Titanacyclopropane

While 49 is illustrated here as an olefin ir-complex, it can alternatively be classified as a titanacyclopropane derivative (50). 

The equilibrium hybrids and NBOs of the final titanacyclopropane ring bonds are shown in Fig. 4.73. Although some bond strain is evident, the degree of bondbending appears appreciably less than that in cyclopropane (cf. Fig. 3.21). The relatively gentle degree of bending of metal hybrids is particularly noteworthy in... 

Figure 4.73 The overlapping NHOs (left) and NBO (right) for (TTiC (upper) and (jCc (lower) bonds of the product titanacyclopropane (cf. Fig. 4.70).

Scheme 16 Some representative methods for the synthesis of titanacyclopropanes and titanacyclopropenes.

Scheme 23 Reactions of titanacyclopropanes and titanacyclopropenes with carbonyl compounds and imines.

Scheme 11.12. Mechanistic test for the formation of a titanacyclopropane-ethylene complex 40 from a titanacyclopentane derivative 39 [103],...

It is remarkable that even cyclobutylmagnesium bromides react cleanly with titanium alkoxides to yield reactive titanacyclopropane intermediates that reductively cyclopropa-nate N,N-dialkylformamides. This constitutes the first synthesis of the highly strained N,N-dialkylbicyclo[2.1.0]pent-5-ylamines (Table 11.9, entries 29—31). 

The reductive cyclopropanation with in situ generated titanacyclopropanes can also be applied to alkyldiformylamines 58, which are easily prepared from inexpensive formamide (57). Both formyl groups are converted to cyclopropyl groups, and the alkyldicyclopropyl-... 

The titanacyclopentane complexes 3a-3e react rapidly in C6D6 solution with tertiary phosphines to form the analogous titanacyclopropane complex containing a phosphine ligand ... 

Lewis acid-promoted allylation, 9, 304 titanacyclopropane and titanacylcopropene reactions,... 

If an alkene is present in the reaction mixture the intermediate, titanacyclopropane, undergoes facile ligand exchange, giving new titanium species, which react further in the catalytic cycle. Thus, various alkenes can be involved in this reaction, inter- or intramolecularly, allowing the preparation of numerous cyclopropane derivatives. In the presence of a chiral titanium alcoholate the reaction can be performed with good enantioselectivity (Scheme 110).319... 

Ally) ethers are selectively cleaved with titanium(lV) isopropoxide and commercially available Grignard reagents like /i-butyl- or cyclohexylmagnesium chloride [Scheme 4.229].432 Neither benzylidene acetals nor more highly substituted allylic ethers suffer under the reaction conditions. A mechanism for the reaction implicates formation of the titanacyclopropane intermediate 229.1 as the first step. Ligand exchange with an unsubstituted allyl ether affords intermediate 229.2. -Elimination to the allyltitanium(lV) alkoxide 2293 followed by hydrolysis returns the deprotected alcohol. The reaction closely resembles an earlier method based on zirconium.433... 

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