.
Angewandte
Communications
Table 1: TEP values and Huynh’s parameter of IAPs 3–7, 11, and 12. The
values of commonly used phosphines and NHCs are included for
comparison.
groups makes IAPs more electron rich than alkylphosphines
and even abnormal N-heterocyclic carbenes. Moreover,
owing to the NHC-nature of the backbone, steric and
electronic fine-tuning of IAPs is easily achieved by drawing
on the versatile class of NHCs as exemplified by using 1,3-
diisopropylbenzimidazolin-2-ylidenamino (NBiPr) and 1,3-
dimesityl-4,5-dimethylimidazolin-2-ylidenamino
(NIMes)
substituents in our studies. We also demonstrate the beneficial
effect of the imidazolin-2-ylidenamino substitution in a series
of IAPs in Suzuki–Miyaura reactions using non-activated aryl
chlorides as challenging substrates.
For the synthesis of IAPs two straightforward procedures
were developed (see Scheme 1 and the Supporting Informa-
[b]
Entry
Ligand L
TEP[a]
[Ni(CO)3L]
dCcarbene
[PdBr2(BiPr)L]
1
2
3
4
5
6
7
8
P(NBiPr)Ph2 (3)
P(NBiPr)2Ph (4)
P(NBiPr)iPr2 (5)
P(NBiPr)2iPr (6)
P(NBiPr)3 (7)
P(NIMes)iPr2 (11)
P(NIMes)2iPr (12)
PPh3
2060.8
2051.0
2053.6
2049.2
2044.3
2047.8
2038.6
(2068.9)
(2059.2)
(2056.1)
(2051.5)
178.0
183.0
181.5
184.5
186.1
183.7
[d]
–
(173.1)
175.9
–
9
10
11
PiPr3
PtBu3
IPr[c]
(177.5)
[a] Values in cmÀ1; measured in CH2Cl2, (literature values[1,19]). [b] Values
in ppm; measured in CDCl3 and internally referenced to the solvent
residual signal at d=77.7 ppm relative to TMS, (literature values[18]).
[c] 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene. [d] trans-[PdBr2-
(BiPr)(12)] not formed.
analysis of the A1 CO stretching frequency of [Ni(CO)3L]
complexes.[1] The observed TEP values indicate that the
donor power of the IAPs is higher than that of the electron-
rich alkylphosphines (Table 1) with the exception of 3 having
a TEP value with the same order of magnitude as PnBu3
(2060.3 cmÀ1). Remarkably, IAPs 6, 7, 11, and 12 are even
better donor ligands than classical NHCs. By analyzing the X-
ray molecular structures of free triaminophosphines and
those of their transition-metal complexes, Woollins et al.
concluded that only two nitrogen lone pairs donate electron
density towards phosphorus, while the third nitrogen lone pair
is oriented such that the nitrogen atom simply acts as an
electron-withdrawing substituent.[12] In contrast to this obser-
vation, the TEP value of NBiPr-substituted phosphines 3–7
increases almost unvaryingly with a higher degree of sub-
stitution (Ph substitution: av. 8.2 cmÀ1, iPr substitution: av.
5.0 cmÀ1). Phosphines 11 and 12, decorated with the NIMes
backbone, show an even more prominent gain in donor
power. Monosubstitution shifts the TEP value (PiPr3:
2059.2 cmÀ1) by 11.4 cmÀ1 (11: 2047.8 cmÀ1) below that of
classical NHCs, and the second substitution by another
9.2 cmÀ1 (12: 2038.6 cmÀ1) below that of abnormal NHCs.
These values support the notion that the NIMes substituent is
a much better p donor than the NBiPr group, and also
indicates that if appropriately substituted, IAPs can surpass
the donor abilities of CAACs or abnormal NHCs.
Scheme 1. Synthesis of IAPs. Reagents and conditions (yields):
a) BrCN, toluene, 1108C, and then KOH (90%); b) nBuLi, THF,
À788C, and then chlorophosphine, room temperature; 3 (94%), 4
(89%), 5 (84%), 6 (86%), 7 (94%); c) nBuLi, THF, À788C, and then
[Fe(C5H4PCl2)2] (81%); d) KOtBu, toluene, À788C, and then TMSN3,
1108C (49%); e) 11: PiPr2Cl, THF, (99%); 12: PCl3, THF, À788C, and
then iPrMgCl, THF, À788C, (87%).
tion). NBiPr-substituted phosphines 3–8 were prepared in
excellent yields through a two-step sequence. The reaction of
diamine 1[16] with cyanogen bromide and subsequent basic
workup afforded imine 2 in 90% yield. Deprotonation of 2
and treatment with the respective chlorophosphine afforded
the IAPs 3–8 as white (3–7) or orange (8) solids in very good
yields. An alternative route to IAPs is based on readily
available imidazolium salts (Scheme 1). Following a synthetic
approach reported by Tamm,[17] deprotonation of the imida-
zolium salt 9 at À788C gives the free NHC, which reacts with
trimethylsilyl azide to afford imine 10 upon N2 elimination in
fair yield. Treatment of 10 with chlorodiisopropylphosphine
afforded IAP 11 upon elimination of trimethylsilyl chloride in
a clean reaction as an off-white solid. The sterically demand-
ing phosphine 12 was prepared by reacting imine 10 with PCl3
and subsequent treatment of the phosphenium chloride salt
with isopropylmagnesium chloride to afford IAP 12 as a white
solid in very good yield.
To demonstrate the reliability of the TEP analysis for the
IAPs, we decided to examine their donor ability by means of
a second method, which is not based on the CO stretching
frequency. Huynh et al. recently reported a parameter that
utilizes the 13C NMR chemical shift of the carbene carbon
atom in trans-[PdBr2(BiPr)L] complexes as a probe for the
measurement of the donor strength of the ligand L.[18] The
To evaluate the donor endowment of the new phosphines,
we determined the Tolman electronic parameter (TEP) by
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 11857 –11860