Insertions of Unsymmetric Alkynes into the Metal−Carbon Bonds of Nickelacycles: What Determines the Regiochemistry?

The seven-membered nickelacycle [NiBr{o-CH=C(CF3)C6H4CH2PPh2-κ2C,P}{PPh(CH2Ph)2}]

“…We have recently reported a systematic study of alkyne insertions into the NiÐC bond of the phosphanickelacycle [NiBr(o-C 6 H 4 CH 2 PPh 2 -2 C,P)(PR 3 )] [PR 3 = PEt 3 for (Ia) and PPh(CH 2 Ph) 2 for (Ib)], in which electronic factors seem to play an important role in determining the geometry of the resulting seven-membered nickelacycles (Edwards et al, 2001). This fact has since been supported by density functional theory (DFT) investigations of the insertion step for several alkynes with model phosphanickelacycles (Macgregor & Wenger, 2002;Bennett et al, 2001). This theoretical work includes the alkyne HCCCF 3 , for which experimental data have only recently become available.…”

“…The insertion reactions of unsymmetrical alkynes with metallacycles have widespread applications in organic synthesis, but the origins of the observed regioselectivities are still unclear (Heck et al, 1990;Abad, 1997;Spencer & Pfeffer, 1998;Cacchi, 1999;Ca  mpora et al, 1999;Larock, 1999a,b;Bennett et al, 2001). We have recently reported a systematic study of alkyne insertions into the NiÐC bond of the phosphanickelacycle [NiBr(o-C 6 H 4 CH 2 PPh 2 -2 C,P)(PR 3 )] [PR 3 = PEt 3 for (Ia) and PPh(CH 2 Ph) 2 for (Ib)], in which electronic factors seem to play an important role in determining the geometry of the resulting seven-membered nickelacycles (Edwards et al, 2001).…”

Three‐Component [2+2+2] Cycloaddition of Carboryne, Unactivated Alkene, and Alkyne via Zirconacyclopentane Mediated by Nickel: One‐Pot Synthesis of Dihydrobenzocarboranes

“…Zirconacyclopentanes [1,2‐{Cp 2 ZrCH 2 CH( n Bu)}‐1,2‐C 2 B 10 H 10 ] ( 1 a ; Cp=C 5 H 5 ) and [1,2‐{Cp 2 ZrCH(Ph)CH 2 }‐1,2‐C 2 B 10 H 10 ] ( 1 b ) were prepared by treatment of [Cp 2 Zr(μ‐Cl)(μ‐C 2 B 10 H 10 )Li(OEt 2 ) 2 ]12 with one equivalent of 1‐hexene or styrene in toluene heated to reflux and isolated in more than 87 % yield (Scheme ). The reversed insertion regioselectivity can be rationalized by electronic effects of the substituents on the alkenes 13. They are very thermally stable and inert toward alkynes even at reflux in toluene for several days, whereas the corresponding nickelacyclopentanes are thermally unstable, leading to the formation of alkenyl carboranes through β‐H elimination upon heating in the absence of other substrates 4.…”

“…For unsymmetrical alkynes, the regioselectivity was dependent on both the polarity of alkynes and relative bulkiness of two substituents. Phenyl‐ and trimethylsilyl‐substituted alkynes 2 e – i afforded very good regioselectivity owing to electronic effects (Table 2, entries 5–9),13, 14 but substituted alkyl alkynes 2 j , k gave 3 j , k as the major regioisomers because of steric effects (Table 2, entries 10 and 11). For alkynes bearing functional groups 2 n – s , the cycloaddition products were formed in low yields, which might be ascribed to the functional group competing with the CC unit for the vacant site at nickel (Table 2, entries 14–19).…”

Direct Nucleophilic Substitution Reaction of Cage B−H Bonds by Grignard Reagents: A Route to Regioselective B4‐Alkylation of o‐Carboranes

“…The aforementioned experimental data show that the presence of two phenyls on each cage carbon atom is crucial to achieve the nucleophilic substitution reaction of B4−H with RMgCl. It is believed that electron‐withdrawing phenyl groups can lower the electron density of the BH vertices, thus facilitating such nucleophilic substitution reactions. To understand the charge distributions in 1,2‐C 2 B 10 H 12 and 1 a , NBO (natural bond orbital) analyses were performed at the B3LYP‐6‐311++G(d,p) level of theory .…”