2.3- dimethylindene

Dimethylindene derivatives were hydroaluminated with BujAlH using Ni(acac)2 as the nickel(O) precursor (Scheme 2-13) [9]. Hydroalumination of the trisubstituted double bond in 9 was also achieved, although more forcing conditions... 

An advantage of nickel catalysts over other metal systems is that the properties of the active species can easily be tuned by the addition of suitable ligands. For example, the presence of PPhj was shown to have a direct influence on the regiochem-istry of hydroalumination of 1,1-dimethylindene la [33]. While the reaction of BU2AIH with la gave a 4 1 mixture of regioisomeric products 13a/13b after deuterolytic workup, the same reaction carried out in the presence of PPh, yielded 13a and 13b in a ratio of >99 1 (Scheme 2-14). 

The use of cationic acceptors such as methylacridinium hexafluorophosphate (4) has been described for the cyclodimerization of 1,1-dimethylindene (5) in dichloromethane to afford the head-to-head anfr-dimer 6 in 80% yield.6... 

The scope and utility of cation radical induced cyclobutanation1 is greatly enhanced by the option of cross additions, the first of which was the formation of a 3 2 mixture of diastereomeric cyclobutanes in the irradiation of an equimolar mixture of phenyl vinyl ether and 1,1-dimethylindene in the presence of tetraphenylpyrylium tetrafluoroboratc in acetonitrile.2 The scope of PET cyclobutanations was further extended in a synthetic sense by the observation of cross additions of electron-rich alkenes to conjugated dienes.3 4 Examples of such reactions are shown below in the formation of compounds 1, 2, 3 and 4. 

Indenes lacking benzylic hydrogens (e.g., 1,1-dimethylindene) give no substitution products upon irradiation with acrylonitrile. 

Farid reported that the crossed addition of 1,1-dimethylindene with phenyl vinyl ether occurs upon irradiation of their acetonitrile solution in the presence... 

An analogous situation arises in the hydroalumination of prochiral alkenes, as already explained in Section 3.11.2.5 and exemplified in the behavior of 1,1-dimethylindene (Scheme 8). The inversion of configuration at the jp -hybridized C—A1 bond (51 52) is so facile that the addition of Bu 2AlH must... 

The cis-stereochemistry of the hydroalumination of olefins is proved by the reaction of a strained cycloolefin, 1,1-dimethylindene, with i-Bu2AlD in the presence of Et20 (to slow down the inversion process of the Al—C bond). The deuterolysis of the Al—C bond (retention of configuration) leads to pure cis-2,3-dideuterio-l,l-dimethylindane ... 

Dimethyl ether, 31 41, 244 formation in zeolites, 42 95-98 Dimethylethylbenzenes, 20 281 Dimethylindan, cyclization, 28 298, 299 Dimethylindene, cyclization, 28 298, 299 Dimethyl methyl phosphonate, catalytic decomposition, 35 159-160 Dimethylnaphthalenes, hydrogenation of, 18 64-102... 

Chloranil forms two types of cycloadducts with 3,3-dimethylindene. In the early stages, oxetane (76) is formed via adduct 76, by addition of the carbonyl oxygen... 

The primary products of the cyclization of 2-phenylpentane are cis- and /ra j-l,3-dimethylindans, 1,3-dimethylindene, and 1-methylnaphthalene ... 

The 1,4-bifunctional intermediate is implicated further by the electron transfer sensitized isomerization of cis- to Irons-1,2-diphenoxylcyclobutane [115], and by the unique CIDNP effects observed during the electron transfer sensitized cleavage of the anti-head-to-head dimer of 3,3-dimethylindene (vide infra) [121]. 

In some cases radical cations may undergo cycloadditions with an acceptor derived intermediate without prior proton transfer. This is observed especially for radical cations without sufficiently acidic protons, although it is not limited to such species. For example, the photoreaction of chloranil with 3,3-dimethylindene results in two types of cycloadducts [141]. In the early stages of the reaction a primary adduct is identified, in which the carbonyl oxygen is connected to the p-position of the indene (type B) in the later stages this adduct is consumed and replaced by an adduct of type A, in which the carbonyl oxygen is connected to the a-position. CIDNP effects observed during the photoreaction indicate that the type B adduct is formed from free indene radical cations, which have lost their spin correlation with the semiquinone anions. 

The ease of cyclobutane cleavage and the detailed mechanism can be affected by the nature of the substituents and the substitution pattern. While the above cyclobutanes are cleaved without a discernible intermediate, the n//-head-to-head dimer of dimethylindene shows a significantly different behavior. This substrate is cleaved in an apparent two-step process, involving a ring-closed radical cation (with spin and charge localized on one indan system) and a ring-opened 1,4-bifunctional radical cation. Apparently, the cleavage of the doubly benzylic cyclobutane bond is reversible. The involvement of more than one dimer radical cation is indicated by a unique polarization pattern (Fig. 13), which is incompatible with any one intermediate, but can be simulated on the basis of two successive radical cations (see Sect. 5.2) [256]. 

Fig. 13. CIDNP effects observed for the cyclobutane signals of the dimethylindene dimer during the photoinduced electron transfer reaction with chloranil (a), and simulated spectra based on the radical pair theory and assuming a ring-opened (extended) dimer radical cation (b), a ring-closed (localized) dimer radical cation (c) and the consecutive ( cooperative ) involvement of open and closed radical cations (d) [256]...

Compared with the parent system and those with identical substitution in all four carbons, the structure of other derivatives should be affected by the substitution pattern and by the nature of the substituents. For 1,2-disubstituted derivatives, structure type C, in which the doubly substituted cyclobutane bond is weakened (and lengthened), or a related structure type in which the bond is cleaved, should be favored. This is born out by several observations mentioned earlier. For example, the geometric isomerization of 1,2-diaryloxycyclobutane (Sect. 4.1) can be rationalized by one-bond rotation in a type C radical ion. Similarly, the fragmentation of the anti-head-to-head dimer of dimethylindene (Sect. 4.4) may involve consecutive cleavage of two cyclobutane bonds in a type C radical ion. The (dialkylbenzene) substituents have a lower ionization potential (IP 9.25 eV) [349] than the cyclobutane moiety (IP 10.7 eV) [350] hence, the primary ionization is expected to occur from one of the aryl groups. 

For dimethylindene (E01 = 1.68 V vs SCE) 29, whose radical cation (29 ) is unreactive vs molecular oxygen, the DCA-sensitized photooxygenation proceeds well in the presence of a nucleophile (water or methanol) leading to a mixture of tram- and cis-hydroxy or methoxy-hydroperoxides 30a, b 31a, b ... 

Althou the evidence presented in support of such reaction is not entirely convincing, in its favour is the existence of a lower oxidation state of antimony which can be readily reached by smooth reduction. In the case of other Lewis acids a lower oxidation state is either unavailable or less easily obtained and the likelyhood of hydride abstraction seems therefore more remote, at least from saturated hydrocarbons. Kennedy s scheme implies however the removal of the allylic hydride ion from an olefin. Although plausible in certain cases (but never proved), this mechanism is obviously impossible with styrene and 1,1 -diphenylethylene With 3-phenylindene it would yield an aryl-substituted allylic carbenium ion which would not be expected to be in equilibrium with its precursor yet, this equilibrium was observed With 2,3-dimethylindene in the same conditions initiation did not take place yet Kennedy s mechanism shouldhave operated without impediments. Finally, with 1,1-diphenylpropene hydride abstraction would have produced an allylic ion incapable of giving back the precursor by reacting with methanol yet Bywater and Worsfold showed that this reversible reaction takes place. 

Farid also reported the first example of an efficient cross addition, that occurring between dimethylindene and phenyl vinyl ether [27]. Essentially all of these reactions were carried out using the PET method. 

For example, the cyclocondensation of cyclopentadiene with 2,5-hexanedione in the presence of sodium affords 4,7-dimethylindene in 65 % yield. Subsequent standard transformations yield the desired bridged-metallocene structures. 

Field, D.J., and Jones. D.W., o-Quinonoid compounds. Part 16. L5-Shift of vinyl groups in 1,3-dimethylindenes. Product studies and migratory apliludes of substituted vinyl groups, J. Chem. Soc., Perkin Trans. L 714. 1980. 

A similar reactivity order was found recently for competition between intramolecular cyclization and intermolecular capture. The AgBF4-assisted solvolysis of trimesityl vinyl chloride 21 in alcohols gives both the ether 23 and 2,3-dimesityl-4,6-dimethylindene, 24 (equation 11 Mes = 2,4,6-Me3C6H2) (80). These compounds are derived from the trimesitylvinyl cation... 

---