Chemistry and catalysis advances in organometallic chemistry and catalysis
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4 2.0
C 15 H 11 OH d 1000 :1 :1 0 T ol. 30 2 8 8 26,400
12,900 10,200
1.14 4 2.0
BnOH 1000
:1 :1 0 T ol.
30 2 8 7 26,100
12,500 10,800
1.17 8 2.0
BnOH 1000
:1 :1 0 T ol.
30 7 1 7 1,457
2,600 2,700
1.17 9 2.0
BnOH 1000
:1 :1 0 T ol.
30 7 2 0 1,714
3,000 3,400
1.12 10 2.0
BnOH 1000
:1 :1 0 T ol.
30 7 5 3 4,543
7,600 8,000
1.11 13 2.0
BnOH 1000
:1 :1 0 T ol.
30 15 83 3,320 10,900
e 8,200
1.20 13 5.0
200 :1 : – THF
30 40 80 240 23,100
f 16,600
1.21 13 5.0
- 200
:1 : – THF 0 6 0 8 8 176 25,400
f 18,100
1.24 10 5.0
- 200
:1 : – THF 0 6 0 7 6 152 21,900
f 6,800
1.55 2 5.0
- 200
:1 : – THF 0 6 0 8 6 172 24,800
f 12,000
1.66 a Isolated yield after
precipitation. b Calculated from M n,theo
= [l -LA] 0 /[ROH]
0 × yield × 144.13
+ M ROH
unless otherwise specified. c Determined by size-exclusion chromatography calibrated versus polystyrene standards, and
corrected by a factor of 0.58. d 9-Anthracenylmethanol. e Calculated from M n,theo
= [l -LA] 0 /([HC
≡ CCH
2 OH]
0 + [Ae] 0 ) × yield × 144.13 + M HC ≡ CCH2OH . f Calculated from
M n,theo
= [l -LA] 0 /[Ae]
0 × yield × 144.13.
369 370 CHARGE-NEUTRAL AND CATIONIC ALKALINE EARTH COMPLEXES 5.4 5.2
5.0 4.8
4.6 4.4
4.2 4.0
3.8 3.6
3.4 f1 (ppm)
3.2 3.0
2.8 2.6
2.4 2.2
2.0 1.8
1.6 1.4
C g e b+b ′ i a f d+h H b b ′ O O O O O O O O O H g h i a c f f e e
d H
H b ′ 2 J bb ′ J ab
ab
ab'
J ab'
2 J bb ′ Figure 28.5 1 H NMR spectrum (500.13 MHz, CDCl 3 , 298 K) of a low molecular weight poly(lactic acid) (PLLA) sample prepared with 4/HC ≡CCH
2 OH ([l-LA] 0 /[4] 0 /[HC
≡CCH 2 OH] 0 = 5000 : 1 : 100; M n,SEC = 4700 g/mol, M w /M n = 1.13). Note the ABX spin system corresponding to H a , H b , and H
b ( 2 J HbHb
= 18.0 Hz, 4
HaHb = 2.5 Hz). TABLE 28.4 Immortal ROP of l-LA with 17–22/ROH Systems a M + ROH
[l-LA] 0 /[M + ] 0 / [ROH]
0 T re , ◦ C
Yield, % b TOF, mol/mol Ae h M n,theo
, g/mol
c M n,SEC
, g/mol
d M w /M n d
BnOH 1000 : 1 : 10 60 8 72 90 10,500
10,700 1.06
17 BnOH
3000 : 1 : 10 60 24 96 120
41,600 30,000
e 1.06
17 BnOH
1000 : 1 : 50 60 8 83 104
2,500 2,400
1.12 18 iPrOH 1000 : 1 : 10 30 1
480 7,000
14,000 1.20
19 iPrOH 1000 : 1 : 10 100 0.05
30 6000
4,400 11,000
1.21 20 BnOH
1000 : 1 : 10 100
3 47 157 6,900 6,700
1.12 21 BnOH
1000 : 1 : 10 100
3 74 247 10,800 9,900
1.17 22 BnOH
1000 : 1 : 10 100
3 50 167 7,300 6,800
1.16 21 BnOH
1000 : 1 : 5 100
1.5 34 227 9,900 9,300
1.09 21 BnOH
1000 : 1 : 10 100
1.5 52 347 7,600 7,500
1.10 21 BnOH
1000 : 1 : 20 100
1.5 57 380 4,200 4,100
1.10 21 BnOH
1000 : 1 : 50 100
1.5 75 500 2,300 2,400
1.10 21 BnOH
2000 : 1 : 5 100
24 85 71 49,100 35,000
e 1.41
a Polymerizations carried out in toluene with [l-LA] 0 = 2.0 M.
b Isolated yield after precipitation. c Calculated from M n,theo = [l-LA]
0 /[ROH]
0 × yield × 144.13 + M ROH , with M BnOH = 108 g/mol and M iPrOH = 60 g/mol. d Determined by size-exclusion chromatography calibrated versus polystyrene standards, and corrected by a factor of 0.58. e Note that the 0.58 factor applied to the correction of PLLA molecular weights determined versus polystyrene standards is inadequate at high molecular weights; higher factors should be utilized. The iROP of l-LA was promoted by 17–19 at 30–60 ◦ C upon addition of an excess of iPrOH. The controlled character of the iROP was established by NMR spectroscopy and MALDI-TOF mass spectrometry; no epimerization of the optically active centers was detected. The Sr and Ba complexes 18 and 19 afforded highly active binary catalysts, allowing rapid conversion of the monomer at the remarkably mild temperature of 30 ◦ C (TOF up to 6000 mol l-LA /mol
Ae h). However, rapid broadening of the molecular weight distributions was observed at high conversion. The Ca derivative 17 offered the best compromise in terms of activity and control, providing a very efficient binary catalyst for well-controlled iROP upon Ae-CATALYZED IMMORTAL RING-OPENING POLYMERIZATIONS OF CYCLIC ESTERS 371 addition of 5–50 equiv iPrOH or BnOH at 60 ◦ C. Full conversion of 3000 equiv of l-LA was achieved within 24 h with excellent control (M n,theo
∼ M n,SEC
; M w /M n = 1.06–1.12). The TOFs were in the range 80–100 mol L–LA /mol
Ca h. There
was no influence of the contents in BnOH on the catalytic activity in the concentration range examined (5–50 equiv vs 17), but the molecular weights decreased linearly with increasing BnOH contents. Effective catalytic systems were also generated upon addition of excess BnOH to the fluorinated complexes 20–22. Partial conversion of 1000 equiv of l-LA was observed at 100 ◦ C with [l-LA] 0 /[Ae
+ ] 0 /[ROH] 0 = 1000 : 1 : 10, with the Sr and Ba derivatives being the most active. General features of the systems 20–22/BnOH included (i) relatively lower catalytic activity than their phenolate counterparts 17–19, requiring higher polymerization temperature (80–100 ◦ C) with TOFs in the range 100–500 mol l-LA
/mol Ae h, (ii) very good agreement between M n , theo and M n , SEC , and generally narrow distributions (M
/M n ∼ 1.10–1.20), (iii) the trend Ca < Sr ∼ Ba, (iv) end-group reliability, and (v) absence of epimerization of the chiral centers. The Sr-based 21/BnOH binary catalyst polymerized 2000 equiv of l-LA and withstood 50 equiv of BnOH. Kinetic studies were conducted by NMR spectroscopy in toluene-d 8 . In the iROP of l-LA (100 ◦ C, [l-LA] 0 = 2.0 M,
[l-LA] 0 /[M] 0 /[BnOH]
0 = 136 : 1 : 6.6) catalyzed by BnOH/20–22, apparent rate constants of 0.0004, 0.0013, and 0.0014 s −1 were calculated for 20, 21, and 22, respectively. The catalytic activity increased with the ionic radius of the metal. This was in line with the trend observed with complexes 17–19 supported by the phenolate ligand {LO
3 } − . The binary catalyst 21/BnOH was selected for further NMR kinetics performed at 100 ◦ C in toluene-d 8 . Partial first orders in monomer, catalyst, and initiator concentrations were determined, which gave the rate law − d[L-LA] d t = k p · [L-LA]
1.0 · [21] 1.0 · [BnOH]
1.0 Eyring analyses (85–100 ◦ C) confirmed first-order kinetics in monomer concentration, and the activation parameters H ‡ = 14.8(5) kcal/mol and S ‡ = −7.6(2.0) cal/K mol were determined. A commonly accepted activated monomer mechanism for iROP, applicable to these binary catalyst systems Ae + /ROH, is depicted in Scheme 28.7. The performances of the {BDI
iPr } − -supported cationic complexes 28 and 29 in the iROP of l-LA (1000 equiv) were assessed in the presence of BnOH (10 equiv) [21]. Both binary catalytic systems exhibited high activities at 30 ◦ C, with
n/x O O H OR
Ae + Ae H
Ae + Ae + + + “ L - LA–Ae + ” + “ L - LA–Ae + ”
′ = 0 to n Monomer activation Exchange/transfer reactions Propagation “ L - LA–Ae + ” + x n O O O O O O O O O O HO O O O O O OR O O
′/x O O H OR O O n ′/x O O
OR Ae + O O n ′/x O O
OR O O n ′/x + 1 O O
OR O O O O O O Ae + (Ae + = alkaline earth cation) Scheme 28.7 Traditional activated monomer mechanism for the immortal ROP of l-LA catalyzed by a binary catalyst Ae + /ROH.
372 CHARGE-NEUTRAL AND CATIONIC ALKALINE EARTH COMPLEXES satisfactory control of all parameters. With the more active 29, 82% conversion was already reached after 10 min, and the macromolecular features of the resulting polymer were well controlled (M n,theo = 11,900 g/mol, M n,SEC = 10,400 g/mol, M w /M n = 1.19). In terms of activity, the 29/BnOH Sr-based binary catalyst (TOF = 4920 mol l-LA /mol
Sr h) outclassed all related cationic systems reported so far. The high activity unveiled by 29/BnOH could be credited to the extreme electrophilicity and large accessibility of the metal center in this cationic Sr complex of relatively low coordination number associated to a poorly coordinating anion.
Significant efforts were devoted by our group in the past 3 years to the conception, preparation, and implementation of ROP catalysts based on the large alkaline earth metals. The main outcomes can be summarized as follows: • We have developed the first families of stable Ae-based ROP catalysts, both heteroleptic charge-neutral and cationic ones. • Specific strategies were introduced to overcome the synthetic difficulties associated with these electropositive metals. • The first discrete, solvent-free cations of the large Ae metals have been synthesized. • Highly electrophilic Ae complexes can be stabilized by intramolecular secondary Ae···H–Si and Ae···F interactions. • These Ae-based complexes mediate very efficiently the ROP of cyclic esters, and are among the most active catalysts known to date. • Charge-neutral catalysts (operating according to a coordination-insertion mechanism) were much more active, but less controlled than their cationic derivatives (operating according to an activated monomer mechanism). 28.4 INTERMOLECULAR HYDROAMINATION OF ACTIVATED ALKENES CATALYZED BY CHARGE-NEUTRAL HETEROLEPTIC COMPLEXES OF LARGE ALKALINE EARTHS Hydroaminations of unsaturated substrates are of upmost interest in bulk and fine chemicals synthesis, notably because of their atom efficiency. Various metals, including late-transition ones, can catalyze this reaction, but d 0 complexes of the rare earths [23] and alkaline earths have shown particularly good abilities, in particular for achieving stereoselective reactions [24]. Most examples that rely on catalysts based on the large alkaline earth metals, typically Ca and, in more seldom cases, Sr, are concerned with intramolecular cyclohydroamination of amino-alkenes [25]. Fewer examples of intermolecular hydroamination reactions of activated alkenes (vinyl aromatics, conjugated dienes) catalyzed by Ae complexes are known. Hill and coworkers [26] showed that in such reactions catalyzed by homoleptic complexes {Ae[N(SiMe 3 )
] 2 } 2 (Ae
= Mg, Ca, Sr, Ba) and {Ae[CH(SiMe 3 ) 2 ] 2 (THF) 2 } (Ae = Ca, Sr), the activity of Ae catalysts does not increase linearly with the size of the metal (Mg 2 + (6), 0.72 ˚ A; Ba
2 + (6), 1.35 ˚ A): the Sr complex was indeed superior to the Ca one, while the Mg and Ba derivatives displayed very poor activities. However, no such experimental data were available for heteroleptic complexes. To get a better insight into this reactivity trend and to eventually determine the real abilities of large Ae elements, our group has developed three families of heteroleptic complexes supported by various monoanionic ancillary ligands—anilido- imino, {LN
1 } (30−32), phenolate-amino-alkoxy {LO 5 } (33 −36), and β-diketiminate {BDI iPr } (37 −39)—and used them for the intermolecular hydroamination of styrene derivatives and isoprene (Scheme 28.8, Table 28.5) [27]. All these heteroleptic complexes proved able to promote fully regioselective (i.e., anti-Markovnikov) addition reactions at a moderate catalyst loading of at most 2 mol% in neat substrates at 60 ◦ C. A clear superiority of these heteroleptic compounds, notably that of Ba complex 32 over the usual bis(amides) Ae[N(SiMe 3 ) 2 ] 2 (THF) 2 (Ae = Ca, Sr) was demonstrated as, under identical conditions, the latter displayed much lower reaction rates. Contrary to expectations based on Hill’s results [26], irrespective of the identity of the ligand, the catalytic activity increased with the size of the metal (Mg ) Ca
< Sr < Ba, as evidenced with anilido-imino-Ae complexes 30–32, phenolate-amino-methoxy complexes 33–36, and β-diketiminate complexes 37–39. For any given metal, the maximal activity was achieved with the {BDI
} − ligand, the anilido-amino one displaying only slightly lower efficiency, whereas the lowest activity was recorded with the phenolate ligand. It must be noted, however, that complexes 30–32 are more readily synthesized than 37–39, which constitutes a key advantage for catalytic applications. The scope of the barium complex 32 was therefore briefly explored. The fastest reaction rates were achieved with pyrrolidine, with turnover frequencies up to 290 h −1 at 0.1 mol% catalyst loading. These values exceed by 1 to 2 orders of Ae-CATALYZED INTERMOLECULAR HYDROAMINATION OF ACTIVATED ALKENES 373 X + HNR 1 R 2 X NR 1 R 2 30–39 (0.1–2 mol–%) Neat, 60 °C +
H N N N Ae Ar Ar (HMe
2 Si)
2 N (THF) n N N Ae Ar Ar (Me 3 Si) 2 N (THF) n O
t Bu Ae N(SiMe 3 ) 2 Ae = Mg Ae = Ca
Ae = Sr Ae = Ba
Ar = 2,6-i Pr 2 -C 6 H 3 Ar = 2,6-i Pr 2 -C 6 H 3 33 34 35 36 n = 0 n = 0 n = 1 n = 0 Ae = Ca
Ae = Sr Ae = Ba
30 31 32 n = 1 n = 2 n = 2 Ae = Ca
Ae = Sr Ae = Ba
37 38 39 n = 1 n = 2 n = 2 (THF)
n N O O Scheme 28.8 Intermolecular hydroamination of styrene derivatives and isoprene with amines catalyzed by heteroleptic Ae complexes 30 −39 [27]. TABLE 28.5 Intermolecular Hydroamination of Styrene and Isoprene with Amines Catalyzed by Heteroleptic Ae Complexes [27] a Complex Amine t, h Conv., %
b {LN
1 }CaN(SiMe 3 )
(THF) (30) BnNH 2
34 {LN
1 }SrN(SiMe 3 )
(THF) 2 (31) BnNH 2 18.5 71 {LN
1 }BaN(SiMe 3 )
(THF) 2 (32) BnNH 2 18.5 86 {LO
5 }MgN(SiMe 3 )
(33) BnNH
2 18.5
1 {LO
5 }CaN(SiMe 3 )
(34) BnNH
2 18.5
6 {LO
5 }SrN(SiMe 3 )
(THF) (35) BnNH 2
24 {LO
5 }BaN(SiMe 3 )
(36) BnNH
2 18.5
37 {BDI
iPr }CaN(SiHMe 2 )
(THF) (37) BnNH 2
29 {BDI
iPr }SrN(SiHMe 2 )
(THF) 2 (38) BnNH 2 2 42 {BDI
iPr }BaN(SiHMe 2 )
(THF) 2 (39) BnNH 2 2 64 {LN
1 }BaN(SiMe 3 )
(THF) 2 (32) (CH 2 ) 4 NH 1 99 {LN
1 }BaN(SiMe 3 )
(THF) 2 c (32) (CH
2 ) 4 NH 2 85 Ca[N(SiMe 3 ) 2 ] 2 (THF) 2 c (CH 2 ) 4 NH 2 <1 Sr[N(SiMe 3 )
] 2 (THF) 2 c (CH 2 ) 4 NH 2 10 {LN 1 }BaN(SiMe 3 ) 2 (THF) 2 d (32) (CH
2 ) 4 NH 1 99 e {LN
1 }BaN(SiMe 3 )
(THF) 2 (32) nHexNH 2 18.5 55 {LN
1 }BaN(SiMe 3 )
(THF) 2 (32) iPr 2 NH 18.5 0 a Reaction conditions unless otherwise specified: [styrene]/[amine]/[catalyst] = 50 : 50 : 1, 10.5 μmol of catalyst, no solvent, T = 60 ◦ C.
Determined by 1 H NMR spectroscopy. c [styrene]/[pyrrolidine]/[3] = 500 : 500 : 1. d Reaction performed with isoprene instead of styrene, with [isoprene]/[pyrrolidine]/[3] = 220 : 50 : 1. e Based on amine conversion. 374 CHARGE-NEUTRAL AND CATIONIC ALKALINE EARTH COMPLEXES [Ba]NR 1
2 HNR
1 R 2 HN(SiMe 3 ) 2 [Ba]
N N H N ‡ + HNR 1 R 2 R 2 R 1 R 1 R 2 R 1 R 2 ≡ [Ba]N(SiMe 3 )
BaN(SiMe 3 ) 2 (THF)
2 N N Scheme 28.9 Possible six-centered concerted mechanistic pathway for styrene/amine intermolecular hydroamination catalyzed by 32 [27]. magnitude those reported to date for intermolecular hydroamination reactions catalyzed by rare-earth [23], Ae [25, 26] or even late-transition metal [28] complexes. The hydroamination of styrene with n-hexylamine also occurred fairly rapidly, in contrast to that with isopropylamine, obviously reflecting the sensitivity to steric factors. Consistent with earlier results with rare-earth [23] and Ae [25, 26] metals, the presence of an electron-donating methoxy substituent group on the aromatic ring in vinyl arenes led to a marked decrease in catalyst activity (5.5 equiv converted after 18.5 h), while the presence of a chlorine atom did not affect much the activity. The reaction of isoprene and pyrrolidine proceeded in the presence of as little as 0.1 mol% of 32 (TOF = 295 h −1
Kinetic studies of the hydroamination of styrene with pyrrolidine catalyzed by 32 revealed an empirical rate law v = k[styrene] 1.0 [pyrrolidine] 1.0 [32] 1.0 . A strong kinetic isotope effect was observed in the reaction of styrene with N-deuterated pyrrolidine catalyzed by 32 (k H /k D = 6.8 and 7.3 at 40 and 60 ◦ C respectively). These results were conciliated in a mechanism, different from that proposed for rare-earth systems [23a], that involves a one-step, noninsertive route with a six-centered transition state via concerted proton transfer onto the unsaturation activated toward the attack of the nucleophile (Scheme 28.9) [25, 26].
Intermolecular hydrophosphination of activated alkenes and alkynes is another hydroelementation process of great interest. Yet, in contrast to hydroamination, rare-earth complexes are not known to catalyze this reaction. Our recent studies have revealed that heteroleptic alkaline earth complexes 30–32, 33–36, and 37–39 (Scheme 28.8) all catalyze the intermolecular anti-Markovnikov hydrophosphination of styrene with secondary phosphines such as HPCy 2 or HPPh 2 (Scheme 28.10; Table 28.6) [27]. As for hydroamination, the activity trend varied according to Ca
with the size of the metal center. However, a different dependence of the performances on the ligand was observed: {BDI
} − < {LN 1 } − ≈ {LO
5 } − . Complex 32 was thus the most active catalyst for this transformation, with TOF values (192 h
−1 with HPPh 2 at 60
◦ C) that outclass those reported with {BDI
}CaN(SiMe 3 )
(THF) (circa = ca 0.5 h −1 at 75 ◦ C) [29]. The reactions with the less basic HPPh 2 were considerably faster than with HPCy 2 .
HPR 2 PR 2 Ae cat (2 mol%) Neat, 60 °C
Intermolecular hydrophosphination of styrene catalyzed by heteroleptic Ae complexes [27].
HYDROPHOSPHONYLATION OF ALDEHYDES AND NONACTIVATED KETONES 375 TABLE 28.6 Intermolecular Hydrophosphination of Styrene Catalyzed by Heteroleptic Ae Complexes [27] a Complex Phosphine t, h Conv., %
b {LN
1 }CaN(SiMe 3 )
(THF) (30) HPCy 2
31 {LN
1 }SrN(SiMe 3 )
(THF) 2 (31) HPCy 2 18.5 41 {LN
1 }BaN(SiMe 3 )
(THF) 2 (32) HPCy 2 18.5 42 {LO
5 }CaN(SiMe 3 )
(34) HPCy
2 18.5
12 {LO
5 }SrN(SiMe 3 )
(THF) (35) HPCy 2
26 {LO
5 }BaN(SiMe 3 )
(36) HPCy
2 18.5
46 {BDI
iPr }CaN(SiHMe 2 )
(THF) (37) HPCy 2
4 {BDI
iPr }SrN(SiHMe 2 )
(THF) 2 (38) HPCy 2 18.5 9 {BDI
iPr }BaN(SiHMe 2 )
(THF) 2 (39) HPCy 2 18.5 18 {LN
1 }CaN(SiMe 3 )
(THF) (30) HPPh 2
42 {LN
1 }SrN(SiMe 3 )
(THF) 2 (31) HPPh 2 0.25 92 {LN
1 }BaN(SiMe 3 )
(THF) 2 (32) HPPh 2 0.25 >96 a Reaction conditions: [styrene]/[phosphine]/[catalyst] = 50 : 50 : 1, 10.5 μmol of catalyst, no additional solvent, T = 60 ◦ C. b Determined by 1 H NMR spectroscopy. 28.6 HYDROPHOSPHONYLATION OF ALDEHYDES AND NONACTIVATED KETONES BY CHARGE-NEUTRAL HOMOLEPTIC AND HETEROLEPTIC COMPLEXES OF LARGE ALKALINE EARTHS Ternary and quaternary α-hydroxy-phosphonates, an important class of biologically active compounds, are commonly obtained by addition of dialkylphosphites onto aldehydes or ketones [30]. Well-defined mono- or bimetallic complexes of rare-earth metals, titanium, or aluminum have emerged over the past two decades as effective catalysts for this so-called hydrophosphonylation of aldehydes [31] and, with more difficulty, that of ketones [31c,d, 32], which are far less reactive because of their lower electrophilicity. In some cases, good enantioselectivities could be achieved thanks to the use of chiral metal-based precatalysts [31, 32]. Despite their several similarities with rare-earth elements, we were surprised to see that discrete complexes of the large Ae metals had never been utilized to catalyze hydrophosphonylation reactions. In fact, we found that hydrophosphonylation of aldehydes and nonactivated ketones could be achieved selectively and rapidly at room temperature using very low catalytic loadings (as low as 0.02 mol%) of the simple Ae[N(SiMe 3 ) 2 ] 2 (THF) 2 complexes (Ae = Ca, Sr, Ba) (Scheme 28.11, Table 28.7) [33]. Indeed these homoleptic Ae precatalysts turned out as N N Ca Ar Ar (Me 3 Si) 2 N (THF) N N Ae Ar Ar (Me 3 Si)
2 N (THF) x O
t Bu Ca N(SiMe 3 ) 2 Ar = 2,6-i Pr 2 -C 6 H 3 Ar = 2,6-i Pr 2 -C 6 H 3 37 Ae = Ca
Ae = Sr Ae = Ba
30 31 32 x = 1 x = 2 x = 2 34 N O O Ae[N(SiMe 3 )
] 2 (THF) 2 Ae = Ca
Ae = Sr Ae = Ba
O R P O O O H + Ae cat (0.05–0.1 mol%) HO R P O O O X X Neat, 23 °C X = H, F, Cl, Br, OMe, NO 2 ;
Scheme 28.11 Ae heteroleptic and homoleptic complexes screened for the hydrophosphonylation of aldehydes and ketones [33]. 376 CHARGE-NEUTRAL AND CATIONIC ALKALINE EARTH COMPLEXES TABLE 28.7 Hydrophosphonylation of Benzaldehyde, Acetophenone, and Related Derivatives with Diethylphosphite Catalyzed by Homoleptic and Heteroleptic Ae Complexes [33] a X R Complex
Cat., mol% Time
Conv., b % TOF c , min −1 H H {BDI iPr }CaN(SiMe 3 )
(37) 0.05
0.3 min >99.9
>6000 H H Ca[N(SiMe 3 ) 2 ] 2 (THF) 2 0.05 0.3 min >99.9
>6000 H Me {LN 1 }Ca[N(SiMe 3 ) 2 ](THF) (30) 0.05 1 min
61 1220
H Me {LN 1 }Sr[N(SiMe 3 )
](THF) 2 (31) 0.05 1 min
65 1300
H Me {LN 1 }Ba[N(SiMe 3 )
](THF) 2 (32) 0.05 1 min
71 1420
H Me {LO 5 }Ca[N(SiMe 3 )
](THF) (34) 0.05 60
1180 H Me {BDI iPr }CaN(SiMe 3 )
(37) 0.05
1 min 55 1100 H Me Ca[N(SiMe 2 H) 2 ] 2 (THF) 0.05 1 min
60 1200
H Me Ca[N(SiMe 3 ) 2 ] 2 0.05 1 min 58 1160 H Me Ca[N(SiMe 3 ) 2 ] 2 (THF) 2 0.05
1 min 60 1200 H Me Sr[N(SiMe 3 ) 2 ] 2 (THF) 2 0.05
1 min 70 1400 H Me Ba[N(SiMe 3 ) 2 ] 2 (THF) 2 0.05
1 min 74 1480 H Me Ca[N(SiMe 3 ) 2 ] 2 (THF) 2 0.1
10 min 93 93 H iPr Ca[N(SiMe 3 )
] 2 (THF) 2 0.1
10 min 10 10 F Me Ca[N(SiMe 3 ) 2 ] 2 (THF) 2 0.1
10 min 85 85 Cl Me Ca[N(SiMe 3 ) 2 ] 2 (THF) 2 0.1
10 min 55 55 Br Me Ca[N(SiMe 3 ) 2 ] 2 (THF) 2 0.1
20 h 38 0.3 NO 2 Me Ca[N(SiMe 3 ) 2 ] 2 (THF) 2 0.1 48 h Traces
- Me Me Ca[N(SiMe 3 ) 2 ] 2 (THF) 2 0.1 48 h Traces
- OMe
Me Ca[N(SiMe 3 )
] 2 (THF) 2 0.1
4 h 33 1.4 a Reaction conditions: neat substrates in 1 : 1 ratio (10–20 mmol) at room temperature. b Conversion determined by 1 H NMR spectroscopy. c Nonoptimized catalyst turnover frequencies. effective as and sometimes more effective than the more sophisticated heteroleptic complexes 30–39 that we used in hydroelementation reactions [27]. The reactions were performed in neat reagents with low catalyst loadings to optimize the overall atom efficiency of the reaction and reduce both organic and metallic wastes. With 0.05 mol% of the calcium heteroleptic complexes 30, 34, or 37 or the simple Ca[N(SiMe 3 ) 2 ] 2 (THF) 2 , quantitative addition of HP(O)(OEt) 2 to an equimolar amount of benzaldehyde was achieved within 20 s. The reactions were too fast to discriminate between these complexes, and full conversion was still obtained over the same short time period when 0.02 mol% of 30 was used. The addition of diethylphosphite to acetophenone also proceeded rapidly, equally using 0.05 mol% of heteroleptic or homoleptic complexes, although, in this case, only partial conversion was obtained after 1 min at room temperature. This evidenced that the catalytic activity increased slightly with the size and electropositive nature of the metal, Ca
influence of the metal and that of the ligand framework appeared at best moderate. Also, no difference was noted between Ca[N(SiMe 3 ) 2 ] 2 (THF) 2 , Ca[N(SiMe 2 H) 2 ] 2 (THF), and Ca[N(SiMe 3 ) 2 ] 2 . Although the less active in the series of homoleptic Ae precatalysts, Ca[N(SiMe 3 ) 2 ] 2 (THF) 2 was prioritized ahead of its heavier congeners on account of the fully biocompatible and nontoxic nature of calcium, to determine the scope of the reaction. Other ketones substituted by groups of varying steric bulk and electronic properties were investigated. The reaction proved obviously quite sensitive to steric considerations. First, the rate was an order of magnitude lower for isobutyrophenone than for acetophenone. Secondly, introduction of an ortho-aryl substituent in acetophenone derivatives affected the conversion very mildly on the basis of electronic factors, but preponderantly on steric grounds (note for instance the similar low reactivity of o-Me and o-NO 2 substrates). Hence, the readily prepared Ae[N(SiMe 3 ) 2 ] 2 (THF) 2 provided efficient and easy access to catalyzed hydrophosphonylation reactions not only with benzaldehydes but also for less reactive, nonactivated ketones, for which turnover frequencies as high as 1200– 1500 min −1 were achieved; these values outclass those reported to date by a considerable margin. ACKNOWLEDGMENTS This research was gratefully supported by the European Research Council (grant FP7-People-2010-IIF, ChemCatSusDe). REFERENCES 377 REFERENCES 1. (a) Uhrich, K. E.; Cannizzaro, S. M.; Langer, R. S.; Shakesheff, K. M. Chem. Rev. 1999, 99 , 3181; (b) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Adv. Mater. 2000, 12 , 1841; (c) Albertsson, A.-C.; Varma, I. K. Biomacromolecules 2003, 4 , 1466; (d) Auras, R.; Lim, L.-T.; Selke, S. E. M.; Tsuji, H. Poly(Lactic Acid): Synthesis, Structures, Properties, Processing and Applications; John Wiley & Sons: Hoboken, New Jersey, 2010. 2. For leading reviews, see: (a) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104 , 6147; (b) Wheaton, C. A.; Hayes, P. G.; Ireland, B. J. Dalton Trans. 2009, 4832; (c) Stanford, M. J.; Dove, A. P. Chem. Soc. Rev. 2010, 39 , 486; (d) Dijkstra, P. J.; Du, H.; Feijen, J. Polym. Chem. 2011, 2 , 520. 3. For reviews, see: (a) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L. Chem. Rev. 2007,
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