Formation of a Unique ansa‐Metallocene Framework by Intramolecular Photochemical [2+2] Cycloaddition of Bis(2‐alkenylindenyl)zirconium Complexes

Abstract

Developing the chemistry of organic functional groups on bent Group 4 metallocene frameworks and related systems has been notoriously difficult due to the high sensitivity of such organometallic compounds toward the typical reaction conditions used.[1] Noteworthy examples include a Mannichtype carbon-carbon coupling reaction [2] and selective transformations of olefinic substituents attached to the metallocene cyclopentadienyl rings, such as metallacyclic olefin coupling [3] and olefin metathesis reactions.[4] Some time ago we had shown that several bis(alkenylcyclopentadienyl)-ZrCl 2 complexes undergo a photochemical [2+2] cycloaddition reaction to yield novel ansa-metallocenes. However, many of these systems displayed unfavorable photostationary equilibria.[5] We have now found that bis(2-alkenylindenyl)-ZrCl 2 complexes undergo a rapid photochemical olefin [2+2] cycloaddition to produce cyclobutylene-bridged ansametallocenes in which the annelated indenyl-arene rings are located at the front side of the bent metallocene wedge in a symmetrical fashion above and below the s-ligand system. A series of typical examples will be described herein.Two synthetic routes were employed to prepare the ligands.[6] Cross-coupling of 2-bromoindene (1) with the alkenyl Grignard reagents 2 a (R = Me) and 2 b (R = Ph) gave the corresponding 2-alkenylindenes 3 a,b in about 90 % yield each (Scheme 1). The cyclohexyl-substituted 2-alkenylindene was prepared by a complementary route starting from 2-indenyl magnesium bromide (4). Addition to cyclohexyl methyl ketone followed by acid-catalyzed dehydration provided both alkene isomers. Chromatographic separation furnished the required compound 3 c in 26 % yield. Deprotonation of 2-alkenylindenes 3 was achieved by treatment with n-butyl lithium (3 a,c) or LDA (3 b) in Et 2 O at 08C. Subsequent reaction of the resulting alkenylindenyl lithium reagents (5 a-c) in toluene with ZrCl 4 then yielded the bis(2-alkenylindenyl)-ZrCl 2 complexes 6 a (R = Me, 60 %), 6 b (R = Ph, 51 %), and 6 c (R = cyclohexyl, 27 %), respectively. Single crystals for X-ray structure analysis were obtained from the complexes 6 a (R = Me) and 6 c (R = cyclohexyl), in both cases from solutions in toluene/pentane at À20 8C. In the crystal 6 a features a "meso-like" rotational structure [7] in which the annelated phenylene rings of the two indenyl ligands are oriented toward the same side of the bent metallocene wedge (Figure 1, left). The CH 2 = C(CH 3 ) substituents at the indenyl 2-positions point to the other lateral sector. The CH 2 = C(CH 3 ) substituents are both oriented in the indenyl ligand plane (q(C9-C1,C10,C11) = À170.6(6)8) with the =CH 2 groups pointing toward the back side of the bent metallocene wedge.In contrast, the cyclohexyl-substituted complex 6 c attains an approximately C 2 -symmetric "rac-like" conformation [7,8] in the solid state, with the CH 2 = C(cyclohexyl) substituents oriented toward opposite lateral metallocene sectors (Figure 1, right). Again, the alkenyl groups are almost coplanar wit...

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

“…The Erker group employed this concept for the synthesis of novel ansa -metallocenes. 170 − 172 The bis(2-alkenylindenyl)zirconium complex 69 for example gave upon direct excitation the cyclobutylene-bridged zirconocene 70 in a yield of 50% ( Scheme 24 ). 170 A related behavior was observed for alkenyl-substituted lithium cyclopentadienides.…”

“… 170 − 172 The bis(2-alkenylindenyl)zirconium complex 69 for example gave upon direct excitation the cyclobutylene-bridged zirconocene 70 in a yield of 50% ( Scheme 24 ). 170 A related behavior was observed for alkenyl-substituted lithium cyclopentadienides. 173 Even a ladderane structure could be generated by employing a bis(butadienylcyclopentadienyl)zirconium complex.…”

“…Remarkably, the cyclopentadienyl ring in bis­(cyclopentadienyl)­metal complexes can serve like the phenyl ring as a suitable substituent to mediate intramolecular [2 + 2] photocycloaddition reactions. The Erker group employed this concept for the synthesis of novel ansa -metallocenes. − The bis­(2-alkenylindenyl)zirconium complex 69 for example gave upon direct excitation the cyclobutylene-bridged zirconocene 70 in a yield of 50% (Scheme ). A related behavior was observed for alkenyl-substituted lithium cyclopentadienides .…”

Ansa-metallocene polymerization catalysts derived from [2+2]cycloaddition reactions of bis(1-methylethenyl-cyclopentadienyl)zirconium systems

“…However, this specific reaction was not synthetically useful for a clean ansa-metallocene catalyst development because of its reversibility under the photochemical conditions to result in a photostationary equilibrium mixture of the open and closed isomers of most investigated examples under practical conditions (23, 24). We later showed that the [2-(1-methylethenyl)indenyl] 2 ZrCl 2 derivative (3) rapidly and completely underwent the intramolecular photolytic [2ϩ2]cycloaddition reaction when irradiated with Pyrex-filtered light to yield 4, from which an interesting homogeneous metallocene ZieglerNatta catalyst system was generated (25)(26)(27)(28). This posed the question as to whether the outcome of the intramolecular [2ϩ2]photocyclization reaction at the group 4 bent metallocenes might significantly depend on the substituent at the alkenyl functional group and that the use of, e.g., the 1-methylalkenyl moiety might actually lead to a synthetically favorable situation.…”

Recent Synthetic Applications of Catalyst-Free Photochemistry

“…In 2004, Erker et al reported an interesting ansa-metallocene framework 126 through the intramolecular [2+2] photocycloaddition of a bis(2-alkenylindenyl)zirconium complex 125. 69…”