Jiang-Tao Fanab,
Xin-Heng Fan*a,
Cai-Yan Gaoa,
Zhenpeng Wangc and
Lian-Ming Yang*a
aBeijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: yanglm@iccas.ac.cn; xinxin9968@iccas.ac.cn; Fax: +8610-62559373
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
cNational Centre for Mass Spectrometry in Beijing, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
First published on 4th November 2019
A simple, mild and efficient protocol was developed for the alkylation of fluorene with alcohols in the presence of t-BuOK as catalyst, affording the desired 9-monoalkylfluorenes with near quantitative yields in most cases.
Then, a more detailed survey of the base-catalysed alkylation reaction was performed by choosing fluorene and p-methoxybenzyl alcohol as model substrates, and the results are summarized in Table 1. After some experimentation, our standard reaction conditions (i.e., in the presence of 50 mol% t-BuOK in toluene at 120 °C under N2 for 3 h) were determined, where a complete conversion and near quantitative yield were achieved (entry 1). The role of bases has been examined: no conversion occurred in the absence of the base (entry 2); KOH (entry 3) can give a high yield of 85%, and thus will be an optional catalyst when a large-scale preparation is considered; and other bases, such as t-BuONa (entry 4), NaOH (entry 5), CsOH·H2O (entry 6) and K2CO3 (entry 7), seemed to be inferior or ineffective. The reaction temperature also was crucial for this reaction since a modest drop of reaction temperatures from 120 °C to 100 °C led to an incomplete conversion, an extremely low yield, and a substantial quantity of 9-benzylidenefluorene byproduct 4 (entry 8).7 Toluene (entry 1) and dioxane (entry 9) were the choice of solvents, but THF (entry 10) was not suitable for the reaction. Subsequently, a systematic investigation was made on the amount of t-BuOK used, suggesting that with increasing the base from 50 mol% (entry 1) to 100 mol% (entry 11), 150 mol% (entry 12) and 200 mol% (entry 13), the desired product decreased and the byproduct 9-fluorenone increased gradually; when it was reduced from 50 mol% (entry 1) to 25 mol% (entry 14), and 10 mol% (entry 15), the reaction could proceed to completion with no side reaction as long as the reaction lasted long enough.
Entry | Variation from the standard conditions | 3cb | 1b | 4b | Fluorenoneb |
---|---|---|---|---|---|
a The standard conditions: fluorene (0.5 mmol), 4-methoxybenzyl alcohol (1.5 mmol), base (0.25 mmol, 50 mol% relative to fluorene), solvent (4 mL), 120 °C, in N2, 3 h.b The crude product examined quantitatively by 1H NMR and qualitatively by TLC; NMR yields determined by 1H NMR analysis using 1,3,5-trimethoxybenzene as an internal standard.c Reaction time: 24 h.d Reaction time: 48 h. | |||||
1 | None | 99 | 0 | 0 | 0 |
2 | With no use of base | 0 | 100 | 0 | 0 |
3 | KOH instead of t-BuOK | 85 | 10 | 0 | 0 |
4 | t-BuONa instead of t-BuOK | ∼5 | 71 | 21 | 0 |
5 | NaOH instead of t-BuOK | Trace | 72 | 25 | 0 |
6 | CsOH·H2O instead of t-BuOK | 26 | 54 | ∼5 | 0 |
7 | K2CO3 instead of t-BuOK | 0 | 100 | 0 | 0 |
8 | 100 °C instead of 120 °C | 10 | 48 | 40 | 0 |
9 | 1,4-Dioxane instead of toluene | 92 | ∼5 | 0 | 0 |
10 | THF instead of toluene | Trace | 88 | 10 | 0 |
11 | 0.5 mmol (100 mol%) t-BuOK | 90 | ∼5 | ∼5 | Trace |
12 | 0.75 mmol (150 mol%) t-BuOK | 60 | ∼5 | ∼5 | 28 |
13 | 1.0 mmol (200 mol%) t-BuOK | 50 | ∼5 | 0 | 40 |
14c | 0.125 mmol (25 mol%) t-BuOK | 99 | 0 | 0 | 0 |
15d | 0.05 mmol (10 mol%) t-BuOK | 99 | 0 | 0 | 0 |
Next, fluorene reacted with a range of representative alcohols to determine the generality of this protocol (Table 2). Generally, benzylic alcohols, whether the nonactivated (3a, 3f, 3k, and 3l), activated (3g, 3h, 3i, and 3j) or deactivated (3b, 3c, and 3d), smoothly underwent the reaction to afford the desired products with almost quantitative conversions and yields. Ortho-substituted benzylic alcohol (3e) needed a prolonged reaction time due to its steric effect, giving an excellent yield of 95%. Additionally, the mild reaction conditions tolerated some functional groups like the fluoro (3g), chloro (3h), bromo (3i and 3s), iodo (3j) or trifluoromethyl (3t) group. As we know, such halogen-containing derivatives would be very useful in organic synthesis as they might be further transformed and compounds containing trifluoromethyl functional groups are important pharmaceutical intermediates. Likewise, the reaction of fused aryl (3k and 3l), heteroaryl (3m) carbinols and piperonyl alcohol (3u) proceeded smoothly in near quantitative conversions under the moderately modified reaction conditions. Although aliphatic alcohols are much less reactive as alkylating reagents than benzylic alcohols,5 that isn't the case in our reaction. Primary aliphatic alcohols (3n, 3o and 3p) were quantitatively converted to the desired 9-monoalkylfluorenes; even sterically congested secondary alcohols such as isopropanol (3q), 1-phenyl ethanol (3r) and cyclohexanol (3v) furnished the corresponding products in high yields at a more elevated temperature of 140 °C.
Entry | Alcohol | Product | Isolated yield (%) |
---|---|---|---|
a Reaction conditions: fluorene (0.5 mmol), alcohols (1.5 mmol), t-BuOK (0.25 mmol), toluene (4 mL), in N2.b 24 h.c t-BuOK (0.375 mmol).d t-BuOK (0.50 mmol).e t-BuOK (0.75 mmol).f 140 °C.g 2-Bromo-9-fluorene was used. | |||
1 | 98 | ||
2 | 99 | ||
3 | 96 | ||
4 | 99 | ||
5b | 95 | ||
6 | 99 | ||
7 | 99 | ||
8 | 99 | ||
9 | 99 | ||
10 | 99 | ||
11b,c | 99 | ||
12b,c | 99 | ||
13b,d | 97 | ||
14b,c | 99 | ||
15b,c | 99 | ||
16b,d | 99 | ||
17b,e,f | 89 | ||
18b,e,f | 90 | ||
19g | 99 | ||
20b,e,f | 85 | ||
21c | 99 | ||
22b,e,f | 81 |
To ascertain the mechanism of the reaction, several additional control experiments were designed and carried out (Scheme 1). In a blank experiment, the freshly distilled p-methoxybenzyl alcohol was treated with potassium tert-butoxide (0.5 equivalents) at 120 °C in N2 for 3 h, affording an around 5% yield of anisaldehyde 5c (Scheme 1-i). 9-Benzylidenefluorene 4c was readily obtained in 72% isolated yield from the reaction of fluorene and benzaldehyde under the standard conditions (Scheme 1-ii). In the process of transfer hydrogenation of 4c with 2c, the target product 3c and equimolar aldehyde 5c (5c/3c = 0.98/1.00 mol mol−1 by 1H NMR analysis of the reaction mixture) would be generated simultaneously (Scheme 1-iii).
Combining our own experimentation with the relevant publications,4,5 we proposed a plausible mechanistic path for this reaction (Scheme 2). As shown in Scheme 2, a small amount of aldehyde 5 corresponding to alcohol 2 would first occur under the reaction conditions given. It was well established that an aldehyde 5 condenses with fluorene 1 to give a dibenzofulvene 4 in the presence of the base.8 Next, potassium alkoxide reduces the exo-double bond with attendant formation of a molecule of aldehyde. This step may formally be regarded as a type of Meerwein–Ponndorf–Verley reduction9 where the exo-double bond of the dipolar fulvene plays the role of the hydrogen acceptor. Finally, the potassium derivative of the product 6 reacts with the alcohol to give 9-alkylfluorene 3.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra07557g |
This journal is © The Royal Society of Chemistry 2019 |