A one-pot cross-pinacol coupling/rearrangement procedure

Article abstract of DOI:10.1002/hlca.201200402

A new catalytic retro-pinacol/cross-pinacol reaction, followed by subsequent rearrangement or deoxygenation of the intermediately formed vicinal diols, is described. This operationally simple one-pot protocol allows isolation of geminal α,α-diphenyl ketones or 1,1-diphenyl alkenes with high yields and selectivities. Copyright

Full text of DOI:10.1002/hlca.201200402

1970
Helvetica Chimica Acta – Vol. 95 (2012)
A One-Pot Cross-Pinacol Coupling/Rearrangement Procedure
by Ulf Scheffler and Rainer Mahrwald*
Institute of Chemistry, Humboldt-University, Brook-Taylor Strasse 2, DE-12489 Berlin
(rainer.mahrwald@rz.hu-berlin.de)
Dedicated to Dieter Seebach on the occasion of his 75th birthday
A new catalytic retro-pinacol/cross-pinacol reaction, followed by subsequent rearrangement or
deoxygenation of the intermediately formed vicinal diols, is described. This operationally simple one-pot
protocol allows isolation of geminal a,a-diphenyl ketones or 1,1-diphenyl alkenes with high yields and
selectivities.
Introduction. – Vicinal diols are not only substructures in a variety of natural
products but they also serve as important functional-group units for the synthesis
various structure elements [1]. A very effective method for the construction of vicinal
diols is the pinacol coupling due to the direct one-step reaction of carbonyl compounds
and thus high atom economy. Usually, the application of pinacol coupling is restricted
to the synthesis of symmetrical diols. In addition, stoichiometric amounts of reducing
metals are required for full conversion.
Recently, we have reported a new method for the construction of pinacol cross-
coupling products. This new methodology is based on a retro-pinacol/cross-pinacol
coupling process. The unsymmetrical 1,2-diols 3 were obtained in excellent yields.
Corresponding symmetrically pinacol products were not detected. The reaction
proceeds with very high yields under real catalytic conditions (Scheme 1) [2].
Here, we report the combination of this new transformation with further reactions.
Scheme 1. Catalytic retro-Pinacol/Pinacol-Cross Coupling
Results and Discussion. – To extend the scope of this new transformation, we have
investigated several suitable reactions for the successive application in a one-pot
procedure. In a first series, we have tested a subsequent Brønsted-acid catalyzed pinacol
rearrangement [3]. This reaction sequence was exemplified with different aldehydes
2a – 2d and ketones 2e – 2k. Several strong acids proved to be catalysts for the
subsequent pinacol rearrangement. para-Toluenesulfonic acid (TsOH) turned out to be
an optimal catalyst and was used for more intensive studies. This transformation is
ꢀ 2012 Verlag Helvetica Chimica Acta AG, Zꢁrich

Helvetica Chimica Acta – Vol. 95 (2012)
1971
characterized by operationally simple and very mild conditions. The initial cross-
pinacol coupling process is monitored by TLC. Upon completion of this process,
20 mol-% TsOH were added to the resulting mixture. Depending on the substrates, the
rearrangement process was completed after 2 – 6 h at room temperature. Results of
these investigations are compiled in Scheme 2.
Scheme 2. One-Pot Cross-Pinacol Coupling/Pinacol Rearrangement
a) 5 – 10 mol-% Ti(OtBu)4/Et3SiCl, 3 equiv. of aldehydes or 4 equiv. ketones, r.t., 8 h – 3 d. b) 20 mol-%
TsOH, 2 – 6 h.
The corresponding ketones 4a – 4k were obtained in high yields under compara-
tively mild conditions1). Regioselectivity of the rearrangement is usually very high.
This is due to the more stable corresponding diphenylcarbenium ions, which are
exclusively formed as intermediates, when used with pinacol-cross-coupling products
3a – 3k. As a consequence a,a-diphenylketones 4a – 4k or 5a – 5k are detected
exclusively as the main products. The ratio of the two possible regioisomers 4a – 4k
and 5a – 5k is dictated by migratory aptitude of the substituents R1 and R2. Based on the
results collected in Scheme 2, the following migration tendency is observed: HꢀPh >
Bn > Alkyl > Me. The extremely high migration tendency of H resulted in a very fast
and selective rearrangement (reactions of aldehydes 2a – 2d, R1 ¼ H; Scheme 2).
Another possible consecutive reaction is the deoxygenation of vicinal diols to yield
the corresponding alkenes [12]. This is of special interest, since the formation of the
corresponding alkenes is not observed during the initial retro-pinacol/pinacol-coupling
reaction (Scheme 1). On the other hand, these compounds were found as by-products
in mixtures of pinacol couplings. In McMurry reactions, alkenes were found as the main
products, alongside varying amounts of the corresponding diols [13].
To exploit the described selectivity, we have adopted the reaction conditions of
Barua and Sharma [4] in a one-pot procedure, as the retro-pinacol/pinacol coupling can
be accomplished in different solvents, also in MeCN. This method for the deoxyge-
nation of vicinal diols represents a mild and operationally easy access to highly
substituted alkenes. Again, the initial cross-pinacol coupling process is monitored by
1
)
Compare with the results in [4 – 11].

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Helvetica Chimica Acta – Vol. 95 (2012)
TLC. Upon completion of this process, 2 equiv. of NaI and Me3SiCl were added to the
mixture. Depending on the substrates, the deoxygenation was completed after 1 – 12 h
at room temperature. Tetrasubstituted alkenes 6e – 6k were isolated in high yields
(Scheme 3)2).
Scheme 3. Synthesis of Alkenes via Cross-retro-Pinacol/Reductive Elimination
a) 5 – 10 mol-% Ti(OtBu)4/Et3SiCl, 3 equiv. of aldehydes or 4 equiv. ketones, r.t., 8 h – 3 d. b) NaI,
Me3SiCl.
When used with 1,2-diols 3a – 3d (pinacol-coupling products of aldehydes 2a – 2d),
the elimination process was observed to only a small degree. The high migration
tendency of H prevents a reductive elimination of secondary alcohols 3a – 3d (R1 ¼ H),
since the diphenylcarbenium ion is also a reactive intermediate during this reductive
elimination. Instead, the corresponding rearranged ketones 4a – 4d were obtained as
the major products. Thus, the reductive elimination is restricted to the deployment of
ketones only.
In conclusion, 1,1-diphenyl alkenes or geminal a,a-diphenyl ketones are accessible
by a new one-pot procedure. This process includes a catalytic retro-pinacol/pinacol-
coupling reaction, followed by rearrangement or reductive elimination. Comparisons
of the results of the one-pot reactions with separately accomplished reaction steps
reveal no relevant difference in yields and selectivity. Thus, by the one-pot procedure
described here, extensive purification steps can be avoided. The products are obtained
in high yields by operationally simple protocols and mild reaction conditions. These
observations further emphasize the efficiency and the advantages of the retro-pinacol/
pinacol-coupling process.
Experimental Part
General. Commercially available aldehydes were freshly distilled before use. All other chemicals
used in the syntheses were purchased from SigmaꢁAldrich and were used without further purification.
Air- and/or moisture-sensitive reactions were carried out in anh. solvents in oven- or flame-dried
glassware. TLC: silica gel 60 F 254 TLC plates; used to monitor the progress of the reactions. Column
2
)
Compounds 4a, 4b, 4c, and 4d are identified as the major products.

Helvetica Chimica Acta – Vol. 95 (2012)
1973
chromatography (CC): silica gel 60 (particle size 0.04 – 0.063 mm). Yields were determined after CC. 1H-
and 13C-NMR: at 500 and 125 MHz, resp.; chemical shifts d in ppm and coupling constants J in Hz. EI-
MS: Concept-H mass spectrometer (MSI). ESI-MS: LTQ-FT mass spectrometer (Thermo Industries).
General Procedure for the Synthesis of a,a-Diphenyl-Ketones 4a – 4k. In a typical experiment, 366 mg
benzopinacol 1 (1.0 mmol) and 3 equiv. of the corresponding aldehyde 2a – 2d (4 equiv. of the
corresponding ketones 2e – 2k) were dissolved in 3.0 ml of dry CH2Cl2. One ml of a separately prepared
soln. containing 0.05m Ti(OtBu)4 and Et3SiCl (0.1m for the reaction of ketones) was added. The resulting
soln. was stirred at ambient temp. in a sealed reaction tube. The reaction was continously monitored by
TLC (hexane/acetone 9 :1). At the end of the reaction (when 1 is no more detectable (1 h to 3 d for
aldehydes or 12 h to 5 d for ketones)), TsOH · H2O (38 mg, 0.2 mmol) was added. Again the reaction was
monitored by TLC and was complete after 2 – 6 h (hexane/acetone 9 :1). The resulting mixture was
extracted with CH2Cl2, and successively by sat. aq. NH4Cl and NaHCO3 soln. The org. layers were
separated, dried (MgSO4), and the solvent was removed in vacuo. The residue was purified by flash CC
(hexane/acetone 19 :1 to 9 :1).
3-Methyl-1,1-diphenylbutan-2-one (4a) [5]. Yield: 93% (over two reaction steps). Colorless oil.
1H-NMR: 1.17 (d, J ¼ 7.2, 6 H); 2.86 (sept., J ¼ 6.9, 1 H); 5.36 (s, 1 H); 7.28 – 7.88 (m, 10 H).13C-NMR:
18.6; 40.9; 62.1; 127.1; 128.6; 128.9; 138.6; 212.1. HR-EI-MS: 238.1358 (Mþ, C17H18Oþ; calc. 238.1358).
4-Methyl-1,1-diphenylpentan-2-one (4b). Yield: 95% (over two reaction steps). Colorless oil.
1H-NMR: 0.85 (t, J ¼ 7.4, 3 H); 1.13 (d, J ¼ 7.0, 3 H); 1.39 – 1.48 (m, 1 H); 1.73 – 1.81 (m, 1 H); 2.68
(pseudo-sext., J ¼ 6.8, 1 H); 5.30 (s, 1 H); 7.27 – 7.36 (m, 10 H).13C-NMR: 11.6; 16.2; 26.0; 47.9, 62.8; 127.1;
128.6; 129.0; 138.6; 211.6. HR-ESI-MS: 275.1406 ([M þ Na]þ, C18H20NaOþ; calc. 275.1412).
3-Ethyl-1,1-diphenylpentan-2-one (4c). Yield: 96% (over two reaction steps). Colorless oil.
1H-NMR: 0.82 (t, J ¼ 7.4, 3 H); 1.48 – 1.60 (m, 2 H); 1.69 – 1.82 (m, 2 H); 2.57 (quint., J ¼ 6.4, 1 H);
5.29 (s, 1 H); 7.28 – 7.38 (m, 10 H). 13C-NMR: 11.6; 23.6; 55.1; 63.2; 127.1; 128.5; 129.1; 138.3; 210.8. HR-
ESI-MS: 289.1561 ([M þ Na]þ, C19H22NaOþ; calc. 289.1568).
1,2,2-Triphenylethanone (4d) [6]. Yield: 91% (over two reaction steps). White solid. M.p. 137.98.
1H-NMR: 5.96 (s, 1 H); 7.15 – 7.26 (m, 10 H); 7.31 – 7.34 (m, 2 H); 7.41 – 7.45 (m, 1 H), 7.92 – 7.94 (m, 2 H).
13C-NMR: 59.4; 127.1; 128.6; 128.7; 129.0; 129.1; 133.0; 136.8; 139.1; 198.2. HR-ESI-MS: 295.1094 ([M þ
Na]þ, C20H16NaOþ; calc. 295.1099).
1
3,3-Diphenylbutan-2-one (4e) [7]. Yield: 83% (over two reaction steps). Colorless oil. H-NMR:
1.92 (s, 3 H); 2.16 (s, 3 H); 7.24 – 7.39 (m, 10 H). 13C-NMR: 26.4; 27.6, 62.2; 126.9; 128.2; 128.3; 143.5;
209.1. HR-ESI-MS: 247.1094 ([M þ Na]þ, C16H16NaOþ; calc. 247.1099).
1
4,4-Diphenylhexan-3-one (4f) [8]. Yield: 87% (over two reaction steps). Colorless oil. H-NMR:
0.71 (t, J ¼ 7.5, 3 H); 0.91 (t, J ¼ 7.5, 3 H); 2.33 – 2.43 (m, 4 H); 7.27 – 7.39 (m, 10 H). 13C-NMR: 9.0; 9.4;
30.0; 32.4; 66.6; 126.7; 128.1; 129.4; 141.5; 211.4. HR-ESI-MS: 275.1406 ([M þ Na]þ, C18H20NaOþ; calc.
275.1412).
2,2-Diphenylcyclohexanone (4g) [9]. Yield: 91% (over two reaction steps). White solid. M.p. 103.28.
1H-NMR: 1.82 – 1.87 (m, 2 H); 1.93 – 1.99 (m, 2 H); 2.51 – 2.57 (m, 2 H); 2.59 – 2.65 (m, 2 H); 7.07 – 7.08
(m, 4 H); 7.24 – 7.34 (m, 6 H). 13C-NMR: 22.1; 27.7; 39.1; 40.6; 63.9; 126.8; 128.3; 128.5; 142.3; 211.2. HR-
ESI-MS: 273.1250 ([M þ Na]þ, C18H18NaOþ; calc. 273.1255).
1,1,1-Triphenylpropan-2-one (4h) [10]. Yield: 92% (over two reaction steps). White solid. M.p.
137.98. 1H-NMR: 2.15 (s, 3 H); 7.26 – 7.37 (m, 15 H). 13C-NMR: 29.7; 73.1; 126.7; 128.1; 130.3; 142.2;
205.9. HR-ESI-MS: 309.1252 ([M þ Na]þ, C21H18NaOþ; calc. 309.1255).
3,3,4-Triphenylbutan-2-one (4i) [11]. Yield: 89% (mixture 4i/5i: 96/4; over two reaction steps).
White solid. M.p. 44.38 (recryst. petroleum ether (PE)/Et2O). 1H-NMR: 2.09 (s, 3 H); 3.71 (s, 2 H);
6.66 – 6.68 (m, 2 H); 6.99 – 7.02 (m, 2 H); 7.06 – 7.07 (m, 1 H); 7.25 – 7.34 (m, 10 H). 13C-NMR: 27.3; 43.5,
68.6; 125.9; 127.0; 127.2; 128.0; 129.8; 130.9; 137.7; 140.3; 207.3. HR-ESI-MS: 323.1407 ([M þ Na]þ,
C22H20NaOþ; calc. 323.1412).
5-Methyl-3,3-diphenylhexan-2-one (4k) [14]. Yield: 84% (mixture 4k/5k: 67/33, over two reaction
1
steps). Colorless oil. H-NMR: 0.67 (d, J ¼ 6.7, 6 H); 1.37 – 1.42 (m, 1 H); 2.06 (s, 3 H); 2.31 (d, J ¼ 5.4,
2 H); 7.21 – 7.37 (m, 10 H). 13C-NMR: 22.4; 24.3; 24.9; 45.9; 66.5; 126.8; 128.1; 129.5; 141.9; 208.0. HR-
ESI-MS: 289.1563 ([M þ Na]þ, C19H22NaOþ; calc. 289.1568).

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Helvetica Chimica Acta – Vol. 95 (2012)
5-Methyl-2,2-diphenylhexan-3-one (5k). Yield: 84% (mixture of 4k and 5k: 67/33; over two reaction
1
steps). H-NMR: 0.81 (d, J ¼ 6.7, 1 H); 1.88 – 1.92 (m, 9 H); 2.07 – 2.12 (m, 1 H); 2.34 (d, J ¼ 6.7, 2 H);
7.21 – 7.37 (m, 10 H). 13C-NMR: 24.3; 26.4; 27.2; 47.9; 62.1; 126.8; 128.2; 128.5; 143.7; 210.4.
General Procedure for the Synthesis of 1,1-Diphenyl Alkenes 6a – 6k. In a typical experiment, 366 mg
of 1 (1.0 mmol) and 3 equiv. of the corresponding aldehyde 2a – 2d (4 equiv. of the corresponding ketone
2e – 2k) were dissolved in 3.0 ml of dry MeCN. One ml of a separately prepared soln. containing 0.05m
Ti(OtBu)4 and Et3SiCl (0.1m for the reaction of ketones) was added. The resulting soln. was stirred at
ambient temp. in a sealed reaction tube.
The reaction was continuously monitored by TLC (hexane/acetone 9 :1). The reaction was complete
after 1 h to 3 d for aldehydes and 12 h to 5 d for ketones (1 was no more detectable).
A soln. of NaI (2.0 mmol) in 4.0 ml of MeCN was added. After 15 min, the mixture was treated with
Me3SiCl (2.0 mmol). The reaction was completed after 1 – 12 h (pinacol-cross-coupling products 3a – 3k
were not detectable). The resulting mixture was extracted with Et2O, and successively washed with sat.
aq. Na2S2O3 and NaCl solns. The org. layers were separated, dried (MgSO4), and the solvent was
removed in vacuo. The residue was purified by flash CC (hexane/Et2O 49 :1 to 9 :1).
1,1,2-Triphenylethene (¼1,1’,1’’-Ethene-1,1,2-triyltribenzene; 6d) [15]. Yield: 44% (over two reaction
steps). White solid. M.p. 71.28. 1H-NMR: 6.89 (s, 1 H); 6.94 – 6.96 (m, 2 H); 7.00 – 7.06 (m, 3 H); 7.12 – 7.15
(m, 2 H); 7.19 – 7.26 (m, 8 H). 13C-NMR: 126.7; 127.4; 127.5; 127.6; 127.9; 128.1; 128.2; 128.6; 129.5; 130.4;
137.4; 140.3; 142.6; 143.4. HR-EI-MS: 256.1252 (Mþ, C20H1þ6 ; calc. 256.1252).
2-Methyl-1,1-diphenylprop-1-ene (¼1,1’-(2-Methylprop-1-ene-1,1-diyl)dibenzene; 6e) [16]. Yield:
1
78% (over two reaction steps). Colorless oil. M.p. 11.08. H-NMR: 1.72 (s, 6 H); 7.05 – 7.11 (m, 6 H);
7.17 – 7.20 (m, 4 H). 13C-NMR: 22.5; 126.0; 127.8; 129.8; 131.0; 137.1; 143.3. HR-EI-MS: 208.1253 (Mþ,
C16Hþ16 ; calc. 208.1252).
2-Ethyl-1,1-diphenylbut-1-ene (¼1,1’-(2-Ethylbut-1-ene-1,1-diyl)dibenzene; 6f) [17]. Yield: 76%
1
(over two reaction steps). Colorless oil. H-NMR: 0.94 (t, J ¼ 7.5, 6 H); 2.07 (q, J ¼ 7.4, 4 H); 7.07 – 7.12
(m, 6 H); 7.18 – 7.21 (m, 4 H). 13C-NMR: 13.3; 24.3; 126.0; 128.0; 129.2; 137.1; 142.1; 143.5. HR-EI-MS:
236.1565 (Mþ, C18H2þ0 ; calc. 236.1565).
1-(Cyclopentylidene(phenyl)methyl)benzene (¼1,1’-(Cyclopentylidenemethanediyl)dibenzene; 6g)
[18]. Yield: 75% (over two reaction steps). White solid. M.p. 62.18. 1H-NMR: 1.56 – 1.60 (m, 4 H);
2.23 – 2.25 (m, 4 H); 7.09 – 7.12 (m, 6 H); 7.19 – 7.22 (m, 4 H). 13C-NMR: 26.9; 33.2; 126.0; 127.9; 129.2;
132.9; 143.4; 143.5. HR-EI-MS: 234.1409 (Mþ, C18H1þ8 ; calc. 234.1409).
1,1,2-Triphenylprop-1-ene (¼1,1’,1’’-Prop-1-ene-1,1,2-triyltribenzene; 6h) [19]. Yield: 84% (over two
reaction steps). White solid. M.p. 90.08. 1H-NMR: 2.06 (s, 3 H); 6.81 – 6.83 (m, 2 H); 6.91 – 6.96 (m, 3 H);
7.00 – 7.03 (m, 2 H); 7.05 – 7.07 (m, 3 H); 7.16 – 7.19 (m, 3 H); 7.25 – 7.28 (m, 2 H). 13C-NMR: 23.3; 125.8;
126.2; 126.4; 126.5; 127.4; 127.8; 128.1; 129.3; 130.0; 130.8; 135.6; 139.3; 143.0; 143.5; 144.0. HR-EI-MS:
270.1408 (Mþ, C21H1þ8 ; calc. 270.1409).
2-Benzyl-1,1-diphenylprop-1-ene (¼1,1’,1’’-(2-Methylprop-1-en-3-yl-1-ylidene)trisbenzene; 6i) [20].
Yield: 77% (over two reaction steps). White solid. M.p. 66.38. 1H-NMR: 1.61 (s, 3 H); 3.43 (s, 2 H); 6.94 –
7.26 (m, 15 H). 13C-NMR: 19.7; 41.4; 125.9; 126.2; 126.4; 127.9; 128.1; 128.3; 128.6; 129.4; 129.6; 133.1;
139.3; 140.5; 143.0; 143.0. HR-EI-MS: 284.1565 (Mþ, C22H2þ0 ; calc. 284.1565).
2,4-Dimethyl-1,1-diphenylpent-1-ene (¼1,1’-(2,4-Dimethylpent-1-ene-1,1-diyl)dibenzene; 6k). Yield:
74% (over two reaction steps). Colorless oil. 1H-NMR: 0.74 (d, J ¼ 6.6, 6 H); 1.68 (s, 3 H); 1.78 – 1.83 (m,
1 H); 1.96 (d, J ¼ 7.5, 2 H); 7.04 – 7.12 (m, 6 H); 7.17 – 7.20 (m, 4 H). 13C-NMR: 19.7; 22.3; 26.7, 44.3; 125.9;
126.0; 127.9; 127.9; 129.6; 129.8; 134.1; 138.6; 143.5; 143.7. HR-EI-MS: 250.1722 (Mþ, C19H2þ2 ; calc.
250.1722).
The authors thank Deutsche Forschungsgemeinschaft, Bayer-Schering Pharma AG, Bayer Services
GmbH, BASF AG, and Sasol GmbH for financial support.
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Received July 30, 2012