Dimethyltitanocene reaction

In the course of studying the reactions of Si-H compounds with dialkyltitanocenes, with a view to the synthesis of new hydridosilyltitanocene complexes, we adventitiously discovered that phenylsilane undergoes facile, quantitative dehydrogenative coupling to a linear poly(phenylsilylene) under the catalytic influence of dimethyltitanocene. The ease with which this reaction proceeds initially induced us to underestimate the significance of the observation. 

Normally, no small cyclopolysilanes are observed in these reactions. Two exceptions we have noted are the very slow reaction of phenylsilane under the influence of Cp2TiMe2 and the reaction of benzylsilane under the influence of dimethyltitanocene at very long reaction times. From both of these reactions we isolate a single isomer of the cyclohexasilane, in ca. 10 per cent yield in the case of the phenylsilane and ca. 60 per cent yield in the case of the benzylsilane. These isomers are believed to be the all-trans isomers. The phenyl derivative is identical to that... 

The compounds 2 and 3, have all been isolated from reactions of phenylsilane with either dimethyltitanocene (1 2) or dimethyl-zirconocene (r ). All of the evidence points to the fact that these compounds are probably resting species and are not involved in the catalytic cycle. They do nevertheless give some indication of the complex series of reactions that transform the dimethyl-metallocene to active catalyst. 

A similar dichotomy was observed in the titanium catalyzed polymerization of primary silanes coupled to the hydrogenation of norbornene (20). At low catalyst concentration (ca. 0.004H), essentially complete conversion of norbornene to an equimolar mixture of norbornane and bis-phenylsilyl- (and/or 1,2-diphenyl-disilyl)norbornane was observed. Under these conditions no evidence for reduction of titanium was obtained. At higher catalyst concentrations (> 0.02M) rapid reduction of the dimethyltitanocene to J, and 2 occurs and the catalytic reaction produces mainly polysilane (DPn ca. 10) and norbornane in ca. 80 per cent yields, and silylated norbornanes in about 20 per cent yield. 

Aluminum-free titanocene-methylidene can be generated by thermolysis of titana-cyclobutanes 6, which are prepared by reaction of the Tebbe reagent with appropriate olefins in the presence of pyridine bases [9]. Alternatively, the titanacyclobutanes are accessible from titanocene dichloride and bis-Grignard reagents [10] or from 71-allyl titanocene precursors [11]. The a-elimination of methane from dimethyltitanocene 7 provides a convenient means of preparing titanocene-methylidene under almost neutral conditions [12] (Scheme 14.5). 

Reactions of titanocene-methylidene generated from titanacyclobutanes with acyl chlorides 55 [46] or acid anhydrides 56 [47] lead initially to the titanium enolates 57 (Scheme 14.24), which then afford aldols upon treatment with the carbonyl compounds. On the other hand, five-membered cyclic anhydrides are methylenated with dimethyltitanocene (Table 14.5, entry 7) [45]. 

Similarly to alkenes, alkynes react with various titanium-methylidene precursors, such as the Tebbe reagent [13,63], titanacydobutanes [9b, 64], and dimethyltitanocene [65] to form the titanium-containing unsaturated cyclic compounds, titanacydobutenes 67 (Scheme 14.29). Alternatively, 2,3-diphenyltitanacydobutene can be prepared by the reaction of the complex titanocene(II) bis(trimethylphosphine) with 1,2-diphenylcyclopropene [66]. Substituent effects in titanacydobutenes [67], the preparation of titanocene-vinylke-tene complexes by carbonylation of titanacydobutenes [68], and titanacyclobutene-vinylcar-bene complex interconversion [69] have been investigated. 

Since the hybridization and structure of the nitrile group resemble those of alkynes, titanium carbene complexes react with nitriles in a similar fashion. Titanocene-methylidene generated from titanacyclobutane or dimethyltitanocene reacts with two equivalents of a nitrile to form a 1,3-diazatitanacyclohexadiene 81. Hydrolysis of 81 affords p-ketoena-mines 82 or 4-amino-l-azadienes 83 (Scheme 14.35) [65,78]. The formation of the azati-tanacyclobutene by the reaction of methylidene/zinc halide complex with benzonitrile has also been studied [44]. 

Catalytic dehydrogenative coupling of organohydrosilanes, a common method for the formation of linear polysilanes, can also be applied to cyclopolysilane synthesis. Thus, benzylsilane reacts to give all-fraws-hexabenzylcyclohexasilane in the presence of dimethyltitanocene (equation 3). However, extremely long reaction times are required and only moderate yields are obtained15. 

Dimethyltitanocene (213), called the Petasis reagent, can be used for alkenation of carbonyls (aldehydes, ketones, esters, thioesters and lactones). This reagent is prepared more easily than the Tebbe reagent by the reaction of titanocene dichloride with MeLi. However, this reagent may not be a carbene complex and its reaction may be explained as a nucleophilic attack of the methyl group at the carbonyl [67], Alkenylsilanes are prepared from carbonyl compounds. Tri(trimethylsilyl)titanacyclobutene (216), as a... 

Dimethyltitanocene, Cp2Ti(CH3)2, (1) m.p. 97°. This reagent is prepared by reaction of titanocene dichloride, Cp2TiCl2, with CH3Li (95% yield). It is stable for several months when stored in the dark in toluene or THF, but decomposes rapidly in the solid state.1... 

In this reaction, acetals are used as dication equivalent of one-carbon unit 72, whereas 71 provides a dianion 73 <95TL5581>. A new stereocontrolled synthesis of substituted tetrahydrofiirans starts with dioxalones 74. A titanium-mediated methylenation using dimethyltitanocene (THF, 60 C) with subsequent treatment of 75 with trialkylaluminium reagents results in the formation of tetrahydrofiirans 76 <95JA6394>. 

Titanacyclobutene complexes result from the reaction of dimethyltitanocene with alkynes (Scheme 29). The reaction can be envisioned to proceed either by initial methane extrusion to form the methylidene, followed by [2+2] cycloaddition, or by initial alkyne insertion to generate the vinylic intermediate followed by 7-elimination. The evidence suggests that the latter mechanism dominates. Diphenylacetylene and bis(trimethylsilyl)acetylene both provide the corresponding titanacyclobutene complex, preferentially abstracting an allylic 7-hydrogen over the... 

One rare example of silicon-bridged early transition-metal complex of type C has been synthesized by the reaction of dimethyltitanocene with PhSiH3 [Eq. (19)].49 The structures of these products containing /r-772-HSiHPh ligands were determined by X-ray crystal structure analysis. 

A review on ynolates, including the synthesis of p-lactones, has appeared <03S2275>. Morita-Baylis-Hillman-type adducts have been converted into a-alkylidene-p-lactones 65, which on reaction with dimethyltitanocene can be transformed into 3-alkylidene-2-methyleneoxetanes <03OL399>. Lactones 66 have been obtained via the cinchona alkaloid-catalyzed dimerization of monosubstituted ketenes <03OL4745>. The PdCh-promoted synthesis of P-lactones 67 have been achieved via cyclocarbonylation of 2-alkynols <03OL4429>. 

Liu and Harrod ° reported on dehydrocoupling of ammonia and silanes catalyzed by dimethyltitanocene. Tertiary silanes, for example, Ph2SiMeH, were transformed into disilazanes. Yet no polysilazanes could be obtained when reacting PhSiH3 under similar conditions. Homodehydrocoupling (which proceeds with Si-Si linking compare Scheme 18.3) effectively competed with amination reactions, indicating that the products obtained by ammonolysis of PhSiHj are polyaminosilanes [SiPh(NH2)] rather than polysilazanes [SiPhH-NH] . 

Martinez, I., Howell, A. R. The reaction of dimethyltitanocene with N-substituted- 3-lactams. Tetrahedron Lett. 2000, 41, 5607-5611. 

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