Platinum catalysed 1,4-diboration of α,β-unsaturated ketones

Article abstract of DOI:10.1039/a705743a

Diborane(4) compounds react with α,β-unsaturated ketones to give the 1,4-addition product in the presence of a platinum catalyst at 80 °C.

Full text of DOI:10.1039/a705743a

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Platinum catalysed 1,4-diboration of a,b-unsaturated ketones
Yvonne G. Lawson,a M. J. Gerald Lesley,a Todd B. Marder,*b† Nicholas C. Norman*a and Craig R. Ricea
a The University of Bristol, School of Chemistry, Cantock’s Close, Bristol, UK BS8 1TS
b The University of Waterloo, Department of Chemistry, Waterloo, Ontario, Canada N2L 3G1
Diborane(4) compounds react with a,b-unsaturated ketones
to give the 1,4-addition product in the presence of a
platinum catalyst at 80 °C.
1H NMR NOE signal enhancement (13.1%) between the vinylic
and methyl protons; both hydroboration of a,b-unsaturated
ketones8 and diboration of dienes5 also produce only the
Z-isomer. Compounds 4a and 4b are both sensitive to
hydrolysis and exposure to H2O affords the hydrolysis products
5a and 5b (Scheme 2).§
In the corresponding reaction between 3a and 2, the
1,4-bis(boronate) ester 4c analogous to 4a,b was not observed
but the hydrolysis product, 5c, was identified by 1H NMR
spectroscopy.¶ This observation indicates that the initial
products formed from reactions involving 2 are more suscept-
ible to hydrolysis than those involving 1 in keeping with
observations made in the diboration of alkynes.5e
In contrast with the rhodium-catalysed hydroboration of
alkenes, alkynes and a,b-unsaturated carbonyl compounds,
which is now well established,1 the related topic of metal-
catalysed diboration reactions has only recently been studied in
any detail. Thus the groups of Baker and Marder,2 Miyaura3 and
Smith4 have shown that alkenes can be diborated in the presence
of rhodium,2 gold2 or platinum3,4 catalysts affording 1,2-bis-
(boronate) ester products, whilst platinum catalysed alkyne
diboration yielding cis-1,2-bis(boronate) alkenes has been
demonstrated by Miyaura and coworkers,5a,b Iverson and
Smith5c,d and Norman and coworkers.5e Additional studies by
Miyaura and coworkers6 have also demonstrated 1,4-addition of
a B–B bond to 1,3-dienes. In all cases, key steps are thought to
involve oxidative addition of the B–B bond of a diborane(4)
compound to the metal centre followed by coordination and
insertion of the organic precursor and subsequent reductive
elimination of the product.7
To date, however, the reaction of a,b-unsaturated carbonyl
compounds with diborane(4) compounds has not been studied,
although previous work has shown that HB(cat) (cat = 1,2-
O2C6H4) reacts with a,b-unsaturated carbonyl compounds in a
1,4 fashion8 to give a synthetically useful9 boron enolate as
shown in Scheme 1. Here we report the reaction of a,b-
unsaturated ketones with the diborane(4) compounds B2(pin)2
110 (pin = OCMe2CMe2O) and B2(cat)2 2.11
As further confirmation of the nature of the products formed
in these reactions, compounds 5a and 5c were oxidised using
NaOH–H2O2 to give the corresponding alcohol 6∑ which was
identified by the comparison of the spectra obtained with lit.
values.12
Ph
HO
O
Me
6
Comparison of 4a,b with the products formed from the
hydroboration of similar a,b-unsaturated ketones shows that the
regiochemistry is similar in both reactions, i.e. 1,4-addition
occurs. However the reaction of diborane(4) compounds with
a,b-unsaturated ketones results in the effective formation of a
hydroxyl group in the b position (i.e. compound 6 in the case of
4a), in contrast to hydroboration where no hydroxyl group is
formed but in which the alkene function is effectively
reduced.
We note finally that boron enolates such as 4a,c are likely to
be useful intermediates in organic synthesis providing starting
materials in processes such as aldol condensations with
aldehydes.9 Reactions, of a,b-unsaturated ketones with chiral
diborane(4) compounds will be reported elsewhere.
T. B. M. thanks NSERC of Canada and N. C. N. thanks the
EPSRC, Laporte plc and The Royal Society for research
support. T. B. M. and N. C. N. also thank NSERC and The
Royal Society for supporting this collaboration via a series of
Bilateral Exchange Awards. Johnson Matthey Ltd. are thanked
for generous supplies of platinum salts.
Me
Me
HB(cat)
O
Ph
O
Ph
B(cat)
Scheme 1
Reaction of trans-4-phenylbut-3-en-2-one 3a or trans-
1,2-diphenylprop-2-en-1-one 3b with 1 equiv. of 1 in the
presence of 5 mol% of [Pt(C2H4)(PPh3)2] at 80 °C gave, after 12
h, the 1,4-bis(boronate) ester products 4a,b quantitatively as
1
judged by H NMR spectroscopy (Scheme 2).‡ Furthermore,
the 1H NMR spectra of 4a and 4b showed that each was present
as only a single isomer which, in the case of 4a and, by
implication 4b, is assumed to be the Z-isomer on the basis of a
Ph
Ph
Ph
Footnotes and References
* E-mail: todd.marder@durham.ac.uk; n.c.norman@bristol.ac.uk
† Present address: The University of Durham, Department of Chemistry,
Science Laboratories, South Road, Durham, UK DH1 3LE.
B2(O2R′)2
H2O
(R′O2)B
(R′O2)B
O
R
O
R
O
R
‡ Synthesis of 4a: to a Schlenk tube charged with 1 (0.020 g, 0.079 mmol),
[Pt(C2H4)(PPh3)2] (5 mol%) and 3a (0.013 g, 0.087 mmol), toluene (5 cm3)
was added and the reaction heated at 80 °C for 12 h. After this time the
toluene was removed by vacuum affording a red oil containing 4a as the
major product (0.030 g, 90%) (the red colour is due to traces of decomposed
catalyst). Compound 4b was prepared similarly. NMR data: 4a: H (300
MHz, C6D6) d 7.6–7.1 (m, 5 H, Ph), 5.40 (dq, 1 H, CNCH, 3JHH 8.7, 4JHH
1.0 Hz), 4.06 [br d, 1 H, CH(B)Ph, 3JHH 8.7 Hz, coupling to Me not resolved
B(O2R′)
+ HOB(O2R′)
3a R = Me
b R = Ph
4a R = Me, O2R′ = pin
b R = Ph, O2R′ = pin
(c R = Me, O2R′ = cat)
5a R = Me, O2R′ = pin
b R = Ph, O2R′ = pin
c R = Me, O2R′ = cat
1
Scheme 2
Chem. Commun., 1997
2051

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owing to broadening resulting from the adjacent boron], 2.05 (dd, 3 H, Me,
125.9 (Ph), 122.5 [C4,5 of B(1,2-O2C6H4)], 112.6 [C3,6 of B(1,2-O2C6H4)],
48.5 (CH2), 28.1 (Me), CH(B)Ph not observed; 11B{1H} (96.3 MHz, C6D6)
d 32.5.
∑ Synthesis of 6: To a solution of 4a (0.050 g, 0.18 mmol) in thf (1 cm3),
samples of EtOH (1 cm3), NaOH(aq) (1 cm3 of a 1 m solution) and H2O2 (30
vol%, 1 cm3) (CAUTION: peroxides and organic solvents can be explosive)
were added and the reaction mixture stirred at room temp. for 12 h. After
this time the crude product was extracted into Et2O (2 3 5 cm3), dried
(MgSO4) and evaporated to dryness affording 6 as an oily solid (0.024 g,
80%).
5
4JHH 1.0, JHH 1.0 Hz), 1.06 [s, 6 H, B(OCMe2CMe2O)], 1.04 [s, 6 H,
B(OCMe2CMe2O)], 1.03 [s, 12 H, OB(OCMe2CMe2O)]; 13C{1H} (75.4
MHz, C6D6) d 146.5 (ipso-Ph), 143.6 [NC(Me)], 129.5, 129.1, 128.9 (Ph),
112.0 (NCH), 83.5, 83.0 [B(OCMe2CMe2O)], 25.3, 24.9, 24.7 [B(OCMe2C-
Me2O)], 21.6 (Me), CH(B)Ph not observed; 11B{1H} (96.3 MHz, C6D6) d
30.8 (1B, CB), 20.0 (1B, OB). 4b: 1H (300 MHz, C6D6) d 7.3–7.0 (m, 10 H,
Ph), 5.82 (d, 1 H, CNCH, 3JHH 8.7 Hz), 3.54 [br d, 1 H, CH(B)Ph, 3JHH 8.7
Hz], 1.16 [s, 6 H, B(OCMe2CMe2O)]; 1.14 [s, 6 H, B(OCMe2CMe2O)],
1.12 [s, 12 H, OB(OCMe2CMe2O)]; 13C{1H} (75.4 MHz, C6D6) d 146.9
[NC(Ph)], 141.9, 141.7 (ipso-Ph), 128.3, 128.2, 128.1, 128.0, 125.3, 124.4
(Ph), 112.8 (NCH), 83.5, 83.3 [B(OCMe2CMe2O)], 30.0 [br, CH(B)Ph],
24.7, 24.6, 23.0 [B(OCMe2CMe2O)]; 11B{1H} (96.3 MHz, C6D6) d 30.6
(1B, CB), 19.5 (1B, OB).
1 K. Burgess and M. J. Ohlmeyer, Chem. Rev., 1991, 91, 1179;
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Chem., Int. Ed. Engl., 1995, 34, 1336.
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689.
4 C. N. Iverson and M. R. Smith, Organometallics, 1997, 16, 2757.
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Soc., 1993, 115, 11018; (b) F. Ozaura, A. Suzuki and N. Miyaura,
Organometallics, 1996, 15, 713; (c) C. N. Iverson and M. R. Smith,
J. Am. Chem. Soc., 1995, 117, 4403; (d) C. N. Iverson and M. R. Smith,
Organometallics, 1996, 15, 5155; (e) M. J. G. Lesley, P. Nguyen,
N. J. Taylor, T. B. Marder, A. J. Scott, W. Clegg and N. C. Norman,
Organometallics, 1996, 15, 5137.
6 T. Ishiyama, M. Yamamoto, and N. Miyaura, Chem. Commun., 1996,
2073.
7 C. Dai, M. J. G. Lesley, T. B. Marder, N. C. Norman and C. R. Rice,
Main Group Chem. News, 1997, in the press.
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Chem., in the press.
§ Synthesis of 5a; Hydrolysis of 4a was achieved by addition of H2O (0.5
cm3) to a solution of 4a (0.20 g, 0.5 mmol) in toluene (2 cm3), removal of
both the solvents by vacuum and extraction of the residue into hexane (3 3
1 cm3) affording 5a as a colourless oil (0.137 g, 100%). Compound 5b was
1
prepared similarly. NMR data: 5a: H (400 MHz, CDCl3) d 7.25 (m, 5 H,
Ph), 3.04 [dd, 1 H, CH(B)Ph/CH2, 3JHH 11.0, 3JHH 18.3 Hz], 2.83 [dd, 1 H,
CH(B)Ph/CH2, 3JHH 5.1, 3JHH 18.3 Hz], 2.64 [dd, 1 H, CH(B)Ph/CH2, 3JHH
5.1, 3JHH 11.0 Hz], 2.14 (s, 3 H, Me), 1.22 [s, 6 H, B(OCMe2CMe2O)], 1.16
[s, 6 H, B(OCMe2CMe2O)]; 13C{1H} (75.4 MHz, C6D6) d 206.6 (CO),
142.7 (ipso-Ph), 128.7, 128.6, 125.7 (Ph), 83.3 [B(OCMe2CMe2O)], 47.5
(CH2), 28.9 (Me), 24.7 [B(OCMe2CMe2O)], 24.6 [B(OCMe2CMe2O)],
1
CH(B)Ph not observed; 11B{1H} (96.3 MHz, C6D6) d 31.3. 5b: H (300
MHz, CDCl3) d 8.00–7.00 (m, 10 H, Ph), 3.49 [dd, 1 H, CH(B)Ph/CH2, 3JHH
10.8, 3JHH 18.3 Hz], 3.35 [dd, 1 H, CH(B)Ph/CH2, 3JHH 5.1, 3JHH 18.3 Hz],
3
3
2.72 [dd, 1 H, CH(B)Ph/CH2, JHH 5.1, JHH 10.8 Hz], 1.17 [s, 6 H,
B(OCMe2CMe2O)], 1.10 [s, 6 H, B(OCMe2CMe2O)]; 13C{1H} (75.4 MHz,
C6D6) d 199.7 (CO), 144.9, 142.0 (ipso-Ph), 132.9, 129.0, 128.5, 128.4,
128.0, 125.6 (Ph), 83.4 [B(OCMe2CMe2O)], 43.3 (CH2), 24.6 [B(OCMe2C-
Me2O)], 24.5 [B(OCMe2CMe2O)], CH(B)Ph not observed; 11B{1H} (96.3
MHz, CDCl3) d 30.8.
¶ NMR data for 5c: 1H (300 MHz, C6D6) d 6.75 (m, 5 H, Ph), 6.45 [m, 4 H,
B(1,2-O2C6H4)], 2.64 [dd, 1 H, CH(B)Ph/CH2, 3JHH 6.1, 3JHH 9.2 Hz], 2.48
[dd, 1 H, CH(B)Ph/CH2, 3JHH 9.2, 3JHH 18.6 Hz], 2.24 [dd, 1 H, CH(B)Ph/
CH2, 3JHH 6.1, 3JHH 18.6 Hz], 1.61 (s, 3 H, Me); 13C{1H} (75.4 MHz, C6D6)
d 210.0 (CO), 149.4 [C1,2 of B(1,2-O2C6H4)], 142.9 (ipso-Ph), 128.7, 128.6,
12 A. Fauve and H. Veschambre, J. Org. Chem., 1988, 53, 5215.
Received in Cambridge, UK, 6th August 1997; 7/05743A
2052
Chem. Commun., 1997