Isopropylidene bridges

Whereas A -benzyl aziridine 69 reacted with the methano-bridged dipolarophile 39 to give exclusively TSA-product 79, use of the bulkier isopropylidene bridged dipolarophile 38 afforded substantial amounts of the TSB-adduct 84 as well as some of the TSA-product 80, an outcome attributed to unfavourable X)(NR TSa interaction in the latter reaction (X large, R small) (Scheme 12a). 

These isomers resulted from the non-stereoselectivity of the initial coupling process typical of the aza-ACE reactions of the 7-isopropylidene-bridged dipolarophile 38, while molecular weight measurements and the presence of an isopropenyl group in the H NMR of each product supported C,A-methano-bridge formation. Such products were considered to arise via the bond reorganisation depicted by the arrows in adduct 156 in which one of the isopropylidene rc-bonds acted as the nucleophile to attack the methylene carbon of the adjacent A-methoxymethyl group. 

The proposal of a conformation at the glacosidic bond with an upside down orientation of the two monomeric constituents has its support from experimental data of completely different origin. Thiem et al. were able to show that the kinetic isopropyli-denation of cellobiose in refluxing pyridine yields an isomer in low yield which has the glycosidic bond fixed via an isopropylidene bridge across the rings from OS to 06. This implies an upside down orientation of both rings compared to its normal conformation [91, 92]. 

It should be emphasised that all open-structure isopropylidene-bridged metallocenes have much lower activities towards ethylene polymerisation... 

Figure 3.27 shows reaction equations and the energy relationships of the hydroboration of enantiomerically pure a-pinene with 9-BBN. The reagent approaches only the side of the C=C double bond that lies opposite the isopropylidene bridge. The addition is thus completely diastereoselective. Moreover, the trialkylborane obtained is a pure enantiomer, since the starting material is a pure enantiomer. It is used as Alpine-Borane for the enantioselec-tive reduction of carbonyl compounds (Section 10.4). 

The addition of hydrogen to (1- and a-pinenes (72 and 73) takes place preferentially from the methylene bridge side, rather than from the isopropylidene bridge side, as might be expected from the consideration of catalyst hindrance. Van Tamelene and Timmons obtained a 84 16 cis.trans mixture in the hydrogenation of 72 over platinum at unspecified conditions.160 The stereoselectivity for the cis isomer is even higher with 73 more than 90% yields of cA-pinane were obtained with platinum catalysts.160,163... 

The isopropylidene bridge points towards the front. What does that mean for the nucleophilic attack of the 1-lithio-l-methoxyallene ... 

First, in a hydrogen-lithium exchange of the 1-H, methoxyallene is converted into a C-nucleophile, which attacks the keto-ftinctionality of 20 to form the tertiary alcohol 22 in 75 % yield. As the r -face of the ketone is blocked by the isopropylidene bridge of the six-membered ring, the nucleophilic attack can only occur from the 5 z-face. 

Recently, Ewen et al. have designed C2-, Cj-, and Ci-symmetric catalysts bearing heterocycle-condensed Cp ligands, as illustrated in Scheme 9, which add electronic effects to the stereocontrol. ° The complexes, containing isopropylidene-bridged cyclopentadienyl and cyclopentyl thiophene ligands, show activity... 

Poly(aryl ketones) (PEEK, PEK, and PEKK) are commercial high temperature polymers offering an excellent combination of properties combined with thermoplastic behavior. Poly(aryl ether ketone) PAEK blends have been reviewed by Harris and Robeson [1989]. Miscibility with PEI (Ultem 1000 GE) and other PI containing isopropylidene bridging units was noted. Arzak et al. [1997] reviewed the performance ofPEEK/PEI blends and noted a synergistic behavior in ductility and impact strength as reported earlier. Utility of these blends as a thermoplastic matrix candidate for advanced composites has been proposed [Harris and Robeson, 1989 Davis et al., 1992]. 

Russian references totally lack any researches on polyestersulfoneketones, not speaking on the production of such polymers in our country. Accounting for this, we have studied the regularities of the synthesis and produced block-copolyestersulfoneketones of some valuable properties [423]. The main structure elements of the polymers are rigid and extremely thermo-stable phenylene groups and flexible, providing for the thermoplastic reprocessibility, ester, sulfone and isopropylidene bridges. 

Products of degradation acetophenone, phenyl acetic acid, phenols, benzoic add, phthalic anhydride and phthalic acid end-groups chain session, photooxidative degradation of the isopropylidene bridge of BPA units, photooxidation of phthalimide units to phthalic anhydride end groups, hydrolysis of phthalic anhydride end groups ... 

Cahx[5]furan 40 with isopropylidene bridges was prepared by the HCl-catalyzed cyclocondensation of the L5 unit 49 with acetone in 45% yield [32,41]. An approach to 40 by the coupling of the L2 and L3 units was not very successfiiL In the condensation of the L2 imit 16, the L3 unit 48, and acetone with and without IiC104 (DME)2, the ratios of C4 1 C5 40 C6 41 were 2.8 1 1.8 and 4.2 1 1.4, respectively [40] C5 40 is always the least favored product. This result also indicates that the efficient synthesis of the larger calixfurans directly from furan and acetone is hampered by the fact that C4 1 constitutes a sink for the growing oligomeric chain. The parent calix[5]furan... 

Calix[6]furan 41 with isopropylidene bridges was also obtained by the cyclocondensation of the linear precursor 50 with acetone in 52% yield [32], Addition of LiC104 or CSCIO4 showed essentially no effect on the yield in the hydrochloric acid-promoted synthesis of 41 [20]. 

Calix[9]furan 43 with isopropylidene bridges was prepared by the HCl-promoted cyclocondensation of the L9 precursor 51 with acetone in 45%... 

For the preparation of larger family of calixfurans, the cyclization of the corresponding linear oUgomers is considered to be the most promising way when such precursors are available. Although not reported yet, the cyclization of the L7 and L8 units with isopropylidene bridges, which were prepared... 

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