exhibit doublets (or triplets where R1 = H) at chemical shifts
consistent with olefinic protons, assignable to the proton that is R2.
Conversely, ester substrates (R3 = alkoxyl) show selectivity for the
3,4-diboration products Xb (C-bound boron enolates) where B2pin2
effectively adds across the CNC double bond of the unsaturated
ester. This is unusual as diboration of alkenes14,15 with Pt(0)14 is
comparatively difficult except for terminal or strained cyclic
systems. In addition, Onozawa and Tanaka16 recently reported the
stoichiometric 1,4-addition of a Pd–B bond to methylvinylketone 1
yielding a Pd–CH2CHNC(Me)OBpin species. This is of interest
because 3,4-diborated products cannot arise from such a pathway
except via a subsequent rearrangement. Compound 4b was
identified by the presence of an AB doublet of doublets at 1.55 ppm.
This is consistent with a CH2 group adjacent to a chiral centre and
thus, the structure is that of the 3,4-diborated product. Ethylcroto-
nate 5 was the only substrate to afford both 3,4- and 1,4-diborated
adducts, the major products being the two diastereomers of 5b .
These were present in a 10 : 1 diastereomeric ratio. Dimethylfumar-
ate 6 (a ligand of the catalyst precursor) is also an active substrate,
with both diastereomers of 6b present in a ratio of 3 : 1.
Dimethylmaleate 7, the cis-isomer of 6, yielded both diastereomers
of 6b in a ratio of 2 : 1, the major isomer being the same as when
6 was used as the substrate. Using Pt(NBE)3 (NBE = norbornene)
as a catalyst precursor for these two substrates led to the formation
of a mixture of 6b and the 1,4-diborated product 6a, which was the
major product when dimethylmaleate was used as the substrate.
This change in selectivity provides evidence that the BIAN ligand
remains bound in the catalytic cycle.
thanks the University of Durham for a Sir Derman Christopherson
Foundation Fellowship.
Notes and references
‡ General procedure: Under an atmosphere of dry nitrogen in a double
length glovebox (Innovative Technology Inc., System One) a solution of
B2pin2 (27.8 mg, 0.11 mmol) in C6D6 (1 mL) was added to a solution of
Pt(BIAN)(DMFU) (3.4 mg, 5 mmol), hexamethylbenzene (18 protons, 0.9
mg, 0.1/18 mmol) and substrate X (0.1 mmol) in C6D6 (1 mL). The reaction
mixtures were stirred at room temperature for 24 h, after which time the
solutions were transferred to NMR tubes fitted with Young’s taps.
Conversions were calculated by 1H NMR spectroscopy, using hexame-
thylbenzene as a quantitative internal standard.
§ Selected characterisation data: 1a : 1H NMR (500 MHz, C6D6) d 4.98 (td,
1 H, CNCH–, 3JHH 7.3 Hz, 4JHH 0.9 Hz), 1.97 (d, 2 H, CH2Bpin, 3JHH 7.3
Hz), 1.87 (d 3 H, Me, 4JHH 0.9 Hz), m/z 324 [M]+; 1c : 1H NMR (500 MHz,
C6D6) d 2.25 (t, 2 H, –CH2CO, 3JHH 7.0 Hz), 1.61 (s, 3 H, CH3CO), 0.97 (t,
2 H, CH2Bpin, 3JHH 7.0 Hz), m/z 183 [M 2 CH3]+; 4b : 1H NMR (500 MHz,
C6D6) d 3.44 (s, 3 H, OCH3), 1.67 (s, 3 H, CH3), 1.55 (AB dd, 2 H,
CH2Bpin, 2JHH 15.7 Hz), m/z 339 [M 2 CH3]+; 6a : 1H NMR (500 MHz,
3
C6D6) d 5.57 (d, 1 H, CHBpinCHN, JHH 10.5 Hz), 5.00 (d, 1 H,
3
CHBpinCHN, JHH 10.5 Hz), 3.46 (s, 3 H, C(O)CH3), 3.16 (s, 3 H,
NC(OBpin)CH3), m/z 398 [M]+; 6b, major isomer: 1H NMR (500 MHz,
C6D6) d 3.42 (s, 3 H, OCH3), 3.05 (s, 1 H, CHBpin), m/z 398 [M]+; 6b,
minor isomer: 1H NMR (500 MHz, C6D6) d 3.45 (s, 3 H, OCH3), 3.28 (s, 1
H, CHBpin), m/z 398 [M]+; 6d : 1H NMR (500 MHz, C6D6) d 3.27 (s, 3 H,
OCH3), 2.28 (s, 2 H, CH2Bpin), m/z 144 [M]+.
¶ An exception to this is for substrates 6 and 7 where both boron units of the
products are a- to a carbonyl group. Thus, hydrolysis results in the product
which would arise from hydrogenation of the CNC double bond 6d
(MeO2CCH2CH2CO2Me).
An investigation of the hydrolysis of all of the primary diboration
1
products was also carried out by H NMR spectroscopy. It was
found that B–O bonds of the 1,4-diborated products hydrolysed
rapidly when the reaction mixtures were exposed to air or
stoichiometric water. B–Ca bonds of the 3,4-diborated products
were stable when exposed to air but hydrolysed slowly upon
addition of stoichiometric water. B–Cb bonds were stable to both
air and moisture. Thus, the primary diborated products Xa and Xb
both hydrolyse to give a species with a boron unit b- to the carbonyl
group,¶ Xc (Scheme 3).
The primary product of the platinum catalysed diboration of a,b-
unsaturated carbonyls is highly substrate-specific and the reaction
is much more complex than previously envisaged. The mechanism
of the reaction, specifically the nature of the platinum-bound
enolate which arises from the addition of the first Pt–B moiety
across the a,b-unsaturated carbonyl, is under investigation. Finally,
we note that the reaction introduces a boron atom b- to a carbonyl
group whilst simultaneously generating an O- or C-bound boron
enolate. The 3,4-diborated products are of particular interest as
there are very few examples in the literature of boronate esters a-
to a carbonyl group.17
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Scheme 3 Hydrolysis of primary diboration products carried out with
stoichiometric quantities of H2O.
C h e m . C o m m u n . , 2 0 0 4 , 1 8 5 4 – 1 8 5 5
1855