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 alpha-olefins can be achieved using a variety of titanium aryloxide catalysts. The titanabicyclic compound [Ti(OC6H3Ph4-2,3,5,6)(2){CH2(C6H10)CH2}] 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-cyclobexenyl)-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)TiCl2] (OAr = 2,6-diphenyl-3,5-dimethyl-phenoxide) activated with 2 equiv of n-butyllithium ((BuLi)-Li-n). 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 (BuLi)-Li-n leads to isomeric mixtures of trimers and tetramers of I,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 beta-hydrogen abstraction/elimination accounts for the observed regio- and stereochemistry. When the alpha-olefin Me3SiCH=CH2 is added to 1,3-CHD, these titanium systems generate the cross-coupled product 5-(beta-trimethylsilylethyl)cyclohexa-1,3-diene followed by the formation of trans-5,6-bis(beta-trimethylsilylethyl)-cyclohexa-1,3-diene. A mechanism involving initial coupling of the 1,3-CHD and Me3SiCH=CH2 at titanium prior to a 1,3-metal shift and beta-hydrogen abstraction/elimination from the initial cyclohexadiene ring is invoked. In contrast cross-coupling of cycloocta-1,3-diene (1,3-COT) with alpha-olefins RCH=CH2 (R = Ph, Bu-n, SiMe3) leads to trans-3-(beta-alkylvinyl)-cyclooctenes. In this case the reaction is proposed to proceed via similar titanacycles with beta-hydrogen abstraction taking place from the alkyl tether instead of the cyclooctene ring.