Alkyl ketenes

Internal Sizing. The most widely used internal sizes are alkyl ketene dimers (AKD), alkenylsuccinic anhydrides (ASA), and rosin-based sizes that are used with papermaker s alum (aluminum sulfate with 14 waters of hydration), polyaluminum chloride (PAG), or polyaluminum siUcosulfate (PAS) (61). The rosin-based sizes are used under acidic conditions. Since the mid 1980 s there has been a steady conversion from acid to alkaline paper production, resulting in static to declining demand for the rosin-based sizing systems. Rosin is a complex mixture of compounds and consists primarily of monocarboxyhc acids with alkylated hydrophenan threne stmctures (62). A main constituent of wood rosin, gum rosin and taH-oil rosin is abietic acid. 

Figure 16 ToF-SIMS images of total ion current, AKD (alkyl ketene dimer) size, optical brightener and hexosan distribution on multipurpose paper surface after coating with Au-Pd. Reproduced from Fardim and Holmbom [70], with permission from Elsevier. 2005.

The trend towards increased calcium carbonate usage and therefore neutral pH paper making has meant that there has been a steady decline in the use of rosin and alum and a concomitant increase in the use of sizes which are effective at higher pH. Commercially the most important in this group are the alkenyl succinic anhydrides (ASA) and the alkyl ketene dimers (AKD). These are able to esterify fibre surfaces directly and are more effective at neutral to high pH. 

Sometimes internal sizing agents such as alkyl ketene dimers and... 

JACS 107 2192 (alkyl and vinyl ketene), 2194 (alkoxy ketene), 4339 (vinyl ketene) (1985) 110 649 (1988) (alkyl ketene)... 

In addition, azaferrocene catalyst 8 has been utilized by Fu and co-workers to perform enantioselective additions of alcohols to prochiral ketenes [51]. Aryl alkyl ketenes are substituted with MeOH to give a-aryl ester products in good enantioselectivities and very good yields, with higher enantiomeric excesses obtained for products with larger alkyl groups (Scheme 12). Use of 2,6-di-f-butylpyridinium triflate as a proton shuttle substantially enhances the enanti-... 

High reactivity was observed for 21b, and 21a was found to be the most selective. In the presence of 10 mol% 21a selectivity factors as high as 6.5 were observed with racemic 1-(1-naphthyl)ethanol as substrate (Scheme 12.6) [18]. The TBS analog of 21a was found to be good catalyst for asymmetric addition of methanol to a variety of prochiral aryl alkyl ketenes [18]. The catalytic asymmetric addition of achiral alcohols to prochiral ketenes is discussed in Section 13.2. 

Figure 1. Postulated sizing reactions of (a) alkyl ketene dimers and (b) alkyl succinic anhydrides. R. and R are short alkyl chains. Cell-OH represents cellulose.

Chiral aryl acetic acids constitute a privileged class of target structures due to their prevalence in bioactive natural products and pharmaceuticals and so, unsurprisingly, they constitute attractive targets for asymmetric synthesis [198]. The face-selective addition of a nucleophile to an aryl alkyl ketene provides a very direct entry for the preparation of such compounds. Although this can be achieved by the use of a chiral nucleophile or acid (cf. Scheme 8.1) [199], catalysis of the addition of an achiral nucleophile is clearly attractive from the standpoint of efficiency. 

With the advice of A. L. Rothschild and others, a series of experiments was begun in the Herty Foundation laboratories that culminated, in December 1959, in the production, on commercial equipment, of a paper meeting the tentative specifications. This was done at the Standard Paper Manufacturing Company in Richmond, Virginia. The pulps were all chemical wood the sizing was alkyl ketene dimer, which is compatible with mild alkalinity and calcium carbonate was added to maintain alkalinity (3). 

The key reaetion intermediate is an insitu alkyl ketene generated by the dehydrohalo-genation of the aeid chloride and is described (1). 

Traditionally papermaking is made at acid pH of about 4.5. Because of this the sizing of paper is carried out with resin acid salts in the presence of alum. Under these conditions, the resin acid anions complex with the aluminium cations and the complex formed is attracted to and deposited on the fibre surface. The purpose of sizing is to render the paper more resistant to water-based printer s ink. Today there is much interest in so-called alkaline sizing at about pH 7, which is preferred for specialist long-life papers. Here sizes such as alkyl ketene dimer replace alum. Alkaline papermaking has the further advantage that fillers such as calcium carbonate can be employed. 

This method has been used for the addition of chlorofluorocarbene to trimethylsilyl enol ethers or trimethylsilyl alkyl ketene acetals. The crude products 8 were rearranged (usually without isolation) to a-fluoro-a,/ -unsaturated aldehydes and ketones or esters of a-fluoro-a,/J-un-saturated acids, respectively, in high yields. ... 

Silyl alkyl ketene acetals, as ester enolate equivalents, are capable of regioselective acylation by acid chlorides. Rathke and Sullivan showed that a variety of acid chlorides reacted with the acetal (3) to give protected -keto esters (Scheme 11). Acid hydrolysis of the silylated products gave the free -keto ester. The reaction was successful with a variety of acyl chlorides, including acetic, butanoic, (2 )-buten-... 

The stereochemistry of cycloaddition of chiral alkyl ketenes with imines has been explored in cycloadditions designed to prepare intermediates for carbapcncm synthesis. Reaction of kctcnc (S)-57, prepared by treatment of the acid chloride with diisopropylethylamine at —40 C. with imine 58 gives 90% of the cw-/3-lactam 59 with d.r. [(35,47 )/(3/f,45 )] 87.5 12.579. The ketene rat-60 reacts with imine 61 to give the ci x-/3-lactam 62, d.r. [(2R, 3R )I(2S, 3S )] 70 30 80. 

Figure 3 The reaction of alkyl ketene dimer with cellulose...

Modern papers are often sized with an alkyl ketene dimer that reacts with hydroxyl groups on the cellulose, and the large alkyl group imparts water-repellency (see Figure 3). The product is a white solid that is emulsified with water for application onto the paper. 

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