Difunctional precursors

The usefulness of the Knorr synthesis arises from the fact that 1,3-dioxo compounds and a-aminoketones are much more easily accessible in large quantities than rational 1,4-difunctional precursors. Such practical syntheses are known for several important hetero-cycles. They are usually limited to certain substitution patterns of the target molecules. 

Dicarbodiimides have not gained the same prominence as diisocyanates as monomers for addition polymers. Dicarbodiimides are obtained from difunctional precursors, such as bis-thioureas. Another synthetic method is the conversion of diisocyanates with iminophosphoranes. The reaction can be condncted stepwise to give an isocyanato-carbodiimide as an intermediate. 

Figure 7. Synthetic strategy to obtain a [3]catenate by cyclodimerization of a difunctional precursor. The circles represent the templating transition metal, and the triangles the terminal diynes.

When the difunctional precursors (diisocyanate, polyol and extender) are allowed to react in a stoichiometric amount, a thermoplastic PU is formed. Thermosetting PU are made by using excess diisocyanate (excess diisocyanate reacts with a urethane structure to form allophanate bonds) or by using a trifunctional extender like glycerin or trimethylol propane [92-94]. The unique feature of PU resin is that the change in UPE between crosslink offers a wide change in properties, especially the strain (which reflects flexibility). For example, a PU system with a molecular weight between two... 

Heterocyde syntheses are often possible from difunctional open-chain precursors, including olefins as 1,2-difunctional reagents, and an appropiate nucleophile or electrophile containing one or more hetero atoms. The choice of the open-chain precursor is usually dictated by the longest carbon chain within the heterocyde to be synthesized. 

The longest carbon chain within a heterocycle indicates possible open-chain precursors. We use this chain as a basis to classify heterocycles as 1,2- to 1,6-difunctional systems. 

Methyl 6-hydroxy-3-methylhexanoate is our 1,6-difunctional target molecule. Obvious precursors are cyclohexene and cyclohexadiene derivatives (section 1.14). Another possible starting material, namely citronellal, originates from the "magic box of readily available natural products (C.G. Overberger, 1967, 1968 E.J. Corey, 1968D R.D. Clark, 1976). 

The 1,6-difunctional hydroxyketone given below contains an octyl chain at the keto group and two chiral centers at C-2 and C-3 (G. Magnusson, 1977). In the first step of the antithesis of this molecule it is best to disconnect the octyl chain and to transform the chiral residue into a cyclic synthon simultaneously. Since we know that ketones can be produced from add derivatives by alkylation (see p. 45ff,), an obvious precursor would be a seven-membered lactone ring, which is opened in synthesis by octyl anion at low temperature. The lactone in turn can be transformed into cis-2,3-dimethyicyclohexanone, which is available by FGI from (2,3-cis)-2,3-dimethylcyclohexanol. The latter can be separated from the commercial ds-trans mixture, e.g. by distillation or chromatography. 

Butane. The VPO of butane (148—152) is, in most respects, quite similar to the VPO of propane. However, at this carbon chain length an important reaction known as back-biting first becomes significant. There is evidence that a P-dicarbonyl intermediate is generated, probably by intramolecular hydrogen abstraction (eq. 32). A postulated subsequent difunctional peroxide may very well be the precursor of the acetone formed. 

Literature articles, which report the formation and evaluation of difunctional cyanoacrylate monomers, have been published. The preparation of the difunctional monomers required an alternative synthetic method than the standard Knoevenagel reaction for the monofunctional monomers, because the crosslinked polymer thermally decomposes before it can revert back to the free monomer. The earliest report for the preparation of a difunctional cyanoacrylate monomer involved a reverse Diels-Alder reaction of a dicyanoacrylate precursor [16,17]. Later reports described a transesterification with a dicyanoacrylic acid [18] or their formation from the oxidation of a diphenylselenide precursor, seen in Eq. 3 for the dicyanoacrylate ester of butanediol, 7 [6]. 

Transformations through 1,2-addition to a formal PN double bond within the delocalized rc-electron system have been reported for the benzo-l,3,2-diazaphospholes 5 which are readily produced by thermally induced depolymerization of tetramers 6 [13] (Scheme 2). The monomers react further with mono- or difunctional acyl chlorides to give 2-chloro-l,3,2-diazaphospholenes with exocyclic amide functionalities at one nitrogen atom [34], Similar reactions of 6 with methyl triflate were found to proceed even at room temperature to give l-methyl-3-alkyl-benzo-l,3,2-diazaphospholenium triflates [35, 36], The reported butyl halide elimination from NHP precursor 13 to generate 1,3,2-diazaphosphole 14 upon heating to 250°C and the subsequent amine addition to furnish 15 (Scheme 5) illustrates another example of the reversibility of addition-elimination reactions [37],... 

Nitrile oxide precursors have been prepared by the reaction of an isocyanate and an alkyl nitroacetate. These precursors release alkanol and carbon dioxide when heated, to liberate the highly reactive nitrile oxide species. An improved synthetic procedure has been developed to afford novel cross-linking agents based on difunctional, trifunctional and aliphatic precursors. Application of these agents to polymer cross-linking has been demonstrated (527). 

The formation of a second ring, based on the generation of a six-membered carbanion followed by alkylation with a difunctional electrophile and further cyclization, was also exploited in the synthesis of hexahydropyrrolo[l,2-tf]pyr-azine-l,4-dione 235 starting from alkoxycarbonyl piperazine-2,5-dione 233. When the key precursor was treated with 2equiv of NaH and 1,3-dibromopropane, the bicyclic compound 234 was obtained in acceptable yield and further transformed into compound 235 by deprotection and decarboxylation (Scheme 30) <2005T8722>. 

Functional perfluoropolyethers11 (Figure 14.7) can also be prepared by direct fluorination in high yields. Difunctional perfluoropolyethers based on fluorinated polyethylene glycol) are of particular interest as possible precursors for elastomers, which should have outstanding high-temperature and low-temperature properties. 

Typically, the most common precursors to new Si-N-P systems are simple silylaminophosphines (eq 1). The difunctional character of these compounds, which is due to the nucleophilic site at phosphorus and a complementary electrophilic site at silicon, makes them very versatile reagents. They have been used in a new synthesis of alkyl and/or phenyl substituted phospha-zenes (R2PN)jj ( ) and have led to the preparation of promising precursors to potentially electrically conducting polymer systems of general formula (RPN)n>... 

Stars with high arm numbers are commonly prepared by the arm-first method. This procedure involves the synthesis of living precursor arms which are then used to initiate the polymerization of a small amount of a difunctional monomer, i.e., for linking. The difunctional monomer produces a crosslinked microgel (nodule), the core for the arms. The number of arms is a complex function of reaction variables. The arm-first method has been widely used in anionic [3-6,32-34], cationic [35-40], and group transfer polymerizations [41] to prepare star polymers having varying arm numbers and compositions. 

Although a plethora of divinyl aromatic compounds have been investigated as precursors for hydrocarbon-soluble dihthium initiators (68), the only system which has been demonstrated to produce a hydrocarbon-soluble dihthium initiator is based on l,3-bis(l-phenylethenyl)benzene (60,81—85). The addition reaction of sec-hutylhthium with l,3-bis(l-phenylethenyl)benzene [34241-86-6] (eq. 16) proceeds rapidly and efficiently to produce the corresponding dilithinm species in toluene (86) or in cyclohexane (82). This dihthium initiator is not only soluble in hydrocarbon media such as cyclohexane, benzene, and toluene (even at —20° C) (84), but also functions as an efficient difunctional initiator for the preparation of homopolymers and triblock copolymers with relatively narrow molecular weight distributions (81—83). However, it is necessary to add a small amount of Lewis base or two equivalents of lithium jw-butoxide to produce narrow, monomodal molecular weight distributions. Lithium JW-butoxide is the preferred additive, since high 1,4-polybutadienes are obtained (60). 

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