	94

Reactions were performed with 0.5 mmol of 1b and 1.2 equiv of THIQ. Yields are isolated yields of chromatographically purified compounds.

In another set of experiments, we evaluated the effect of 3 Å molecular sieves (3 Å MS) on the reaction rate (Table 1, entries 7–9). Compared to the reaction without additives (Table 1, entry 1), addition of 3 Å MS resulted in a marked reduction of reaction time and a slight increase in yield (Table 1, entry 7). The reaction still progressed with a satisfactory rate down to 70 °C, but stalled at 60 °C (Table 1, entries 8, 9). A powerful cooperative effect was observed when the title reaction was conducted at 60 °C in the presence of both benzoic acid (20 mol %) and 3 Å MS. These conditions allowed for the isolation of 2b in 91% yield after only 2 h (Table 1, entry 10, compare to entry 5). The use of 4 Å MS in an otherwise identical experiment gave very similar results (Table 1, entry 11). Remarkably, the reaction was found to proceed efficiently even at room temperature. In fact, the highest yield of 2b (96%) was observed in this instance (Table 1, entry 13). A reduction in the loading of benzoic acid to 10 mol % had a negative effect on the yield of 2b (Table 1, entry 14), whereas an increase to 50 mol % did not offer any significant advantages over the 20 mol % catalyst loading (Table 1, entry 15).[9]

Reactions of THIQ with a range of aldehydes derived from differently substituted salicylaldehydes were evaluated under the optimized conditions (Scheme 2). In all cases, products were obtained in good yields at room temperature or slightly elevated temperatures (products 2b–h). Importantly, other benzylic amines including 6,7-dimethoxy-THIQ, tryptoline, isoindoline, and 3-pyrroline also underwent the title reaction under equally mild conditions. Interestingly, even sterically demanding 1-aryl THIQ’s and a 1-aryl-tryptoline readily participated in the reaction sequence to generate polycyclic amines containing tetrasubstituted carbon centers (products 2q–s). Finally, the oxygen linker present in all products discussed so far could be replaced with nitrogen or carbon linkers (e.g., products 2t and 2u). Notably, in all cases, products were isolated as single diastereomers.

Scheme 2

Substrate Scope for the [3 + 2]-Cycloaddition with Benzylic Amines

The scope of the [3 + 2]-cycloaddition could be readily expanded to nonbenzylic amines (Scheme 3). Not surprisingly, higher reaction temperatures were found to be required for these more challenging substrates. While pyrrolidine was sufficiently reactive at reflux temperature in toluene,[10] superior results were obtained under microwave conditions. Following a reaction time of 30 min at 160 °C, product 4a was obtained in 82% yield. Under similar conditions, piperidine, azepane, morpholine, thiomorpholine, and indoline[2i] also underwent the corresponding [3 + 2]-cycloadditions. Other aldehydes including those with nitrogen or carbon linkers also participated in efficient product formation.

Scheme 3

Substrate Scope for the [3 + 2]-Cycloaddition with Less Reactive Amines

In order to explore the regioselectivity of the dipolar cycloaddition with nonsymmetrical amines, the reaction was conducted with 2-substituted pyrrolidines (Scheme 4). A reaction with 2-methylpyrrolidine resulted in the formation of the two diastereomeric products 5a and 5b in an approximately 5:1 ratio. Interestingly, no regioisomeric product was observed that would have resulted from the functionalization of the C–H bond α to the methyl group in 2-methylpyrrolidine. This preference for the functionalization of a secondary over an electronically favorable tertiary C–H bond is presumably due to sterical reasons. Slightly higher overall yields were observed in the related reaction with 2-phenylpyrrolidine. Here, two regioisomeric products 6a and 6b were obtained in 52% and 25% yield, respectively, amounting to a 2-fold preference for the functionalization of a secondary over an electronically favorable tertiary benzylic C–H bond.

Scheme 4

Regioselectivity in the [3 + 2]-Cycloaddition with Nonsymmetrical Amines

Lastly, we wanted to establish whether a stereospecific [3 + 2]-cycloaddition could be accomplished with a chiral nonracemic amine substrate, as this would add further value to this type of transformation. To this end, enantioenriched (S)-3-isopropyl-THIQ (88% ee)[11] was allowed to react with aldehyde 1b (Scheme 5). In the event, product 7 was isolated as a single diastereomer in 86% yield and with 90% ee, illustrating the feasibility of the asymmetric approach.

Scheme 5

Asymmetric Variant of the [3 + 2]-Cycloaddition

In conclusion, we have demonstrated that simple cyclic amines engage in reaction cascades with aldehydes bearing a pendent dipolarophile to provide polycyclic products via amine α-C–H bond functionalization. The key step is a highly diastereoselective [3 + 2]-cycloaddition of in situ generated azomethines ylides. The overall process is facilitated by the combined action of benzoic acid and molecular sieves. This approach provides a valuable alternative to the widely used decarboxylative versions of these transformations, in particular in those instances where the corresponding amino acids are not readily available.