Bifunctional starting

Suzuki cross-coupling has found applications in the preparation of specialty polymers, too. Rigid rod polymers may have very useful properties (the well-known Kevlar, poly(p-phenyleneterephtalamide) belongs to this family, too) but they are typically difficult to synthetize, characterize and process. Such materials with good solubility in organic solvents [38] or in water [39] were obtained in the reactions of bifunctional starting compounds under conventional Suzuki conditions with [Pd(PPh3)4] and [Pd(TPPMS)3] catalysts, respectively (Scheme 6.15). 

It should be noted that the 1,3-bifunctional starting compounds 4-7 are quite difficult to obtain. Moreover, the direction of acid-catalyzed cycliza-tion of 3-acylaminopropanols depends considerably on their structure. Thus, N-acylaminoalcohols 8 can be cyclized to give either 5,6-dihydro-4//-1,3-oxazinium salts 10 or 2-oxazoline derivatives 12 as well as mixtures thereof (82MI1, 82MI2 see also 78AHC1). 

In a third type of block copolymer formation. Scheme (3), the initiator s azo group is decomposed in the presence of monomer A in a first step. The polymer formed contains active sites different from azo functions. These sites may, after a necessary activation step, start the polymerization of the second monomer B. Actually, route (3) of block copolymer formation is a vice versa version of type (1). It has been shown in a number of examples that one starting bifunctional azo compound can be used for block copolymer synthesis following either path. Reactions of type (3) are tackled in detail in Section III of this chapter. 

PAMAM dendrimers are synthesized in a multistep process. Starting from a multifunctional amine (for example ammonia, ethylenediamine, or tris(2-amino-ethyl)amine) repeated Michael addition of methylacrylate and reaction of the product with ethylenediamine leads to dendrimers of different generation numbers [1,9]. Two methylacrylate monomers are added to each bifunctional ethylenediamine generating a branch at each cycle. Unreacted ethylenediamine has to be completely removed at each step to prevent the initiation of additional dendrimers of lower generation number. Excess methylacrylate has also to be removed. Bridging between two branches of the same or of two different dendrimers by ethylenediamine can also be a problem, and has to be avoided by choosing appropriate reaction conditions. 

A second strategy is to attach a linker (also referred to as a handle or anchor) to the resin followed by assembly of the molecule. A linker is bifunctional spacer that serves to link the initial synthetic unit to the support in two discrete steps (Fig. 3). To attach a linker to a chloromethyl-PS resin, a phenol functionality such as handle 4 is used to form an ether bond (Fig. 4). To attach the same handle to an amino-functionalized support, acetoxy function 5 or a longer methylene spacer of the corresponding phenol is applied to form an amide bond. Both of these resins perform similarly and only differ in their initial starting resin [4], An alternative approach is to prepare a preformed handle in which the first building block is prederivatized to the linker and this moiety is attached to the resin. For peptide synthesis, this practice is common for the preparation of C-terminal peptide acids in order to reduce the amount of racemization of the a-carbon at the anchoring position [5],... 

The possibility of coordinating functionalized TTFs onto polynuclear core is a very stimulating issue because it is now well established that polynuclear cores, with some restrictions of course, can act as SMMs. We started a systematic investigation of polynuclear paramagnetic complexes with TTF CH=CH—py ligands to scan the possibility to access to bifunctional molecules which can act at the same time as SMM and single component metal. We succeeded in coordinating our modified TTFs to several homo- or heteropolynuclear complexes. This opens new perspectives in the field of multifunctional materials. The size of these molecules, which is of the order of 4 nm, is another important aspect in the field of molecular scale electronic. 

Other molecules in which the presence of a C2 axis of symmetry also simplifies the syntheses are squalene (5) rmd P-carotene (6). Notice that in such cases the simplification derives from the fact that the syntheses are convergent and reflexive i.e., syntheses which start from identical precursors [5]. In practice, the synthesis of these two substances may be carried out not only by dimerisation of two identical moieties [10] [11] -which may lead to a mixture of isomers-, but preferentially by addition of two identical fragments to a central bifunctional unit according to the scheme -1- + C. = Cjj (squalene Cji -t-Cg + Cj] = 30 [12] p-carotene ... 

Thus to provide a route to polymer chains, it is necessary for the starting materials to be bifunctional. Choosing the synthesis of terylene (Fig. 5.1) as an example ... 

Diazirines are the cyclic isomers of the alphatic diazo compounds. Both the diaziridines and the diazirines are starting materials for the synthesis of alkyl hydrazines. 3,3-Pentamethyl-enediaziridine can be hydrolyzed quantitatively to hydrazine. Methylamine may be substituted for ammonia in the procedure resulting in l-methyl-3,3-pentamethylenediaziridine (m.p. 35-36°, yield 62% of theoretical) and then methyl hydrazine. Use of ethylenediamine leads to ethylene bis-hydrazine via a bifunctional diaziridine (m.p. 143-144°, yield 48% of theoretical). Ammonia can also be replaced by w-propylamine or cydo-hexylamine cyclohexanone by acetone. 

In the research aiming at the application of the addition reaction, Pringle and co-workers found that bifunctional phosphine starting materials like H2PCH2CH2PH2 spontaneously react with formaldehyde in the absence of the catalyst, forming water-soluble chelating phosphines [9]. 

Naphthalene-based bifunctional Lewis acids that involve boron and a heavier group 13 element have also been prepared starting from the boron/tin derivative 30 (Scheme 15). Thus, the transmetalation reaction of 30 with gallium trichloride or indium trichloride in tetrahydrofuran (THF) results in high yields of l-(dichlorogallium)-8-(dimesitylboron)naphthalenediyl 35 and l-(dichloroindium)-8-(dimesitylboron)... 

This reaction encompasses a number of interesting features (general Brpnsted acid/ Brpnsted base catalysis, bifunctional catalysis, enantioselective organocatalysis, very short hydrogen bonds, similarity to serine protease mechanism, oxyanion hole), and we were able to obtain a complete set of DFT based data for the entire reaction path, from the starting catalyst-substrate complex to the product complex. 

The cobalt-catalyzed synthesis is also applicable to bifunctional nitriles [Eq.(20)]. The starting materials containing different bridging groups afford the bis(2-pyridyl) derivatives. The reaction proceeds stepwise in the... 

Cho et al. describes an alternative synthesis (Scheme 20) of the 2,5 DKP scaffold 92 = 123 via a bifunctional dipeptide 120, an aldehyde 121, and an isocyanide 122 [42]. These commercially available starting materials were added in equimolar amounts to trifluoroethanol at 40°C under nitrogen. The reaction was brought to room temperature and allowed to complete. The standard Ugi workup was used followed by column chromatography. Yields were shown in the range of 21-87%. When this reaction was done in a microwave, time taken for the reaction to complete was decreased significantly and yields increased by a factor of 4. The substitution pattern of this and the previously described (in Scheme 15) are identical however, the reactions use different types of starting materials, for example, dipeptide vs. N- and C-protected amino acids. Thus different stereochemical outcomes can be expected for the two syntheses. 

Condensation polymerizations (polycondensations) are stepwise reactions between bifunctional or polyfunctional components, with elimination of small molecules such as water, alcohol, or hydrogen and the formation of macromo-lecular substances. For the preparation of linear condensation polymers from bifunctional compounds (the same considerations apply to polyfunctional compounds which then lead to branched, hyperbranched, or crosslinked condensation polymers) there are basically two possibilities. One either starts from a monomer which has two unlike groups suitable for polycondensation (AB type), or one starts from two different monomers, each possessing a pair of identical reactive groups that can react with each other (AABB type). An example of the AB type is the polycondensation of hydroxycarboxylic acids ... 

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