Chemistry and catalysis advances in organometallic chemistry and catalysis
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Scheme 7.8 Pd/Ni co-catalyzed [2 + 2 + 2] cycloaddition of 1,3-dehydro-o-carborane with alkynes. react with the carboryne precursor 1-Li-2-Me-3-I-1,2-C 2 B
H 9 . Substituents at the cage C(2) position also affect the yields of [2 + 2 + 2] cycloaddition products. Results show that steric effects of substituents are much less significant than electronic effects; for example, an electron-withdrawing group such as phenyl leads to a big drop in the yield. X-ray structures indicate that the six-membered C 5 B rings in the C,B-substituted benzocarboranes are planar with alternative short and long bonds, similar to those observed in 1,2-benzocarboranes [14, 20], suggesting that there is no substantial π-delocalization in such rings and that the C 4 unit may be described as a butadiene moiety. A possible reaction mechanism for such a cooperative catalysis is proposed in Scheme 7.9. Since Ni(0) can hardly insert into the cage B–I bond, the Pd-1,3-dehydro-o-carborane H is formed via oxidative addition of B–I on Pd(0) followed by subsequent LiI elimination. Indicated by the aforementioned experiments that a two-component catalyst system is much more effective than Pd species alone in catalyzing the reaction of 1,3-dehydro-o-carborane with alkynes, a transmetalation process between Pd and Ni may occur, affording a more reactive nickel-1,3-dehydro-o-carborane intermediate I [24]. Such a transmetalation process can be evidenced by the following reactions: ( η 2
2 B 10 H 10 )Ni(PPh 3 ) 2 can be observed by treatment of ( η 2
2 B 10 H 10 )Pd(PPh 3 ) 2 with 1 equiv of Ni(cod) 2 in toluene at room temperature as indicated by 31 P NMR;
and the addition of 20 mol% of Ni(cod) 2 to the mixture of ( η 2 -o-C 2 B 10 H 10 )Pd(PPh 3 ) 2 and 3-hexyne leads to the isolation of benzocarborane in 18% yield, while ( η 2
2 B 10 H 10 )Pd(PPh 3 ) 2 alone does not show any activity toward alkyne. The relatively high activity of the Ni species may probably be ascribed to the weaker Ni–B bond over the Pd–B one or the Ni–B bonding pair being more nucleophilic [25]. In the reaction with unsymmetrical alkyne PhC ≡CEt, the electronically controlled regioselective insertion into the Ni–B bond gives the nickelacyclopentene intermediate J. The alkyne insertion can be viewed as a nucleophilic attack of the M–C/B σ -bond on one of the two alkyne carbons. The exclusive insertion of the first equivalent of alkyne into the Ni–B bond with Me Me I G I H Ni 0 Pd 0 LiI Ph Et Ph(Et) Et(Ph) Ph Me Et Ni
Li Me
Li Me I [Pd] [Ni]
Me [Ni]
Et Ph Ph Et K R 1 R 1 Et Ph Ph Et Ph Et Ph Et +
Proposed mechanism for Pd/Ni co-catalyzed formation of 1,3-benzocarboranes.
88 TRANSITION-METAL-PROMOTED FUNCTIONALIZATION OF CARBORANES electronically controlled regioselectivity is well supported by the absence of 2-Me-1,3-[EtC =C(C
6 H 5 )-C(Et) =C(C
6 H 5 )]-1,2- C 2 B 10 H 10 in the products, as an M–B bond is much more nucleophilic than an M–C one [25]. The nucleophilic attack in nature also explains the regioselectivity observed in the insertion of unsymmetrical alkynes, in which the boron is bonded to the carbon having the electron-donating ethyl substituent. Subsequent insertion of the second molecule of PhC ≡CEt into the Ni–C vinyl bond in both head-to-tail (major) and head-to-head (minor) manners followed by reductive elimination affords the final products (Scheme 7.9). It is noted that the insertion of alkynes into the M–C cage
bond in J is prohibited for steric reasons. This work offers a new methodology for cage B-functionalization of carboranes and demonstrates that metal-1,3-o-carboryne can be viewed as a new kind of boron nucleophile.
X-ray structures of metal–carborynes show that electronic configurations of a metal center can have large effects on the bonding interactions between the metal atom and carboryne unit, which may in turn influence their chemical properties [26]. Structural analyses also indicate that the interactions between the Ni atom and carboryne have more π character than that in Zr–carboryne complexes [27], which may facilitate the reactivity studies on these metal complexes. As a result, the Ni–carboryne complexes can react well with alkynes and alkenes, but they are inert toward polar unsaturated molecules such as nitriles and carbodiimides. On the other hand, the Zr–carboryne can react with a variety of polar unsaturated molecules, affording insertion products. The first zirconium– carboryne complex [ {η 5
σ -Me 2 C(C 9 H 6 )(C 2 B 10 H 10 ) }ZrCl(η
3 -C 2 B 10 H 10 )][Li(THF) 4 ] was prepared from the reaction of in situ generated [ η 5 : σ -Me
2 C(C
9 H 6 )(C 2 B 10 H 10 )]ZrCl 2 with 1 equiv of Li 2 C 2 B 10 H 10 [11]. The anionic nature of this molecule results in inertness toward unsaturated molecules. On the other hand, treatment of Cp 2 ZrCl 2 with
1 equiv of Li 2 C 2 B 10 H 10 in ether gives the ate-complex Cp 2 Zr(
μ-Cl)(μ-C 2 B 10 H 10 )Li(OEt 2 ) 2 (9) in 70% isolated yield [28]. It can be viewed as a precursor of zirconocene– carboryne Cp 2 Zr( η 2 -C 2 B 10 H 10 ). In addition, a series of neutral group 4 metal–carboryne complexes bearing amidinato ligands L, ( η 2 -C 2 B 10 H 10 )M(L) 2 , were synthesized by salt metathesis reactions between Li 2 C 2 B 10 H 10 and diamidinato group 4 metal dichloride complexes MCl 2 (L)
2 (L = [η 2 -R 2 C(NR 1 ) 2 ]) [29].
Treatment of ( η 2 -C 2 B 10 H 10 )M(L) 2 (L = Cp, [η 2 -R 2 C(NR
1 ) 2 ]) with PhCN, CyN =C=NCy, PhN 3 , Ph
2 C =O, n BuN
=C=S, and PhN
=C=O affords the corresponding monoinsertion products L 2 Zr[ σ :σ -N=C(Ph)(C 2 B 10 H 10 )] (10), L 2 Zr[ σ :σ - CyNC(
=NCy)(C 2 B 10 H 10 )] (11), L 2 Zr[ η 2 : σ -(PhNN=N)(C 2 B 10 H 10 )] (12), L 2 Zr[ σ :σ -OC(Ph) 2 (C 2 B 10 H 10 )] (13), L 2 Zr[
σ :σ - n BuNC(
=S)(C 2 B 10 H 10 )] (14), and L 2 Zr[ σ :σ -PhNC(=O)(C 2 B 10 H 10 )] (15), respectively, in moderate to high yields (Scheme 7.10) [28, 29]. In these reactions, unsaturated molecules insert into only one Zr–C cage bond, and the other Zr–C cage bond remains inert. No double insertion products are observed even under forced reaction conditions in the presence of an excess amount of substrates. This reaction offers an efficient route to zirconaheterocycles incorporating a carboranyl unit.
In attempts to isolate Cp 2 Zr(
η 2 -C 2 B 10 H 10 )L, when pyridine is added to Cp 2 Zr(
μ-Cl)(μ-C 2 B 10 H 10 )Li(OEt 2 ) 2 (9), an unexpected C–H activation product, Cp 2 Zr[ η 2 (C,N)-pyridine]( σ -C 2 B 10 H 11 ) (16), is isolated in 90% yield. If deuterated pyridine C 5 D
N is used, the α –deuterium of pyridine is transferred to o-carborane, leading to the formation of Cp 2
η 2 (C,N)-pyridine-d 4 ]( σ -C 2 B 10 H 10 D) (16-d 5 ) in 87% isolated yield (Scheme 7.11) [30]. Other pyridine derivatives such as 2-bromo-pyridine, 2,4-lutidine, and quinolinecan can all react with 9 to afford the corresponding α-C–H activation products Cp 2 Zr[ η 2 (C,N)-(2-bromopyridine)]( σ -C 2 B 10 H 11 ), Cp 2 Zr[ η 2 (C,N)-(2,4- lutidine)]( σ -C
2 B 10 H 11 ), and Cp 2 Zr(
η 2 (C,N)-quinoline)( σ -C 2 B 10 H 11 ), respectively. These results are similar to those observed in the interaction of Cp 2 Zr(CH
3 )(THF)
+ with pyridines [31]. It is noted that the interaction of 9 with acridine generates an insertion product Cp 2 Zr {2-[9-(σ-10(N)-dihydroacridine)]}(σ-1-C 2 B 10 H 10 ). Treatment of 9 with 1 equiv of 2-(1-hexynyl)pyridine in toluene at room temperature affords the α-C–H activation product Cp 2 Zr
2 (C,N)-[2-(1- n BuC
≡C)pyridine]}(σ -C 2 B 10 H 11 ) (17) in 56% isolated yield. However, if 2 equiv of CuI is added to the above reaction, the α-C–H activation (σ -bond metathesis) reaction is completely blocked, and, instead, the alkyne insertion product 1,2-[Cp 2 ZrC(2-pyridinyl) =CBu n ]-1,2-C
2 B 10 H 10 (18) is generated (Scheme 7.11) [30]. These results suggest that the coordination of pyridine to the Zr atom activates the α-C–H bond, leading to the formation of α-C–H activation ( σ -bond metathesis) products, and the Cu atom can compete for Zr’s binding site of pyridine to block the α-C–H activation path and facilitate the alkyne insertion.
REACTION OF ZIRCONOCENE–CARBORYNE WITH ALKYNES AND ALKENES 89 C N L 2 Zr Ph C N L 2 Zr 11 N Cy Cy N N L 2 Zr 12 N Ph DCC PhN
3 L 2 Zr PhCN
C O L 2 Zr Ph Ph PhNCO
n BuNCS
L = Cp or C N L 2 Zr 14 O Ph C N L 2 Zr S Bu n N N R 1 R 2 R 1 Ph 2 CO 10 13 15 Scheme 7.10 Reaction of zirconocene– carboryne with polar unsaturated molecules. Li Cl
2 Zr OEt 2 OEt
2 9 Cp 2 Zr H Cp 2 Zr N N D D D D D D Cp 2 Zr N D D D D N N Bu n H Cp 2 Zr N Bu n N Bu n CuI
Cp 2 Zr Bu n N
16-d 5 17 18 Scheme 7.11 Reaction of zirconocene– carboryne with pyridines. 7.9 REACTION OF ZIRCONOCENE– CARBORYNE WITH ALKYNES AND ALKENES Zirconacycles are in general much more stable than their nickel analogs and often serve as very important and versatile intermediates for the construction of the carbon– carbon bond [32]. The carborane version of zirconacyclopentenes 1,2- [Cp
2 ZrC(R)
=C(R)]-1,2-C 2 B 10 H 10 (19) can be obtained by the treatment of complex 9 with various kinds of alkynes (Scheme 7.12) in refluxing toluene [33]. This reaction cannot proceed in donor solvents such as Et 2 O and THF, suggesting that the coordination of alkyne to the Zr atom is essential for the subsequent insertion. 90 TRANSITION-METAL-PROMOTED FUNCTIONALIZATION OF CARBORANES R 2
1 Cp 2 Zr R 2 R 1
Toluene Δ
Δ Cp 2 Zr (R 1 , R 2 = alkyl, aryl) R 2 R 1 Li Cl Cp 2 Zr OEt 2 OEt 2 9 Cp 2 Zr R Cp 2 Zr
R or
R = Alkyl R = Aryl
R R = Alkyl Cp 2
N R Cp 2 Zr R 20 20' L δ + δ − δ + δ − δ + δ − Scheme 7.12 Reaction of zirconocene–carboryne with alkynes and alkenes. Both symmetrical and unsymmetrical alkynes bearing alkyl or aryl substituents are compatible with this reaction, in which steric factors play an important role. Me 3 SiC
≡CSiMe 3 is inert toward 9, and PhC ≡CPh offers a much lower yield than linear alkynes. In case of unsymmetrical alkynes, the excellent regioselectivity of the insertion is generally determined by the polarity of alkynes. This reaction can tolerate many functional groups such as vinyl, chloro, amido, alkoxyl, and tetrahydro-2-pyranyl. X-ray structures show that the sum of the five interior angles of the zirconacyclopentene ring is very close to 540 o , suggestive of a planar geometry. These structural features resemble those of zirconacyclopentadienes. In contrast to Ni–carborynes, no double insertion products are formed even under forced reaction conditions in the presence of an excess amount of alkynes. On the other hand, reaction of complex 9 with terminal alkenes RCH =CH
2 in refluxing toluene gives zirconacyclopentanes 1,2-[Cp 2
2 ]-1,2-C
2 B 10 H 10 (20, R = aryl) or 1,2-[Cp 2 ZrCH 2 CH(R)]-1,2-C 2 B
H 10 (20 , R = alkyl) in good to high isolated yields with high regioselectivity (Scheme 7.12) [34]. Disubstituted alkenes such as α-methylstyrene and cyclohexene cannot react with 9 probably because of steric reasons. A reaction pathway for the formation of zirconacycles, involving the Zr–carboryne Cp 2 Zr( η 2 -C 2 B 10 H 10 ), has been proposed (Scheme 7.12). In general, electron-withdrawing aryl substituents go to the α position (via intermediate M), ZR/NI CO-MEDIATED [2 + 2 + 2] CYCLOADDITION OF CARBORYNE WITH TWO DIFFERENT ALKYNES 91 Cp 2 Zr Et Et XylNC H 3 + O CuCl I 2 I H Et Et 22 H H Et Et Et Et 21 N CuCl 2 I I CuCl Δ
Et Et
Et Et
19a Scheme 7.13 Reactivity study of zirconacyclopentene incorporating a carboranyl unit. whereas the electron-donating alkyl substituents prefer the β position (via intermediate N). High temperatures are required for the reactions, which can not only promote the dissociation of LiCl from 9 forming the zirconocene– carboryne intermediate but also facilitate the insertion reactions between carboryne and the coordinated alkyne or alkene via the intermediates L, M, or N. Zirconacyclopentene 19 can be converted to a variety of functionalized carboranes. Scheme 7.13 outlines their representative reactivity patterns in the example of 1,2-[Cp 2 ZrC(Et) =C(Et)]-1,2-C 2 B 10 H 10 (19a) [35]. It can react readily with 2,6-(CH 3 )
C 6 H 3 NC to give a Zr–C vinyl bond insertion product 1,2-[(2 ,6 -Me 2 C 6 H 3 N =)CC(Et)=C(Et)]-1,2-C 2 B 10 H 10 (21) in refluxing toluene. Hydrolysis of 19a under acidic media affords alkenylcarborane 1-[HC(Et) =C(Et)]-1,2-C 2 B
H 11 (22). Interaction of 19a with I 2 in the presence of CuCl generates a monoiodosubstituted product 1-[CI(Et) =C(Et)]-1,2- C 2 B 10 H 11 (23) but not the disubstituted species 1-I-2-[CI(Et) =C(Et)]-1,2-C 2 B 10 H 10 . It is suggested that, after transmetalation to Cu(I), only the Cu–C vinyl bond is reactive toward I 2 whereas the Cu–C cage bond is inert probably because of steric reasons [9, 36]. This is very different from zirconacyclopentadienes, in which the diiodo-substituted compound is the major product in the presence of CuCl [37]. Reaction of 19a with o-diiodobenzene in the presence of CuCl produces naphthalocarborane 1,2-[o-C 6 H 4 C(Et)
=C(Et)]-1,2-C 2 B 10 H 10 (24). Treatment of 19a with CuCl 2 in toluene at 80 ◦ C gives the C–C coupling product 1,2-[C(Et) =C(Et)]-1,2-C 2 B
H 10 (25). It is suggested that the intermediate 1,2-[CuC(Et) =C(Et)]-1,2-C 2 B 10 H 10 is formed by the transmetalation of 19a to Cu(II). Subsequent reductive elimination affords the four-membered ring product and Cu mirror. These results indicate that zirconacyclopentenes incorporating a carboranyl unit resemble their analogous zirconacyclopentadienes Cp 2 Zr[C(R)
=C(R)-C(R)=C(R)] in some reactions [38], while they have some unique properties of their own due to the presence of highly sterically demanding carboranyl unit. Download 11.05 Mb. Do'stlaringiz bilan baham: 
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