Titanium Aryloxide Catalyzed Cross-Coupling and Oligomerization Reactions Involving 1,3-Cyclohexadiene, 1,3-Cyclooctadiene, and α-Olefins

The dimerization/oligomerization and cross coupling of 1,3-cyclohexadiene (1,3-CHD) with α-olefins can be achieved using a variety of titanium aryloxide catalysts. The titanabicyclic compound [Ti(OC 6 H 3 Ph 4 -2,3,5,6) 2 {CH 2 (C 6 H 10 )CH 2 }] initiates the rapid, non-Diels−Alder catalytic dimerization of 1,3-CHD to produce exclusively threo -5-(3-cyclohexenyl)-1,3-cyclohexadiene. Following dimerization of the majority of 1,3-cyclohexadiene into 8, the titanium catalyst then isomerizes (1,5-shift) 8 into 1-(3-cyclohexenyl)-1,3-cyclohexadiene 9 and eventually into a 70/30 mixture (GC analysis) of 9 and 2-(3-cyclohexenyl)-1,3-cyclohexadiene 10 . Further cross coupling of dimers 8 − 10 with themselves and 1,3-CHD leads to trimers (C 18 species) and tetramers (C 24 ). This reaction can also be catalyzed by the dichlorides [Ti(OAr) 2 Cl 2 ] (OAr = 2,6-diphenyl-, 2,3,5,6-tetraphenyl-, 2,6-diphenyl-3,5-dimethyl-, and 2,6-di-isopropyl-phenoxide) and [Cp(OAr)TiCl 2 ] (OAr = 2,6-diphenyl-3,5-dimethyl-phenoxide) activated with 2 equiv of n -butyllithium (Bu n Li). In the case of the catalyst system [Ti(OAr) 4 ] or [Ti(OAr) 2 Cl 2 ] (OAr = 2,6-dimethylphenoxide), activation with 2 equiv of Bu n Li leads to isomeric mixtures of trimers and tetramers of 1,3-CHD with detectable amounts of pentamers following dimerization. The dimerization product is argued to originate via initial coupling of 1,3-CHD at the Ti metal center to produce 9-titana-octahydrofluorenes. A 1,3-metal shift followed by β-hydrogen abstraction/elimination accounts for the observed regio- and stereochemistry. When the α-olefin Me 3 SiCH CH 2 is added to 1,3-CHD, these titanium systems generate the cross-coupled product 5-(β-trimethylsilylethyl)-cyclohexa-1,3-diene followed by the formation of trans -5,6-bis(β-trimethylsilylethyl)-cyclohexa-1,3-diene. A mechanism involving initial coupling of the 1,3-CHD and Me 3 SiCH CH 2 at titanium prior to a 1,3-metal shift and β-hydrogen abstraction/elimination from the initial cyclohexadiene ring is invoked. In contrast cross-coupling of cycloocta-1,3-diene (1,3-COT) with α-olefins RCH CH 2 (R = Ph, Bu n, SiMe 3 ) leads to trans -3-(β-alkylvinyl)-cyclooctenes. In this case the reaction is proposed to proceed via similar titanacycles with β-hydrogen abstraction taking place from the alkyl tether instead of the cyclooctene ring.