Indan formation

Dihydronaphthalene (DHN) is frequently assumed to be an intermediate in hydrogen transfer reactions. While this appears reasonable, efforts to detect and/or measure this intermediate have never been very successful. Assuming that DHN is present, we have briefly explored its role in hydrogen transfer and methyl indan formation. 

DHN. Apparently DHN both thermally dehyrogenates to naphthalene and disproportionates to Tetralin and naphthalene. In all of the runs, there was a sizable amount of hydrogen released when the reactors were opened. When DHN was heated at 450°C for either 15 or 180 minutes, the ratio of naphthalene to etralin was 1.8. Increased methyl indan formation occurred with time. With the introduction of dibenzyl, the anticipated [2] increased isomerization of T etralin to methyl indan occurred. These results suggest that the rearrangement of hydroaromatics does not proceed through the dihydro-... 

Indan formation [Reactions (13)-(15)] represents an intramolecular alkylation of the aromatic ring by a carbanion. 

Indan formation is significant in polymerizations of styrene, a-methylstyrene, and p-alkyl-substituted styrenes, but is less important in p-me-thoxystyrene polymerizations [14], Indan formation is not possible in polymerizations of styrene derivatives substituted at both ortho positions and is suppressed in polymerizations of 3,5-disubstituted styrene derivatives due to steric hindrance [105]. Indan formation consumes initiator in attempts at polymerizing isobutene with cumyl chloride and p-dicumyl chloride initiators [280]. 

Nuyken has optimized indan formation of diisopropenylbenzene to prepare polyindans with both high glass transition temperatures and high thermal stability [Eq. (94)] [291]. 

Indan formation is favored by the relatively high equilibrium monomer concentration of the corresponding a-methylstyrene ([ML 1 mol/L at ambient temperature). For example, a-methylstyrene itself is dimerized rather than polymerized at [M]0 < 1 mol/L, and the resulting unsaturated dimers cyclize efficiently to provide dimeric indans, especially at elevated temperatures. 

Although indan formation is significant in styrene polymerizations, j3-proton elimination is much faster than intramolecular alkylation [292]. Unsaturated styrene and a-methylstyrene dimers are prepared quantitatively under high dilution at elevated temperatures without cyclization to indan derivatives [293]. In this case, the carbocations must be quenched before intramolecular cyclization becomes significant at high conversion. However, indan formation competes with depropagation at temperatures above 50° C, which is much too high for unsaturated styrene dimers (D =) to homopolymerize. As outlined in Eq. (95), the unsaturated dimers form indans (Din) in the presence of acid. 

At lower temperatures (- 70° C) with lower equilibrium monomer concentrations, unsaturated dimers rapidly form tetramers without producing a significant amount of trimers [295]. This indicates that the unsaturated dimer prefers to dimerize, with depropagation and indan formation less probable. 1,2-Hydride shifts may also occur, as indicated by the large variety of dimer stereoisomers formed, as well as by spectral changes in the stopped-flow studies of the dimerization. Eq. (96) outlines the variety of isomeric indan derivatives formed at later stages of the reaction by a combination of hydride shifts (cf., Section V.A.4) and intramolecular cyclizations. 

Figure 5 Effect of temperature on rate constants of propagation, depropagation, transfer to monomer, transfer to triflate anion, and indan formation in the carbocat-ionic polymerization of styrene (From Ref. 292).

Scheme 8.13 Chain transfer by indane formation during the uncontrolled cationic polymerization of styrene.

Addition of further monomer results in an increase in molecular weight. The danger of termination by indane formation seems to be reduced by adding the monomer in small portions [102,103] ... 

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